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.

No abstract

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].