arXiv subjects

Ishwaree P. Neupane

Publications and source records attributed to Ishwaree P. Neupane.

At least 37 records · Page 2Linked to original sources

arXiv2006

On compatibility of string effective action with an accelerating universe

In this paper, we fully investigate the cosmological effects of the moduli dependent one-loop corrections to the gravitational couplings of the string effective action to explain the cosmic acceleration problem in early (and/or late) universe. These corrections comprise a Gauss-Bonnet (GB) invariant multiplied by universal non-trivial functions of the common modulus $σ$ and the dilaton $ϕ$. The model exhibits several features of cosmological interest, including the transition between deceleration and acceleration phases. By considering some phenomenologically motivated ansatzs for one of the scalars and/or the scale factor (of the universe), we also construct a number of interesting inflationary potentials. In all examples under consideration, we find that the model leads only to a standard inflation ($w \geq -1$) when the numerical coefficient $δ$ associated with modulus-GB coupling is positive, while the model can lead also to a non-standard inflation ($w<-1$), if $δ$ is negative. In the absence of (or trivial) coupling between the GB term and the scalars, there is no crossing between the $w< -1$ and $w> -1$ phases, while this is possible with non-trivial GB couplings, even for constant dilaton phase of the standard picture. Within our model, after a sufficient amount of e-folds of expansion, the rolling of both fields $ϕ$ and $σ$ can be small. In turn, any possible violation of equivalence principle or deviations from the standard general relativity may be small enough to easily satisfy all astrophysical and cosmological constraints.

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arXiv2006

Addendum to "Hyperbolic Space Cosmologies"

In our earlier paper [JHEP 0310 (2003) 058], we considered higher dimensional cosmological models with hyperbolic spaces. In particular the eternal accelerating expansion was obtained by studying small perturbation around the critical non-accelerated solution for $D > 10$. In this addendum, we show that there is also such a solution in the critical case $D = 10$.

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arXiv2006

Dynamical Relaxation of Dark Energy: A Solution to Early Inflation, Late-time Acceleration and the Cosmological Constant Problem

In recent years different explanations are provided for both an inflation and a recent acceleration in the expansion of the universe. In this Letter we show that a model of physical interest is the modification of general relativity with a Gauss-Bonnet term coupled to a dynamical scalar-field as predicted by certain versions of string theory. This construction provides a model of evolving dark energy that naturally explains a dynamical relaxation of the vacuum energy (gravitationally repulsive potential) to a small value (exponentially close to zero) after a sufficient number of e-folds. The model also leads to a small deviation from the $w=-1$ prediction of non-evolving dark energy.

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arXiv2005

Cosmic Acceleration from M Theory on Twisted Spaces

In a recent paper [I.P. Neupane and D.L. Wiltshire, Phys. Lett. B 619, 201 (2005).] we have found a new class of accelerating cosmologies arising from a time--dependent compactification of classical supergravity on product spaces that include one or more geometric twists along with non-trivial curved internal spaces. With such effects, a scalar potential can have a local minimum with positive vacuum energy. The existence of such a minimum generically predicts a period of accelerated expansion in the four-dimensional Einstein-conformal frame. Here we extend our knowledge of these cosmological solutions by presenting new examples and discuss the properties of the solutions in a more general setting. We also relate the known (asymptotic) solutions for multi-scalar fields with exponential potentials to the accelerating solutions arising from simple (or twisted) product spaces for internal manifolds.

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arXiv2005

Accelerating cosmologies from compactification with a twist

It is demonstrated by explicit solutions of the (4+n)-dimensional vacuum Einstein equations that accelerating cosmologies in the Einstein conformal frame can be obtained by a time-dependent compactification of string/M-theory, even in the case that internal dimensions are Ricci-flat, provided one includes one or more geometric twists. Such acceleration is transient. When both compact hyperbolic internal spaces and geometric twists are included, however, the period of accelerated expansion may be made arbitrarily large.

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arXiv2004

Accelerating Cosmologies from Exponential Potentials

An exponential potential of the form $V\sim \exp(-2c ϕ/M_p)$ arising from the hyperbolic or flux compactification of higher-dimensional theories is of interest for getting short periods of accelerated cosmological expansions. Using a similar potential but derived for the combined case of hyperbolic-flux compactification, we study the four-dimensional flat (and open) FLRW cosmologies and give analytic (and numerical) solutions with exponential behavior of scale factors. We show that, for the M-theory motivated potentials, the cosmic acceleration of the universe can be eternal if the spatial curvature of the 4d spacetime is negative, while the acceleration is only transient for a spatially flat universe. We also comment on the size of the internal space and its associated geometric bounds on massive Kaluza-Klein excitations.

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arXiv2004

Cosmic Acceleration and M Theory Cosmology

This is a short overview of spatially flat (or open) four-dimensional accelerating cosmologies for some simple exponential potentials obtained by string or M theory compactification on some non-trivial curved spaces, which may lead to some striking results, e.g., the observed cosmic acceleration and the scale of the dark energy from first principles.

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arXiv2004

Thermodynamic and gravitational instability on hyperbolic spaces

We study the properties of anti--de Sitter black holes with a Gauss-Bonnet term for various horizon topologies (k=0, \pm 1) and for various dimensions, with emphasis on the less well understood k=-1 solution. We find that the zero temperature (and zero energy density) extremal states are the local minima of the energy for AdS black holes with hyperbolic event horizons. The hyperbolic AdS black hole may be stable thermodynamically if the background is defined by an extremal solution and the extremal entropy is non-negative. We also investigate the gravitational stability of AdS spacetimes of dimensions D>4 against linear perturbations and find that the extremal states are still the local minima of the energy. For a spherically symmetric AdS black hole solution, the gravitational potential is positive and bounded, with or without the Gauss-Bonnet type corrections, while, when k=-1, a small Gauss-Bonnet coupling, namely, α<< {l}^2 (where l is the curvature radius of AdS space), is found useful to keep the potential bounded from below, as required for stability of the extremal background.

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arXiv2003

Inflation from String/M-Theory Compactification?

We present some exact scalar potentials for the dimensionally reduced theory and examine the possibility of obtaining accelerating 4d cosmology from String/M-theory, more generally, hyperbolic and flux compactification. In the hyperbolic case, even in the zero-flux limit, the scalar potential is positive for the 4d effective theory as required to get an accelerating universe, and thereby evading the ``no-go theorem'' given for static internal space. When we turn on the gauge fields as source terms at the cosmological background with potential V\propto exp(-2cϕ), we find eternally accelerating cosmologies when the 4d space-time is flat and c\geq 1, or hyperbolic and 1<c<\sqrt{2}.

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arXiv2003

Hyperbolic Space Cosmologies

We present a systematic study of accelerating cosmologies obtained from M/string theory compactifications of hyperbolic spaces with time-varying volume. A set of vacuum solutions where the internal space is a product of hyperbolic manifolds is found to give qualitatively the same accelerating four-dimensional FLRW universe behavior as a single hyperbolic space. We also examine the possibility that our universe is a hyperbolic space and provide exact Milne type solutions, as well as intersecting S-brane solutions. When both the usual 4D spacetime and the m-dimensional internal space are hyperbolic, we find eternally accelerating cosmologies for $m\geq 7$, with and without form field backgrounds. In particular, the effective potential for a magnetic field background in the large 3 dimensions is positive definite with a local minimum and thus enhances the eternally accelerating expansion.

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arXiv2003

Warped brane-world compactification with Gauss-Bonnet term

In the Randall-Sundrum (RS) brane-world model a singular delta-function source is matched by the second derivative of the warp factor. So one should take possible curvature corrections in the effective action of the RS models in a Gauss-Bonnet (GB) form. We present a linearized treatment of gravity in the RS brane-world with the Gauss-Bonnet modification to Einstein gravity. We give explicit expressions for the Neumann propagator in arbitrary D dimensions and show that a bulk GB term gives, along with a tower of Kaluza-Klein modes in the bulk, a massless graviton on the brane, as in the standard RS model. Moreover, a non-trivial GB coupling can allow a new branch of solutions with finite Planck scale and no naked bulk singularity, which might be useful to avoid some of the previously known ``no--go theorems'' for RS brane-world compactifications.

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arXiv2003

A Note on Acceleration from Product Space Compactification

We study compactifications of Einstein gravity on product spaces in vacuum and their acceleration phases. Scalar potentials for the dimensionally reduced effective theory are found to be of exponential form and exact solutions are obtained for a class of product spaces. The inflation in our solutions is not sufficient for the early universe. We comment on the possibility of obtaining sufficient inflation by compactification in general.

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arXiv2003

Black hole entropy in string-generated gravity models

The Euclidean action and entropy are computed in string-generated gravity models with quadratic curvatures, and used to argue that a negative mass extremal metric is the background for hyperbolic (k=-1) black hole spacetimes, k being the curvature constant of the event horizon. The entropy associated with a black hole is always positive for k=(0,1) family. The positivity of energy condition also ensures that the k=-1 (extremal) entropy is non-negative.

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arXiv2003

Completely localized gravity with higher curvature terms

In the intersecting braneworld models, higher curvature corrections to the Einstein action are necessary to provide a non-trivial geometry (brane tension) at the brane junctions. By introducing such terms in a Gauss-Bonnet form, we give an effective description of localized gravity on the singular delta-function branes. There exists a non-vanishing brane tension at the four-dimensional brane intersection of two 4-branes. Importantly, we give explicit expressions of the graviton propagator and show that the Randall-Sundrum single-brane model with a Gauss-Bonnet term in the bulk correctly gives a massless graviton on the brane as for the RS model. We explore some crucial features of completely localized gravity in the solitonic braneworld solutions obtained with a choice (ξ=1) of solutions. The no-go theorem known for Einstein's theory may not apply to the ξ=1 solution. As complementary discussions, we provide an effective description of the power-law corrections to Newtonian gravity on the branes or at the common intersection thereof.

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arXiv2002

No ghost state of Gauss-Bonnet interaction in warped backgrounds

A general solution to the Einstein field equations with Gauss-Bonnet(GB) term in the $AdS_5$ bulk background implies that the GB coupling $α$ can take either sign (+ or -), though a positive $α$ will be more meaningful. By considering linearized gravity with the GB term in the Randall-Sundrum(RS) a singular 3-brane model, we study the gravitational interactions between matter sources localized on the brane. With a correctly defined boundary condition on the brane, we find a smooth behavior of graviton propagator and hence the zero-mode solution as a 4d massless graviton localized on the brane with correct momentum and tensor structures. The coupling $α$ modifies the graviton propagators both on the brane and in the bulk. The issue on ghost state of the GB term is resolved, and we find that there is no real ghost (negative norm) state of the GB term in the RS single brane picture. The latter condition leads to a consistency in the coupling between the brane matter and the bulk gravity. We also elucidate about the possibilities for behavior of a test particle on the brane and in the bulk.

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arXiv2002

Anti-de Sitter Black Holes, Thermal Phase Transition and Holography in Higher Curvature Gravity

We study anti-de Sitter black holes in the Einstein-Gauss-Bonnet and the generic R^2 gravity theories, evaluate different thermodynamic quantities, and also examine the possibilities of Hawking-Page type thermal phase transitions in these theories. In the Einstein theory, with a possible cosmological term, one observes a Hawking-Page phase transition only if the event horizon is a hypersurface of positive constant curvature (k=1). But, with the Gauss-Bonnet or/and the (Riemann)^2 interaction terms, there may occur a similar phase transition for a horizon of negative constant curvature (k=-1). We examine the finite coupling effects, and find that N>5 could trigger a Hawking-Page phase transition in the latter theory. For the Gauss-Bonnet black holes, one relates the entropy of the black hole to a variation of the geometric property of the horizon based on first law and Noether charge. With (Riemann)^2 term, however, we can do this only approximately, and the two results agree when, r_H>>L, the size of the horizon is much bigger than the AdS curvature scale. We establish some relations between bulk data associated with the AdS black hole and boundary data defined on the horizon of the AdS geometry. Following a heuristic approach, we estimate the difference between Hubble entropy {\cal S}_H and Bekenstein-Hawking entropy {\cal S}_{BH} with (Riemann)^2 term, which, for k=0 and k=-1, would imply {\cal S}_{BH}\leq {\cal S}_H.

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arXiv2002

Effective Lagrangian from Higher Curvature Terms: Absence of vDVZ Discontinuity in AdS Space

We argue that the van Dam-Veltman-Zakharov discontinuity arising in the $M^2 \to 0$ limit of the massive graviton through an explicit Pauli-Fierz mass term could be absent in anti de Sitter space. This is possible if the graviton can acquire mass spontaneously from the higher curvature terms or/and the massless limit $M^2\to 0$ is attained faster than the cosmological constant $Λ\to 0$. We discuss the effects of higher-curvature couplings and of an explicit cosmological term ($Λ$) on stability of such continuity and of massive excitations.

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arXiv2001

Gravitational potential correction with Gauss-Bonnet interaction

A treatment of linearized gravity with the Einstein-Gauss-Bonnet interaction terms in $D\geq$ 5 is given in the Randall-Sundrum brane background. This Letter has outlined some interesting features of the brane world gravity and Newtonian potential correction with the GB interaction term. We find that the GB coupling $α$ renormalizes the effective four-dimensional Newton constant on the brane, and also additionally contributes to the correction term of the Newtonian potential. Indeed, the GB term does not affect the massless graviton mode and the Einstein gravity on the brane, and quite interestingly, such term in $D\geq 5$ appears to give more information about the necessary boundary condition(s) to be satisfied by the zero mode wavefunction on the brane(s). small (and preferably positive) GB coupling $α$.

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