Effect of ambiguities in loop cosmology on primordial power spectrum

Tuesday, Feb. 4th.
Parampreet Singh, LSU

Title: Effect of ambiguities in loop cosmology on primordial power spectrum
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By Jorge Pullin, LSU.


Cosmology is the study of the universe as a whole. You might ask, how can they study the universe, such a complicated system? The answer: very coarsely. One ignores many degrees of freedom and concentrates on a few. In its simplest incarnation, the study of cosmology is what is known as a minisuperspace approximation. One freezes all degrees of freedom except a handful. In the simplest case, one only concentrates on the universe's size. Since this is just one number, the equations for it become very simple.

However, we would like to study more features of the universe. To this aim, a technique used is known as perturbations. One assumes that the universe is simple enough that one can concentrate on its size, this will constitute the "background" on which small deviations are considered. One can then write equations for those small deviations that are simple enough to deal with. Such approach has led to spectacular predictions.

Perhaps the most striking ones ar the predictions for the anisotropies of the cosmic microwave background.  This is composed by light that arrivesat the Earth after traveling all the way from the Big Bang. Because the universe has expanded in the meantime, it has "cooled" (its wavelength havs become larger) and that is why we receive it as microwaves, which have comparatively large wavelengths compared to ordinary light. It turns out if one looks into two different directions of the sky the "temperature" (wavelength) of the microwaves that are incoming are exactly the same. They agree to one part in 100,000. The tiny disagreements however, are not random in nature, they have patterns in their structure. And those patterns have been measured with microwave satellites. And they agree remarkably well with the predictions of perturbation theory.

 Loop quantum cosmology is the application of loop quantum gravity techniques to cosmology. The resulting quantum cosmologies have been studied with tiny perturbations living in them. The results are that the predictions are almost the same of the classical theory, but with some deviations that at the moment are unobservable experimentally.

When one quantizes theories, there is not a single procedure you can follow. Different procedures lead to slightly different theories, with different predictions. This talk concentrated on how such differences in the treatment of the background solution impact the predictions on the anisotropies of the cosmic microwave background. The main conclusion is that, in spite of the ambiguities in the quantization, the resulting predictions exhibit robustness, enhancing our confidence on their physical plausibility. These predictions are perhaps the closest we are to an experimental test of quantum gravity so it is very important that they do not have significant ambiguities in them.