Cosmo Connection

Abstract of the talk.

Abstract of the talk.

Metastable cosmic strings are well-motivated in grand unified theories and can generate a stochastic gravitational-wave background relevant for the signal observed by pulsar timing arrays. This talk revisits their gravitational-wave spectrum by distinguishing two physically different time scales: the breaking of closed string loops through monopole nucleation, and the collapse of the long-string network. Treating these scales independently leads to a three-parameter description and reveals qualitatively new spectral behavior. The framework provides new analytical and numerical templates for future PTA analyses.

Abstract of the talk.

Abstract of the talk.

Pulsar timing array (PTA) collaborations, including NANOGrav, have reported evidence for a stochastic gravitational-wave background, potentially originating from inflation. The corresponding spectrum depends on the poorly constrained reheating era through the transfer function. We construct an agnostic reheating model that introduces the averaged equation-of-state parameter \(w_{\rm rh}\) and the temperature dependence \(q\) of the inflaton decay rate as free, model-independent parameters, combined with a blue-tilted, cutoff primordial tensor spectrum. To make the resulting parameter space tractable for fitting, we develop an analytic fit function for the transfer function, additionally accounting for perturbativity bounds and backreaction of the gravitational-wave energy density on the reheating dynamics. We then confront this model with the NANOGrav 15-year data set, assuming the reheating-induced spectral feature falls within the PTA frequency band.

Abstract of the talk.

We investigate the treatment of real intermediate states (RIS) in the freeze-in production of dark matter through unstable s-channel mediators, where a double-counting issue arises when decay and annihilation processes are included simultaneously within the semiclassical Boltzmann approach. While several ad hoc RIS subtraction schemes are available to mitigate this problem, we study it from a first-principles perspective. In this talk, I will present a description based on the Closed-Time-Path formulation of non-equilibrium quantum field theory, which is intrinsically free of the double-counting problem. I will discuss how unstable mediators can be described by resummed propagators with thermal mass shifts and modified widths induced by the surrounding plasma, and how the resulting dark matter production rate connects to the familiar Boltzmann description. Finally, I will present numerical examples in several toy models to illustrate the impact of these effects and identify the regimes in which the full CTP description becomes essential.