We have had a preliminary understanding What Does The Universe Look Like On Very Large Scales.
Humans’ understanding of the universe begins with the sun, the moon, and the shining starry sky.
Although they all have the name of “star”, the identities of the members are quite complicated. Among them are planets like Venus, stars like Sirius, and galaxies containing tens of billions of stars at every turn-they only look like a star because of the distance.
Although these stars are different from us, due to the limited resolution of human eyes, they all look like they are distributed on a huge celestial sphere. What we see is their projection on a two-dimensional sphere.
What is the law of their distribution in the sky? Since ancient times, it has been a topic of discussion. The ancients did not understand that the stars are divided into planets, stars, and galaxies, and sometimes they link celestial bodies of different natures together to fabricate the myth of constellations. This can be regarded as their initial attempt to find the distribution of stars, but now we know that these “rules” are wrong.
Principle of Cosmology:
The universe is uniform Since Copernicus put forward the theory of the center of the sun, with the help of increasingly powerful telescopes. People have realized that the distribution of celestial bodies on a certain scale follows a hierarchy: satellites revolve around planets; planets revolve around stars; stars are composed again. The galaxies revolve around the black hole in the center; galaxies continue to form galaxy clusters… According to our intuitive thinking, this hierarchical structure similar to “a group of small leaders subordinate to a big leader” should continue indefinitely.
However, at the very beginning of modern cosmology, it put forward a basic hypothesis called “the principle of cosmology”. This hypothesis holds that: What Does The Universe Look Like On Very Large Scales. The reason for making this assumption is that this kind of universe is the simplest.
But does this principle contradict the hierarchical structure of the galaxy mentioned earlier? To answer this question, we need to figure out What Does The Universe Look Like On Very Large Scales.
Suppose you randomly drop a bowl of peas in a small room. In your opinion, they are evenly distributed on the ground. But at this time, if a bug is found in a pea, what will it look like?
Its eyesight is limited, and it can only see a bean under its eyelids. When it lifts its head, it only feels the space around it. If it can speak, it will tell you: “The matter of the entire universe is concentrated on the bean where I am, and everything else is empty.”
Why do you and the insects have such different opinions? This is because you can see the entire room from a global perspective, and the bug can only see the bean-sized area under its nose. Or put it this way: your observation scales are different. You look at a Universe Look Like On Very Large Scales, while bugs are viewed at a small scale. From a small scale, the distribution of matter in the “universe” is uneven, but from a large scale, it is uniform.
Therefore, whether the universe is uniform or not, the conclusions drawn from different scales will be different. The principle of cosmology does not deny that, from a small scale, the distribution of galaxies has an uneven hierarchical structure, but it believes that there is a scale. From this scale, the hierarchical structure ends and the universe is uniform.
3D map depicting the universe
In the past research, people used to draw the observed galaxy in a two-dimensional celestial coordinate system because they did not know the distance of the galaxy. In such a picture, their distribution looks very uniform, which seems to confirm the principle of cosmology.
But after astronomers learned how to measure the distance of galaxies, the situation became uncertain.
Since Hubble made that epoch-making discovery, we know that galaxies far away are far away from us. The speed of their retreat is proportional to the distance, which is Hubble’s law V=HD (V is the retreating speed, D is the distance, and H is the Hubble constant). And we also know that when the heavenly body is far away from us, the spectrum emitted by it will move toward the end of the long-wavelength, which is the “redshift”. Based on the amount of redshift, we can calculate the speed at which the celestial body leaves us, and then apply Hubble’s law to calculate its distance to us.
In this way, astronomers measure the three-dimensional coordinates of each galaxy and draw them into a three-dimensional map of the universe. Two of the dimensions represent the position of the galaxy on the celestial sphere, and the third dimension represents its distance in the depth direction.
A three-dimensional map of the universe can more truly reflect the distribution of cosmic matter than a two-dimensional map, allowing astronomers to test the principles of cosmology in a stricter sense.
However, the three-dimensional map of the universe drawn by this method was beyond their expectations. First of all, what catches our eyes are some holes. The so-called “hole” is a space where the density of the universe (the number of galaxies contained in a unit volume) is much smaller than the average density. Their scale is generally 30 million light-years.
Second, it is the area where the density of the universe is greater than or equal to the average density, which cosmologists call the “Great Wall”. Most galaxies are concentrated on these winding “Great Walls”. Each “Great Wall” varies in thickness everywhere and can extend for hundreds of millions of light-years. At their intersection, the galaxies are more densely distributed, called “knots.”
In the 1970s, people have realized that the proportion of visible matter in the universe is very small, and more than 84% are invisible dark matter. This means that dark matter is the protagonist in the activities that shape the universe.
Although astronomers know very little about the dark matter so far, they have performed computer simulations of the evolution of the universe in the 1980s based on their limited knowledge. The results showed that under the action of dark matter, galaxies were formed first, then clusters of galaxies, and later, the entire bubble structure containing “holes” and “Great Walls” came out. The large-scale structure of the universe obtained by simulation is very similar to the observed one.
In the simulation, it can be seen that in this evolution, dark matter has played a “scaffolding” role. Before stars and galaxies are formed, dark matter pulls some gas to its side by its gravity, and then the gas collapses under the action of gravity and collapses to a certain extent to ignite the internal nuclear fusion, thus forming stars. Thousands of stars further from the galaxy.
What Does The Universe Look Like On Very Large Scales
The existence of this large-scale structure shows that the distribution of cosmic matter is not uniform at least on the scale of galaxies.
However, cosmologists are ultimately reluctant to admit their mistakes easily. They said: “The universe looks uneven because the scale of observation we chose is not large enough. If we observe from a larger scale, it might be uniform.”-If you want to convince the previous one on the pea The bug, I must say the same.
Larger scales than galaxies are of course galaxy clusters or superclusters. However, for distant celestial bodies, it is too difficult to judge whether it is a galaxy or a cluster of galaxies. Astronomers have adopted a lazy approach. They take any galaxy on the three-dimensional map of the universe as the center, draw a sphere with a radius of R, and count the star coefficients contained in the sphere. Then compare with the star coefficients that should be contained in the sphere when the galaxies are evenly distributed.
They found that when R is smaller, the deviation is larger. But as R increases, the deviation becomes smaller and smaller; when R is greater than 350 million light-years, it tends to be the same. So they concluded that when the scale is greater than 350 million light-years, the distribution of cosmic matter should be uniform. This conclusion implies this meaning: there is no structure larger than 350 million light-years in the universe.
Unfortunately, in recent years, astronomy has discovered more and more structures larger than 350 million light-years in size in the universe.
Let’s take a look at these giants below:
1. A Giant Hole
In 2007, American astronomers discovered that there is a huge hole in the universe. It has a diameter of 600 to 1 billion light-years. There are neither stars, gas clouds, or other ordinary space matter in the cavity, nor the mysterious dark matter.
Although astronomers have known before that the universe is full of holes, the diameter of this giant hole is almost dozens of times larger than those found.
2. Large Quasar group
Quasars are a very active type of galaxy nucleus and are the brightest celestial bodies in the universe. Its most notable feature is that the poles emit strong, bright jets, which are said to be emitted by the supermassive black hole in the center of the galaxy when it swallows food.
Since the 1980s, people have discovered the phenomenon of quasars clustering together. This large-scale structure is called a “large quasar group”. The large quasar group U1.28 discovered in 1991 is composed of 34 quasars, spanning approximately 2 billion light-years. Another quasar group U1.1 discovered in 2011 spans 2.5 billion light-years. In 2013, a larger group of quasars was discovered again. This super-large quasar group named U1.27 is composed of 73 quasars, spanning 4 billion light-years. This size is 1,600 times the distance from the Milky Way to the nearest Andromeda Galaxy.
3. The “Great Wall” of the universe
As mentioned earlier, the three-dimensional map of the universe is full of “holes” and “Great Walls.” Some of these “Great Walls” are 500 million light-years long.
Since then, longer “Great Walls” have been discovered one after another. The “Great Wall of Saron” discovered in 2003 is 1.4 billion light-years long. The “Wuxian-North Mianzu Great Wall” discovered in 2013 is 15 billion light-years long and is the largest structure of the universe discovered so far.
The existence of these large-scale structures has pushed the principles of cosmology to the forefront again and again. People can’t help but deeply doubt: Is the universe uniform? Where is the limit of the large-scale structure? What if the universe is not uniform? These questions are testing the patience of cosmologists.
Are we in a Cosmic Hole?
Dimus Clifton of the University of Cambridge in the United Kingdom made an amazing point: We may live in a cosmic void. He said that if we were to assume that we were in a huge cosmic hole, “accelerating the expansion of the universe” might be a false proposition.
As we all know, astronomers discovered in the 1990s that distant supernovae look dimmer than theoretically predicted. In other words, they are farther away from us than expected. They believe that a repulsive force opposite to the nature of gravity is responsible for this. They call it “dark energy”.
What is dark energy? The current mainstream view is that this is a kind of vacuum energy-in quantum mechanics, an empty vacuum also has energy. However, the dark energy value calculated later based on observations was 120 orders of magnitude smaller than predicted by quantum physics.
According to Clifton, the problem may not be in the observations of cosmologists, but in a hypothesis, they have made, that is, the “principle of cosmology.” This hypothesis says that the distribution of matter in the universe is uniform. We can imagine that in a uniform universe, there is neither a special point nor a special direction. Even where we are, it is not special.
But what if we are indeed in a special area? For example, in the center of a huge hole, where we are in an area where the density of matter is much smaller than that of the rest of the universe, then the space-time structure around us may be very different from other places.
We know that when distant stars pass through massive celestial bodies, they will converge. The stars seem to be brighter. This is the gravitational lensing effect. But what happens when the starlight in the distance passes through a large hole? The opposite effect of the gravitational lens will occur, that is, the starlight diverges and the stars look dim. This may explain why the supernova is dimmer than expected. The accelerated expansion of the universe and dark energy have become unnecessary assumptions.
Clifton’s idea is of course largely a guess. Even if his theory can explain the dimming of supernovae, it cannot explain other evidence supporting the existence of dark energy.
However, whether right or wrong, Clifton’s approach gives us a revelation: in cosmology, even fundamental hypotheses like the principle of cosmology must be tested by experiments. We should not take a certain hypothesis for granted just because we cherish it.