Matter is capable of organizing itself into many different levels of complexity . Quarks combine to form protons and neutrons . These, along with electrons, combine to form the nearly 100 types of naturally occurring atoms . Atoms combine to form molecules , which are capable of forming from cells of millions of different living beings to minerals, gases and compounds in general that form all the inert matter on Earth and any other planet. They can also accumulate to form entire planets or even stars . And the stars accumulate to form stellar globules and galaxies , which accumulate to form galaxy clusters and superclusters . But does this trend continue ad infinitum? Or is there an end to the structure of the universe?
In recent decades, several galaxy surveys have been carried out with the aim of creating a map of the galaxies that populate the universe and establishing what structures these galaxies form. The most recent of these is known as the Dark Energy Spectroscopic Instrument, or DESI , which aims to study the structure of the universe with the goal of learning more about the prevalence of dark energy in it. Well, these surveys give us images like the one below. In this image the Milky Way is in the center and each colored dot represents a different galaxy . As you can imagine, this map shows huge scales, billions of light years from side to side, containing the information of millions of galaxies.
This image has been created by measuring the redshift of light from millions of different galaxies . In the same way that the sound emitted by an ambulance, a train or a police siren is distorted when it travels at a certain speed with respect to the listener, the light emitted by a galaxy , or any other object, is also distorted. is distorted by moving at a certain speed . Specifically, what happens is that the frequency changes. In this way, if an object emits light and approaches you, it will increase the frequency of the light you see, while if it moves away it will decrease that frequency . Therefore, any object approaching will have its frequencies “shifted towards blue”, since the color blue corresponds to a light with high frequency and any object moving away will have its frequencies “shifted towards red” , since this color is light with a lower frequency. Due to the expansion of the universe, all galaxies (except the closest ones, such as Andromeda) move away from ours and move away faster the further away they are, so their light will be shifted or shifted towards the red.
Observing the map made by DESI we can point out several things. First of all, we observe more galaxies in the vicinity of the Milky Way than in the more distant regions . This is to be expected, since it will be easier for us to detect those galaxies that are closer to ours. So this is, in principle, a limitation of our technology rather than a feature of the universe. We also note that there are two regions where no galaxies have been detected . These correspond to the hidden regions behind the disk of the Milky Way , where the interstellar gas present in them prevents us from observing these distant galaxies.
On the other hand, we can see that there is a certain structure and that we can observe regions where there seems to be more concentration of galaxies and others where there seems to be a relative absence of them . At the scales of the map presented, these regions may seem tiny, but the reality is that these agglomerations and voids are hundreds of millions of light years in size. However, above these scales we do not observe any distinguishable structure , there do not seem to be clumps beyond hundreds of light years. In addition to surveys such as the one carried out by DESI, where a large portion of the sky is covered but up to relatively small distances, there are also surveys that study a smaller region of the sky but manage to detect galaxies at much greater distances .
All the surveys carried out to date seem to show us a universe that is homogeneous on large scales . That is, they tell us that if we were to take a volume a billion light-years across and put it anywhere in the universe, its contents would be basically identical no matter where we put it. This cannot be extended to smaller scales. If, for example, we take a volume of one cubic meter, it will not be the same to place it on the surface of the Earth, inside the Sun or in the intergalactic void. Even if we take a volume of a million light years, it will not be the same to place it in the center of a particularly dense cluster of galaxies than in the most remote region of one of those voids that we mentioned above.
In addition to being homogeneous, the universe also appears to be isotropic, meaning that it looks the same in any direction we look . This is also shown by these polls. No matter which direction we choose in the night sky (as long as we can see something in that direction), we count the same number of galaxies. Well, this homogeneity and isotropy of the universe is what is known as the cosmological principle . This principle will therefore imply that the universe will not have an edge , since this would imply that it is not homogeneous everywhere and it will not have a center either , since it would cease to be isotropic from any point other than that same center. Until today this cosmological principle has worked for us, although of course it is not an absolute and fundamental law of the universe.
References:
A. Becker, 2022, Dark Energy Spectroscopic Instrument (DESI) Creates Largest 3D Map of the Cosmos, Berkeley Lab
M. Levi et al, 2013, The DESI Experiment, arXiv:1308.0847
