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Subject: search.filters.subject.Dark energy

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    Machian model of dark energy
    (2002-03-02) Vishwakarma, R. G.
    Einstein believed that Mach’s principle should play a major role in finding a meaningful spacetime geometry, though it was discovered later that his field equations gave some solutions which were not Machian. It is shown, in this essay, that the kinematical Λ mod- els, which are invoked to solve the cosmological constant problem, are in fact consistent with Mach’s ideas. One particular model in this category is described which results from the microstructure of space- time and seems to explain the current observations successfully and also has some benefits over the conventional models. This forces one to think whether the Mach’s ideas and the cosmological constant are interrelated in some way.
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    Theoretical models of dark energy
    (2002-04-01) Sahni, Varun
    Observations of high redshift type Ia supernovae indicate that the universe is accelerating, fueled by an unknown form of dark energy having large negative pressure p < 0. The simplest example of dark energy is the cosmological constant (p ¼ q K=8pGÞ. The cosmological constant arises at a fundamental level from one-loop quantum effects which generate a K-term many orders of magnitude larger than the observed value of dark energy 10 47 GeV4 . This leads to the cosmological constant problem . Dynamical models of dark energy include scalar fields with exponential and power law potentials. Dark energy can also be generated in extra-dimensional braneworld models. Model-inde- pendent methods which attempt to reconstruct dark energy from supernova observations are discussed. 2002 Elsevier Science Ltd. All rights reserved
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    Statefinder - anew geometrical diagnostic of dark energy
    (2002-02-21) Sahni, Varun; Saini, Tarun Deep; Starobinsky, A. A.
    We introduce a new cosmological diagnostic pair {r, s} called Statefinder. The Statefinder is a geometrical diagnostic and allows us to characterize the properties of dark energy in a model independent manner. The Statefinder is dimensionless and is constructed from the scale factor of the Universe and its time derivatives only. The parameter r forms the next step in the hierarchy of geometrical cosmological parameters after the Hubble parameter H and the deceleration parameter q, while s is a linear combination of q and r chosen in such a way that it does not depend upon the dark energy density. The Statefinder pair {r, s} is algebraically related to the equation of state of dark energy and its first time derivative. The Statefinder pair is calculated for a number of existing models of dark energy having both constant and variable w. For the case of a cosmological constant the Statefinder acquires a particularly simple form. We demonstrate that the Statefinder diagnostic can effectively differentiate between different forms of dark energy. We also show that the mean Statefinder pair can be determined to very high accuracy from a SNAP-type experiment.
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    Consequences on some dark energy-candidates from SN 1997ff
    (2001-07-28) Vishwakarma, R. G.
    We examine the status of various dark energy-models in light of the recently observed SN 1997ff at z ≈ 1.7. The modified data still fit a pure cosmological constant Λ or a quintessence with an equation of state similar to that of Λ. The kinematical Λ-models, Λ ∼ S−2 and Λ ∼ H2 , also fit the data reasonably well and require less dark energy density (hence more matter energy density) than is required by the constant Λ-model. However, the model Λ ∼ S−2 with low energy density becomes unphysical as it cannot accommodate higher redshift objects. We also examine an alternative explanation of the data, viz., the absorp- tion by the intervening whisker-like dust and find that the quasi-steady state model and the FRW model Ωm0 = 0.33, without any dark energy also fit the data reasonably well. We notice that the addition of SN 1997ff to the old data has worsened the fit to most of the models, except a closed FRW model with a constant Λ and a closed quintessence-model with ωφ = −0.82, and the models have started departing from each other as we go above z = 1. However, to make a clear discrimination possible, a few more supernovae with z > 1 are required. We have also calculated the age of the universe in these models and find that, in the models with a constant Λ, the expansion age is uncomfortably close to the age of the globular clusters. Quintessence-models show even lower age. The kinematical Λ-models are, however, interesting in this con- nection (especially the model Λ ∼ H2 ), which give remarkably large age of the universe.
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    Cosmology with tachyon field as dark energy
    (2011-07-06) Bagla, J. S.; Jassal, H. K.; Padmanabhan, T.
    We present a detailed study of cosmological effects of homogeneous tachyon matter coexisting with non-relativistic matter and radiation, concentrating on the inverse square potential and the expo- nential potential for the tachyonic scalar field. A distinguishing feature of these models (compared to other cosmological models) is that the matter density parameter and the density parameter for tachyons remain comparable even in the matter dominated phase. For the exponential potential, the solutions have an accelerating phase, followed by a phase with a(t) ∝ t 2/3 as t → ∞. This elimi- nates the future event horizon present in ΛCDM models and is an attractive feature from the string theory perspective. A comparison with supernova Ia data shows that for both the potentials there exists a range of models in which the universe undergoes an accelerated expansion at low redshifts and are also consistent with requirements of structure formation. They do require fine tuning of parameters but not any more than in the case of ΛCDM or quintessence models.
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    Cosmological constant - The weight of the vacuum
    (2011-07-06) Padmanabhan, T.
    Recent cosmological observations suggest the existence of a positive cosmological constant Λ with the magnitude Λ(G~/c3) ≈ 10−123. This review discusses several aspects of the cosmological constant both from the cosmological (sections 1–6) and field theoretical (sections 7–11) perspectives. After a brief introduction to the key issues related to cosmological constant and a historical overview, a summary of the kinematics and dynamics of the standard Friedmann model of the universe is provided. The observational evidence for cosmological constant, especially from the supernova results, and the constraints from the age of the universe, structure for- mation, Cosmic Microwave Background Radiation (CMBR) anisotropies and a few others are described in detail, followed by a discussion of the theoretical models (quintessence, tachyonic scalar field, ...) from different perspectives. The latter part of the review (sections 7–11) concentrates on more conceptual and fundamental aspects of the cosmological constant like some alternative interpretations of the cosmological constant, relaxation mechanisms to reduce the cosmological constant to the currently observed value, the geometrical structure of the de Sitter space- time, thermodynamics of the de Sitter universe and the role of string theory in the cosmological constant problem.
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    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.
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    Is the present expansion of the universe really accelerating?
    (2011-07-05) Vishwakarma, R. G.
    The current observations are usually explained by an accelerating ex- pansion of the present universe. However, with the present quality of the supernovae Ia data, the allowed parameter space is wide enough to accommodate the decelerating models as well. This is shown by considering a particular example of the dark energy equation-of-state wφ ≡ pφ/ρφ = −1/3, which is equivalent to modifying the geometrical curvature index k of the standard cosmology by shifting it to (k − α) where α is a constant. The resulting decelerating model is consistent with the recent CMB observations made by WMAP, as well as, with the high redshift supernovae Ia data including SN 1997ff at z = 1.755. It is also consistent with the newly discovered supernovae SN 2002dc at z = 0.475 and SN 2002dd at z = 0.95 which have a general tendency to improve the fit.
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    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).
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    What is needed of a tachyon if it is to be the dark energy?
    (2011-07-06) Copeland, E. J.; Garousi, M. R.; Sami, M.; et al.
    We study a dark energy scenario in the presence of a tachyon field φ with potential V (φ) and a barotropic perfect fluid. The cosmological dynamics crucially depends on the asymptotic behavior of the quantity λ = −MpVφ/V 3/2 . If λ is a constant, which corresponds to an inverse square potential V (φ) ∝ φ−2 , there exists one stable critical point that gives an acceleration of the universe at late times. When λ → 0 asymptotically, we can have a viable dark energy scenario in which the system approaches an “instantaneous” critical point that dynamically changes with λ. If |λ| approaches infinity asymptotically, the universe does not exhibit an acceleration at late times. In this case, however, we find an interesting possibility that a transient acceleration occurs in a regime where |λ| is smaller than of order unity.