Testing theories of gravity and supergravity with inflation and observations of the cosmic microwave background

Abstract

Abstract. Cosmological and astrophysical observations lead to the emerging picture of a universe that is spatially flat and presently undertaking an accelerated expansion. The observations supporting this picture come from a range of measurements encompassing estimates of galaxy cluster masses, the Hubble diagram derived from type-Ia supernovae observations, the measurements of Cosmic Microwave Background radiation anisotropies, etc. The present accelerated expansion of the universe can be explained by admitting the existence of a cosmic fluid, with negative pressure. In the simplest scenario this unknown component of the universe, the Dark Energy, is represented by the cosmological constant (Λ), and accounts for about 70% of the global energy budget of the universe. The remaining 30% consists of a small fraction of baryons (4%) with the rest cold Dark Matter (CDM). The Lambda Cold Dark Matter (ΛCDM) model, i.e. General Relativity with cosmological constant, is in good agreement with observations. It can be assumed as the first step towards a new standard cosmological model. However, despite the satisfying agreement with observations, the ΛCDM model presents several lacks of congruence and shortcomings, and therefore theories beyond Einsteins General Relativity are called for. Many extensions of Einstein's theory of gravity have been studied and proposed with various motivations like the quest for a quantum theory of gravity to extensions of anomalies in observations at the solar system, galactic and cosmological scales. These extensions include adding higher powers of Ricci curvature R, coupling the Ricci curvature with scalar fields and generalized functions of R. In addition when viewed from the perspective of Supergravity (SUGRA) many of these theories may originate from the same SUGRA theory, but interpreted in different frames. SUGRA therefore serves as a good framework for organizing and generalizing theories of gravity beyond General Relativity. All these theories when applied to inflation (a rapid expansion of early Universe in which primordial gravitational waves might be generated and might still be detectable by the imprint they left or by the ripples that persist today) can have distinct signatures in the Cosmic Microwave Background radiation temperature and polarization anisotropies. We give a review of ΛCDM cosmology and survey the theories of gravity beyond Einsteins General Relativity, specially which arise from SUGRA, and study the consequences of these theories in the context of inflation and put bounds on the theories and the parameters therein from the observational experiments like PLANCK, Keck/BICEP etc. The possibility of testing these theories in the near future in CMB observations and new data coming from colliders like the LHC, provides an unique opportunity for constructing verifiable models of particle physics and General Relativity.

PeV IceCube signals and Dark Matter relic abundance in modified cosmologies

“…For example, considering the f (R) gravity, with f (R) = R + a R n , we have found that IceCube data and DM relic abundance can be explained provided n < 1, However, such a value is not favored by recent Planck release, which require n > 1 (and in fact the Starobisnky model n = 2 is one of the favorite candidate for Inflation) [114,115]. Results here discussed do not allow to distinguish between f (T ) and BD theories of gravity.…”

“…The signal of inflation is in the cosmic microwave anisotropy spectrum while the only experimental evidence of the radiation era is the successful predictions of big-bang nucleosynthesis (BBN) which occurs around T∼ 1 MeV at t ∼ 1 s. Observations from Type Ia Supernovae [32,33], CMB radiation [34,35], and the large scale structure [36,37], suggest that there are strong evidences that the present cosmic expansion of the Universe is accelerating. The latter is ascribed to the existence of Dark Energy (DE), an exotic form of energy characterized by a negative pressure that at late times dominates over the cold and dark matter, driving the Universe to the observed accelerating phase, and new ingredients, such as DM and Dark Energy (DE), are required [38][39][40][41][42][43][44][45][46][47][48][49][50]. Both inflation and dark energy have motivated the extension of Einstein's theory to a general f (R) theory [51].…”

Consequences of f(?) Cosmology in Thermal Leptogenesis and Gravitino Late Abundance

“…the N 1 scattering rates in which the Higgs φ processes in tand s-channels are involved), and Γ W is the washout-rate. The quantities Ω = D, S, W are proportional to λ † λ, so that the reduced rates entering (12) are such that [52] (12), are dependent on A(T). However, in this paper we are mainly interested to fulfill the conditions allowing the out-of-equilibrium, and the consequent constraints on the parameter spaces of the non-standard cosmology, therefore the numerical analysis of the N 1 and N B−L quantities are beyond the scope of the present paper, and will be faced elsewhere (in the regime to which we are interested in this paper, i.e., A(T) ∼ T (ν = 1) and A(T M 1 ) ∼ O(10 − 10 2 ), see next Section, one gets H(z)…”

“…Modified theories of gravity have been proposed in the last years as a possible solution of the shortcomings related to the Cosmological Standard Model, based on General Relativity (GR). This is the case, for example, of higher order curvature invariants than the simple Ricci scalar R that allow to get inflationary behaviors, removing the primordial singularity, and to explain the flatness and horizon problems [1] (see also [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]). This approach and, of course, all those related to it, are fundamentally motivated by the fact that, at high curvature regimes, further curvature invariants have to be considered for constructing self-consistent effective actions in curved spacetime [19][20][21].…”

Cosmological curvature acceleration

“…From a fundamental physics point of view, such curvature invariants are derived as quantum corrections from renormalisation of gravity in curved spacetime [65]. Other curvature invariants, as R μν R μν , R μνσρ R μνσρ have also been taken into account in literature [66][67][68][69][70][71][72], in particular the Gauss-Bonnet topological invariant G which is related to the emergence of the trace anomaly in curved spacetime [65,73]. In general, if both R and G are present in the gravitational action, all the curvature budget is considered, if we do not take into account further derivative terms like R and others [8].…”