Observing the end of inflation

by Prof. Sriramkumar L. (Dept. of Physics, IIT-Madras, Chennai.)

Thursday 16 Jul 2026, 16:00 → 17:00 Asia/Kolkata

AG-80 (Hybrid) (https://tifr-res-in.zoom.us/j/96517028121?pwd=QMdJVhNRIpmyN5hvb5btsXXdeBdgP2.1 Meeting ID: 965 1702 8121 Passcode: 648752)

The primary gravitational waves (PGWs) are generated in the early universe from the quantum vacuum during inflation. In slow roll inflation, the power spectrum (PS) of PGWs over large scales, which leave the Hubble radius during inflation, is nearly scale-invariant. However, over very small scales, which never leave the Hubble radius, the PS of PGWs rises quadratically. In this talk, I will first show that, over such small scales, the PS of PGWs exhibits a similar behavior even when it is evaluated at any time after inflation. I will argue that the PS has to be regularized in order to truncate the quadratic rise over small scales or, equivalently, at high frequencies. Thereafter, I will describe the method of adiabatic regularization to arrive at the PS of PGWs over a wide range of frequencies. Assuming instantaneous transitions from inflation to the epochs of radiation and matter domination, I will show that the process of regularization curtails the quadratic rise and the regularized PS of PGWs oscillates with a fixed amplitude about a vanishing mean value over small scales. Later, considering a smoother transition from inflation to radiation domination, I will explicitly show that the smoother transition leads to a power-law suppression in the amplitude of the oscillations (about the zero mean value) of the regularized PS of PGWs over scales that always remain within the Hubble radius. Finally, using the Born approximation, I will illustrate that, on small scales, the suppression in the regularized PS of PGWs occurs more and more sharply as the transition is smoothed further and further. With the help of specific examples, I will also show that, in the case of infinitely continuous transitions, the regularized PS of PGWs exhibits an exponential suppression on small scales. I will conclude by pointing out that the observation of the exponential drop in the PS of PGWs can, in principle, help us determine the energy scale as well as the time of the end of inflation.