Quantum Gravity and Astrophysics: The Microwave Background and Other Thermal Sources

The problem of formulating a fully consistent quantum theory of gravity has not yet been solved. Even before we are able to work out the details of a complete theory, however, we do know some important qualitative features to be expected in any quantum theory. Fluctuations of the metric, for example, are expected and are associated with fluctuations of the lightcone. Lightcone fluctuations affect the arrival time of signals from distant sources in potentially measurable ways, broadening the spectra. In the work described here, we start with a thermal spectrum and derive the form of spectral changes expected in a wide class of quantum theories of gravity. The results can be applied to any thermal spectrum. We focus on their application to the cosmic microwave background (CMB) because the CMB offers two advantages: (1) deviations from a thermal spectrum are well constrained, and (2) the radiation emanates from the most distant source of light, the surface of last scattering. We use the existing CMB data to derive an upper bound on the value of ∆ t, the mean spread in arrival times due to metric fluctuations: ∆t < 2.1 × 10 s at the 95% confidence limit. This limit applies to a wide range of quantum theories of gravity, and thus serves to falsify theories predicting a larger spread in arrival times. We find this limit rules out at least one quantum gravity