## Thursday, November 17, 2016

### Alternatives to the Big Bang

The Big Bang theory has a problem, which can be explained by the following set of questions:
• The farthest we can see is the cosmic microwave background radiation, which originated about 380,000 years after the Big Bang. We cannot see directly what happened before, because it is hidden behind that radiation.
• It is true that we cannot see, but we can deduce what happened in those first 380,000 years by applying the standard physical theory, i.e. general relativity. It is also possible to check those deductions, for they offer predictions, such as the average composition of the cosmos, which fit well with the experimental data.
• The problem is, general relativity does not take us to time zero, the Big Bang itself. This theory can be applied only from 5×10-44 seconds after the Big Bang (the Planck time), as quantum effects were predominant before that time, and we do not have a physical theory that unifies quantum mechanics with general relativity.
What happened between t=0 (if it existed) and t=5×10-44 seconds? That is the question. It is true that this lapse is very small, but it would be the most important one in the universe: from what happened there, depends everything that happened later.
This problem has been given various solutions, among which we can mention the following three:
 Martin Bojowald
1. We have no idea. Until we have a new physical theory that unifies general relativity and quantum mechanics, we cannot say anything about what could have happened in that time span.
2. Various theories of quantum gravity attempt to create this new physical theory we lack, although none of them has been able so far to make checkable predictions. The first, now abandoned, was made by James Hartle and Stephen Hawking and proposed that time before the Planck time was imaginary, so it would not have gone bye (an imaginary time would be equivalent to space), and time zero did never exist. For all practical purposes, real time would have started precisely at t=5×10-44 seconds. Similar theories (such as string theory) usually keep this idea.
3. Loop quantum gravity, developed by Martin Bojowald, proposes that time is a quantum variable, i.e. it advances by leaps, with an interval equal to the Planck time, so that it can be represented very easily by a sequence of integers, where 1 corresponds to the Planck time, 2 to twice this time, and so on. According to this theory, there was a time zero, but we can know nothing about it, it is indeterminate. In addition, there would also have been negative times (-1 would correspond to an interval equal to Planck time, prior to time zero), so the universe, according to this model, would have infinite duration.
In short: we do not know whether there was or not an initial Big Bang (time zero). According to some, there was, and the density of the universe would have been infinite (a singularity). Others say that there was no time zero, because real time did not begin until t=5×10-44 seconds later. For others it did exist, although we cannot know anything about it, and there was an infinite previous time, about which perhaps we might deduce something.
As none of these theories has been able to offer testable predictions, I think the most reasonable option for a scientist is the first one. Therefore, John Mather’s words quoted in the previous article (there is no time zero) are, at best, questionable and risky.

You will note that I have not said anything about the inflationary model of the cosmos in these two articles. This is because it does not have to do with what happened before Planck time, but with something that is supposed to have happened later, so this is not the right place to consider it.

Manuel Alfonseca