Quantum gravity in the sky?

by Abhay Ashtekar and Brajesh Gupt.

Quantum gravity effects in the very early Universe can leave observable imprints.


Abhay Ashtekar (picture taken as a postdoc at Oxford University) is the Eberly Professor of Physics and the Director of the Institute for Gravitation and the Cosmos at the Pennsylvania State University.

The inflationary paradigm traces the genesis of the large-scale structure of the cosmos to astonishingly early times. However, at the onset of inflation spacetime curvature is only about 10-14 times the Planck curvature where quantum gravity effects dominate. Therefore, it is natural to ask if the earlier, pre-inflationary phase of dynamics would change observable predictions of standard inflation. The answer is often assumed to be in the negative. Our CQG paper shows that this conclusion is premature. Specifically, in Loop Quantum Cosmology (LQC) there is an unforeseen interplay between the ultraviolet effects that tame the big bang singularity, and dynamics of infrared modes of cosmological perturbations. As a result, imprints of the quantum spacetime geometry in the Planck regime can manifest themselves at the largest angular scales in the CMB.

In LQC, quantum geometry effects dominate in the Planck regime, replacing the big bang by a quantum bounce, where scalar curvature reaches its finite and universal upper bound. Therefore the radius of curvature has a non-zero lower bound, r_{\rm LQC}. Over the last 7 years, techniques have been developed to describe dynamics of the cosmological perturbations on this quantum background geometry, thereby facing the trans-Planckian issues squarely. Standard inflation assumes Continue reading