ICTS

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.
 

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.
 

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.
 

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.

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.

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.

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.

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.

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. 
 

I will discuss phase transitions in Twin Higgs (TH) models. Previous studies found that phase transitions cannot be first-order. We show that strong first-order phase transitions (FOPTs) can occur provided that appropriate source of Z_2 symmetry breaking between the twin and Standard Model (SM) sectors is present. I will discuss explicit models in which this can be realised. The strongest FOPTs are found in scenarios with large twin fermion Yukawa couplings and light twin sfermions in the framework of supersymmetric TH models. I will present predictions for gravitational waves. I will also discuss the EW symmetry non-restoration scenario in which the EW symmetry breaking occurs at temperatures much above the EW scale and implications for darkogenesis (baryogenesis from dark sector).

In this talk, I will present recent progress in the study of domain wall networks. First, in terms of their gravitational relics - gravitational waves (GWs) and primordial black holes - that might be behind the recent signals observed at Pulsar Timing Arrays. Second, I will discuss the isotropic birefringence effect that domain wall networks coupled to photons cause on the polarization of the CMB, with striking connections to the recent evidence found in the CMB data.

In this talk, I will discuss a phenomenon called cosmic inflation in which the Universe went through accelerated exponential expansion to solve the horizon problem of Cosmological Microwave Background within a billionth of a trillionth of a trillionth of a second, in the very early Universe. This accelerated expansion, in its minimal form, is driven by a scalar field (inflaton) and it takes place when this scalar field slowly rolls down a potential well. However, the origin of this scalar field and the correct form of the scalar potential remains an open question in cosmology. I will present a string theory motivated model where the inflaton is connected to the geometry of the internal space -- the overall volume of it drives the inflation. In particular, I will present a construction where the overall volume modulus (scalar field) is dynamically stabilized to an exponentially large value only via perturbative corrections, also known as perturbative large volume scenario (LVS). In this framework, the robustness of the single-field inflationary model is checked against possible sub-leading corrections. In the later part of my talk, I will focus on the global embedding of the fibre inflation in perturbative LVS and show how our constructions pose less challenge in realizing a successful period of inflation.

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.

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.