Talks

By utilizing constraints on primordial magnetic fields (PMFs), their contributions to secondary gravitational waves (GWs), and recent results from pulsar timing arrays (PTAs), we establish bounds on the reheating epoch. Our analysis reveals that the combined spectral density of both primary and secondary GWs (generated by PMFs) can generally be characterized as a broken power law with five distinct indices. The spectral behavior of the GWs varies significantly based on the equation of state (EoS), resulting in unique shapes that can readily distinguish different scenarios. Furthermore, we demonstrate that the GWs produced are sufficient to account for the recently observed signals in PTAs under specific reheating scenarios. Importantly, these signals are also likely to be detected by future GW observatories, enhancing our ability to decode the early universe and shed light on inflationary magnetogenesis mechanisms.

The recent detection of very low-frequency stochastic gravitational wave background (SGWB) by Pulsar Timing Array collaborations like NANOGrav and IPTA has initiated many studies to understand the possible cosmological origin of such a signal. Amongst other candidates, the existence of primordial black holes (PBHs) in the early universe has also been pointed out as a very promising channel to generate such a signal. The most studied mechanism in this context is the formation of near-solar mass primordial black holes which leads to an amplification of SGWB in the relevant frequency range. This particular mechanism suffers from the overproduction of PBHs which can be overcome if we consider the PBHs to form during an extended non-standard reheating phase, instead of the standard radiation era. On the other hand, the resonant amplification of SGWB due to the domination of very low mass PBHs (10-10^5 g) before BBN can also effectively explain the NANOGrav data. We compare these different channels of SGWB generation with Bayesian analysis and find both these scenarios are statistically favored when individually compared with the astrophysical supermassive black hole binary merger model.

Galactic Dark Matter (DM) particles can get captured inside celestial bodies if they have some non-zero but weak interaction with the nucleons. Due to their significant size and lifetime, these celestial bodies can capture huge amounts of DM particles, and eventually, an overly dense dark core is created. This core can further collapse and form a minuscule Balck Hole (BH) that can eat up the entire celestial body over time and form a similar mass BH. Depending on the DM- nucleon interaction cross-section, this theory can be studied in non-compact stars like the Sun, and Jupiter, and compact objects like Neutron stars (NS). We show constraints on DM parameter space using gravitational wave detectors like LIGO (ground-based) and LISA (space-based), by studying low-mass (1-2.5 M_{solar}) compact object mergers and close stellar binaries in their inspiral phase respectively. We will argue how these gravitational wave experiments can work as a direct detection experiment for DM searches.

If a set of massive objects collide in space and the fragments disperse, possibly forming black holes, then this process will emit gravitational waves. Computing the detailed gravitational wave-form associated with this process is a complicated problem, not only due to the non-linearity of gravity but also due to the fact that during the collision and subsequent fragmentation the objects could undergo complicated non-gravitational interactions. Nevertheless the classical soft graviton theorem determines the power law fall-off of the wave-form at late and early times, including logarithmic corrections, in terms of only the momenta of the incoming and outgoing objects without any reference to what transpired during the collision. In this short review I shall explain the results and very briefly outline the derivation of these results.

In this talk I will discuss gravitational waves from the sound generated during the cosmic first order phase transitions. I will discuss some of the recent developments in the determination of the spectrum from this source. I will also discuss how to detect such gravitational waves with ground and space based detectors.