Browsing by Author "Sahni, Varun"

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Anisotropy dissipation in brane-world inflation
(2001-02-14) Maartens, Roy; Sahni, Varun; Saini, Tarun Deep
We examine the behavior of an anisotropic brane-world in the presence of inﬂationary scalar ﬁelds. We show that, contrary to naive expectations, a large anisotropy does not adversely aﬀect inﬂation. On the contrary, a large initial anisotropy introduces more damping into the scalar ﬁeld equation of motion, resulting in greater inﬂation. The rapid decay of anisotropy in the brane-world signiﬁcantly increases the class of initial conditions from which the observed universe could have originated. This generalizes a similar result in general relativity. A unique feature of Bianchi I brane-world cosmology appears to be that for scalar ﬁelds with a large kinetic term the initial expansion of the universe is quasi-isotropic. The universe grows more anisotropic during an intermediate transient regime until anisotropy ﬁnally disappears during inﬂationary expansion.
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.
Behaviour of lagrangian approximations in spherical voids
(2015-02-07) Sahni, Varun; Shandarin, S.F.
We study the behaviour of spherical voids in Lagrangian perturbation theories L(n), of which the Zel'dovich approximation is the lowest order solution L(1). We find that at early times higher order L(n), give an increasingly accurate picture of void expansion. However, at late times particle trajectories in L (2) begin to turnaround and converge leading to the contraction of a void, a sign of pathological behaviour. By contrast particle trajectories in L(3) are well behaved and this approximation gives results in excellent agreement with the exact top-hat solution as long as the void is not too underdense. For very underdense voids, L (3) evacuates the void much too rapidly, leading us to conclude that the Zel'dovich approximation L(1) remains the best approximation to apply to the late-time study of voids. The behaviour of high-order Lagrangian approximations in spherical voids is typical for asymptotic (semiconvergent) series and may be generic for Lagrangian perturbation theory.
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.
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.
Can dark energy be decaying?
(2011-07-05) Ujjaini, Alam; Sahni, Varun; Starobinsky, A. A.
We explore the fate of the universe given the possibility that the density associated with ‘dark energy’ may decay slowly with time. Decaying dark energy is modeled by a homogeneous scalar ﬁeld which couples minimally to gravity and whose potential has at least one local quadratic maximum. Dark energy decays as the scalar ﬁeld rolls down its potential, consequently the current acceleration epoch is a transient. We examine two models of decaying dark energy. In the ﬁrst, the dark energy potential is modeled by an analytical form which is generic close to the potential maximum. The second potential is the cosine, which can become negative as the ﬁeld evolves, ensuring that a spatially ﬂat universe collapses in the future. We examine the feasibility of both models using observations of high redshift type Ia supernovae. A maximum likelihood analysis is used to ﬁnd allowed regions in the {m, φ0} plane (m is the tachyon mass modulus and φ0 the initial scalar ﬁeld value; m ∼ H0 and φ0 ∼ MP by order of magnitude). For the ﬁrst model, the time for the potential to drop to half its maximum value is larger than ∼ 8 Gyrs. In the case of the cosine potential, the time left until the universe collapses is always greater than ∼ 18 Gyrs (both estimates are presented for Ω0m = 0.3, m/H0 ∼ 1, H0 ≃ 70 km/sec/Mpc, and at the 95.4% conﬁdence level).
Case for a Positive Cosmological Lambda-term
(2000-04-25) Sahni, Varun; Starobinsky, A. A.
Recent observations of Type 1a supernovae indicating an accelerat- ing universe have once more drawn attention to the possible existence, at the present epoch, of a small positive Λ-term (cosmological constant). In this paper we review both observational and theoretical aspects of a small cosmological Λ-term. We discuss the current observational situation focusing on cosmolog- ical tests of Λ including the age of the universe, high redshift supernovae, gravitational lensing, galaxy clustering and the cosmic microwave background. We also review the theoretical debate surrounding Λ: the generation of Λ in models with spontaneous symmetry breaking and through quantum vacuum polarization eﬀects – mechanisms which are known to give rise to a large value of Λ hence leading to the ‘cosmological constant problem’. More recent at- tempts to generate a small cosmological constant at the present epoch using either ﬁeld theoretic techniques, or by modeling a dynamical Λ-term by scalar ﬁelds are also extensively discussed. Anthropic arguments favouring a small Λ-term are brieﬂy reviewed. A comprehensive bibliography of recent work on Λ is provided.
Case for dynamical dark energy revisited
(2011-07-06) Alam, Ujjaini; Sahni, Varun; Starobinsky, A. A.
We investigate the behaviour of dark energy using the recently released supernova data of Riess et al., 2004 and a model independent parameterization for dark energy (DE). We ﬁnd that, if no priors are imposed on Ω0m and h, DE which evolves with time provides a better ﬁt to the SNe data than ΛCDM. This is also true if we include results from the WMAP CMB data. From a joint analysis of SNe+CMB, the best-ﬁt DE model has w0 < ∼ − 1 at the present epoch and the transition from deceleration to acceleration occurs at zT = 0.39±0.03. However, DE evolution becomes weaker if the ΛCDM based CMB results Ω0m = 0.27 ± 0.04, h = 0.71 ± 0.06 are incorporated in the analysis. In this case, zT = 0.57±0.07. Our results also show that the extent of DE evolution is sensitive to the manner in which the supernova data is sampled.
Confronting braneworld cosmology with supernova data and baryon oscillations
(2006-03-18) Alam, Ujjaini; Sahni, Varun
Braneworld cosmology has several attractive and distinctive features. For instance the eﬀective equation of state in braneworld models can be both quintessence-like (w0 ≥ −1) as well as phantom like (w0 ≤ −1). Models with w0 ≥ −1 (w0 ≤ −1) are referred to as Brane 2 (Brane 1) and correspond to complementary embeddings of the brane in the bulk. (The equation of state in Brane 1 can successfully cross the ‘phantom divide’ at w = −1.) In this paper we compare the predictions of braneworld models to two recently released supernova data sets: the ‘Gold’ data (Riess et al., 2004) and the data from the Supernova Legacy Survey (SNLS) (Astier et al., 2005). We also incorporate the recent discovery of the baryon acoustic peak in the Sloan Digital Sky Survey (Eisenstein et al., 2005) into our analysis. Our main results are that braneworld models satisfy both sets of SNe data. Brane 1 (with w0 ≤ −1) shows very good agreement with data for values of the matter density bounded from below: Ω0m > ∼ 0.25 (Gold) and Ω0m > ∼ 0.2 (SNLS). On the other hand Brane 2 (with w0 ≥ −1) shows excellent agreement with data for values of the matter density which are bounded from above: Ω0m < ∼ 0.45 (Gold) and Ω0m < ∼ 0.35 (SNLS). The DGP model is excluded at 3σ by SNLS and at 1σ by the Gold dataset. Braneworld models with future ‘quiescent’ singularities (at which the Hubble parameter and the matter density remain ﬁnite but higher derivatives of the expansion factor diverge) are excluded by both datasets.
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
Cosmological constant problem and quintessence
(2011-07-06) Sahni, Varun
I brieﬂy review the cosmological constant problem and the issue of dark energy (or quintessence). Within the framework of quantum ﬁeld theory, the vacuum expectation value of the energy momentum tensor formally diverges as k4. A cutoﬀ at the Planck or electroweak scale leads to a cosmological constant which is, respectively, 10123 or 1055 times larger than the observed value, Λ/8πG ≃ 10−47 GeV4. The absence of a fundamental symmetry which could set the value of Λ to either zero or a very small value leads to the cosmological constant problem. Most cosmological scenario’s favour a large time-dependent Λ-term in the past (in order to generate inﬂation at z ≫ 1010), and a small Λ-term today, to account for the current acceleration of the universe at z < ∼ 1. Constraints arising from cosmological nucleosynthesis, CMB and structure formation constrain Λ to be sub-dominant during most of the intermediate epoch 1010 < z < 1. This leads to the cosmic coincidence conundrum which suggests that the acceleration of the universe is a recent phenomenon and that we live during a special epoch when the density in Λ and in matter are almost equal. Time varying models of dark energy can, to a certain extent, ameliorate the ﬁne tuning problem (faced by Λ), but do not resolve the puzzle of cosmic coincidence. I brieﬂy review tracker models of dark energy, as well as more recent brane inspired ideas and the issue of horizons in an accelerating universe. Model independent methods which reconstruct the cosmic equation of state from supernova observations are also assessed. Finally, a new diagnostic of dark energy – ‘Stateﬁnder’, is discussed.
Cosmological Surprises from Braneworld models of Dark Energy
(2005-04-01) Sahni, Varun
Properties of Braneworld models of dark energy are reviewed. Braneworld models admit the following interesting possibilities: (i) The eﬀective equation of state can be w ≤ −1 as well as w ≥ −1. In the former case the expansion of the universe is well behaved at all times and the universe does not run into a future ‘Big Rip’ singularity which is usually encountered by Phantom models. (ii) For a class of Braneworld models the acceleration of the universe can be a transient phenomenon. In this case the current acceleration of the universe is sandwiched between two matter dominated epochs. Such a braneworld does not have a horizon in contrast to LCDM and most Quintessence models. (iii) For a speciﬁc set of parameter values the universe can either originate from, or end its existence in a Quiescent singularity, at which the density, pressure and Hubble parameter remain ﬁnite, while the deceleration parameter and all invariants of the Riemann tensor diverge to inﬁnity within a ﬁnite interval of cosmic time. (iv) Braneworld models of dark energy can loiter at high redshifts: 6 < ∼ z < ∼ 40. The Hubble parameter decreases during the loitering epoch relative to its value in LCDM. As a result the age of the universe at loitering dramatically increases and this is expected to boost the formation of high redshift gravitationally bound systems such as 109 M⊙ black holes at z ∼ 6 and lower-mass black holes and/or Population III stars at z > 10, whose existence could be problematic within the LCDM scenario. (v) Braneworld models with a time-like extra dimension bounce at early times thereby avoiding the initial ‘Big Bang singularity’. (vi) Both Inﬂation and Dark Energy can be successfully uniﬁed within a single scheme (Quintessential Inﬂation)
Cosmology
(2013-07-16) Sahni, Varun; Souradeep, Tarun
Daha vaidnyanikanna India citation award
(2012-05-01) Padmanabhan, T.; Sahni, Varun
Dark energy
(2006-01-10) Sahni, Varun
The cosmological constant problem as well as the case for dark energy are briefly reviewed and some theoretical models of dark energy are discussed in detail. These include: the cosmological constant, quintessence, the Chaplygin gas and Braneworld models. I also discuss model independent measures of dark energy and conclude by mentioning some properties of the Statefinder diagnostic which can successfully differentiate between different families of dark energy models.
Dark matter and dark energy
(2011-07-06) Sahni, Varun
Abstract. I brieﬂy review our current understanding of dark matter and dark en- ergy. The ﬁrst part of this paper focusses on issues pertaining to dark matter includ- ing observational evidence for its existence, current constraints and the ‘abundance of substructure’ and ‘cuspy core’ issues which arise in CDM. I also brieﬂy describe MOND. The second part of this review focusses on dark energy. In this part I dis- cuss the signiﬁcance of the cosmological constant problem which leads to a predicted value of the cosmological constant which is almost 10123 times larger than the ob- served value Λ/8πG ≃ 10−47 GeV4 . Setting Λ to this small value ensures that the acceleration of the universe is a fairly recent phenomenon giving rise to the ‘cosmic coincidence’ conundrum according to which we live during a special epoch when the density in matter and Λ are almost equal. Anthropic arguments are brieﬂy dis- cussed but more emphasis is placed upon dynamical dark energy models in which the equation of state is time dependent. These include Quintessence, Braneworld models, Chaplygin gas and Phantom energy. Model independent methods to deter- mine the cosmic equation of state and the Stateﬁnder diagnostic are also discussed. The Stateﬁnder has the attractive property ... a /aH3 = 1 for LCDM, which is helpful for diﬀerentiating between LCDM and rival dark energy models. The review ends with a brief discussion of the fate of the universe in dark energy models.
Density perturbations, gravity waves and the cosmic microwave background
(2014-07-30) Souradeep, Tarun; Sahni, Varun
Did the Universe loiter at high redshifts?
(2005-04-01) Sahni, Varun
We show that loitering at high redshifts (z > ∼ 6) can easily arise in braneworld models of dark energy which, in addition to being spatially ﬂat, also accelerate at late times. Loitering is characterized by the fact that the Hubble parameter dips in value over a narrow redshift range which we shall refer to as the ‘loitering epoch’. During loitering, density perturbations are expected to grow rapidly. In addition, since the expansion of the universe slows down, its age near loitering dramatically increases. An early epoch of loitering is expected to boost the formation of high redshift gravitationally bound systems such as 109M⊙ black holes at z ∼ 6 and lower-mass black holes and/or Population III stars at z > 10, whose existence could be problematic within the LCDM scenario. Loitering models also help to reduce the redshift of reionization from its currently (high) value of zreion ≃ 17 in LCDM cosmology, thus alleviating a signiﬁcant source of tension between observations of the high-redshift universe and theoretical model building. Currently a loitering universe accelerates with an eﬀective equation of state w < −1 thus mimicking phantom dark energy. Unlike phantom, however, the late-time expansion of the universe in our model is singularity free, and a universe that loitered in the past will approach a LCDM model symptotically in the distant future.