Why a Bouncing Universe Theory Doesn't Work

In the science news this week there has been coverage of a paper by Neil Turok and Steffen Gielen, a paper purporting to show that the Big Bang might have been a “Big Bounce,” occurring after a contraction of the universe. The idea of a “Big Bounce” is part of a cyclical model of the universe.

A cyclical universe theory is one that says that the universe passes through a series of phases or cycles, with each cycle being repeated over and over again. Depending on the theory, each cycle may last billions or trillions of years.

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 paper by Turok and Gielen deals purely with whether or not a Big Bounce would occur after a Big Crunch. The paper claims that such a Big Crunch might actually lead to a Big Bounce in which the universe starts expanding again. Such a thing actually seems incredibly improbable. Previously cosmologists have said it is overwhelming more likely that a contracting, collapsing universe would eventually collapse into one or more black holes, without resulting in another phase of cosmic expansion.

The authors claimed to have reached their conclusion based on simple assumptions. In an article in the magazine New Scientist, Turok is quoted as follows:

“The spirit of our work is to focus on simplicity,” Turok says. “We’re not adding bells and whistles to the physics we already know.”

In the abstract of their paper, the authors refer to their theory as “natural,” and in this article Turok is quoted as saying the theory involves only “minimal assumptions.”

Turok's claims about being minimal, natural and simple are quite misleading. The assumptions he is making are unnatural, absurdly complicated and highly speculative. He is indeed adding a whole bunch of speculative bells and whistles to the physics we already know.

The press release for Turok's study makes clear that it based on the idea of conformal symmetry. But conformal symmetry is a highly speculative and unproven idea. The speculative nature of Turok's paper is also shown on page 2 of the paper, where he starts speculating about anti-gravity. He might as well have been speculating about unicorns and fairies. Gravity is a known feature of nature. There has never been the slightest evidence for any such thing as anti-gravity. Discussion of anti-gravity belongs in science fiction novels.

The following passage from page 2 of the paper shows that Turok's claims of simplicity are bogus:

Once anisotropies and inhomogeneities are included, generically there are

no regular, real “bounce" solutions; but there are regular, complex solutions which are deformations of the classical bounces. We claim these are legitimate saddle points of the path integral and provide a consistent semiclassical description of a quantum bounce.

Here Turok seems to be basically admitting that the straightforward answer he gets (as to whether a collapsing universe would bounce) is “no bounce,” and that he can only get a bouncing universe by resorting to “complex solutions” – in other words, weird, ornate speculations which he introduces through absurdly complicated speculative mathematics. This is the opposite of simplicity.

You won't be able to follow Turok's speculative mathematics, but it is easy to explain a reason why the notion of a cyclical universe (with periodic “Big Bounces” at the end of contraction phases) does not work. The reason is simple: scientists have determined that the expansion of our universe is accelerating, which means there will be no Big Crunch in our future.  The expansion of the universe will continue forever, and the universe will never begin to contract. 

The diagram below shows (in black) the cyclical, bouncing universe theory, the idea of an oscillating universe. But the red arrow shows what scientists actually observe. The acceleration of the universe's expansion means there will be no Big Crunch in the future. If there isn't one in the future, it's not plausible to maintain that there was a previous cycle of the universe in which a Big Crunch turned into a Big Bang.

Anyone trying to come up with a theory of a cyclical universe has the deck stacked against him. You have the Big Bang, the unexplained one-of-a-kind event beginning the universe. You have the Second Law of Thermodynamics, which tells us entropy always increases over long time periods. You have the fact that the current entropy level in the universe is vastly lower than it would be if the universe were many times older than 13 billion years. You have the acceleration of the universe's expansion, which tells us that there will be no further opportunities for an event like the Big Bang, when everything was densely packed. These are not facts that lend credence to any cyclical theory of a universe. Nature seems to be screaming at us that the universe is not eternal and has not existed forever.

Scientists say that at the time of the Big Bang, the entropy level of our universe must have been incredibly low. Since the Second Law of Thermodynamics says entropy always increases, for you to have a universe such as ours after 13 billions years means the entropy of our universe must have incredibly low at the the time of the Big Bang. Anti-gravity speculations such as Turok's are futile, as they provide no explanation as to how we could have ended up with a universe with very, very low entropy at the time of the Big Bang, if the Big Bang had been a Big Bounce from a previous phase of the universe lasting several times longer than 13 billion years. Turok's paper completely ignores this crucial issue, and doesn't even mention entropy.