Emmanuel N. Saridakis

Cosmological perturbations in f(T) gravity

We investigate the cosmological perturbations in f(T) gravity. Examining the pure gravitational perturbations in the scalar sector using a diagonal vierbein, we extract the corresponding dispersion relation, which provides a constraint on the f(T) Ansaetze that lead to a theory free of instabilities. Additionally, upon inclusion of the matter perturbations, we derive the fully perturbed equations of motion, and we study the growth of matter overdensities. We show that f(T) gravity with f(T) constant coincides with General Relativity, both at the background as well as at the first-order perturbation level. Applying our formalism to the power-law model we find that on large subhorizon scales (O(100 Mpc) or larger), the evolution of matter overdensity will differ from {Lambda}CDM cosmology. Finally, examining the linear perturbations of the vector and tensor sectors, we find that (for the standard choice of vierbein) f(T) gravity is free of massive gravitons.

f(T) gravity mimicking dynamical dark energy. Background and perturbation analysis

We investigate f(T) cosmology in both the background, as well as in the perturbation level, and we present the general formalism for reconstructing the equivalent one-parameter family of f(T) models for any given dynamical dark energy scenario. Despite the completely indistinguishable background behavior, the perturbations break this degeneracy and the growth histories of all these models differ from one another. As an application we reconstruct the f(T) equivalent for quintessence, and we show that the deviation of the matter overdensity evolution is strong for small scales and weak for large scales, while it is negligible for large redshifts.

Quintessence and phantom cosmology with nonminimal derivative coupling

We investigate cosmological scenarios with a nonminimal derivative coupling between the scalar field and the curvature, examining both the quintessence and the phantom cases in zero and constant potentials. In general, we find that the universe transits from one de Sitter solution to another, determined by the coupling parameter. Furthermore, according to the parameter choices and without the need for matter, we can obtain a big bang, an expanding universe with no beginning, a cosmological turnaround, an eternally contracting universe, a big crunch, a big rip avoidance, and a cosmological bounce. This variety of behaviors reveals the capabilities of the present scenario.

New agegraphic dark energy in Hořava-Lifshitz cosmology

We investigate the new agegraphic dark energy scenario in a universe governed by Ho?ava-Lifshitz gravity. We consider both the detailed and non-detailed balanced version of the theory, we impose an arbitrary curvature, and we allow for an interaction between the matter and dark energy sectors. Extracting the differential equation for the evolution of the dark energy density parameter and performing an expansion of the dark energy equation-of-state parameter, we calculate its present and its low-redshift value as functions of the dark energy and curvature density parameters at present, of the Ho?ava-Lifshitz running parameter ?, of the new agegraphic dark energy parameter n, and of the interaction coupling b. We find that w0 = ?0.82+0.08?0.08 and w1 = 0.08+0.09?0.07. Although this analysis indicates that the scenario can be compatible with observations, it does not enlighten the discussion about the possible conceptual and theoretical problems of Ho?ava-Lifshitz gravity.

Observational constraints on holographic dark energy with varying gravitational constant

We use observational data from Type Ia Supernovae (SN), Baryon Acoustic Oscillations (BAO), Cosmic Microwave Background (CMB) and observational Hubble data (OHD), and the Markov Chain Monte Carlo (MCMC) method, to constrain the cosmological scenario of holographic dark energy with varying gravitational constant. We consider both flat and non-flat background geometry, and we present the corresponding constraints and contour-plots of the model parameters. We conclude that the scenario is compatible with observations. In 1σ we find ΩΛ0 = 0.72+0.03−0.03, Ωk0 = −0.0013+0.0130−0.0040, c = 0.80+0.19−0.14 and ΔG≡G/G = −0.0025+0.0080−0.0050, while for the present value of the dark energy equation-of-state parameter we obtain w0 = −1.04+0.15−0.20.

Dynamics of the anisotropic Kantowsky–Sachs geometries in Rn gravity

We construct general anisotropic cosmological scenarios governed by an f(R) gravitational sector. Focusing then on Kantowski–Sachs geometries in the case of Rn-gravity, and modeling the matter content as a perfect fluid, we perform a detailed phase-space analysis. We find that at late times the universe can result in a state of accelerating expansion, and additionally, for a particular n-range (2 < n < 3), it exhibits phantom behavior. Furthermore, isotropization has been achieved independently of the initial anisotropy degree, showing in a natural way why the observable universe is so homogeneous and isotropic, without relying on a cosmic no-hair theorem. Moreover, contracting solutions also have a large probability to be the late-time states of the universe. Finally, we can also obtain the realization of the cosmological bounce and turnaround, as well as of cyclic cosmology. These features indicate that anisotropic geometries in modified gravitational frameworks present radically different cosmological behaviors compared to the simple isotropic scenarios.

Cyclic cosmology from Lagrange-multiplier modified gravity

We investigate cyclic and singularity-free evolutions in a universe governed by Lagrange-multiplier modified gravity, in both scalar-field cosmology and the f(R) one. In the scalar case, cyclicity can be induced by a suitably reconstructed simple potential, and the matter content of the Universe can be successfully incorporated. In the case of f(R)-gravity, cyclicity can be induced by a suitably reconstructed second function f2(R) of a very simple form; however, the matter evolution cannot be analytically handled. Furthermore, we study the evolution of cosmological perturbations for the two scenarios. For the scalar case the system possesses no wavelike modes due to a dust-like sound speed, while for the f(R) case there exists an oscillation mode of perturbations which indicates a dynamical degree of freedom. Both scenarios allow for stable parameter spaces of cosmological perturbations through the bouncing point.

Overall observational constraints on the running parameter λ of Hořava-Lifshitz gravity

We use observational data from Type Ia Supernovae (SNIa), Baryon Acoustic Oscillations (BAO), and Cosmic Microwave Background (CMB), along with requirements of Big Bang Nucleosynthesis (BBN), to constrain the running parameter λ of Hořava-Lifshitz gravity, which determines the flow between the Ultra-Violet and the Infra-Red. We consider both the detailed and non-detailed balance versions of the gravitational sector, and we include the matter and radiation sectors. Allowing for variation of all the parameters of the theory, we construct the likelihood contours and we conclude that in 1σ confidence λ is restricted to |λ−1|∼<0.02, while its best fit value is |λ{sub b.f}−1| ≈ 0.002. Although this observational analysis restricts the running parameter λ very close to its IR value 1, it does not enlighten the discussion about the theorys possible conceptual and theoretical problems.

Quintom scenario with mixed kinetic terms

We examine an extension of the quintom scenario of dark energy, in which a canonical scalar field and a phantom field are coupled through a kinetic interaction. We perform a phase-space analysis and show that the kinetic coupling gives rise to novel cosmological behavior. In particular, we obtain both quintessence-like and phantomlike late-time solutions, as well as solutions that cross the phantom divide during the evolution of the Universe.

Inflation in Entropic Cosmology: Primordial Perturbations and non-Gaussianities

We investigate thermal inflation in double-screen entropic cosmology. We find that its realization is general, resulting from the system evolution from non-equilibrium to equilibrium. Furthermore, going beyond the background evolution, we study the primordial curvature perturbations arising from the universe interior, as well as from the thermal fluctuations generated on the holographic screens. We show that the power spectrum is nearly scale-invariant with a red tilt, while the tensor-to-scalar ratio is in agreement with observations. Finally, we examine the non-Gaussianities of primordial curvature perturbations, and we find that a sizable value of the non-linearity parameter is possible due to holographic statistics on the outer screen.