tag:theconversation.com,2011:/us/topics/redshift-27068/articlesRedshift – The Conversation2023-10-16T12:30:12Ztag:theconversation.com,2011:article/2058102023-10-16T12:30:12Z2023-10-16T12:30:12ZWhy is space so dark even though the universe is filled with stars?<figure><img src="https://images.theconversation.com/files/538108/original/file-20230718-17-5jcl17.jpg?ixlib=rb-1.1.0&rect=26%2C6%2C996%2C676&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">This age old question has been dubbed Olbers' paradox.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/the-milky-way-appears-over-the-valle-de-la-luna-in-the-news-photo/1418507439?adppopup=true">John Moore via Getty Images News</a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><em><a href="https://theconversation.com/us/topics/curious-kids-us-74795">Curious Kids</a> is a series for children of all ages. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskidsus@theconversation.com">curiouskidsus@theconversation.com</a>.</em></p>
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<p><strong>Why is space so dark despite all of the stars in the universe? – Nikhil, age 15, New Delhi</strong></p>
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<p>People have been asking why space is dark despite being filled with stars for so long that this question has a special name – <a href="https://lambda.gsfc.nasa.gov/product/suborbit/POLAR/cmb.physics.wisc.edu/tutorial/olbers.html">Olbers’ paradox</a>.</p>
<p>Astronomers estimate that there are about <a href="https://theconversation.com/how-many-stars-are-there-in-space-165370">200 billion trillion stars</a> in the observable universe. And many of those stars are as bright or even brighter than our sun. So, why isn’t space filled with dazzling light?</p>
<p><a href="http://www.astrojack.com/">I am an astronomer</a> who studies stars and planets – including those outside our solar system – and their motion in space. The study of distant stars and planets helps <a href="https://scholar.google.com/citations?user=pF3HbeQAAAAJ&hl=en&oi=ao">astronomers like me</a> understand why space is so dark.</p>
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<p>You might guess it’s because a lot of the stars in the universe are very far away from Earth. Of course, it is true that the farther away a star is, the less bright it looks – <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/isq.html">a star 10 times farther away looks 100 times dimmer</a>. But it turns out this isn’t the whole answer. </p>
<h2>Imagine a bubble</h2>
<p>Pretend, for a moment, that the universe is so old that the light from even the farthest stars has had time to reach Earth. In this imaginary scenario, all of the stars in the universe are not moving at all.</p>
<p>Picture a large bubble with Earth at the center. If the bubble were about 10 <a href="https://exoplanets.nasa.gov/faq/26/what-is-a-light-year/">light years</a> across, it would contain about <a href="https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs">a dozen stars</a>. Of course, at several light years away, many of those stars would look pretty dim from Earth. </p>
<p>If you keep enlarging the bubble to 1,000 light years across, then to 1 million light years, and then 1 billion light years, the farthest stars in the bubble will look even more faint. But there would also be more and more stars inside the bigger and bigger bubble, all of them contributing light. Even though the farthest stars look dimmer and dimmer, there would be a lot more of them, and the whole night sky should look very bright.</p>
<p>It seems I’m back where I started, but I’m actually a little closer to the answer.</p>
<h2>Age matters</h2>
<p>In the imaginary bubble illustration, I asked you to imagine that the stars are not moving and that the universe is very old. But the universe is only about <a href="https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question28.html">13 billion years old</a>. </p>
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<a href="https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Image of lightly colored galaxies and stars against dark background" src="https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=613&fit=crop&dpr=1 600w, https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=613&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=613&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=770&fit=crop&dpr=1 754w, https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=770&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/538112/original/file-20230718-39873-q38o2g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=770&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">Galaxies as they appeared approximately 13.1 billion years ago, taken by the James Webb Space Telescope.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/image-released-by-nasa-on-july-11-2022-shows-galaxy-cluster-news-photo/1241872380?adppopup=true">NASA/ESA/CSA/STScI/Handout from Xinhua News Agency via Getty Images</a></span>
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<p>Even though that’s an amazingly long time in human terms, it’s short in astronomical terms. It’s short enough that the light from stars more distant than about 13 billion light years hasn’t actually reached Earth yet. And so the actual bubble around Earth that contains all the stars we can see only extends out to about <a href="https://science.nasa.gov/observable-universe">13 billion light years from Earth</a>.</p>
<p>There just are not enough stars in the bubble to fill every line of sight. Of course, if you look in some directions in the sky, you can see stars. If you look at other bits of the sky, you can’t see any stars. And that’s because, in those dark spots, the stars that could block your line of sight are so far away their light hasn’t reached Earth yet. As time passes, light from these more and more distant stars will have time to reach us. </p>
<h2>The Doppler shift</h2>
<p>You might ask whether the night sky will eventually light up completely. But that brings me back to the other thing I told you to imagine: that all of the stars are not moving. The universe is actually expanding, with the most distant galaxies <a href="https://starchild.gsfc.nasa.gov/docs/StarChild/questions/redshift.html">moving away from Earth at nearly the speed of light</a>. </p>
<p>Because the galaxies are moving away so fast, the light from their stars is pushed into colors the human eye can’t see. This effect is called the <a href="https://starchild.gsfc.nasa.gov/docs/StarChild/questions/redshift.html">Doppler shift</a>. So, even if it had enough time to reach you, <a href="https://svs.gsfc.nasa.gov/12856">you still couldn’t see</a> the light from the most distant stars with your eyes. And the night sky would not be completely lit up. </p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/ikgRZt1BSyk?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The Doppler shift, also known as the redshift, is a phenomenon in which light from objects that are moving away from an observer appears more toward the red end of the spectrum.</span></figcaption>
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<p>If you wait even longer, eventually the stars will all burn out – <a href="https://astronomy.swin.edu.au/cosmos/m/main+sequence+lifetime">stars like the sun last only about 10 billion years</a>. Astronomers hypothesize that in the distant future – a thousand trillion years from now – the universe will go dark, <a href="https://en.wikipedia.org/wiki/The_Five_Ages_of_the_Universe">inhabited by only stellar remnants</a> like white dwarfs and black holes.</p>
<p>Even though our night sky isn’t completely filled with stars, we live in a very special time in the universe’s life, when we’re lucky enough to enjoy a rich and complex night sky, filled with light and dark.</p>
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<p class="fine-print"><em><span>Brian Jackson receives federally funded research grants from NASA. </span></em></p>An astronomer explains why space looks so dark despite containing 200 billion trillion stars.Brian Jackson, Associate Professor of Astronomy, Boise State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2117272023-10-04T19:53:51Z2023-10-04T19:53:51ZGravitational distortion of time helps tell modified gravity apart from a dark force<figure><img src="https://images.theconversation.com/files/550733/original/file-20230927-15-pmj07x.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C1200%2C878&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/mission_pages/gpb/gpb_012.html">(NASA)</a></span></figcaption></figure><iframe style="width: 100%; height: 100px; border: none; position: relative; z-index: 1;" allowtransparency="" allow="clipboard-read; clipboard-write" src="https://narrations.ad-auris.com/widget/the-conversation-canada/gravitational-distortion-of-time-helps-tell-modified-gravity-apart-from-a-dark-force" width="100%" height="400"></iframe>
<p>With his theory of General Relativity in 1915, Albert Einstein revolutionized how we think about our universe. Rather than the cosmos simply providing the room for the planets and stars to orbit each other, space and time themselves were now dynamical entities in one ever-evolving play with matter and light. </p>
<p>Einstein’s equations described how <a href="https://www.space.com/17661-theory-general-relativity.html">stars, galaxies and all other matter curve or warp space and time</a>. The galaxies and the light rays then travel in this distorted space-time according to <a href="https://www.britannica.com/biography/Leonhard-Euler">the equation provided by the 18th-century Swiss mathematician Leonhard Euler</a>.</p>
<p>With the help of modern telescopes, we can watch this dance and compare it to the choreography scripted by the two giants of science, Einstein and Euler. But can we differentiate a universe where Einstein’s equations were violated from a universe where Euler’s equation were modified? In other words, if what we observed with telescopes disagreed with what Einstein and Euler prescribed, would we be able to tell which one of the two was wrong?</p>
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Read more:
<a href="https://theconversation.com/rippling-space-time-how-to-catch-einsteins-gravitational-waves-7058">Rippling space-time: how to catch Einstein's gravitational waves</a>
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<h2>A universe filled with unknowns</h2>
<p>You may wonder why one would want to doubt Einstein or Euler in the first place. After all, existing observations have beautifully confirmed the validity of their theories. The reason to put those to the test comes from the fact that our universe is filled with unknowns. </p>
<p>In the 1930s, the Swiss-American astrophysicist Fritz Zwicky observed that there was five times more matter in the universe than we can detect with our telescopes. He called this new matter “dark matter.” </p>
<p>Nearly 100 years later, <a href="https://doi.org/10.1038/509560a">we still don’t know what dark matter is</a>: we have never detected a particle of dark matter and we don’t know how it moves. It is therefore legitimate to question if it behaves as ordinary matter and obeys Euler’s law. Could it be affected by other forces and interactions, which would change the Euler equation? </p>
<p>Then, in 1998, two groups of astrophysicists observed that <a href="https://doi.org/10.1063/1.3232196">the expansion of our universe</a> <a href="https://doi.org/10.1086/300499">is accelerating</a>, contrary to the deceleration expected because of the gravitational attraction between galaxies. </p>
<p>As of today, we don’t know what causes this strange behaviour: is it due to the presence of yet another “dark” substance that has repulsive gravity? Or is it due to gravity itself, meaning Einstein’s predictions of how it behaves over very large distances would be wrong? Testing Einstein’s and Euler’s equations is therefore the logical consequence of the mysteries we face.</p>
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<a href="https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="multicoloured galaxies against a black background" src="https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=386&fit=crop&dpr=1 600w, https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=386&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=386&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=485&fit=crop&dpr=1 754w, https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=485&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=485&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">A composite image showing the distribution of dark matter, galaxies and hot gas in the core of the merging galaxy cluster Abell 520, formed from a violent collision of massive galaxy clusters.</span>
<span class="attribution"><a class="source" href="https://images.nasa.gov/details/GSFC_20171208_Archive_e001774">(NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University))</a></span>
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<h2>Vast distances of the universe</h2>
<p>Checking if Einstein’s gravity works over the vast distances of the universe has become an active field of research. Theoreticians propose new ideas for how gravity could work differently, while astronomers use increasingly advanced facilities to provide the data needed to test them. </p>
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Read more:
<a href="https://theconversation.com/we-tested-einsteins-theory-of-gravity-on-the-scale-of-the-universe-heres-what-we-found-194118">We tested Einstein's theory of gravity on the scale of the universe – here's what we found</a>
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<p>Researchers have identified a particular “smoking gun” signature of modified gravity known as the “<a href="https://doi.org/10.1103/PhysRevD.77.103513">gravitational slip</a>.” General Relativity predicts that the pathways of light and matter should bend in the same way when travelling through the same distorted space-time. </p>
<p>This is much like the fact that different objects fall at the same rate in Earth’s gravity (if the air resistance could be neglected) — <a href="https://www.sciencenews.org/article/galileo-gravity-experiment-atoms-general-relativity-einstein">something Galileo famously demonstrated from the tower of Pisa</a>. By comparing the way galaxies fall into gravitational wells to how the light from these galaxies is deflected by gravitational lensing, one can deduce if they feel the same gravity. </p>
<p>If one finds them to be different, we would say there was a <a href="https://doi.org/10.48550/arXiv.0802.1068">gravitational slip</a>. Measuring the slip is one of the main targets of Euclid, <a href="https://www.esa.int/Science_Exploration/Space_Science/Euclid">a wide-angle space telescope launched by the European Space Agency on a Space-X rocket</a>. </p>
<p>But what if Euclid found that there was a slip? Could we be certain that it occurs due to a modification of gravity, or could it also be due to a modification of Euler’s equation? The latter would be different if, for example, the dark matter in the galaxies were subject to a new force.</p>
<h2>Gravitational slips</h2>
<p>The two of us approached this question from different perspectives: one involved developing tests of <a href="https://www.sfu.ca/physics/cosmology/TestingGravity2023/">modified gravity</a>, while the other investigated the subtle corrections General Relativity adds to <a href="https://doi.org/10.1103/PhysRevD.84.063505">what we measure with galaxy surveys</a>. </p>
<p>To our surprise, while both of us came into this thinking that the answer would be obvious, our initial answers were opposite to each other. After intensive discussion, we eventually came to an agreement, <a href="https://doi.org/10.1038/s41550-023-02003-y">resulting in a paper published in <em>Nature Astronomy</em></a>. </p>
<p>Our conclusion was that, despite the common expectation, measuring the gravitational slip would not allow one to distinguish a modification of Einstein’s laws from a modification of Euler’s equation. </p>
<p>However, the distinction may be possible if one could measure the effect called “gravitational redshift,” which should be possible with telescopes such as the <a href="https://www.desi.lbl.gov/">Dark Energy Spectroscopic Instrument</a> and the upcoming <a href="https://www.skao.int/en">Square Kilometer Array</a>.</p>
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Read more:
<a href="https://theconversation.com/canadas-participation-in-the-worlds-largest-radio-telescope-means-new-opportunities-in-research-and-innovation-201341">Canada's participation in the world's largest radio telescope means new opportunities in research and innovation</a>
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<p>One of our key realizations was that to determine if the measured gravitational slip signals a breakdown of General Relativity, one would need to measure the velocity of normal matter when it is not confined to a galaxy. In practice, however, we can only observe the light from stars that reside in galaxies, and hence move together with the dark matter. </p>
<p>Telescopes can only measure the collective motion of a galaxy that contains both normal matter and dark matter. So, if a galaxy were to fall into a gravitational potential in a way that was not consistent with our expectations, we would be unable to tell if it’s because the dark matter is doing something, or because gravity was modified. </p>
<h2>Light and gravity</h2>
<p>There <em>is</em> a way to probe the gravitational potential directly through the way it distorts time via gravitational redshift.</p>
<p>The time kept by a clock <a href="https://www.washingtonpost.com/national/health-science/an-atomic-clock-is-used-to-measure-not-time-but-the-height-of-mountains/2018/02/23/5a845166-11c3-11e8-9570-29c9830535e5_story.html">that’s on top of a tall mountain is different from that of a clock at the sea level</a>. These differences are extremely tiny but are, in fact, very important in the design of satellite navigation systems. </p>
<p>When the light from a galaxy escapes the gravitational potential it is falling into, its colour shifts closer to red. This gravitational redshift is solely due to time distortion. Gravitational lensing, which differs from redshift, is due to both space and time distortions, as opposed to just time. </p>
<p>We need to have both lensing and redshift in order to isolate the gravitational slip. It is this ability to separate the distortion of space and time from the distortion of time alone that is key to measuring true gravitational slip.</p>
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<figcaption><span class="caption">Swiss cosmologists explain how the distortion of time, an effect predicted by General Relativity, can be measured.</span></figcaption>
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<p>A measurement of the gravitational redshift is impossible if one cannot easily keep track if a pair of galaxies swapped their positions. While it’s not that hard to tell any two galaxies measured by a telescope apart, when running a statistical analysis on a catalogue of millions of galaxies, you can quickly lose the ability to assign any identity to the galaxies; at some point they are all treated as points on the sky. </p>
<p>Techniques have, however, been developed to split galaxies into different populations and <a href="https://doi.org/10.48550/arXiv.1309.1321">keep track of swaps between them</a>. In time, new technologies will be able to detect the tiny effects of gravitational redshift, and consequently distinguish a modification of Euler’s equation for dark matter from a modification of gravity.</p><img src="https://counter.theconversation.com/content/211727/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Levon Pogosian receives funding from the Natural Sciences and Engineering Research Council of Canada.</span></em></p><p class="fine-print"><em><span>Camille Bonvin receives funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program and from the Swiss National Science Foundation.</span></em></p>The gravitational field can affect space and time: the stronger gravity is, the slower time moves. This prediction of General Relativity can be used to reveal hidden forces acting on dark matter.Levon Pogosian, Professor of Physics, Simon Fraser UniversityCamille Bonvin, Associate professor, Cosmology and Astroparticle Physics, Université de GenèveLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2105812023-08-28T16:53:29Z2023-08-28T16:53:29ZHow old is the universe exactly? A new theory suggests that it’s been around for twice as long as believed<figure><img src="https://images.theconversation.com/files/543889/original/file-20230822-10860-2cqa28.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C1912%2C1000&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Galaxy NGC 6822, neighbouring the Milky Way galaxy, being studied to learn more about stars and dust in the early universe.
</span> <span class="attribution"><a class="source" href="https://esawebb.org/images/potm2307a/">(NASA/James Webb Space Telescope)</a></span></figcaption></figure><iframe style="width: 100%; height: 100px; border: none; position: relative; z-index: 1;" allowtransparency="" allow="clipboard-read; clipboard-write" src="https://narrations.ad-auris.com/widget/the-conversation-canada/how-old-is-the-universe-exactly-a-new-theory-suggests-that-its-been-around-for-twice-as-long-as-believed" width="100%" height="400"></iframe>
<p>Early universe observations by the <a href="https://webbtelescope.org/home">James Webb Space Telescope</a> (JWST) cannot be explained by current cosmological models. These models estimate the universe to be 13.8 billion years in age, based on the <a href="https://www.space.com/52-the-expanding-universe-from-the-big-bang-to-today.html">big-bang expanding universe concept</a>.</p>
<p>My research proposes a model that determines <a href="https://doi.org/10.1093/mnras/stad2032">the universe’s age to be 26.7 billion years</a>, which accounts for the JWST’s “<a href="https://www.usatoday.com/story/news/nation/2023/07/14/universe-may-older-than-thought-study-shows/70411343007/">impossible early galaxy</a>” observations. </p>
<p>Impossible early galaxies refer to the fact that some galaxies dating to the cosmic dawn — 500 to 800 million years after the big bang — have discs and bulges similar to those which have passed through a long period of evolution. And smaller in size galaxies are <a href="https://phys.org/news/2023-02-impossible-giant-baby-galaxies-early.html">apparently more massive than larger ones</a>, which is quite the opposite of expectation.</p>
<h2>Frequency and distance</h2>
<p>This age estimate is derived from the universe’s expansion rate by measuring the redshift of spectral lines in the light emitted by distant galaxies. An earlier explanation of the redshift was based on the hypothesis that light loses energy as it travels cosmic distances. This <a href="https://www.science.org/content/article/tired-light-hypothesis-gets-re-tired">“tired light” explanation</a> was rejected as it could not explain many observations.</p>
<p>The redshift of light is similar to <a href="https://www.britannica.com/science/Doppler-effect">the Doppler effect</a> on sound: noises appear to have higher frequency (pitch) when approaching, and lower when receding. Redshift, a lower light frequency, indicates when an object is receding from us; the larger the galaxy distance, the higher the recessional speed and redshift.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/6L3vDeG8ZP0?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">A NASA animation showing how light from distant galaxies is stretched by the expansion of the universe.</span></figcaption>
</figure>
<p>An alternative explanation for the redshift was due to the Doppler effect: distant galaxies are receding from us at speeds proportional to their distance, indicating that the universe is expanding. The expanding universe model became favoured by most astronomers after two astronomers working for Bell Labs, Arno Penzias and Robert Wilson, <a href="https://doi.org/10.1086/148307">accidentally discovered cosmic microwave background (CMB)</a> radiation in 1964, which the steady-state model could not satisfactorily explain.</p>
<p>The rate of expansion essentially determines the age of the universe. Until the launch of the Hubble Space Telescope in the 1990s, uncertainty in the expansion rate estimated the universe’s age ranging from seven to 20 billion years. Other observations led to the currently accepted value of 13.8 billion years, putting the big-bang model on the cosmology pedestal.</p>
<h2>Limitations of previous models</h2>
<p>Research published last year proposed to resolve the <a href="https://doi.org/10.3390/galaxies10060108">impossible early galaxy problem using the tired light model</a>. However, tired light cannot satisfactorily explain other cosmological observations like <a href="https://doi.org/10.3847/1538-4357/abb140">supernovae redshifts</a> and <a href="https://www.space.com/33892-cosmic-microwave-background.html">uniformity of the cosmic microwave background</a>.</p>
<p>I attempted to combine the standard big-bang model with the tired light model to see how it fits the supernovae data and the JWST data, but it did not fit the latter well. It did, however, increase the universe’s age to 19.3 billion years.</p>
<p>Next, I tried creating a hybrid model comprising the tired light and a cosmological model I had developed based on <a href="https://doi.org/10.1093/acprof:oso/9780198722892.003.0012">the evolving coupling constants proposed by British physicist Paul Dirac in 1937</a>. This fitted both the data well, but almost doubled the universe’s age.</p>
<p>The new model stretches galaxy formation time 10 to 20 fold over the standard model, giving enough time for the formation of well-evolved “impossible” early galaxies as observed.</p>
<p>As with any model, it will need to provide a satisfactory explanation for all those observations that are satisfied by the standard cosmological model.</p>
<h2>Mixing models</h2>
<p>The approach of mixing two models to explain new observations is not new. Isaac Newton considered that light propagates as particles in his theory of light, which prevailed until it was replaced by the wave theory of light in the 19th century to explain diffraction patterns observed with monochromatic light.</p>
<p>Albert Einstein resurrected the particle-like nature of light to explain the photoelectric effect — that light has dual characteristics: particle-like in some observations and wave-like in others. It has since become well-established that all particles have such dual characteristics.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/543891/original/file-20230822-28-xeifp6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="a galaxy forming — bright lights clustered against a black background" src="https://images.theconversation.com/files/543891/original/file-20230822-28-xeifp6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/543891/original/file-20230822-28-xeifp6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/543891/original/file-20230822-28-xeifp6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/543891/original/file-20230822-28-xeifp6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/543891/original/file-20230822-28-xeifp6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/543891/original/file-20230822-28-xeifp6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/543891/original/file-20230822-28-xeifp6.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>
<figcaption>
<span class="caption">A galaxy estimated to be as young as 500 million years old, making it one of the youngest galaxies seen.</span>
<span class="attribution"><a class="source" href="https://esahubble.org/images/opo0435a/">(NASA/ESA Hubble Space Telescope)</a></span>
</figcaption>
</figure>
<p>Another way of measuring the age of the universe is to estimate the age of stars in globular clusters in our own galaxy — <a href="https://www.space.com/19915-milky-way-galaxy.html">the Milky Way</a>. Globular clusters include up to a million stars, all of which appear to have formed at the same time in the early universe.</p>
<p>Assuming all galaxies and clusters started to form simultaneously, the age of the oldest star in the cluster should provide the age of the universe (less the time when the galaxies began to form). For some stars such as Methuselah, believed to be oldest in the galaxy, astrophysical modeling yields <a href="https://www.space.com/how-can-a-star-be-older-than-the-universe.html">an age greater than the age of the universe determined using the standard model</a>, which is impossible.</p>
<p>Einstein believed that the universe is the same observed from any point at any time — homogeneous, isotropic and timeless. To explain the observed redshift of distant galaxies in such a steady-state universe, which appeared to increase in proportion to their distance (Hubble’s law), Swiss astronomer Fritz Zwicky, <a href="https://ui.adsabs.harvard.edu/abs/2017JAHH...20....2K/abstract">proposed the tired light theory in 1929</a>.</p>
<h2>New information</h2>
<p>While some Hubble Space Telescope observations did point towards the impossible early galaxy problem, it was not until the launch of JWST in December 2021, and the data it provided since mid-2022, that this problem was firmly established.</p>
<p>To defend the standard big-bang model, astronomers have tried to resolve the problem <a href="https://www.livescience.com/space/cosmology/james-webb-telescope-finds-evidence-of-celestial-monster-stars-the-size-of-10000-suns-lurking-at-the-dawn-of-time">by compressing the timeline for forming massive stars</a> and primordial black holes <a href="https://doi.org/10.1038/d41586-023-02460-5">accreting mass at unphysically high rates</a>. </p>
<p>However, a consensus is developing towards new physics to explain these JWST observations.</p><img src="https://counter.theconversation.com/content/210581/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Rajendra Gupta receives funding from Macronix Research Corporation.</span></em></p>A new hypothesis suggests that the universe may be twice as old as we had believed. Observations from the James Webb Space Telescope provide new information on the rate of the universe’s expansion.Rajendra Gupta, Adjunct professor, Physics, L’Université d’Ottawa/University of OttawaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1258862019-10-28T15:13:10Z2019-10-28T15:13:10ZDark energy: new experiment may solve one of the universe’s greatest mysteries<figure><img src="https://images.theconversation.com/files/298818/original/file-20191027-113980-g2b9tz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Star trails take shape around the story Mayall Telescop dome in Arizona.</span> <span class="attribution"><span class="source">P. Marenfeld and NOAO/AURA/NSF).jpg</span></span></figcaption></figure><p>As an astronomer, there is no better feeling than achieving “first light” with a new instrument or telescope. It is the culmination of years of preparations and construction of new hardware, which for the first time collects light particles from an astronomical object. This is usually followed by a sigh of relief and then the excitement of all the new science that is now possible. </p>
<p>On October 22, the <a href="https://www.desi.lbl.gov/">Dark Energy Spectroscopic Instrument (DESI</a>) on the <a href="https://www.noao.edu/outreach/kptour/mayall.html">Mayall Telescope</a> in Arizona, US, achieved first light. This is a huge leap in our ability to measure galaxy distances – enabling a new era of mapping the structures in the universe. As its name indicates, it may also be key to solving one of the biggest questions in physics: what is the mysterious force dubbed “dark energy” that makes up the <a href="https://theconversation.com/the-experiments-trying-to-crack-physics-biggest-question-what-is-dark-energy-52917">70% of the universe</a>?</p>
<p>The cosmos is clumpy. Galaxies live together in groups of a few to tens of galaxies. There are also clusters of a few hundreds to thousands of galaxies and superclusters that contain many such clusters. </p>
<p>This hierarchy of the universe has been known from the first maps of the universe, which looked like a <a href="http://tdc-www.harvard.edu/zcat">“stickman”</a> in graphs by the pioneering Centre for Astrophysics (CfA) <a href="https://en.wikipedia.org/wiki/CfA_Redshift_Survey">Redshift Survey</a>. These striking images were the first glimpse of large-scale structures in the universe, some spanning hundreds of millions of light years across. </p>
<p>The CfA survey was laboriously constructed one galaxy at a time. This involved measuring the spectrum of the galaxy light – a splitting of the light by wavelength, or colour – and identifying the fingerprints of certain chemical elements (mostly hydrogen, nitrogen and oxygen).</p>
<p>These chemical signatures are systematically shifted to longer redder wavelengths due to the expansion of the universe. This “red shift” was first detected by the astronomer <a href="https://en.wikipedia.org/wiki/Vesto_Slipher">Vesto Slipher</a> and gave rise to the <a href="https://theconversation.com/how-fast-is-the-universe-really-expanding-new-measurements-help-tackle-the-mystery-123338">now famous Hubble’s Law</a> – the observation that more distant galaxies appear to be moving away at a faster rate. This means that galaxies that are close by appear to be moving away relatively slowly by comparison – they are less redshifted than galaxies far away. Therefore, measuring the redshift of a galaxy is a way to measure its distance. </p>
<p>Crucially, the exact relationship between redshift and distance depends on the expansion history of the Universe which can be calculated theoretically using our theory of gravity and our assumptions of the matter and energy density of the universe.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/298817/original/file-20191027-113972-1hmy009.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/298817/original/file-20191027-113972-1hmy009.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=597&fit=crop&dpr=1 600w, https://images.theconversation.com/files/298817/original/file-20191027-113972-1hmy009.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=597&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/298817/original/file-20191027-113972-1hmy009.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=597&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/298817/original/file-20191027-113972-1hmy009.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=750&fit=crop&dpr=1 754w, https://images.theconversation.com/files/298817/original/file-20191027-113972-1hmy009.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=750&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/298817/original/file-20191027-113972-1hmy009.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=750&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">SDSS map. Each dot is a galaxy;.</span>
<span class="attribution"><span class="source">M. Blanton and SDSS</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>All these assumptions were ultimately tested at the turn of the century with the combination of new observations of the universe, including new 3D maps from larger redshift surveys. In particular, the <a href="https://www.sdss.org/">Sloan Digital Sky Survey (SDSS)</a> was the first dedicated redshift survey telescope to measure over a million galaxy redshifts, mapping the large scale structure in the universe to unprecedented detail.</p>
<p>The SDSS maps included hundreds of superclusters and filaments and helped make an unexpected discovery – dark energy. They showed that the matter density of the universe was much less than expected from the <a href="https://theconversation.com/the-cmb-how-an-accidental-discovery-became-the-key-to-understanding-the-universe-45126">Cosmic Microwave Background</a>, which is the light left over from the Big Bang. That meant there must be an unknown substance, dubbed dark energy, driving an accelerated expansion of the Universe and become increasingly devoid of matter. </p>
<h2>The puzzle</h2>
<p>The combination of all these observations heralded a new era of cosmological understanding with a universe consisting of 30% matter and 70% dark energy. But despite the fact that most physicists have now accepted that there is such a thing as dark energy, we <a href="https://theconversation.com/how-crashing-neutron-stars-killed-off-some-of-our-best-ideas-about-what-dark-energy-is-88998">still do not know its exact form</a>. </p>
<p>There are several possibilities though. Many researchers believe that the energy of the vacuum simply has some particular value, dubbed a “cosmological constant”. Other options include the possibility that Einstein’s hugely successful <a href="https://theconversation.com/study-finds-dark-matter-and-dark-energy-may-not-exist-heres-what-to-make-of-it-88181">theory of gravity is incomplete</a> when applied on the huge scale of the entire universe. </p>
<p>New instruments like DESI will help take the next step in resolving the mystery. It will measure tens of millions of galaxy redshifts, spanning a huge volume of the universe up to ten billion light years from Earth. Such an amazing, detailed map should be able to answer a few key questions about dark energy and the creation of the large scale structures in the universe.</p>
<p>For example, it should be able to tell us if dark energy is just a cosmological constant. To do this it will measure the ratio of pressure that dark energy puts on the universe to the energy per unit volume. If dark energy is a cosmological constant, this ratio should be constant in both cosmic time and location. For other explanations, however, this ratio would vary. Any indication that it is not a constant would be revolutionary and spark intense theoretical work. </p>
<p>DESI should also be able to constrain, and even kill, many theories of modified gravity, possibly providing an emphatic confirmation of Einstein’s Theory of General Relativity on the largest scales. Or the opposite – and again that would spark a revolution in theoretical physics.</p>
<p>Another important theory that will be tested with DESI is Inflation, which predicts that tiny random quantum fluctuations of energy density in the primordial universe were exponentially expanded during a short period of intense growth to become the seeds of the large scale structures we see today. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/298824/original/file-20191027-113998-100tkb8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/298824/original/file-20191027-113998-100tkb8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=504&fit=crop&dpr=1 600w, https://images.theconversation.com/files/298824/original/file-20191027-113998-100tkb8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=504&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/298824/original/file-20191027-113998-100tkb8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=504&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/298824/original/file-20191027-113998-100tkb8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=633&fit=crop&dpr=1 754w, https://images.theconversation.com/files/298824/original/file-20191027-113998-100tkb8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=633&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/298824/original/file-20191027-113998-100tkb8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=633&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A team at a vendor in Santa Rosa Calif poses behind a DESI lens.</span>
<span class="attribution"><span class="source">VIAVI Solutions</span></span>
</figcaption>
</figure>
<p>DESI is only one of several next generation dark energy missions and experiments coming in the next decade, so there’s certainly reason to be optimistic that we could soon solve the mystery of dark energy. New satellite missions <a href="https://sci.esa.int/web/euclid">like Euclid</a>, and massive ground based observatories like the <a href="https://www.lsst.org/">Large Synoptic Survey Telescope</a>, will also offer insights. </p>
<p>There will also be other redshift instruments like DESI including <a href="https://www.eso.org/public/unitedkingdom/teles-instr/paranal-observatory/surveytelescopes/vista/4most/">4MOST at the European Southern Observatory</a>. Together, these will provide hundreds of millions of redshifts across the whole sky leading to an unimaginable map of our cosmos.</p>
<p>It seems a long time ago now when I wrote my PhD thesis based on just 700 galaxy redshifts. It really goes to show it’s an exciting time to be an astronomer.</p><img src="https://counter.theconversation.com/content/125886/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Bob Nichol receives funding from the UK Space Agency and Science and Technology Facilities Council (STFC).
I am a member of the Euclid, 4MOST, DESI and LSST scientific collaborations. </span></em></p>Will we have to rewrite Einstein’s theory of gravity? The DESI experiment could find out.Bob Nichol, Professor of Astrophysics and Pro Vice-Chancellor (Research and Innovation), University of PortsmouthLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/939942018-07-02T10:40:57Z2018-07-02T10:40:57ZObserving the universe with a camera traveling near the speed of light<figure><img src="https://images.theconversation.com/files/225012/original/file-20180626-112641-fpyvp6.jpg?ixlib=rb-1.1.0&rect=51%2C67%2C1120%2C731&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">What could a 'relativistic camera' capture on the way to Alpha Centauri?</span> <span class="attribution"><a class="source" href="https://images.nasa.gov/details-GSFC_20171208_Archive_e000214.html">ESA/NASA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Astronomers strive to observe the universe via ever more advanced techniques. Whenever researchers invent a new method, unprecedented information is collected and people’s understanding of the cosmos deepens.</p>
<p>An ambitious program to blast cameras far beyond the solar system was announced in April 2016 by internet investor and science philanthropist Yuri Milner, late physicist Stephen Hawking and Facebook CEO Mark Zuckerberg. Called “<a href="https://breakthroughinitiatives.org/initiative/3">Breakthrough Starshot</a>,” the idea is to send a bunch of tiny nano-spacecraft to the sun’s closest stellar neighbor, the three-star Alpha Centauri system. Traveling at around 20 percent the speed of light – so as fast as 100 million miles per hour – the craft and their tiny cameras would aim for the smallest but closest star in the system, Proxima Centari, and its planet Proxima b, 4.26 light-years from Earth.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/xRFXV4Z6x8s?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Breakthrough Starshot aims to establish proof of concept for a ‘nanocraft’ driven by a light beam.</span></figcaption>
</figure>
<p>The Breakthrough Starshot team’s goal will rely on a number of as-yet unproven technologies. The plan is to use light sails to get these spacecraft further and faster than anything that’s come before – lasers on Earth will push the tiny ships via their super-thin and reflective sails. I have another idea that could piggyback on this technology as the project is gearing up: Researchers could get valuable data from these mobile observatories, even directly test Einstein’s theory of special relativity, long before they get anywhere close to Alpha Centauri.</p>
<h2>Technical challenges abound</h2>
<p>Achieving Breakthrough Starshot’s goal is by no means an easy task. The project relies on continuing technological development on three independent fronts.</p>
<p>First, researchers will need to dramatically decrease the size and weight of microelectronic components to make a camera. Each nanocraft is planned to be no more than <a href="http://breakthroughinitiatives.org/concept/3">a few grams in total</a> – and that will have to include not just the camera, but also other payloads including power supply and communication equipment.</p>
<p>Another challenge will be to build thin, ultra-light and highly reflective materials to serve as the “sail” for the camera. One possibility is to have <a href="https://en.wikipedia.org/wiki/Solar_sail">a single-layer graphene sail – just a molecule thick, only 0.345 nanometer</a>.</p>
<p>The Breakthrough Starshot team will benefit from the rising power and falling cost of laser beams. Lasers with <a href="http://breakthroughinitiatives.org/concept/3">100-Gigawatt power</a> are needed to accelerate the cameras from the ground. Just as wind fills a sailboat’s sails and pushes it forward, the photons from a high-energy laser beam can propel an ultralight reflective sail forward as they bounce back.</p>
<p>With the projected technology development rate, it will likely be at least two more decades before scientists can launch a camera traveling with a speed a significant fraction of the speed of light. </p>
<p>Even if such a camera could be built and accelerated, several more challenges must be overcome in order to fulfill the dream of reaching the Alpha Centauri system. Can researchers aim the cameras correctly so they reach the stellar system? Can the camera even survive the near 20-year journey without being damaged? And if it beats the odds and the trip goes well, will it be possible to transmit the data – say, images – back to Earth over such a huge distance?</p>
<h2>Introducing ‘relativistic astronomy’</h2>
<p>My collaborator Kunyang Li, a graduate student at Georgia Institute of Technology, and I <a href="https://doi.org/10.3847/1538-4357/aaa9b7">see potential in all these technologies</a> even before they’re perfected and ready to head out for Alpha Centauri. </p>
<p>When a camera travels in space at close to the speed of light – what could be called “relativistic speed” – Einstein’s special theory of relativity plays a role in how the images taken by the camera will be modified. Einstein’s theory states that in different “rest frames” observers have different measures of the lengths of space and time. That is, space and time are relative. How differently the two observers measure things depends on how fast they’re moving with respect to each other. If the relative speed is close to the speed of light, their observations can differ significantly. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=471&fit=crop&dpr=1 754w, https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=471&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/225009/original/file-20180626-112611-j9xz0u.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=471&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 Doppler effect explains how a source moving away from you will stretch the wavelengths of its light and look redder, while if it’s moving closer the wavelengths will shorten and look bluer.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Redshift_blueshift.svg">Aleš Tošovský</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Special relativity also affects many other things physicists measure – for example, the frequency and intensity of light and also the size of an object’s appearance. In the rest frame of the camera, the entire universe is moving at a good fraction of the speed of light in the opposite direction of the camera’s own motion. To an imaginary person on board, thanks to the different spacetimes experienced by him and everyone back on Earth, the light from a star or galaxy would appear bluer, brighter and more compact, and the angular separation between two objects would look smaller. </p>
<p>Our idea is to take advantage of these features of special relativity to observe familiar objects in the relativistic camera’s different spacetime rest frame. This can provide a new mode to study astronomy – what we’re calling “relativistic astronomy.”</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=462&fit=crop&dpr=1 600w, https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=462&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=462&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=581&fit=crop&dpr=1 754w, https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=581&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/225412/original/file-20180628-117382-s24ih1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=581&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Observed image of nearby galaxy M51 on the left. On the right, how the image would look through a camera moving at half the speed of light: brighter, bluer and with the stars in the galaxy closer together.</span>
<span class="attribution"><a class="source" href="https://doi.org/10.3847/1538-4357/aaa9b7">Zhang & Li, 2018, The Astrophysical Journal, 854, 123</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>What could the camera capture?</h2>
<p>So, a relativistic camera would naturally serve as a <a href="https://www.quora.com/What-do-spectrographs-help-astronomers-determine-How-is-this-important">spectrograph</a>, allowing researchers to look at an intrinsically redder band of light. It would act as a lens, magnifying the amount of light it collects. And it would be a wide-field camera, letting astronomers observe more objects within the same field of view of the camera.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1060&fit=crop&dpr=1 600w, https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1060&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1060&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1332&fit=crop&dpr=1 754w, https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1332&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/225392/original/file-20180628-117374-fbv8fx.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1332&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An example of redshift: On the right, absorption lines occur closer to the red end of the spectrum.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Redshift.svg">Georg Wiora</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Here’s one example of the kind of data we could gather using the relativistic camera. Due to the expansion of the universe, the light from the early universe is redder by the time it reaches Earth than when it started. Physicists call this effect redshifting: As the light travels, its wavelength stretches as it expands along with the universe. Red light has longer wavelengths than blue light. All this means that to see red-shifted light from the young universe, one must use the difficult-to-observe infrared wavelengths to collect it.</p>
<p>Enter the relativistic camera. To a camera moving at close to the speed of light, such redshifted light becomes bluer – that is, it’s now blueshifted. The effect of the camera’s motion counteracts the effect of the universe’s expansion. Now an astronomer could catch that light using the familiar visible light camera. The same Doppler boosting effect also allows the faint light from the early universe to be amplified, aiding detection. Observing the spectral features of distant objects can allow us to reveal the history of the early universe, especially <a href="https://www.symmetrymagazine.org/article/what-ended-the-dark-ages-of-the-universe">how the universe evolved after it became transparent</a> 380,000 years after the Big Bang.</p>
<p>Another exciting aspect of relativistic astronomy is that humankind can directly test the principles of special relativity using macroscopic measurements for the first time. Comparing the observations collected on the relativistic camera and those collected from ground, astronomers could precisely test the fundamental predictions of Einstein’s relativity regarding change of frequency, flux and light travel direction in different rest frames.</p>
<p>Compared with the ultimate goals of the Starshot project, observing the universe using relativistic cameras should be easier. Astronomers wouldn’t need to worry about aiming the camera, since it could get interesting results when sent in any direction. The data transmission problem is somewhat alleviated since the distances wouldn’t be as great. Same with the technical difficulty of protecting the camera.</p>
<p>We propose that trying out relativistic cameras for astronomical observations could be a forerunner of the full Starshot project. And humankind will have a new astronomical “observatory” to study the universe in an unprecedented way. History suggests that opening a new window like this will unveil many previously undetected treasures.</p><img src="https://counter.theconversation.com/content/93994/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Bing Zhang does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>An astronomer suggests an idea to piggyback on the ambitious Breakthrough Starshot project that aims to send nano spacecraft to Alpha Centauri at a major fraction of the speed of light.Bing Zhang, Professor of Astrophysics, University of Nevada, Las VegasLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/581472016-04-28T20:13:04Z2016-04-28T20:13:04ZKitchen Science: gastrophysics brings the universe into your kitchen<figure><img src="https://images.theconversation.com/files/120659/original/image-20160429-28029-1y1mkuk.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">You can learn a lot about the cosmos in the kitchen.</span> <span class="attribution"><span class="source">Shutterstock/Wikimedia</span></span></figcaption></figure><p>This title will probably come as a surprise to you. <a href="https://theconversation.com/au/topics/astrophysics">Astrophysics</a> concerns enormous scales of space, time and energy; kitchens are relatively small, homely places. </p>
<p>So how can there be a connection between astrophysics and the kitchen?</p>
<p>Have you ever dropped a just-opened plastic bottle of milk or fruit juice on the kitchen bench and had the contents jumped up and hit you in the face? I have. And when it happened, I suddenly realised there was a connection with the physics of a type II <a href="https://theconversation.com/au/topics/supernova">supernova</a> explosions.</p>
<p>When a massive star – roughly ten times or more the mass of our sun – comes to the end of its life, it is blown apart in a catastrophic explosion known as a type II supernova. </p>
<p>The explosion is triggered by the sudden collapse of the iron core at the centre of the star. The rest of the star follows and slams into the rebounding core, which creates a shockwave that propagates through the star back to the surface. When the shockwave reaches the surface there is no more star to push on, and so the outer layers of the star are ejected violently into space.</p>
<p>On the kitchen bench, dropping a plastic bottle causes the base to flex and push on the liquid above, creating a shock wave. When the shockwave reaches the top of the bottle there is no more liquid to push on, and so the liquid is ejected up into the air.</p>
<h2>Baking the universe</h2>
<p>You can also see the evolution of the universe while baking. </p>
<p>Astrophysicists often use the “raisin loaf” analogy to explain the expansion of the universe. The dough represents space and the raisins galaxies. </p>
<p>Imagine sitting on a raisin in the middle of the dough as it bakes in the oven. As the loaf expands we would see every other raisin move away from us. When we look out into the universe (almost) every galaxy is moving away from our home galaxy, the Milky Way. </p>
<p>At home I often make what I call Hubble Damper in honour of <a href="http://hubblesite.org/the_telescope/hubble_essentials/edwin_hubble.php">Edwin Hubble</a>, who first ascertained that galaxies are moving away from the Earth. He also determined that the farther the galaxy, the faster the recession. </p>
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<a href="https://images.theconversation.com/files/120444/original/image-20160428-30976-1kxit49.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/120444/original/image-20160428-30976-1kxit49.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/120444/original/image-20160428-30976-1kxit49.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/120444/original/image-20160428-30976-1kxit49.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/120444/original/image-20160428-30976-1kxit49.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/120444/original/image-20160428-30976-1kxit49.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/120444/original/image-20160428-30976-1kxit49.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/120444/original/image-20160428-30976-1kxit49.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>
<figcaption>
<span class="caption">Delicious and educational!</span>
<span class="attribution"><span class="source">Stephen Hughes</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>For added realism I add craisins (dried cranberries) rather than raisins. Craisins are red. Galaxies are <a href="http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/redshift.html">redshifted</a>.</p>
<p>If you are interested in making your own Hubble Damper, here is the recipe. Mix together:</p>
<ul>
<li>450g self–raising flour</li>
<li>a pinch of salt</li>
<li>80g diced chilled butter</li>
<li>185ml water</li>
<li>170g craisins. </li>
</ul>
<p>Bake for 30 minutes at 200°C. Tap the base of the bread; if it sounds hollow, it’s done.</p>
<h2>Sunset in a glass</h2>
<p>One of the first “science” questions children often ask is: “Why is the sky blue?” And it’s a question dreaded by many parents. </p>
<p>If you have one of those children, here is something you can do in your kitchen to explain why the sky is blue. Get hold of a glass of water – a glass with straight sides is best. Put a drop of milk into the water and stir. </p>
<p>Get hold of a LED torch and shine it through the water. When you look at the beam of light end-on, you will see that it is has a yellow tint. If you can’t see the LED light, dilute the milky water. </p>
<p>The fact that the torch beam has a yellow tint means that blue light has been extracted from the beam, which is what we call scattering. </p>
<p>If you perform the experiment at night, switch off the lights in the kitchen and look at the glass of water from the side. You will notice a blue glow. This is the light scattered by the milk. Congratulations, you’ve just made your own blue sky and sunset in your kitchen.</p>
<p>In the atmosphere, molecules in the air scatter blue light from the sun more than the red wavelengths. The blue light comes into our eyes from all directions, which is why the sky is blue. When the sun is close to the horizon, the sunlight has to pass through a greater thickness of atmosphere and more blue light is scattered making the sun appear red.</p>
<p>After my light bulb moment of dropping the milk bottle I decided to explore other connections between astrophysics and the physics of cooking and kitchen appliances. Over the course of about three years I wrote an eBook book called <a href="http://www.stephenhughes.com.au/portfolio/24/">Gastrophysics</a>, which explores the connections, and I learnt a bit of cooking at the same time.</p>
<p><em>This article is part of the <a href="https://theconversation.com/au/topics/kitchen-science">Kitchen Science</a> series, exploring the amazing physics and chemistry going on in our kitchens every day. If you’re an academic with an idea for a Kitchen Science article, <a href="mailto:tim.dean@theconversation.edu.au">get in touch!</a></em></p><img src="https://counter.theconversation.com/content/58147/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Stephen Hughes is the author of the eBook Gastrophysics, upon which this article is based.</span></em></p>From supernovae explosions to the expansion of the universe and why the sky is blue: you can learn a lot about the universe in the kitchen.Stephen Hughes, Senior Lecturer in Physics, Queensland University of TechnologyLicensed as Creative Commons – attribution, no derivatives.