Primordial gravitational waves offer unique insights into the inflationary period and subsequent thermal history of the Universe. The spectrum of primordial high-frequency gravitational waves is...Show morePrimordial gravitational waves offer unique insights into the inflationary period and subsequent thermal history of the Universe. The spectrum of primordial high-frequency gravitational waves is highly sensitive to the processes in the early Universe and can be significantly suppressed during an epoch of early matter domination (EMD) induced by new long-lived massive particles. This damping effect is studied with numerical and analytic methods. The relative energy density of gravitational waves today is found to scale with the wavenumber k as k^(-2) for waves crossing the horizon during the EMD epoch. The overall damping between the start and the end of the EMD epoch is given by m^(4/3) Γ^(-2/3)M^(-2/3), where m and Γ are the mass and decay width of the long-lived particles correspondingly, and M is the Planck mass. For concrete examples of EMD, models with inflaton decay and heavy neutral leptons are considered. Experimental observation of stochastic gravitational wave background could probe early cosmological events and constrain new physics scenarios.Show less
The ever-increasing accuracy of telescopes like Planck has allowed for the measurements of anisotropies in the CMB. For this project, the influence of heavy neutral leptons (HNLs) on the upper...Show moreThe ever-increasing accuracy of telescopes like Planck has allowed for the measurements of anisotropies in the CMB. For this project, the influence of heavy neutral leptons (HNLs) on the upper bound of the neutrino mass sum, as determined from the CMB anisotropies and additional datasets, has been studied. Specifically, an HNL with mass mN ∼ 500 MeV and lifetime τN ∼ 0.05 s is considered, which would cause a change of the number of relativistic species to Neff = 2.45 and of the primordial helium fraction to YP = 0.26. With these conditions, the mass bound is found to be ∑ mν < 0.12 at 95% CL, while the bound in standard ΛCDM + ∑ mν resulted in ∑ mν < 0.13 eV at 95% CL. The bound thus decreases slightly. This can be explained if Neff and ∑ mν have distinct effects, and other pa- rameters’ effects are able to counter changes due to the change in Neff, so that the mass sum does not need to change much. HNLs thus have a tiny effect on the mass bound, but there may also be no effect, as the results could be slightly off if we have not run the fitting program long enough.Show less
Despite the success of the Standard Model in the last few decades, we know it is not complete. There is strong motivation for assuming the existence of aditional heavy neutral leptons, which can...Show moreDespite the success of the Standard Model in the last few decades, we know it is not complete. There is strong motivation for assuming the existence of aditional heavy neutral leptons, which can account for active neutrino masses and possibly also have cosmological implications. In this work I consider the Standard Model with two neutral lepton singlets (sterile neutrinos) with degenerated masses in the range $20MeV - 2GeV$. The constraints on the active-sterile neutrino mixing angles are evaluated based on recent neutrino oscillations data. Using these constraints the bounds from accelerator experiments are reanalyzed for the case of the considered model. Finally, the results are compared with cosmological constraints coming from Big Bang nucleosynthesis and the $\nu MSM$ resonant leptogenesis.Show less
