Talks

In the primordial plasma, at temperatures above the scale of electroweak symmetry breaking, the presence of chiral asymmetries is expected to induce the development of helical hypermagnetic fields through the phenomenon of chiral plasma instability. It results in magnetohydrodynamic turbulence due to the high conductivity and low viscosity and sources gravitational waves that survive in the universe today as a stochastic polarized gravitational wave background. In this article, we show that this scenario only relies on Standard Model physics, and therefore the observable signatures, namely the relic magnetic field and gravitational background, are linked to a single parameter controlling the initial chiral asymmetry. We estimate the magnetic field and gravitational wave spectra, and validate these estimates with 3D numerical simulations.
 

Multi-field models of inflation with negative field-space curvature may lead to geometrical destabilization of non-adiabatic, or spectator, scalar perturbations. This phenomenon can occur at the end of inflation, e.g. in alpha-attractor models of inflation, or during inflation. Recent numerical lattice simulations shed light onto dynamics of the coupled scalar perturbations when such geometrical destabilization occurs. In the end-of-inflation geometrical destabilization, a rapid growth of the spectator perturbations can lead to preheating and associated production of gravitational waves, to the extent that alpha attractor T-models can be constrained or even ruled out by present observations. The middle-of-inflation geometrical destabilization turns out a short-lived phenomenon and a negative feedback loop prevents field fluctuations from growing indefinitely. As a result, fields undergoing geometrical destabilization are merely shifted to a new classical configuration corresponding to a uniform value of the spectator field within a Hubble patch.
 

Pulsar timing arrays are on the cusp of making a detection of the stochastic gravitational wave background at nanohertz frequencies, with strong evidence presented in the datasets of five PTAs. While an astrophysical supermassive black-hole binaries driven background has been the most favoured source, current amplitudes appear to be in tension with classical models using quasi-circularised SMBHBs. This raises the intriguing possibility that the current background is at least partially driven by cosmological or "new physics" sources. I will present the current state of PTA experiments and the constraints on cosmological backgrounds, as well as future prospects.
 

In these two lectures, I shall discuss the generation of primary gravitational waves (GWs) during inflation and secondary GWs sourced by enhanced scalar perturbations during the epoch of radiation domination. In the first lecture, after a quick introduction to inflation and reheating, I shall describe the origin of GWs from the quantum vacuum during inflation and also discuss their evolution post inflation. I shall begin the second lecture by describing the generation of scalar spectra with enhanced power on small scales due to a brief phase of ultra slow roll during the later stages of inflation. Thereafter, I shall outline the manner in which the enhanced scalar power on small scales can generate secondary GWs of strengths comparable to the sensitivities of the ongoing and forthcoming GW observatories. I shall conclude by highlighting that the scalar-induced, secondary GWs generated during reheating can possibly explain the recent observations by the pulsar timing arrays that suggest a stochastic background of GWs.

Neutron star (NS) can capture dark matter (DM) particles, which can lead to black hole formation destroying the entire star. For bosonic DM, the black hole formation can happen either from a Bose-Einstein condensate (BEC) state or a non-BEC state. The nascent black holes have different initial mass depending on the thermal state of DM particles. As its consequence, the destruction of the host star takes different time for depending on the intial mass of the black hole. This time can be inferred from gravitational wave (GW) signals of a binary system. We show the DM parameter space for BEC formation and discuss its probe via GW detectors.

The memory effect in gravitational wave (GW) signals is the phenomenon, wherein the relative position of two inertial GW detectors undergoes a permanent displacement owing to the passage of GWs through them. Measurement of the memory signal is an important target for future observations as it establishes a connection between observations with field-theoretic results like the soft-graviton theorems. Theoretically, the memory signal is predicted at the leading order quadrupole formula for sources like binaries in hyperbolic orbits. This can be in the realm of observations by Advanced LIGO, Einstein-Telescope, or LISA for black holes with masses ∼O(10^3M⊙) scattered by the supermassive black hole at the galactic center. Apart from the direct memory component, there is a nonlinear memory signal in the secondary GW emitted from the primary GW chirp signals emitted by coalescing binaries.

In this talk, I will discuss the computation of the gravitational wave signals and their energy spectrum using the field-theoretic method for eccentric elliptical and hyperbolic binary orbits. The field-theoretic calculation gives us the gravitational waveforms of linear and nonlinear memory signals directly in frequency space. The frequency domain templates are useful for extracting signals from the data.

We discuss an SO(10) model where a dimension five operator induces kinetic mixing between the abelian subgroups at the unification scale. We discuss gauge coupling unification and proton decay in this model, as well as the appearance of superheavy quasistable strings, which can explain the PTA data.

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.
 

In these two lectures, I shall discuss the generation of primary gravitational waves (GWs) during inflation and secondary GWs sourced by enhanced scalar perturbations during the epoch of radiation domination. In the first lecture, after a quick introduction to inflation and reheating, I shall describe the origin of GWs from the quantum vacuum during inflation and also discuss their evolution post inflation. I shall begin the second lecture by describing the generation of scalar spectra with enhanced power on small scales due to a brief phase of ultra slow roll during the later stages of inflation. Thereafter, I shall outline the manner in which the enhanced scalar power on small scales can generate secondary GWs of strengths comparable to the sensitivities of the ongoing and forthcoming GW observatories. I shall conclude by highlighting that the scalar-induced, secondary GWs generated during reheating can possibly explain the recent observations by the pulsar timing arrays that suggest a stochastic background of GWs.