A new cosmic scenario with gravitationally induced particle creation is proposed. In this model the Universe evolves from an early to a late time de Sitter era, with the recent accelerating phase driven only by the negative creation pressure associated with the cold dark matter component. The model can be interpreted as an attempt to reduce the so-called cosmic sector (dark matter plus dark energy) and relate the two cosmic accelerating phases (early and late time de Sitter expansions). A detailed thermodynamic analysis including possible quantum corrections is also carried out. For a very wide range of the free parameters, it is found that the model presents the expected behavior of an ordinary macroscopic system in the sense that it approaches thermodynamic equilibrium in the long run (i.e., as it nears the second de Sitter phase). Moreover, an upper bound is found for the Gibbons-Hawking temperature of the primordial de Sitter phase. Finally, when confronted with the recent observational data, the current 'quasi'-de Sitter era, as predicted by the model, is seen to pass very comfortably the cosmic background tests.PACS numbers: 98.80.-k, 95.35.+d, 95.36.+x

We combine current measurements of the local expansion rate, H0, and Big Bang Nucleosynthesis (BBN) estimates of helium abundance with the latest cosmic microwave background (CMB) data from the Planck Collaboration to discuss the observational viability of the scale invariant HarrisonZeldovch-Peebles (HZP) spectrum. We also analyze some of its extensions, namely, HZP + YP and HZP + N ef f , where YP is the primordial helium mass fraction and N ef f is the effective number of relativistic degrees of freedom. We perform a Bayesian analysis and show that the latter model is favored with respect to the standard cosmology for values of N ef f lying in the interval 3.70 ± 0.13 (1σ), which is currently allowed by some independent analyses. , long before realistic physical mechanisms of generation of density perturbations have been proposed. Such a spectrum, characterized by a spectral index n s = 1, was proven in accordance with the early CMB data but became less attractive from the observational point of view as new and more precise data became available. The most recent result, using data from the second release of the Planck collaboration, shows that n s = 1 at 5.6σ [4] 1 . Theoretically, it is undeniable that the confirmation of this result, although not definitely proving the inflationary scenario [8], has important consequences and points to the success of the theory of the quantum origin of cosmological perturbations and the early cosmic acceleration [9,10], which is the current paradigm for the early universe.

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We analyze the H 0 -tension problem in the context of models of the early universe that predict a blue tilted spectrum of primordial gravitational waves (GWs), which is a positive value of the tensor tilt n T . By considering the GW's contribution, N GW eff , to the effective number of relativistic degrees of freedom, N eff , and assuming standard particle physics, we discuss the effects of N GW eff on the background expansion, especially the constraints on the Hubble parameter H 0 . We analyze three scenarios that take into account the contribution of N GW eff using recent data of cosmic microwave background, baryon acoustic oscillation, the latest measurement of the local expansion rate, along with the LIGO constraints on the tensor to scalar ratio, r, and the tensor index. For the models explored, we show that an additional contribution from the primordial GW's background to N eff does not solve but alleviates the current H 0 -tension problem.The contribution of N GW eff to the radiation content of the universe also affects the predictions of the primordial nucleosynthesis (BBN) [31,32].

We discuss a model of non perturbative decay of dark energy. We suggest the possibility that this model can provide a mechanism from the field theory to realize the energy transfer from dark energy into dark matter, which is the requirement to alleviate the coincidence problem. The advantage of the model is the fact that it accommodates a mean life compatible with the age of the universe. We also argue that supersymmetry is a natural set up, though not essential.

In a previous communication [1] it was shown that a joint analysis of Cosmic Microwave Background (CMB) data and the current measurement of the local expansion rate favours a model with a scale invariant spectrum (HZP) over the minimal ΛCDM scenario provided that the effective number of relativistic degrees of freedom, Neff, is taken as a free parameter. Such a result is basically obtained due to the Hubble Space Telescope (HST) value of the Hubble constant, H0 = 73.24 ± 1.74 km . s−1.Mpc−1 (68% C.L.), as the CMB data alone discard the HZP+Neff model. Although such a model is not physically motivated by current scenarios of the early universe, observations pointing to a scale invariant spectrum may indicate that the origin of cosmic perturbations lies in an unknown physical process. Here, we extend the previous results performing a Bayesian analysis using joint CMB, HST, and Baryon Acoustic Oscillations (BAO) measurements. In order to take into account the well-known tension on the value of the fluctuation amplitude parameter, σ8, we also consider Cluster Number counts (CN) and Weak Lensing (WL) data. We use two different samples of BAO data, which are obtained using two-point spatial (BAO 2PCF) and angular (BAO 2PACF) correlation functions. Our results show that the joint CMB+HST+WL+NC dataset favor the extensions of the ΛCDM model over its minimal parameterization. Also, analysis with the BAO 2PCF always discard the HZP+Neff model with respect to standard scenario, whereas the combinations using BAO 2PACF favor the former model. We, therefore, find that all dataset disfavor the ΛCDM model with respect to the HZP+Neff extension, the only exception being the joint analysis with BAO (2PCF).

We discuss the effect of super-Hubble cosmological fluctuations on the locally measured Hubble expansion rate. We consider a large bare cosmological constant in the early universe in the presence of scalar field matter (the dominant matter component), which would lead to a scale-invariant primordial spectrum of cosmological fluctuations. Using the leading order gradient expansion we show that the expansion rate measured by a (secondary) clock field which is not comoving with the dominant matter component obtains a negative contribution from infrared fluctuations, a contribution whose absolute value increases in time. This is the same effect which a decreasing cosmological constant would produce. This supports the conclusion that infrared fluctuations lead to a dynamical relaxation of the cosmological constant. Our analysis does not make use of any perturbative expansion in the amplitude of the inhomogeneities.

Swampland conjecture has been recently proposed to connect early time cosmological models with the string landscape, and then to understand if related scalar fields and potentials can come from some fundamental theory in the high energy regime. In this paper, we discuss swampland criteria for f (R) gravity considering models where duality symmetry is present. In this perspective, specific f (R) models can naturally belong to the string landscape. In particular, it is possible to show that duality is a Noether symmetry emerging from dynamics. The selected f (R) models, satisfying the swampland conjecture, are consistent, in principle, with both early and late-time cosmological behaviors.