tag:theconversation.com,2011:/id/topics/advanced-virgo-28471/articlesAdvanced Virgo – The Conversation2019-05-03T05:53:05Ztag:theconversation.com,2011:article/1162672019-05-03T05:53:05Z2019-05-03T05:53:05ZWe’ve detected new gravitational waves, we just don’t know where they come from (yet)<figure><img src="https://images.theconversation.com/files/272121/original/file-20190501-117601-6mspoo.jpg?ixlib=rb-1.1.0&rect=425%2C189%2C3144%2C1782&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A visualisation of a binary neutron star merger.</span> <span class="attribution"><a class="source" href="https://svs.gsfc.nasa.gov/12740">NASA's Goddard Space Flight Center/CI Lab</a></span></figcaption></figure><p>The hunt for <a href="https://theconversation.com/au/topics/gravitational-waves-9473">gravitational waves</a> is back on with the <a href="https://www.ligo.caltech.edu/news/ligo20190502">announcement overnight</a> of the detection of signals from what’s thought to be the merger of two <a href="http://astronomy.swin.edu.au/cosmos/N/Neutron+Star">neutron stars</a>, the incredibly dense remains of a collapsed star.</p>
<p>The signals were actually picked up on Thursday April 25 — ANZAC Day here in Australia — from a binary merger named <a href="https://gracedb.ligo.org/superevents/S190425z/view/">S190425z</a>), only the second ever neutron star merger to be observed. </p>
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<p>The twin detectors of the Laser Interferometer Gravitational-Wave Observatory (<a href="https://www.ligo.caltech.edu/page/ligo-detectors">LIGO</a>) — in Washington and Louisiana in the United States — along with Virgo, located at the European Gravitational Observatory (<a href="http://www.ego-gw.it/public/virgo/virgo.aspx">EGO</a>) in Italy, only resumed their operations on April 1 after a year and a half of upgrades. The latest result shows the hunt is back with a bang. </p>
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Read more:
<a href="https://theconversation.com/signals-from-a-spectacular-neutron-star-merger-that-made-gravitational-waves-are-slowly-fading-away-94294">Signals from a spectacular neutron star merger that made gravitational waves are slowly fading away</a>
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<p>This is the third observing run (named O3) and soon after the merger signal was detected, astronomers around the world started searching for a host galaxy, but this time there was an extra challenge.</p>
<h2>Where is the signal coming from?</h2>
<p>When LIGO detects <a href="http://astronomy.swin.edu.au/cosmos/G/Gravitational+Waves">gravitational waves</a> — the ripples in space-time predicted by Albert Einstein — we can work out some information quite accurately, such as the mass of merging neutron stars.</p>
<p>The images (below) of all the signals detected in the first and second observing runs of the detectors (named O1 and O2) show how each signal is unique. These differences allow us to work out the masses and distances to the objects.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/271925/original/file-20190501-39926-j0843w.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/271925/original/file-20190501-39926-j0843w.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/271925/original/file-20190501-39926-j0843w.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=342&fit=crop&dpr=1 600w, https://images.theconversation.com/files/271925/original/file-20190501-39926-j0843w.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=342&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/271925/original/file-20190501-39926-j0843w.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=342&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/271925/original/file-20190501-39926-j0843w.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=430&fit=crop&dpr=1 754w, https://images.theconversation.com/files/271925/original/file-20190501-39926-j0843w.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=430&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/271925/original/file-20190501-39926-j0843w.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=430&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Gravitational wave events detected by LIGO before O3. Each signal is different, revealing the properties of the merging objects.</span>
<span class="attribution"><a class="source" href="https://www.ligo.org/detections/O1O2catalog.php">LIGO/VIrgo/Georgia Tech/S. Ghonge & K. Jani</a></span>
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<p>But one thing that is harder to work out is <em>where</em> the signal is coming from?</p>
<p>We do this by triangulating the signal received at the three detectors (the two LIGO detectors in the US and the Virgo detector in Italy).</p>
<p>For the first detection of merging binary neutron stars, <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.161101" title="GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral">GW170817</a>, we got lucky. We were able to narrow down the signal to a region of 28 square degrees on the sky (about 140 times the area of the full Moon).</p>
<p>But S190425z was only detected in a single LIGO detector and Virgo, and hence the localisation region was 10,000 square degrees. That’s about a quarter of the entire sky.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/271926/original/file-20190501-39956-1y9s9ft.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/271926/original/file-20190501-39956-1y9s9ft.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/271926/original/file-20190501-39956-1y9s9ft.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/271926/original/file-20190501-39956-1y9s9ft.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/271926/original/file-20190501-39956-1y9s9ft.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/271926/original/file-20190501-39956-1y9s9ft.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/271926/original/file-20190501-39956-1y9s9ft.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/271926/original/file-20190501-39956-1y9s9ft.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">LIGO localisation for the neutron star merger S190425z. The area covers about a quarter of the entire sky.</span>
<span class="attribution"><span class="source">LIGO</span></span>
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<p>The neutron star merger is also estimated to have happened about 500 million light-years away from Earth. </p>
<h2>Needle in a haystack</h2>
<p>Astronomers around the world, including Australian teams, have been using telescopes from the outback of Western Australia to The Canary Islands in the Atlantic Ocean, to search for possible counterparts: galaxies that could be hosting the neutron star merger.</p>
<p>To do this we had to work out which of the 45,000 possible galaxies in the region would be the most likely hosts.</p>
<p>No confirmed matches have been found, so far, but on the way, we’ve found lots of other interesting events such as new supernovae - the explosions that occur when massive stars die.</p>
<p>This effort is an integral part of the Australian gravitational-wave hunting team at <a href="https://www.ozgrav.org/">OzGrav</a>. OzGrav supports more than 100 scientists and engineers who are making critical contributions to improving LIGO instrumentation, data analysis software, and interpretation of the results.</p>
<h2>How far can LIGO see now?</h2>
<p>The recent upgrades of LIGO and Virgo mean astronomers can now detect gravitational waves from binary neutron star mergers further than ever before, up to 500 million light years away.</p>
<p>Any signals we detect from these distant mergers would have left their host galaxy around the time the first fish evolved on Earth (two hundred million years before dinosaurs came along).</p>
<p>Every second counts when astronomers are trying to use gravitational wave triggers to capture the last moments as neutron stars collide. </p>
<p>The team at the University of Western Australia node of OzGrav has developed a real-time search program (called “SPIIR”) to trigger gravitational waves from the LIGO-Virgo data within ten seconds. </p>
<p>The team has already identified four gravitational-wave candidates, and in the future it may even be possible to eventually alert astronomers before the emission of any light from a merger.</p>
<h2>Beating the noise</h2>
<p>An important part of the LIGO O3 upgrade was the installation of instruments called “quantum squeezers”. The squeezers are based on an Australian National University design and ANU OzGrav scientists were part of the team that installed and commissioned them.</p>
<p>One of the most significant engineering challenges in building LIGO is reducing noise that can drown out the miniscule gravitational-wave signals. This noise comes from many different sources, such as seismic noise from earthquakes, ocean waves and even vehicle traffic.</p>
<p>Another source of noise is quantum noise, due to the discrete nature of light. The squeezers dampen this quantum noise by changing the quantum properties of the light used by LIGO to detect ripples in the fabric of spacetime. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/272118/original/file-20190501-117582-1i6qqvl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/272118/original/file-20190501-117582-1i6qqvl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/272118/original/file-20190501-117582-1i6qqvl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/272118/original/file-20190501-117582-1i6qqvl.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/272118/original/file-20190501-117582-1i6qqvl.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/272118/original/file-20190501-117582-1i6qqvl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/272118/original/file-20190501-117582-1i6qqvl.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/272118/original/file-20190501-117582-1i6qqvl.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">LIGO team members (left-to-right: Fabrice Matichard, Sheila Dwyer, Hugh Radkins) install in-vacuum equipment as part of the squeezed-light upgrade.</span>
<span class="attribution"><span class="source">Nutsinee Kijbunchoo/ANU</span></span>
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<h2>Another event detected</h2>
<p>With the third observing run now well underway, we’re already seeing the results of these improvements to LIGO instrumentation and software.</p>
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<em>
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Read more:
<a href="https://theconversation.com/how-we-found-a-white-dwarf-a-stellar-corpse-by-accident-114089">How we found a white dwarf – a stellar corpse – by accident</a>
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<p>In addition to the technical improvements there’s another marked contrast with previous observing runs: all detections are being released to the astronomy community, and the wider public, straight away. </p>
<p>In the midst of the excitement about S190425z there was <a href="https://gracedb.ligo.org/superevents/S190426c/view/">another gravitational-wave alert</a> a day later - a candidate signal with properties that suggest it could be a merger of a neutron star and a black hole.</p>
<p>This was picked up by all three detectors but as yet we also have no host identified for this, so we are not yet sure of the nature of this event. But it’s another hint of the exciting results yet to come.</p><img src="https://counter.theconversation.com/content/116267/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Tara Murphy works for the University of Sydney. She receives funding from the Australian Research Council.</span></em></p><p class="fine-print"><em><span>Eric Thrane works for Monash University. He receives funding from the Australian Research Council.</span></em></p><p class="fine-print"><em><span>Qi Chu works for the University of Western Australia. She receives funding from Australian Research Council.</span></em></p>The signal came in on ANZAC Day, ripples in space-time from the merger of two neutron stars an estimated 500-million light years away. But where it happened is still a mystery.Tara Murphy, Professor, University of SydneyEric Thrane, Associate professor, Monash UniversityQi Chu, Research fellow, The University of Western AustraliaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1079622018-12-03T13:06:06Z2018-12-03T13:06:06ZNew detections of gravitational waves brings the number to 11 – so far<figure><img src="https://images.theconversation.com/files/248354/original/file-20181203-194928-1wx57ti.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Ripples in space-time caused by massive events such this artist rendition of a pair of merging neutron stars.</span> <span class="attribution"><span class="source">Carl Knox, OzGrav</span>, <span class="license">Author provided</span></span></figcaption></figure><p>Four new detections of gravitational waves have been announced at the <a href="https://www.elisascience.org/news/conferences/gravitational-wave-physics-and-astronomy-workshop-gwpaw">Gravitational Waves Physics and Astronomy Workshop</a>, at the University of Maryland in the United States. </p>
<p>This brings the total number of detections to 11, since the first <a href="https://theconversation.com/gravitational-waves-discovered-the-universe-has-spoken-54237">back in 2015</a>.</p>
<p>Ten are from binary black hole mergers and one from the merger of two neutron stars, which are the dense remains of stellar explosions. One black hole merger was extraordinarily distant, and the most powerful explosion ever observed in astronomy.</p>
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Read more:
<a href="https://theconversation.com/the-search-for-the-source-of-a-mysterious-fast-radio-burst-comes-relatively-close-to-home-105735">The search for the source of a mysterious fast radio burst comes relatively close to home</a>
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<p>The latest news comes just a month after doubts were raised about the initial detection. In late October an article in New Scientist, headlined <a href="https://www.newscientist.com/article/mg24032022-600-exclusive-grave-doubts-over-ligos-discovery-of-gravitational-waves/">Exclusive: Grave doubts over LIGO’s discovery of gravitational waves</a>, raised the idea that it “might have been an illusion”. </p>
<p>So how confident are we that we are detecting gravitational waves, and not seeing an illusion?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/248361/original/file-20181203-194944-1pglcj3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/248361/original/file-20181203-194944-1pglcj3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/248361/original/file-20181203-194944-1pglcj3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/248361/original/file-20181203-194944-1pglcj3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/248361/original/file-20181203-194944-1pglcj3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/248361/original/file-20181203-194944-1pglcj3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/248361/original/file-20181203-194944-1pglcj3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/248361/original/file-20181203-194944-1pglcj3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=603&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Artist’s conception shows two merging black holes.</span>
<span class="attribution"><span class="source">LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)</span></span>
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<h2>Open to scrutiny</h2>
<p>All good scientists understand that scrutiny and scepticism is the power of science. All theories and all knowledge are provisional, as science slowly homes in on our best understanding of the truth. There is no certainty, only probability and statistical significance. </p>
<p>Years ago the team searching for gravitational waves with the Laser Interferometer Gravitational-Wave Observatory (LIGO), determined the levels of statistical significance needed to make a claim of detection. </p>
<p>For each signal we determine the false alarm rate. This tells you how many years you would need to wait before you have an even chance of a random signal mimicking your real signal.</p>
<p>The weakest signal detected so far has a false alarm rate of one every five years, so still there is a chance that it could have been accidental. </p>
<p>Other signals are much stronger. For the three strongest signals detected so far you would have to wait from 1,000 times to 10 billion billion times the age of the universe for the signals to occur by chance.</p>
<h2>Knowing what to listen out for</h2>
<p>The detection of gravitational waves is a bit like acoustic ornithology.</p>
<p>Imagine you study birds and want to determine the population of birds in a forest. You know the calls of the various bird species. </p>
<p>When a bird call matches your predetermined call, you jump with excitement. Its loudness tells you how far away it is. If it was very faint against the background noise, you may be uncertain.</p>
<p>But you need to consider the <a href="https://wildambience.com/wildlife-sounds/superb-lyrebird/">lyre birds</a> that mimic other species. How do you know that sound of a kookaburra isn’t actually made by a lyre bird? You have to be very rigorous before you can claim there is a kookaburra in the forest. Even then you will only be able to be confident if you make further detections.</p>
<p>In gravitational waves we use memorised sounds called templates. There is one unique sound for the merger of each possible combination of black hole masses and spins. Each template is worked out using Einstein’s theory of gravitational wave emission.</p>
<p>In the hunt for gravitational waves, we are searching for these rare sounds using <a href="https://www.ligo.caltech.edu/page/facilities">two LIGO detectors</a> in the US and a third detector, <a href="http://public.virgo-gw.eu/language/en/">Virgo</a>, in Italy.</p>
<p>To avoid missing signals or claiming false positives, the utmost rigour is needed to analyse the data. Huge teams look over the data, search for flaws, criticise each other, review computer codes and finally review proposed publications for accuracy. Separate teams use different methods of analysis, and finally compare results.</p>
<p>Next comes reproducibility – the same result recorded again and again. Reproducibility is a critical component of science.</p>
<h2>The signals detected</h2>
<p>Before LIGO made its first public announcement of gravitational waves, two more signals had been detected, each of them picked up in two detectors. This increased our confidence and told us that there is a population of colliding black holes out there, not just a single event that could be something spurious.</p>
<p>The <a href="https://theconversation.com/gravitational-waves-discovered-the-universe-has-spoken-54237">first detected gravitational wave</a> was astonishingly loud and it matched a pre-determined template. It was so good that LIGO spent many weeks trying to work out if it was possible for it to have been a prank, deliberately injected by a hacker. </p>
<p>While LIGO scientists eventually convinced themselves that the event was real, further discoveries greatly increased our confidence. In August 2017 a signal was detected by the two LIGO detectors and the Virgo detector in Italy.</p>
<p>On August 17 last year a completely different, but long predicted type of signal was observed from a <a href="https://theconversation.com/at-last-weve-found-gravitational-waves-from-a-collapsing-pair-of-neutron-stars-85528">coalescing pair of neutron stars</a>, accompanied by the predicted burst of gamma rays and light.</p>
<h2>The black hole mergers</h2>
<p>Now the LIGO-Virgo collaboration has completed the analysis of all the data since September 2015. </p>
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<figcaption><span class="caption">The ten black hole mergers.</span></figcaption>
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<p>For each signal we determine the mass of the two colliding black holes, the mass of the new black hole that they create, and rather roughly, the distance and the direction. </p>
<p>Each signal has been seen in two or three detectors almost simultaneously (they were separated by milliseconds).</p>
<p>Eight of the 20 initial black holes have masses between 30 and 40 Suns, six are in the 20s, three are in the teens and only two are as low as 7 to 8 Suns. Only one is near 50, the biggest pre-collision black hole yet seen.</p>
<p>These are the numbers that will help us work out where all these black holes were made, how they were made, and how many are out there. To answer these big questions we need many more signals.</p>
<figure class="align-center zoomable">
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<figcaption>
<span class="caption">Graphic showing the masses of recently announced gravitational-wave detections and black holes and neutron stars.</span>
<span class="attribution"><span class="source">LIGO-Virgo / Frank Elavsky / Northwestern</span></span>
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</figure>
<p>The weakest of the new signals, GW170729, was detected on July 29, 2017. It was the collision of a black hole 50 times the mass of the Sun, with another 34 times the mass of the Sun.</p>
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Read more:
<a href="https://theconversation.com/explainer-why-you-can-hear-gravitational-waves-when-things-collide-in-the-universe-92356">Explainer: why you can hear gravitational waves when things collide in the universe</a>
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<p>This was by far the most distant event, having taken place, most likely, 5 billion years ago – before the birth of Earth and the Solar system 4.6 billion years ago. Despite the weak signal, it was the most powerful gravitational explosion discovered, so far. </p>
<p>But because the signal was weak, this is the detection with the false alarm rate of one every five years.</p>
<p>LIGO and Virgo are improving their sensitivity year by year, and will be finding many more events. </p>
<p>With planned new detectors we anticipate ten times more sensitivity. Then we expect to be detecting new signals about every five minutes.</p><img src="https://counter.theconversation.com/content/107962/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Blair receives funding from the Australian Research Council. He is affiliated with the Gravity Discovery Centre Foundation that operates an exciting self-funded education centre near Gingin, Western Australia where you can find out much more about black holes, neutron stars and gravitational waves. </span></em></p>More ripples in space-time have been detected from merging pairs of black holes, one of which was the most massive and distant gravitational-wave source ever observed.David Blair, Emeritus Professor, ARC Centre of Excellence for Gravitational Wave Discovery, OzGrav, The University of Western AustraliaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/861042017-10-24T20:18:24Z2017-10-24T20:18:24ZCosmic alchemy: Colliding neutron stars show us how the universe creates gold<figure><img src="https://images.theconversation.com/files/191637/original/file-20171024-30571-frs0vu.jpg?ixlib=rb-1.1.0&rect=96%2C0%2C803%2C573&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Illustration of hot, dense, expanding cloud of debris stripped from the neutron stars just before they collided.</span> <span class="attribution"><a class="source" href="https://svs.gsfc.nasa.gov/12740">NASA's Goddard Space Flight Center/CI Lab</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>For thousands of years, humans have searched for a way to turn matter into gold. <a href="https://doi.org/10.1086/660139">Ancient alchemists</a> considered this precious metal to be the highest form of matter. As human knowledge advanced, the mystical aspects of alchemy gave way to the sciences we know today. And yet, with all our advances in science and technology, the origin story of gold remained unknown. Until now. </p>
<p>Finally, scientists know how the universe makes gold. Using our <a href="https://doi.org/10.3847/2041-8213/aa91c9">most advanced telescopes and detectors</a>, we’ve seen it created in the cosmic fire of the two colliding stars first detected by LIGO via the gravitational wave they emitted.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/191658/original/file-20171024-30605-ei0pxa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/191658/original/file-20171024-30605-ei0pxa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/191658/original/file-20171024-30605-ei0pxa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/191658/original/file-20171024-30605-ei0pxa.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/191658/original/file-20171024-30605-ei0pxa.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/191658/original/file-20171024-30605-ei0pxa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=425&fit=crop&dpr=1 754w, https://images.theconversation.com/files/191658/original/file-20171024-30605-ei0pxa.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=425&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/191658/original/file-20171024-30605-ei0pxa.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=425&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The electromagnetic radiation captured from GW170817 now confirms that elements heavier than iron are synthesized in the aftermath of neutron star collisions.</span>
<span class="attribution"><a class="source" href="https://www.caltech.edu/news/caltech-led-teams-strike-cosmic-gold-80074">Jennifer Johnson/SDSS</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
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</figure>
<h2>Origins of our elements</h2>
<p>Scientists have been able to piece together where many of the elements of the periodic table come from. The Big Bang <a href="https://doi.org/10.1146/annurev.nucl.56.080805.140437">created hydrogen</a>, the lightest and most abundant element. As stars shine, they fuse hydrogen into heavier elements like carbon and oxygen, the elements of life. In their dying years, stars create the common metals – aluminum and iron – and blast them out into space in different types of <a href="https://doi.org/10.1146/annurev.astro.38.1.191">supernova</a> <a href="https://doi.org/10.1146/annurev-astro-082708-101737">explosions</a>.</p>
<p>For decades, scientists have theorized that these stellar explosions also explained the origin of the heaviest and most rare elements, like gold. But they were missing a piece of the story. It hinges on the object left behind by the death of a massive star: a neutron star. Neutron stars pack one-and-a-half times the mass of the sun into a ball only 10 miles across. A teaspoon of material from their surface would weigh 10 million tons.</p>
<p>Many stars in the universe are in binary systems – two stars bound by gravity and orbiting around each other (think Luke’s home planet’s suns in “Star Wars”). A pair of massive stars might eventually end their lives as a pair of neutron stars. The neutron stars orbit each other for hundreds of millions of years. But Einstein says that their dance cannot last forever. Eventually, they must collide.</p>
<h2>Massive collision, detected multiple ways</h2>
<p>On the morning of August 17, 2017, a ripple in space passed through our planet. It was detected by the LIGO and Virgo gravitational wave detectors. This cosmic disturbance came from a pair of city-sized neutron stars colliding at one third the speed of light. The <a href="https://doi.org/10.1103/PhysRevLett.119.161101">energy of this collision</a> surpassed any atom-smashing laboratory on Earth.</p>
<p>Hearing about the collision, astronomers around the world, <a href="http://kilonova.org/about.html">including</a> <a href="https://dabrown.expressions.syr.edu/">us</a>, jumped into action. Telescopes large and small scanned the patch of sky where the gravitational waves came from. Twelve hours later, three telescopes caught sight of a brand new star – called a kilonova – in a galaxy called NGC 4993, about 130 million light years from Earth.</p>
<p>Astronomers had captured the light from the cosmic fire of the colliding neutron stars. It was time to point the world’s biggest and best telescopes toward the new star to see the visible and infrared light from the collision’s aftermath. In Chile, the Gemini telescope swerved its large 26-foot mirror to the kilonova. NASA steered the Hubble to the same location.</p>
<figure>
<img src="http://kilonova.org/img/DECam_fading_kn_final.gif">
<figcaption><span class="caption">Movie of the visible light from the kilonova fading away in the galaxy NGC 4993, 130 million light years away from Earth.</span></figcaption>
</figure>
<p>Just like the embers of an intense campfire grow cold and dim, the afterglow of this cosmic fire quickly faded away. Within days the visible light faded away, leaving behind a warm infrared glow, which eventually disappeared as well. </p>
<h2>Observing the universe forging gold</h2>
<p>But in this fading light was encoded the answer to the age-old question of how gold is made.</p>
<p>Shine sunlight through a prism and you will see our sun’s spectrum – the colors of the rainbow spread from short wavelength blue light to long wavelength red light. This spectrum contains the fingerprints of the elements bound up and forged in the sun. Each element is marked by a unique fingerprint of lines in the spectrum, reflecting the different atomic structure.</p>
<p>The spectrum of the kilonova contained the fingerprints of the heaviest elements in the universe. Its light carried the telltale signature of the neutron-star material decaying into platinum, gold and other so-called <a href="https://en.wikipedia.org/wiki/R-process">“r-process” elements</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/191689/original/file-20171024-30613-1iljobe.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/191689/original/file-20171024-30613-1iljobe.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/191689/original/file-20171024-30613-1iljobe.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/191689/original/file-20171024-30613-1iljobe.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/191689/original/file-20171024-30613-1iljobe.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/191689/original/file-20171024-30613-1iljobe.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/191689/original/file-20171024-30613-1iljobe.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/191689/original/file-20171024-30613-1iljobe.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Visible and infrared spectrum of the kilonova. The broad peaks and valleys in the spectrum are the fingerprints of heavy element creation.</span>
<span class="attribution"><span class="source">Matt Nicholl</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>For the first time, humans had seen alchemy in action, the universe turning matter into gold. And not just a small amount: This one collision created at least 10 Earths’ worth of gold. You might be wearing some gold or platinum jewelry right now. Take a look at it. That metal was created in the atomic fire of a neutron star collision in our own galaxy billions of years ago – a collision just like the one seen on August 17.</p>
<p>And what of the gold produced in this collision? It will be blown out into the cosmos and mixed with dust and gas from its host galaxy. Perhaps one day it will form part of a new planet whose inhabitants will embark on a millennia-long quest to understand its origin.</p><img src="https://counter.theconversation.com/content/86104/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Duncan Brown receives funding from the National Science Foundation and the Research Corporation for Science Advancement.</span></em></p><p class="fine-print"><em><span>Edo Berger receives funding from the National Science Foundation and NASA. </span></em></p>Until the recent observation of merging neutron stars, how the heaviest elements come to be was a mystery. But their fingerprints are all over this cosmic collision.Duncan Brown, Professor of Physics, Syracuse UniversityEdo Berger, Professor of Astronomy, Harvard UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/857272017-10-16T14:12:17Z2017-10-16T14:12:17ZLIGO announcement vaults astronomy out of its silent movie era into the talkies<figure><img src="https://images.theconversation.com/files/190302/original/file-20171016-27747-10xe6lo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Supercomputer simulation of a pair of neutron stars colliding.</span> <span class="attribution"><a class="source" href="https://svs.gsfc.nasa.gov/11530">NASA/AEI/ZIB/M. Koppitz and L. Rezzolla</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>When <a href="https://theconversation.com/what-happens-when-ligo-texts-you-to-say-its-detected-one-of-einsteins-predicted-gravitational-waves-53259">LIGO detected its first gravitational wave</a> back in September 2015, I was pretty excited to say the least. As part of a decades-long endeavor, our whole team was ecstatic to observe gravitational waves – <a href="https://www.ligo.caltech.edu/page/what-are-gw">which are literally ripples in space</a> – caused by two black holes smashing together. It was the first time that Einstein’s predictions about these tiny ripples were directly confirmed. Just this month, the <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/2017/">Nobel Prize in physics was awarded to three of the founders</a> of our international collaborative effort – Rainer Weiss, Kip Thorne and Barry Barish – in recognition of this first observation.</p>
<p>It may be hard to believe, but today I am even more excited than I was in 2015. For the first time ever, astrophysicists have discovered <a href="https://doi.org/10.1103/PhysRevLett.119.161101">gravitational waves originating from an entirely new source</a>: merging neutron stars.</p>
<p>That’s not all. This new event, GW170817, was accompanied by a host of other observations across the electromagnetic spectrum <a href="https://doi.org/10.3847/2041-8213/aa920c">including gamma-rays</a>, X-rays, visible light and <a href="https://doi.org/10.1126/science.aap9855">radio waves</a>. Before, we had detected only gravitational waves on their own, without any other corroborating observations of the source event. This groundbreaking announcement from the <a href="http://www.ligo.org/">LIGO Scientific Collaboration</a> and the <a href="http://www.virgo-gw.eu/">Virgo collaboration</a> heralds the beginning of <a href="https://doi.org/10.3847/2041-8213/aa91c9">a new era in “multi-messenger” astronomy</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/190371/original/file-20171016-31016-11kx1ph.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190371/original/file-20171016-31016-11kx1ph.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190371/original/file-20171016-31016-11kx1ph.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190371/original/file-20171016-31016-11kx1ph.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190371/original/file-20171016-31016-11kx1ph.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190371/original/file-20171016-31016-11kx1ph.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190371/original/file-20171016-31016-11kx1ph.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190371/original/file-20171016-31016-11kx1ph.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=603&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Various telescopes are focused on different energy wavelengths along the electromagnetic spectrum.</span>
<span class="attribution"><a class="source" href="http://chandra.harvard.edu/resources/illustrations/elec_mag_spec.html">NASA/CXC</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
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<p>Until gravitational waves were discovered, astronomy was essentially in its silent film era. Gravitational waves provide something like a long-awaited soundtrack for our universe. The 2015 breakthrough and subsequent gravitational wave observations never managed to synchronize the sights and sounds of the cosmos, though. That changed with the detection of GW170817. Today we celebrate astronomy’s version of “the talkie” with the simultaneous observation of gravitational waves and electromagnetic radiation from the same source.</p>
<h2>A text alert with big implications</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/190284/original/file-20171015-3532-1r6xfxl.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190284/original/file-20171015-3532-1r6xfxl.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/190284/original/file-20171015-3532-1r6xfxl.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=303&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190284/original/file-20171015-3532-1r6xfxl.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=303&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190284/original/file-20171015-3532-1r6xfxl.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=303&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190284/original/file-20171015-3532-1r6xfxl.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=380&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190284/original/file-20171015-3532-1r6xfxl.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=380&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190284/original/file-20171015-3532-1r6xfxl.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=380&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Original text indicated a potential compact binary coalescence (CBC) – what we know know as the first binary neutron star merger, named GW170817.</span>
<span class="attribution"><span class="source">Chad Hanna</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>On August 17, 2017 at 8:47 a.m. EDT, I received a text message from the LIGO real-time analysis system that indicated a significant gravitational wave candidate had been identified. My text message notification is a simple “Hey!” I’m not a huge texter, so a large fraction of the messages I receive tend to be about gravitational waves. As you might imagine, I have a somewhat Pavlovian response to that “Hey!” </p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/190285/original/file-20171015-3511-1e3ik35.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190285/original/file-20171015-3511-1e3ik35.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/190285/original/file-20171015-3511-1e3ik35.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=431&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190285/original/file-20171015-3511-1e3ik35.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=431&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190285/original/file-20171015-3511-1e3ik35.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=431&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190285/original/file-20171015-3511-1e3ik35.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=542&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190285/original/file-20171015-3511-1e3ik35.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=542&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190285/original/file-20171015-3511-1e3ik35.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=542&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Institute for Gravitation and the Cosmos, where Penn State researchers keep an eye on real-time updates to the gravitational wave candidate event database (GraCEDb).</span>
<span class="attribution"><span class="source">Chad Hanna</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>At Penn State, we have a small conference room devoted to LIGO real-time analysis where we monitor gravitational wave searches. When I turned to look at our display of the candidate event database, I was shocked. Not only was this new gravitational wave extremely significant, but there was also a coincident gamma-ray burst (GRB).</p>
<p>Given that the LIGO and Virgo collaborations had already detected four gravitational waves known as <a href="https://doi.org/10.1103/PhysRevLett.116.061102">GW150914</a>, <a href="https://doi.org//10.1103/PhysRevLett.116.241103">GW151226</a>, <a href="https://doi.org//10.1103/PhysRevLett.118.221101">GW170104</a> and <a href="https://doi.org//10.1103/PhysRevLett.119.141101">GW170814</a>, it wasn’t that surprising to see another significant gravitational wave candidate that August morning – but one that coincided with a gamma-ray burst was simply surreal.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/190250/original/file-20171015-3511-thqs2y.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190250/original/file-20171015-3511-thqs2y.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190250/original/file-20171015-3511-thqs2y.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=402&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190250/original/file-20171015-3511-thqs2y.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=402&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190250/original/file-20171015-3511-thqs2y.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=402&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190250/original/file-20171015-3511-thqs2y.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=505&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190250/original/file-20171015-3511-thqs2y.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=505&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190250/original/file-20171015-3511-thqs2y.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=505&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">LIGO’s previous detections of gravitational waves came from colliding black holes. The latest detection is not pictured and lasted much longer than the scale of this graphic, more than 100 seconds.</span>
<span class="attribution"><a class="source" href="https://www.ligo.caltech.edu/image/ligo20170927d">LIGO/Caltech/MIT/LSC</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Until that day, LIGO and Virgo had detected gravitational waves only from binary black holes – astronomical phenomena that are unlikely to be associated with an electromagnetic counterpart such as a gamma-ray burst. Despite tremendous effort, researchers hadn’t ever been able to confidently correlate a gravitational wave event with other astronomical observations. </p>
<p>I immediately dialed into the LIGO rapid response teleconference channel and opened my laptop. There I saw the missing link, which was not immediately clear from the text message: The estimated masses of each object that had merged were just 1.2 and 1.5 times the mass of our sun. That’s firmly within the expectations of a new gravitational wave source type: neutron stars.</p>
<p>We had always hoped that one day LIGO would detect a gravitational wave signal simultaneously with other telescopes. In fact, we even thought that two colliding neutron stars would be the most promising source. To increase our chances, LIGO and Virgo have developed a program over the last decade to rapidly analyze gravitational wave data and alert a <a href="http://ligo.org/scientists/GWEMalerts.php">worldwide team of astronomers</a> to our findings so that they can observe the area of interest. LIGO and Virgo also receive alerts of transient astronomical phenomena so researchers can undertake a deeper search of gravitational wave data. It looked like this was finally it.</p>
<h2>Chasing down the confirming data</h2>
<p>For all that had gone right that morning, a few things were bound to go wrong.</p>
<p>We recovered the gravitational wave signal in the LIGO Washington detector data only in real time. Unfortunately, the LIGO Louisiana detector had suffered from a burst of instrumental noise right around the time that the neutron star merger signal had arrived. The Virgo detector data from Italy was clean, but the transatlantic data transfer had stopped due to a network connection outage.</p>
<p>Our group proceeded anyway, and the LIGO rapid response team quickly assembled a notice to be sent to our <a href="https://gcn.gsfc.nasa.gov/gcn3/21505.gcn3">over 70 observing partners</a> – astronomers from all over the world. This was the first notice of around 200 and counting that helped to firmly establish gravitational wave multi-messenger astronomy as a brand new field.</p>
<p>The binary neutron star signal lasted over 100 seconds in LIGO’s data. That’s long enough to recover almost all of the signal in spite of the instrumental noise in the LIGO Louisiana data, which affected only the very end of the detected signal.</p>
<p>Eventually, we were able to analyze all three gravitational wave detector data streams to figure out when the signal arrived at each one. Then we triangulated the gravitational wave source on the sky to a sufficiently small area that astronomers could survey the entire region. </p>
<p>We were fortunate that the <a href="https://www.nasa.gov/content/fermi-gamma-ray-space-telescope">Fermi Gamma-ray Space Telescope</a> was already pointing in the direction of this new gravitational wave when it arrived at Earth. However, astronomers using ground-based telescopes had to wait for nighttime.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/190419/original/file-20171016-30971-sdpddh.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190419/original/file-20171016-30971-sdpddh.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190419/original/file-20171016-30971-sdpddh.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=297&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190419/original/file-20171016-30971-sdpddh.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=297&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190419/original/file-20171016-30971-sdpddh.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=297&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190419/original/file-20171016-30971-sdpddh.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=373&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190419/original/file-20171016-30971-sdpddh.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=373&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190419/original/file-20171016-30971-sdpddh.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=373&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Composite images of the optical counterpart to GW170817. Each image is 1.5 arcseconds on a side. Images are taken two weeks apart.</span>
<span class="attribution"><a class="source" href="http://ligo.org/detections/GW170817.php">Soares-Santos et al. and DES Collaboration</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>About 10 hours after the initial alert, the first news of a visible light counterpart emerged and was independently confirmed by many facilities: A new bright spot that hadn’t been there previously was found in a galaxy in the direction of the gravitational wave. Over the coming hours, days and weeks, we also learned that there were ultraviolet counterparts, X-ray counterparts and even radio waves all associated with the binary neutron star merger. Each complementary observation revealed a new part of the story of how the immensely energetic neutron star matter was flying off into space after the collision.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/190427/original/file-20171016-30966-1qjsqhu.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190427/original/file-20171016-30966-1qjsqhu.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/190427/original/file-20171016-30966-1qjsqhu.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190427/original/file-20171016-30966-1qjsqhu.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190427/original/file-20171016-30966-1qjsqhu.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190427/original/file-20171016-30966-1qjsqhu.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190427/original/file-20171016-30966-1qjsqhu.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190427/original/file-20171016-30966-1qjsqhu.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">We can pinpoint sources like GW170817 by triangulating the three signals. The rapid Hanford-Livingston localization is shown in blue, and the final Hanford-Livingston-Virgo localization is in green. The gray rings are one-sigma triangulation constraints from the three detector pairs.</span>
<span class="attribution"><a class="source" href="http://ligo.org/detections/GW170817.php">LIGO/Virgo/NASA/Leo Singer (Milky Way image: Axel Mellinger)</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>We were lucky to pin down the location of the gravitational wave quickly enough to identify the observational counterparts so early. Nevertheless, next time around, we hope that gravitational wave identification can happen even sooner and at a more favorable time of day so we don’t miss out on the earliest optical emission. Perhaps one day we’ll even be able to use the early gravitational wave emission leading up to a neutron star collision to predict where on the sky they’ll merge and have telescopes already pointed in that direction, ready for the show.</p>
<h2>Future of multi-messenger astronomy with LIGO</h2>
<p>GW170817 confirmed the hypothesis that at least some gamma-ray bursts are in fact <a href="http://theconversation.com/why-astrophysicists-are-over-the-moon-about-observing-merging-neutron-stars-84957">caused by merging neutron stars</a>. It shattered our expectations for how frequently we would be able to associate gamma-ray bursts with gravitational waves and how prevalent other electromagnetic counterparts would be. After all, we’ve been <a href="https://www.ligo.caltech.edu/page/timeline">operating advanced LIGO for only two years</a>!</p>
<p>Our most optimistic hopes have come true with this new gravitational wave, and the team hopes to have many more opportunities like this in the next few years. The future of <a href="https://www.nsf.gov/about/congress/reports/nsf_big_ideas.pdf#page=7">multi-messenger astronomy</a> is very, very bright.</p><img src="https://counter.theconversation.com/content/85727/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Chad Hanna receives funding from the National Science Foundation and the Charles E. Kaufman Foundation of The Pittsburgh Foundation.</span></em></p>A LIGO team member describes how the detection of a gravitational wave from a new source – merging neutron stars – vaults astronomy into a new era of ‘multi-messenger’ observations.Chad Hanna, Assistant Professor of Physics, Penn StateLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/849572017-10-16T14:03:49Z2017-10-16T14:03:49ZWhy astrophysicists are over the moon about observing merging neutron stars<figure><img src="https://images.theconversation.com/files/190212/original/file-20171013-3555-ldwh37.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Simulation of two neutron stars merging.</span> <span class="attribution"><a class="source" href="https://svs.gsfc.nasa.gov/10740">NASA/AEI/ZIB/M. Koppitz and L. Rezzolla</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>When <a href="https://www.ligo.caltech.edu/">LIGO</a>, the Laser Interferometer Gravitational-Wave Observatory, first detected <a href="https://www.ligo.caltech.edu/page/what-are-gw">gravitational waves</a> from merging black holes, it opened up a new window in astrophysics and provided the most powerful confirmation yet of Einstein’s theory of general relativity. Now LIGO has done it again, together with the <a href="https://www.ego-gw.it/public/about/whatIs.aspx">Virgo interferometer</a>, this time by <a href="https://doi.org/10.1103/PhysRevLett.119.161101">observing merging neutron stars</a> – something astrophysicists had known must happen but had never been able to detect definitively until now.</p>
<p>Observing two neutron stars smash together is important for much more than just the thrill of discovery. This news may confirm a longstanding theory: that some gamma-ray bursts (GRBs for short), which are among the most energetic, luminous events in the universe, are the result of merging neutron stars. And it is in the crucible of these mergers that most heavy elements may be forged. Researchers can’t produce anything like the temperatures or pressures of neutron stars in a laboratory, so observation of these exotic objects provides a way to test what happens to matter at such extremes.</p>
<p>Astronomers are excited because for the first time they have gravitational waves and light signals stemming from the same event. These truly independent measurements are separate avenues that together add to the physical understanding of the neutron star merger.</p>
<h2>Gravitational waves just one part of this news</h2>
<p>The LIGO project has thus far announced the detection of four mergers of binary black holes – observed via the gravitational waves they emitted. These are ripples in the fabric of spacetime propagating in all directions, like waves emanating out from a pebble dropped in a pond. Encoded in the gravitational wave signal is information about the pre- and post-merger masses of the objects. Black holes are much more massive than neutron stars, so the energy they release as gravitational waves is much higher. Because light cannot escape from a black hole, you expect (and see) no light from these mergers.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Artist’s rendering of a gamma-ray burst, the most energetic form of light.</span>
<span class="attribution"><a class="source" href="https://svs.gsfc.nasa.gov/12055">NASA/Swift/Cruz deWilde</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>The merger of neutron stars should produce both a gravitational wave and a short gamma-ray burst signal. These brief, incredibly intense flashes of gamma-ray light are seen from galaxies across the universe. They come in two types, classified by their duration. Short GRBs are thought to <a href="https://doi.org/10.1016/j.physrep.2007.02.005">come from the mergers of neutron stars</a>, while long GRBs are known to be coincident with supernovas.</p>
<p>Key to unlocking the mystery of any astronomical object is knowing its distance. In recent years, astronomers have <a href="https://doi.org/10.1086/498107">identified the host galaxies</a> of a <a href="https://doi.org/10.1086/512664">handful of short GRBs</a>. Determining those galaxies’ distances allows astronomers to calculate the power emitted in gamma-rays during the burst, and to determine (or rule out) physical scenarios that could produce that power.</p>
<p>But for LIGO to detect two <a href="https://doi.org/10.1103/PhysRevD.93.112004">neutron stars spiraling in toward each other and merging</a>, it would need to happen relatively nearby – within around 250 million light-years. That such an event was not detected during the first year and a half of LIGO observations already lets astronomers place a constraint on how frequently they happen in the nearby universe.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Galaxy NGC 4993 seemed unassuming enough….</span>
<span class="attribution"><a class="source" href="http://stdatu.stsci.edu/dss/index.html">Palomar Observatory – Space Telescope Science Institute Digital Sky Survey</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>So the rumor of a merging neutron star detection by LIGO with a coincident short gamma-ray burst (<a href="https://gcn.gsfc.nasa.gov/other/170817A.gcn3">GRB170817A</a>) seen by NASA’s <a href="https://fermi.gsfc.nasa.gov/">Fermi Gamma-ray Space Telescope</a> spread through the astronomical community like wildfire this past summer. Astronomers watched from the sidelines as most of the major telescopes in (and above) the world slewed toward an otherwise unremarkable old, nearby (130 million light-years) elliptical galaxy named NGC 4993.</p>
<h2>What we’ve known about neutron stars</h2>
<p>Most stars end their lives relatively calmly; no longer supported by the fusion of hydrogen into helium, their outer layers glide slowly off into space while their cores collapse to the very limits allowed by normal matter – burning embers the size of the Earth called white dwarf stars.</p>
<p>For the rare stars whose masses are a bit higher, 10 to 20 times that of the sun, the picture is a bit different. These stars die the way they lived: quickly and violently, ejecting their outer layers as supernovas and leaving behind something far stranger – a neutron star.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=945&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=945&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=945&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1187&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1187&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1187&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Nobel Prize-winning physicist Subrahmanyan Chandrasekhar.</span>
<span class="attribution"><a class="source" href="http://www.apimages.com/metadata/Index/AP-A-IL-CX3-OBIT-CHANDRASEKHAR/a3ada89cc6e0da11af9f0014c2589dfb/2/0">AP Photo</a></span>
</figcaption>
</figure>
<p>The details of this story were worked out in 1930 by then 19-year-old Indian astrophysicist <a href="http://chandra.harvard.edu/about/chandra.html">Subrahmanyan Chandrasekhar</a>. He determined precisely how far you can compress normal matter before the relentless pressure of gravity forces electrons into the nuclei of their atoms where they merge with protons to form neutrons. Instead of an Earth-sized remnant, a massive star’s core collapses further to become a highly compressed ball of exotic matter as small as a city but whose mass can be twice that of the sun.</p>
<p>Neutron stars rotate incredibly rapidly. The collapse from millions to tens of kilometers in extent increases their spin due to conservation of angular momentum, like an ice skater pulling in her arms. While the parent star may have rotated once a month, a newly born neutron star can spin hundreds of times per second.</p>
<p>This rapid spinning led to their initial discovery. 50 years ago, Antony Hewish and Jocelyn Bell Burnell <a href="https://www.atnf.csiro.au/outreach/education/everyone/pulsars/index.html">discovered the first radio pulsar</a>: a neutron star emitting radio waves which appear to observers as pulses as the star rotates, like a lighthouse. <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1974/">Hewish would win the 1974 Nobel Prize in physics</a> for this discovery, while Bell Burnell was controversially overlooked.</p>
<p>But what are neutron stars really made of? Are they neutrons all the way through or can they break down further again, into what physicists call “quark soup”? The answer lies in measuring their size. A larger neutron star is mostly neutrons, a smaller star has a more complicated interior made of quarks – the building blocks of protons and neutrons. Untangling how this works is important for our understanding of the fundamental properties of subatomic particles. <a href="https://www.nasa.gov/nicer">A new telescope on the International Space Station</a> aims to address this question by targeting neutron stars and measuring their sizes.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=393&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=393&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=393&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=494&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=494&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=494&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The orbiting neutron stars rapidly lose energy by emitting gravitational waves and merge after about three orbits, or in less than 8 milliseconds. A black hole forms and the magnetic field becomes more organized, eventually producing structures capable of supporting the jets that power short gamma-ray bursts.</span>
<span class="attribution"><a class="source" href="https://svs.gsfc.nasa.gov/10740">NASA/AEI/ZIB/M. Koppitz and L. Rezzolla</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>When neutron stars merge</h2>
<p>Over half of all stars are part of binary pairs, and massive stars are more likely to occur in binaries. These pairs of massive stars will co-evolve, and when they die, a pair of neutron stars may remain, orbiting one another.</p>
<p>An orbiting pair of neutron stars loses energy by emitting gravitational waves, and over time this loss of energy will cause them to migrate closer and closer until they eventually collide. While the eventual merger is nearly instantaneous, the gradual inspiral takes tens to hundreds of millions of years, so we expect to see mergers in more evolved galaxies – like NGC 4993, for instance – rather than those that are still rapidly forming new stars.</p>
<p>For decades, it has been <a href="https://doi.org/10.1086/181612">suggested that merging neutron stars</a> may <a href="https://doi.org/10.3847/2041-8205/829/1/L13">provide a mechanism for producing most of the elements</a> on the periodic table heavier than iron. These so-called r-process elements must form in a neutron-rich environment, and have been formed by humans only during the explosion of nuclear bombs.</p>
<p>The signal from such an event is suspected to rapidly cascade through the electromagnetic spectrum, from gamma-rays to X-rays, visible light and infrared. Known as kilonovas, <a href="https://doi.org/10.1038/nature12505">these afterglows have been seen</a> from past short GRBs.</p>
<p>Finally all the pieces fall into place with this gravitational wave detected by the LIGO and Virgo teams, and all the subsequent supporting observations made by astronomers around the world. We know the neutron star masses, the duration of the event, and the distance of the host galaxy. This not only confirms the hypothesis that <a href="https://doi.org/10.3847/2041-8213/aa920c">merging neutron stars produce short GRBs</a>; it lays the foundation for astronomers to produce models of the merger backed both by fundamental physics and real world observations. It’s a rare event to see something new for the first time, and rarer still that it confirms a longstanding theory.</p><img src="https://counter.theconversation.com/content/84957/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Roy Kilgard has received funding from NASA through the Space Telescope Science Institute and from the Smithsonian Astrophysical Observatory.</span></em></p>The gravitational wave itself is the least exciting part of the announcement from LIGO and Virgo. Observing this new source answers many longstanding questions.Roy Kilgard, Research Associate Professor of Astronomy, Wesleyan UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/610562016-06-15T17:29:16Z2016-06-15T17:29:16ZGravitational waves found again: here’s how they could whisper the universe’s secrets<figure><img src="https://images.theconversation.com/files/126774/original/image-20160615-14054-1os72zd.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.youtube.com/watch?v=s7Oq8_9QlHQ">YouTube</a></span></figcaption></figure><p>The international team of physicists and astronomers responsible for the <a href="https://theconversation.com/gravitational-waves-found-the-inside-story-54589">discovery</a> of gravitational waves <a href="http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102">back in February</a> has <a href="http://link.aps.org/doi/10.1103/PhysRevLett.116.241103">announced</a> the detection of a second strong signal from the depths of space. It is further confirmation that gravitational waves both exist and tell us a whole new story about how the universe came to be the way it is.</p>
<p>In 1915 Albert Einstein put forward his <a href="http://www.space.com/17661-theory-general-relativity.html">general theory of relativity</a>, making the bold step of equating gravity to distortions of time and space caused by the presence of mass. He and others went on to predict that accelerating masses would generate ripples of distortion, flowing out as gravitational waves, though it was thought these would be too weak to ever measure. </p>
<p>Like the first detection, this second fleeting signal came from two black holes in a tight orbit – what’s known as a <a href="http://heavy.com/news/2016/02/binary-black-hole-merger-gravitational-waves-what-are-speed-of-light-general-relativity/">binary black hole system</a>. After spending many millions of years orbiting each other in ever decreasing circles, these black holes collided in a fraction of a second over a billion years ago.</p>
<p>They had total mass around three times smaller than in the first detection – roughly 22 times the mass of our sun – but once again the collision shook the structure of the universe so violently that we detected the waves of stretched and squeezed space and time as they passed through us at the speed of light on Boxing Day last year. </p>
<p>Even so, this did not produce the kind of clearly visible waveform that heralded the first detection. It only stood out strongly once processed by the software the team developed over many years to extract the maximum amount of science from our three gravitational wave detectors – the two that comprise <a href="https://theconversation.com/gravitational-waves-found-the-inside-story-54589">Advanced LIGO</a> (Laser Interferometer Gravitational Wave Observatory) in the US and <a href="http://www.virgo-gw.eu/">Advanced Virgo</a> in Italy. </p>
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<h2>Questions and possible answers</h2>
<p>The existence of two black holes with such high masses came as a surprise to many astronomers at the time of the first announcement, since the formation routes for such black holes are unclear. It was in fact the first direct evidence that binary black holes exist and can collide and merge. </p>
<p>The existence of massive black holes raises many questions: were they formed in the Big Bang or through the collapse of matter at a later date? Did they form from a pair of <a href="http://www.bbc.co.uk/earth/story/20151020-our-sun-is-big-but-some-stars-make-it-look-like-a-dust-mote">supermassive stars</a> or did they join together by chance once the stars had become black holes? Are they related to the formation of the <a href="http://www.cosmotography.com/images/supermassive_blackholes_drive_galaxy_evolution_2.html">supermassive black holes</a> that appear to exist in the cores of nearly all galaxies? And what are the implications for the collapse of matter into stars and galaxies and the formation of structure in the universe? </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/126602/original/image-20160614-22388-m1tvy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/126602/original/image-20160614-22388-m1tvy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/126602/original/image-20160614-22388-m1tvy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/126602/original/image-20160614-22388-m1tvy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/126602/original/image-20160614-22388-m1tvy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/126602/original/image-20160614-22388-m1tvy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/126602/original/image-20160614-22388-m1tvy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/126602/original/image-20160614-22388-m1tvy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Black hole at the centre of a galaxy.</span>
<span class="attribution"><a class="source" href="http://www.shutterstock.com/cat.mhtml?lang=en&language=en&ref_site=photo&search_source=search_form&version=llv1&anyorall=all&safesearch=1&use_local_boost=1&autocomplete_id=&search_tracking_id=fZkGRhE-L3qh0SdUuAgLjg&searchterm=gravitational%20waves&show_color_wheel=1&orient=&commercial_ok=&media_type=images&search_cat=&searchtermx=&photographer_name=&people_gender=&people_age=&people_ethnicity=&people_number=&color=&page=1&inline=340299842">Malll Themd</a></span>
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<p>When it comes to answering questions such as these, the number of events we see is important. Everything we have seen to date was the result of the inaugural four-month run of Advanced LIGO – <a href="https://dcc.ligo.org/LIGO-P1600088/public">two spectacular deaths</a> of binary black hole systems (and possibly also a third one, though the signal is not strong enough for us to say with certainty). </p>
<p>The tantalising implication is that we will see many more in future. This would let us see the full range of masses of binary black holes, and also how rapidly they spin and how their spin axes are aligned. These are crucial factors in determining how black holes are formed. </p>
<p>As if that wasn’t enough, these signals give us something else. It’s basic, but in astronomy it is a very highly prized number: the distance to their source. There are remarkably few reliable ways to measure the biggest distances in the universe and gravitational astronomy provides one that is entirely new. As the field advances and new telescopes become available, this could help us map the expansion of the universe and examine the elusive ideas of “dark matter” and “dark energy” that run through current cosmological theories.</p>
<p>When the Advanced LIGO detectors <a href="https://www.caltech.edu/news/dedication-advanced-ligo-46822">return to operation</a> towards the end of the year it will be with <a href="http://relativity.livingreviews.org/Articles/lrr-2016-1/">even greater sensitivity</a> and they will observe for longer, this time in conjunction with Advanced Virgo. As a result we can reasonably hope to see approximately ten further binary black hole signals by this time next year. By 2018 we <a href="http://arxiv.org/abs/1602.03842">predict that</a> we will see many tens, maybe even hundreds of such signals, allowing us to study the population of these enigmatic sources more deeply. </p>
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<a href="https://images.theconversation.com/files/126603/original/image-20160614-22377-153li9k.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/126603/original/image-20160614-22377-153li9k.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/126603/original/image-20160614-22377-153li9k.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/126603/original/image-20160614-22377-153li9k.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/126603/original/image-20160614-22377-153li9k.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/126603/original/image-20160614-22377-153li9k.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/126603/original/image-20160614-22377-153li9k.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/126603/original/image-20160614-22377-153li9k.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Northern leg of LIGO interferometer in Washington, US.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/LIGO#/media/File:Northern_leg_of_LIGO_interferometer_on_Hanford_Reservation.JPG">Wikimedia</a></span>
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<p>The improved sensitivity and the third detector in Italy also open the door to even more exciting discoveries to come. We are hoping to observe gravitational waves from neutron stars. These are the remnants of lower-mass stars whose lower gravity prevented them from collapsing all the way to black holes. Unlike a black hole, a neutron star is a highly compressed ball of nuclear matter and its detailed structure is set by particle physics.</p>
<p>Observing gravitational waves either from <a href="http://iopscience.iop.org/article/10.1088/0004-637X/785/2/119/meta">single</a> neutron stars or <a href="http://journals.aps.org/prd/abstract/10.1103/PhysRevD.88.062001">binary pairs</a> would reveal the physics of matter under the most extreme conditions in the universe. Having three detectors working in conjunction will also <a href="http://relativity.livingreviews.org/Articles/lrr-2016-1/">mean that</a> we can be more accurate about the position in the sky from which a wave is coming. That will help us to identify any signals from these sources seen by <a href="https://arxiv.org/abs/1602.08492">conventional telescopes</a> – the flash from the collision or its aftermath. This will multiply the scientific return from both kinds of signals many times over, for example by giving us more insight into the structure of the universe. </p>
<p>Looking ahead a few years, we will have <a href="http://www.outerplaces.com/science/item/12379-one-step-closer-to-detecting-gravitational-waves-in-space">space-based</a> gravitational wave observatories of the kind pioneered by the recent highly successful <a href="http://sci.esa.int/lisa-pathfinder/">LISA Pathfinder Mission</a> (which announced its own <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.231101">astonishing performance results</a> earlier in June). This would allow us to see a new spectrum of sources at lower frequencies, but could also reveal these black hole binaries earlier in their lives, and allow us to follow them for years before they finally coalesce. In short, the spectacular results from this first short run of LIGO are just the beginning for gravitational astronomy.</p><img src="https://counter.theconversation.com/content/61056/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Graham Woan works for the University of Glasgow. He receives funding from STFC and is affiliated with the LIGO Scientific Collaboration.
</span></em></p>State of the art detectors have found another signal from a pair of collapsing black holes – the consequences could be momentous.Graham Woan, Professor of Astrophysics, University of GlasgowLicensed as Creative Commons – attribution, no derivatives.