Naureen Goheer - Academia.edu (original) (raw)
Papers by Naureen Goheer
Physical Review D, 2008
We investigate whether or not an Einstein Static universe is a solution to the cosmological equat... more We investigate whether or not an Einstein Static universe is a solution to the cosmological equations in f (R) gravity. It is found that only one class of f (R) theories admits an Einstein Static model, and that this class is neutrally stable with respect to vector and tensor perturbations for all equations of state on all scales. Scalar perturbations are only stable on all scales if the matter fluid equation of state satisfies c 2 s > √ 5−1 6 ≈ 0.21. This result is remarkably similar to the GR case, where it was found that the Einstein Static model is stable for c 2 s > 1 5 .
Abstract: Working within the theory of modified Gauss-Bonnet gravity, we show that FLRW--like pow... more Abstract: Working within the theory of modified Gauss-Bonnet gravity, we show that FLRW--like power--law solutions only exist for a very special class of f (G) theories. Furthermore, we point out that any transition from decelerated to accelerated expansion must pass through ...
The Astrophysical Journal, 2009
Motivated by the observed shortfall of baryons in the local universe, we investigate the ability ... more Motivated by the observed shortfall of baryons in the local universe, we investigate the ability of high resolution cosmic microwave background (CMB) experiments to detect hot gas in the outer regions of nearby group halos. We construct hot gas models with the gas in hydrostatic equilibrium with the dark matter and described by a polytropic equation of state. We also consider models that add entropy to the gas in line with constraints from X-ray observations. We calculate the thermal Sunyaev-Zel'dovich (SZ) signal in these halos and compare it to the anticipated sensitivities of forthcoming SZ survey experiments such as ACT, PLANCK and SPT. Using a multi-frequency Wiener filter we derive SZ detectability limits as a function of halo mass and redshift in the presence of galactic and extragalactic foregrounds and the CMB. We find that group-sized halos with virial masses below 10 14 M⊙ can be detected at z ∼ < 0.05 with the threshold mass dropping to 3−4×10 13 M⊙ at z ∼ < 0.01. The SZ distortion of nearby group-sized halos can thus be mapped out to the virial radius by these CMB experiments, beyond the sensitivity limits of X-ray observations. These measurements will provide a unique probe of hot gas in the outer regions of group halos, shedding insight into the local census of baryons and the injection of entropy into the intragroup medium from nongravitational feedback.
Physical Review D, 2009
We show that within the class of f (R) gravity theories, FLRW power-law perfect fluid solutions o... more We show that within the class of f (R) gravity theories, FLRW power-law perfect fluid solutions only exist for R n gravity. This significantly restricts the set of exact cosmological solutions which have similar properties to what is found in standard General Relativity.
Physical Review D, 2004
It has recently been suggested that an isotropic singularity may be a generic feature of brane co... more It has recently been suggested that an isotropic singularity may be a generic feature of brane cosmologies, even in the inhomogeneous case. Using the covariant and gauge -invariant approach we present a detailed analysis of linear perturbations of the isotropic model F b which is a past attractor in the phase space of homogeneous Bianchi models on the brane. We find that for matter with an equation of state parameter γ > 1, the dimensionless variables representing generic anisotropic and inhomogeneous perturbations decay as t → 0, showing that the model F b is asymptotically stable in the past. We conclude that brane universes are born with isotropy naturally built -in, contrary to standard cosmology. The observed large -scale homogeneity and isotropy of the universe can therefore be explained as a consequence of the initial conditions if the brane -world paradigm represents a description of the very early universe.
Physical Review D, 2003
In this paper we study the cosmological dynamics of Randall -Sundrum braneworld type scenarios in... more In this paper we study the cosmological dynamics of Randall -Sundrum braneworld type scenarios in which the five -dimensional Weyl tensor has a non -vanishing projection onto the three -brane where matter fields are confined. Using dynamical systems techniques, we study how the state space of Friedmann-Lemaître-Robertson-Walker (FLRW) and Bianchi type I scalar field models with an exponential potential is affected by the bulk Weyl tensor, focusing on the differences that appear with respect to standard general relativity and also Randall -Sundrum cosmological scenarios without the Weyl tensor contribution.
Physical Review D, 2002
In this work we consider Randall -Sundrum brane -world type scenarios, in which the spacetime is ... more In this work we consider Randall -Sundrum brane -world type scenarios, in which the spacetime is described by a five -dimensional manifold with matter fields confined in a domain wall or threebrane. We present the results of a systematic analysis, using dynamical systems techniques, of the qualitative behaviour of Friedmann -Lemaître -Robertson -Walker type models, whose matter is described by a scalar field with an exponential potential. We construct the state spaces for these models and discuss how their structure changes with respect to the general -relativistic case, in particular, what new critical points appear and their nature and the occurrence of bifurcation.
Journal of Cosmology and Astroparticle Physics, 2010
Recently, spatially inhomogeneous cosmological models have been proposed as an alternative to the... more Recently, spatially inhomogeneous cosmological models have been proposed as an alternative to the ΛCDM model, with the aim of reproducing the late time dynamics of the Universe without introducing a cosmological constant or dark energy. This paper investigates the possibility of distinguishing such models from the standard ΛCDM using background or large scale structure data. It also illustrates and emphasizes the necessity of testing the Copernican principle in order to confront the tests of general relativity with the large scale structure.
Classical and Quantum Gravity, 2008
In this paper we address important issues surrounding the choice of variables when performing a d... more In this paper we address important issues surrounding the choice of variables when performing a dynamical systems analysis of alternative theories of gravity. We discuss the advantages and disadvantages of compactifying the state space, and illustrate this using two examples. We first show how to define a compact state space for the class of LRS Bianchi type I models in R n -gravity and compare to a non-compact expansion-normalised approach. In the second example we consider the flat Friedmann matter subspace of the previous example, and compare the compact analysis to studies where non-compact non-expansionnormalised variables were used. In both examples we comment on the existence of bouncing or recollapsing orbits as well as the existence of static models.
Classical and Quantum Gravity, 2009
We study the dynamics of homogeneous isotropic FRW cosmologies with positive spatial curvature in... more We study the dynamics of homogeneous isotropic FRW cosmologies with positive spatial curvature in f (R)-gravity, paying special attention to the existence of Einstein static models and only study forms of f (R) = R n for which these static models have been shown to exist. We construct a compact state space and identify past and future attractors of the system and recover a previously discovered future attractor corresponding to an expanding accelerating model. We also discuss the existence of universes which have both a past and future bounce, a phenomenon which is absent in General Relativity.
Classical and Quantum Gravity, 2007
In this paper we study the dynamics of orthogonal spatially homogeneous Bianchi cosmologies in R ... more In this paper we study the dynamics of orthogonal spatially homogeneous Bianchi cosmologies in R n -gravity. We construct a compact state space by dividing the state space into different sectors. We perform a detailed analysis of the cosmological behaviour in terms of the parameter n, determining all the equilibrium points, their stability and corresponding cosmological evolution. In particular, the appropriately compactified state space allows us to investigate static and bouncing solutions. We find no Einstein static solutions, but there do exist cosmologies with bounce behaviours. We also investigate the isotropisation of these models and find that all isotropic points are flat Friedmann like.
Physical Review D, 2004
We present the complete set of propagation and constraint equations for the kinematic and nonloca... more We present the complete set of propagation and constraint equations for the kinematic and nonlocal first order quantities which describe general linear inhomogeneous and anisotropic perturbations of a flat FRW braneworld with vanishing cosmological constant and decompose them in the standard way into their scalar, vector and tensor contributions. A detailed analysis of the perturbation dynamics is performed using dimensionless variables that are specially tailored for the different regimes of interest; namely, the low energy GR regime, the high energy regime and the dark energy regime. Tables are presented for the evolution of all the physical quantities, making it easy to do a detailed comparison of the past asymptotic behaviour of the perturbations of these models. We find results that exactly match those obtained in the analysis of the spatially inhomogeneous G2 braneworld cosmologies presented recently; i.e., that isotropization towards the F b model occurs for γ > 4/3.
Physical Review D, 2008
We investigate whether or not an Einstein Static universe is a solution to the cosmological equat... more We investigate whether or not an Einstein Static universe is a solution to the cosmological equations in f (R) gravity. It is found that only one class of f (R) theories admits an Einstein Static model, and that this class is neutrally stable with respect to vector and tensor perturbations for all equations of state on all scales. Scalar perturbations are only stable on all scales if the matter fluid equation of state satisfies c 2 s > √ 5−1 6 ≈ 0.21. This result is remarkably similar to the GR case, where it was found that the Einstein Static model is stable for c 2 s > 1 5 .
Abstract: Working within the theory of modified Gauss-Bonnet gravity, we show that FLRW--like pow... more Abstract: Working within the theory of modified Gauss-Bonnet gravity, we show that FLRW--like power--law solutions only exist for a very special class of f (G) theories. Furthermore, we point out that any transition from decelerated to accelerated expansion must pass through ...
The Astrophysical Journal, 2009
Motivated by the observed shortfall of baryons in the local universe, we investigate the ability ... more Motivated by the observed shortfall of baryons in the local universe, we investigate the ability of high resolution cosmic microwave background (CMB) experiments to detect hot gas in the outer regions of nearby group halos. We construct hot gas models with the gas in hydrostatic equilibrium with the dark matter and described by a polytropic equation of state. We also consider models that add entropy to the gas in line with constraints from X-ray observations. We calculate the thermal Sunyaev-Zel'dovich (SZ) signal in these halos and compare it to the anticipated sensitivities of forthcoming SZ survey experiments such as ACT, PLANCK and SPT. Using a multi-frequency Wiener filter we derive SZ detectability limits as a function of halo mass and redshift in the presence of galactic and extragalactic foregrounds and the CMB. We find that group-sized halos with virial masses below 10 14 M⊙ can be detected at z ∼ < 0.05 with the threshold mass dropping to 3−4×10 13 M⊙ at z ∼ < 0.01. The SZ distortion of nearby group-sized halos can thus be mapped out to the virial radius by these CMB experiments, beyond the sensitivity limits of X-ray observations. These measurements will provide a unique probe of hot gas in the outer regions of group halos, shedding insight into the local census of baryons and the injection of entropy into the intragroup medium from nongravitational feedback.
Physical Review D, 2009
We show that within the class of f (R) gravity theories, FLRW power-law perfect fluid solutions o... more We show that within the class of f (R) gravity theories, FLRW power-law perfect fluid solutions only exist for R n gravity. This significantly restricts the set of exact cosmological solutions which have similar properties to what is found in standard General Relativity.
Physical Review D, 2004
It has recently been suggested that an isotropic singularity may be a generic feature of brane co... more It has recently been suggested that an isotropic singularity may be a generic feature of brane cosmologies, even in the inhomogeneous case. Using the covariant and gauge -invariant approach we present a detailed analysis of linear perturbations of the isotropic model F b which is a past attractor in the phase space of homogeneous Bianchi models on the brane. We find that for matter with an equation of state parameter γ > 1, the dimensionless variables representing generic anisotropic and inhomogeneous perturbations decay as t → 0, showing that the model F b is asymptotically stable in the past. We conclude that brane universes are born with isotropy naturally built -in, contrary to standard cosmology. The observed large -scale homogeneity and isotropy of the universe can therefore be explained as a consequence of the initial conditions if the brane -world paradigm represents a description of the very early universe.
Physical Review D, 2003
In this paper we study the cosmological dynamics of Randall -Sundrum braneworld type scenarios in... more In this paper we study the cosmological dynamics of Randall -Sundrum braneworld type scenarios in which the five -dimensional Weyl tensor has a non -vanishing projection onto the three -brane where matter fields are confined. Using dynamical systems techniques, we study how the state space of Friedmann-Lemaître-Robertson-Walker (FLRW) and Bianchi type I scalar field models with an exponential potential is affected by the bulk Weyl tensor, focusing on the differences that appear with respect to standard general relativity and also Randall -Sundrum cosmological scenarios without the Weyl tensor contribution.
Physical Review D, 2002
In this work we consider Randall -Sundrum brane -world type scenarios, in which the spacetime is ... more In this work we consider Randall -Sundrum brane -world type scenarios, in which the spacetime is described by a five -dimensional manifold with matter fields confined in a domain wall or threebrane. We present the results of a systematic analysis, using dynamical systems techniques, of the qualitative behaviour of Friedmann -Lemaître -Robertson -Walker type models, whose matter is described by a scalar field with an exponential potential. We construct the state spaces for these models and discuss how their structure changes with respect to the general -relativistic case, in particular, what new critical points appear and their nature and the occurrence of bifurcation.
Journal of Cosmology and Astroparticle Physics, 2010
Recently, spatially inhomogeneous cosmological models have been proposed as an alternative to the... more Recently, spatially inhomogeneous cosmological models have been proposed as an alternative to the ΛCDM model, with the aim of reproducing the late time dynamics of the Universe without introducing a cosmological constant or dark energy. This paper investigates the possibility of distinguishing such models from the standard ΛCDM using background or large scale structure data. It also illustrates and emphasizes the necessity of testing the Copernican principle in order to confront the tests of general relativity with the large scale structure.
Classical and Quantum Gravity, 2008
In this paper we address important issues surrounding the choice of variables when performing a d... more In this paper we address important issues surrounding the choice of variables when performing a dynamical systems analysis of alternative theories of gravity. We discuss the advantages and disadvantages of compactifying the state space, and illustrate this using two examples. We first show how to define a compact state space for the class of LRS Bianchi type I models in R n -gravity and compare to a non-compact expansion-normalised approach. In the second example we consider the flat Friedmann matter subspace of the previous example, and compare the compact analysis to studies where non-compact non-expansionnormalised variables were used. In both examples we comment on the existence of bouncing or recollapsing orbits as well as the existence of static models.
Classical and Quantum Gravity, 2009
We study the dynamics of homogeneous isotropic FRW cosmologies with positive spatial curvature in... more We study the dynamics of homogeneous isotropic FRW cosmologies with positive spatial curvature in f (R)-gravity, paying special attention to the existence of Einstein static models and only study forms of f (R) = R n for which these static models have been shown to exist. We construct a compact state space and identify past and future attractors of the system and recover a previously discovered future attractor corresponding to an expanding accelerating model. We also discuss the existence of universes which have both a past and future bounce, a phenomenon which is absent in General Relativity.
Classical and Quantum Gravity, 2007
In this paper we study the dynamics of orthogonal spatially homogeneous Bianchi cosmologies in R ... more In this paper we study the dynamics of orthogonal spatially homogeneous Bianchi cosmologies in R n -gravity. We construct a compact state space by dividing the state space into different sectors. We perform a detailed analysis of the cosmological behaviour in terms of the parameter n, determining all the equilibrium points, their stability and corresponding cosmological evolution. In particular, the appropriately compactified state space allows us to investigate static and bouncing solutions. We find no Einstein static solutions, but there do exist cosmologies with bounce behaviours. We also investigate the isotropisation of these models and find that all isotropic points are flat Friedmann like.
Physical Review D, 2004
We present the complete set of propagation and constraint equations for the kinematic and nonloca... more We present the complete set of propagation and constraint equations for the kinematic and nonlocal first order quantities which describe general linear inhomogeneous and anisotropic perturbations of a flat FRW braneworld with vanishing cosmological constant and decompose them in the standard way into their scalar, vector and tensor contributions. A detailed analysis of the perturbation dynamics is performed using dimensionless variables that are specially tailored for the different regimes of interest; namely, the low energy GR regime, the high energy regime and the dark energy regime. Tables are presented for the evolution of all the physical quantities, making it easy to do a detailed comparison of the past asymptotic behaviour of the perturbations of these models. We find results that exactly match those obtained in the analysis of the spatially inhomogeneous G2 braneworld cosmologies presented recently; i.e., that isotropization towards the F b model occurs for γ > 4/3.