On generalized Lemaitre–Tolman–Bondi metric: Fractal matter at the end of matter–antimatter recombination

Many recent researches have investigated the deviations from the Friedmannian cosmological model, as well as their consequences on unexplained cosmological phenomena, such as dark matter and the acceleration of the Universe. On one hand, a first-order perturbative study of matter inhomogeneity returned a partial explanation of dark matter and dark energy, as relativistic effects due to the retarded potentials of far objects. On the other hand, the fractal cosmology, now approximated by a Lemaitre–Tolman–Bondi (LTB) metric, results in distortions of the luminosity distances of SNe Ia, explaining the acceleration as apparent. In this work, we extend the LTB metric to ancient times. The origin of the fractal distribution of matter is explained as the matter remnant after the matter–antimatter recombination epoch. We show that the evolution of such a inhomogeneity necessarily requires a dynamical generalization of LTB, and we propose a particular solution.

Clustering of hotspots in the cosmic microwave background

The physics behind the origin and composition of the Cosmic Microwave Background (CMB) is a well-established topic in the field of Cosmology. Literature on CMB anisotropies reveal consistency with Gaussianity [1], but these were conducted on full multi-frequency temperature maps. In this thesis, we utilise clustering algorithms to specifically conduct statistical analyses on the distribution of hotspots in the CMB. We describe a series of data processing and clustering methodologies conducted, with results that conclusively show that the counts-in-cells distribution of hotspots in the CMB does not follow a Poisson distribution. Rather, the distribution exhibits a much closer fit to both the Negative Binomial Distribution (NBD) and the Gravitational Quasi-Equilibrium Distribution (GQED). From this result, we conclude that structure likely existed in the early universe, from the period of the recombination Epoch, possibly opening new insights in the field of galaxy formation.

Quantum Faraday Effect in the Universe

We study the Quantum Faraday rotation starting from the photon self-energy in the presence of a constant magnetic field. The Faraday angle is calculated in the non-degenerate regime and for weak field limit. Two physical scenarios, possibly characterized by these conditions, are the recombination epoch and the jets originated in pulsars. We discuss the resonant behavior that the Faraday angle exhibits in these scenarios and investigate the possibility of detecting cosmic magnetic fields through this resonant mechanism.

A conservative assessment of the current constraints on dark matter annihilation from cosmic rays and CMB observations

In view of the current interest in combining different observations to constraint annihilating weakly interacting massive particle (WIMP) dark matter (DM), we examine the relation between the Sommerfeld effect at the recombination epoch and in the galactic halo. By considering an up-to-date collection of interpolations of cosmic rays (CRs) lepton data (AMS-02 2014, Fermi and PAMELA), as DM annihilation signals, we show that current CRs measurements and recent Planck 2015 constraints from CMB anisotropies almost overlap for DM masses of the order of few TeV, although great theoretical uncertainties afflict CRs and DM descriptions. Combining CRs fits we obtain proper minimal regions allowed by CMB observations, especially for [Formula: see text] and [Formula: see text] annihilation channels, once assumed viable values of the efficiency factor for energy absorption at recombination: the results are consistent with those obtained by the Planck collaboration but allow a slightly larger overlap between CRs constraints from the lepton sector and CMB. Incoming AMS-02 measurements of CRs antiprotons will help to clarify the conundrum.

Cosmological birefringence constraints from the Planck 2015 CMB likelihood

The Planck 2015 cosmic microwave background (CMB) likelihood is employed to constrain the cosmological birefringence angle, i.e. the in vacuo rotation of the linear polarization that CMB photons would experience during propagation in case of parity violating coupling within the electromagnetic sector of the Standard Model of particle physics. We find [Formula: see text] which is well compatible with no parity violation mechanism in action from recombination epoch to present.

Recombination of H and He in Yang–Mills Gravity

We investigate some aspects of the thermal history of the early universe according to Yang–Mills Gravity (YMG); a gauge theory of gravity set in flat space–time. Specifically, equations for the ionization fractions of hydrogen and singly ionized helium during the recombination epoch are deduced analytically and then solved numerically. By considering several approximations, we find that the presence of primordial helium and its interaction with Lyman series photons has a much stronger effect on the overall free electron density in YMG than it does in the standard, General Relativity (GR)-based, model. Compared to the standard model, recombination happens over a much larger range of temperatures, although there is still a very sharp temperature of last scattering around 2000 K. The ionization history of the universe is not directly observable, but knowledge of it is necessary for CMB power spectrum calculations. Such calculations will provide another rigorous test of YMG and will be explored in detail in an upcoming paper.