Cosmic superclusters, the Universe’s largest structures, don’t actually exist
“The idea of a supercluster and the name for ours, “Laniakea,” will persist for a long time. But just because we named it doesn’t make it real. Billions of years from now, all the different components will simply be strewn farther and farther apart from one another, and in the farthest futures of our imaginings, they’ll disappear from our view and reach entirely.”
Galaxies don’t just exist in isolation in our Universe, but are often found bound together as a part of even grander structures. Our own Milky Way is bound in a galactic group (our local group), nearby are larger groups and galaxy clusters, and on still larger scales, cosmic superclusters appear to encompass as many as 100,000 individual galaxies. Yet it isn’t sufficient to simply see what appears to be a collection and draw an imaginary line around it. You can’t just give something a name and proclaim that it’s meaningful because you defined it. Instead, for a collection of objects in space, they need to be gravitationally bound together and connected. Thanks to dark energy, these superclusters aren’t.
ASTRONOMERS MAKE THE LARGEST MAP OF THE UNIVERSE YET
Astronomers with the Sloan Digital Sky Survey (SDSS) have created the first map of the large-scale structure of the universe based entirely on the positions of quasars. Quasars are the incredibly bright and distant points of light powered by supermassive black holes.
“Because quasars are so bright, we can see them all the way across the universe,” said Ashley Ross of the Ohio State University, the co-leader of the study. “That makes them the ideal objects to use to make the biggest map yet.”
The amazing brightness of quasars is due to the supermassive black holes found at their centers. As matter and energy fall into a quasar’s black hole, they heat up to incredible temperatures and begin to glow. It is this bright glow that is detected by a dedicated 2.5-meter telescope here on Earth.
“These quasars are so far away that their light left them when the universe was between three and seven billion years old, long before the Earth even existed,” said Gongbo Zhao from the National Astronomical Observatories of Chinese Academy of Sciences, the study’s other co-leader.
To make their map, scientists used the Sloan Foundation Telescope to observe an unprecedented number of quasars. During the first two years of the SDSS’s Extended Baryon Oscillation Spectroscopic Survey (eBOSS), astronomers measured accurate three-dimensional positions for more than 147,000 quasars.
The telescope’s observations gave the team the quasars’ distances, which they used to create a three-dimensional map of where the quasars are. But to use the map to understand the expansion history of the universe, they had to go a step further, using a clever technique involving studying “baryon acoustic oscillations” (BAOs). BAOs are the present-day imprint of sound waves which travelled through the early universe, when it was much hotter and denser than the universe we see today. But when the universe was 380,000 years old, conditions changed suddenly and the sound waves became “frozen” in place. These frozen waves are left imprinted in the three-dimensional structure of the universe we see today.
The good news about these frozen waves – the original baryon acoustic oscillations – is that the process that produced them is simple. Thus, we have a good understanding of what BAOs must have looked like at that ancient time. When we look at the three-dimensional structure of the universe today, it contains these same BAOs grown out to a huge scale by the expansion of the universe. The observed size of the BAO can be used as a “standard ruler” to measure distances. Just as by using the apparent angle of a meter stick viewed from the other side of a football field, you can estimate the length of the field. “You have meters for small units of length, kilometres or miles for distances between cities, and we have the BAO scale for distances between galaxies and quasars in cosmology,” explained Pauline Zarrouk, a PhD student at the Irfu/CEA, University Paris-Saclay, who measured the projected BAO scale.
Astronomers from the SDSS have previously used the BAO technique on nearby galaxies and then on intergalactic gas distributions to push this analysis farther and farther back in time. The current results cover a range of times where they have never been observed before, measuring the conditions when the universe more than two billion years before the Earth formed.
The results of the new study confirm the standard model of cosmology that researchers have built over the last twenty years. In this standard model, the universe follows the predictions of Einstein’s general theory of relativity – but includes components whose effects we can measure, but whose causes we do not understand. Along with the ordinary matter that makes up stars and galaxies, the universe includes dark matter – invisible yet still affected by gravity – and a mysterious component called “dark energy.” Dark energy is the dominant component at the present time, and it has special properties that cause the expansion of the universe to speed up.
“Our results are consistent with Einstein’s theory of general relativity,” said Hector Gil-Marin, a researcher from the Laboratoire de Physique Nucléaire et de hautes Énergies in Paris who undertook key parts of the analysis. “We now have BAO measurements covering a range of cosmological distances, and they all point to the same thing: the simple model matches the observations very well.”
Even though we understand how gravity works, we still do not understand everything – there is still the question of what exactly dark energy is. “We would like to understand dark energy further,” said Will Percival from the University of Portsmouth, who is the eBOSS survey scientist. “Surveys like eBOSS are helping us to build up our understanding of how dark energy fits into the story of the universe.”
The eBOSS experiment is still continuing, using the Sloan Telescope at Apache Point Observatory in New Mexico, USA. As astronomers with eBOSS observe more quasars and nearby galaxies, the size of their map will continue to increase. After eBOSS is complete, a new generation of sky surveys will begin, including the Dark Energy Spectroscopic Instrument (DESI) and the European Space Agency Euclid satellite mission. These will increase the fidelity of the maps by a factor of ten compared with eBOSS, revealing the universe and dark energy in unprecedented detail.
IMAGE….A slice through largest-ever three-dimensional map of the universe. Earth is at the left, and distances to galaxies and quasars are labeled by the lookback time to the objects (lookback time means how long the light from an object has been traveling to reach us here on Earth). The locations of quasars (galaxies with supermassive black holes) are shown by the red dots, and nearer galaxies mapped by SDSS are also shown (yellow). The right-hand edge of the map is the limit of the observable Universe, from which we see the cosmic microwave background (CMB) – the light “left over” from the Big Bang. The bulk of the empty space in between the quasars and the edge of the observable universe are from the “dark ages,” prior to the formation of most stars, galaxies, or quasars. Credit: Anand Raichoor (Ecole Polytechnique Federale de Lausanne, Switzerland) and the SDSS Collaboration
Missing Matter Found, But Doesn’t Dent Dark Matter
“But this doesn’t eliminate the need for dark matter; it doesn’t touch that undiscovered 27% of matter in the Universe, not in the slightest. It’s another piece of that 5% that we know is out there, that we’re struggling to put together. It’s just protons, neutrons, and electrons, existing in about six times the abundance within these filaments as compared to the cosmic average. The fact that this filamentary structure contains normal matter at all is further evidence for dark matter, since without it there’d be no gravitationally overdense regions to hold the extra normal matter in place. In this case, the WHIM traces the dark matter, further confirming what we know must be out there.”
It’s no secret that if we look at the matter we see in the Universe, the story doesn’t add up. On all scales, from individual galaxies to pairs, groups and clusters of galaxies, all the way up to the large-scale structure of the Universe, the matter we see is insufficient to explain the structures we get. There has to be more matter, both normal (atom-based) matter and dark (non-interacting) matter, to make our theory and predictions match. In a wonderful new pair of papers, two independent teams have detected the warm-hot intergalactic medium along the large-scale structure filaments in the Universe. With six times the normal matter density, this accounts for a significant fraction of the missing normal matter in the Universe! It’s estimated that 50-90% of the baryons in the Universe are part of the WHIM, and this could be the first step towards detecting them. But it doesn’t touch or change the dark matter at all; we still need it and still don’t have it.
Scientists Are Using the Universe as a “Cosmological Collider”
Physicists are capitalizing on a direct connection between the largest cosmic structures and the smallest known objects to use the universe as a “cosmological collider” and investigate new physics.
The three-dimensional map of galaxies throughout the cosmos and the leftover radiation from the Big Bang – called the cosmic microwave background (CMB) – are the largest structures in the universe that astrophysicists observe using telescopes. Subatomic elementary particles, on the other hand, are the smallest known objects in the universe that particle physicists study using particle colliders.
A team including Xingang Chen of the Harvard-Smithsonian Center for Astrophysics (CfA), Yi Wang from the Hong Kong University of Science and Technology (HKUST) and Zhong-Zhi Xianyu from the Center for Mathematical Sciences and Applications at Harvard University has used these extremes of size to probe fundamental physics in an innovative way. They have shown how the properties of the elementary particles in the Standard Model of particle physics may be inferred by studying the largest cosmic structures. This connection is made through a process called cosmic inflation.
Cosmic inflation is the most widely accepted theoretical scenario to explain what preceded the Big Bang. This theory predicts that the size of the universe expanded at an extraordinary and accelerating rate in the first fleeting fraction of a second after the universe was created. It was a highly energetic event, during which all particles in the universe were created and interacted with each other. This is similar to the environment physicists try to create in ground-based colliders, with the exception that its energy can be 10 billion times larger than any colliders that humans can build.
Inflation was followed by the Big Bang, where the cosmos continued to expand for more than 13 billion years, but the expansion rate slowed down with time. Microscopic structures created in these energetic events got stretched across the universe, resulting in regions that were slightly denser or less dense than surrounding areas in the otherwise very homogeneous early universe. As the universe evolved, the denser regions attracted more and more matter due to gravity. Eventually, the initial microscopic structures seeded the large-scale structure of our universe, and determined the locations of galaxies throughout the cosmos.
In ground-based colliders, physicists and engineers build instruments to read the results of the colliding events. The question is then how we should read the results of the cosmological collider.
“Several years ago, Yi Wang and I, Nima Arkani-Hamed and Juan Maldacena from the Institute of Advanced Study, and several other groups, discovered that the results of this cosmological collider are encoded in the statistics of the initial microscopic structures. As time passes, they become imprinted in the statistics of the spatial distribution of the universe’s contents, such as galaxies and the cosmic microwave background, that we observe today,” said Xingang Chen. “By studying the properties of these statistics we can learn more about the properties of elementary particles.”
As in ground-based colliders, before scientists explore new physics, it is crucial to understand the behavior of known fundamental particles in this cosmological collider, as described by the Standard Model of particle physics.
“The relative number of fundamental particles that have different masses – what we call the mass spectrum – in the Standard Model has a special pattern, which can be viewed as the fingerprint of the Standard Model,” explained Zhong-Zhi Xiangyu. “However, this fingerprint changes as the environment changes, and would have looked very different at the time of inflation from how it looks now.”
The team showed what the mass spectrum of the Standard Model would look like for different inflation models. They also showed how this mass spectrum is imprinted in the appearance of the large-scale structure of our universe. This study paves the way for the future discovery of new physics.
“The ongoing observations of the CMB and large-scale structure have achieved impressive precision from which valuable information about the initial microscopic structures can be extracted,” said Yi Wang. “In this cosmological collider, any observational signal that deviates from that expected for particles in the Standard Model would then be a sign of new physics.”
The current research is only a small step towards an exciting era when precision cosmology will show its full power.
“If we are lucky enough to observe these imprints, we would not only be able to study particle physics and fundamental principles in the early universe, but also better understand cosmic inflation itself. In this regard, there are still a whole universe of mysteries to be explored,” said Xianyu.
This research is detailed in a paper published in the journal Physical Review Letters on June 29, 2017, and the preprint is available online.
Inflation Isn’t Just Science, It’s The Origin Of Our Universe
“3.) There should be fluctuations on scales larger than light could have traveled since the Big Bang. This is another consequence of inflation, but there’s no way to get a coherent set of fluctuations on large scales like this without something stretching them across cosmic distances. The fact that we see these fluctuations in the cosmic microwave background and in the large-scale structure of the Universe — and didn’t know about them until the COBE satellite in the 1990s — further validates inflation.”
Beginning in 1979, a new idea arose in theoretical physics, seeking to improve upon the idea of the Big Bang: cosmic inflation. Recently, a number of physicists, including one of inflation’s cofounders, Paul Steinhardt, have come out with vitriol against the theory of inflation, calling it not even science. It’s true that inflation may not be the final answer to absolutely everything in the Universe, as there are a number of indeterminate predictions and a number of puzzles it fails to adequately solve. But that does not mean it isn’t science! In fact, inflation is the best theory we have to explain the initial conditions that the Big Bang has been observed to begin with. In addition, inflationary cosmology makes a number of powerful predictions that give rise to observables within our Universe, and a great many of them have been subsequently tested and validated. For reproducing the successes of the pre-existing theory, for explaining phenomena the old theory could not, and making new, testable, successful predictions, inflation hits all the points a scientific theory could aspire to.
How Does Earth Move Through Space? Now We Know, On Every Scale
“Ask a scientist for our cosmic address, and you’ll get quite a mouthful. Here we are, on planet Earth, which spins on its axis and revolves around the Sun, which orbits in an ellipse around the center of the Milky Way, which is being pulled towards Andromeda within our local group, which is being pushed around inside our cosmic supercluster, Laniakea, by galactic groups, clusters, and cosmic voids, which itself lies in the KBC void amidst the large-scale structure of the Universe. After decades of research, science has finally put together the complete picture, and can quantify exactly how fast we’re moving through space, on every scale.”
It’s hard to believe, but despite being at rest here on the surface of Earth, we’re actually hurtling through the Universe in a variety of impressive ways. The Earth spins on its axis, giving someone at the equator a speed of some 1700 km/hr. Yet at even faster speeds, the Earth orbits the Sun, the Sun moves through the Milky Way, and there’s a great cosmic motion that applied to the Milky Way galaxy beyond even that. For a long time, we’ve been able to measure the total effect of all these motions, summed up, by measuring our motion relative to the cosmic microwave background: the leftover glow from the Big Bang. But it’s only very, very recently that we’ve identified the source of all the gravitational causes of this motion. While we’ve known of stars, galaxies, and the large-scale structure of where matter is, it’s new that we’ve quantified the effects of these great cosmic voids.
Study reveals substantial evidence of holographic universe
A UK, Canadian and Italian study has provided what researchers believe is the first observational evidence that our universe could be a vast and complex hologram.
Theoretical physicists and astrophysicists, investigating irregularities in the cosmic microwave background (the ‘afterglow’ of the Big Bang), have found there is substantial evidence supporting a holographic explanation of the universe – in fact, as much as there is for the traditional explanation of these irregularities using the theory of cosmic inflation.
The Huge-LQG is a possible structure that could be one of the largest in the known universe. Having originally been identified as the largest, the Hercules-Corona Borealis Great Wall is bigger at 10 billion light years.
The Huge-LQG consists of 73 quasars, a quasar being a class of active galactic nuclei is essentially a superheated region of gas and dust that surrounds a supermassive black hole typically being 10-10,000 times the size of the Schwarzschild radius of the black hole. The existence of this structure defies Einstein’s cosmological principal which states that at large scales, the universe is approximately homogenous (meaning that the fluctuation in matter density throughout space can be considered small). It’s around 9 billion light years away from us, has a length of 1.24 gigaparsecs which is 4.0443 billion light years and a solar mass of 6.1 quintillion (that’s 6.1 quintillion times the mass of our sun and our sun is approximately 2 nonillion kg’s)!
On April 24, 1990, the Hubble Space Telescope was launched into orbit.
“No matter what Hubble reveals — planets, dense star fields, colorful interstellar nebulae, deadly black holes, graceful colliding galaxies, the large-scale structure of the Universe — each image establishes your own private vista on the cosmos.”- Neil deGrasse Tyson
Astronomers discover mysterious alignment of black holes
Deep radio imaging by researchers in the University of Cape Town and University of the Western Cape, in South Africa, has revealed that supermassive black holes in a region of the distant universe are all spinning out radio jets in the same direction – most likely a result of primordial mass fluctuations in the early universe. The astronomers publish their results in a new paper in Monthly Notices of the Royal Astronomical Society.
The new result is the discovery – for the first time – of an alignment of the jets of galaxies over a large volume of space, a finding made possible by a three-year deep radio imaging survey of the radio waves coming from a region called ELAIS-N1 using the Giant Metrewave Radio Telescope (GMRT).
“SARASWATI”– ONE OF THE MOST MASSIVE LARGE-
SCALE STRUCTURES IN THE UNIVERSE DISCOVERED
** Synopsis: Indian astrophysicists identify megastructure of galaxies 4 billion light-years away; several scholars and faculty of Indian universities involved. **
A team of astronomers from the Inter University Centre for Astronomy & Astrophysics (IUCAA) and Indian Institute of Science Education and Research (IISER), both in Pune, India, and members of two other Indian universities, have identified a previously unknown, extremely large supercluster of galaxies located in the direction of constellation Pisces. This is one of the largest known structures in the nearby universe and is at a distance of 4,000 million (400 crore) light-years away from us.
This novel discovery is being published in the latest issue of the Astrophysical Journal, the premier research journal of the American Astronomical Society.
Large-scale structures in the universe are found to be hierarchically assembled, with galaxies, together with associated gas and dark matter, being clumped in clusters, which are organized with other clusters, smaller groups, filaments, sheets, and large empty regions (“voids”) in a pattern called the “cosmic web,” which spans the observable universe.
Superclusters are the largest coherent structures in the cosmic web. A supercluster is a chain of galaxies and galaxy clusters, bound by gravity, often stretching to several hundred times the size of clusters of galaxies, consisting of tens of thousands of galaxies. This newly discovered ‘Saraswati’ supercluster, for instance, extends over a scale of 600 million light-years and may contain the mass equivalent of over 20 million billion Suns.
When astronomers look far away, they see the universe from long ago, since light takes a while to reach us. The Saraswati supercluster is observed as it was when the universe was 10 billion years old.
The long-popular “cold dark matter” model of the evolution of the universe predicts that small structures like galaxies form first, which congregate into larger structures. Most forms of this model do not predict the existence of large structures such as the “Saraswati” supercluster within the current age of the universe. The discovery of these extremely large structures thus forces astronomers into re-thinking the popular theories of how the universe got its current form, starting from a more-or-less uniform distribution of energy after the Big Bang. In recent years, the discovery of the present-day universe being dominated by “dark energy,” which behaves very differently from gravitation, might play a role in the formation of these structures.
It is believed that galaxies are formed mostly on the filaments and sheets that are part of the cosmic web, and many of the galaxies travel along these filaments, ending up in the rich clusters, where the crowded environment switches off their star formation and aids in the transformation of galaxies to disky blue spiral galaxies to red elliptical galaxies. Since there is an extensive variation of environment within a supercluster, galaxies travel through these varied environments during their “lifetime.” To understand their formation and evolution, one needs to identify these superclusters and closely study the effect of their environment on the galaxies. This is a very new research area – with the aid of observations of new observational facilities, astronomers are now beginning to understand galaxy evolution. This discovery will greatly enhance this field of research.
“Saraswati” (or “Sarasvati”), a word that has proto-Indo-European roots, is a name found in ancient Indian texts to refer to the major river around which the people of the ancient Indian civilization lived. It is also the name of the celestial goddess who is the keeper of the celestial rivers. In modern India, Saraswati is worshipped as the goddess of knowledge, music, art, wisdom and nature – the muse of all creativity.
Our own galaxy is part of a supercluster called the Laniakea supercluster, announced in 2014 by Brent Tully at the University of Hawaii and collaborators.
Interestingly, Somak Raychaudhury, currently director of IUCAA, Pune, who is a co-author of this paper, also discovered the first massive supercluster of galaxies on this scale (the “Shapley concentration”), during his PhD research at the University of Cambridge. In his paper, published in the journal ‘Nature’ in 1989, he had named the supercluster after the American astronomer Harlow Shapley, in recognition of his pioneering survey of galaxies, from the southern hemisphere, in which this massive superstructure was first imaged, way back in 1932.
Joydeep Bagchi from IUCAA, the lead author of the paper, and co-author Shishir Sankhyayan (PhD scholar at IISER, Pune) said, ‘‘We were very surprised to spot this giant wall-like supercluster of galaxies, visible in a large spectroscopic survey of distant galaxies, known as the Sloan Digital Sky Survey. This supercluster is clearly embedded in a large network of cosmic filaments traced by clusters and large voids. Previously only a few comparatively large superclusters have been reported, for example the ‘Shapley concentration’ or the ‘Sloan Great Wall’ in the nearby universe, while the ‘Saraswati’ supercluster is a far more distant one. Our work will help to shed light on the perplexing question: how such extreme large scale, prominent matter-density enhancements had formed billions of years in the past when the mysterious dark energy had just started to dominate structure formation.’’
TOP IMAGE….The distribution of galaxies, from Sloan Digital Sky Survey (SDSS), in Saraswati supercluster. It is clearly visible that the density of galaxies is very high in the Saraswati supercluster region. The typical size of a galaxy here is around 250,000 light years. The galaxy sizes are increased for representation
LOWER IMAGE….Two most massive clusters of galaxies in the Saraswati supercluster : “ABELL 2631” cluster (left) and “ZwCl 2341.1+0000” cluster (right). “ABELL 2631” resides in the core of the Saraswatisupercluster. The Saraswati supercluster has a total of 43 clusters of galaxies
Ask Ethan: How Do We Know The Universe Is 13.8 Billion Years Old?
“You’ve heard the story before: the Universe began with the Big Bang 13.8 billion years ago, and formed atoms, stars, galaxies, and eventually planets with the right ingredients for life. Looking at distant locations in the Universe is also looking back in time, and somehow, through the power of physics and astronomy, we’ve figured out not only how the Universe began, but its age. But how do we know how old the Universe is? That what Thys Hauptfleisch wants to know for this week’s Ask Ethan:
Ethan, how was the 13.8 billion years calculated? (In English please!)”
There’s a unique relationship between everything that exists in the Universe today – the stars and galaxies, the large-scale structure, the leftover glow from the Big Bang, the expansion rate, etc. – and the amount of time that’s passed since it all began. When it comes to our Universe, there really was a day without a yesterday, but how do we know exactly how much time has passed between then and now? There are two ways: one complex and one simple. The complex way is to determine all the matter and energy components making up the Universe, to measure how the Universe has expanded over the entirety of its cosmic history, and then, in the context of the Big Bang, to deduce how old the Universe must be. The other is to understand stars, measure them, and determine how old the oldest ones are.
“Thanks to the power of gravitational lensing, where intervening mass acts like a lens to background light, distorting and magnifying it, we were able to reconstruct the mass. Lo and behold, it appeared (in blue) well-separated from where the X-rays and therefore the gas (in pink) was. And when we reconstructed how much of that mass is present in the form of dark matter, we find that it’s almost all of it. Again, normal matter, even if we change the laws of gravity, can’t account for these observations. Fast-forward to the present day, and we’ve found a great number of these colliding clusters that all show the same separation between the X-ray emitting normal matter and the mass, present in the form of dark matter.”
“Dark matter is the most mysterious, non-interacting substance in the Universe. Its gravitational effects are necessary to explain the rotation of galaxies, the motions of clusters, and the largest scale-structure in the entire Universe. But on smaller scales, it’s too sparse and diffuse to impact the motion of the Solar System, the matter here on Earth, or the origin and evolution of humans in any meaningful way. Yet the gravity that dark matter provides is an absolute necessity for allowing our galaxy to hold onto the raw ingredients that made life like us and planets like Earth possible at all. Without dark matter, the Universe would likely have no signs of life at all.”
Making up some 85% of the mass in our Universe, dark matter is necessary to explain the motions of individual galaxies, the grouping and clustering of assemblies of galaxies, the large-scale structure of the Universe and more. But on a much closer-to-home level, dark matter may be absolutely essential to the origin of life, too! Without dark matter, supernova explosions and starburst events would still create copious amounts of heavy elements, driven outwards by winds and the force of the explosions. But it’s the extra gravity of the dark matter that prevents most of this material from escaping, and allows it to take part in the formation of future generations of stars, to participate in rocky planet formation, and to deliver the ingredients necessary for life.
Ask Ethan: Is this actually a hole in the Universe?
“What do you do about people and entities who actively harm the amount of knowledge that the general populace has in the world? After all, the opposite of knowledge isn’t ignorance, but rather misinformation posing as knowledge.”
There are plenty of scientific myths that go around, including many that were generated recently by so-called science communicators that actively harm public knowledge. One of them was a now-famous image of a dark nebula silhouetted against a star field, claiming that this was a hole in the Universe a billion light years across with no matter in it. Not only is the image itself a completely different picture – that of a tiny molecular gas cloud just 500 light years away – but the study that led to the conclusion of a “hole in the Universe” has that as only one of many possible interpretations. Far more likely is that we’re simply looking at a large, underdense region that’s well within the range of what’s normal and expected for our Universe.