arXiv subjects

Marc Kamionkowski

Publications and source records attributed to Marc Kamionkowski.

At least 253 records · Page 14Linked to original sources

arXiv2007

Red Density Perturbations and Inflationary Gravitational Waves

We study the implications of recent indications for a red spectrum of primordial density perturbations for the detection of inflationary gravitational waves (IGWs) with forthcoming cosmic microwave background experiments. We find that if inflation occurs with a single field with an inflaton potential minimized at V=0, then Planck will be able to detect IGWs at better than 2$σ$ confidence level, unless the inflaton potential is a power law with a very weak power. The proposed satellite missions of the Cosmic Vision and Inflation Probe programs will be able to detect IGWs from all the models we have surveyed at better than 5$σ$ confidence level. We provide an example of what is required if the IGW background is to remain undetected even by these latter experiments.

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arXiv2007

A Telescope Search for Decaying Relic Axions

A search for optical line emission from the two-photon decay of relic axions was conducted in the galaxy clusters Abell 2667 and 2390, using spectra from the VIMOS (Visible Multi-Object Spectrograph) integral field unit at the Very Large Telescope. New upper limits to the two-photon coupling of the axion are derived, and are at least a factor of 3 more stringent than previous upper limits in this mass window. The improvement follows from larger collecting area, integration time, and spatial resolution, as well as from improvements in signal to noise and sky subtraction made possible by strong-lensing mass models of these clusters. The new limits either require that the two-photon coupling of the axion be extremely weak or that the axion mass window between 4.5 eV and 7.7 eV be closed. Implications for sterile-neutrino dark matter are discussed briefly also.

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arXiv2007

Dark Matter and Dark Energy

This is a short review, aimed at a general audience, of several current subjects of research in cosmology. The topics discussed include the cosmic microwave background (CMB), with particular emphasis on its relevance for testing inflation; dark matter, with a brief review of astrophysical evidence and more emphasis on particle candidates; and cosmic acceleration and some of the ideas that have been put forward to explain it. A glossary of technical terms and acronyms is provided.

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arXiv2007

Galaxy surveys, inhomogeneous reionization, and dark energy

We examine the effect of inhomogeneous reionization on the galaxy power spectrum and the consequences for probing dark energy. To model feedback during reionization, we apply an ansatz setting the galaxy overdensity proportional to the underlying ionization field. Thus, inhomogeneous reionization may leave an imprint in the galaxy power spectrum. We evolve this imprint to low redshift and use the Fisher-matrix formalism to assess the effect on parameter estimation. We show that a combination of low- (z=0.3) and high- (z=3) redshift galaxy surveys can constrain the size of cosmological HII regions during reionization. This imprint can also cause confusion when using baryon oscillations or other features of the galaxy power spectrum to probe the dark energy. We show that when bubbles are large, and hence detectable, our ability to constrain w can be degraded by up to 50%. When bubbles are small, the imprint has little or no effect on measuring dark-energy parameters.

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arXiv2006

Tidal Tails Test the Equivalence Principle in the Dark Sector

Satellite galaxies currently undergoing tidal disruption offer a unique opportunity to constrain an effective violation of the equivalence principle in the dark sector. Theories in which cold dark matter (CDM) couples to a light scalar field naturally lead to a long-range force between dark matter particles. An inverse-square-law force of this kind would manifest itself as a violation of the equivalence principle in the dynamics of CDM compared to baryons in the form of gas or stars. In a previous paper, we showed that an attractive force would displace stars outwards from the bottom of the satellite's gravitational potential well, leading to a higher fraction of stars being disrupted from the tidal bulge further from the Galactic center. Since stars disrupted from the far (near) side of the satellite go on to form the trailing (leading) tidal stream, an attractive dark-matter force will produce a relative enhancement of the trailing stream compared to the leading stream. This distinctive signature of a dark-matter force might be detected through detailed observations of the tidal tails of a disrupting satellite, such as those recently performed by the Two-Micron All-Sky Survey (2MASS) and Sloan Digital Sky Survey (SDSS) on the Sagittarius (Sgr) dwarf galaxy. Here we show that this signature is robust to changes in our models for both the satellite and Milky Way, suggesting that we might hope to search for a dark-matter force in the tidal features of other recently discovered satellite galaxies in addition to the Sgr dwarf.

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arXiv2006

Solar System tests DO rule out 1/R gravity

Shortly after the addition of a 1/R term to the Einstein-Hilbert action was proposed as a solution to the cosmic-acceleration puzzle, Chiba showed that such a theory violates Solar System tests of gravity. A flurry of recent papers have called Chiba's result into question. They argue that the spherically-symmetric vacuum spacetime in this theory is the Schwarzschild-de Sitter solution, making this theory consistent with Solar System tests. We point out that although the Schwarzschild-de Sitter solution exists in this theory, it is not the unique spherically-symmetric vacuum solution, and it is not the solution that describes the spacetime in the Solar System. The solution that correctly matches onto the stellar-interior solution differs from Schwarzschild-de Sitter in a way consistent with Chiba's claims. Thus, 1/R gravity is ruled out by Solar System tests.

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arXiv2006

Galilean Equivalence for Galactic Dark Matter

Satellite galaxies are tidally disrupted as they orbit the Milky Way. If dark matter (DM) experiences a stronger self-attraction than baryons, stars will preferentially gain rather than lose energy during tidal disruption leading to an enhancement in the trailing compared to the leading tidal stream. The Sgr dwarf galaxy is seen to have roughly equal streams, challenging models in which DM and baryons accelerate differently by more than 10%. Future observations and a better understanding of DM distribution should allow detection of equivalence violation at the percent level.

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arXiv2006

Report of the Dark Energy Task Force

Dark energy appears to be the dominant component of the physical Universe, yet there is no persuasive theoretical explanation for its existence or magnitude. The acceleration of the Universe is, along with dark matter, the observed phenomenon that most directly demonstrates that our theories of fundamental particles and gravity are either incorrect or incomplete. Most experts believe that nothing short of a revolution in our understanding of fundamental physics will be required to achieve a full understanding of the cosmic acceleration. For these reasons, the nature of dark energy ranks among the very most compelling of all outstanding problems in physical science. These circumstances demand an ambitious observational program to determine the dark energy properties as well as possible.

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arXiv2006

Supermassive Black Hole Merger Rates: Uncertainties from Halo Merger Theory

The merger of two supermassive black holes is expected to produce a gravitational-wave signal detectable by the satellite LISA. The rate of supermassive-black-hole mergers is intimately connected to the halo merger rate, and the extended Press-Schechter formalism is often employed when calculating the rate at which these events will be observed by LISA. This merger theory is flawed and provides two rates for the merging of the same pair of haloes. We show that the two predictions for the LISA supermassive-black-hole-merger event rate from extended Press-Schechter merger theory are nearly equal because mergers between haloes of similar masses dominate the event rate. An alternative merger rate may be obtained by inverting the Smoluchowski coagulation equation to find the merger rate that preserves the Press-Schechter halo abundance, but these rates are only available for power-law power spectra. We compare the LISA event rates derived from the extended Press-Schechter merger formalism to those derived from the merger rates obtained from the coagulation equation and find that the extended Press-Schechter LISA event rates are thirty percent higher for a power spectrum spectral index that approximates the full Lambda-CDM result of the extended Press-Schechter theory.

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arXiv2006

Cosmological bounds on dark matter-neutrino interactions

We investigate the cosmological effects of a neutrino interaction with cold dark matter. We postulate a neutrino that interacts with a ``neutrino interacting dark matter'' (NIDM) particle with an elastic-scattering cross section that either decreases with temperature as $T^2$ or remains constant with temperature. The neutrino--dark-matter interaction results in a neutrino--dark-matter fluid with pressure, and this pressure results in diffusion-damped oscillations in the matter power spectrum, analogous to the acoustic oscillations in the baryon-photon fluid. We discuss the bounds from the Sloan Digital Sky Survey on the NIDM opacity (ratio of cross section to NIDM-particle mass) and compare with the constraint from observation of neutrinos from supernova 1987A. If only a fraction of the dark matter interacts with neutrinos, then NIDM oscillations may affect current cosmological constraints from measurements of galaxy clustering. We discuss how detection of NIDM oscillations would suggest a particle-antiparticle asymmetry in the dark-matter sector.

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arXiv2006

What mass are the smallest protohalos?

We calculate the kinetic-decoupling temperature for weakly interacting massive particles (WIMPs) in supersymmetric (SUSY) and universal-extra-dimension (UED) models that can account for the cold-dark-matter abundance determined from cosmic microwave background measurements. Depending on the parameters of the particle-physics model, a wide variety of decoupling temperatures is possible, ranging from several MeV to a few GeV. These decoupling temperatures imply a range of masses for the smallest protohalos much larger than previously thought -- ranging from 10^{-6} earth masses to 10^{2} earth masses. We expect the range of protohalos masses derived here to be characteristic of most particle-physics models that can thermally accommodate the required relic abundance of WIMP dark matter, even beyond SUSY and UED.

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arXiv2006

A new cosmic microwave background constraint to primordial gravitational waves

Primordial gravitational waves (GWs) with frequencies > 10^{-15} Hz contribute to the radiation density of the Universe at the time of decoupling of the cosmic microwave background (CMB). The effects of this GW background on the CMB and matter power spectra are identical to those due to massless neutrinos, unless the initial density-perturbation amplitude for the gravitational-wave gas is non-adiabatic, as may occur if such GWs are produced during inflation or some post-inflation phase transition. In either case, current observations provide a constraint to the GW amplitude that competes with that from big-bang nucleosynthesis (BBN), although it extends to much lower frequencies (~10^{-15} Hz rather than the ~10^{-10} Hz lower limit from BBN): at 95% confidence-level, Omega_gw h^2 < 6.9 x 10^{-6} for homogeneous (i.e., non-adiabatic) initial conditions. Future CMB experiments, like Planck and CMBPol, should allow sensitivities to Omega_gw h^2 < 1.4 x 10^{-6} and Omega_gw h^2 < 5 x 10^{-7}, respectively.

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arXiv2006

Direct detection of the inflationary gravitational wave background

Inflation generically predicts a stochastic background of gravitational waves over a broad range of frequencies, from those accessible with cosmic microwave background (CMB) measurements, to those accessible directly with gravitational-wave detectors, like NASA's Big-Bang Observer (BBO) or Japan's Deci-Hertz Interferometer Gravitational-wave Observer (DECIGO), both currently under study. Here we investigate the detectability of the inflationary gravitational-wave background at BBO/DECIGO frequencies. To do so, we survey a range of slow-roll inflationary models consistent with constraints from the CMB and large-scale structure (LSS). We go beyond the usual assumption of power-law power spectra, which may break down given the 16 orders of magnitude in frequency between the CMB and direct detection, and solve instead the inflationary dynamics for four classes of inflaton potentials. Direct detection is possible in a variety of inflationary models, although probably not in any in which the gravitational-wave signal does not appear in the CMB polarization. However, direct detection by BBO/DECIGO can help discriminate between inflationary models that have the same slow-roll parameters at CMB/LSS scales.

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arXiv2006

Dark Matter and the CACTUS Gamma-Ray Excess from Draco

The CACTUS atmospheric Cherenkov telescope collaboration recently reported a gamma-ray excess from the Draco dwarf spheroidal galaxy. Draco features a very low gas content and a large mass-to-light ratio, suggesting as a possible explanation annihilation of weakly interacting massive particles (WIMPs) in the Draco dark-matter halo. We show that with improved angular resolution, future measurements can determine whether the halo is cored or cuspy, as well as its scale radius. We find the relevant WIMP masses and annihilation cross sections and show that supersymmetric models can account for the required gamma-ray flux. The annihilation cross section range is found to be not compatible with a standard thermal relic dark-matter production. We compute for these supersymmetric models the resulting Draco gamma-ray flux in the GLAST energy range and the rates for direct neutralino detection and for the flux of neutrinos from neutralino annihilation in the Sun. We also discuss the possibility that the bulk of the signal detected by CACTUS comes from direct WIMP annihilation to two photons and point out that a decaying-dark-matter scenario for Draco is not compatible with the gamma-ray flux from the Galactic center and in the diffuse gamma-ray background.

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arXiv2005

Cluster magnetic fields from large-scale-structure and galaxy-cluster shocks

The origin of the micro-Gauss magnetic fields in galaxy clusters is one of the outstanding problem of modern cosmology. We have performed three-dimensional particle-in-cell simulations of the nonrelativistic Weibel instability in an electron-proton plasma, in conditions typical of cosmological shocks. These simulations indicate that cluster fields could have been produced by shocks propagating through the intergalactic medium during the formation of large-scale structure or by shocks within the cluster. The strengths of the shock-generated fields range from tens of nano-Gauss in the intercluster medium to a few micro-Gauss inside galaxy clusters.

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arXiv2005

Pair Correlations and Merger Bias

We study analytically the possibility that mergers of haloes are more highly clustered than the general population of haloes of comparable masses. We begin by investigating predictions for merger bias within the extended Press-Schechter formalism and discuss the limitations and ambiguities of this approach. We then postulate that mergers occur whenever two objects form within a (small) fixed distance of each other. We therefore study the clustering of pairs of points for a highly biased population in the linear regime, for the overall mass distribution in the quasilinear regime, and (using the halo model of clustering) in the nonlinear regime. Biasing, quasilinear evolution, and nonlinear clustering all lead to nonzero reduced (or connected) three-point and four-point correlation functions. These higher-order correlation functions can in many cases enhance the clustering of close pairs of points relative to the clustering of individual points. If close pairs are likely to merge, then the clustering of mergers may be enhanced. We discuss implications for the observed clustering of luminous z=3 galaxies and for correlations of active galactic nuclei and galaxy clusters.

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arXiv2005

Dynamical Friction and Cooling Flows in Galaxy Clusters

We investigate a model of galaxy clusters in which the hot intracluster gas is efficiently heated by dynamical friction (DF) of galaxies. We allow for both subsonic and supersonic motions of galaxies and use the gravitational drag formula in a gaseous medium presented by Ostriker (1999). The energy lost by the galaxies is either redistributed locally or into a Gaussian centered on the galaxy. We find that the condition of hydrostatic equilibrium and strict energy balance yields a trivial isothermal solution T_iso, independent of radius, or rising temperature distributions provided T_iso/gamma < T < T_iso, where gamma is the adiabatic index of the gas. The isothermal temperature corresponds to the usual scaling relation between the gas temperatures and the velocity dispersions of galaxies. However the minimal temperature associated with the rising solutions is ~ 0.5 T_vir, larger than that inferred from observations, the radial distribution of galaxy masses notwithstanding. Heating by supersonically moving galaxies cannot suppress thermal instability, although it can lengthen the growth time up to the level comparable to the ages of clusters when Mach number of galaxies is less than about two. We show using numerical hydrodynamic simulations that DF-induced heating is generally unable to produce stable equilibrium cores by evolving arbitrary non-equilibrium clusters, although it can lengthen the cooling time. We conclude that DF-induced heating alone is an unlikely solution to the cooling flow problem, although it can still be an important heat supplier, considerably delaying cooling catastrophe. We discuss other potential consequences of DF of galaxies in galaxy clusters.

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arXiv2005

Theory of Cosmic Microwave Background Polarization

These lectures introduce some of the basic theory of cosmic microwave background (CMB) polarization with the primary aim of developing the theory of CMB polarization from inflationary gravitational waves, as well as some of the related theory of weak gravitational lensing (cosmic shear) of CMB polarization. We begin with production of polarization by Thomson scattering. We then discuss tensor-harmonic analysis (the ``grad-curl'' or ``E-B'' decomposition) on a flat and full sky in some detail. The Boltzmann/Einstein equations required to predict the CMB temperature/polarization pattern due to primordial gravitational waves are derived. We show that gravitational waves produce a curl component of the CMB polarization while density perturbations (at linear order) do not. We then show how cosmic shear induces a curl component from a curl-free surface of last scattering. We describe, though in less detail, how higher-order correlations can be used to subtract the cosmic-shear--induced curl. Several exercises are provided.

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