Daniel E. Holz

Toward a halo mass function for precision cosmology: The Limits of universality

We measure the mass function of dark matter halos in a large set of collisionless cosmological simulations of flat ΛCDM cosmology and investigate its evolution at -->z 2. Halos are identified as isolated density peaks, and their masses are measured within a series of radii enclosing specific overdensities. We argue that these spherical overdensity masses are more directly linked to cluster observables than masses measured using the friends-of-friends algorithm (FOF), and are therefore preferable for accurate forecasts of halo abundances. Our simulation set allows us to calibrate the mass function at -->z = 0 for virial masses in the range -->1011 h−1 M☉ ≤ M≤ 1015 h−1 M☉ to 5%, improving on previous results by a factor of 2-3. We derive fitting functions for the halo mass function in this mass range for a wide range of overdensities, both at -->z = 0 and earlier epochs. Earlier studies have sought to calibrate a universal mass function, in the sense that the same functional form and parameters can be used for different cosmologies and redshifts when expressed in appropriate variables. In addition to our fitting formulae, our main finding is that the mass function cannot be represented by a universal function at this level or accuracy. The amplitude of the universal function decreases monotonically by 20%-50%, depending on the mass definition, from -->z = 0 to 2.5. We also find evidence for redshift evolution in the overall shape of the mass function.

Precision Determination of the Mass Function of Dark Matter Halos

The predicted mass function of dark matter halos is essential in connecting observed galaxy-cluster counts and models of galaxy clustering to the properties of the primordial density field. We determine the mass function in the concordance ΛCDM cosmology, as well as its uncertainty, using sixteen 10243 particle nested-volume dark matter simulations spanning a mass range of over 5 orders of magnitude. Using the nested volumes and single-halo tests, we find and correct for a systematic error in the friends-of-friends halo-finding algorithm. We find a fitting form and full error covariance for the mass function that successfully describes the simulations mass function and is well behaved outside the simulations resolutions. Estimated forecasts of uncertainty in cosmological parameters from future cluster-count surveys receive a negligible contribution from remaining statistical uncertainties in the central cosmology multiplicity function. There exists a potentially nonnegligible cosmological dependence (nonuniversality) of the halo multiplicity function.

Gravitational Wave Emission from Core Collapse of Massive Stars

We derive estimates for the characteristics of gravitational radiation from stellar collapse, using recent models of the core collapse of Chandrasekhar-massed white dwarfs (accretion-induced collapse), core-collapse supernovae and collapsars, and the collapse of very massive stars (300 M?). We study gravitational wave emission mechanisms using several estimation techniques, including two-dimensional numerical computation of quadrupole wave emission, estimates of bar-mode strength, estimates of r-mode emission, and estimates of waves from black hole ringing. We also review the rate at which the relevant collapses are believed to occur, which has a major impact on their relevance as astrophysical sources. Although the latest supernova progenitor simulations produce cores rotating much slower than those used in the past, we find that bar-mode and r-mode instabilities from core-collapse supernovae remain among the leading candidate sources for second-generation detectors at the Laser Interferometer Gravitational-Wave Observatory (LIGO II). Accretion-induced collapse (AIC) of a white dwarf could produce gravitational wave signals similar to those from core collapse. In the models that we examine, such collapses are not unstable to bar modes; we note that models recently examined by Liu & Lindblom, which have slightly more angular momentum, are certainly unstable to bar formation. Because AIC events are probably 1000 times less common than core-collapse supernovae, the typical AIC event will be much farther away, and thus the observed waves will be much weaker. In the most optimistic circumstances, we find that it may be possible to detect gravitational waves from the collapse of 300 M? Population III stars.

The Effect of Metallicity on the Detection Prospects for Gravitational Waves

Data from the Sloan Digital Sky Survey (~300,000 galaxies) indicate that recent star formation (within the last 1 billion years) is bimodal: half of the stars form from gas with high amounts of metals (solar metallicity) and the other half form with small contribution of elements heavier than helium (~10%-30% solar). Theoretical studies of mass loss from the brightest stars derive significantly higher stellar-origin black hole (BH) masses (~30-80 M ☉) than previously estimated for sub-solar compositions. We combine these findings to estimate the probability of detecting gravitational waves (GWs) arising from the inspiral of double compact objects. Our results show that a low-metallicity environment significantly boosts the formation of double compact object binaries with at least one BH. In particular, we find the GW detection rate is increased by a factor of 20 if the metallicity is decreased from solar (as in all previous estimates) to a 50-50 mixture of solar and 10% solar metallicity. The current sensitivity of the two largest instruments to neutron star-neutron star (NS-NS) binary inspirals (VIRGO: ~9 Mpc; LIGO: ~18) is not high enough to ensure a first detection. However, our results indicate that if a future instrument increased the sensitivity to ~50-100 Mpc, a detection of GWs would be expected within the first year of observation. It was previously thought that NS-NS inspirals were the most likely source for GW detection. Our results indicate that BH-BH binaries are ~25 times more likely sources than NS-NS systems and that we are on the cusp of GW detection.

Exploring short gamma-ray bursts as gravitational-wave standard sirens

Recent observations support the hypothesis that a large fraction of “short-hard” gamma-ray bursts (SHBs) are associated with the inspiral and merger of compact binaries. Since gravitational-wave (GW) measurements of well-localized inspiraling binaries can measure absolute source distances with high accuracy, simultaneous observation of a binary’s GWs and SHB would allow us to directly and independently determine both the binary’s luminosity distance and its redshift. Such a “standard siren” (the GW analog of a standard candle) would provide an excellent probe of the relatively nearby (z . 0.3) universe’s expansion, independent of the cosmological distance ladder, and thus complementing other standard candles. Previous work explored this idea using a simplified formalism to study measurement by advanced GW detector networks, incorporating a high signal-to-noise ratio limit to describe the probability distribution for measured parameters. In this paper we eliminate this simplification, constructing distributions with a Markov Chain Monte Carlo technique. We assume that each SHB observation gives both the source sky position and the time of coalescence, and we take both binary neutron stars and black hole-neutron star coalescences as plausible SHB progenitors. We examine how well parameters (particularly the luminosity distance) can be measured from GW observatations of these sources by a range of ground-based detector networks. We find that earlier estimates overstate how well distances can be measured, even at fairly large signal-to-noise ratio. The fundamental limitation to determining distance to these sources proves to be the gravitational waveform’s degeneracy between luminosity distance and source inclination. Despite this, we find that excellent results can be achieved by measuring a large number of coalescing binaries, especially if the worldwide network consists of many widely separated detectors. Advanced GW detectors will be able to determine the absolute luminosity distance to an accuracy of 10–30% for NS-NS and NS-BH binaries out to 600 and 1400 Mpc, respectively. Subject headings: cosmology: distance scale—cosmology: theory—gamma rays: bursts—gravitational waves

No evidence for dark energy dynamics from a global analysis of cosmological data

We use a variant of principal component analysis to investigate the possible temporal evolution of the dark energy equation of state, w(z). We constrain w(z) in multiple redshift bins, utilizing the most recent data from type Ia supernovae, the cosmic microwave background, baryon acoustic oscillations, the integrated Sachs-Wolfe effect, galaxy clustering, and weak lensing data. Unlike other recent analyses, we find no significant evidence for evolving dark energy; the data remain completely consistent with a cosmological constant. We also study the extent to which the time evolution of the equation of state would be constrained by a combination of current- and future-generation surveys, such as Planck and the Joint Dark Energy Mission.

Collisional dark matter and scalar phantoms

Abstract As has been previously proposed, a minimal modification of the standard SU (3)× SU (2)× U (1) theory provides a viable dark matter candidate. Such a particle, a scalar gauge singlet, is naturally self-interacting — making it of particular interest given recent developments in astrophysics. We review this dark matter candidate, with reference to the parameter ranges currently under discussion.

Ultra-high precision cosmology from gravitational waves

We show that the Big Bang Observer (BBO), a proposed space-based gravitational-wave (GW) detector, would provide ultraprecise measurements of cosmological parameters. By detecting ∼3×10^5 compact-star binaries, and utilizing them as standard sirens, BBO would determine the Hubble constant to ∼0.1%, and the dark-energy parameters w_0 and w_a to ∼0.01 and ∼0.1, respectively. BBO’s dark-energy figure-of-merit would be approximately an order of magnitude better than all other proposed, dedicated dark-energy missions. To date, BBO has been designed with the primary goal of searching for gravitational waves from inflation, down to the level Ω_(GW)∼10^(-17); this requirement determines BBO’s frequency band (deci-Hz) and its sensitivity requirement (strain measured to ∼10^(-24)). To observe an inflationary GW background, BBO would first have to detect and subtract out ∼3×10^5 merging compact-star binaries, out to a redshift z ∼ 5. It is precisely this carefully measured foreground which would enable high-precision cosmology. BBO would determine the luminosity distance to each binary to ∼ percent accuracy. In addition, BBO’s angular resolution would be sufficient to uniquely identify the host galaxy for the majority of binaries; a coordinated optical/infrared observing campaign could obtain the redshifts. Combining the GW-derived distances and the electromagnetically-derived redshifts for such a large sample of objects, out to such high redshift, naturally leads to extraordinarily tight constraints on cosmological parameters. We emphasize that such “standard siren” measurements of cosmology avoid many of the systematic errors associated with other techniques: GWs offer a physics-based, absolute measurement of distance. In addition, we show that BBO would also serve as an exceptionally powerful gravitational-lensing mission, and we briefly discuss other astronomical uses of BBO, including providing an early warning system for all short/hard gamma-ray bursts.

ON THE ORIGIN OF THE HIGHEST REDSHIFT GAMMA-RAY BURSTS

GRB 080913 and GRB 090423 are the most distant gamma-ray bursts (GRBs) known to date, with spectroscopically determined redshifts of z = 6.7 and z = 8.1, respectively. The detection of bursts at this early epoch of the universe significantly constrains the nature of GRBs and their progenitors. We perform population synthesis studies of the formation and evolution of early stars, and calculate the resulting formation rates of short- and long-duration GRBs at high redshift. The peak of the GRB rate from Population II stars occurs at z approx 7 for a model with efficient/fast mixing of metals, while it is found at z approx 3 for an inefficient/slow metallicity evolution model. We show that in the redshift range 6 approx< z approx< 10, essentially all GRBs originate from Population II stars, regardless of the metallicity evolution model. These stars (having small, but non-zero metallicity) are the most likely progenitors for both long GRBs (collapsars) and short GRBs (neutron star-neutron star or blackhole-neutron star mergers) at this epoch. Although the predicted intrinsic rates of long and short GRBs are similar at these high redshifts, observational selection effects lead to higher (a factor of approx10) observed rates for long GRBs. Wemorexa0» conclude that the two recently observed high-z GRB events are most likely long GRBs originating from Population II collapsars.«xa0less

Hydrostatic Expansion and Spin Changes during Type I X-Ray Bursts

We present calculations of the spin-down of a neutron star atmosphere due to hydrostatic expansion during a Type I X-ray burst. We show that (i) Cumming and Bildsten overestimated the spin-down of rigidly-rotating atmospheres by a factor of two, and (ii) general relativity has a small (5-10%) effect on the angular momentum conservation law. We rescale our results to different neutron star masses, rotation rates and equations of state, and present some detailed rotational profiles. Comparing with recent observations of large frequency shifts in MXB 1658-298 and 4U 1916-053, we find that the spin-down expected if the atmosphere rotates rigidly is a factor of two to three less than the observed values. If differential rotation is allowed to persist, we find that the upper layers of the atmosphere spin down by an amount comparable to the observed values; however, there is no compelling reason to expect the observed spin frequency to be that of only the outermost layers. We conclude that hydrostatic expansion and angular momentum conservation alone cannot account for the largest frequency shifts observed during Type I bursts.