Showing papers on "Cosmology published in 2001"

The mass function of dark matter haloes

Abstract: We combine data from a number of N-body simulations to predict the abundance of dark haloes in cold dark matter (CDM) universes over more than four orders of magnitude in mass. A comparison of different simulations suggests that the dominant uncertainty in our results is systematic and is smaller than 10–30 per cent at all masses, depending on the halo definition used. In particular, our 'Hubble volume' simulations of τCDM and ΛCDM cosmologies allow the abundance of massive clusters to be predicted with uncertainties well below those expected in all currently planned observational surveys. We show that for a range of CDM cosmologies and for a suitable halo definition, the simulated mass function is almost independent of epoch, of cosmological parameters and of the initial power spectrum when expressed in appropriate variables. This universality is of exactly the kind predicted by the familiar Press–Schechter model, although this model predicts a mass function shape that differs from our numerical results, overestimating the abundance of 'typical' haloes and underestimating that of massive systems.

Trans-Planckian problem of inflationary cosmology

Abstract: In most current models of inflation based on a weakly self-coupled scalar matter field minimally coupled to gravity, the period of inflation lasts so long that, at the beginning of the inflationary period, the physical wavelengths of comoving scales which correspond to the present large-scale structure of the Universe were smaller than the Planck length. Thus, the usual computations of the spectrum of fluctuations in these models involve extrapolating low-energy physics (both in the matter and gravitational sector) into regions where this physics is not applicable. In this article we study the dependence of the usual predictions of inflation for the spectrum of cosmological fluctuations on the hidden assumptions about super-Planck scale physics. We introduce a class of modified dispersion relations to mimic possible effects of super-Planck scale physics, and find that, given an initial state determined by minimizing the energy density, for dispersions relations introduced by Unruh the spectrum is unchanged, whereas for a class of dispersion relations similar to those used by Corley and Jacobson (which involve a more radical departure from the usual linear relation) important deviations from the usual predictions of inflation can be obtained. Some implications of this result for the unification of fundamental physicsmore » and early Universe cosmology are discussed.« less

Cosmology from MAXIMA-1, BOOMERANG, and COBE DMR cosmic microwave background observations.

Abstract: Recent results from BOOMERANG-98 and MAXIMA-1, taken together with COBE DMR, provide consistent and high signal-to-noise measurements of the cosmic microwave background power spectrum at spherical harmonic multipole bands over 2

Big Bang Nucleosynthesis Predictions for Precision Cosmology

Abstract: The determination of the primeval deuterium abundance has opened a precision era in big bang nucleosynthesis, making accurate predictions more important than ever before. We present in analytic form new, more precise predictions for the light-element abundances and their error matrix. Using our predictions and the primeval deuterium abundance, we infer a baryon density of ΩBh2 = 0.020 ± 0.002 (95% confidence level) and find no evidence for stellar production (or destruction) of 3He beyond burning D into 3He. Conclusions about 4He and 7Li currently hinge on possible systematic errors in their measurements.

The Reionization of the Universe by the First Stars and Quasars

Abstract: ▪ Abstract The formation of the first stars and quasars marks the transformation of the universe from its smooth initial state to its clumpy current state. In popular cosmological models, the first sources of light began to form at a redshift z = 30 and reionized most of the hydrogen in the universe by z = 7. Current observations are at the threshold of probing the hydrogen reionization epoch. The study of high-redshift sources is likely to attract major attention in observational and theoretical cosmology over the next decade.

Cosmic shear statistics and cosmology

Abstract: We report a measurement of cosmic shear correlations using an eective area of 6.5 deg 2 of the VIRMOS deep imaging survey in progress at the Canada-France-Hawaii Telescope. We measured various shear correlation functions, the aperture mass statistic and the top-hat smoothed variance of the shear with a detection signicance exceeding 12 . We present results on angular scales from 3 arcsec to half a degree. The lensing origin of the signal is conrmed through tests that rely on the scalar nature of the gravitational potential. The dierent statistical measures give consistent results over the full range of angular scales. These important tests of the measurements demonstrate that the measured correlations could provide accurate constraints on cosmological parameters, subject to the systematic uncertainty in the source redshift distribution. The measurement over more than two decades of scale allows one to evaluate the eect of the shape of the power spectrum on cosmological parameter estimation. The degeneracy on 8 0 can be broken if priors on the shape of the linear power spectrum (parameterized by ) are assumed. For instance, with = 0 :21 and at the 95% condence level, we obtain 0:65 0: 7a nd0 < 0:4 for flat (-CDM) models. We discuss how these results would scale if the assumed source redshift distribution needed to be modied with forthcoming measurements of photometric redshifts. From the tangential/radial mode decomposition we can set an upper limit on the intrinsic shape alignment, which has recently been suggested as a possible contribution to the lensing signal. Within the error bars, there is no detection of intrinsic shape alignment for scales larger than 1 0 .

Power-spectrum normalization from the local abundance of rich clusters of galaxies

Abstract: The number density of rich galaxy clusters still provides the most robust way of normalizing the power spectrum of dark matter perturbations on scales relevant to large-scale structure. We revisit this constraint in the light of several recent developments: (1) the availability of well-defined samples of local clusters with relatively accurate X-ray temperatures; (2) new theoretical mass functions for dark matter haloes, which provide a good fit to large numerical simulations; (3) more accurate mass–temperature relations from larger catalogues of hydrodynamical simulations; (4) the requirement to consider closed as well as open and flat cosmologies to obtain full multiparameter likelihood constraints for CMB and SNe studies. We present a new sample of clusters drawn from the literature and use this sample to obtain improved results on σ8, the normalization of the matter power spectrum on scales of 8 h−1 Mpc, as a function of the matter density and cosmological constant in a universe with general curvature. We discuss our differences with previous work, and the remaining major sources of uncertainty. Final results on the normalization, approximately independent of power spectrum shape, can be expressed as constraints on σ at an appropriate cluster normalization scale RCl. We provide fitting formulas for RCl and σ(RCl) for general cosmologies, as well as for σ8 as a function of cosmology and shape parameter Γ. For flat models we find approximately σ8≃(0.495−0.037+0.034)ΩM−0.60 for Γ=0.23, where the error bar is dominated by uncertainty in the mass–temperature relation.

Cosmic microwave background anisotropy power spectrum statistics for high precision cosmology

Abstract: As the era of high precision cosmology approaches, the empirically determined power spectrum of the microwave background anisotropy $C_l$ will provide a crucial test for cosmological theories. We present an exact semi-analytic framework for the study of the ampling statistics of the $C_l$ resulting from observations with partial sky coverage and anisotropic noise distributions. This includes space-borne, air-borne and ground-based experiments. We apply this theory to demonstrate its power for constructing fast but unbiased approximate methods for the joint estimation of cosmological parameters. Further applications, such as a test for possible non-Gaussianity of the underlying theory and a ``poor man's power spectrum estimator'' are suggested. An appendix derives recursion relations for the efficient computation of the couplings between spherical harmonics on the cut sky.

String theory and hybrid inflation / acceleration

Abstract: We find a description of hybrid inflation in (3+1)-dimensions using brane dynamics of Hanany-Witten type. P-term inflation/acceleration of the universe with the hybrid potential has a slow-roll de Sitter stage and a waterfall stage which leads towards an = 2 supersymmetric ground state. We identify the slow-roll stage of inflation with a non-supersymmetric `Coulomb phase' with Fayet-Iliopoulos term. This stage ends when the mass squared of one of the scalars in the hypermultiplet becomes negative. At that moment the brane system starts undergoing a phase transition via tachyon condensation to a fully Higgsed supersymmetric vacuum which is the absolute ground state of P-term inflation. A string theory/cosmology dictionary is provided, which leads to constraints on parameters of the brane construction from cosmological experiments. We display a splitting of mass levels reminiscent of the Zeeman effect due to spontaneous supersymmetry breaking.

Constraints from the Lyα Forest Power Spectrum

Abstract: We use published measurements of the transmission power spectrum of the Ly? forest to constrain several parameters that describe cosmology and thermal properties of the intergalactic medium (IGM). A six-parameter grid is constructed using particle-mesh dark matter simulations together with scaling relations to make predictions for the gas properties. We fit for all parameters simultaneously and identify several degeneracies. We find that the temperature of the IGM can be well determined from the falloff of the power spectrum at small scales. We find a temperature of around 2 ? 104 K, dependent on the slope of the gas equation of state. We see no evidence for evolution in the IGM temperature. We place constraints on the amplitude of the dark matter fluctuations. However, contrary to previous results, the slope of the dark matter power spectrum is poorly constrained. This is because of uncertainty in the effective Jeans smoothing scale, which depends on the temperature as well as the thermal history of the gas.

A Serendipitous Galaxy Cluster Survey with XMM: Expected Catalog Properties and Scientific Applications

Abstract: This paper describes a serendipitous galaxy cluster survey that we plan to conduct with the XMM X-ray satellite. We have modeled the expected properties of such a survey for three di†erent cosmological models, using an extended Press-Schechter formalism combined with a detailed characterization of the expected capabilities of the European Photon Imaging Camera (EPIC) camera on board XMM. We estimate that, over the 10 yr design lifetime of XMM, the EPIC camera will image a total of ^800 deg2 in Ðelds suitable for the serendipitous detection of clusters of galaxies. For the presently favored low-density model with a cosmological constant, our simulations predict that this survey area would yield a catalog of more than 8000 clusters, ranging from poor to very rich systems, with around 750 detections above z\ 1. A low-density open universe yields similar numbers, though with a di†erent redshift distribution, while a critical-density universe gives considerably fewer clusters. This dependence of catalog properties on cosmology means that the proposed survey will place strong constraints on the values of and The survey would also facilitate a variety of follow-up projects, including the quan- )0 )" . tiÐcation of evolution in the cluster X-ray luminosity-temperature relation, the study of high-redshift galaxies via gravitational lensing, follow-up observations of the Sunyaev-Zeldovich e†ect, and foreground analyses of cosmic microwave background maps.

Modelling the uv/x-ray cosmic background with CUBA

Abstract: In this paper, I will describe the features of the numerical code CUBA, aimed at the solution of the radiative transfer equation in a cosmological context. CUBA will be soon available for public use at the URL this http URL, allowing for several user-supplied input parameters, such as favourite cosmology, luminosity functions, Type II object evolution, stellar spectra, and many others. I will also present some new results of the UV/X-ray cosmic background as produced by the observed populations of QSOs and star forming galaxies, updating and extending our previous works. The background evolution is complemented with a number of derived quantities such as the ionization and thermal state of the IGM, the HeII opacity, the HI and HeII ionization rates, and the HI, HeII and Compton heating rates.

A Practical Theorem on Gravitational Wave Backgrounds

Abstract: There is an extremely simple relationship between the spectrum of the gravitational wave background produced by a cosmological distribution of discrete gravitational wave sources, the total time-integrated energy spectrum of an individual source, and the present-day comoving number density of remnants. Stated in this way, the background is entirely independent of the cosmology, and only weakly dependent on the evolutionary history of the sources. This relationship allows one easily to compute the amplitude and spectrum of cosmic gravitational wave backgrounds from a broad range of astrophysical sources, and to evaluate the uncertainties therein.

A Measurement of H0 from the Sunyaev-Zeldovich Effect

Abstract: We present a determination of the Hubble constant, H0, from measurements of the Sunyaev-Zeldovich effect in an orientation-unbiased sample of seven z < 01 galaxy clusters With improved X-ray models and a more accurate 32 GHz calibration, we obtain H0 = 64 ± 14sys km s-1 Mpc-1 for a standard cold dark matter (CDM) cosmology, or H0 = 66 ± 15sys km s-1 Mpc-1 for a flat ΛCDM cosmology In combination with X-ray cluster measurements and the big bang nucleosynthesis value for ΩB, we find ΩM = 032 ± 005

Anisotropy dissipation in brane-world inflation

Abstract: 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 Blanchi type 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. ©2001 The American Physical Society.

Stress-energy tensor for trans-Planckian cosmology

Abstract: This article presents the derivation of the stress-energy tensor of a free scalar field with a general non-linear dispersion relation in curved spacetime. This dispersion relation is used as a phenomelogical description of the short distance structure of spacetime following the conventional approach of trans-Planckian modes in black hole physics and in cosmology. This stress-energy tensor is then used to discuss both the equation of state of trans-Planckian modes in cosmology and the magnitude of their backreaction during inflation. It is shown that gravitational waves of trans-Planckian momenta but subhorizon frequencies cannot account for the form of cosmic vacuum energy density observed at present, contrary to a recent claim. The backreaction effects during inflation are confirmed to be important and generic for those dispersion relations that are liable to induce changes in the power spectrum of metric fluctuations. Finally, it is shown that in pure de Sitter inflation there is no modification of the power spectrum except for a possible magnification of its overall amplitude independently of the dispersion relation.

Analyses of Type Ia Supernova Data in Cosmological Models with a Local Void

Abstract: The data for type Ia supernovae obtained by the High-z SN Search Team and Supernova Cosmology Project are analyzed using inhomogeneous cosmological models with a local void on scales of about 200 Mpc, to derive the best-fit values of model parameters and the confidence contours The Χ 2 fitting is found to be slightly better than that in homogeneous models It is shown that (1) the best-fit values are most sensitive to the ratio R of the outer Hubble constant, H I I 0 , to the inner Hubble constant, H I 0 , (2) the best-fit outer density parameter, Ω I I 0 , and cosmological constant parameter, λ I I 0 , are, respectively, increasing and decreasing functions of R, and (3) (Ω I I 0 , All) can be (1, 0) for R 08 Moreover, it is shown that these models are naturally consistent with the new supernova data (SN1997ff) with z = 17

Evolution of Cosmological Perturbations in a Brane-Universe

Abstract: In our present Letter, we analyze the impact of the existence of extra dimensions on cosmology, in particular, on the evolution of cosmological perturbations. For a five-dimensional anti--de Sitter spacetime where ordinary matter is confined to a brane-universe, the equations governing the cosmological perturbations are presented in a form very close to the equations of standard cosmology. Two types of corrections appear: corrections due to the unconventional evolution of the homogeneous solution, which change the background-dependent coefficients of the equations, and corrections due to the curvature along the fifth dimension, which act as source terms in the evolution equations.

Large extra dimensions and cosmological problems

Abstract: We consider a variant of the brane-world model in which the universe is the direct product of a Friedmann-Robertson-Walker (FRW) space and a compact hyperbolic manifold of dimension $dg~2.$ Cosmology in this space is particularly interesting. The dynamical evolution of the space-time leads to the injection of a large entropy into the observable (FRW) universe. The exponential dependence of surface area on distance in hyperbolic geometry makes this initial entropy very large, even if the CHM has a relatively small diameter (in fundamental units). The very large statistical averaging inherent in the collapse of the initial entropy onto the brane acts to smooth out initial inhomogeneities. This smoothing is then sufficient to account for the current homogeneity of the universe. With only mild fine-tuning, the current flatness of the universe can also then be understood. Finally, recent brane-world approaches to the hierarchy problem can be readily realized within this framework.

The 2dF QSO Redshift Survey — II. Structure and evolution at high redshift

Abstract: use a sample of 10 558 QSOs taken from the preliminary data release catalogue of the 2dF QSO Redshift Survey (2QZ). The two-point redshift-space correlation function of QSOs, jQðsÞ; is shown to follow a power law on scales s . 1–35 h Mpc: Fitting a power law of the form jQðsÞ 1⁄4 ðs/ s0Þ to the QSO clustering averaged over the redshift interval 0:3 , z # 2:9; we find s0 1⁄4 3:99 20:34 h Mpc and g 1⁄4 1:58 20:09 for an Einstein–de Sitter cosmology. The effect of a significant cosmological constant, l0, is to increase the separation of QSOs, so that with V0 1⁄4 0:3; l0 1⁄4 0:7 the power law extends to .60 h 21 Mpc and the best fit is s0 1⁄4 5:69 20:50 h Mpc and g 1⁄4 1:56 20:09: These values, measured at a mean redshift of z 1⁄4 1:49; are comparable to the clustering of local optically selected galaxies. We compare the clustering of 2QZ QSOs with generic cold dark matter (CDM) models with shape parameter Geff. Standard CDM with Geff 1⁄4 0:5 is ruled out in both Einstein–de Sitter and cosmological constant dominated cosmologies, where Geff . 0:2–0:4 and Geff . 0:1–0:2 respectively are the allowable ranges. We measure the evolution of QSO clustering as a function of redshift. For V0 1⁄4 1 and l0 1⁄4 0 there is no significant evolution in comoving coordinates over the redshift range of the 2QZ. QSOs thus have similar clustering properties to local galaxies at all redshifts that we sample. In the case of V0 1⁄4 0:3 and l0 1⁄4 0:7, QSO clustering shows a marginal increase at high redshift, s0 being a factor of ,1.4 higher at z . 2:4 than at z . 0:7. Although the clustering of QSOs is measured on large scales where linear theory should apply, the evolution of QSO clustering does not follow the linear theory predictions for growth via gravitational instability (rejected at the .99 per cent confidence level). A redshift-dependent bias is required to reconcile QSO clustering observations with theory. A simple biasing model, in which QSOs have cosmologically long lifetimes (or alternatively form in peaks above a constant threshold in the density field), is acceptable in an V0 1⁄4 1 cosmology, but is only marginally acceptable if V0 1⁄4 0:3 and l0 1⁄4 0:7. Biasing models in which QSOs are assumed to form over a range in redshift, based on the Press–Schechter formalism, are consistent with QSO clustering evolution for a minimum halo mass of ,10 and ,10 M( in an Einstein–de Sitter and cosmological constant dominated universe, respectively. However, until an accurate, physically motivated model of QSO formation and evolution is developed, we should be cautious in interpreting the fits to these biasing models.

Lensing at cosmological scales: a test of higher dimensional gravity

Abstract: Recent developments in gravitational lensing astronomy have paved the way to genuine mappings of the gravitational potential at cosmological scales. We stress that comparing these data with traditional large scale structure surveys will provide us with a test of gravity at such scales. These constraints could be of great importance in the framework of higher dimensional cosmological models.

An Holographic cosmology

Abstract: We present a new cosmological model, based on the holographic principle, which shares many of the virtues of inflation. The very earliest semiclassical era of the universe is dominated by a dense gas of black holes, with equation of state $p=\rho$. Fluctuations lead to an instability to a phase with a dilute gas of black holes, which later decays via Hawking radiation to a radiation dominated universe. The quantum fluctuations of the initial state give rise to a scale invariant spectrum of density perturbations, for a range of scales. We point out a problem, that appears to prevent the range of scales predicted by the model from coinciding with the range where such a spectrum has been observed. We speculate that this may be related to our field theoretic treatment of fluctuations in the highly holographic $p=\rho$ background. The monopole problem is solved in a manner completely different from inflationary models, and a relic density of highly charged extremal black monopoles is predicted. We discuss the nature of the entropy and flatness problems in our model.

Cosmology and Particle Astrophysics

Abstract: The Observable Universe.- Special Relativity.- General Relativity.- Cosmological Models.- Gravitational Lensing.- Particles and Fields.- Phase Transitions.- Thermodynamics in the Early Universe.- Thermal Relics from the Big Bang.- The Accelerating Universe.- The Cosmic Microwave Background Radiation and Growth of Structure.- Cosmic Rays.- Cosmic Gamma-Rays.- The Role of Neutrinos.- Gravitational Waves.

Cosmological expansion on a dilatonic brane-world

Abstract: In this paper we study brane-world scenarios with a bulk scalar field, using a covariant formalism to obtain a 4D Einstein equation via projection onto the brane. We discuss, in detail, the effects of the bulk on the brane and how the scalar field contribute to the gravitational effects. We also discuss choice of conformal frame and show that the frame selected by the induced metric provides a natural choice. We demonstrate our formalism by applying it to cosmological scenarios of Randall-Sundrum and Horava-Witten type models. Finally we consider the cosmology of models where the scalar field couples non-minimally to the matter on the brane. This gives rise to a novel scenario where the universe expands from a finite scale factor with an initial period of accelerated expansion, thus avoiding the singularity and flatness problem of the standard big bang model.

Nonlinear Gravitational Growth of Large-Scale Structures Inside and Outside Standard Cosmology

Abstract: We reconsider the problem of gravitational structure formation inside and outside general relativity (GR), in both the weakly and strongly nonlinear regime We show how these regimes can be explored observationally through clustering of high-order cumulants and through the epoch of formation, abundance, and clustering of collapse structures, using Press-Schechter formalism and its extensions We address the question of how different these predictions are when using a nonstandard theory of gravity We study examples of cosmologies that do not necessarily obey Einstein's field equations: scalar-tensor theories (STT), such as Brans-Dicke (BD), parametrized with ω, a nonstandard parameterization of the Hubble law, H2 = a-3(1+), or a nonstandard cosmic equation of state p = γρ, where γ can be chosen irrespective of the cosmological parameters (ΩM and ΩΛ) We present some preliminary bounds on γ, ω, and from observations of the skewness and kurtosis in the Automated Plate Measuring (APM) Galaxy Survey This test is independent of the overall normalization of rms fluctuations We also show how abundances and formation times change under these assumptions Upcoming data on nonlinear growth will place strong constraints on such variations from the standard paradigm

Non-uniform reionization by galaxies and its effect on the cosmic microwave background

Abstract: We present predictions for the reionization of the intergalactic medium (IGM) by stars in high-redshift galaxies, based on a semi-analytic model of galaxy formation. We calculate ionizing luminosities of galaxies, including the effects of absorption by interstellar gas and dust on the escape fraction ƒesc, and follow the propagation of the ionization fronts around each galaxy in order to calculate the filling factor of ionized hydrogen in the IGM. For a ΛCDM cosmology, with parameters of the galaxy formation model chosen to match observations of present-day galaxies, and a physical calculation of the escape fraction, we find that the hydrogen in the IGM will be reionized at redshift z=6.1 if the IGM has uniform density, but only by z=4.5 if the IGM is clumped. If instead we assume a constant escape fraction of 20 per cent for all galaxies, then we find reionization at z=4.5 and 7.8 for the same two assumptions about IGM clumping. We combine our semi-analytic model with an N-body simulation of the distribution of dark matter in the Universe in order to calculate the evolution of the spatial and velocity distribution of the ionized gas in the IGM, and use this to calculate the secondary temperature anisotropies induced in the cosmic microwave background (CMB) by scattering off free electrons. The models predict a spectrum of secondary anisotropies covering a broad range of angular scales, with fractional temperature fluctuations ∼10−7-10−6 on arcminute scales. The amplitude depends strongly on the total baryon density, and less sensitively on ƒesc. The amplitude also depends somewhat on the geometry of reionization, with models in which the regions of highest gas density are reionized first giving larger CMB fluctuations than the case where galaxies ionize surrounding spherical regions, and models where low-density regions reionize first giving the smallest fluctuations. Measurement of these anisotropies can therefore put important constraints on the reionization process, in particular, the redshift evolution of the filling factor, and should be a primary objective of a next generation submillimetre telescope such as the Atacama Large Millimeter Array.