Showing papers on "Cosmology published in 2000"

Cosmological Reionization by Stellar Sources

Abstract: I use cosmological simulations that incorporate a physically motivated approximation to three- dimensional radiative transfer that recovers correct asymptotic ionization front propagation speeds for some cosmologically relevant density distributions to investigate the process of the reionization of the universe by ionizing radiation from protogalaxies. Reionization proceeds in three stages and occupies a large redshift range from z D 15 until z D 5. During the —rst, ii preoverlap ˇˇ stage, H II regions gradually expand into the low-density intergalactic medium, leaving behind neutral high-density protrusions. During the second, ii overlap ˇˇ stage, that occurs in about 10% of the Hubble time, H II regions merge and the ionizing background rises by a large factor. During the third, ii postoverlap ˇˇ stage, remaining high-density regions are being gradually ionized as the required ionizing photons are being produced. Residual —uctuations in the ionizing background reach signi—cant (more than 10%) levels for the Lya forest absorption systems with column densities above 1014¨1015 cm~2 at z \ 3¨4. Subject headings: cosmology: theorygalaxies: formationintergalactic medium ¨ large-scale structure of universe

LETTER TO THE EDITOR: Cosmic holography + Cosmic holography

Abstract: A version of holographic principle for the cosmology is proposed, which dictates that the particle entropy within the cosmological apparent horizon should not exceed the gravitational entropy associated with the apparent horizon. It is shown that, in the Friedmann-Robertson-Walker (FRW) cosmology, the open Universe as well as a restricted class of flat cases are compatible with the principle, whereas closed Universe is not. It is also found that inflationary universe after the big-bang is incompatible with the cosmic holography.

Relativistic Cosmology. I

Abstract: The paper presents some general relations obtaining in relativistic cosmology. It appears from these that a simple change over to anisotropy without the introduction of spin does not solve any of the outstanding difficulties of isotropic cosmological models.

Brane world cosmology

Abstract: A simple model of the brane-world cosmology has been proposed, which is characterized by four parameters, namely, the bulk cosmological constant, the spatial curvature of the universe, the radiation strength arising from bulk space-time and the breaking parameter of 2-symmetry. The bulk space-time is assumed to be locally static five-dimensional analogue of the Schwarzschild-Anti-de Sitter space-time, and then the location of three-brane is determined by the metric junction. The resulting Friedmann equation recovers the standard cosmology, and a new term arises if 2-symmetry is not assumed, which behaves as a cosmological term in the early universe, next turns to a negative curvature term, and finally damps rapidly.

Dwarf Galaxy Rotation Curves and the Core Problem of Dark Matter Halos

Abstract: The standard cold dark matter (CDM) model has recently been challenged by the claim that dwarf galaxies have dark matter halos with constant density cores. Consequently, numerous alternative dark matter candidates have recently been proposed. In this paper, we scrutinize the observational evidence for the incongruity between dwarf galaxies and the CDM model. To this end, we analyze the rotation curves of 20 late-type dwarf galaxies studied by Swaters (1999). Taking the effects of beam-smearing and adiabatic contraction into account, we fit mass models to these rotation curves with dark matter halos with different cusp slopes, ranging from constant density cores to r^{-2} cusps. Uncertainties in the stellar mass-to-light ratio and the limited spatial sampling of the halo's density distribution hamper a unique mass decomposition. Consequently, the rotation curves in our sample cannot be used to discriminate between dark halos with constant density cores and r^{-1} cusps. We show that the dwarf galaxies analyzed here are consistent with cold dark matter halos in a LCDM cosmology, and that there is thus no need to abandon the idea that dark matter is cold and collisionless. However, the data is also consistent with any alternative dark matter model that produces dark matter halos with central cusps less steep than r^{-1.5}. In fact, we argue that based on existing rotation curves alone at best weak limits can be obtained on cosmological parameters and/or the nature of the dark matter.

Cosmic γ-ray background from structure formation in the intergalactic medium

Abstract: The Universe is filled with a diffuse background of γ-ray radiation1, the origin of which remains one of the unsolved puzzles of cosmology. Less than one-quarter of the γ-ray flux can be attributed to unresolved discrete sources2,3, such as active galactic nuclei; the remainder appears to constitute a truly diffuse background. Here we show that the shock waves induced by gravity in the gas of the intergalactic medium, during the formation of large-scale structures like filaments and sheets of galaxies, produce a population of highly relativistic electrons. These electrons scatter a small fraction of the cosmic microwave background photons in the local Universe up to γ-ray energies, thereby providing the γ-ray background. The predicted diffuse flux agrees with the observed background across more than four orders of magnitude in photon energy, and the model predicts that the γ-ray background, though generated locally, is isotropic to better than five per cent on angular scales larger than a degree. Moreover, the agreement between the predicted and observed background fluxes implies a mean cosmological density of baryons that is consistent with Big Bang nucleosynthesis.

Why the Universe is Just So

Abstract: Some properties of the world are fixed by physics derived from mathematical symmetries, while others are selected from an ensemble of possibilities. Several successes and failures of ‘‘anthropic’’ reasoning in this context are reviewed in light of recent developments in astrobiology, cosmology, and unification physics. Specific issues raised include our space-time location (including the reason for the present age of the universe), the time scale of biological evolution, the tuning of global cosmological parameters, and the origin of the Large Numbers of astrophysics and the parameters of the standard model. Out of the 20 parameters of the standard model, the basic behavior and structures of the world (nucleons, nuclei, atoms, molecules, planets, stars, galaxies) depend mainly on five of them: m e , m u , m d , a, and a G (where m proton and a QCD are taken as defined quantities). Three of these appear to be independent in the context of Grand Unified Theories (that is, not fixed by any known symmetry) and at the same time have values within a very narrow window which provides for stable nucleons and nuclei and abundant carbon. The conjecture is made that the two light quark masses and one coupling constant are ultimately determined even in the ‘‘final theory’’ by a choice from a large or continuous ensemble, and the prediction is offered that the correct unification scheme will not allow calculation of (m d2m u)/m proton from first principles alone.

Gamma-Ray Background from Structure Formation in the Intergalactic Medium

Abstract: The universe is filled with a diffuse and isotropic extragalactic background of gamma-ray radiation, containing roughly equal energy flux per decade in photon energy between 3 MeV-100 GeV. The origin of this background is one of the unsolved puzzles in cosmology. Less than a quarter of the gamma-ray flux can be attributed to unresolved discrete sources, but the remainder appears to constitute a truly diffuse background whose origin has hitherto been mysterious. Here we show that the shock waves induced by gravity during the formation of large-scale structure in the intergalactic medium, produce a population of highly-relativistic electrons with a maximum Lorentz factor above 10^7. These electrons scatter a small fraction of the microwave background photons in the present-day universe up to gamma-ray energies, thereby providing the gamma-ray background. The predicted diffuse flux agrees with the observed background over more than four decades in photon energy, and implies a mean cosmological density of baryons which is consistent with Big-Bang nucleosynthesis.

Supernova explosions in the Universe

Abstract: During the lifetime of our Milky Way galaxy, there have been something like 100 million supernova explosions, which have enriched the Galaxy with the oxygen we breathe, the iron in our cars, the calcium in our bones and the silicon in the rocks beneath our feet. These exploding stars also influence the birth of new stars and are the source of the energetic cosmic rays that irradiate us on the Earth. The prodigious amount of energy (approximately 10(51), or approximately 2.5 x 10(28) megatonnes of TNT equivalent) and momentum associated with each supernova may even have helped to shape galaxies as they formed in the early Universe. Supernovae are now being used to measure the geometry of the Universe, and have recently been implicated in the decades-old mystery of the origin of the gamma-ray bursts. Together with major conceptual advances in our theoretical understanding of supernovae, these developments have made supernovae the centre of attention in astrophysics.

A different approach to cosmology

Abstract: Modern cosmology began with the solutions to Einstein's theory of gravity discovered by Aleksandr Friedmann and Georges Lemaitre in the 1920s. When combined with the Hubble redshift‐distance relation, these solutions could be interpreted as showing that we live in an expanding universe. By 1930, the scientific establishment and much of the lay public believed in this expanding cosmos. It then requires only time reversal and elementary logic to conclude that the universe must originally have been so compact that we can talk of a beginning. Lemaitre tried to describe this state as the “primeval atom.”

General primordial cosmic perturbation

Abstract: We consider the most general primordial cosmological perturbation in a universe filled with photons, baryons, neutrinos, and a hypothetical cold dark matter (CDM) component within the framework of linearized perturbation theory. We present a careful discussion of the different allowed modes, distinguishing modes which are regular at early times, singular at early times, or pure gauge. As well as the familiar growing and decaying adiabatic modes and the baryonic and CDM isocurvature modes, we identify two neutrino isocurvature modes. In the first, the ratio of neutrinos to photons varies spatially but the net density perturbation vanishes. In the second the photon-baryon plasma and the neutrino fluid have a spatially varying relative bulk velocity balanced so that the net momentum density vanishes. Possible mechanisms which could generate the two neutrino isocurvature modes are discussed. If one allows the most general regular primordial perturbation, all quadratic correlators of observables such as the microwave background anisotropy and matter perturbations are completely determined by a $5\ifmmode\times\else\texttimes\fi{}5,$ real, symmetric matrix-valued function of comoving wave number. In a companion paper we examine prospects for detecting or constraining the amplitudes of the most general allowed regular perturbations using present and future CMB data.

Compact hyperbolic extra dimensions: branes, kaluza-klein modes, and cosmology

Abstract: We reconsider theories with low gravitational (or string) scale M where Newton’s constant is generated via new large-volume spatial dimensions, while Standard Model states are localized to a 3brane. Utilizing compact hyperbolic manifolds (CHM’s) we show that the spectrum of Kaluza-Klein (KK) modes is radically altered. This allows an early universe cosmology with normal evolution up to substantial temperatures , and completely negates the constraints on M arising from astrophysics. Furthermore, an exponential hierarchy between the usual Planck scale and the true fundamental scale of physics can emerge with only O(1) coecients. The linear size of the internal space remains small. The proposal has striking testable signatures.

Cosmological expansion in the Randall-Sundrum brane world scenario

Abstract: The cosmology of the Randall-Sundrum scenario for a positive tension brane in a 5D universe with localized gravity has been studied previously In the radiation-dominated universe, it was suggested that there are two solutions for the cosmic scale factor $a(t):$ the standard solution $a\ensuremath{\sim}{t}^{1/2},$ and a solution $a\ensuremath{\sim}{t}^{1/4},$ which is incompatible with standard big bang nucleosynthesis In this paper, we reconsider expansion of the Universe in this scenario We derive and solve a first order, linear differential equation for ${H}^{2},$ the square of the expansion rate of the Universe, as a function of a The differences between our equation for ${H}^{2}$ and the relationship found in standard cosmology are (i) there is a term proportional to density squared (a fact already known), which is small when the density is small compared to the brane tension, and (ii) there is a contribution which acts like a relativistic fluid We show that this second contribution is due to gravitational degrees of freedom in the bulk Thus, we find that there need not be any conflict between cosmology of the Randall-Sundrum scenario and the standard model of cosmology We discuss how reheating at the end of inflation leads to the correct relationship between matter density and expansion rate, ${H}^{2}\ensuremath{\rightarrow}8\ensuremath{\pi}G{\ensuremath{\rho}}_{m}/3,$ and the conditions that must be met for the expansion rate of the universe to be close to its standard model value around the epoch of cosmological nucleosynthesis

A critical cosmological constant from millimeter extra dimensions

Abstract: We consider `brane universe' scenarios with standard-model fields localized on a 3-brane in 6 spacetime dimensions. We show that if the spacetime is rotationally symmetric about the brane, local quantities in the bulk are insensitive to the couplings on the brane. This potentially allows compactifications where the effective 4-dimensional cosmological constant is independent of the couplings on the 3-brane. We consider several possible singularity-free compactification mechanisms, and find that they do not maintain this property. We also find solutions with naked spacetime singularities, and we speculate that new short-distance physics can become important near the singularities and allow a compactification with the desired properties. The picture that emerges is that standard-model loop contributions to the effective 4-dimensional cosmological constant can be cut off at distances shorter than the compactification scale. At shorter distance scales, renormalization effects due to standard-model fields renormalize the 3-brane tension, which changes a deficit angle in the transverse space without affecting local quantities in the bulk. For a compactification scale of order 10−2 mm, this gives a standard-model contribution to the cosmological constant in the range favored by cosmology.

Nonminimal derivative couplings and inflation in generalized theories of gravity

Abstract: We study extended theories of gravity where nonminimal derivative couplings of the form Rφ, kφ, l are present in the Lagrangian. We show how and why the other couplings of similar structure may be ruled out and then deduce the field equations and the related cosmological models. Finally, we get inflationary solutions which do follow neither from any effective scalar field potential nor from a cosmological constant introduced “by hand”, and we show the de Sitter space–time to be an attractor solution. PACS number(s): 04.50.+h, 98.80.Cq Keyword(s): Cosmology, Alternative Theories of Gravity, Nonminimal Coupling. E-mail: capozziello@vaxsa.csied.unisa.it E-mail: lambiase@physics.unisa.it E-mail: hjschmi@rz.uni-potsdam.de

The Cosmological Constant

Abstract: This is a review of the physics and cosmology of the cosmological constant. Focusing on recent developments, I present a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy.

Supernovae, Dark Energy and the Accelerating Universe

Abstract: or millennia, cosmology has been a theorist’s domain, where elegant theory was only occasionally endangered by inconvenient facts. Early in the 20th century, Albert Einstein gave us new conceptual tools to rigorously address the questions of the origins, evolution, and fate of the universe. In recent years, technology has developed to the point where these concepts from general relativity can be substantiated and elaborated by measurements. For example, measurement of the remnant glow from the hot, dense beginnings of the expanding universe—the cosmic microwave background—is yielding increasingly detailed data about the first half-million years and the overall geometry of the cosmos (see the news story on page 21 of this issue). The standard model of particle physics has also begun to play a prominent role in cosmology. The widely accepted idea of exponential inflation in the immediate aftermath of the Big Bang was built on the predicted effect of certain putative particle fields and potentials on the cosmic expansion. Measuring the history of cosmic expansion is no easy task, but in recent years, a specific variety of supernovae, type Ia, has given us a first glimpse at that history—and surprised us with an unexpected plot twist.

Constraints from the Lyman alpha forest power spectrum

Abstract: We use published measurements of the transmission power spectrum of the Lyman alpha forest to constrain several parameters that describe cosmology and thermal properties of the intergalactic medium (IGM). A 6 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 fall-off of the power spectrum at small scales. We find a temperature around 20.000 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 due to uncertainty in the effective Jeans smoothing scale, which depends on the temperature as well as the thermal history of the gas.

The cosmic microwave background radiation temperature at a redshift of 2.34

Abstract: The existence of the cosmic microwave background radiation is a fundamental prediction of hot Big Bang cosmology, and its temperature should increase with increasing redshift. At the present time (redshift z = 0), the temperature has been determined with high precision to be T(CMBR)(0) = 2.726 +/- 0.010 K. In principle, the background temperature can be determined using measurements of the relative populations of atomic fine-structure levels, which are excited by the background radiation. But all previous measurements have achieved only upper limits, thus still formally permitting the radiation temperature to be constant with increasing redshift. Here we report the detection of absorption lines from the first and second fine-structure levels of neutral carbon atoms in an isolated cloud of gas at z = 2.3371. We also detected absorption due to several rotational transitions of molecular hydrogen, and fine-structure lines of singly ionized carbon. These constraints enable us to determine that the background radiation was indeed warmer in the past: we find that T(CMBR)(z = 2.3371) is between 6.0 and 14 K. This is in accord with the temperature of 9.1 K predicted by hot Big Bang cosmology.

Type Ia supernovae, evolution, and the cosmological constant

Abstract: We explore the possible role of evolution in the analysis of data on Type Ia supernovae (SNe Ia) at cosmological distances. First, using a variety of simple sleuthing techniques, we find evidence that the properties of the high- and low-redshift SNe Ia observed so far differ from one another. Next, we examine the effects of allowing for an uncertain amount of evolution in the analysis, using two simple phenomenological models for evolution and prior probabilities that express a preference for no evolution but allow it to be present. One model shifts the magnitudes of the high-redshift SNe Ia relative to the low-redshift SNe Ia by a fixed amount. A second, more realistic, model introduces a continuous magnitude shift of the form δm(z) = β ln(1 + z) to the SNe Ia sample. The result is that cosmological models and evolution are highly degenerate with one another, so that the incorporation of even very simple models for evolution makes it virtually impossible to pin down the values of ΩM and ΩΛ, the density parameters for nonrelativistic matter and for the cosmological constant, respectively. The Hubble constant, H0, is unaffected by evolution. We evaluate the Bayes factor for models with evolution versus models without evolution, which, if one has no prior predilection for or against evolution, is the odds ratio for these two classes of models. The resulting values are always of order 1, in spite of the fact that the models that include evolution have additional parameters; thus, the data alone cannot discriminate between the two possibilities. Simulations show that simply acquiring more data of the same type as are available now will not alleviate the difficulty of separating evolution from cosmology in the analysis. What is needed is a better physical understanding of the SN Ia process, and the connections among the maximum luminosity, rate of decline, spectra, and initial conditions, so that physical models for evolution may be constructed, and confronted with the data. Moreover, we show that if SNe Ia evolve with time, but evolution is neglected in analyzing data, then, given enough SNe Ia, the analysis hones in on values of ΩM and ΩΛ that are incorrect. Using Bayesian methods, we show that the probability that the cosmological constant is nonzero (rather than zero) is unchanged by the SNe Ia data when one accounts for the possibility of evolution, provided that we do not discriminate among open, closed, and flat cosmologies a priori. The case for nonzero cosmological constant is stronger if the universe is presumed to be flat but still depends sensitively on the degree to which the peak luminosities of SNe Ia evolve as a function of redshift.

Enlarged quintessence cosmology

Abstract: We show that the combination of a fluid with a bulk dissipative pressure and quintessence matter can simultaneously drive an accelerated expansion phase and solve the coincidence problem of our current Universe. We then study some scenarios compatible with the observed cosmic acceleration.

Generalized second law in cosmology from causal boundary entropy

Abstract: A classical and quantum mechanical generalized second law of thermodynamics in cosmology implies constraints on the effective equation of state of the universe in the form of energy conditions, obeyed by many known cosmological solutions, forbids certain cosmological singularities, and is compatible with entropy bounds. This second law is based on the conjecture that causal boundaries and not only event horizons have geometric entropies proportional to their area. In string cosmology the second law provides new information about nonsingular solutions.

Is the radiation temperature-redshift relation of the standard cosmology in accordance with the data?

Abstract: The radiation temperature–redshift relation for Friedmann–Robertson–Walker geometries is rediscussed in connection with recent observational data based on the fine-structure splitting of atomic and singly ionized carbon lines in quasar absorption-line systems. Indirect measurement of T(z) is one of the most powerful cosmological tests available because it may exclude even the presence of a cosmological constant. Unlike recent claims, we argue that the temperature at high z may be smaller than the standard prediction, thereby opening a window to alternative (big bang) models. By including new ingredients like a phenomenological decaying vacuum energy density and gravitational ‘adiabatic’ photon creation as well as late inflationary models driven by a scalar field, a new temperature law is deduced and its predictions are compared with the standard result.

Large scale cosmic shear measurements

Abstract: We present estimates of the gravitational lensing shear variance obtained from images taken at the CFHT using the UH8K CCD mosaic camera. Six fields were observed for a total of 1 hour each in V and I, resulting in catalogs containing 20; 000 galaxies per field, with properly calibrated and optimally weighted shear estimates. These were averaged in cells of sizes ranging from 1 0 :875 to 30 0 to obtain estimates of the cosmic shear varianceh 2 i, with uncertainty estimated from the scatter among the estimates for the 6 fields. Our most reliable estimator for cosmic shear is provided by the cross-correlation of the shear measured in the two passbands. At scales< 10 0 the results are in good agreement with those of Van Waerbeke et al. (2000), Bacon et al. (2000) and Wittman et al. (2000) and with currently fashionable cosmological models. At larger scales the shear variance falls below the theoretical predictions, and on the largest scales we find a null detection of shear variance averaged in 30 0 cells of h 2 i =( 0:28 1:84) 10 -5 . Subject headings: Cosmology: observations — dark matter — gravitational lensing — large-scale structure of Universe — galaxies: photometry

Geometrodynamics of variable-speed-of-light cosmologies

Abstract: Variable-speed-of-light (VSL) cosmologies are currently attracting interest as an alternative to inflation We investigate the fundamental geometrodynamic aspects of VSL cosmologies and provide several implementations which do not explicitly break Lorentz invariance (no ``hard'' breaking) These ``soft'' implementations of Lorentz symmetry breaking provide particularly clean answers to the question ``VSL with respect to what?'' The class of VSL cosmologies we consider are compatible with both classical Einstein gravity and low-energy particle physics These models solve the ``kinematic'' puzzles of cosmology as well as inflation does, but cannot by themselves solve the flatness problem, since in their purest form no violation of the strong energy condition occurs We also consider a heterotic model (VSL plus inflation) which provides a number of observational implications for the low-redshift universe if \ensuremath{\chi} contributes to the ``dark energy'' either as CDM or quintessence These implications include modified gravitational lensing, birefringence, variation of fundamental constants and rotation of the plane of polarization of light from distant sources