Modern Physics Letters A

The inflaton coupling to a vector field via the $f{(\phi )}^2{F}_{\mu \nu }{F}^{\mu \nu }$ term is used in several contexts in the literature, such as to generate primordial magnetic fields, to produce statistically anisotropic curvature perturbation, to support anisotropic inflation, and to circumvent the $\eta$-problem. In this work, I perform dynamical analysis of this system allowing for the most general Bianchi I initial conditions. I also confirm the stability of attractor fixed points along phase–space directions that had not been investigated before.

There are possibilities to violate symmetries (e.g. parity and rotational invariance) in the primordial cosmological fluctuations. Such symmetry breakings can imprint very rich signatures in late-time phenomena, which may be possible to observe. Especially, Cosmic Microwave Background (CMB) will change its face drastically, corresponding to the symmetry-breaking types, since the harmonic-space representation is very sensitive to the statistical, spin and angular dependences of cosmological perturbations. Here, we discuss (1) general responses of CMB to the symmetry breakings, (2) some theoretical models creating interesting CMB signatures, and (3) aspects of the estimation from observational data.

We explore a mechanism to generate local contributions to the curvature perturbation in isolated patches of the cosmic microwave background (CMB). The mechanism, based on the generation of an out-of-equilibrium configuration of a fluctuating scalar field of mass m $\sim$H during a sustained stage of fast-roll inflation, has been recently shown to be capable of accounting for some of the most robust large-angle anomalies detected in the CMB. In this paper, we show in detail how the embedding of the mechanism into models including vector fields can result in the breaking of statistical isotropy in isolated patches of the CMB.

The existence of a primordial spectrum of gravitational waves is a generic prediction of inflation. Here, I will discuss under what conditions the coupling of a pseudoscalar inflaton to a U(1) gauge field can induce, in a two-step process, gravitational waves with unusual properties such as: (i) a net chirality, (ii) a blue spectrum, (iii) large non-Gaussianities even if the scalar perturbations are approximately Gaussian and (iv) being detectable in the (relatively) near future by ground-based gravitational interferometers.

We review work related to the breaking of statistical isotropy during inflation due to the presence of a vector field carrying a coherent vacuum expectation value over our Hubble patch. We also review isotropic signatures of vector fields during inflation.

If the spacetime metric has anisotropic spatial curvature, one can still expand the universe as if it were isotropic, provided that the energy–momentum tensor satisfies a certain constraint. This leads to the so-called shear-free (SF) metrics, which have the interesting property of violating the cosmological principle while still preserving the isotropy of the cosmic microwave background (CMB) radiation. In this work, we show that SF cosmologies correspond to an attractor solution in the space of models with anisotropic spatial curvature. Through a rigorous definition of linear perturbation theory in these spacetimes, we show that SF models represent a viable alternative to explain the large-scale evolution of the universe, leading, in particular to a kinematically equivalent Sachs–Wolfe (SW) effect. Alternatively, we discuss some specific signatures that SF models would imprint on the temperature spectrum of CMB.

Gauge-flation model at zeroth-order in cosmological perturbation theory offers an interesting scenario for realizing inflation within a particle physics context, allowing us to investigate interesting possible connections between inflation and the subsequent evolution of the Universe. Difficulties, however, arise at the perturbative level, thus motivating a modification of the original model. In order to agree with the latest Planck observations, we modify the model such that the new dynamics can produce a relation between the spectral index n${}_s$ and the tensor-to-scalar ratio r allowed by the data. By including an identical mass term for each of the fields of the system, we find interesting dynamics leading to slow-roll inflation of the right length. The presence of the mass term has the potential to modify the n${}_s$ versus r relation so as to agree with the data. As a first step, we study the model at zeroth-order in cosmological perturbation theory, finding the conditions required for slow-roll inflation and the number of e-foldings of inflation. Numerical solutions are used to explore the impact of the mass term. We conclude that the massive version of gauge-flation offers a viable inflationary model.

We investigate the effects of modified gravity theories, in particular, the symmetron and f(R) gravity, on the nonlinear regime of structure formation. In particular, we investigate the velocity dispersion of galaxy clusters as a function of the halo masses, how the matter power spectra depend on the coupling, range and screening scale of the fifth force, and on possible ways of detecting violations of the equivalence principle using the mass inferred via lensing methods versus the mass inferred via dynamical methods.

We present and expand the simplest possible quantum cosmological bouncing model already discussed in previous works: the trajectory formulation of quantum mechanics applied to cosmology (through the Wheeler–De Witt equation) in the Friedmann–Lemaître–Robertson–Walker (FLRW) minisuperspace without spatial curvature. The initial conditions that were previously assumed were such that the wave function would not change its functional form but instead provide a dynamics to its parameters. Here, we consider a more general situation, in practice consisting of modified Gaussian wave functions, aiming at obtaining a nonsingular bounce from a contracting phase. Whereas previous works consistently obtain very symmetric bounces, we find that it is possible to produce highly non-symmetric solutions, and even cases for which multiple bounces naturally occur. We also introduce a means of treating the shear in this category of models by quantizing in the Bianchi I minisuperspace.

The high precision measurements of the cosmic microwave background by the Planck survey yielded tight constraints on cosmological parameters and the statistics of the density fluctuations at the time of recombination. This provides the means for a critical study of structure formation in the Universe by comparing the microwave background results with present epoch measurements of the cosmic large-scale structure. It can reveal subtle effects such as how different forms of Dark Matter may modify structure growth. Currently most interesting is the damping effect of structure growth by massive neutrinos. Different observations of low redshift matter density fluctuations provided evidence for a signature of massive neutrinos. Here we discuss the study of the cosmic large-scale structure with a complete sample of nearby, X-ray luminous clusters from our REFLEX cluster survey. From the observed X-ray luminosity function and its reproduction for different cosmological models, we obtain tight constraints on the cosmological parameters describing the matter density, ${\mathrm{\Omega }}_m$, and the density fluctuation amplitude, ${\sigma }_8$. A comparison of these constraints with the Planck results shows a discrepancy in the framework of a pure $\mathrm{\Lambda }$CDM model, but the results can be reconciled, if we allow for a neutrino mass in the range of 0.17 eV to 0.7 eV. Also some others, but not all of the observations of the nearby large-scale structure provide evidence or trends for signatures of massive neutrinos. With further improvement in the systematics and future survey projects, these indications will develop into a definitive measurement of neutrino masses.