Talks

In this talk, I will discuss a couple of string theory-inspired models that predict an enhancement of the primordial gravitational wave spectrum during and after inflation. These models offer unique opportunities to probe the physics of the early universe and quantum gravity through potentially detectable signatures in current and future experiments.

First-order phase transitions, which take place when the symmetries are predominantly broken (and masses are then generated) through radiative corrections, produce observable gravitational waves and primordial black holes. I illustrate a model-independent approach that is valid for large-enough supercooling to quantitatively describe these phenomena in terms of few parameters, which are computable once the model is specified.  In particular, in this talk, I describe the abundance, mass and spin of the produced primordial black holes in terms of the above-mentioned parameters. I identify regions of that parameter space allowed by the observational constrains where primordial black holes can account for a fraction of the (or the entire) dark matter abundance.

I will give an introduction to Gravitational Waves from the perspective of Field Theory. I will then go on to discuss soft-graviton theorems and memory effect. I will also cover the theory of Stochastic Gravitational Waves and derive the Dellings-Down relation.

Domain walls are a defect that arises when a vacuum manifold is discontinuous. They are often regarded as a problem - literally the "domain wall problem" - but if you can get rid of them, they could be an interesting source of gravitational waves. If the domain walls result from a breaking a global symmetry, the most common way of doing so always struck me as contrived - having an unnaturally small bias term. Quantum gravity is expected to violate all global symmetries - but the process is generally a non-perturbative process like an instanton/wormhole. This means the effective scale of explicit global symmetry breaking is many orders of magnitude above the Planck scale. This makes gravitational waves from domain walls natural. Moreover, if dark matter is protected by a global symmetry which is violated by the same mechanism, one can acquire an independent measurement of a qualitative feature of quantum gravity. Finally, the domain walls themselves can catalyze primordial black hole production, making quantum gravity the indirect source of dark matter.

We discuss the possibility of attaining the SM Higgs mass along with the perturbativity of the dimensionless couplings  for inert models The possibility of first order phase transition demands a lower bare mass to the extended scalars. The possibility of dark matter  in these models often demands the SM higgs portal coupling or combinations to be low. The possibility of having a charged Higgs boson  also puts more collider bounds. We show the allowed region of parameters for the inert models. 
 

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.

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.