Description
The recent detection of the stochastic gravitational wave background
(SGWB) by the pulsar timing arrays (PTAs) has opened up the possibility of
directly probing the very early universe. A possible source of the observed
background are the secondary GWs generated by excess scalar power on small
scales. However, when such scalar-induced secondary GWs are assumed to be
generated during the epoch of radiation domination to explain the PTA data, it has
been found that the process leads to the excessive production of primordial black
holes (PBHs). In this talk, I shall discuss the production of PBHs and the
scalar-induced secondary GWs during the phase of reheating, which precedes the
standard epoch of radiation domination. I shall consider an inflationary scalar
power spectrum with a broken power law form and construct scenarios wherein the
spectral densities of scalar-induced secondary GWs fit the NANOGrav 15-year
data quite well without leading to the overproduction of PBHs. I shall conclude with
a brief summary and outlook.
