Conformal invariant cosmological perturbations via the covariant approach:multicomponent universe

In recent years there has been a lot of interest in discussing frame depeudences/independences of the cosmological perturbations under the conforlnal transformations. This problem has previously been investigated in Lerlns of the cow^riant approach for a single component universe, and it was found that tile covariant approach is very powerful to pick out the perturbative variables which are both gauge and conformal invariant. In this work, we extend the covariant approach to a universe with multicomponent fluids. We find that similar results can be derived, as e, xpected. In addition, some other interesting perturbations are also identified to be conformal invariant, such as entropy perturbation between two different components.

Braneworld Inflation with Induced Gravity and Curvature Effect

We construct a general braneworld inflation scenario where the inflaton field evolves on the DGP brahe and curvature effects are taken into account via incorporation of the Gauss-Bonnet term in the bulk action. While induced gravity on the DGP brane modifies the IR limit of general relativity, the Gauss-Bonnet term in the bulk action modifies the UV sector of the theory. In this setup, the dynamics of perturbations on the brane are studied with details and some confrontation with recent observations are discussed. While the Einstein-Gauss-Bonnet inflation scenario favors only a red spectrum of the scalar perturbation, pure DGP and GBIG inflation models have the capacity to realize the blue spectrum too. In addition, the GBIG inflation scenario in the large field limit requires a smaller number of e-folds than other proposed scenarios in the same situation. For the tensor-to-scalar ratio, the GBIG inflation scenario g/yes a better fit with observationally supported value of R≈ 0.24.

Tensor perturbations from bounce inflation scenario in f(Q)gravity

In this paper,we construct a bounce inflation cosmological scenario in the framework of the modified symmetric teleparallel gravity,namely f(Q)theory,and investigate the tensor perturbations therein.As is well-known,the tensor perturbations generated in the very early Universe(inflation and pre-inflation regions)can account for the primordial gravitational waves(PGWs)that are to be detected by the next generation of GW experiments.We discuss the stability condition of the tensor perturbations in the bounce inflation process and investigate in detail the evolution of the perturbation variable.The general form of the tensor power spectrum is obtained both for large as well as small scale modes.As a result,we show both kinds of modes(short or long wavelength modes),and the tensor spectrum may get a positive tilt in the parametric range where the tensor perturbation proves to be stable—this interestingly hints an enhancement of gravitational waves’amplitude in the background of the f(Q)bounce-inflation scenario.Moreover,we study the LQC-like scenario as a specific case of our model,in which,the primordial tensor power spectrum turns out to be nearly scale-invariant on both small and large scales.

Spatial covariant gravity with two degrees of freedom in the presence of an auxiliary scalar field:Perturbation analysis

We investigate a class of gravity theories respecting only spatial covariance,termed spatially covariant gravity,in the presence of an auxiliary scalar field.We examine the conditions on the Lagrangian required to eliminate scalar degrees of freedom,allowing only two tensorial degrees of freedom to propagate.Instead of strict constraint analysis,in this paper,we employ the perturbation method and focus on the necessary conditions to evade the scalar mode at the linear order in perturbations around a cosmological background.Beginning with a general action and solving the auxiliary perturbation variables in terms of a would-be dynamical scalar mode,we derive the condition to remove its kinetic term,thus ensuring that no scalar mode propagates.As an application of the general condition,we study a polynomial-type Lagrangian as a concrete example,in which all monomials are spatially covariant scalars containing two derivatives.We find that the auxiliary scalar field is essential,and new terms in the Lagrangian are allowed.Our analysis provides insights into constructing gravity theories with two degrees of freedom in the extended framework of spatially covariant gravity.

The observational constraint on constant-roll inflation

We discuss the constant-roll inflation with constant ∈1 and constant η. By using the method of Bessel function approximation, the analytical expressions for the scalar and tensor power spectra, the scalar and tensor spectral tilts, and the tensor to scalar ratio are derived up to the first order of ∈1. The model with constant ∈2 is ruled out by the observations at the 3σ confidence level, and the model with constant η is consistent with the observations at the 1σ confidence level. The potential for the model with constant η is also obtained from the Hamilton-Jacobi equation. Although the observations constrain the constant-roll inflation to be the slow-roll inflation, the ns-r results from the constant-roll inflation are not the same as those from the slow-roll inflation even when η- 0.01.

Doubly coupled matter fields in massive bigravity

In the context of massive （bi-）gravity, non-minimal matter couplings have been proposed. These couplings are special in the sense that they are free of the Boulware-Deser ghost below the strong coupling scale and can be used consistently as an effective field theory. Furthermore, they enrich the phenomenology of massive gravity. We consider these couplings in the framework of bimetric gravity and study the cosmological implications for background and linear tensor, vector, and scalar Previous works have investigated special branches of solutions. Here we perform a complete perturbation analysis for the general background equations of motion, completing previous analyses.