Assuming a nongravitational interaction among the dark fluids of our Universe—namely, dark matter and dark energy—we study a specific interaction model in the background of a spatially flat Friedmann-Lemaître-Robertson-Walker geometry. We find that the interaction model solves the background evolution in an analytic way when the dark energy takes a constant barotropic equation of state, $w_x$. In particular, we analyze two separate interaction scenarios, namely, when the dark energy is a fluid other than the vacuum energy (i.e., $w_x\ne -1$) and when it is vacuum energy itself (i.e., $w_x=-1$). We find that the interacting model with $w_x\ne -1$ produces stable perturbations at large scales for $w_x<-1$ with the coupling strength $\xi <0$. Both scenarios are constrained by the latest astronomical data. The analyses show that a very small interaction with the coupling strength is allowed, and within the 68.3% confidence region $\xi =0$ is recovered. The analyses further show that a large coupling strength significantly affects the large-scale dynamics of the Universe, while according to the observational data the interaction models are very well consistent with $\mathrm{\Lambda }$ cosmology. Furthermore, we observe that for the vacuum interaction scenario, the tension on $H_0$ is not released while for the interacting dark energy scenario with $w_x<-1$, the tension on $H_0$ seems to be released partially because of the high error bars in $H_0$. Finally, we conclude the work by calculating the Bayesian evidence, which shows that $\mathrm{\Lambda }\mathrm{CDM}$ cosmology is favored over the two interacting scenarios.

We investigate warm inflationary scenario in which the accelerated expansion of the early Universe is driven by chameleon-like scalar fields. Due to the non-minimal coupling between the scalar field and the matter sector, the energy-momentum tensor of each fluid component is not conserved anymore, and the generalized balance equation is obtained. The new source term in the energy equation can be used to model warm inflation. On the other hand, if the coupling function varies slowly, the model reduces to the standard model used for the description of cold inflation. To test the validity of the warm chameleon inflation model, the results for warm inflationary scenarios are compared with the observational Planck2018 Cosmic Microwave Background data. In this regard, the perturbation parameters such as the amplitude of scalar perturbations, the scalar spectral index and the tensor-to-scalar ratio are derived at the horizon crossing in two approximations, corresponding to the weak and strong dissipative regimes. As a general result it turns out that the theoretical predictions of the chameleon warm inflationary scenario are consistent with the Planck 2018 observations.

It is investigated the reconstruction during the slow-roll inflation in the most general class of scalar-torsion theories whose Lagrangian density is an arbitrary function $$f(T,\phi )$$f ( T , ϕ ) of the torsion scalar T of teleparallel gravity and the inflaton $$\phi $$ϕ . For the class of theories with Lagrangian density $$f(T,\phi )=-M_{pl}^{2} T/2 - G(T) F(\phi ) - V(\phi )$$f ( T , ϕ ) = - M pl 2 T / 2 - G ( T ) F ( ϕ ) - V ( ϕ ) , with $$G(T)\sim T^{s+1}$$G ( T ) ∼ T s + 1 and the power s as constant, we consider a reconstruction scheme for determining both the non-minimal coupling function $$F(\phi )$$F ( ϕ ) and the scalar potential $$V(\phi )$$V ( ϕ ) through the parametrization (or attractor) of the scalar spectral index $$n_{s}(N)$$n s ( N ) and the tensor-to-scalar ratio r(N) as functions of the number of $$e-$$e - folds N. As specific examples, we analyze the attractors $$n_{s}-1 \propto 1/N$$n s - 1 ∝ 1 / N and $$r\propto 1/N$$r ∝ 1 / N , as well as the case $$r\propto 1/N (N+\gamma )$$r ∝ 1 / N ( N + γ ) with $$\gamma $$γ a dimensionless constant. In this sense and depending on the attractors considered, we obtain different expressions for the function $$F(\phi )$$F ( ϕ ) and the potential $$V(\phi )$$V ( ϕ ) , as also the constraints on the parameters present in our model and its reconstruction.

The reconstruction of a warm inflationary universe model from the scalar spectral index [equation] and the tensor to scalar ratio r( N) as a function of the number of e-folds Nis studied. Under a...

A unified multiscalar field model with three flat regions is discussed. The three flat regions are the inflation, early- and late dark energy epochs. The potential is obtained by a spontaneous breaking of scale invariance generated by non-Riemannian measures of integration (or two measures theories (TMT)). We define the scale invariant couplings of the scalar fields to the different measures through exponential potentials. Spontaneous breaking of scale invariance takes place when integrating the fields that define the measures. When going to the Einstein frame we obtain: (i) An effective potential for the scalar fields with three flat regions which allows for a unified description of both early Universe inflation (in the higher-energy density flat region) as well as of present dark energy epoch which can be realized with a double phase, i.e., in two flat regions. (ii) In the slow-roll inflation, only one field combination, the “dilaton,” which transforms under scale transformations, has nontrivial dynamics; the orthogonal one, which is scale invariant, remains constant. The corresponding perturbations of the dilaton are calculated. (iii) For a reasonable choice of the parameters the present model perturbations conforms to the Planck Collaboration data. (iv) In the late Universe we define scale-invariant couplings of Dark Matter to the dilaton. These couplings define a matter-induced potential for the dilaton and extremizing this potential determines the scale-invariant scalar field, while all exotic noncanonical behavior of the Dark Matter as well as any possible fifth force disappear. (v) We calculate the evolution of the late Universe under these conditions with the realization of two different possible realizations of $\mathrm{\Lambda }$ cold dark matter-type scenarios depending on the flat region in the late Universe. These two phases could appear at different times in the history of the Universe. (vi) From the Planck data, we find the constraints on the parameters during the inflationary epoch and these values are used to obtain constraints relevant to the present epoch.