Before the Big Bang: A Twin Universe?

esocid writes "Until very recently, asking what happened at or before the Big Bang was considered by physicists to be a religious question. General relativity theory just doesn't go there — at T=0, it spews out zeros, infinities, and errors — and so the question didn't make sense from a scientific view. But in the past few years, a new theory called Loop Quantum Gravity (LQG) has emerged. The theory suggests the possibility of a "quantum bounce," where our universe stems from the collapse of a previous universe. This may be similar with beliefs of Physicist Neil Turok of Cambridge University who has theorized about a cyclic universe, constantly expanding and compressing."

Comments

not just loop quantum gravity

[bcrowell]

There's nothing particularly special about loop quantum gravity that makes it possible to avoid having a singularity at the big bang. Loop quantum gravity is just one theory of quantum gravity. The best known theory of quantum gravity is string theory. In pretty much any theory of quantum gravity, the classical picture of the big bang singularity is going to get heavily modified. The conditions of the big bang are pretty much the only conditions under which you really need a theory of quantum gravity (unless you're really clever about finding some other situation, like black hole evaporation, where quantum gravitational effects come in). In all theories of quantum gravity, there's a scale called the Planck scale, and when you go beyond that scale (e.g., the universe is hot enough so that the wavelengths of particles are on the order of the Planck length), mysterious stuff happens. Because of this, it's reasonably plausible that the big bang singularity is eliminated in any theory of quantum gravity.

Old attempts to make a theory of a rebounding big bang (with, e.g., a cyclic universe) had various technical problems, which have been solved in recent years. In a rebounding big bang, there are issues to worry about such as what happens to causality, entropy, and the thermodynamic arrow of time. E.g., you could imagine that a universe cycles through a series of big bangs, and that each cycle is a lot like the one before, or you could imagine that the second law of thermodynamics operates across rebounds, so that each cycle has more entropy than the one before. You could imagine that there could be cause and effect relationships extending across rebounds, or that that could be prevented by the laws of physics. Some people believe that there's an unsolved "entropy problem" in the current standard big bang theory. Here is a good FAQ about cyclic models.