We show that the f(T) gravitational paradigm, in which gravity is described by an arbitrary function of torsion scalar, can provide a mechanism for realizing bouncing cosmologies, thereby avoiding Big Bang singularity. After constructing simplest version matter bounce, we investigate scalar and tensor modes cosmological perturbations. Our results metric perturbations sector lead to background-dependent sound speed, distinguishable feature from Einstein gravity. Additionally, obtain...

We investigate the cosmological perturbations in f(T) gravity. Examining pure gravitational scalar sector using a diagonal vierbien, we extract corresponding dispersion relation, which provides constraint on ansatzes that lead to theory free of instabilities. Additionally, upon inclusion matter perturbations, derive fully perturbed equations motion, and study growth overdensities. show gravity with constant coincides General Relativity, both at background as well first-order perturbation...

It is possible that the properties of HII regions during reionization depend sensitively on many poorly constrained quantities (the nature ionizing sources, clumpiness gas in IGM, degree to which photo-ionizing feedback suppresses abundance low mass galaxies, etc.), making it extremely difficult interpret upcoming observations this epoch. We demonstrate actual situation more encouraging, using a suite radiative transfer simulations, post-processed outputs from 1024^3, 94 Mpc N-body...

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

We examine dark energy models in which a quintessence or phantom field, $\phi$, rolls near the vicinity of local minimum maximum, respectively, its potential $V(\phi)$. Under approximation that $(1/V)(dV/d\phi) \ll 1$, [although $(1/V)(d^2 V/d\phi^2)$ can be large], we derive general expression for equation state parameter $w$ as function scale factor these models. The dynamics field depends on value extremum, describes curvature. For models, when V/d\phi^2)<3/4$ at minimum, $w(a)$ evolves...

We examine hilltop quintessence models, in which the scalar field is rolling near a local maximum potential, and $w\ensuremath{\approx}\ensuremath{-}1$. first derive general equation for evolution of $\ensuremath{\phi}$ limit where solve this case to $w$ as function scale factor; these solutions depend on curvature potential its maximum. Our result excellent agreement ($\ensuremath{\delta}w\ensuremath{\lesssim}0.5%$) with all particular cases examined. It works particularly well...

We derive slow-roll conditions for thawing k-essence with a separable Lagrangian $p(X,\ensuremath{\phi})=F(X)V(\ensuremath{\phi})$. examine the evolution of equation state parameter, $w$, as function scale factor $a$, case where $w$ is close to $\ensuremath{-}1$. find two distinct cases, corresponding $X\ensuremath{\approx}0$ and ${F}_{X}\ensuremath{\approx}0$, respectively. For $\ensuremath{\phi}$ hence described by only parameters, $w(a)$ model independent coincides similar behavior seen...

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 cosmological scenarios governed by Horava-Lifshitz gravity. consider both detailed non-detailed balance versions gravitational sector, we include matter radiation sectors. conclude that detailed-balance scenario cannot be ruled out point view, however corresponding likelihood contours...

We reexamine the evolution of thermal relic particle abundances for case where interaction rate depends on velocities. For Sommerfeld enhancement, we show that standard analytic approximation, modified in a straightforward way, provides an estimate abundance is accurate to within 10% (in comparison <1% error non-Sommerfeld-enhanced case). examine effect kinetic decoupling when velocity. pure p-wave annihilation, increase abundance, but negligible temperature much less than chemical...

We perform a combined perturbation and observational investigation of the scenario non-minimal derivative coupling between scalar field curvature. First we extract necessary condition that ensures absence instabilities, which is fulfilled more sufficiently for smaller values. Then using Type Ia Supernovae (SNIa), Baryon Acoustic Oscillations (BAO), Cosmic Microwave Background (CMB) observations, show that, contrary to its significant effects on inflation, term has negligible effect universe...

We investigate the clustering properties of a dynamical dark energy component. In cosmic mix pressureless fluid and light scalar field, we follow linear evolution spherical matter perturbations. find that field tends to form underdensities in response gravitationally collapsing matter. thoroughly these voids for variety initial conditions, explain physics behind their formation, consider possible observational implications. Detection will clearly rule out cosmological constant as main source...

Oscillating scalar fields, with an oscillation frequency much greater than the expansion rate, have been proposed as models for dark energy. We examine these models, particular emphasis on evolution of ratio to rate. show that this always increases time if energy density declines less rapidly background density. This allows us classify oscillating in terms epoch at which exceeds is effectively rapid oscillations begin. There are three basic types behavior: early begin during matter-dominated...

We examine dark energy models in which a phantom field $\phi$ is rolling near local minimum of its potential $V(\phi)$.We require that $(1/V)(dV/d\phi) \ll 1$, but $(1/V)(d^2 V/d\phi^2)$ can be large. Using techniques developed the context hilltop quintessence, we derive general expression for $w$ as function scale factor, and case, find dynamics depend on value mimimum. Our result gives within 1% true (numerically-derived) all particular cases examined. $w(a)$ reduces to previously-derived...

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 Hořava-Lifshitz gravity, which determines flow between Ultra-Violet Infra-Red. consider both detailed non-detailed balance versions gravitational sector, we include matter radiation sectors. Allowing for variation all parameters theory, construct likelihood contours...

We study the persistence of warps in galactic discs presence massive haloes. A disc is approximated by a set rings, while halo represented conventional n-body simulation. confirm conclusion Nelson &38; Tremaine that responds strongly to an embedded precessing disc. This response invalidates approximations made derivation classical 'modified tilt' modes. show causes line nodes starts from modified tilt mode wind up within few dynamical times. explain this finding terms probable spectrum true...

We study the creation of solitons from particles, using lambdaphi(4) model as a prototype. consider scattering small, identical, wave pulses, which are equivalent to sequence and find that kink-antikink pairs created for large region in parameter space. also at low velocities is favorable creating have energy compared mass particle.

The effect of quintessence perturbations on the ISW is studied for a mixed dynamical scalar field dark energy (DDE) and pressureless perfect fluid matter. A new general methodology developed to track growth perturbations, which uses only equation state (EoS) parameter $w_{\rm DDE} (z) \equiv p_{\rm DDE}/ρ_{\rm DDE}$ DDE, initial values relative entropy perturbation (between matter DDE) intrinsic DDE as inputs. We also derive relation between rest frame sound speed $\hat{c}_{s,{\rm DDE}}^2$...

We study the scale-dependent evolution of growth function $\ensuremath{\delta}(a,k)$ cosmological perturbations in dark energy models based on general relativity. This scale dependence is more prominent scales $100{h}^{\ensuremath{-}1}\text{ }\text{ }\mathrm{Mpc}$ or larger. derive a new parametrization which generalizes well-known Newtonian approximation result...

We investigate two possible effects of the tidal field induced by a spherical cluster on its elliptical galaxy members: modification ellipticity and isophotal alignment in radial direction misaligned prolate galaxy. Numerical N-body simulations have been performed for circular galactic orbits. The properties stars' zero--velocity surfaces perturbed galaxies are explored briefly, adiabaticity to external is discussed. For choice parameters characteristic rich clusters we find that below or...

Models that lead to a cosmological stiff fluid component, with density ${\ensuremath{\rho}}_{S}$ scales as ${a}^{\ensuremath{-}6}$, where $a$ is the scale factor, have been proposed recently in variety of contexts. We calculate numerically effect such on primordial element abundances. Because energy decreases factor more rapidly than radiation, it produces relatively larger change helium-4 abundance other abundances, relative changes produced by an additional radiation component. show varies...

We propose a simple dark energy model with the following properties: predicts late-time radiation component that is not ruled out by current observational data, but which produces distinctive time-dependent equation of state $w(z)$ for $z&lt;3$. The field can be coupled strongly enough to standard particles detected in colliders, and requires only modest additional particle content little or no fine-tuning other than new scale order milli-electron volts.

We examine the accuracy of growth equation $\stackrel{\ifmmode\ddot\else\textasciidieresis\fi{}}{\ensuremath{\delta}}+2H\stackrel{\ifmmode \dot{}\else \textperiodcentered \fi{}}{\ensuremath{\delta}}\ensuremath{-}4\ensuremath{\pi}G\ensuremath{\rho}\ensuremath{\delta}=0$, which is ubiquitous in cosmological literature, context Newtonian gauge. By comparing predicted by this to a numerical solution linearized Einstein equations $\ensuremath{\Lambda}\mathrm{CDM}$ scenario, we show that while...

We examine a dark energy model where scalar unparticle degree of freedom plays the role quintessence. In particular, we study has standard kinetic term and simple mass potential, evolution is slowly rolling field value order scale (${\ensuremath{\lambda}}_{u}$). how $w$ depends on parameters $B$ (a function scaling dimension ${d}_{u}$), initial ${\ensuremath{\phi}}_{i}$ (or equivalently, ${\ensuremath{\lambda}}_{u}$) present matter density ${\ensuremath{\Omega}}_{m0}$. use observational data...