arXiv subjects

Craig J. Hogan

Publications and source records attributed to Craig J. Hogan.

71 records · Page 4Linked to original sources

arXiv1999

Gravitational Lensing by Cold Dark Matter Catastrophes

Intrinsically cold particle dark matter inevitably creates halos with sharp discontinuities in projected surface density caused by the projection of fold catastrophes onto the sky. In principle, these imperfections can be detected and measured with gravitational lensing through discontinuities in image magnification and image structure. Lens solutions are discussed for the most common universal classes of discontinuities. Edges caused by cold particles such as condensed axions and thermal WIMPs are very sharp, respectively about $10^{-12}$ and $10^{-7}$ of the halo scale. Their structure can be resolved by stellar and quasi-stellar sources which show sudden changes in brightness or even sudden disappearances (sometimes within hours) as edges are crossed. Images of extended objects such as edge-on galaxies or jets can show sudden bends at an edge, or stretched, parity-inverted reflection symmetry about a sharp line. Observational strategies and prospects are briefly discussed.

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arXiv1999

A Java Calculator of Standard Big Bang Nucleosynthesis

A simple Java applet is presented which allows quick and easy computation and plotting of the predictions of light element abundances in SBBN, including errors in the predictions propagated from input reaction rates correlated and calibrated via Monte Carlo. The applet, which requires Java 1.1-compatible browsers, can be found at http://www.astro.washington.edu/research/bbn/

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arXiv1998

The Full Re-Ionization of Helium

Observations of resolved HeII Lyman alpha absorption in spectra of two QSO's suggest that the epoch of helium ionization occurred at z~3. Proximity zones in the spectra of the quasars (z=3.18, 3.285) at 304 A resemble Stromgren spheres, suggesting that the intergalactic medium is only singly ionized in helium. We present models of the proximity effect which include the full physics of the ionization, heating and cooling and an accurately simulated inhomogeneous gas distribution. In these models the underdense intergalactic medium is heated to at least 10,000-20,000 K after cooling to as low as a few 1000 K due to cosmological expansion, with higher temperatures achieved farther away from the quasar due to absorption-hardened ionizing spectra. The quasars turn on for a few times 10^7 years with a fairly steady flux output at 228 A comparable to the 304 A flux output directly observed with HST. The recoveries in the spectra occur naturally due to voids in the IGM and may provide a fairly model-independent probe of the baryon density.

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arXiv1998

Cosmological Gravitational Wave Backgrounds

An overview is presented of possible cosmologically distant sources of gravitational wave backgrounds, especially those which might produce detectable backgrounds in the LISA band between 0.1 and 100 mHz. Examples considered here include inflation-amplified vacuum fluctuations in inflaton and graviton fields, bubble collisions in first-order phase transitions, Goldstone modes of classical self-ordering scalars, and cosmic strings and other gauge defects. Characteristic scales and basic mechanisms are reviewed and spectra are estimated for each of these sources. The unique impact of a LISA detection on fundamental physics and cosmology is discussed.

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arXiv1998

Mapping Low-Density Intergalactic Gas: a Third Helium Lyman-alpha Forest

We present a new HST/STIS spectrum of the z=3.18 quasar PKS 1935-692 and summarize the spectral features shortwards of 304A in the rest frame likely to be caused by foreground HeII Lyman-alpha absorption. In accord with previous results on two other quasars at similar redshifts, we demonstrate a correlation with the HI Lyman-alpha forest absorption, and show that much of the helium absorption is caused by a comparable quantity of more diffuse gas with Omega~0.01, that is not detected in HI. The helium ionization zone around the quasar is detected as well as a void seen in both HI and HeII. The properties of the absorption are in broad agreement with those of the other quasars and with models of the protogalactic gas distribution and ionization at this redshift.

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arXiv1998

Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant

We present observations of 10 type Ia supernovae (SNe Ia) between 0.16 < z < 0.62. With previous data from our High-Z Supernova Search Team, this expanded set of 16 high-redshift supernovae and 34 nearby supernovae are used to place constraints on the Hubble constant (H_0), the mass density (Omega_M), the cosmological constant (Omega_Lambda), the deceleration parameter (q_0), and the dynamical age of the Universe (t_0). The distances of the high-redshift SNe Ia are, on average, 10% to 15% farther than expected in a low mass density (Omega_M=0.2) Universe without a cosmological constant. Different light curve fitting methods, SN Ia subsamples, and prior constraints unanimously favor eternally expanding models with positive cosmological constant (i.e., Omega_Lambda > 0) and a current acceleration of the expansion (i.e., q_0 < 0). With no prior constraint on mass density other than Omega_M > 0, the spectroscopically confirmed SNe Ia are consistent with q_0 <0 at the 2.8 sigma and 3.9 sigma confidence levels, and with Omega_Lambda >0 at the 3.0 sigma and 4.0 sigma confidence levels, for two fitting methods respectively. Fixing a ``minimal'' mass density, Omega_M=0.2, results in the weakest detection, Omega_Lambda>0 at the 3.0 sigma confidence level. For a flat-Universe prior (Omega_M+Omega_Lambda=1), the spectroscopically confirmed SNe Ia require Omega_Lambda >0 at 7 sigma and 9 sigma level for the two fitting methods. A Universe closed by ordinary matter (i.e., Omega_M=1) is ruled out at the 7 sigma to 8 sigma level. We estimate the size of systematic errors, including evolution, extinction, sample selection bias, local flows, gravitational lensing, and sample contamination. Presently, none of these effects reconciles the data with Omega_Lambda=0 and q_0 > 0.

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arXiv1997

Extragalactic Abundances of Hydrogen, Deuterium and Helium: New Steps, Missteps and Next Steps

Estimates of the deuterium abundance in quasar absorbers are reviewed, including a brief account of incorrect claims published by the author and a brief review of the problem of hydrogen contamination. It is concluded that the primordial abundance may be universal with a value $(D/H)_P\approx 10^{-4}$, within about a factor of two, corresponding to $Ω_B h_{0.7}^2\approx 0.02$ or $η_{10}\approx 2.7$ in the Standard Big Bang. This agrees with current limits on primordial helium, $Y_P\le 0.243$, which are shown to be surprisingly insensitive to models of stellar enrichment. It also agrees with a tabulated sum of the total density of baryons in observed components. Much lower primordial deuterium ($\approx 2\times 10^{-5}$) is also possible but disagrees with currently estimated helium abundances; the larger baryon density in this case fits better with current models of the Lyman-$α$ forest but requires the bulk of the baryons to be in some currently uncounted form.

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arXiv1997

A Bayesian Estimate of the Primordial Helium Abundance

We introduce a new statistical method to estimate the primordial helium abundance, Y_p from observed abundances in a sample of galaxies which have experienced stellar helium enrichment. Rather than using linear regression on metal abundance we construct a likelihood function using a Bayesian prior, where the key assumption is that the true helium abundance must always exceed the primordial value. Using a sample of measurements compiled from the literature we find estimates of Y_p between 0.221 and 0.236, depending on the specific subsample and prior adopted, consistent with previous estimates either from a linear extrapolation of the helium abundance with respect to metallicity, or from the helium abundance of the lowest metallicity HII region, I Zw 18. We also find an upper limit which is insensitive to the specific subsample or prior, and estimate a model-independent bound Y_p < 0.243 at 95% confidence, favoring a low cosmic baryon density and a high primordial deuterium abundance. The main uncertainty is not the model of stellar enrichment but possible common systematic biases in the estimate of Y in each individual HII region.

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arXiv1997

Deuterium and Helium Absorption at High Redshift: Mapping the Abundance, Density and Ionization of Primordial Gas

Spectra of quasars at high redshift with high resolution and high signal-to-noise allow in favorable circumstances detection of absorption by deuterium and ultimately measurement of its primordial abundance. Ultraviolet spectra of high redshift quasars allow measurement of absorption by the most abundant cosmic absorber, singly ionized helium, thereby mapping gas even in the most rarefied cosmic voids. These new techniques already provide significant constraints on cosmological models but will soon become much more precise.

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arXiv1997

Resolving the Helium Lyman-$α$ Forest: Mapping Intergalactic Gas and Ionizing Radiation at $z\approx 3$

We present a new, high resolution HST/GHRS spectrum of quasar Q0302-003, and use the HeII Lyman-$α$ absorption, together with a high resolution Keck spectrum of the HI Lyman-$α$ forest, to probe the distribution and ionization state of foreground gas just below the quasar redshift $z\approx 3.3$. The main new conclusions from the current data are: 1. The HeII Lyman-$α$ forest is detected; 2. The ``diffuse'' (redshift-space-filling) medium is also detected, and must have a low density ($Ω\le 0.01 (h/0.7)^{-3/2}$) consistent with standard primordial nucleosynthesis and models of early gas collapse into protogalaxies; 3. The intergalactic ionizing spectrum is soft ($η\ge 100$), although the intergalactic helium is probably mostly doubly ionized by $z=3.3$; 4. The helium abundance is within a factor of a few of standard Big Bang predictions, over a large volume of space at high redshift.

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arXiv1997

Statistical Measurement of Primordial Deuterium Abundance

Lyman series absorption features and corresponding metal lines in three high-resolution quasar spectra are fitted in the standard way with thermally broadened discrete redshift components. A candidate sample is assembled, of 10 velocity components in 7 separate complexes, which display features attributable to deuterium. Three of these components in 2 complexes were previously published; the fits to the seven others are described here in detail. To estimate the contamination of this sample by hydrogen ``interlopers'', a control sample of ``pretendium'' candidates is assembled from the same data, and in the same way in all respects but one: the control sample candidates are drawn from the red side of hydrogen absorbers, rather than the blue side where the deuterium feature appears. The control sample is therefore drawn from a population with the same statistical properties as the plausible interlopers, including their joint correlationsin velocity, column density and width. The properties of the two samples are compared statistically, using the Doppler parameters and column densities from the line fits, and are found to be significantly different. The D candidates have narrower Doppler parameters, consistent with the deuterium identification. The column densities of the D sample (but not the P sample) are consistent with a universal cosmic abundance; the weighted mean abundance for the new candidates presented here is $ < log (D/H) > = -3.75 \pm 0.51 $.

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arXiv1996

Big Bang Nucleosynthesis and the Observed Abundances of Light Elements

The predictions of Standard Big Bang Nucleosynthesis are summarized and compared with observations of abundances of helium in HII regions, deuterium in quasar absorbers, deuterium and helium-3 in the Galaxy, and lithium in metal-poor stars. It is concluded that the prospects are good for a precise test of the theory.

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arXiv1996

High Deuterium Abundance in a New Quasar Absorber

We present a new analysis of an absorption system in the spectrum of Q0014+813, with particular attention to a single cold cloud at redshift $z=2.797957$. Features are identified at this redshift corresponding to SiIII, SiIV, CIV, DI, and HI, all consistent with purely thermal broadening at $T\approx 19,000$K. The deuterium identification is confirmed by its narrow width and precise agreement with the silicon and carbon redshifts. The HI column is well constrained by a slightly damped profile in the blue wing of Ly$α$, and the DI column by its saturation, leading to a 95\% confidence lower limit $(D/H)> 0.7\times 10^{-4}$. The abundance is measured from line fits to be $(D/H) = 1.9_{-0.9}^{+1.6}\times 10^{-4}$ , in agreement with the high deuterium abundance previously found in the $z=3.32$ system in the same quasar spectrum. This system strengthens the case for a high universal primordial deuterium abundance and low cosmic baryon density ($η=1.7\times 10^{-10}$, $Ω_bh^2=0.006$), for which Standard Big Bang Nucleosynthesis gives concordant predictions for helium-4 and lithium-7 abundances. Implications for Galactic chemical evolution and the baryonic and nonbaryonic dark matter problems are briefly discussed.

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arXiv1995

Cosmic Deuterium and Baryon Density

Quasar absorption lines now permit a direct probe of deuterium abundances in primordial material, with the best current estimate $ (D/H)=1.9\pm 0.4 \times 10^{-4}$. If this is the universal primordial abundance $(D/H)_p$, Standard Big Bang Nucleosynthesis yields an estimate of the mean cosmic density of baryons, $η_{10}= 1.7\pm 0.2$ or $ Ω_b h^2= 6.2\pm 0.8\times 10^{-3}$, leading to SBBN predictions in excellent agreement with estimates of primordial abundances of helium-4 and lithium-7. Lower values of $(D/H)_p$ derived from Galactic chemical evolution models may instead be a sign of destruction of deuterium and helium-3 in stars. The inferred baryon density is compared with known baryons in stars and neutral gas; about two thirds of the baryons are in some still-unobserved form such as ionized gas or compact objects. Galaxy dynamical mass estimates reveal the need for primarily nonbaryonic dark matter in galaxy halos. Galaxy cluster dynamics imply that the total density of this dark matter, while twenty or more times the baryon density, is still well below the critical value, unless both baryons and galaxies are concentrated in galaxy clusters relative to the dark matter.

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arXiv1995

Cosmic Fluctuations and Dark Matter from Scalar Field Oscillations

Scale-invariant fluctuations and cold dark matter could originate from two different modes of a single scalar field, fluctuations from massless Goldstone oscillations and matter from massive Higgs modes. Matching the fluctuations and dark matter density observed requires a heavy scale ($ϕ_0\approx 10^{16}$GeV) for the potential minimum and an extremely small self coupling ($λ\approx 10^{-83}$). Mode coupling causes the dark matter to form in lumps with nonnegligible velocities, leading to early collapse of dense dark matter ``miniclusters'' and halos on the scale of compact dwarf galaxies.

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arXiv1994

Primordial Deuterium Abundance and Cosmic Baryon Density

The comparison of cosmic abundances of the light elements with the density of baryonic stars and gas in the universe today provides a critical test of big bang theory and a powerful probe of the nature of dark matter. A new technique allows determination of cosmic deuterium abundances in quasar absorption clouds at large redshift, allowing a new test of big bang homogeneity in diverse, very distant systems. The first results of these studies are summarized, along with their implications. The quasar data are confronted with the apparently contradictory story from the helium-3 abundances measured in our Galaxy. The density of baryonic stars and gas in the universe today is reviewed and compared with the big bang prediction.

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arXiv1994

Giant Branch Mixing and the Ultimate Fate of Primordial Deuterium in the Galaxy

The observed cosmic abundances of light elements are most consistent with each other, and with the predictions of big bang nucleosynthesis,if, contrary to the usual assumption, galactic chemical evolution reduces $(D+\ ^3He)/H$ with time. Chemical evolution models which do this require that low mass stars destroy $^3He$ in the envelope gas that they return to the interstellar medium. A simple argument based on the rates of limiting nuclear reactions shows that the same giant branch mixing process which appears to be needed to explain the observed $^{12}C/ ^{13}C$ and $C/N$ ratios in 1-- 2$\msol$ stars would indeed also probably destroy $^3He$ by a large factor in the bulk of the envelope material. The conclusion is that Galactic $^3He/H$ estimates should not be trusted for setting an upper limit on primordial $(D+ ^3He)/H$. This removes the strongest lower bound on the cosmic baryon density from big bang nucleosynthesis and the only argument for abundant baryonic dark matter.

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