arXiv subjects

Marc Kamionkowski

Publications and source records attributed to Marc Kamionkowski.

At least 343 records · Page 19Linked to original sources

arXiv1997

Detectability of Inflationary Gravitational Waves with Microwave Background Polarization

Inflation predicts specific relations between the amplitudes and spectral indices of the primordial spectrum of density (scalar metric) perturbations and gravitational waves (tensor metric perturbations). Detection of a stochastic gravitational-wave background is essential for identifying this unmistakable signature. Polarization of the cosmic microwave background can isolate these tensor modes in a model-independent way and thereby circumvent the cosmic-variance limit to detecting a small tensor signal with only a temperature map. Here we assess the detectability of a gravity-wave background with a temperature and polarization map. For detector sensitivities better than $10-20 μK \sqrt{sec}$, the sensitivity to a tensor signal is always dominated by the polarization map. With a detector sensitivity of order $1 μK \sqrt{sec}$, polarization could improve on a temperature-map sensitivity to tensor modes by two to three orders of magnitude. Even a small amount of reionization substantially enhances tensor-mode detectability. We also argue that the sensitivity of the Planck Surveyor to tensor modes is significantly improved with polarization, even taking into account the resulting degradation of the intensity determination in the high-frequency channels.

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arXiv1997

Getting Around Cosmic Variance

Cosmic microwave background (CMB) anisotropies probe the primordial density field at the edge of the observable Universe. There is a limiting precision (``cosmic variance'') with which anisotropies can determine the amplitude of primordial mass fluctuations. This arises because the surface of last scatter (SLS) probes only a finite two-dimensional slice of the Universe. Probing other SLSs observed from different locations in the Universe would reduce the cosmic variance. In particular, the polarization of CMB photons scattered by the electron gas in a cluster of galaxies provides a measurement of the CMB quadrupole moment seen by the cluster. Therefore, CMB polarization measurements toward many clusters would probe the anisotropy on a variety of SLSs within the observable Universe, and hence reduce the cosmic-variance uncertainty.

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arXiv1996

Statistics of Cosmic Microwave Background Polarization

We present a formalism for analyzing a full-sky temperature and polarization map of the cosmic microwave background. Temperature maps are analyzed by expanding over the set of spherical harmonics to give multipole moments of the two-point correlation function. Polarization, which is described by a second-rank tensor, can be treated analogously by expanding in the appropriate tensor spherical harmonics. We provide expressions for the complete set of temperature and polarization multipole moments for scalar and tensor metric perturbations. Four sets of multipole moments completely describe isotropic temperature and polarization correlations; for scalar metric perturbations one set is identically zero, giving the possibility of a clean determination of the vector and tensor contributions. The variance with which the multipole moments can be measured in idealized experiments is evaluated, including the effects of detector noise, sky coverage, and beam width. Finally, we construct coordinate-independent polarization two-point correlation functions, express them in terms of the multipole moments, and derive small-angle limits.

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arXiv1996

Particle Dark Matter

Several ideas for new physics beyond the standard model may provide particle candidates for the dark matter in the Galactic halo. The two leading candidates are an axion and a weakly-interacting massive particle (WIMP), such as the neutralino in supersymmetric extensions of the standard model. Several possibilities for detection of such particles are discussed. An assessment of the relative merits of various WIMP-detection techniques is given. I then review the prospects for improving our knowledge of the amount, distribution, and nature of the dark matter in the Universe from future maps of the cosmic microwave background.

hep-ph↗

arXiv1996

A Probe of Primordial Gravity Waves and Vorticity

A formalism for describing an all-sky map of the polarization of the cosmic microwave background is presented. The polarization pattern on the sky can be decomposed into two geometrically distinct components. One of these components is not coupled to density inhomogeneities. A non-zero amplitude for this component of polarization can only be caused by tensor or vector metric perturbations. This allows unambiguous identification of long-wavelength gravity waves or large-scale vortical flows at the time of last scattering.

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arXiv1996

Indirect Detection of a Light Higgsino Motivated by Collider Data

Kane and Wells recently argued that collider data point to a Higgsino-like lightest supersymmetric partner which would explain the dark matter in our Galactic halo. They discuss direct detection of such dark-matter particles in laboratory detectors. Here, we argue that such a particle, if it is indeed the dark matter, might alternatively be accessible in experiments which search for energetic neutrinos from dark-matter annihilation in the Sun. We provide accurate analytic estimates for the rates which take into account all relevant physical effects. Currently, the predicted signal falls roughly one to three orders of magnitude below experimental bounds, depending on the mass and coupling of the particle; however, detectors such as MACRO, super-Kamiokande, and AMANDA will continue to take data and should be able to rule out or confirm an interesting portion of the possible mass range for such a dark-matter particle within the next five years.

hep-ph↗

arXiv1996

Matter/Microwave Correlations in an Open Universe

In an intriguing recent paper, Crittenden and Turok proposed cross-correlating the cosmic microwave background (CMB) with tracers of the matter density to probe the existence of a cosmological constant. Here I emphasize that a similar cross-correlation arises in an open Universe and, depending on the redshift distribution of the tracer population and the matter density, may be comparable to or stronger than that in a flat cosmological-constant Universe with the same matter density. The two cases can be distinguished through cross-correlation with tracer populations with different redshift distributions.

astro-ph↗

arXiv1996

Comment on ``The Dispersion Velocity of Galactic Dark Matter Particles''

In a recent Letter (astro-ph/9605001), Cowsik et al. claim that a self-consistent treatment of the dark Galactic halo requires that the local velocity dispersion of dark-matter particles be $600$ km~s$^{-1}$ or greater, more than a factor of two larger than the canonical value of 270 km~s$^{-1}$. If true, this would significantly affect rates and signatures for detection of baryonic and nonbaryonic dark matter. We believe that this work is incorrect, probably because not all the observational constraints were taken into account and because the models were forced to satisfy arbitrary constraints on the halo density.

astro-ph↗

arXiv1996

Determining Cosmological Parameters from the Microwave Background

Recently funded satellites will map the cosmic microwave background radiation with unprecedented sensitivities and angular resolutions. Assuming only primordial adiabatic scalar and tensor perturbations, we evaluate how accurately experiments of this type will measure the basic cosmological parameters $Ω$ (the total density of the Universe), $Ω_b$ (the baryon density), $h$ (the Hubble constant), and $Λ$ (the cosmological constant). The proposed experiments are capable of measuring these parameters at the few-percent level. We briefly discuss the generality of these estimates and complications arising in actual data analysis.

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arXiv1996

Cosmological-Parameter Determination with Microwave Background Maps

The angular power spectrum of the cosmic microwave background (CMB) contains information on virtually all cosmological parameters of interest, including the geometry of the Universe ($Ω$), the baryon density, the Hubble constant ($h$), the cosmological constant ($Λ$), the number of light neutrinos, the ionization history, and the amplitudes and spectral indices of the primordial scalar and tensor perturbation spectra. We review the imprint of each parameter on the CMB. Assuming only that the primordial perturbations were adiabatic, we use a covariance-matrix approach to estimate the precision with which these parameters can be determined by a CMB temperature map as a function of the fraction of sky mapped, the level of pixel noise, and the angular resolution. For example, with no prior information about any of the cosmological parameters, a full-sky CMB map with $0.5^\circ$ angular resolution and a noise level of 15 $μ$K per pixel can determine $Ω$, $h$, and $Λ$ with standard errors of $\pm0.1$ or better, and provide determinations of other parameters which are inaccessible with traditional observations. Smaller beam sizes or prior information on some of the other parameters from other observations improves the sensitivity. The dependence on the the underlying cosmological model is discussed.

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arXiv1996

Do We Know the Geometry of the Universe?

It is quite remarkable that seventy years after Hubble discovered the expansion of the Universe, we still have no idea in which of the three Friedmann-Robertson-Walker geometries we live. Most of the current literature has focussed on flat or open models. Here, we construct a viable model of the Universe which has closed geometry even though the nonrelativistic-matter density is less than critical. Furthermore, in this model, the cosmic microwave background could come from a causally-connected region at the antipode of the closed Universe. This model illustrates that the geometry of the Universe is unconstrained by current data. We discuss observations which may reliably determine the geometry of the Universe in the near future.

astro-ph↗

arXiv1996

Parallax-Shifted Microlensing Events from Ground-Based Observations of the Galactic Bulge

The parallax effect in ground-based microlensing (ML) observations consists of a distortion to the standard ML light curve arising from the Earth's orbital motion. In most cases, the resolution in current ML surveys is not accurate enough to observe this effect, but parallax could conceivably be detected with frequent followup observations of ML events in progress. We calculate the expected fraction of events where parallax distortions will be detected by such observations, adopting Galactic models consistent with the observed ML timescale ($t_0$) distributions. We study the dependence of the rates for parallax-shifted events on the sampling frequency and on the photometric precision. For example, we find that for hourly observations with typical photometric errors of 0.01 mag, 6\% of events where the lens is in the bulge, and 31\% of events where the lens is in the disk, (or $\approx 10$\% of events overall) will give rise to a measurable parallax shift at the 95\% confidence level. These fractions may be increased by improved photometric accuracy and increased sampling frequency. Parallax measurements yield the reduced transverse speed, $\tilde{v}$, which gives both the relative transverse speed and lens mass as functions of distance. We give examples of the accuracies with which $\tilde{v}$ may be measured in typical parallax events. Using only the 3 standard ML parameters to fit ML light curves which may be shape-distorted by parallax or blending, can result in incorrect inferred values for these quantities. We find that the inferred timescales from such fits tend to shift the event duration distribution by $\approx 10$\% towards shorter $t_0$ for events with disk lenses, but do not affect bulge lenses. In both cases, the impact-parameter distribution is depressed slightly at the low and high ends.

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arXiv1996

A Low-Density Closed Universe

Matter with an equation of state $p=-ρ/3$ may arise in certain scalar field theories, and the energy density of this matter decreases as $a^{-2}$ with the scale factor $a$ of the Universe. In this case, the Universe could be closed but still have a nonrelativistic-matter density $Ω_0<1$. Furthermore, the cosmic microwave background could come from a causally-connected region at the other side of the Universe. This model is currently viable and might be tested by a host of forthcoming observations.

astro-ph↗

arXiv1996

Future Cosmic Microwave Background Constraints to the Baryon Density

We discuss what can be learned about the baryon density from an all-sky map of the cosmic microwave background (CMB) with sub-degree angular resolution. With only minimal assumptions about the primordial spectrum of density perturbations and the values of other cosmological parameters, such a CMB map should be able to distinguish between a Universe with a baryon density near 0.1 and a baryon-dominated Universe. With additional reasonable assumptions, it is conceivable that such measurements will constrain the baryon density to an accuracy similar to that obtained from BBN calculations.

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arXiv1995

Rates for Color Shifted Microlensing Events

If the objects responsible for gravitational microlensing (ML) of Galactic-bulge stars are faint dwarfs, then blended light from the lens will distort the shape of the ML light curve and shift the color of the observed star during the event. The resolution in current surveys is not accurate enough to observe this effect, but it should be detected with frequent and precise followup observations. We calculate the expected rates for ML events where the shape distortions will be observable by such followup observations, assuming that the lenses are ordinary main-sequence stars in a bar and in the disk. We study the dependence of the rates for color-shifted (CS) events on the frequency of followup observations and on the precision of the photometry for a variety of waveband pairings. We find that for hourly observations in $B$ and $K$ with typical photometric errors of 0.01 mag, 28\% of the events where a main-sequence bulge star is lensed, and 7\% of the events where the source is a bulge giant, will give rise to a measurable CS at the 95\% confidence level. For observations in $V$ and $I$, the fractions become 18\% and 5\%, respectively, but may be increased to 40\% and 13\% by improved photometric accuracy and increased sampling frequency. We outline how the mass, distance, and transverse speed of the lens can be obtained, giving examples of typical errors. We discuss how CS events can be distinguished from events where the source is blended with a binary companion.

astro-ph↗

arXiv1995

Weighing the Universe with the Cosmic Microwave Background

Variations in $Ω$, the total density of the Universe, leave a clear and distinctive imprint on the power spectrum of temperature fluctuations in the cosmic microwave background (CMB). This signature is virtually independent of other cosmological parameters or details of particular cosmological models. We evaluate the precision with which $Ω$ can be determined by a CMB map as a function of sky coverage, pixel noise, and beam size. For example, assuming only that the primordial density perturbations were adiabatic and with no prior information on the values of any other cosmological parameters, a full-sky CMB map at $0.5^\circ$ angular resolution and a noise level of $15\,μ{\rm K}$ per pixel can determine $Ω$ with a variance of 5\%. If all other cosmological parameters are fixed, $Ω$ can be measured to better than 1\%.

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arXiv1995

Solar Neutrinos: Radiative Corrections in Neutrino-Electron Scattering Experiments

Radiative corrections to the electron recoil-energy spectra and to total cross sections are computed for neutrino-electron scattering by solar neutrinos. Radiative corrections change monotonically the electron recoil spectrum for incident \b8 neutrinos, with the relative probability of observing recoil electrons being reduced by about 4 \% at the highest electron energies. For $p-p$ and $\be7$ neutrinos, the recoil spectra are not affected significantly. Total cross sections for solar neutrino-electron scattering are reduced by about 2 \% compared to previously computed values. We also calculate the recoil spectra from $^{13}$N and $^{15}$O neutrinos including radiative corrections.

astro-ph↗

arXiv1995

Gamma Rays from Neutralino Annihilation

We calculate the flux of cosmic gamma rays expected {}from the annihilation of neutralinos in the Galactic halo. Our calculation of the annihilation cross section to two photons improves the existing calculations by inclusion of exact one-loop diagrams for the amplitudes involving Higgs boson and chargino states as well as those involving fermion and sfermion states. A survey of supersymmetric parameter space shows that numerous models would be observable at the $3σ$ level with an air Cerenkov telescope with an exposure of $10^4 \meter^2 \yr$.

hep-ph↗