No, the Universe Hasn't "Got Its Bounce Back"

The main theory of a cyclical universe has been the theory of an oscillating universe. To understand this theory, you must understand the concept of critical density. Scientists have long said that if the density of the universe in mass-energy is less than a particular density called the critical density, the universe will keep expanding. But if the density of the universe is greater than this critical density, the universe's expansion will one day slow down and then reverse. If that were to happen, the expansion of the universe (in which the distance between galaxies increases) would turn into a contraction of the universe (in which the distance between galaxies decreases). At the end of the period of contraction would be a Big Crunch in which all of the universe ends up crunched together in a very dense state.

According to the theory of an oscillating universe, this Big Crunch would turn into a Big Bounce – another Big Bang that would start the universe expanding again. The oscillating universe theory is the idea that such cosmic phases of expansion and contraction have continued indefinitely – perhaps forever.

The visual below illustrates the idea:

Although it gained a few adherents in the 1970's and the 1980's, the theory of an oscillating universe has for quite some time been what we might call a dead horse. The bullet in the head of this dead horse was the discovery during the 1990's that the expansion of the universe is accelerating. This means that the current expansion of the universe will continue forever, and there will be no Big Crunch in the future. If the current expansion of the universe will continue forever, then we cannot be living in a cyclical universe that continually goes through cycles of expansion and contraction.

But now science writer Natalie Wolchover is trying to flog this dead horse back into life. She has an article on the Quanta web side with the very misleading title “How the Universe Got Its Bounce Back.” You would think from this title that there has been some new evidence or some swing of opinion causing the idea of a bouncing, oscillating universe to become popular again. Nothing of the sort has happened.

Wolchover refers to a “growing number” of cosmologists interested in the idea that the Big Bang was a Big Bounce. This is a standard rhetorical device of people making dubious claims that some dubious idea is becoming popular: they usually make the claim that there is some “growing number” of experts supporting the idea. A search of scientific papers at the Arxiv physics paper server reveals no such trend. In 2016 there were 10 papers (out of many thousands) referring to a bouncing universe in their title; and in 2017 there were 8 such papers. Hardly much of an upsurge in interest. By comparison, during 2017 there were 659 physics papers referring to “acceleration.” 
 

Wolchover also tries to create a bandwagon impression by referring to the “bounce cosmology community.” Judging from the number of papers published, this “community” might have trouble filling the seats at an average high school cafeteria lunch table. Then Wolchover makes the claim that cosmologists are “highly polarized” about whether the Big Bang was a Big Bounce. This is not correct, and the concept of a Big Bounce is merely the speculation of a handful of thinkers, most of whom do not even seem to claim it is a likelihood.

Wolchover mentions the cyclical theories of Paul Steinhardt, failing to tell us that Steinhardt's very speculative theories (which he calls the “ekpyrotic universe” and the “anamorphic universe”) have attracted virtually no support, with pretty much no one (other than a few of his co-authors) out there selling his theories other than himself. Wolchover also refers us to a recent paper by Graham, Kaplan and Rajendran.

When I looked up the paper by Graham, Kaplan and Rajendran I found a paper with the catchy title “The Born Again Universe.” But the abstract reveals that the authors are just giving us some madcap speculation, for it reveals they are speculating about “vorticity in compact extra dimensions” and “an NEC violating fluid of Kaluza-Klein excitations of the higher dimensional metric.” They're not just imagining undiscovered extra dimensions, but a spooky spinning vortex in undiscovered extra dimensions, along with some kind of weird wet wackiness (and the paper also speculates about the sci-fi concept of wormholes). This is no more substantial than speculating about purple giants living near the star Alpha Centauri. We should not at all mistake this type of groundless runaway speculation for either a scientific finding or something that is worthy of our attention. 

Not to be confused with actual science

In case you didn't know, there is a breed of modern theoretical physicists that love to create incredibly elaborate speculations, like science fiction writers imagining in great detail some imaginary planet. Such flights of fancy should only be published in journals of metaphysics or mathematics or in journals with titles such as “The Journal of Cosmological Speculations.” But we have a bad situation in which these little physicists' daydreams (with as many imaginary details as a science fiction story) can get published in physics journals, where people might mistake such works of imagination for actual science. 

The reason why such speculations are very rarely worthy of anyone's attention is that at any moment there are more than 1,000,000,000,000,000,000,000,000,000,000 possible states that matter, energy, time and space might take; so when some physicist  speculates (with lots of specific details) about just one of these possible states (without providing evidence that such a state exists) there is only a microscopic chance that such a speculation matches reality.  

Einstein's theory of general relativity (which has held up extraordinarily well to observational tests) excludes the possibility of a Big Bounce. There could have been no such bounce if Einstein's theory of general relativity is correct. All attempts to imagine an eternal cyclical universe are doomed to failure. The first reason is that we live in an accelerating universe destined to expand forever. If there is only one cycle of endless expansion ahead of us in time, it makes no sense to believe there were an infinite number of cycles in the past, alternating between expansion and contraction.

The second reason for rejecting a cyclical universe has to do with entropy. The Second Law of Thermodynamics tells us that entropy is always increasing. To end up with a universe such as ours after 13 billion years means the universe must have had an extraordinarily low entropy at its beginning, which is exactly what cosmologists such as Roger Penrose have pointed out.  As discussed here, in a book Penrose states that “the improbability of the universe conditions that we actually seem to find ourselves in” is roughly 1 in 10 to the 10 to the 124^th power, which is a probability almost infinitely smaller than the 1 in 10 to the 10 to the 60^th power estimate he made for the chance of a planet with all of Earth's biology appearing suddenly from random particle collisions.

Cosmologists have never explained this low entropy, and the problem of explaining it becomes many times worse if you imagine previous cycles of the universe (and infinitely worse if you imagine an infinite number of such cycles). This is because entropy would grow very much in each cycle of the cyclical universe, and there would be no magic mechanism reverting entropy to zero during a Big Bounce. This problem has been pointed out many times to theorists of cyclical universes, who just continue to ignore it, concentrating on their madcap speculations trying to squeeze a bounce out of a universe showing no signs of being compatible with such a notion.