13.8 billion years ago, the universe began with a hot Big Bang. Since then, it has expanded and cooled, up to the present day. From our point of view, we can observe the Universe within a radius of 46 billion years, thanks to the limitation of the speed of light and the expansion of the Universe. And although this distance is enormous, it is finite. But this is only the part that we see. What is beyond it, and is it possible that there is infinity?
Adam Stevens wants to know:
What do you think about the infinity of the universe? Many cosmologists have told me that the infinity of the universe has not been proven. How can this be proven empirically?
First, we can learn more than what we see within 46 billion light years.
The further we look in any direction, the further we look into the depths of time. The nearest galaxy, located 2.5 million light-years from us, is visible to us as it was 2.5 million years ago, since the light travels from there to our eyes from the moment it is emitted. More distant galaxies are visible to us as they were tens of millions, hundreds of millions or even billions of years ago. Looking even further, we have seen the light of the universe since its younger days. So if we look at the light emitted 13.8 billion years ago, a relic of the Big Bang, we see the relic radiation.
The fluctuation pattern is extremely confusing, with different mean temperatures at different angular scales. It also encrypts a huge amount of information about the Universe, including an amazing fact: the curvature of space, as far as we can judge, is absent, that is, it is flat. If space had a positive curvature, as if we lived on the surface of a four-dimensional sphere, we would see the convergence of distant rays of light. If it had negative curvature, as on the surface of a four-dimensional saddle, we would see distant rays of light diverge. Instead, the rays of light move as they did, and fluctuations tell us about an ideal plane.
From a set of data on the relict radiation and large-scale structures of the Universe (accessible through the study of baryon acoustic oscillations), we can conclude that if the Universe is finite and closed on itself, it must be at least 250 times larger than what we can see. Since we live in three dimensions, an increase in radius 250 times means an increase in volume 250 ^ 3 times, or 15 million times more space. But this is still not an infinite volume. The minimum estimate for the size of the universe is 11 trillion light years in all directions, which is an awful lot, but still not infinite.
There is reason to believe that it is even larger. A hot Big Bang may mark the beginning of the observable universe, but not the birth of space and time. Before the Big Bang, the universe was going through a period of cosmic inflation. Instead of being filled with matter and radiation and being hot, the universe was:
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• was filled with energy inherent in the space itself,
• expanded at an exponential rate, • created new space so quickly that the smallest physical size, the Planck length, stretched to the size of the Universe observed today every 10 ^ -32 s.
In our region of the Universe, inflation is really over. But there are several questions, the answers to which are unknown to us, that have a huge impact on the size of the universe and its finiteness or infinity.
1) What size was the section of the Universe after inflation that gave rise to our hot Big Bang? Observing today's Universe and the homogeneity of the Big Bang afterglow, the closeness of the Universe to a plane, fluctuations stretching across the Universe at all scales, etc., etc., we can learn a lot. We can calculate the upper limit on the energy scale on which inflation occurred, how much inflation has increased the universe, the lower limit on the duration of inflation. But that pocket of the expanding Universe, from which our part originated, could very much exceed the lower limit! It can be hundreds, millions, googols times larger than what we can observe - or be truly infinite. Without the ability to observe more than is available to us now, we will not get enough information to answer this question.
2) Is the idea of "eternal inflation" correct? If you consider the possibility that inflation is a quantum field, then at any moment of exponential expansion, there is a possibility that inflation will end, leading to the Big Bang, and the likelihood that inflation will continue, creating more space. Such calculations are available to us (within the framework of certain assumptions), and they lead to the conclusion: if we need enough inflation before the creation of the observable universe, then inflation will always create even more space that will continue to expand, unlike the sections where it ends and there will be a Big Bang. And although our observable Universe could have appeared after the end of inflation in our region 13.8 billion years ago, there are regions where inflation continues - and creates more and more space,and generates more and more Big Bangs - to this day. This idea is known as eternal inflation, and it is generally accepted in the physical community. So how big is the entire unobservable universe today?
3) How long did inflation last before it ended and the Big Bang occurred? We only have access to the Universe created by the end of inflation and our hot Big Bang. We know that inflation should have continued for at least 10 ^ -32 s, but most likely it went on longer. But how much? Seconds? Years? Billions of years? Infinitely? Has the universe always been subject to inflation? Did inflation start? Did it follow from a previous state that lasted forever? Or perhaps space and time emerged from nothing a limited time ago? There are many possibilities, but the answer cannot be verified at this time.
Based on our best observations, we know that the universe is much larger than the observable portion. We suspect that even more of the Universe extends beyond these limits, the same as ours, with the same laws of physics, types of structures (stars, galaxies, clusters, filaments, voids, etc.), and with the same chances of a complex a life. The bubble in which inflation has ended must be finite, and the larger, expanding space-time must contain an exponentially huge number of such bubbles. But, even if this entire Universe, or the Multiverse, is so incredibly huge, it may not be infinite. In fact, if inflation did not last indefinitely, the universe must be finite.
But the biggest problem is that we only have access to the information contained within the observable part of the Universe, in these 46 billion light years in all directions. The answer to the biggest question - whether the universe is finite or infinite - can be encoded in the universe, but we can't access a large enough part of it to know. Until we either solve this issue, or come up with a clever way to expand the possibilities of physics, all this will be in the realm of possibilities.