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Can the universe be infinite? 13.8 billion years ago, the universe began with a hot Big Bang.

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The logarithmic image of the observable universe in the artist's view

Since then, it has been expanding and cooling down to the present day. From our point of view, we can observe the universe within a radius of 46 billion years, due to the limitation of the speed of light and the expansion of the universe. And although this distance is huge, it is finite. But this is only the part that we see. What is beyond it, and is it possible that there lies infinity? Adam Stevens wants to know:

What do you think about the infinity of the universe? Many cosmologists told me that the infinity of the universe is not proven. And how can this be proved empirically at all?

First, we can learn more than what we see within 46 billion light years.


Watching the objects further from us, we see They are more and more deeply immersed in the past

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 away, is visible to us as it was 2.5 million years ago, since the light on the journey from there to our eyes since its emission requires just that much time. More distant galaxies are visible to us as they were tens of millions, hundreds of millions or even billions of years ago. Looking further, we see the light of the universe from the time when she was younger. So if we look at the light emitted 13.8 billion years ago on the relic of the Big Bang, we will see relic radiation.


Only a few hundred microns separates the hottest areas from The most cold, but the correlation of fluctuations in scale and strength contains a huge amount of information about the early universe

The picture of fluctuations is extremely complicated, it contains different average temperatures on different angular scales. It also encodes a huge amount of information about the universe, including a striking fact: the curvature of space, as far as we can tell, is missing, that is, it is flat. If the 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 he had a negative curvature, as on the surface of a four-dimensional saddle, we would see the distant rays of light diverge. Instead, the rays of light move as they move, and the fluctuations tell us about the ideal plane.


The temperature of the hot and cold sections, and their scales, tell us about the curvature of the universe. As far as we can judge, it is flat.

From the set of data on CMB and large-scale structures of the Universe (available through the study of baryonic acoustic oscillations), we can conclude that if the universe is finite and self-contained, it must Be at least 250 times greater than the part that we can see. Since we live in three dimensions, an increase in radius of 250 times means an increase in volume of 250 3 times, or 15 million times more space. But it's still not an infinite volume. The minimal estimate of the size of the universe, 11 trillion light-years in all directions, which is terribly many, but still not infinite.


The universe we observe is 46 billion light years in all directions, But behind this border there is necessarily something else.

There are reasons to believe that it is even greater. A hot Big Bang can mark the beginning of the observable universe, but not the birth of space and time. Before the Big Bang the universe was experiencing a period of cosmic inflation. Instead of being filled with matter and radiation, and being hot, the universe was:

• was filled with the energy inherent in the space itself,
• Expanded at an exponential rate,
• created a new space so quickly that the smallest physical size, the Planck length, stretched to the size of the universe observed today every 10 -32


Inflation Leads to an exponential growth of space, because of which existing curved space may seem flat

In our region of the universe, inflation really ended. But there are several questions, the answers to which are unknown to us, having a huge influence on the size of the universe and its finiteness or infinity.


Inflation prepared everything for the Big Bang and spawned the observed universe, but Only a small fraction of a second of the impact of inflation on our Universe is available to us for measurements

1) What size was the site of the universe after inflation that gave rise to our hot Big Bang? Observing today's universe and the homogeneity of the afterglow of the Big Bang, the closeness of the universe to the plane, the fluctuations that stretched across the universe on all scales, etc., etc., we can learn a great deal. We can calculate the upper limit of the energy scale at which inflation occurred, how much inflation 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, gugolas times more than what we can observe – or be really infinite. Without the ability to observe more than is available to us now, we do not get enough information to answer this question.


Inflation ends (above) when the balloon descends into the lowland. But the inflationary field is quantum (in the middle), and it stretches in time. In many areas of space (purple, red, blue), inflation will end, and in many others (green, blue) will continue, possibly to infinity (below)

2) Is the idea of ​​"eternal inflation" ? If you consider the possibility that inflation is a quantum field, then at any time of exponential expansion there is a probability of the end of inflation, which will lead to the Big Bang, and the likelihood that inflation will continue, creating more and 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 creating the observable universe, then inflation will always create an even greater space that will continue to expand, unlike the sites where it ends and There will be a Big Bang. And although our observable universe could appear 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 Great explosions – to this day. This idea is known as eternal inflation, and it is generally accepted in the physical community. So what size is the entire unobserved universe to date?


Although inflation could end in more than half of all sites at any time (marked in red X), a sufficient number of sites continue forever Expand, causing inflation to continue forever, even though no two Universes will ever collide

3) How long did inflation last until it ended and there was a Big Bang? Only the universe created by the end of inflation and our hot Big Bang is accessible to us. We know that inflation should have lasted no less than 10 -32 s, but, most likely, it lasted longer. But how much? Seconds? Years? Billions of years? Infinitely? Has the universe always been exposed to inflation? Did inflation start? Did she follow from the previous state, which lasted forever? Or, perhaps, space and time emerged from nothing limited time ago? There are many possibilities, but at the moment it is impossible to verify the answer.


A huge number of individual regions where the Big Bang occurred was divided by areas of expanding space in infinite inflation. We do not know how to test, measure or gain access to what lies outside the observable universe

Judging from the best observations, we know that the universe is much larger than the observable part. We suspect that beyond these limits is spread even more of the universe, the same as ours, with the same laws of physics, types of structures (stars, galaxies, clusters, threads, voids, etc.), and with the same chances for a complex a life. The size of the bubble in which inflation has ended should be finite, and in larger, expanding space-time, an exponentially large number of such bubbles must be contained. But, let this entire Universe, or the Multiverse, be so unimaginably huge, it may not be infinite. In fact, if inflation did not continue for an infinite time, the universe must be finite.


The part of the universe that we observe is huge, but this is only a tiny part of all existing

But the biggest problem is that we have access only to the information contained within the observable part of the universe, in these 46 billion light years in all directions. The answer to the largest question – the finite or infinite universe – can be encoded in the universe, but we can not access a large enough part of it to find out. While we either do not solve this question, or we do not come up with a clever way to expand the possibilities of physics, all this will be in the field of possibilities.

Ethan Siegel – astrophysicist, popularizer of science, author of the blog Starts With A Bang! He wrote books "Outside the Galaxy" [Beyond The Galaxy]and "Treknologia: Science of the Star Trek" [Treknology].