Ask Ethan: Could the universe start with a Big Rebound?

A “Big Bounce” requires a re-collapse phase followed by an expansion phase.

Thanks to the progress of science over the past hundred years, we have been able to establish the origin of the Universe, the causes of its current state and its fate in the distant future. But our knowledge also has limitations: how far into the past we can look, and how far the future of the evolution of the Universe we can predict with a certain degree of confidence. Outside of these limitations lie the greatest mysteries. Our reader asks about one of these riddles:

I read your post about the thermal death of the Universe , and I wondered what you think about the theory of the Big Rebound ?

The answer to this question consists of three parts: what we know, what is considered possible, and what we consider the most likely (for compelling reasons).


Clustering scheme to which galaxies in our Universe aspire
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Our Universe is currently filled with stars, galaxies, black holes, dark matter, dark energy and radiation. There are both clusters and crowds of matter in it, as well as giant voids (voids). It expands, cools, and it has a certain number of particles, organized in a certain way at any time. Based on our knowledge of its composition, its expansion and the laws of physics, we can extrapolate the state of the Universe both in the past and in the future. Going into the past, we find that it was more uniform, hot, dense, less crumpled, more energetic and uniform. Going to the future, we see that it freezes, empties, becomes more clogged, sparse, less energetic. We know this with great precision.


Since the hot Big Bang, our Universe has undergone tremendous growth and evolution, and continues to do so.

To look at it from the other side, we can turn to the entropy of the observable part of the Universe. The concept of entropy is quite difficult to understand; you can imagine it this way: this is the number of possible ways in which a state of a certain system can be formed. Today we can calculate the entropy of the Universe and get a number of the order of 10 ¹⁰⁴ k, where k is the Boltzmann constant . Basically, this amount consists of supermassive black holes in the centers of galaxies; for example, the entropy of one supermassive black hole in the center of the Milky Way alone is 10 ⁹¹ k. In the early stages of the universe, these black holes did not exist (they had not yet formed), so its entropy was much less. In the distant future, the Universe will reach even higher entropy values, when all of them will decay due to Hawking radiation (until this happens). When radiation prevailed in the universe, about 13.8 billion years ago, its entropy was only 10 ⁸⁸ k. When in the distant future the last black hole falls apart, the entropy will take the value 10 ¹²³ k. The laws of thermodynamics, according to which entropy always grows, correspond to what is happening with the Universe.


There are several options for the distant future of the universe; but if, as our data says, dark energy is indeed constant, then it will continue to develop along the red curve.

What about opportunities? In the future, the Universe will forever expand, accelerate, but may also experience a discontinuity, make a tunnel transition to a new quantum state, or recollapse into a singularity. In the past, it could exist in an inflationary state before the hot Big Bang (in a state with even less entropy, of the order of 10 ¹⁵ k), but we do not know anything about the period that lasted up to the time point 10 ^-33 s from the beginning of the Universe. Did she have a single beginning in which space and time arose? Or have they always existed? At the annual meeting of the American Astronomical Community, cosmologist Sean Carroll described in detail the four possibilities of the origin of the Universe, which did not originate in a singularity.


In the classical general theory of relativity, it is difficult to avoid singularities. But in quantum theories of gravity, for example, admitting the existence of additional dimensions, bounce options are possible.

1. String rebound. In GR, extrapolation into the past to a state with an arbitrarily high temperature, high density, or small size, you inevitably come to a singularity, and all definitions of time and space stop working. But in quantum extensions of general relativity, for example, loop quantum gravity , string theory, or cosmology of branes , it is also possible to “bounce off” from a previous contracting state into a hot, dense, and expanding state.

2. Cyclic cosmology. It looks like a string bounce, only in this version bounces are performed constantly. The universe expands, reaches its maximum size, shrinks — all this time, the entropy grows — and then it re-collapses and then bounces again.

3. Cosmological hibernation. Instead of the rapid expansion that our Universe is demonstrating today or showing during the inflation period, the Universe could have been in an approximately constant state of rest for a long time. This requires something exotic, like degravity (when gravity is temporarily turned off), or cosmology of string gas.

4. Cosmological reproduction. In this case, the Universe creates the previous space-time, which has many options for location and properties, but which did not begin with a singularity. In this case, one of the offspring grows into our universe.


A huge number of areas where the Big Bangs occurred are separated by an ever-expanding space that is in a phase of perpetual inflation.

The possibility of a Big Rebound is clearly to be considered, and many are engaged in it. But it has a big problem, as with the options 1, 2 and 3 described above: our Universe had to be born with low entropy, and nobody canceled the second law of thermodynamics . Either in the past, the entropy of the Universe decreased, which would be the greatest violation of the second law of thermodynamics, or in the past, the entropy was even smaller - precisely adjusted to be almost zero.

In the first variant, the string rebound, the entropy should decrease. In cyclic rebounds, entropy should always increase. This means that in the last cycle preceding the rebound, there must be even less entropy than it was at the birth of our Universe; that in this cycle the entropy grew, and that the next rebound would begin with an even greater entropy value than the one with which our Universe will end. And of all these scenarios, only the fourth avoids problems with entropy. To understand how this works, imagine that the Universe is in a state with large entropy, with many variations and fluctuations.


Particles in the diagram below very rarely go into the state shown above, but small fluctuations, or entropy drops, are quite possible.

This is a fairly universal condition; it is the least accurately tuned initial state from which one can start the counting, and he also has much in common with most physical systems that can be imagined — for example, with a room filled with gas molecules at a relatively high temperature. We can not expect that all the molecules at the same time will be in one half of the room, and the second will remain empty. This option is not only less preferable from the point of view of thermodynamics, but also extremely unlikely from the point of view of statistics. But you wouldn’t be surprised that if in any area the size of a fist would be several billion more or less molecules than average, or it would have a little more (or less) energy or entropy than average. If you limit yourself to studying very small sections that have, say, a size on the order of a virus (and they are 5 nm in size), you can find a section with extremely low or even negligible entropy. The total entropy of the system should still grow, but a very small area may have very small, even negligible, entropy at any time.


Although inflation can end in more than 50% of the plots at any given time (indicated by red X), a sufficient number of regions will continue to expand forever, and inflation will go on forever, while no two universes will ever collide

It is possible that tiny patches with fluctuations, where the entropy decreases quite strongly, may give rise to a new universe, where inflation occurs.


Inflation led to the emergence of the Big Bang and the observable Universe, to which we have access, but the fluctuations that occurred during inflation have increased in the structures existing today

Inflation has such an amazing property - when it starts, it creates more and more space at a surprisingly high speed, and it continues to grow exponentially. There are areas in which inflation will end, and will generate a hot Big Bang, and a space filled with matter, antimatter and radiation, such as our visible Universe. But there are areas that will continue to expand. The universe could begin with a singularity, when space and time came from a state that had neither space nor time from the outside (as far as the concept of “appeared” and “outside” can be used in the absence of space and time), but it could appear and not from singularity. However, as long as we have the second law of thermodynamics, that is, until the total entropy of the system can decrease, the ideas of the “big rebound” are hampered by a very big obstacle. The lack of evidence for the recollapse, together with the theoretical difficulties faced by the rebound variant, the best that physics can offer to describe the birth of the Universe is the reproduction variant.

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