International Journal of Modern Physics D

Even though Planck data released in 2013 (P13) is not compatible with Background Imaging of Cosmic Extragalactic Polarization (B2) and some local cosmological observations, including Supernova Legacy Survey (SNLS) samples and H₀ prior from Hubble Space Telescope (HST) etc. Wilkinson Microwave Anisotropy Probe 9-year data (W9) is consistent with all of them in the base six-parameter ΛCDM + tensor cosmology quite well. In this paper, we adopt the combinations of B2+W9 and B2+W9+SNLS+BAO+HST to constrain the cosmological parameters in the base six-parameter ΛCDM + tensor model with n_t = -r/8, where r and n_t are the tensor-to-scalar ratio and the tilt of relic gravitational wave spectrum, and BAO denotes Baryon Acoustic Oscillation (BAO). We find that the Harrison–Zel'dovich (HZ) scale invariant scalar power spectrum is consistent with both data combinations, chaotic inflation is marginally disfavored by the data at around 2σ level, but the power-law inflation model and the inflation model with inverse power-law potential can fit the data nicely.

Inflation and dark energy are two of the most relevant aspects of modern cosmology. These different epochs provide the universe is passing through accelerated phases soon after the Big–Bang and at present stage of its evolution. In this review paper, we discuss that both eras can be, in principle, described by a geometric picture, under the standard of f(R) gravity. We give the fundamental physics motivations and outline the main ingredients of f(R) inflation, quintessence and cosmography. This wants to be a quick summary of f(R) paradigm without claiming of completeness.

Perhaps the deepest mystery of our accelerating universe in expansion is the existence of a tiny and rigid cosmological constant, Λ. Its size is many orders of magnitude below the expected one in the standard model of particle physics. This is a very welcome fact, namely if we care at all about our own existence and fate. However, we do not have a minimally satisfactory explanation for our good fortune and for the failing of the SM at that crucial point. To start with, an expanding universe is not expected to have a static vacuum energy density. We should rather observe a mildly dynamical behavior δΛ(t) ~ R ~ H²(t) with the expansion rate H. At the same time, it is natural to think that the huge value of the primeval vacuum energy (presumably connected to some GUT) was responsible for the initial inflationary phase. In the traditional inflaton models such phase is inserted by hand in the early epoch of the cosmic evolution, and it is assumed to match the standard ΛCDM regime during the radiation epoch. Here, instead, we consider a class of dynamical vacuum models which incorporate in a single vacuum structure the rapid stage of inflation, followed by the radiation and cold matter epochs, until achieving our dark energy universe. The early behavior of such "running vacuum model" bares resemblance with Starobinsky's inflation in the early universe and is very close to the concordance model for the entire post-inflationary history. Most remarkably, the inflationary period in the terminates with "graceful exit" and the large entropy problem can be solved. The model is compatible with the latest cosmological data on Hubble expansion and structure formation, and at the same time presents distinctive observational features that can be tested in the near future.

Gravitino-condensate-induced inflation via the super-Higgs effect is a UV-motivated scenario for both inflating the early universe and breaking local supersymmetry dynamically, entirely independent of any coupling to external matter. As an added benefit, this also removes the (as of yet unobserved) massless Goldstino associated to global supersymmetry breaking from the particle spectrum. In this review, we detail the pertinent properties and outline previously hidden details of the various steps required in this context in order to make contact with current inflationary phenomenology. The class of models of SUGRA we use to exemplify our approach are minimal four-dimensional N = 1 supergravity and extensions thereof with broken conformal symmetry. Therein, the gravitino condensate itself can play the role of the inflation, however the requirement of slow-roll necessitates unnaturally large values of the wave function renormalization. Nevertheless, there is an alternative scenario that may provide Starobinsky-type inflation, occurring in the broken-SUGRA phase around the nontrivial minima of the gravitino-condensate effective potential. In this scenario higher curvature corrections to the effective action, crucial for the onset of an inflationary phase, arise as a result of integrating out massive quantum gravitino fields in the path integral. The latter scenario is compatible with Planck satellite phenomenology but not with BICEP2 data.

A primordial gravitational wave background is a hallmark of inflationary cosmology. The recent announcement made by the BICEP2 collaboration of a possible measurement of B-mode polarization of the CMB on degree scales has produced an abundance of ideas and speculations on how such a signal constrains the inflationary paradigm, or possible alternative mechanisms of gravitational wave production. Here the possibility of a contribution to the gravitational wave background from the relaxation of a scalar field after a global phase transition is reviewed. The general contribution to the overall power is shown, and it is then demonstrated that if the BICEP2 result were to hold, this mechanism could at best produce a very small fraction of the measured tensor power.

We propose a large class of nonsingular cosmologies of arbitrary spatial curvature whose cosmic history is determined by a primeval dynamical Λ(t)-term. For all values of the curvature, the models evolve between two extreme de Sitter phases driven by the relic time-varying vacuum energy density. The transition from inflation to the radiation phase is universal and points to a natural solution of the graceful exit problem regardless of the values of the curvature parameter. The flat case recovers the scenario recently discussed in the literature [Perico et al., Phys. Rev. D88 (2013) 063531]. The early de Sitter phase is characterized by an arbitrary energy scale H_I associated to the primeval vacuum energy density. If H_I is fixed to be nearly the Planck scale, the ratio between the relic and the present observed vacuum energy density is ρ_vI/ρ_v0 ≃ 10¹¹⁸.