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Anirudh Pradhan - One of the best experts on this subject based on the ideXlab platform.
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anisotropic bulk viscous string Cosmological Models of the universe under a time dependent deceleration parameter
Pramana, 2020Co-Authors: Archana Dixit, Anirudh PradhanAbstract:We investigate a new class of LRS Bianchi type-II Cosmological Models by revisiting the paper of Mishra et al (Int. J. Theor. Phys. 52, 2546 (2013)) by considering a new deceleration parameter (DP) depending on the time in string cosmology for the modified gravity theory suggested by Saez–Ballester (Phys. Lett. 113, 467 (1986)). We have considered the energy–momentum tensor proposed by Letelier (Phys. Rev. 28, 2414 (1983)) for bulk viscous and perfect fluid under some assumptions. To make our Models consistent with recent astronomical observations, we have used the scale factor (Sharma et al, Astron Astrophys. 19, 55 (2018), Garg et al, Int. J. Geo. Meth. Mod. Phys. 16, 1950007 (2019)) $$a(t)=\exp {[\frac{1}{\beta }\sqrt{2 \beta t + k}]}$$, where $$\beta $$ and k are positive constants and it provides a time-varying DP. By using the recent constraints ($$H_{0}=73.8$$ and $$q_{0} = -0.54$$) from SN Ia data in combination with BAO and CMB observations (Giostri et al, JCAP 3, 27 (2012), arXiv:1203.3213v2[astro-ph.CO]), we affirm $$\beta = 0.0062$$ and $$k = 0.000016$$. For these constraints, we have substantiated a new class of Cosmological transit Models for which the expansion takes place from the early decelerated phase to the current accelerated phase. Also, we have studied some physical, kinematic and geometric behaviour of the Models, and have found them consistent with observations and well-established theoretical results. We have also compared our present results with those of Mishra et al (Int. J. Theor. Phys. 52, 2546 (2013)) and observed that the results in this paper are much better, stable under perturbation and in good agreement with Cosmological reflections.
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anisotropic bulk viscous string Cosmological Models of the universe under a time dependent deceleration parameter
arXiv: General Physics, 2019Co-Authors: Archana Dixit, Anirudh PradhanAbstract:We investigate a new class of LRS Bianchi type-II Cosmological Models by revisiting in the paper of Mishra {\it et al} (2013) by considering a new deceleration parameter (DP) depending on the time in string cosmology for the modified gravity theory suggested by S$\acute{a}$ez \& Ballester (1986). We have considered the energy-momentum tensor proposed by Leteliar (1983) for bulk viscous and perfect fluid under some assumptions. To make our Models consistent with recent astronomical observations, we have used scale factor (Sharma {\it et al} 2018; Garg {\it et al} 2019) $ a(t)=\exp{[\frac{1}{\beta}\sqrt{2 \beta t + k}]}$, where $\beta $ and $k$ are positive constants and it provides a time-varying DP. By using the recent constraints ($H_{0}=73.8$, and $q_{0} = -0.54$) from SN Ia data in combination with BAO and CMB observations (Giostri {\it et al}, arXiv:1203.3213v2[astro-ph.CO]), we affirm $\beta = 0.0062$ and $k = 0.000016$. For these constraints, we have substantiated a new class of Cosmological transit Models for which the expansion takes place from early decelerated phase to the current accelerated phase. Also, we have studied some physical, kinematic and geometric behavior of the Models, and have found them consistent with observations and well established theoretical results . We have also compared our present results with those of Mishra {\it et al} (2013) and observed that the results in this paper are much better, stable under perturbation and in good agreement with Cosmological reflections.
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frw Cosmological Models in brans dicke theory of gravity with variable q and dynamical varlambda term
Astrophysics and Space Science, 2016Co-Authors: Avtar Chand, R K Mishra, Anirudh PradhanAbstract:Exact solution of modified Einstein’s field equations are considered within the scope of spatially homogeneous and isotropic Fraidmann-Robertson-Walker (FRW) space-time filled with perfect fluid in the frame work of Brans-Dicke scalar-tensor theory of gravity. In this paper we have investigated the flat, open and closed FRW Models and the effect of dynamic Cosmological term on the evolution of the universe. Two types of FRW Cosmological Models are obtained by setting the power law between the scalar field $\phi$ and the scale factor $a$ and deceleration parameter (DP) $q$ as a time dependent. The concept of time dependent DP with some proper assumptions yield two type of the average scale factors (i) $a(t)=[\sinh(\alpha t)]^{\frac{1}{n}}$ and (ii) $a(t)=[t^{\alpha}e^{t}]^{\frac{1}{n}}$ , $\alpha$ and $n\neq 0$ are arbitrary constants. In case (i), for $0 < n \leq 1$ , it generates a class of accelerating Models while for $n > 1$ , the Models of the universe exhibit phase transition from early decelerating to present accelerating phase and the transition redshift $z_{t}$ has been calculated and found to be in good agreement with the results from recent astrophysical observations. In case (ii), for $n \geq 2$ and $\alpha = 1$ , we obtain a class of transit Models of the universe from early decelerating to present accelerating phase. Taking into consideration the observational data, we conclude that the Cosmological constant behaves as a positive decreasing function of time. The physical and geometric properties of the Models are also discussed with the help of graphical presentations.
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string Cosmological Models from early deceleration to current acceleration phase with varying g and lambda
European Physical Journal Plus, 2012Co-Authors: Chanchal Chawla, R K Mishra, Anirudh PradhanAbstract:Thepresent study deals with spatially homogeneous and anisotropic Bianchi-I Cosmological Models representing massive strings with variable G and decaying vacuum energy density \( \Lambda\) . The energy-momentum tensor, as formulated by Letelier (Phys. Rev. D 20, 1294 (1979); Phys. Rev. D 28, 2414 (1983)), has been used to construct massive string Cosmological Models for which we assume the expansion scalar in the Models is proportional to one of the components of shear tensor and barotropic EoS. The Einstein field equations have been solved by considering the time-dependent deceleration parameter which yields a scale factor \( a(t) = (\sinh(\alpha t))^{\frac{1}{n}}\) , where n is a positive constant. For n > 1 , this generates a transition of the Universe from the early decelerating phase to the recent accelerating phase and the transition redshift zt has been calculated. The study reveals that massive strings dominate the early Universe evolving with deceleration and in the later phase they disappear, which is in good agreement with current astronomical observations. The Cosmological constant \( \Lambda\) is found to be a positive decreasing function of time which is corroborated by results from recent Supernovae Ia observations. The physical and geometric properties of the Models have been also discussed in detail.
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bianchi type ii string Cosmological Models in normal gauge for lyra s manifold with constant deceleration parameter
Indian Journal of Physics, 2012Co-Authors: Shilpi Agarwal, Rajesh Pandey, Anirudh PradhanAbstract:The present study deals with a spatially homogeneous and anisotropic Bianchi-II Cosmological Models representing massive strings in normal gauge for Lyra’s manifold by applying the variation law for generalized Hubble’s parameter that yields a constant value of deceleration parameter. The variation law for Hubble’s parameter generates two types of solutions for the average scale factor, one is of power-law type and other is of the exponential form. Using these two forms, Einstein’s modified field equations are solved separately that correspond to expanding singular and non-singular Models of the universe respectively. The energy-momentum tensor for such string as formulated by Letelier (1983) is used to construct massive string Cosmological Models for which we assume that the expansion (�) in the model is proportional to the component � 1 1 of the shear
Christian Wagner - One of the best experts on this subject based on the ideXlab platform.
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the coyote universe iii simulation suite and precision emulator for the nonlinear matter power spectrum
The Astrophysical Journal, 2010Co-Authors: Earl Lawrence, Katrin Heitmann, David Higdon, Martin White, Salman Habib, Christian Wagner, Brian J WilliamsAbstract:Many of the most exciting questions in astrophysics and cosmology, including the majority of observational probes of dark energy, rely on an understanding of the nonlinear regime of structure formation. In order to fully exploit the information available from this regime and to extract Cosmological constraints, accurate theoretical predictions are needed. Currently, such predictions can only be obtained from costly, precision numerical simulations. This paper is the third in a series aimed at constructing an accurate calibration of the nonlinear mass power spectrum on Mpc scales for a wide range of currently viable Cosmological Models, including dark energy Models with w ≠ –1. The first two papers addressed the numerical challenges and the scheme by which an interpolator was built from a carefully chosen set of Cosmological Models. In this paper, we introduce the "Coyote Universe" simulation suite which comprises nearly 1000 N-body simulations at different force and mass resolutions, spanning 38 w CDM cosmologies. This large simulation suite enables us to construct a prediction scheme, or emulator, for the nonlinear matter power spectrum accurate at the percent level out to k 1 h Mpc–1. We describe the construction of the emulator, explain the tests performed to ensure its accuracy, and discuss how the central ideas may be extended to a wider range of Cosmological Models and applications. A power spectrum emulator code is released publicly as part of this paper.
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the coyote universe ii Cosmological Models and precision emulation of the nonlinear matter power spectrum
The Astrophysical Journal, 2009Co-Authors: Katrin Heitmann, David Higdon, Martin White, Salman Habib, Brian J Williams, Earl Lawrence, Christian WagnerAbstract:The power spectrum of density fluctuations is a foundational source of Cosmological information. Precision Cosmological probes targeted primarily at investigations of dark energy require accurate theoretical determinations of the power spectrum in the nonlinear regime. To exploit the observational power of future Cosmological surveys, accuracy demands on the theory are at the 1% level or better. Numerical simulations are currently the only way to produce sufficiently error-controlled predictions for the power spectrum. The very high computational cost of (precision) N-body simulations is a major obstacle to obtaining predictions in the nonlinear regime, while scanning over Cosmological parameters. Near-future observations, however, are likely to provide a meaningful constraint only on constant dark energy equation of state, "wCDM", cosmologies. In this paper, we demonstrate that a limited set of only 37 Cosmological Models—the "Coyote Universe" suite—can be used to predict the nonlinear matter power spectrum to 1% over a prior parameter range set by current cosmic microwave background observations. This paper is the second in a series of three, with the final aim to provide a high-accuracy prediction scheme for the nonlinear matter power spectrum for wCDM cosmologies.
Woei Chet Lim - One of the best experts on this subject based on the ideXlab platform.
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cosmic microwave background limits on spatially homogeneous Cosmological Models with a Cosmological constant
Classical and Quantum Gravity, 2007Co-Authors: A A Coley, Woei Chet LimAbstract:We investigate the effect of dark energy on the limits on the shear anisotropy in spatially homogeneous Bianchi Cosmological Models obtained from measurements of the temperature anisotropies in the cosmic microwave background. We shall primarily assume that the dark energy is modelled by a Cosmological constant. In general, we find that there are tighter bounds on the shear than in Models with no Cosmological constant, although the limits are (Bianchi) model dependent. In addition, there are special spatially homogeneous Cosmological Models whose rate of expansion is highly anisotropic, but whose cosmic microwave background temperature is measured to be exactly isotropic at one instant of time.
Brian J Williams - One of the best experts on this subject based on the ideXlab platform.
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the coyote universe iii simulation suite and precision emulator for the nonlinear matter power spectrum
The Astrophysical Journal, 2010Co-Authors: Earl Lawrence, Katrin Heitmann, David Higdon, Martin White, Salman Habib, Christian Wagner, Brian J WilliamsAbstract:Many of the most exciting questions in astrophysics and cosmology, including the majority of observational probes of dark energy, rely on an understanding of the nonlinear regime of structure formation. In order to fully exploit the information available from this regime and to extract Cosmological constraints, accurate theoretical predictions are needed. Currently, such predictions can only be obtained from costly, precision numerical simulations. This paper is the third in a series aimed at constructing an accurate calibration of the nonlinear mass power spectrum on Mpc scales for a wide range of currently viable Cosmological Models, including dark energy Models with w ≠ –1. The first two papers addressed the numerical challenges and the scheme by which an interpolator was built from a carefully chosen set of Cosmological Models. In this paper, we introduce the "Coyote Universe" simulation suite which comprises nearly 1000 N-body simulations at different force and mass resolutions, spanning 38 w CDM cosmologies. This large simulation suite enables us to construct a prediction scheme, or emulator, for the nonlinear matter power spectrum accurate at the percent level out to k 1 h Mpc–1. We describe the construction of the emulator, explain the tests performed to ensure its accuracy, and discuss how the central ideas may be extended to a wider range of Cosmological Models and applications. A power spectrum emulator code is released publicly as part of this paper.
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the coyote universe ii Cosmological Models and precision emulation of the nonlinear matter power spectrum
The Astrophysical Journal, 2009Co-Authors: Katrin Heitmann, David Higdon, Martin White, Salman Habib, Brian J Williams, Earl Lawrence, Christian WagnerAbstract:The power spectrum of density fluctuations is a foundational source of Cosmological information. Precision Cosmological probes targeted primarily at investigations of dark energy require accurate theoretical determinations of the power spectrum in the nonlinear regime. To exploit the observational power of future Cosmological surveys, accuracy demands on the theory are at the 1% level or better. Numerical simulations are currently the only way to produce sufficiently error-controlled predictions for the power spectrum. The very high computational cost of (precision) N-body simulations is a major obstacle to obtaining predictions in the nonlinear regime, while scanning over Cosmological parameters. Near-future observations, however, are likely to provide a meaningful constraint only on constant dark energy equation of state, "wCDM", cosmologies. In this paper, we demonstrate that a limited set of only 37 Cosmological Models—the "Coyote Universe" suite—can be used to predict the nonlinear matter power spectrum to 1% over a prior parameter range set by current cosmic microwave background observations. This paper is the second in a series of three, with the final aim to provide a high-accuracy prediction scheme for the nonlinear matter power spectrum for wCDM cosmologies.
Katrin Heitmann - One of the best experts on this subject based on the ideXlab platform.
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the coyote universe iii simulation suite and precision emulator for the nonlinear matter power spectrum
The Astrophysical Journal, 2010Co-Authors: Earl Lawrence, Katrin Heitmann, David Higdon, Martin White, Salman Habib, Christian Wagner, Brian J WilliamsAbstract:Many of the most exciting questions in astrophysics and cosmology, including the majority of observational probes of dark energy, rely on an understanding of the nonlinear regime of structure formation. In order to fully exploit the information available from this regime and to extract Cosmological constraints, accurate theoretical predictions are needed. Currently, such predictions can only be obtained from costly, precision numerical simulations. This paper is the third in a series aimed at constructing an accurate calibration of the nonlinear mass power spectrum on Mpc scales for a wide range of currently viable Cosmological Models, including dark energy Models with w ≠ –1. The first two papers addressed the numerical challenges and the scheme by which an interpolator was built from a carefully chosen set of Cosmological Models. In this paper, we introduce the "Coyote Universe" simulation suite which comprises nearly 1000 N-body simulations at different force and mass resolutions, spanning 38 w CDM cosmologies. This large simulation suite enables us to construct a prediction scheme, or emulator, for the nonlinear matter power spectrum accurate at the percent level out to k 1 h Mpc–1. We describe the construction of the emulator, explain the tests performed to ensure its accuracy, and discuss how the central ideas may be extended to a wider range of Cosmological Models and applications. A power spectrum emulator code is released publicly as part of this paper.
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the coyote universe ii Cosmological Models and precision emulation of the nonlinear matter power spectrum
The Astrophysical Journal, 2009Co-Authors: Katrin Heitmann, David Higdon, Martin White, Salman Habib, Brian J Williams, Earl Lawrence, Christian WagnerAbstract:The power spectrum of density fluctuations is a foundational source of Cosmological information. Precision Cosmological probes targeted primarily at investigations of dark energy require accurate theoretical determinations of the power spectrum in the nonlinear regime. To exploit the observational power of future Cosmological surveys, accuracy demands on the theory are at the 1% level or better. Numerical simulations are currently the only way to produce sufficiently error-controlled predictions for the power spectrum. The very high computational cost of (precision) N-body simulations is a major obstacle to obtaining predictions in the nonlinear regime, while scanning over Cosmological parameters. Near-future observations, however, are likely to provide a meaningful constraint only on constant dark energy equation of state, "wCDM", cosmologies. In this paper, we demonstrate that a limited set of only 37 Cosmological Models—the "Coyote Universe" suite—can be used to predict the nonlinear matter power spectrum to 1% over a prior parameter range set by current cosmic microwave background observations. This paper is the second in a series of three, with the final aim to provide a high-accuracy prediction scheme for the nonlinear matter power spectrum for wCDM cosmologies.
