Research Projects

In modern cosmology, the inflationary scenario is the most elegant theory to resolve the horizon and flatness problems and simultaneously provide an explanation for the observed anisotropies in the cosmic microwave background (CMB). Inflation is an early epoch during which the universe expands nearly exponentially. The epoch is assumed to be driven by one or more scalar fields (often referred to as the inflaton), which slowly roll down a nearly flat potential. At the end of inflation, the energy of the inflation is transferred to other degrees of freedom through reheating, eventually leading to a thermal bath of particles associated with the standard model (responsible for the visible universe) and dark matter (which constitutes the invisible universe). But, reheating may not always be efficient enough to lead to the conventional hot big bang model. To achieve the transition to the radiation-dominated era, one may need to invoke another phase prior to reheating, which is a highly non-linear, non-perturbative and non thermal regime that is referred to as preheating. Interestingly, the preheating phase, driven by the oscillating scalar field, can lead to resonances and rapid growth of fragmented states of the inflaton known as oscillons or Q-balls. The early stages of the universe are completely opaque to photons. But, they permit gravitational waves (GWs) to propagate, though they affect their evolution. The dynamics of (p)reheating can, say, affect the primary GWs generated during inflation. Also, in some scenarios, secondary GWs can be produced during inflation as well as in the post-inflationary universe. Hence, the GWs from the early universe carry distinct imprints of each epoch of the universe. The recent detection of GWs from merging compact binaries has opened up a new window to probe the early universe. There is widespread expectation that we should be able to unravel the physics of the primordial universe through observations of the GW background, in a manner similar to the CMB. There are many mechanisms for the generation of GWs in the early universe and we shall focus on the three of them in this proposal. They can be generated during inflation from the quantum vacuum (viz. the primary GWs we mentioned above), which leave their signatures as anisotropies in the CMB. They can also be generated due to enhanced amplitude of scalar perturbations (essential to produce a significant number of primordial black holes (PBHs), when the modes reenter the Hubble radius during the radiation-dominated era (which are known as secondary GWs). Apart from these effects, the resonant processes during reheating can generate GWs due to the instabilities that arise. In this project, we intend to study the evolution mechanism of the early universe through GWs at preheating era, reheating era and possible existence of primordial black hole(PBH) in the frameworks of large-N limit axionic inflation(cold and warm) models and quintessence axionic model.