The next cosmology seminar takes place on **Thursday 2nd** of
November, seminar **room E349** at **2pm**. Our guest speaker will be
**Suvashis Maity** from the Indian Institute of Technology (IIT)
Madras (Chennai, India).

At 2pm, room E349, Suvashis Maity (IIT Madras), will be talking about

### Ultra slow roll inflation and loop corrections to the scalar power spectrum

*Recently, there have been efforts to examine the contributions to the
scalar power spectrum due to the loops arising from the cubic order
terms in the action describing the perturbations, specifically in
inflationary scenarios that permit a brief epoch of ultra slow roll
(USR). A phase of USR inflation leads to significant observational
consequences, such as the copious production of primordial black
holes. In this talk, I shall discuss the loop contributions to the
scalar power spectrum in scenarios of USR inflation arising due to
the quartic order terms in the action describing the scalar
perturbations. I shall initially describe the computation of the loop
contributions to the scalar power spectrum due to the dominant term
in the action at the quartic order. Thereafter, I shall consider a
scenario wherein a phase of USR is sandwiched between two stages of
slow roll inflation and describe the behavior of the loop
contributions in situations involving late, intermediate and early
epochs of USR. In the inflationary scenario involving a late phase of
USR, for reasonable choices of the parameters, I shall show that the
loop corrections are negligible for the entire range of wave
numbers. In the intermediate case, the contributions from the loops
prove to be scale invariant over large scales, and we find that these
contributions can amount to 30% of the power spectrum at the leading
order. In the case wherein USR sets in early, we find that the loop
contributions could be negative and can dominate the power spectrum
at the leading order, which indicates a breakdown of the perturbative
expansion. I shall conclude with a brief summary and outlook.*