tag:theconversation.com,2011:/us/topics/advanced-ligo-28470/articlesAdvanced LIGO – The Conversation2017-10-16T14:02:45Ztag:theconversation.com,2011:article/856472017-10-16T14:02:45Z2017-10-16T14:02:45ZHow we discovered gravitational waves from ‘neutron stars’ – and why it’s such a huge deal<figure><img src="https://images.theconversation.com/files/190387/original/file-20171016-31010-1rr1trx.jpg?ixlib=rb-1.1.0&rect=0%2C243%2C1710%2C1324&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's illustration of two merging neutron stars.</span> <span class="attribution"><span class="source">National Science Foundation/LIGO/Sonoma State University/A. Simonnet.</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Rumours have been <a href="https://www.scientificamerican.com/article/rumors-swell-over-new-kind-of-gravitational-wave-sighting/">swirling for weeks</a> that scientists have detected <a href="https://theconversation.com/explainer-gravitational-waves-and-why-their-discovery-is-such-a-big-deal-53239">gravitational waves</a> – tiny ripples in space and time – from a source other than colliding black holes. Now we can finally confirm that we’ve observed such waves produced by the violent collision of two massive, ultra-dense stars more than 100m light years from the Earth. </p>
<p>The discovery was made on August 17 by the <a href="https://theconversation.com/gravitational-waves-discovered-how-did-the-experiment-at-ligo-actually-work-54510">global network of advanced gravitational-wave interferometers</a> – comprising the twin LIGO detectors in the US and their European cousin, Virgo, in Italy. It is hugely important, not least because it helps solve some big mysteries in astrophysics – including the cause of bright flashes of light known as “<a href="https://theconversation.com/flash-aah-aah-could-a-gamma-ray-burst-eradicate-all-life-on-earth-5291">gamma ray bursts</a>” and perhaps even the origins of heavy elements such as gold.</p>
<p>As a member of the LIGO scientific collaboration, I was immediately in raptures as soon as I saw the initial data. And the period that followed was definitely the most intense and sleep deprived, but also incredibly exciting, two months of my career. </p>
<p>The announcement comes just weeks after three scientists <a href="https://theconversation.com/scientists-behind-the-discovery-of-gravitational-waves-win-the-2017-nobel-prize-for-physics-66457">were awarded the Nobel Prize in Physics</a> for their foundational work leading to the discovery of gravitational waves, first announced in February 2016. Since then, detecting gravitational waves from colliding black holes has started to feel like familiar territory – <a href="https://theconversation.com/experiments-simultaneously-detect-gravitational-waves-and-help-open-up-a-new-era-of-astronomy-84818">with four further such events detected</a>. But as far as we know, colliding black holes offer purely a window on the dark side of the universe. We haven’t been able to register light from these events with any other instruments.</p>
<p>But GW170817 – the catchy title for the event of August 17 — changes all that. That’s because the source of the waves this time was two “<a href="https://www.nasa.gov/mission_pages/GLAST/science/neutron_stars.html">neutron stars</a>” – incredibly dense stellar remnants the size of a city, each weighing more than the sun. These stars whizzed around each other at a sizeable fraction of the speed of light before merging in a cataclysmic collision that we’ve now seen shake the very fabric of space and time.</p>
<h2>Mysteries solved</h2>
<p>The cosmic concerto was just beginning, however. Astronomers have long suspected that the merger of two neutron stars could be the overture to a short <a href="https://theconversation.com/flash-aah-aah-could-a-gamma-ray-burst-eradicate-all-life-on-earth-5291">gamma ray burst</a> – an intense flash of gamma-ray light that releases more energy in a fraction of a second than the sun will pump out in ten billion years. For several decades we have observed these gamma ray bursts, but without knowing for sure what causes them.</p>
<p>However, just 1.7 seconds after the gravitational waves from GW170817 arrived at the Earth, <a href="https://www.nasa.gov/content/fermi-gamma-ray-space-telescope/">NASA’s Fermi satellite</a> observed a short burst of gamma rays in the same general region of the sky. LIGO and Virgo had found the smoking gun, and the link between neutron star collisions and short gamma ray bursts was finally and clearly established.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.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">
<figcaption>
<span class="caption">Many hands make light (and gravity) work. NASA’s Fermi satellite was instrumental in the discovery.</span>
<span class="attribution"><span class="source">NASA</span></span>
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</figure>
<p>The combination of gravitational-wave and gamma-ray observations allowed the position of the cosmic explosion to be pinpointed to less than 30 square degrees on the sky – or about 100 times the size of the full moon. This, in turn, allowed a whole barrage of astronomical telescopes sensitive to light across the entire electromagnetic spectrum to search this small patch of sky for the aftermath of the explosion. And sure enough this was found – in an unfashionable backwater towards the edge of a fairly <a href="https://en.wikipedia.org/wiki/NGC_4993">unassuming galaxy called NGC4993</a>, in the constellation of Hydra. </p>
<p>Over the next few days and weeks astronomers watched agog as the embers from the explosion glowed brightly and faded, beautifully matching the pattern expected for <a href="http://theconversation.com/we-beat-a-cyber-attack-to-see-the-kilonova-glow-from-a-collapsing-pair-of-neutron-stars-85660">a so-called “kilonova”</a>. This is produced when material rich in subatomic particles known as neutrons from the initial merger is ejected at great speed by the gamma ray burst. This ploughs into the surrounding region of space, triggering the production of heavy radioactive elements. </p>
<p>These unstable elements typically split up (decay) to a stable state by emitting radiation. This is what causes the glow of the kilonova, which we have now confirmed by mapping it out in exquisite detail. Our observations also strongly support the theory that the stable end-products of these chains of reactions include copious amounts of precious metals like gold and platinum. While we’ve suspected neutron stars to be key to <a href="https://www.smithsonianmag.com/science-nature/all-the-gold-in-the-universe-could-come-from-the-collisions-of-neutron-stars-13474145/">producing these elements in space</a>, that hypothesis now looks a whole lot more convincing. Indeed, the kilonova that formed from the embers of GW170817 could have produced as much gold as the entire mass of the Earth – that is 1,000 trillion tonnes.</p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"919936738893602816"}"></div></p>
<p>By observing a kilonova “up close and personal” for the very first time, and seeing how well it fits into the unfolding astronomical storyboard that began with the neutron star merger, astronomers have taken a huge leap forward in our understanding of these violent cosmic events. </p>
<p>The idea that we are all made of stardust is increasingly appreciated in popular culture – in everything from documentaries to song lyrics. But the mind-blowing concept that the gold in our wedding rings and Rolex watches is made of neutron stardust is about to catch on. Perhaps even more exciting, however, is the enormous potential now unlocked by this radical, new approach to studying the cosmos.</p>
<p>By working together collaboratively – using instruments that operate not just across the entire spectrum of light but are sensitive to gravitational waves and even neutrinos too – astronomers are poised to fully open a completely new “multi-messenger” window on the universe, with many further discoveries to be made and cosmic mysteries to be solved. For example, we have already used our observations to make the first ever joint measurement of the expansion rate of the universe, using both gravitational waves and light. Our paper will appear in Nature on October 16.</p>
<p>More results will also surely follow soon. The exciting new era of multi-messenger astronomy just started with a bang.</p><img src="https://counter.theconversation.com/content/85647/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Martin Hendry is a member of the LIGO scientific collaboration.</span></em></p>The discovery of tiny ripples in space from the violent collision of dense stars could help solve many mysteries – including where the gold in our jewellery comes from.Martin Hendry, Professor of Gravitational Astrophysics and Cosmology, University of GlasgowLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/785712017-06-01T16:41:34Z2017-06-01T16:41:34ZLIGO detects more gravitational waves, from even more ancient and distant black hole collisions<figure><img src="https://images.theconversation.com/files/171733/original/file-20170531-25676-1n7a6j4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's conception of two merging black holes, spinning in a nonaligned fashion.</span> <span class="attribution"><span class="source">LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>For the third time in a year and a half, the Advanced Laser Interferometer Gravitational Wave Observatory <a href="https://doi.org/10.1103/PhysRevLett.118.221101">has detected gravitational waves</a>. Hypothesized by Einstein a century ago, the <a href="https://theconversation.com/what-happens-when-ligo-texts-you-to-say-its-detected-one-of-einsteins-predicted-gravitational-waves-53259">identification of these ripples in space-time</a> – for the third time, no less – is fulfilling the promise of an area of astronomy that has enticed scientists for decades, but had always seemed to lie just out of our reach.</p>
<p>As a gravitational-wave astrophysicist and member of the <a href="http://www.ligo.org">LIGO Scientific Collaboration</a>, I am naturally thrilled to see the vision of so many of us becoming a reality. But I’m accustomed to finding my own work more interesting and exciting than other people do, so the extent to which the whole world seems to be fascinated by this accomplishment came as something of a surprise. The excitement is well-deserved, though. By <a href="https://doi.org/10.1103/PhysRevLett.116.061102">detecting these gravitational waves</a> for the first time, we’ve not only directly verified a key prediction of Einstein’s theory of general relativity in convincing and spectacular fashion, but we’ve opened up an entirely new window that will revolutionize our understanding of the cosmos.</p>
<p>Already these discoveries have affected our understanding of the universe. And LIGO is just getting started.</p>
<h2>Tuning in to the universe</h2>
<p>At its core, this new way of understanding the universe stems from our newfound ability to hear its soundtrack. Gravitational waves aren’t actually sound waves, but the analogy is apt. Both types of waves carry information in a similar way, and both are completely independent phenomena from light. </p>
<p>Gravitational waves are ripples in space-time that propagate outward from intensely violent and energetic processes in space. They can be generated by objects that don’t shine, and they can travel through dust, matter or anything else, without being absorbed or distorted. They carry unique information about their sources that reaches us in a pristine state, giving us a true sense of the source that can’t be obtained in any other way. </p>
<p>General relativity tells us, among other things, that some stars can become so dense that they close themselves off from the rest of the universe. These extraordinary objects are called black holes. General relativity also predicted that when pairs of black holes orbit tightly around each other in a binary system, they stir up space-time, the very fabric of the cosmos. It’s this disturbance of space-time that sends energy across the universe in the form of gravitational waves.</p>
<p>That loss of energy causes the binary to tighten further, until eventually the two black holes smash together and form a single black hole. This spectacular collision generates more power in gravitational waves than is radiated as light by all the stars in the universe combined. These catastrophic events last only tens of milliseconds, but during that time, they are the most powerful phenomena since the Big Bang.</p>
<p>These waves carry information about the black holes that can’t possibly be gained in any other way, since telescopes can’t see objects that don’t emit light. For each event, we are able to measure the black holes’ masses, their rate of rotation or “spin,” and details about their locations and orientations with varying degrees of certainty. This information allows us to learn how these objects were formed and evolved across cosmic time. </p>
<p>While we have previously had strong evidence for the existence of black holes <a href="https://en.m.wikipedia.org/wiki/Sagittarius_A*">based on the effect of their gravity on surrounding stars and gas</a>, the detailed information from gravitational waves is invaluable for learning about the origins of these spectacular events.</p>
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<a href="https://images.theconversation.com/files/171743/original/file-20170601-25676-195envm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/171743/original/file-20170601-25676-195envm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/171743/original/file-20170601-25676-195envm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/171743/original/file-20170601-25676-195envm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/171743/original/file-20170601-25676-195envm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/171743/original/file-20170601-25676-195envm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/171743/original/file-20170601-25676-195envm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/171743/original/file-20170601-25676-195envm.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>
<figcaption>
<span class="caption">Aerial view of the LIGO gravitational wave detector in Livingston, Louisiana.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/pennstatelive/26661493514">LIGO</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<h2>Detecting the tiniest fluctuations</h2>
<p>In order to detect these incredibly quiet signals, researchers constructed two LIGO instruments, one in <a href="https://www.ligo.caltech.edu/WA">Hanford, Washington</a> and the other 3,000 miles away in <a href="https://www.ligo.caltech.edu/LA">Livingston, Louisiana</a>. They’re designed to leverage the unique effect that gravitational waves have on whatever they encounter. When gravitational waves pass by, they change the distance between objects. There are gravitational waves going through you right now, forcing your head, feet and everything in between to move back and forth in a predictable – but imperceptible – way.</p>
<p>You can’t feel this effect, or even see it with a microscope, because the change is so incredibly tiny. The gravitational waves that we can detect with LIGO change the distance between each end of the 4-kilometer-long detectors by only 10⁻¹⁸ meters. How small is this? A thousand times smaller than the size of a proton – which is why we can’t expect to see it even with a microscope.</p>
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<a href="https://images.theconversation.com/files/171742/original/file-20170601-23531-1dsgmm5.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/171742/original/file-20170601-23531-1dsgmm5.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/171742/original/file-20170601-23531-1dsgmm5.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=792&fit=crop&dpr=1 600w, https://images.theconversation.com/files/171742/original/file-20170601-23531-1dsgmm5.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=792&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/171742/original/file-20170601-23531-1dsgmm5.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=792&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/171742/original/file-20170601-23531-1dsgmm5.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=995&fit=crop&dpr=1 754w, https://images.theconversation.com/files/171742/original/file-20170601-23531-1dsgmm5.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=995&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/171742/original/file-20170601-23531-1dsgmm5.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=995&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 scientists working on its optics suspension.</span>
<span class="attribution"><span class="source">LIGO Laboratory</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>To measure such a minute distance, <a href="https://theconversation.com/gravitational-waves-discovered-how-did-the-experiment-at-ligo-actually-work-54510">LIGO uses a technique called “interferometry.”</a> Researchers split the light from a single laser into two parts. Each part then travels down one of two perpendicular arms that are each 2.5 miles long. Finally, the two join back together and are allowed to interfere with each other. The instrument is carefully calibrated so that, in the absence of a gravitational wave, the interference of the laser results in nearly perfect cancellation – no light comes out of the interferometer.</p>
<p>However, a passing gravitational wave will stretch one arm at the same time as it squeezes the other arm. With the relative lengths of the arms changed, the interference of the laser light will no longer be perfect. It’s this tiny change in the amount of interference that Advanced LIGO is actually measuring, and that measurement tells us what the detailed shape of the passing gravitational wave must be. </p>
<p><audio preload="metadata" controls="controls" data-duration="11" data-image="" data-title="The sound of two black holes colliding" data-size="166960" data-source="LIGO" data-source-url="https://soundcloud.com/newyorktimes/the-sound-of-two-black-holes-colliding" data-license="" data-license-url="">
<source src="https://cdn.theconversation.com/audio/320/ligo-chirp-1080p.m4a" type="audio/mp4">
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<div class="audio-player-caption">
The sound of two black holes colliding.
<span class="attribution"><a class="source" rel="nofollow" href="https://soundcloud.com/newyorktimes/the-sound-of-two-black-holes-colliding">LIGO</a><span class="download"><span>163 KB</span> <a target="_blank" href="https://cdn.theconversation.com/audio/320/ligo-chirp-1080p.m4a">(download)</a></span></span>
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<p>All gravitational waves have the shape of a “chirp,” where both the amplitude (akin to the loudness) and the frequency, or pitch, of the signals increase with time. However, the characteristics of the source are encoded in the precise details of this chirp and how it evolves with time.</p>
<p>The shape of the gravitational waves that we observe, in turn, can tell us details about the source that could not be measured in any other way. With the first three confident detections by Advanced LIGO, we’ve already found that black holes are more common than we ever expected, and that the most common variety, which forms directly from the collapse of massive stars, can be more massive than we previously thought was possible. All this information helps us understand how massive stars evolve and die. </p>
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<a href="https://images.theconversation.com/files/171739/original/file-20170601-23531-85y4rx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/171739/original/file-20170601-23531-85y4rx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/171739/original/file-20170601-23531-85y4rx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=415&fit=crop&dpr=1 600w, https://images.theconversation.com/files/171739/original/file-20170601-23531-85y4rx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=415&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/171739/original/file-20170601-23531-85y4rx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=415&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/171739/original/file-20170601-23531-85y4rx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=522&fit=crop&dpr=1 754w, https://images.theconversation.com/files/171739/original/file-20170601-23531-85y4rx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=522&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/171739/original/file-20170601-23531-85y4rx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=522&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 three confirmed detections by LIGO (GW150914, GW151226, GW170104), and one lower-confidence detection (LVT151012), point to a population of stellar-mass binary black holes that, once merged, are larger than 20 solar masses – larger than what was known before.</span>
<span class="attribution"><span class="source">LIGO/Caltech/Sonoma State (Aurore Simonnet)</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Black holes becoming less of a black box</h2>
<p>This most recent event, which we detected on Jan. 4, 2017, is the most distant source we’ve observed so far. Because gravitational waves travel at the speed of light, when we look at very distant objects, we also look back in time. This most recent event is also the most ancient gravitational wave source we’ve detected so far, having occurred over two billion years ago. Back then, the universe itself was 20 percent smaller than it is today, and multicellular life had not yet arisen on Earth.</p>
<p>The mass of the final black hole left behind after this most recent collision is 50 times the mass of our sun. Prior to the first detected event, which weighed in at 60 times the mass of the sun, astronomers didn’t think such massive black holes could be formed in this way. While the second event was only 20 solar masses, detecting this additional very massive event suggests that such systems not only exist, but may be relatively common.</p>
<p>In addition to their masses, black holes can also rotate, and their spins affect the shape of their gravitational-wave emission. The effects of spin are more difficult to measure, but this most recent event shows evidence not only for spin, but potentially for spin that is not oriented around the same axis as the binary’s orbit. If the case for such misalignment can be made stronger by observing future events, it will have significant implications for our understanding of how these black hole pairs form.</p>
<p>In the coming years, we will have more instruments like LIGO listening for gravitational waves in <a href="http://public.virgo-gw.eu/language/en/">Italy</a>, in <a href="http://gwcenter.icrr.u-tokyo.ac.jp/en/">Japan</a> and in <a href="http://www.gw-indigo.org/tiki-index.php?page=LIGO-India">India</a>, learning even more about these sources. My colleagues and I are still eagerly awaiting the first detection of a binary containing at least one <a href="https://theconversation.com/explainer-what-is-a-neutron-star-29341">neutron star</a> – a type of dense star that was not quite massive enough to collapse all the way to a black hole.</p>
<p>Most astronomers predicted that pairs of neutron stars would be observed before black-hole pairs, so their continued absence would present a challenge to theorists. Their eventual detection will facilitate a host of new possibilities for discoveries, including the prospect of better understanding extremely dense states of matter, and potentially observing a unique light signature using conventional telescopes from the same source as the gravitational-wave signal.</p>
<p>We also expect to detect gravitational waves within the next few years from space, using very precise natural clocks called pulsars, which send <a href="https://en.wikipedia.org/wiki/Pulsar_timing_array">blasts of radiation our way at very regular intervals</a>. Eventually <a href="https://lisa.nasa.gov">we plan to place</a> <a href="https://www.elisascience.org">extremely large interferometers in orbit</a>, where they can evade the persistent rumbling of the Earth, which is a limiting source of noise for the Advanced LIGO detectors.</p>
<p>Nearly every time scientists have built new telescopes or particle accelerators, they’ve discovered things no one could have predicted. As exciting as the known prospects for discovery are in this new field of gravitational-wave astrophysics, as a theorist I’m most excited by the unknown wonders that still lie in store for us.</p><img src="https://counter.theconversation.com/content/78571/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Sean McWilliams is a member of the LIGO Scientific Collaboration, and receives funding from the National Science Foundation. </span></em></p>These ripples in the very fabric of the universe were hypothesized by Einstein a century ago. Now scientists have detected them for the third time in a year and a half – ushering in a new era in astrophysics.Sean McWilliams, Assistant Professor of Physics and Astronomy, West Virginia 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.