Talks

When cosmic strings are formed during inflation, they regrow to reach a scaling regime, leaving distinct imprints on the stochastic gravitational wave background (SGWB). Such signatures, associated with specific primordial features, could be detectable by upcoming gravitational wave observatories like LISA, Einstein Telescope (ET), and others. Our analysis explores scenarios where cosmic strings form either before or during inflation. We examine how the number of e-folds experienced by the cosmic strings during inflation correlates with the predictions of inflationary models observable in CMB measurements. This correlation provides a testable link between inflationary physics and associated gravitational wave signals in a complementary manner. Focusing on α-attractor models of inflation,with the Polynomial α-attractor serving as an illustrative example, we find constraints, for instance, on the spectral index ns to 0.962 ≲ ns ≲ 0.972 for n = 1, 0.956 ≲ ns ≲ 0.968 for n = 2, 0.954 ≲ ns ≲ 0.965 for n = 3 and 0.963 ≲ ns ≲ 0.964 for n = 4 which along with the GW signals from LISA are capable of detecting local cosmic strings that have suffered ∼ 34 − 47 e-folds of inflation consistent with current Planck data and also testable in the upcoming CMB experiments like LiteBIRD and CMB-S4.

we investigate the impact of scalar fluctuations ($\chi$) non-minimally coupled to gravity, $\xi\chi^2 R$, as a potential source of secondary gravitational waves (SGWs).
Our study reveals that when reheating EoS $\wre < 1/3$ and $\xi \lesssim 1/6$ or $\wre > 1/3$ and $\xi \gtrsim 1/6$, the super-horizon modes of scalar field experience a \textit{Tachyonic instability}
during the reheating phase. Such instability causes a substantial growth in the scalar field amplitude leading to pronounced production of SGWs in the low and intermediate-frequency ranges that are strong enough to be detected by PLANCK and future gravitational wave detectors. Such growth in super-horizon modes of the scalar field and associated GW production may have a significant effect on the strength of the tensor fluctuation at the Cosmic Microwave Background (CMB) scales (parametrized by $r$) and the number of relativistic degrees of freedom (parametrized by $\dneff$) at the time of CMB decoupling.
To prevent such overproduction, the PLANCK constraints on tensor-to-scalar ratio $r \leq 0.036$ and $\dneff \leq 0.284$ yield a strong lower bound on $\xi$ for $\wre < 1/3$, and upper bound on the value of $\xi$ for $\wre > 1/3$. Taking into account all the observational constraints we found the value of $\xi$ should be $ \gtrsim 0.02$ for $\wre =0$, and $\lesssim 4.0$ for $\wre \geq 1/2$ for a wide range of reheating temperature within $10^{-2} \lesssim \Tre \lesssim 10^{14}$ GeV, and for a wide range of inflationary energy scales. Further, as one approaches $\wre$ towards $1/3$, the value of $\xi$ remains unconstrained. Finally, we identify the parameter regions in $(\Tre,\xi)$ plane which can be probed by the upcoming GW experiments namely BBO, DECIGO, LISA, and ET.

A stochastic gravitational wave background (GWB) produced in the early universe would necessarily exhibit anisotropies analogous to the CMB. In multi-field inflationary scenarios, anisotropies in GWB could differ significantly from those of the CMB if sourced by a quantum field different from the one sourcing CMB. In these scenarios, however, the more interesting case of highly anisotropic GWB typically comes at the cost of suppressed isotropic GWB signal. In this talk, I will present models of modified post-inflationary cosmology in which this tradeoff is made less severe. Such models significantly improve the detection prospects of these highly anisotropic GWBs at future GW experiments.

The remnant of a binary black hole coalescence is a perturbed black hole, which equilibrates by emitting ringdown gravitational waves. These ringdown waves not only encode information in their frequencies about the spacetime structure of the remnant black hole metric, but also have imprints in their amplitudes of the progenitor black hole binary's properties. In this talk I will highlight recent results in black hole ringdown data analysis and new understanding of astrophysical ringdown amplitudes gleaned from numerical relativity simulations, and will then discuss how ringdown analyses may become a vital tool for making inferences of binary black hole properties - even in situations where the gravitational wave signal from the binary inspiral itself is never directly observed.

In these lectures, I discuss how GWs are produced by cosmological phase transitions. We start by a short recap of gravitational waves and then move to early attempts using pairs of bubbles and the envelope approximation. Next we cover the energy budget of phase transitions and the impact of sound waves. Finally I comment on recent developments.