We consider general curvature-invariant modifications of the Einstein-Hilbert action that become important only in regions extremely low space-time curvature. investigate far future evolution universe such models, examining possibilities for cosmic acceleration and other ultimate destinies. The models generically possess de Sitter space as an unstable solution exhibit interesting set attractor solutions which, some cases, provide alternatives to dark energy models.

We present a wide class of models which realise bounce in spatially flat Friedmann universe standard General Relativity. The key ingredient the theories we consider is noncanonical, minimally coupled scalar field belonging to with Kinetic Gravity Braiding / Galileon-like self-couplings. In these models, smoothly evolves from contraction expansion, suffering neither ghosts nor gradient instabilities around turning point. end-point evolution can be radiation-domination era or an inflationary...

We present a nonsingular bouncing cosmology using single scalar field matter with non-trivial potential and non-standard kinetic term. The sources dynamical attractor solution Ekpyrotic contraction which washes out small amplitude anisotropies. At high energy densities the evolves into ghost condensate, leading to bounce. Following bounce there is smooth transition standard expanding radiation dominated phases. Using linear cosmological perturbation theory we track each Fourier mode of...

Over the past decade it has become clear that fundamental strings are not only degrees of freedom in string theory. D-branes also part spectrum states. In this paper we explore some possible effects on early Universe cosmology, starting with two key assumptions: firstly initial state corresponded to a dense, hot gas which all were thermal equilibrium, and secondly topology background space admits one-cycles. We argue by t duality context cosmological singularities present. derive equation...

Reheating is a transition era after the end of inflation, during which inflaton converted into particles that populate Universe at later times. No direct cosmological observables are normally traceable to this period reheating. Indirect bounds can however be derived. One possibility consider evolution for observable CMB scales from time Hubble crossing present time. Depending upon model, duration and final temperature reheating, as well its equation state, may directly linked inflationary...

We discuss the question of whether or not inflationary spacetimes can be geodesically complete in infinite past. Geodesic completeness is a necessary condition for averting an initial singularity during eternal inflation. It frequently argued that cosmological models which are expanding sufficiently fast (having average Hubble expansion rate $H_{avg}>0$) must incomplete null and timelike past directions. This well-known conjecture relies on specific bounds integral parameter over...

The Weyl double copy relates vacuum solutions in general relativity to Abelian gauge fields Minkowski spacetime. In a previous work, we showed how the can be extended provide treatment of external gravitational sources consistent with classical Kerr-Schild copy. Using this generalization, here complete double-copy analysis electrovacuum Petrov type-D spacetimes. This includes first charged C metric, whose single-copy interpretation invokes two-potential formalism electrodynamics. We also...

We find static charged black hole solutions in nonlinear massive gravity. In the parameter space of two gravitational potential parameters $(\alpha, \beta)$ we show that below Compton wavelength reduce to Reissner-Nordstr\"om via Vainshtein mechanism weak field limit. simplest case with $\alpha=\beta=0$ solution exhibits vDVZ discontinuity but ordinary General Relativity is recovered deep inside horizon due existence electric charge. For $\alpha\neq0$ and $\beta=0$, post-Newtonian evolves...

One of the major pillars modern cosmology theory is a period accelerating expansion in early universe. This expansion, or inflation, must be sustained for at least 30 e--foldings. mechanism used to drive acceleration addition new energy field, called Inflaton; often this scalar field. We propose an alternative which, like our approach explain late-time universe, uses entropy and temperature intrinsic information holographically stored on surface enclosing observed space. The due both cases...

We study a recently proposed scenario for the early universe:Subluminal Galilean Genesis. prove that without any other matter present in spatially flat Friedmann universe, perturbations of Galileon scalar field propagate with speed at most equal to light. This proof applies all cosmological solutions — whole phase space. However, more realistic situation, when one includes which is not directly coupled Galileon, there always exists region space where these superluminally, indeed arbitrarily...

This paper proposes a method to test Einstein's equivalence principle using the profile of black hole shadows. The shadow in edge-on view is found have different appearances at wavelengths observed light.

We present new traversable wormhole and non-singular black hole solutions in pure, scale-free $R^2$ gravity. These exotic require no null energy condition violating or "exotic" matter are supported only by the vacuum of theory. It is well known that $f(R)$ theories gravity may be recast as dual Einstein frame. The we found when conformal transformation required to move frame singular. For quadratic gravity, factor identically zero for spacetimes with $R=0$. Solutions this case argued arise...

We apply the classical double-copy procedure to a class of regular, nonsingular black hole solutions. give several examples, paying particular attention string-theory-corrected solution emerging from T-duality. Nonperturbative stringy corrections introduce an ultraviolet (UV) zero-point length cutoff which results in spacetimes. Apart UV regulator, is equivalent Bardeen spacetime. extend this include asymptotic de Sitter background. All Yang-Mills field theory quantities associated with...

The Weyl double copy relates exact solutions in general relativity to gauge theory, formulated the spinorial language. To date, is understood and employed only for vacuum spacetimes, hence theories. In this Letter, we propose an extension that provides a systematic procedure treating gravitational sources. We show extended gives new perspective Kerr-Newman black hole class of Petrov type D electrovac spacetimes.

A bstract In the standard derivation of Kerr-Schild double copy, geodicity vector and stationarity spacetime are presented as assumptions that necessary for single copy to satisfy Maxwell’s equations. However, it is well known vacuum Einstein equations imply geodesic shear-free, possesses a distinguished field simultaneously Killing full flat background, but need not be timelike with respect background metric. We show gauge obtained by contracting this graviton solves Maxwell equations,...

Modified gravitational theories can provide alternatives to dark energy as an explaination for the observed late-time cosmic acceleration. Several examples of low-curvature corrections Einstein-Hilbert action are studied. These models generically contain unstable de Sitter solutions and, depending on parameters theory, exhibit late time accelerating attractor solutions.

We study the cosmological implications of including angular motion in Dirac–Born–Infeld brane inflation scenario. The non-canonical kinetic terms action give an interesting alternative to slow-roll inflation, and cycling branes can drive periods accelerated expansion Universe. present explicit numerical solutions demonstrating Klebanov–Strassler throat. find that demanding sufficient takes place throat is conflict with keeping brane's total energy low enough local gravitational back-reaction...

There is now overwhelming observational evidence that our Universe accelerating in its expansion. I discuss how modified gravitational models can provide an explanation for this observed late-time cosmic acceleration. We consider specific low-curvature corrections to the Einstein-Hilbert action. Many of these generically contain unstable de Sitter solutions and, depending on parameters theory, attractor solutions.

We propose a warm inflationary model in the context of relativistic D-brane inflation warped throat, which has Dirac-Born-Infeld (DBI) kinetic term and is coupled to radiation through dissipation term. The perturbation freezes at sound horizon power spectrum determined by combination dissipative parameter speed parameter. thermal ameliorates eta problem softens theoretical constraints from extra-dimensional volume observational bounds on tensor-to-scalar ratio. DBI can lead appreciable...

We examine bounds on adiabatic and isocurvature density fluctuations from $\mu$-type spectral distortions of the cosmic microwave background (CMB). Studies such distortion are complementary to CMB measurements index its running, will help constrain these parameters significantly smaller scales. show that a detection order $\mu \sim 10^{-7}$ would strongly be at odds with standard cosmological model nearly scale-invariant spectrum perturbations. Further, we find given current constraints mode...

We present a class of spherically symmetric vacuum solutions to an asymptotically safe theory gravity containing high-derivative terms. find quantum corrected Schwarzschild-(anti)-de Sitter with running gravitational coupling parameters. The evolution the couplings is determined by their corresponding renormalization group flow equations. These black holes exhibit properties classical Schwarzschild solution at large length scales. At center, metric factor remains smooth but curvature...