“Big Bang” has become a household expression, but for physicists it’s primarily a Big Headache. Exactly what happened in the first moments of our universe is still not understood. In this early phase, when matter densities were extremely high, quantum fluctuations of space and time were large. We know that much, but we still do not know to describe these fluctuations which would require a quantum theory of gravity. Without that, we cannot reliably tell what banged, if anything.It is generally expected that quantum gravity will remove the Big Bang singularity that general relativity predicts was the origin of our universe, but we don’t know with what it will be replaced. However, while we do not yet have a full theory of quantum gravity, different models for the early universe have been investigated. These are models based on, but not strictly speaking derived from, theoretical approaches to quantum gravity. The best known of these models are string cosmology and loop quantum cosmology, based on string theory and loop quantum gravity respectively.
Loop quantum cosmology in particular is well known for replacing the Big Bang with a “Big Bounce”: When the density of matter reaches a certain critical density (related to the Planck scale) contraction turns back into expansion. For a recent status update on string cosmology, see here.
A completely different approach to quantum gravity that we discussed recently is Causal Dynamical Triangulation which avoids singularities by discretizing space and time into chunks of finite size. In this approach it was recently found that space-time can exist in different phases, much like water exists in different phases. In the early universe, temperatures were high, and space-time might have been in a different phase, one in which space-time falls apart into causally disconnected pieces.
|Phase diagram of space-time in CDT. See earlier post for details|
It is thus very interesting that a similar behavior was recently found in loop quantum cosmology, an approach which a priori doesn’t have anything to do with Causal Dynamical Triangulation.
Jakub Mielczarek argues that the modification that arises through a loop-quantization of space-time can be rewritten in a suggestively simply way, as a density-dependence of the speed of light. A brief summary are these conference proceedings:
- J. Mielczarek,
Asymptotic silence in loop quantum cosmology
AIP Conf. Proc. 1514 (2012) 81, [arXiv:1212.3527].
The full length paper is here. It’s very technical, but the main conclusion is this: The higher the density, the slower the speed of light. At half the critical density, the speed of light reaches zero – this means points become causally disconnected. But things become even more interesting when the density becomes larger than half the critical density and increases towards the critical density. In this range the speed of light becomes an imaginary number and its square becomes negative. This means that time stops existing and turns into space. Physicists say space-time becomes Euclidean.
This finding realizes the so-called “no-boundary” proposal by Hartle and Hawking, and it also matches well with even earlier, quite general, considerations of what should happen nearby a singularity. In the classical theory, the causal disconnect happens only asymptotically and was dubbed ‘asymptotic silence’. In the quantized case, the causal disconnect happens at a finite time and replaces the singularity, and thus the big bang, by a singularity free region, a “moment of silence.”
I find this an intriguing development because here we have several different routes that point towards the same behavior at high density, much like is the case with dimensional reduction. I will not be surprised if further theoretical support for the moment of silence appears in the soon future. The big question is of course if traces of this silent beginning of the universe are left in observables like structure formation or the cosmic microwave background.