Research Projects

Cosmic inflation, an accelerated expansion at the start of the universe, provides an attractive explanation for the horizon problem. Inflation is mostly driven by a scalar field called inflaton. The inflaton, being a quantum field, inherently contains scalar quantum fluctuations that accounts for the seed perturbations later, required to explain the observed CMB anisotropies and the large scale structure. In addition to the scalar perturbations, tensor perturbations or primary gravitational wave(s) (PGW) are also generated due to quantum fluctuations of metric during inflation, which subsequently propagate freely (due to their extremely weak interaction). PGW carries information about the very early universe, otherwise inaccessible from other messengers like photons or neutrinos, hence is the key probe of early universe evolution. Recent detection of astrophysical GW via interferometers (Ligo, Virgo, Kagra) and stochastic GW via PTA (NANOGrav, EPTA) have substantially strengthened this pursuit. However, many aspects of PGW regarding the spectral energy density (SED) and potential observability are not yet well understood. A naive calculation of SED of PGW exhibits a quartic rise at small scales and redshifts as radiation, significantly contributing to the effective number of relativistic degrees of freedom around BBN, which is tightly constrained from CMB. So, it has been argued that the SED needs to be regularized, in order to truncate this rise. Adiabatic regularization is best suited for the scenario at hand and we are employing it. However, when this technique is applied for a scenario with an abrupt transition from inflation to radiation domination, even after regularization, the SED remains flat at small scales. It was only recently understood that the smoothness of the transition determines the behaviour of regularized small scale SED. We aim to investigate the connection of smoothness of transitions and the small scale SED behaviour in greater detail. Similar to SED or two-point function (power spectrum), higher order correlation functions should also have rise at small scales, hence requiring some regularization procedure. We believe that regularization of such correlations remains unexplored, hence is a novel direction to pursue. Our insights of regularisation and smoothing are also applicable to vector fields. In the study of magnetogenesis, the EM fields are also tainted by a rise at small scales, resulting in possible back-reaction, which can disrupt the background evolution itself. We intend to adapt our knowledge of regularisation and smoothing to this context.