13,8 billion years ago in the hot Big Bang our universe was born. Since then, it has expanded and cooled down to the present day. From our point of view, we can look at 46 billion light years in all directions, thanks to the speed of light and the expansion of space. And although this is a huge distance, it is not infinite. Because we do not see further. What lies beyond the horizon of these 46 billion light years and can the universe be infinite?
First of all, it is worth noting that we do not know for sure whether the universe is finite or infinite. But we know for sure that beyond what we can observe, there are many, notes physicist Ethan Siegel in his article on Medium.com.
The further objects that we observe in the universe, the farther back in time we go, until the time when atoms did not exist, until the Big Bang
Looking as far as possible, we also move back in time. The nearest galaxy, 2.5 million light-years away, appears before us, as it was 2.5 million years ago, because the light needs just that much time to get to our eyes from wherece it was emitted. Many galaxies are seen to us as they were tens of millions, hundreds of millions or even billions of years ago. Looking as far as possible into space, we see the light as it was in the young days of the universe. Why then do not we look at the very beginning, see how it was 13.8 billion years ago? We not only looked, but also found something: the cosmic microwave background, the afterglow of the Big Bang.
It turned out that at that time the universe was almost perfectly homogeneous, but some areas were more or less dense than the average, to 1 part of 30,000. That’s enough to form stars, galaxies, galactic clusters and cosmic voids that we see Today. But in those early imperfections that we see from this space image, there is incredibly much information about the universe. For example, a striking fact: the curvature of space, as far as we know, is absolutely flat. If the space were curved, as if we lived on the surface of a four-dimensional sphere, the distant rays of light would merge. If the space were concave, like the surface of a four-dimensional saddle, the far rays would be divergent. But no, the distant rays of light move in the direction specified initially,
The values of hot and cold spots, as well as their scales, indicate the curvature of the universe. We came to the conclusion that it is perfectly flat
From the limitations associated with both the cosmic microwave background and the large-scale structure of the universe in the aggregate, we can conclude that if the universe is finite and closes on itself, it must be at least 250 times larger than the part that we observe. Since we live in three dimensions, 250 times the radius means (250) 3 volumes, which is 15 million times more space. And yet, no matter how large this number may seem, it is not infinite. The lower boundary of the universe will be at least 11 trillion light years in all directions, and this is a lot, but still of course.
And of course, we have reasons to believe that the Universe is much more than that. The Big Bang could mark the beginning of the observable universe, to which we are accustomed, but it does not necessarily mean the birth of space-time itself. Before the Big Bang, the universe was experiencing a period of cosmic inflation. Instead of being filled with matter and radiation in a hot state, the universe was different:
- filled with energy inherent in the space itself;
- Expanded at a constant exponential rate;
- created a new space so quickly that the smallest physical length, Planck length, could be stretched to the size of the currently observed universe in just 10 -32 seconds.
Inflation leads to the fact that space expands exponentially, which can very quickly lead to the fact that any previously curved space is flat.
In our region of the universe, inflation ended, it’s true. But there are three questions that we do not know the answer to. They are extremely important for determining how great the universe really is and whether it is infinite or not.
How big was the universe after the inflation in which the Big Bang was born?
Looking at our Universe today, on the uniform afterglow of the Big Bang, on the plane of the Universe and on the fluctuations that stretched across the Universe on all scales, we can extract some information. We can determine the upper limit of the energy scales in which inflation took place; we can find out how much the universe had to go through inflation; we can find out the lower limit of how long inflation should have lasted.
But the pocket with the inflationary universe, which gave birth to us, can be much more than this lower limit! It can be hundreds, millions, or googles times more than we observe, or truly endless. And yet, not being able to observe much of the universe, we do not have enough information to make a decision.
Is the idea of ”eternal inflation” correct?
Assuming that inflation should be a quantum field, at any given point at this stage of exponential expansion, there is a possibility that inflation will end, which will lead to the Big Bang, and the probability of continuing inflation with creating more space. Our calculations lead us to the inevitable conclusion: in order for inflation to produce the universe we observe, it must always create more space in which inflation will continue, compared to the regions in which inflation ended with the Big Bang.
Although our observable universe could appear as a result of the end of inflation in our area of space 13.8 billion years ago, there remain areas in which inflation continues, creating more and more space, even today. This idea is known as eternal inflation and is generally accepted by the community of theoretical physicists. But how big should the entire unobserved universe be then?
How long did inflation last until it ended with the Big Bang?
We can only see the observable universe, generated by the end of inflation and the Big Bang. We know that inflation should have lasted for at least 10 -32 seconds or so, but surely it could have lasted longer. But how much longer? Seconds? Years? Billions of years? Eternity? Is the universe always in a state of inflation? Did inflation start? Did she come from a previous state that was eternal? Or did all the space and time come from nothing definite time ago? All can be, and for all these options there is no definitive and verifiable answer.
As far as we know, the universe is much larger than the part that we observe. Outside the observed we should expect much more of a universe similar to ours, with the same laws of physics, the same constants, cosmic structures and the chances of the emergence of a complex life. There must be other “bubbles” in which inflation has come to an end, a lot of bubbles that are enclosed in even more space-time, undergoing endless inflation. And yet, no matter how large this universe – or the multiverse – is, it may not be infinite. Most likely, the universe has its end, its extent, though speculatively large.
The only problem is that we do not have enough information to finally answer this question. We only know how to access the information available within our observable universe: in these 46 billion light years in all directions. The answer to the question exciting us can be coded in the Universe itself, but we simply can not reach it. For now.