Browsing by Author "Shtanov, Yuri"

Now showing 1 - 12 of 12

APSIS - an Artificial Planetary System in Space to probe extra-dimensional gravity and MOND
(2008-01) Sahni, Varun; Shtanov, Yuri
A proposal is made to test Newton’s inverse-square law using the perihelion shift of test masses (planets) in free fall within a spacecraft located at the Earth–Sun L2 point. Such an Artiﬁcial Planetary System In Space (APSIS) will operate in a dragfree environment with controlled experimental conditions and minimal interference from terrestrial sources of contamination. We demonstrate that such a space experiment can probe the presence of a ‘hidden’ ﬁfth dimension on the scale of a micron, if the perihelion shift of a ‘planet’ can be measured to sub-arc-second accuracy. Some suggestions for spacecraft design are made.
Bouncing Braneworlds
(2011-07-05) Shtanov, Yuri; Sahni, Varun
We study cosmological braneworld models with a single timelike extra di- mension. Such models admit the intriguing possibility that a contracting braneworld experiences a natural bounce without ever reaching a singular state. This feature persists in the case of anisotropic braneworlds under some additional and not very restrictive assumptions. Generalizing our study to braneworld models containing an induced brane curvature term, we ﬁnd that a FRW-type singularity is once again absent if the bulk extra dimension is timelike. In this case, the universe either has a non-singular origin or com- mences its expansion from a quasi-singular state during which both the Hubble parameter and the energy density and pressure remain ﬁnite while the cur- vature tensor diverges. The non-singular and quasi-singular behaviour which we have discovered diﬀers both qualitatively and quantitatively from what is usually observed in braneworld models with spacelike extra dimensions and could have interesting cosmological implications.
Brane corresponding to the nariai bulk
(2011-07-05) Dadhich, Naresh; Shtanov, Yuri
We consider the ﬁve-dimensional bulk spacetime with negative Λ described by the Nariai metric (which is not conformally ﬂat) and match it with a vacuum brane satisfying the proper boundary conditions. It is shown that the brane metric corresponds to a cloud of string dust of constant energy density.
Braneworld dynamics in Einstein Gauss Bonnet gravity
(2007-10-10) Maeda, Hideki; Sahni, Varun; Shtanov, Yuri
We discuss the cosmological evolution of a braneworld in ﬁve dimensional Gauss– Bonnet gravity. Our discussion allows the ﬁfth (bulk) dimension to be space-like as well as time-like. The resulting equations of motion have the form of a cubic equation in the H2, (ρ + σ)2 plane, where σ is the brane tension and ρ is the matter density. This allows us to conduct a comprehensive pictorial analysis of cosmological evolution for the Gauss–Bonnet brane. The many interesting properties of this braneworld include the possibility of accelerated expansion at late times. For a ﬁnite region in parameter space the accelerated expansion can be phantom-like so that w < −1. At late times, this branch approaches de Sitter space (w = −1) and avoids the big-rip singularities usually present in phantom models. For a time-like extra dimension the Gauss–Bonnet brane can bounce and avoid the initial singularity.
Braneworld models of dark energy
(2011-07-06) Sahni, Varun; Shtanov, Yuri
We explore a new class of braneworld models in which the scalar curvature of the (induced) brane metric contributes to the brane action. The scalar curvature term arises generically on account of one-loop eﬀects induced by matter ﬁelds residing on the brane. Spatially ﬂat braneworld models can en- ter into a regime of accelerated expansion at late times. This is true even if the brane tension and the bulk cosmological constant are tuned to satisfy the Randall–Sundrum constraint on the brane. Braneworld models admit a wider range of possibilities for dark energy than standard LCDM. In these models the luminosity distance can be both smaller and larger than the lu- minosity distance in LCDM. Whereas models with dL ≤ dL(LCDM) imply w = p/ρ ≥ −1 and have frequently been discussed in the literature, mod- els with dL > dL(LCDM) have traditionally been ignored, perhaps because within the general-relativistic framework, the luminosity distance has this property only if the equation of state of matter is strongly negative (w < −1). Within the conventional framework, ‘phantom energy’ with w < −1 is beset with a host of undesirable properties, which makes this model of dark en- ergy unattractive. Braneworld models, on the other hand, have the capacity to endow dark energy with exciting new possibilities (including w < −1) without suﬀering from the problems faced by phantom energy. For a subclass of parameter values, braneworld dark energy and the acceleration of the universe are transient phenomena. In these models, the universe, after the current period of acceleration, re-enters the matter-dominated regime so that the deceleration parameter q(t) → 0.5 when t ≫ t0, where t0 is the present epoch. Such models could help reconcile an accelerating universe with the requirements of string/M-theory.
Cosmic acceleration and extra dimensions
(2008-11) Sahni, Varun; Shtanov, Yuri
Brane cosmology presents many interesting possibilities including: phantom acceleration (w < −1), self-acceleration, uniﬁcation of dark energy with inﬂation, transient acceleration, loitering cosmology, new singularities at which the Hubble parameter remains ﬁnite, cosmicmimicry, etc. The existence of a time-like extra dimension can result in a singularity-free cyclic cosmology
Cosmic mimicry: Is LCDM a braneworld in disguise?
(2006-01-01) Sahni, Varun; Shtanov, Yuri; Viznyuk, Alexander
Generalizing the cosmic energy equation
(2011-01-01) Shtanov, Yuri; Sahni, Varun
We generalize the cosmic energy equation to the case when massive particles in- teract via a modified gravitational potential of the form .(a, r), which is allowed to explicitly depend on the cosmological time through the expansion factor a(t). Using the nonrelativistic approximation for particle dynamics, we derive the equation for the cosmological expansion which has the form of the Friedmann equation with a renormalized gravitational constant. The generalized Layzer–Irvine cosmic energy equation and the associated cosmic virial theorem are applied to some recently pro- posed modifications of the Newtonian gravitational interaction between dark-matter particles. We also draw attention to the possibility that the cosmic energy equation may be used to probe the expansion history of the universe thereby throwing light on the nature of dark matter and dark energy.
Gravitational instability on the brane: the role of boundary conditions
(2007-11-12) Shtanov, Yuri; Viznyuk, Alexander; Sahni, Varun
An outstanding issue in braneworld theory concerns the setting up of proper boundary conditions for the brane–bulk system. Boundary conditions (BC’s) employing regulatory branes or demanding that the bulk metric be nonsingular have yet to be implemented in full generality. In this paper, we take a diﬀerent route and specify boundary conditions directly on the brane thereby arriving at a local and closed system of equations (on the brane). We consider a one-parameter family of boundary conditions involving the anisotropic stress of the projection of the bulk Weyl tensor on the brane and derive an exact system of equations describing scalar cosmological perturbations on a generic braneworld with induced gravity. Depending upon our choice of boundary conditions, perturbations on the brane either grow moderately (region of stability) or rapidly (instability). In the instability region, the evolution of perturbations usually depends upon the scale: small scale perturbations grow much more rapidly than those on larger scales. This instability is caused by a peculiar gravitational interaction between dark radiation and matter on the brane. Generalizing the boundary conditions obtained by Koyama and Maartens, we ﬁnd for the Dvali–Gabadadze–Porrati model an instability, which leads to a dramatic scale-dependence of the evolution of density perturbations in matter and dark radiation. A diﬀerent set of BC’s, however, leads to a more moderate and scale-independent growth of perturbations. For the mimicry braneworld, which expands like LCDM, this class of BC’s can lead to an earlier epoch of structure formation.
New vistas in braneworld cosmology
(2002-03-21) Sahni, Varun; Shtanov, Yuri
Traditionally, higher-dimensional cosmological models have sought to pro- vide a description of the fundamental forces in terms of a unifying geometri- cal construction. In this essay we discuss how, in their present incarnation, higher-dimensional ‘braneworld’ models might provide answers to a number of cosmological puzzles including the issue of dark energy and the nature of the big-bang singularity
Quantum effects, soft singularities and the fate of the universe in a braneworld cosmology
(2006-03-24) Tretjakov, P. V.; Toporensky, A.; Shtanov, Yuri; et al.
Recycling the universe using scalar fields
(2001-07-02) Kanekar, Nissim; Sahni, Varun; Shtanov, Yuri
We examine the behaviour of a closed oscillating universe ﬁlled with a homogeneous scalar ﬁeld and ﬁnd that, contrary to naive expectations, such a universe expands to larger vol- umes during successive expansion epochs. This intriguing be- haviour introduces an arrow of time in a system which is time- reversible. The increase in the maximum size of the universe s closely related to the work done on/by the scalar ﬁeld dur- ng one complete oscillatory cycle which, in turn, is related to the asymmetry in the scalar ﬁeld equation of state dur- ng expansion and collapse. Our analysis shows that scalar ﬁelds with polynomial potentials V (φ) = λφq , q > 1 lead to a growing oscillation amplitude for the universe: the increase n amplitude between successive oscillations is more signiﬁ- cant for smaller values of q. Such behaviour allows for the eﬀective recycling of the universe. A recycled universe can be quite old and can resolve the ﬂatness problem. These results have strong bearing on cosmological models in which the role of dark matter is played by a scalar ﬁeld. They are also rele- vant for chaotic inﬂationary models of the early universe since they demonstrate that, even if the universe fails to inﬂate the ﬁrst time around, it will eventually do so during future oscil- atory cycles. Thus, the space of initial conditions favourable or chaotic inﬂation increases signiﬁcantly.