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Quantum Field Theory and Cosmology

Final Report Summary - QFT&COSMO (Quantum field theory and cosmology)

The main results that the researcher achieved so far related to QFT&COSMO project objectives can be summarised in four parts:

1. producing a graph, by numerically solving geodesic equations with particular dark matter density profile, in order to give an idea for gravitational wave (GW) observers about what to expect for time delay between GWs and photons. This time lag between GWs and other massless particles (photons/neutrinos) should exist if the class of models called 'dark matter emulators' are correct. We still haven't seen any GWs in laboratories by direct observation, despite the fact that there were possible sources during the last century. The whole data analysis is based on an assumption that the GW signal and the external trigger are coincident in time within a small time window. The results of the researcher show that this assumption might not be correct and serves as a guide to GW observers to know what to expect for events in our Milky Way.
2. finding an explicit form of the graviton propagator on de Sitter space. The main result that was achieved is the explicit form of the graviton propagator in D dimensions on de Sitter space by adding a de Sitter invariant gauge fixing term to the action.
3. calculating the $\zeta$-$\zeta$ correlator to seek for possible time dependence. This result is related to the discussion in the community about the behaviour of the $\zeta$-$\zeta$ correlator after first horizon crossing. The researcher showed that for some particular cases it indeed is possible to get time dependence for the correlator after first horizon crossing.
4. calculating the two relevant Feynmann diagrams for the self-mass-squared of a conformally coupled scalar interacting with gravitons at one loop order in D dimensions on locally de Sitter space.

These results should have an impact on the cosmology and quantum gravity community. The first result is a test that should be doable with current observatories such as Laser Interferometer GW Observatory (LIGO) and Virgo or at least the advanced versions of them. If the observers see a time lag that would imply an invalidation of Einstein's theory and dark matter and if they see that GW's and photons are almost coincident in time that would rule out all dark matter emulator models. The result, in either case, has a potential impact on the scientific community. The other three results are related to quantum effects during inflation and we already can observe a power spectrum and any correction to it, even if they are very small due to quantum effects, surely have a potential impact. There is a growing interest in this field where many researchers are desperately looking for some enhanced effects and possible implications of those, such as non-gaussianity, possible tilts on the observed spectrum, quantum corrections to B polarisation modes and many other observable effects. The field of cosmology is one of the most active and important field of both gravitational and high energy physics. With the new data set coming up with Planck experiment it is exciting, important and relevant time to work on this subject. This gives us a chance of, once unimaginable, testing various predictions of quantum theories of gravity with the help of cosmological data provided to us.