Showing papers by "Institute of Cosmology and Gravitation, University of Portsmouth published in 2005"

Cosmological parameter analysis including SDSS Lyα forest and galaxy bias: Constraints on the primordial spectrum of fluctuations, neutrino mass, and dark energy

Abstract: We combine the constraints from the recent Ly$\ensuremath{\alpha}$ forest analysis of the Sloan Digital Sky Survey (SDSS) and the SDSS galaxy bias analysis with previous constraints from SDSS galaxy clustering, the latest supernovae, and 1st year WMAP cosmic microwave background anisotropies. We find significant improvements on all of the cosmological parameters compared to previous constraints, which highlights the importance of combining Ly$\ensuremath{\alpha}$ forest constraints with other probes. Combining WMAP and the Ly$\ensuremath{\alpha}$ forest we find for the primordial slope ${n}_{s}=0.98\ifmmode\pm\else\textpm\fi{}0.02$. We see no evidence of running, $dn/d\mathrm{ln} k=\ensuremath{-}0.003\ifmmode\pm\else\textpm\fi{}0.010$, a factor of $3$ improvement over previous constraints. We also find no evidence of tensors, $rl0.36$ ($95%$ c.l.). Inflationary models predict the absence of running and many among them satisfy these constraints, particularly negative curvature models such as those based on spontaneous symmetry breaking. A positive correlation between tensors and primordial slope disfavors chaotic inflation-type models with steep slopes: while the $V\ensuremath{\propto}{\ensuremath{\phi}}^{2}$ model is within the 2-sigma contour, $V\ensuremath{\propto}{\ensuremath{\phi}}^{4}$ is outside the 3-sigma contour. For the amplitude we find ${\ensuremath{\sigma}}_{8}=0.90\ifmmode\pm\else\textpm\fi{}0.03$ from the Ly$\ensuremath{\alpha}$ forest and WMAP alone. We find no evidence of neutrino mass: for the case of $3$ massive neutrino families with an inflationary prior, $\ensuremath{\sum}_{}^{}{m}_{\ensuremath{ u}}l0.42$ eV and the mass of lightest neutrino is ${m}_{1}l0.13$ eV at $95%$ c.l. For the 3 massless $+1$ massive neutrino case we find ${m}_{\ensuremath{ u}}l0.79$ eV for the massive neutrino, excluding at $95%$ c.l. all neutrino mass solutions compatible with the LSND results. We explore dark energy constraints in models with a fairly general time dependence of dark energy equation of state, finding ${\ensuremath{\Omega}}_{\ensuremath{\lambda}}=0.72\ifmmode\pm\else\textpm\fi{}0.02$, $\mathrm{w}(z=0.3)=\ensuremath{-}{0.98}_{\ensuremath{-}0.12}^{+0.10}$, the latter changing to $\mathrm{w}(z=0.3)=\ensuremath{-}{0.92}_{\ensuremath{-}0.10}^{+0.09}$ if tensors are allowed. We find no evidence for variation of the equation of state with redshift, $\mathrm{w}(z=1)=\ensuremath{-}{1.03}_{\ensuremath{-}0.28}^{+0.21}$. These results rely on the current understanding of the Ly$\ensuremath{\alpha}$ forest and other probes, which need to be explored further both observationally and theoretically, but extensive tests reveal no evidence of inconsistency among different data sets used here.

Cosmological structure formation in a homogeneous dark energy background

Abstract: There is now strong evidence that the current energy density of the Universe is dominated by dark energy with an equation of state w

Detecting extra dimensions with gravity-wave spectroscopy: the black-string brane world.

Abstract: Using the black string between two branes as a model of a brane-world black hole, we compute the gravity-wave perturbations and identify the features arising from the additional polarizations of the graviton. The standard four-dimensional gravitational wave signal acquires late-time oscillations due to massive modes of the graviton. The Fourier transform of these oscillations shows a series of spikes associated with the masses of the Kaluza-Klein modes, providing in principle a spectroscopic signature of extra dimensions.

Escaping the big rip

Abstract: We discuss dark energy models which might describe effectively the actual acceleration of the universe. More precisely, for a four-dimensional Friedmann–Lemaitre–Robertson–Walker (FLRW) universe we consider two situations. For the first of them, we model dark energy as phantom energy described as a perfect fluid satisfying the equation of state P = (β−1)ρ (with β<0 and constant). In this case the universe reaches a 'big rip' independently of the spatial geometry of the FLRW universe. In the second situation, the dark energy is described as a phantom (generalized) Chaplygin gas which violates the dominant energy condition. Contrary to the previous case, for this material content a FLRW universe would never reach a 'big rip' singularity (indeed, the geometry is asymptotically de Sitter). We also show how this dark energy model can be described in terms of scalar fields, corresponding to a minimally coupled scalar field, a Born–Infeld scalar field and a generalized Born–Infeld scalar field. Finally, we introduce a phenomenologically viable model where dark energy is described as a phantom generalized Chaplygin gas.

Active Learning For Identifying Function Threshold Boundaries

Abstract: We present an efficient algorithm to actively select queries for learning the boundaries separating a function domain into regions where the function is above and below a given threshold. We develop experiment selection methods based on entropy, misclassification rate, variance, and their combinations, and show how they perform on a number of data sets. We then show how these algorithms are used to determine simultaneously valid 1 - α confidence intervals for seven cosmological parameters. Experimentation shows that the algorithm reduces the computation necessary for the parameter estimation problem by an order of magnitude.

Non-Gaussianity from cosmic magnetic fields

Abstract: Magnetic fields in the early Universe could have played an important role in sourcing cosmological perturbations. While not the dominant source, even a small contribution might be traceable through its intrinsic non-Gaussianity. Here we calculate analytically the one-, two-, and three-point statistics of the magnetic stress energy resulting from tangled Gaussian fields, and confirm these with numerical realizations of the fields. We find significant non-Gaussianity, and importantly predict higher order moments that will appear between the scalar, vector, and tensor parts of the stress energy (e.g., scalar-tensor-tensor moments). Such higher order cross correlations are a generic feature of nonlinear theories and could prove to be an important probe of the early Universe.

FRW quantum cosmology with a generalized Chaplygin gas

Abstract: Cosmologies with a Chaplygin gas have recently been explored with the objective of explaining the transition from a dust dominated epoch towards an accelerating expansion stage. In this context, we consider the hypothesis that this transition involves a quantum mechanical process. Our analysis is entirely analytical, with the objective of finding explicit mathematical expressions for the different quantum mechanical states and their cosmological implications. We employ a Friedmann-Robertson-Walker (FRW) minisuperspace model, characterized by two Lorentzian sectors, separated by a classically forbidden region. This is the configuration associated with the evolution of a generalized Chaplygin gas in a FRW universe. The Hartle-Hawking and Vilenkin wave functions are then computed, together with the transition amplitudes towards the accelerating epoch. Furthermore, for specific initial conditions we found that the generalized Chaplygin gas parameters become related through an expression involving an integer $n$. We also introduce a phenomenological association between some brane-world scenarios and a FRW minisuperspace cosmology with a generalized Chaplygin gas. The aim is to promote a discussion and subsequent research on the quantum creation of brane cosmologies from such a perspective. Additional results in this paper suggest that the brane tension would become related with the generalized Chaplygin gas parameters through another expression involving an integer.

Testing for double inflation with WMAP

Abstract: With the WMAP data we can now begin to test realistic models of inflation involving multiple scalar fields. These naturally lead to correlated adiabatic and isocurvature (entropy) perturbations with a running spectral index. We present the first full (9 parameter) likelihood analysis of double inflation with WMAP data and find that despite the extra freedom, supersymmetric hybrid potentials are strongly constrained with less than 7% correlated isocurvature component allowed when standard priors are imposed on the cosomological parameters. As a result we also find that Akaike and Bayesian model selection criteria rather strongly prefer single-field inflation, just as equivalent analysis prefers a cosmological constant over dynamical dark energy in the late universe. It appears that simplicity is the best guide to our universe.

Induced gravity with a nonminimally coupled scalar field on the brane

Abstract: We present the four-dimensional equations on a brane with a scalar field nonminimally coupled to the induced Ricci curvature, embedded in a five-dimensional bulk with a cosmological constant. This is a natural extension to a brane-world context of scalar-tensor (Brans-Dicke) gravity. In particular we consider the cosmological evolution of a homogeneous and isotropic (FRW) brane. We identify low-energy and strong-coupling limits in which we recover effectively four-dimensional evolution. We find de Sitter brane solutions with both constant and evolving scalar field. We also consider the special case of a conformally coupled scalar field for which it is possible (when the conformal energy density exactly cancels the effect of the bulk black hole) to recover a conventional four-dimensional Friedmann equation for all energy densities.

Are black holes overproduced during preheating

Abstract: We provide a simple but robust argument that primordial black hole production generically does not exceed astrophysical bounds during the resonant preheating phase after inflation. This conclusion is supported by fully nonlinear lattice simulations of various models in two and three dimensions which include rescattering but neglect metric perturbations. We examine the degree to which preheating amplifies density perturbations at the Hubble scale and show that, at the end of the parametric resonance, power spectra are universal, with no memory of the power spectrum at the end of inflation. In addition, we show how the probability distribution of density perturbations changes from exponential on very small scales to Gaussian when smoothed over the Hubble scale - the crucial length for studies of primordial black hole formation - hence justifying the standard assumption of Gaussianity.

The Environmental Dependence of Galaxy Colors in Intermediate-Redshift X-Ray-selected Clusters

Abstract: We present a wide-field imaging study of the colors of bright galaxies (

A late-accelerating universe with no dark energy—and a finite-temperature big bang

Abstract: Brane-world models offer the possibility of explaining the late acceleration of the universe via infra-red modifications to general relativity, rather than a dark energy field. However, one also expects ultra-violet modifications to general relativity, when high-energy stringy effects in the early universe begin to grow. We generalize the DGP brane-world model via an ultra-violet modification, in the form of a Gauss–Bonnet term in the bulk action. The combination of infra-red and ultra-violet modifications produces an intriguing cosmology. The DGP feature of late-time acceleration without dark energy is preserved, but there is an entirely new feature—there is no infinite-temperature big bang in the early universe. The universe starts with finite density and pressure, from a 'quiet' and 'sudden' curvature singularity.

Optimizing cosmological surveys in a crowded market

Abstract: Optimizing the major next-generation cosmological surveys (such as SNAP, KAOS, etc.) is a key problem given our ignorance of the physics underlying cosmic acceleration and the plethora of surveys planned. We propose a Bayesian design framework which (1) maximizes the discrimination power of a survey without assuming any underlying dark-energy model, (2) finds the best niche survey geometry given current data and future competing experiments, (3) maximizes the cross section for serendipitous discoveries and (4) can be adapted to answer specific questions (such as ``is dark energy dynamical?''). Integrated parameter-space optimization (IPSO) is a design framework that integrates projected parameter errors over an entire dark energy parameter space and then extremizes a figure of merit (such as Shannon entropy gain which we show is stable to off-diagonal covariance matrix perturbations) as a function of survey parameters using analytical, grid or MCMC techniques. We discuss examples where the optimization can be performed analytically. IPSO is thus a general, model-independent and scalable framework that allows us to appropriately use prior information to design the best possible surveys.

Towards a wave-extraction method for numerical relativity. II. The quasi-Kinnersley frame

Abstract: The Newman-Penrose formalism may be used in numerical relativity to extract coordinate-invariant information about gravitational radiation emitted in strong-field dynamical scenarios. The main challenge in doing so is to identify a null tetrad appropriately adapted to the simulated geometry such that NewmanPenrose quantities computed relative to it have an invariant physical meaning. In black hole perturbation theory, the Teukolsky formalism uses such adapted tetrads, those which differ only perturbatively from the background Kinnersley tetrad. At late times, numerical simulations of astrophysical processes producing isolated black holes ought to admit descriptions in the Teukolsky formalism. However, adapted tetrads in this context must be identified using only the numerically computed metric, since no background Kerr geometry is known a priori. To do this, this paper introduces the notion of a quasi-Kinnersley frame. This frame, when space-time is perturbatively close to Kerr, approximates the background Kinnersley frame. However, it remains calculable much more generally, in space-times nonperturbatively different from Kerr. We give an explicit solution for the tetrad transformation which is required in order to find this frame in a general space-time.

Asymmetric brane-worlds with induced gravity

Abstract: The Randall-Sundrum scenario, with a $1+3$-dimensional brane in a five-dimensional bulk spacetime, can be generalized in various ways. We consider the case where the ${Z}_{2}$ symmetry at the brane is relaxed, and in addition the gravitational action is generalized to include an induced gravity term on the brane. We derive the complete set of equations governing the gravitational dynamics for a general brane and bulk, and identify how the asymmetry and the induced gravity act as effective source terms in the projected field equations on the brane. For a Friedmann brane in an anti-de Sitter bulk, the solution of the Friedmann equation is given by the solution of a quartic equation. We find the perturbative solutions for small asymmetry, which has an effect at late times.

The cross-correlation between the microwave and X-ray backgrounds: foregrounds and systematics

Abstract: Recently, we presented evidence for a cross-correlation of the WMAP satellite map of the cosmic microwave background (CMB) and the HEAO1 satellite map of the hard X-ray background (XRB) with a dimensionless amplitude of 0.14 ± 0.05 normalized to the product of the rms fluctuations of the CMB and XRB. Such a correlation is expected in a universe dominated by a cosmological constant via the integrated Sachs‐Wolfe (ISW) effect and the level of the correlation observed is consistent with that predicted by the currently favoured Lambda cold dark matter model of the Universe. As this offers independent confirmation of the cosmological model, it is important to verify the origin of the correlation. Here we explore in detail some possible foreground sources of the correlation. The present evidence all supports an ISW origin. Ke yw ords: cosmic microwave background ‐ cosmology: observations.

Sounding the dark cosmos

Abstract: Perhaps the greatest surprise in cosmology since Edwin Hubble's original discovery of the expansion of the universe in the 1930s has been the compelling evidence that the rate of cosmic expansion has been accelerating rather than slowing down in the past few billion years. Before 1998, models of an accelerating universe were very unfashionable; following General Relativity, cosmic acceleration appeared to require the cosmos to be dominated by energy with negative pressure or by Einstein's admitted “greatest blunder”– the cosmological constant, Λ. However, the magnitude of the cosmological constant required just to accelerate the cosmos today is 120 orders of magnitude smaller than the natural quantum gravity scale set by the Planck mass, which would make it the single worst theoretically estimated quantity in the history of science! But in 1998, contrary evidence turned cosmology on its head. Two independent teams showed that distant supernovae were fainter and hence more distant than could be explained in a decelerating universe, thereby forcing cosmologists to confront the possibility of acceleration. Since then, evidence for acceleration has steadily mounted and improved, yet our knowledge of the underlying physics of the process has gone almost nowhere. We still do not know whether Einstein's theory of gravity is wrong, whether the acceleration is caused by the cosmological constant or by a completely new form of matter such as the scalar fields often invoked (but as yet undetected) as sources of acceleration during the first few fractions of a second after the Big Bang. Deciding between these three possible sources of acceleration is one of the major goals of cosmology in the next decade, with scores of dedicated surveys and experiments planned or in progress attempting to address the nature of “dark energy”, as the mysterious source of acceleration is called. There are several major new supernovae searches (ESSENCE, SNLS, SDSS-II) that, by 2008, will enhance our understanding of dark energy by measuring the distances to several hundred new Type Ia supernovae (SNIa) (Matheson et al. 2005; Mullivan 2005; Sako 2005). SNIa measurements are, however, fundamentally limited by the fact that we do not physically understand the mechanisms of supernova explosion. Also there are many possible astrophysical uncertainties, such as progenitor bias, absorption of emitted photons by intervening dust and possible redshift evolution of SNIa. In addition, SNIa distance measurements are not ideal dark energy discriminators because, given a dark energy model, one must perform two integrals over redshift to derive the corresponding predicted distance (to compare with the observed distance provided by the SNIa). This double integral smears out any interesting or distinctive fingerprints of the underlying physics, implying that one needs a very large number of SNIa to be able to differentiate between different dark energy models. This is exactly the plan with future SNIa surveys such as the Large Synoptic Survey Telescope (LSST), Dark Energy Survey (DES) and the SuperNova Acceleration Probe (SNAP), a satellite mission that would fly after 2014 and would detect nearly 2000 SNIa in the redshift range 0 < z < 1.7. An alternative and highly profitable approach is to measure the Hubble expansion rate, as a function of redshift, i.e. H(z). This would be advantageous since H(z) is linked to the dark energy models through a single redshift integral only, making model discrimination easier. Measuring H(z) is ambitious, however. It has taken decades of heated debate to reach a consensus for the local (i.e. z= 0) value, H0, of this rate. How can we therefore hope to track the evolution of H with time? We will show that new methods will not only detect H(z), but measure it to an accuracy of less than 3% at certain redshifts, significantly better than we know its value at z= 0, i.e. today

Coupling of radial and nonradial oscillations of relativistic stars: Gauge-invariant formalism

Abstract: Linear perturbation theory is appropriate to describe small oscillations of stars, while a mild nonlinearity is still tractable perturbatively but requires one to consider mode coupling, i.e., to take into account second order effects. It is natural to start to look at this problem by considering the coupling between linear radial and nonradial modes. A radial pulsation may be thought of as an important component of an overall mildly nonlinear oscillation, e.g., of a protoneutron star. Radial pulsations of spherical compact objects do not per se emit gravitational waves but, if the coupling between the existing first order radial and nonradial modes is efficient in driving and possibly amplifying the nonradial oscillations, one may expect the appearance of nonlinear harmonics, and gravitational radiation could then be produced to a significant level. More in general, mode coupling typically leads to an interesting phenomenology, thus it is worth investigating in the context of star perturbations. In this paper we develop the relativistic formalism to study the coupling of radial and nonradial first order perturbations of a compact spherical star. From a mathematical point of view, it is convenient to treat the two sets of perturbations as separately parametrized, using a 2-parameter perturbative expansion of the metric, the energy-momentum tensor and Einstein equations in which $\ensuremath{\lambda}$ is associated with the radial modes, $ϵ$ with the nonradial perturbations, and the $\ensuremath{\lambda}ϵ$ terms describe the coupling. This approach provides a well-defined framework to consider the gauge dependence of perturbations, allowing us to use $ϵ$ order gauge-invariant nonradial variables on the static background and to define new second order $\ensuremath{\lambda}ϵ$ gauge-invariant variables representing the result of the nonlinear coupling. We present the evolution and constraint equations for our variables outlining the setup for numerical computations, and briefly discuss the surface boundary conditions in terms of the second order $\ensuremath{\lambda}ϵ$ Lagrangian pressure perturbation.

Ringing the Randall-Sundrum braneworld: Metastable gravity wave bound states

Abstract: In the Randall-Sundrum scenario, our universe is a 4-dimensional ``brane`` living in a 5-dimensional bulk spacetime. By studying the scattering of bulk gravity waves, we show that this brane rings with a characteristic set of complex quasinormal frequencies, much like a black hole. To a bulk observer these modes are interpreted as metastable gravity wave bound states, while a brane observer views them as a discrete spectrum of decaying massive gravitons. Potential implications of these scattering resonances are discussed.

Delocalization of brane gravity by a bulk black hole

Abstract: We investigate the analogue of the Randall–Sundrum braneworld in the case when the bulk contains a black hole. Instead of the static vacuum Minkowski brane of the RS model, we have an Einstein static vacuum brane. We find that the presence of the bulk black hole has a dramatic effect on the gravity that is felt by brane observers. In the RS model, the 5D graviton has a stable localized zero mode that reproduces 4D gravity on the brane at low energies. With a bulk black hole, there is no such solution—gravity is delocalized by the 5D horizon. However, the brane does support a discrete spectrum of metastable massive bound states, or quasinormal modes, as was recently shown to be the case in the RS scenario. These states should dominate the high frequency component of the bulk gravity wave spectrum on a cosmological brane. We expect our results to generalize to any bulk spacetime containing a Killing horizon.

The general relativistic magnetohydrodynamic dynamo equation

Abstract: The magnetohydrodynamic dynamo equation is derived within general relativity, using the covariant 1 + 3 approach, for a plasma with finite electrical conductivity. This formalism allows for a clear division and interpretation of plasma and gravitational effects, and we are not restricted to a particular space‐time geometry. The results should be of interest in astrophysics and cosmology, and the formulation is well suited to gauge-invariant perturbation theory. Moreover, the dynamo equation is presented in some specific limits. In particular, we consider the interaction of gravitational waves with magnetic fields, and present results for the evolution of the linearly growing electromagnetic induction field, as well as the diffusive damping of these fields. Ke yw ords: gravitation ‐ gravitational waves ‐ MHD.

Amplitude of dark energy perturbations

Abstract: We propose a model which produces dark energy perturbations large enough to explain the lack of power seen at the quadrupole scale in the cosmic microwave background. If the dark energy is frozen from horizon exit during inflation until dark energy domination, then it is not possible to have perturbations in the dark energy which are large enough. We propose using a tachyonic amplification mechanism to overcome this. The dark energy is taken to be a complex scalar field, where the radial field has a Mexican hat potential. During inflation, the radial component is trapped near the maximum of its potential. At the end of inflation, it rolls down to the minimum. The dark energy today is taken to be a pseudo-Nambu-Goldstone boson. The perturbations generated during inflation are amplified by the rolling of the radial field. We also examine the use of the variable decay mechanism in order to generate an anticorrelation between the dark energy perturbations and the curvature perturbation. We show that using this mechanism then constrains the properties of the dark energy and its evolution from redshift one until today.

The speciality index as invariant indicator in the BKL mixmaster dynamics

Abstract: The long-standing difficulty in general relativity of classifying the dynamics of cosmological models, e.g. as chaotic, is directly related to the gauge freedom intrinsic to relativistic spacetime theories: in general the invariance under diffeomorphisms makes any analysis of dynamical evolution dependent on the particular choice of time slicing one uses. We show here that the speciality index, a scalar dimensionless curvature invariant that has been mainly used in numerical relativity as an indicator of the special or non-special Petrov-type character of a spacetime, is a time-independent quantity (a pure number) at each Kasner step of the Belinski–Khalatnikov–Lifshitz (BKL) map approximating the mixmaster cosmology. Thus the BKL dynamics can be characterized in terms of the speciality index, i.e. in terms of curvature invariants directly related to observables. Possible applications for the associated mixmaster dynamics are discussed.

Coupled boundary and bulk fields in anti-de Sitter spacetime

Abstract: We investigate the dynamics of a boundary field coupled to a bulk field with a linear coupling in an anti-de Sitter bulk spacetime bounded by a Minkowski (Randall-Sundrum) brane. An instability criterion for the coupled boundary and bulk system is found. There exists a tachyonic bound state when the coupling is above a critical value, determined by the masses of the brane and bulk fields and AdS curvature scale. This bound state is normalizable and localized near the brane, and leads to a tachonic instability of the system on large scales. Below the critical coupling, there is no tachyonic state and no bound state. Instead, we find quasinormal modes which describe stable oscillations, but with a finite decay time. Only if the coupling is tuned to the critical value does there exist a massless stable bound state, as in the case of zero coupling for massless fields. We discuss the relation to gravitational perturbations in the Randall-Sundrum brane-world

The environmental dependence of galaxy colors in intermediate redshift X-ray-selected clusters

Abstract: We present a wide-field imaging study of the colors of bright galaxies in 12 X-ray selected clusters and groups of galaxies at z ~ 0.3. The systems cover one of the largest ranges in X-ray luminosity (Lx ~ 10^43 - 10^45 erg/s), and hence mass, of any sample studied at this redshift. We find that the `red' galaxies form a tight color-magnitude relation (CMR) and that neither the slope nor zero-point of this relation changes significantly over the factor of 100 in X-ray luminosity of our sample. Using stellar population synthesis models we find our data allow a maximum possible change of 2 Gyrs in the typical age of the galaxies on the CMR over the range of Lx of our sample. We also measure the fraction of blue galaxies (fb) relative to the CMR in our clusters and find a low value of fb ~ 0.1 and find that there is no correlation between fb and Lx over our large Lx range. However, both the CMR and fb do depend on cluster radius, with the zero-point of the CMR shifting blueward in B-R by 0.10 +/- 0.036 magnitudes out to 0.75 times the virial radius, equivalent to a luminosity weighted age gradient of ~ 2.5 Gyrs per log(radius). It thus appears that the global cluster environment, in the form of cluster mass (Lx), has little influence on the properties of bright cluster galaxies, whereas the local environment, in the form of galaxy density (radius), has a strong effect. The range of ~ 100 in Lx corresponds to a factor of ~ 40 in ram-pressure efficiency, suggesting that ram-pressure stripping, or other mechanisms that depend on cluster mass like tidal stripping or harassment, are unlikely to be solely responsible for changing the galaxy population from the `blue' star forming galaxies, that dominate low density environments, to the `red' passive galaxies that dominate cluster cores.(abridged)

A late-accelerating universe with no dark energy - and a finite-temperature big bang

Abstract: Brane-world models offer the possibility of explaining the late acceleration of the universe via infra-red modifications to General Relativity, rather than a dark energy field. However, one also expects ultra-violet modifications to General Relativity, when high-energy stringy effects in the early universe begin to grow. We generalize the DGP brane-world model via an ultra-violet modification, in the form of a Gauss-Bonnet term in the bulk action. The combination of infra-red and ultra-violet modifications produces an intriguing cosmology. The DGP feature of late-time acceleration without dark energy is preserved, but there is an entirely new feature - there is no hot big bang in the early universe. The universe starts with finite density and pressure, from a "sudden" curvature singularity.

Universal fitting formulae for baryon oscillation surveys

Abstract: The next generation of galaxy surveys will attempt to measure the baryon oscillations in the clustering power spectrum with high accuracy. These oscillations encode a preferred scale which may be used as a standard ruler to constrain cosmological parameters and dark energy models. In this paper we present simple analytical fitting formulae for the accuracy with which the preferred scale may be determined in the tangential and radial directions by future spectroscopic and photometric galaxy redshift surveys. We express these accuracies as a function of survey parameters such as the central redshift, volume, galaxy number density and (where applicable) photometric redshift error. These fitting formulae should greatly increase the efficiency of optimizing future surveys, which requires analysis of a potentially vast number of survey configurations and cosmological models. The formulae are calibrated using a grid of Monte Carlo simulations, which are analyzed by dividing out the overall shape of the power spectrum before fitting a simple decaying sinusoid to the oscillations. The fitting formulae reproduce the simulation results with a fractional scatter of 7% (10%) in the tangential (radial) directions over a wide range of input parameters. We also indicate how sparse-sampling strategies may enhance the effective survey area if the sampling scale is much smaller than the projected baryon oscillation scale.

Cosmological structure formation in a homogeneous dark energy background

Abstract: There is now strong evidence that the current energy density of the Universe is dominated by dark energy with an equation of state w<-1/3, which is causing accelerated expansion. The build-up of structure within such Universes is subject to significant ongoing study, particularly through the spherical collapse model. This paper aims to review and consolidate progress for cosmologies in which the dark energy component remains homogeneous on the scales of the structures being modelled. The equations presented are designed to allow for dark energy with a general time-varying equation of state w(a). In addition to reviewing previous work, a number of new results are introduced: A new fitting formula for the linear growth factor in constant w cosmologies is given. A new formalism for determining the critical density for collapse is presented based on the initial curvature of the perturbation. The commonly used approximation to the critical density for collapse based on the linear growth factor is discussed for a general dark energy equation of state. Virialisation within such cosmologies is also considered, and the standard assumption that energy is conserved between turn-around and virialisation is questioned and limiting possiblities are presented.