A giant GRB ring and the alleged large-scale structure associated with it. It is possible, of course, that this is only a pseudostructure, and we are deceiving ourselves, believing that this formation extends for many billions of light
years.For almost the entire history of mankind, one of the ideas about our place in the Universe has not been disputed for a long time: our planet, the Earth, is the immovable center of space. All observations corresponded to this:
- the skies - including stars, nebulae and the Milky Way, revolve over our heads;
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The idea that the Earth rotates on its axis and moves in an orbit around the Sun seemed like some kind of funny invention, which was seriously considered by only a few ancient philosophers like Aristarchus of Samos or Archimedes. Ptolemy's geocentric scheme worked better than any other model in describing the motion of celestial bodies until Kepler postulated elliptical orbits in the 17th century.
However, the probably more powerful revolution occurred a hundred years earlier, when Nicolaus Copernicus revived the idea that the Earth should be taken away from a privileged position at the center of the universe. Copernicus principle today , saying that neither we, nor anyone else, occupy a special place in the Universe, is the cornerstone of modern cosmology. But is he correct? Let's take a closer look at the evidence.
Mars movement from December 2013 to July 2014. Mars moved from the lower right corner of the diagram to the upper left until February, then slowed down, stopped, walked back, then slowed down again in May and stopped, and finally returned to the first path. This used to be considered evidence of epicycles , but now we know that this is not the case.
Copernicus' model of the solar system, first formulated 500 years ago, offered an interesting alternative to the conventional explanation.
According to one of the classical ideas, all other planets, except for the Earth, moved around the Sun in a circle, while rotating in small circles, which in turn moved in large circles. Such a scheme gave a certain trajectory for each planet, according to which most of the year the planets in the sky moved in a certain direction in relation to the stars, but for a while they seemed to stop, reverse the movement, and then went back to normal. direction.
This phenomenon is known as the retrograde motion of the planets., has long served as evidence against circular heliocentric orbits. One of the greatest breakthroughs of Copernicus - at least based on historical evidence, because the treatise of Aristarchus of Samos did not reach us - was the demonstration that if the inner planets move in orbits faster than the outer ones, then their periodic retrograde motion can be explained without resorting to epicycles or circles in circles.
One of the greatest mysteries of the 16th century was why the planets move retrograde. It could be explained by either Ptolemy's geocentric model (left) or Copernicus's heliocentric model (right). True, none of them succeeded in describing the system with arbitrary precision.
If the Earth does not need to occupy a special position in the Universe, then it, and everyone else, should be governed by the same physical laws. The planets revolve around the Sun, the moon around the planets, and even all objects falling on the Earth's surface must be subject to the same universal laws. It took over a century to go from the original idea of Copernicus to the discovery of the first successful law of gravity. It took more than a hundred years to test it directly. However, Copernicus's heliocentric model turned out to be correct.
Today we have extended the Copernican principle to the limit. Our planet, our solar system, our place in the Galaxy, the location of the Milky Way in the Universe - and indeed all planets, stars and galaxies should not stand out in any way. In the Universe, not only must the same laws operate all the time and everywhere - there must not be anything special or prominent at any place and in any direction in the entire cosmos.
Simulation of the large-scale structure of the universe. Cosmologists are just beginning to tackle the problem of determining which regions were massive and dense enough to correspond to star clusters, galaxies, galactic clusters, and on what scale and under what conditions they were formed.
This, of course, is also just an assumption. We assume that the universe is the same in all directions - isotropic - and the same in all places - homogeneous, at least on the largest scales, but to test this in practice, we need to solve two problems.
1. We need to quantify these quantities. It is one thing to declare that the Universe is isotropic and homogeneous, and quite another to understand at what level it is isotropic and homogeneous, and at what level does anisotropy and inhomogeneity begin to play a role? After all, if you measure the average density of the Universe, you get something about one proton per cubic meter - only the Earth is 10 30 times denser than the average value, which obviously implies the inhomogeneity of the Universe!
2. We need to measure the Universe and check everything. We expect that on a large cosmic scale, the Universe will be very close to ideal homogeneity - isotropy and homogeneity. However, at all scales, anisotropy and inhomogeneity should manifest, and observations should demonstrate how accurately the universe is imperfect.
And if the theory does not strongly coincide with observations, we will have a problem that will cause us to question the validity of Copernicus' principle.
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As far as we understand, the Universe did not just emerge from the Big Bang, but from the state known as cosmic inflation, which preceded and gave rise to the Big Bang. During inflation, the Universe did not consist of matter and radiation, it was dominated by the form of energy inherent in the very fabric of space. Quantum fluctuations with the expansion of the Universe have spread throughout its entire volume. When this phase and inflation ended, the inherent energy of space turned into matter, antimatter and radiation, giving rise to the Big Bang.
Quantum fluctuations in this important transition turned into density fluctuations: areas where the density was slightly higher or slightly lower than average. Based on the fluctuations we observe in the relict radiation and the large-scale structure of the Universe, we know that their level is on the order of 1 / 30,000, and rarely, in about 0.01% of cases, you can find a fluctuation four times larger. On all scales, large and small, the universe was born almost perfectly homogeneous - almost, but not quite.
As satellites improve, their ability to probe smaller scales, more frequency ranges, and smaller CMB temperature differences increases. Note the existence of fluctuations on the left side of the graph: even on the largest scales, the universe was not born perfectly homogeneous.
If you want gravitationally bound structures to form in your universe, regardless of scale, you have to wait. Enough time should pass before:
- areas with an initially higher density have grown, only slightly exceeding the average;
- and this will happen only when the cosmic horizon, that is, the distance that light can travel from one end to the other, becomes greater than the spatial scale of these fluctuations;
- and they should rise from 0.003% to 68% - this is a critical value leading to gravitational collapse and rapid (non-linear) gravitational growth;
- and only then can such observable features as quasars, galaxies and enriched clouds of interstellar gas appear.
On average, this means that when certain distances are exceeded, the chances of getting connected cosmic structures are small, and at distances smaller than these scales, such structures should be quite common. While the full likelihood of what exactly is likely, as well as the likelihood that it will happen, is still poorly understood, the general expectation is that large, connected cosmic structures should disappear at scales of over 1–2 billion light years. Models and observations of galaxies give similar large-scale clustering patterns.
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However, observations did not give us exactly the picture we expected. In the years leading up to 2010, our search for large-scale structures revealed the existence of giant "walls" of the Universe: galaxies grouping on cosmic scales and forming connected structures stretching for hundreds of millions of light years - up to a maximum of 1.4 billion light years. But over the past decade, several structures have been discovered that seem to go beyond this limit:
- The Huge LQG , or Huge Quasar Group, is a group of 73 quasars that form a structure about 4 billion light years across;
- the great wall of Hercules - the Northern Corona - a cluster of about 20 GRBs, giving out a structure that stretches for 10 billion light years;
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The coarse structure found by observation seems to disprove homogeneity on a large scale. Black spots are ionized gaseous magnesium, detected by the absorption of light from background quasars (blue dots). But whether this is actually a real unified structure is not yet clear.
At first glance, these structures are huge - even too huge to fit into our usual picture of the Universe. But we need to be very, very careful with claims that our universe is not homogeneous on a large scale - especially because we have a lot of evidence against it. In his seminal work, cosmologist Sesh Nadatur, after a detailed study of these structures, made two interesting assumptions:
If you throw a large number of matches on the floor, you can find patterns associated with grouping in them. If there are sequences of several matches on the floor in a row, it will be easy to mistake them for a large-scale structure.
Several recent works are trying to answer the first question, but the second remains unanswered. One way to imagine this problem is to imagine that you have a box with a very large number of matches, and you knock it down on the floor, letting the matches roll freely on the floor. The resulting structure will be partially, but not completely random. You will see patterns associated with grouping in it.
Some of the matches will be kept separately. Some will line up with 2, 3, 4 or even 5 matches in a row. You will even come across sequences of 8-10 matches that you did not expect to see.
However, what happens if you have one group of 4-5 matches in a row, located close to another such group? There is a risk that you may think you have found a group of 8-10 matches, especially if your correlation search tools are not perfect. Although we already have quite a few examples of such structures, whose sizes have exceeded our expectations, none of the structures over 1.4 billion light years in length has not yet been considered definitely real.
The picture shows two large clusters of quasars: the Cloves-Camposano group of quasars(in red) and a huge group of quasars (in black). Just two degrees from them, another group was found. But it is not yet clear whether these quasars are independent, or are part of one large-scale structure.
Some important points about the homogeneity of the universe on the largest scale are missed by most people - and even most astronomers. For example, we still don't have enough data. We have not identified most of the galaxies behind these quasars, gas clouds and gamma-ray bursts. Limiting ourselves to qualitative data from observations of galaxies, we find no structures larger than 1.4 billion light years across.
In addition, the Universe was not born perfectly homogeneous, it had imperfections on all scales. A few large, rare (but not strong) fluctuations may be the simplest explanation for our observation of these large-scale structures, exceeding in size what we predicted.
If it turns out that these unexpectedly large structures are in fact real, this will shake not only the assumptions associated with homogeneity, but also the very foundations of modern cosmology and the Copernican principle. However, there are several more obstacles to overcome before this evidence is conclusive. This is an interesting research topic - but just as you shouldn't bet on preliminary results that disprove Einstein's theory, you shouldn't bet against Copernicus either.