Our Universe began with the Big Bang, but this does not mean that we have drawn it correctly for ourselves. Most of us imagine this as a real explosion: when everything starts with hot and dense, and then cools and cools, while individual fragments fly further and further. But this is not at all true. Therefore, the question arises: does the Universe have a center? Is cosmic background radiation really the same distance from us wherever you look? After all, if the Universe is expanding, then this expansion must have started somewhere?
Let's think for a moment about the physics of explosion and what our universe would be like if it started with it.
The first stages of the explosion during the Trinity nuclear test, 16 milliseconds after the explosion. The top of the fireball is at a height of 200 meters. July 16, 1945
The explosion starts at a point and quickly expands outward. The fastest moving material comes out the fastest and therefore spreads the fastest. The further you are from the center of the explosion, the less material will catch up with you. The energy density decreases as time passes, but farther from the explosion it decreases faster, because the energy material is more rarefied in the vicinity. No matter where you are, you will always be able - if you are not destroyed - to reconstruct the center of the explosion.
The large-scale structure of the universe changes over time as tiny defects grow and form the first stars and galaxies, and then merge to form the large, modern galaxies that we see today. The further you look, the younger the universe.
But this is not the universe that we see. The universe looks the same at large and small distances: the same densities, the same energies, the same galaxies, etc. Distant objects that move away from us at high speeds do not coincide in age with objects that are closer to us and move with lower speeds; they seem younger. And at a great distance, objects become not less, but more. And if we look at how everything in the universe moves, we can see that despite what we see tens of billions of light years away, we have reconstructed the center right where we are.
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The Laniakei Supercluster, marked in red for the position of the Milky Way, represents only one billionth of the volume of the observable universe. If the universe began with an explosion, the Milky Way would be exactly in the center
Does this mean that we, of all the trillions of galaxies in the universe, are at the center of the Big Bang? And that the original "explosion" was tuned that way - with irregular, inhomogeneous energy densities, "points of reference" and a mysterious 2.7 K glow - so that we were in its center? How generous it would be for the Universe to adjust itself so that we are at this incredibly unrealistic point at the start.
During an explosion in space, the outer material will be removed the fastest, which means that it will be the fastest to demonstrate other properties, moving away from the center, since it will lose energy and density faster
But general relativity tells us that this is not an explosion, but an expansion. The universe began with a hot, dense state and it was its tissue that expanded. There is a misconception that it had to start from one point, but no. The whole area had such properties - filled with matter, energy, etc. - and then simply universal gravity entered into action.
These properties were the same everywhere and everywhere - density, temperature, number of galaxies, etc. But if we could see this, we would find evidence of an evolving universe. Since the Big Bang happened immediately and everywhere a certain time ago in a certain region of space, and this region is all that we can see, if we look from our point of view - we see a region of space that is not too different from our own position in the past. It's hard to understand, but try it.
Looking back at great cosmic distances is like looking back in time. It has been 13.8 billion years since the Big Bang where we are now, but the Big Bang has also occurred elsewhere. Light traveling in time from those galaxies means we see distant regions as they were in the past.
Galaxies, whose light has reached us for a billion years, are visible to us as they were a billion years ago; the galaxies that appear to us ten billion years later look as they were exactly this time ago. 13.8 billion years ago, the universe was full of radiation, not matter, and when neutral atoms first formed, this radiation did not go anywhere, cooled down and went through redshift due to the expansion of the universe. What we see as the cosmic microwave background is not only the afterglow of the Big Bang, but it is visible from anywhere in the universe.
The universe doesn't have to have a center. What we call the "region" of space in which the Big Bang took place can be infinity. If there is a center, it can literally be anywhere, and we would not know about it, because we are not observing enough of the Universe to get complete information. We would need to see the edge, the fundamental anisotropy (when different directions look different) in the temperatures and numbers of galaxies, and our universe at the largest scales seems the same everywhere and in all directions.
There is no place from which the universe began to expand, there is a time when the universe began to expand. This is exactly what the Big Bang was: the state into which the entire observable Universe passed at a certain moment. That is why looking in all directions is looking back in time. That is why the Universe is homogeneous in all directions. That is why our history of cosmic evolution can be traced as far as our observatories can see.
Perhaps the universe has a finite shape and size, but if this is so, then this information is not available to us. Part of the universe we observe is finite, and this information is not contained in it. If you think of the universe as a balloon, a loaf of bread, or something else by analogy, do not forget that we can only access a tiny part of the real universe. All we see is a small part of it. And whether it is finite or infinite, it does not stop expanding and decompaction.
The universe is not expanding in any way; it just becomes less dense.
ILYA KHEL