How was it when the cosmic web took shape?



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A simulation of the large-scale structure of the Universe. Identifying which regions are dense and massive enough to match clusters of stars, galaxies, clusters of galaxies and determining when and under what conditions they form is a challenge that cosmologists are just beginning to face.Dr. Zarija Lukic

One of the strangest facts about the Universe is how it has changed dramatically over time. Today, we see a universe full of large galaxies containing hundreds of billions of stars, clustered together in a huge cosmic web. Further back in time to the Big Bang, however, everything was extremely smooth and even with very little clumping or grouping to speak. Get back enough, in fact, and you will not find any galaxy or star.

This makes sense from a qualitative point of view. The Universe was born with tiny imperfections, gravitation grows as the Universe expands, and depending on how and where gravity wins, we get these huge galaxies and clusters of galaxies separated by regions that contain nothing: cosmic voids. But the structure did not form at once, and the larger structures formed last. This is & nbsp; the cosmic reason why.

The evolution of large-scale structure in the Universe, from an initial and uniform state to the grouped universe we know today. The type and abundance of dark matter would provide a very different Universe if we alter what our Universe possesses. Note the fact that the small scale structure appears early in all cases, while the structure on larger scales does not come until much later.Angulo et al. 2008, via Durham University

Imagine the Universe as it was in these early stages. It is full of matter and radiation that distribute almost perfectly evenly everywhere you look. In the aftermath of the Big Bang, a typically superdense region had an average density of 100.003%, while a typically low density had 99.997% of the average density. When we describe the primitive universe as uniform, this is the level of uniformity that we attain.

These superdensities and subdensities were almost exactly the same on all scales. Whether you look at a region a few miles or a few light years or a few million or billions light-years in size, that same 1 part-30,000 float describes the super-dense and sub-dense regions with which the Universe began.

Overdensate regions grow and grow over time, but are limited in their growth by the small initial magnitudes of the superdensities, the cosmic scale at which superdensities are encountered (and the time that takes the gravitational force through them), and the presence of radiation that is still energetic, which prevents the structure from growing more rapidly. It takes tens to hundreds of millions of years to form the first stars; small clusters of matter exist long before that, however.Aaron Smith / TACC / UT-Austin

But that does not stay that way for long. Gravity immediately begins to preferentially attract mass to the regions of excess density as compared to all others. The sub-dense regions more readily yield their matter to the surrounding regions, comparatively denser.

However, although the law of gravity is universal and the same on all scales, the Universe does not form clusters of stars, galaxies and clusters of galaxies at one and the same time. In fact, it takes less than 100 million years for the first stars to form, but billions of years – more than ten times as long – before we form the huge clusters of galaxies that populate the Universe.

The fluctuations in the cosmic microwave background, as measured by COBE (large scales), WMAP (in intermediate scales) and Planck (in small scales), are all consistent with not only arising from an invariant set of quantum fluctuations scale, but be so low in magnitude that they could not have arisen from a dense, arbitrarily warm state. The horizontal line represents the initial spectrum of fluctuations (of inflation), while the wavy line represents how the interactions of gravity and radiation / matter shaped the expanding Universe in the early stages.NASA / WMAP Science Team

This may seem counterintuitive, but there is a simple reason for this that appears in the first picture we have of the infantile universe: gravity is a force of infinite reach, but it does not propagate at infinite speeds. It spreads only at the speed of light, which means that if you want to have an impact on a region of space that takes 100 million years to reach the speed of light, it will not be able to sense its presence until 100 million years.

That is why, on the cosmic microwave bottom chart above, the larger (left) scales have temperature fluctuations that are completely flat: gravitation has not yet affected them. This first massive peak is where gravitational contraction is occurring now, but there was not enough collapse to trigger backwardness by the radiation. And the peaks and valleys in addition represent a splash on scales smaller than the current cosmic horizon.

The cosmic web is driven by dark matter, which could arise from particles created in the early stage of the Universe that do not decompose but remain stable to the present day. Smaller scales collapse first, while larger scales require longer cosmic times to become excessively dense enough to form structure.Ralf Kaehler, Oliver Hahn and Tom Abel (KIPAC)

This all translates into a detailed script of how the large-scale structure in the Universe forms. We can divide it into some general rules.

  • The structure will form on smaller scales first: stars before galaxies, galaxies before clusters, clusters before superclusters.
  • This characteristic scale, where the density fluctuations are the largest, will correspond to a distance scale today, where we are more likely to see correlations of galaxies than on shorter or longer scales.
  • If there is some sort of acceleration phase that comes later in the Universe, this will cause a cut in the formation of the structure: a maximum and maximum scale for the structure.
  • And once you become gravitationally connected, you must remain connected gravitationally even as the expansion of the Universe continues indefinitely.

Based on our observations of the distant universe, all these predictions are confirmed.

An illustration of the first stars that transform into the universe. Without metals to cool the stars, only the largest clusters within a large mass cloud can become stars. Until there is enough time for gravity to affect larger scales, only small scales can form structures at the beginning.NASA

The first stars, as we understand them, appear when the Universe is between 50 and 100 million years old. It takes many millions of solar masses (but less than a billion) to start the gravitational collapse to the stars for the primordial material in the Universe, which means that even the densest regions of all will not develop stars until tens of millions of years have arisen. past.

It will take extra time for these individual star clusters to unite to create galaxies so that these galaxies are grounded to create evolved galaxies and clusters of galaxies, and for these clusters to merge into clusters of galaxies. This is what we mean when we talk about the cosmic web and the large-scale structure of the Universe: it has to be constructed from small scales (where gravity first acts) to large ones.

Although this is how the structure forms in the Universe, giving rise to a network of filaments where clusters exist in the nexus, the network appears in smaller scales first. Larger scales show no structure until the Universe has aged the most because of the extremely large amount of time it takes a gravitational signal to cross hundreds of millions or billions of light years.

At present, we have an observable Universe that is approximately 92 billion light-years across. And the scale at which we are most likely to see these correlations of galaxies reaches about 500 million light years, which means that if you put your finger across any galaxy and look at a distance, you are more likely to find another galaxy to 500 million light-years away than 400 or 600 million light-years away.

An illustration of clustering patterns due to acoustic oscillations of baryon, where the probability of finding a galaxy at some distance from any other galaxy is governed by the relationship between dark matter and normal matter. As the Universe expands, this characteristic distance expands also, allowing to measure the Hubble constant, the density of dark matter and even the scalar spectral index. The results agree with the data of the CMB, and a universe composed of 27% of dark matter, as opposed to 5% of normal matter.ZOSIA ROSTOMIAN

In addition, the large-scale features we recognize as clusters of galaxies should not be present in the early stages. For many hundreds of millions of years, there should be no clusters of galaxies, and should take billions of years to see large collections of galaxies clustered in bona fide galaxy clusters.

In addition, those appearing in these primitive times must have a mass less than those that appear later. Overall, this is confirmed spectacularly by observations, with the first clusters of known massive galaxies appearing well after massive galaxies are abundant. As we look closely, we find clusters of galaxies that are more massive and contain far more galaxies than the more distant ones.

The giant cluster of neighboring galaxies, Abell 2029, houses the IC 1101 galaxy at its core. With 5.5 million light-years, more than 100 trillion stars and the mass of almost one quadrillion suns, it is the largest known galaxy of all. The further we look, the lower the clusters of mass galaxies, while the first cluster of prototypes we encounter is still more than a billion years after the Big Bang.Digitized Sky Research 2, NASA

More spectacularly, there seems to be a limit to the size and mass of structures. You may have heard of our local supercluster: Laniakea, which contains the Milky Way, the local group, the Virgo clump, and many other groups and groups that appear to be arranged in a web-like structure. If you were to map all this, you might be tempted to conclude that Laniakea is real, and that this massive object is an even larger structure than the large clusters of galaxies that we see in the Universe.

However, it is nothing more than a ghost. Laniakea is just an apparent structure; it is not connected gravitationally. At the largest cosmic scales, dark energy dominates the gravitational force, and has been doing so for the last 6 billion years. If an object had not grown gravitationally to a sufficient density, so that it would collapse under its own power until then, it would never do so.

The Laniakea supercluster, containing the Milky Way (red dot), on the outskirts of the Virgo cluster (large white collection near the Milky Way). Despite the misleading looks of the image, this is not a real structure, since dark energy will drive away most of these clusters, fragmenting them over time.Tully, R.B., Courtois, H., Hoffman, Y & amp; Orchard & Egrave; de, D. Nature 513, 71-73 (2014)

Laniakea, like all the huge scale structures of superclusters, is being torn apart by the expansion of the Universe. It takes, on average, about 2 to 3 billion years for these large clusters of galaxies to grow at densities sufficient to collapse gravitationally. The most massive may contain thousands of galaxies the size of the Milky Way nowadays, but there are no monsters spanning tens of billions of light years or containing tens of thousands of Milky Way within them. The accelerated expansion of the Universe is simply too much for gravity to overcome.

The cosmic web of dark matter and the large-scale structure it forms. Normal matter is present, but it is only 1/6 of the total matter. The other 5/6 are dark matter, and no amount of normal matter will get rid of it. If there were no dark energy in the Universe, the structure would continue to grow and grow on larger and larger scales as time passed, but in its presence there are no structures exceeding billions of light-years in size.The millennium simulation, V. Springel et al.

Although the seeds necessary for the cosmic structure were planted in the earliest stages of the Universe, it takes time and the right resources for these seeds to grow. Seeds for small-scale structures germinate first, as the gravitational force propagates at the speed of light, causing the denser regions to grow in the first stellar clusters after only a few tens of millions of years. Over time, the seeds of the galaxy-scale structure also grow, taking hundreds of millions of years to produce galaxies within the Universe.

But clusters of galaxies, growing from seeds of the same magnitude on larger distance scales, take billions of years. Over time, the Universe is 7.8 billion years old, the accelerated expansion took over, explaining why there are no higher boundary structures than galaxy clusters. The cosmic web is no longer growing as it was before, but it is being destroyed by dark energy. Enjoy what we have while we have it; the Universe will never be structured again!


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A simulation of the large-scale structure of the Universe. Identifying which regions are dense and massive enough to match clusters of stars, galaxies, clusters of galaxies and determining when and under what conditions they form is a challenge that cosmologists are just beginning to face.Dr. Zarija Lukic

One of the strangest facts about the Universe is how it has changed dramatically over time. Today, we see a universe full of large galaxies containing hundreds of billions of stars, clustered together in a huge cosmic web. Further back in time to the Big Bang, however, everything was extremely smooth and even with very little clumping or grouping to speak. Get back enough, in fact, and you will not find any galaxy or star.

This makes sense from a qualitative point of view. The Universe was born with tiny imperfections, gravitation grows as the Universe expands, and depending on how and where gravity wins, we get these huge galaxies and clusters of galaxies separated by regions that contain nothing: cosmic voids. But the structure did not form at once, and the larger structures formed last. That is the cosmic reason why.

The evolution of large-scale structure in the Universe, from an initial and uniform state to the grouped universe we know today. The type and abundance of dark matter would provide a very different Universe if we alter what our Universe possesses. Note the fact that the small scale structure appears early in all cases, while the structure on larger scales does not come until much later.Angulo et al. 2008, via Durham University

Imagine the Universe as it was in these early stages. It is full of matter and radiation that distribute almost perfectly evenly everywhere you look. In the aftermath of the Big Bang, a typically superdense region had an average density of 100.003%, while a typically low density had 99.997% of the average density. When we describe the primitive universe as uniform, this is the level of uniformity that we attain.

These superdensities and subdensities were almost exactly the same on all scales. Whether you look at a region a few miles or a few light years or a few million or billions light-years in size, that same 1 part-30,000 float describes the super-dense and sub-dense regions with which the Universe began.

Overdensate regions grow and grow over time, but are limited in their growth by the small initial magnitudes of the superdensities, the cosmic scale at which superdensities are encountered (and the time that takes the gravitational force through them), and the presence of radiation that is still energetic, which prevents the structure from growing more rapidly. It takes tens to hundreds of millions of years to form the first stars; small clusters of matter exist long before that, however.Aaron Smith / TACC / UT-Austin

But that does not stay that way for long. Gravity immediately begins to preferentially attract mass to the regions of excess density as compared to all others. The sub-dense regions more readily yield their matter to the surrounding regions, comparatively denser.

However, although the law of gravity is universal and the same on all scales, the Universe does not form clusters of stars, galaxies and clusters of galaxies at one and the same time. In fact, it takes less than 100 million years for the first stars to form, but billions of years – more than ten times more – before we form the clusters of mass galaxies that populate the Universe.

The fluctuations in the cosmic microwave background, as measured by COBE (large scales), WMAP (in intermediate scales) and Planck (in small scales), are all consistent with not only arising from an invariant set of quantum fluctuations scale, but be so low in magnitude that they could not have arisen from a dense, arbitrarily warm state. The horizontal line represents the initial spectrum of fluctuations (of inflation), while the wavy line represents how the interactions of gravity and radiation / matter shaped the expanding Universe in the early stages.NASA / WMAP Science Team

This may seem counterintuitive, but there is a simple reason for this that appears in the first picture we have of the infantile universe: gravity is a force of infinite reach, but it does not propagate at infinite speeds. It spreads only at the speed of light, which means that if you want to have an impact on a region of space that takes 100 million years to reach the speed of light, it will not be able to sense its presence until 100 million years.

That is why, on the cosmic microwave bottom chart above, the larger (left) scales have temperature fluctuations that are completely flat: gravitation has not yet affected them. This first massive peak is where gravitational contraction is occurring now, but there was not enough collapse to trigger backwardness by the radiation. And the peaks and valleys in addition represent a splash on scales smaller than the current cosmic horizon.

The cosmic web is driven by dark matter, which could arise from particles created in the early stage of the Universe that do not decompose but remain stable to the present day. Smaller scales collapse first, while larger scales require longer cosmic times to become excessively dense enough to form structure.Ralf Kaehler, Oliver Hahn and Tom Abel (KIPAC)

This all translates into a detailed script of how the large-scale structure in the Universe forms. We can divide it into some general rules.

  • The structure will form on smaller scales first: stars before galaxies, galaxies before clusters, clusters before superclusters.
  • This characteristic scale, where the density fluctuations are the largest, will correspond to a distance scale today, where we are more likely to see correlations of galaxies than on shorter or longer scales.
  • If there is some sort of acceleration phase that comes later in the Universe, this will cause a cut in the formation of the structure: a maximum and maximum scale for the structure.
  • And once you become gravitationally connected, you must remain connected gravitationally even as the expansion of the Universe continues indefinitely.

Based on our observations of the distant universe, all these predictions are confirmed.

An illustration of the first stars that transform into the universe. Without metals to cool the stars, only the largest clusters within a large mass cloud can become stars. Until there is enough time for gravity to affect larger scales, only small scales can form structures at the beginning.NASA

The first stars, as we understand them, appear when the Universe is between 50 and 100 million years old. It takes many millions of solar masses (but less than a billion) to start the gravitational collapse to the stars for the primordial material in the Universe, which means that even the densest regions of all will not develop stars until tens of millions of years have arisen. past.

It will take extra time for these individual star clusters to unite to create galaxies so that these galaxies are grounded to create evolved galaxies and clusters of galaxies, and for these clusters to merge into clusters of galaxies. This is what we mean when we talk about the cosmic web and the large-scale structure of the Universe: it has to be constructed from small scales (where gravity first acts) to large ones.

Although this is how the structure forms in the Universe, giving rise to a network of filaments where clusters exist in the nexus, the network appears in smaller scales first. Larger scales show no structure until the Universe has aged the most because of the extremely large amount of time it takes a gravitational signal to cross hundreds of millions or billions of light years.

At present, we have an observable Universe that is approximately 92 billion light-years across. And the scale at which we are most likely to see these correlations of galaxies reaches about 500 million light years, which means that if you put your finger across any galaxy and look at a distance, you are more likely to find another galaxy to 500 million light-years away than 400 or 600 million light-years away.

An illustration of clustering patterns due to acoustic oscillations of baryon, where the probability of finding a galaxy at some distance from any other galaxy is governed by the relationship between dark matter and normal matter. As the Universe expands, this characteristic distance expands also, allowing to measure the Hubble constant, the density of dark matter and even the scalar spectral index. The results agree with the data of the CMB, and a universe composed of 27% of dark matter, as opposed to 5% of normal matter.ZOSIA ROSTOMIAN

In addition, the large-scale features we recognize as clusters of galaxies should not be present in the early stages. For many hundreds of millions of years, there should be no clusters of galaxies, and it must take billions of years to see large collections of galaxies clustered in clusters of authentic galaxies.

In addition, those appearing in these primitive times must have a mass less than those that appear later. Overall, this is confirmed spectacularly by observations, with the first clusters of known massive galaxies appearing well after massive galaxies are abundant. As we look closely, we find clusters of galaxies that are more massive and contain far more galaxies than the more distant ones.

The giant cluster of neighboring galaxies, Abell 2029, houses the IC 1101 galaxy at its core. With 5.5 million light-years, more than 100 trillion stars and the mass of almost one quadrillion suns, it is the largest known galaxy of all. The further we look, the lower the clusters of mass galaxies, while the first cluster of prototypes we encounter is still more than a billion years after the Big Bang.Digitized Sky Research 2, NASA

More spectacularly, there seems to be a limit to the size and mass of structures. You may have heard of our local supercluster: Laniakea, which contains the Milky Way, the local group, the Virgo clump, and many other groups and groups that appear to be arranged in a web-like structure. If you were to map all this, you might be tempted to conclude that Laniakea is real, and that this massive object is an even larger structure than the large clusters of galaxies that we see in the Universe.

However, it is nothing more than a ghost. Laniakea is just an apparent structure; it is not connected gravitationally. At the largest cosmic scales, dark energy dominates the gravitational force, and has been doing so for the last 6 billion years. If an object had not grown gravitationally to a sufficient density, so that it would collapse under its own power until then, it would never do so.

The Laniakea supercluster, containing the Milky Way (red dot), on the outskirts of the Virgo cluster (large white collection near the Milky Way). Despite the misleading looks of the image, this is not a real structure, since dark energy will drive away most of these clusters, fragmenting them over time.Tully, R.B., Courtois, H., Hoffman, Y and Pomarede, D. Nature 513, 71-73 (2014)

Laniakea, like all the huge scale structures of superclusters, is being torn apart by the expansion of the Universe. It takes, on average, about 2 to 3 billion years for these large clusters of galaxies to grow at densities sufficient to collapse gravitationally. The most massive may contain thousands of galaxies the size of the Milky Way nowadays, but there are no monsters spanning tens of billions of light years or containing tens of thousands of Milky Way within them. The accelerated expansion of the Universe is simply too much for gravity to overcome.

The cosmic web of dark matter and the large-scale structure it forms. Normal matter is present, but it is only 1/6 of the total matter. The other 5/6 are dark matter, and no amount of normal matter will get rid of it. If there were no dark energy in the Universe, the structure would continue to grow and grow on larger and larger scales as time passed, but in its presence there are no structures exceeding billions of light-years in size.The millennium simulation, V. Springel et al.

Although the seeds necessary for the cosmic structure were planted in the earliest stages of the Universe, it takes time and the right resources for these seeds to grow. Seeds for small-scale structures germinate first, as the gravitational force propagates at the speed of light, causing the denser regions to grow in the first stellar clusters after only a few tens of millions of years. Over time, the seeds of the galaxy-scale structure also grow, taking hundreds of millions of years to produce galaxies within the Universe.

But clusters of galaxies, growing from seeds of the same magnitude on larger distance scales, take billions of years. Over time, the Universe is 7.8 billion years old, the accelerated expansion took over, explaining why there are no higher boundary structures than galaxy clusters. The cosmic web is no longer growing as it was before, but it is being destroyed by dark energy. Enjoy what we have while we have it; the Universe will never be structured again!


More readings about how the Universe was when:






























































































































































































































































































































































































































































































































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