Alleviating tensions in the cosmic microwave background using loop quantum cosmology

Tuesday, Feb. 18th.
Brajesh Gupt/Abhay Ashtekar, TACC/PSU

Title: Alleviating tensions in the CMB using LQC.
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By Jorge Pullin, LSU

The cosmic microwave background is radiation that reaches us from the big bang. Its wavelength (temperature) is incredibly uniform. If one looks in one direction in the sky and then another, the temperature is the same to one part in 100,000. But the tiny temperature differences between microwaves coming from different directions have been measured and they are not completely random. If one looks in one direction in the sky and then considers the ring of all possible directions a certain angle away from the original one and one averages the temperature along the ring, one does not get zero, as would be the case if the deviations were random. If one plots that deviation as a function of the angle, one obtains a curve with clear features (Credit NASA/WMAP team):

Remarkably, this curve can be rather straightforwardly predicted by the so called inflationary model. In it, the universe suffers a period of rapid expansion. If one considers a quantum field living in the universe in its simplest state (the vacuum) at the beginning of inflation and one evolves it through inflation, the field will develop correlations and those correlations are the ones observed. In the above figure the red dots are experimental points obtained by the WMAP satellite of NASA and the green curve is the prediction of inflation. The agreement is astonishing.

In spite of the agreement, there are some anomalies. If you look at the curve for large angles (left of the diagram) the points do not align as well as in the rest. Another anomaly is the lensing amplitude anomaly. It appears when one studies more complicated correlations than the one discussed before. That one was what is called the "two point" correlation function for the two directions in the sky one looks at. There are more complicated correlations involving three and four points. In the latter, the predictions of standard inflation scenario in the standard cosmological model do not square as well with observations, though the discrepancies are small.

Loop quantum gravity slightly modifies the predictions of inflation. In loop quantum gravity the Big Bang gets replaced by a "bounce" from a previous universe. In such a scenario, there is no good reason to put the quantum field in its simplest state at the outset of inflation. It would be much more naturally to either put it at the bounce or at the beginning of the previous universe. It turns out that things do not change much if one chooses one or the other of those options. The important thing is that by the time inflation starts, the field is not in a vacuum anymore and that modifies the correlations one sees in the cosmic microwave background.

This talk argued that the different correlations that loop quantum gravity predicts actually allow to solve the two anomalies we described above. Loop quantum gravity is not the only model that explains the anomalies, but compared to others, it is much cleaner in that includes essentially no free parameters to tweak and it is therefore more remarkable that it agrees with nature than other models with more freedom to tweak.