Speaker: Keisuke Inomata

Title: Effects of an early matter dominated era on gravitational waves induced by scalar perturbations

Scalar perturbations can induce not only the fluctuations of cosmic microwave background (CMB) and the large scale structures (LSSs), but also gravitational waves (GWs) through their non-linear interactions. Observations of the induced GWs enable us to access small-scale ($k\gtrsim 1$\,Mpc$^{-1}$) scalar perturbations, which are difficult to measure through CMB or LSS observations. For this reason, many authors have focused on the induced GWs recently.
In this seminar, I discuss how the existence of an early matter dominated (eMD) era, preceding the radiation dominated (RD) era, changes the spectrum of the induced GWs. I carefully take into account the evolutions of the scalar perturbations before, during, and after the transition from an eMD era to the RD era. To make the discussion concrete, I consider two scenarios for the transition: the gradual transition and the sudden transition. In the gradual (sudden) transition scenario, the transition from an eMD era to the RD era occurs with the timescale comparable to (much shorter than) the Hubble timescale at that time. As a result, I find that the induced GWs are suppressed in the gradual transition scenario, or on the other hand are much enhanced in the sudden transition scenario. In particular, in the sudden transition scenario, the enhanced GWs could be detected by future observations and be used to determine the reheating temperature.

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Last-modified: 2020-04-06 (月) 09:52:33 (380d)