arXiv subjects

Daniel E. Holz

Publications and source records attributed to Daniel E. Holz.

At least 109 records · Page 6Linked to original sources

arXiv2008

Beyond Two Dark Energy Parameters

Our ignorance of the dark energy is generally described by a two-parameter equation of state. In these approaches a particular {\it ad hoc} functional form is assumed, and only two independent parameters are incorporated. We propose a model-independent, multi-parameter approach to fitting the dark energy, and show that next-generation surveys will constrain the equation of state in three or more independent redshift bins to better than 10%. Future knowledge of the dark energy will surpass two numbers (e.g., [$w_0$,$w_1$] or [$w_0$,$w_a$]), and we propose a more flexible approach to the analysis of present and future data.

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arXiv2008

Close Pairs as Proxies for Galaxy Cluster Mergers

Galaxy cluster merger statistics are an important component in understanding the formation of large-scale structure. Unfortunately, it is difficult to study merger properties and evolution directly because the identification of cluster mergers in observations is problematic. We use large N-body simulations to study the statistical properties of massive halo mergers, specifically investigating the utility of close halo pairs as proxies for mergers. We examine the relationship between pairs and mergers for a wide range of merger timescales, halo masses, and redshifts (0<z<1). We also quantify the utility of pairs in measuring merger bias. While pairs at very small separations will reliably merge, these constitute a small fraction of the total merger population. Thus, pairs do not provide a reliable direct proxy to the total merger population. We do find an intriguing universality in the relation between close pairs and mergers, which in principle could allow for an estimate of the statistical merger rate from the pair fraction within a scaled separation, but including the effects of redshift space distortions strongly degrades this relation. We find similar behavior for galaxy-mass halos, making our results applicable to field galaxy mergers at high redshift. We investigate how the halo merger rate can be statistically described by the halo mass function via the merger kernel (coagulation), finding an interesting environmental dependence of merging: halos within the mass resolution of our simulations merge less efficiently in overdense environments. Specifically, halo pairs with separations less than a few Mpc/h are more likely to merge in underdense environments; at larger separations, pairs are more likely to merge in overdense environments.

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arXiv2008

Lensing and Supernovae: Quantifying The Bias on the Dark Energy Equation of State

The gravitational magnification and demagnification of Type Ia supernovae (SNe) modify their positions on the Hubble diagram, shifting the distance estimates from the underlying luminosity-distance relation. This can introduce a systematic uncertainty in the dark energy equation of state (EOS) estimated from SNe, although this systematic is expected to average away for sufficiently large data sets. Using mock SN samples over the redshift range $0 < z \leq 1.7$ we quantify the lensing bias. We find that the bias on the dark energy EOS is less than half a percent for large datasets ($\gtrsim$ 2,000 SNe). However, if highly magnified events (SNe deviating by more than 2.5$σ$) are systematically removed from the analysis, the bias increases to $\sim$ 0.8%. Given that the EOS parameters measured from such a sample have a 1$σ$ uncertainty of 10%, the systematic bias related to lensing in SN data out to $z \sim 1.7$ can be safely ignored in future cosmological measurements.

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arXiv2007

CMB Cluster Lensing: Cosmography with the Longest Lever Arm

We discuss combining gravitational lensing of galaxies and the cosmic microwave background (CMB) by clusters to measure cosmographic distance ratios, and hence dark energy parameters. Advantages to using the CMB as the second source plane, instead of galaxies, include: a well-determined source distance, a longer lever arm for distance ratios, typically up to an order of magnitude higher sensitivity to dark energy parameters, and a decreased sensitivity to photometric redshift accuracy of the lens and galaxy sources. Disadvantages include: higher statistical errors, potential systematic errors, and the need for disparate surveys that overlap on the sky. Ongoing and planned surveys, such as the South Pol Telescope in conjunction with the Dark Energy Survey, can potentially reach the statistical sensitivity to make interesting consistency tests of the standard cosmological constant model. Future measurements that reach 1% or better precision in the convergences can provide sharp tests for future supernovae distance measurements.

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arXiv2007

Narrowing Constraints with Type Ia Supernovae: Converging on a Cosmological Constant

We apply a parameterization-independent approach to fitting the dark energy equation-of-state (EOS). Utilizing the latest type Ia supernova data, combined with results from the cosmic microwave background and baryon acoustic oscillations, we find that the dark energy is consistent with a cosmological constant. We establish independent estimates of the evolution of the dark energy EOS by diagonalizing the covariance matrix. We find three independent constraints, which taken together imply that the equation of state is more negative than -0.2 at the 68% confidence level in the redshift range 0 1.2 and rolls to a value of -1 today, and ``thawing'' models where the EOS is near -1 at high redshifts, but rapidly evolves to values greater than -0.85 at z < 0.2. Finally, we propose a parameterization-independent figure-of-merit, to help assess the ability of future surveys to constrain dark energy. While previous figures-of-merit presume specific dark energy parameterizations, we suggest a binning approach to evaluate dark energy constraints with a minimum number of assumptions.

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arXiv2006

Direct Reconstruction of the Dark Energy Scalar-Field Potential

While the accelerated expansion of the Universe is by now well established, an underlying scalar field potential possibly responsible for this acceleration remains unconstrained. We present an attempt to reconstruct this potential using recent SN data, under the assumption that the acceleration is driven by a single scalar field. Current approaches to such reconstructions are based upon simple parametric descriptions of either the luminosity distance or the dark energy equation of state. We find that these various approximations lead to a range of derived evolutionary histories of the dark energy equation of state (although there is considerable overlap between the different potential shapes allowed by the data). Instead of these indirect reconstruction schemes, we discuss a technique to determine the potential directly from the data by expressing it in terms of a binned scalar field. We apply this technique to a recent SN dataset, and compare the results with model-dependent approaches. In a similar fashion to direct estimates of the dark energy equation of state, we advocate direct reconstruction of the scalar field potential as a way to minimize prior assumptions on the shape, and thus minimize the introduction of bias in the derived potential.

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arXiv2006

The Clustering of Massive Halos

The clustering properties of dark matter halos are a firm prediction of modern theories of structure formation. We use two large volume, high-resolution N-body simulations to study how the correlation function of massive dark matter halos depends upon their mass and formation history. We find that halos with the lowest concentrations are presently more clustered than those of higher concentration, the size of the effect increasing with halo mass; this agrees with trends found in studies of lower mass halos. The clustering dependence on other characterizations of the full mass accretion history appears weaker than the effect with concentration. Using the integrated correlation function, marked correlation functions, and a power-law fit to the correlation function, we find evidence that halos which have recently undergone a major merger or a large mass gain have slightly enhanced clustering relative to a randomly chosen population with the same mass distribution.

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arXiv2006

Short GRB and binary black hole standard sirens as a probe of dark energy

Observations of the gravitational radiation from well-localized, inspiraling compact object binaries can measure absolute source distances with high accuracy. When coupled with an independent determination of redshift through an electromagnetic counterpart, these standard sirens can provide an excellent probe of the expansion history of the Universe and the dark energy. Short gamma-ray bursts, if produced by merging neutron star binaries, would be standard sirens with known redshifts detectable by ground-based GW networks such as LIGO-II, Virgo, and AIGO. Depending upon the collimation of these GRBs, a single year of observation of their gravitational waves can measure the Hubble constant to about 2%. When combined with measurement of the absolute distance to the last scattering surface of the cosmic microwave background, this determines the dark energy equation of state parameter w to 9%. Similarly, supermassive binary black hole inspirals will be standard sirens detectable by LISA. Depending upon the precise redshift distribution, 100 sources could measure w at the 4% level.

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arXiv2005

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 $Λ$CDM cosmology, as well as its uncertainty, using sixteen $1024^3$-particle nested-volume dark-matter simulations, spanning a mass range of over five 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 have negligible contribution from remaining statistical uncertainties in the central cosmology multiplicity function. There exists a potentially non-negligible cosmological dependence (non-universality) of the halo multiplicity function.

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arXiv2005

Cosmology with coalescing massive black holes

The gravitational waves generated in the coalescence of massive binary black holes will be measurable by LISA to enormous distances. Redshifts z~10 or larger (depending somewhat on the mass of the binary) can potentially be probed by such measurements, suggesting that binary coalescences can be made into cosmological tools. We discuss two particularly interesting types of probes. First, by combining gravitational-wave measurements with information about the universe's cosmography, we can study the evolution of black hole masses and merger rates as a function of redshift, providing information about the growth of structures at high redshift and possibly constraining hierarchical merger scenarios. Second, if it is possible to associate an ``electromagnetic'' counterpart with a coalescence, it may be possible to measure both redshift and luminosity distance to an event with less than ~1% error. Such a measurement would constitute an amazingly precise cosmological standard candle. Unfortunately, gravitational lensing uncertainties will reduce the quality of this candle significantly. Though not as amazing as might have been hoped, such a candle would nonetheless very usefully complement other distance-redshift probes, in particular providing a valuable check on systematic effects in such measurements.

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arXiv2005

Using gravitational-wave standard sirens

Gravitational waves (GWs) from supermassive binary black hole (BBH) inspirals are potentially powerful standard sirens (the GW analog to standard candles) (Schutz 1986, 2002). Because these systems are well-modeled, the space-based GW observatory LISA will be able to measure the luminosity distance (but not the redshift) to some distant massive BBH systems with 1-10% accuracy. This accuracy is largely limited by pointing error: GW sources generally are poorly localized on the sky. Localizing the binary independently (e.g., through association with an electromagnetic counterpart) greatly reduces this positional error. An electromagnetic counterpart may also allow determination of the event's redshift. In this case, BBH coalescence would constitute an extremely precise (better than 1%) standard candle visible to high redshift. In practice, gravitational lensing degrades this precision, though the candle remains precise enough to provide useful information about the distance-redshift relation. Even if very rare, these GW standard sirens would complement, and increase confidence in, other standard candles.

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arXiv2004

Safety in numbers: Gravitational Lensing Degradation of the Luminosity Distance-Redshift Relation

Observation of the expansion history of the Universe allows exploration of the physical properties and energy density of the Universe's various constituents. Standardizable candles such as Type Ia supernovae remain one of the most promising and robust tools in this endeavor, by allowing for a direct measure of the luminosity distance-redshift curve, and thereby producing detailed studies of the dark energy responsible for the Universe's currently accelerating expansion. As such observations are pushed to higher redshifts, the observed flux is increasingly affected by gravitational lensing magnification due to intervening structure along the line-of-sight. We simulate and analyze the non-Gaussian probability distribution function of de/amplification due to lensing of standard candles, quantify the effect of a convolution over many independent sources (which acts to restore the intrinsic average (unlensed) luminosity due to flux conservation), and compute the additional uncertainty due to lensing on derived cosmological parameters. For example, the ``degradation factor'' due to lensing is a factor of three reduction in the effective number of usable supernovae at z=1.5 (for sources with intrinsic flux dispersion of 10%). We also derive a useful expression for the effective increased dispersion in standard candles due to lensing, as a function of redshift.

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arXiv2004

How black holes get their kicks: Radiation recoil in binary black hole mergers

Gravitational waves from the coalescence of binary black holes carry linear momentum, causing center of mass recoil. This ``radiation rocket'' has important implications for systems with escape speeds of order the recoil velocity. We describe new recoil calculations using high precision black hole perturbation theory to estimate the magnitude of the recoil for the slow ``inspiral'' coalescence phase; coupled with a cruder calculation for the final ``plunge'', we estimate the total recoil imparted to a merged black hole. We find that velocities of many tens to a few hundred km/sec can be achieved fairly easily. The recoil probably never exceeds about 500 km/sec.

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arXiv2004

Consequences of gravitational radiation recoil

Coalescing binary black holes experience an impulsive kick due to anisotropic emission of gravitational waves. We discuss the dynamical consequences of the recoil accompanying massive black hole mergers. Recoil velocities are sufficient to eject most coalescing black holes from dwarf galaxies and globular clusters, which may explain the apparent absence of massive black holes in these systems. Ejection from giant elliptical galaxies would be rare, but coalescing black holes are displaced from the center and fall back on a time scale of order the half-mass crossing time. Displacement of the black holes transfers energy to the stars in the nucleus and can convert a steep density cusp into a core. Radiation recoil calls into question models that grow supermassive black holes from hierarchical mergers of stellar-mass precursors.

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arXiv2004

How black holes get their kicks: Gravitational radiation recoil revisited

Gravitational waves from the coalescence of binary black holes carry away linear momentum, causing center of mass recoil. This "radiation rocket" effect has important implications for systems with escape speeds of order the recoil velocity. We revisit this problem using black hole perturbation theory, treating the binary as a test mass spiraling into a spinning hole. For extreme mass ratios (q = m1/m2 << 1) we compute the recoil for the slow inspiral epoch of binary coalescence very accurately; these results can be extrapolated to q ~ 0.4 with modest accuracy. Although the recoil from the final plunge contributes significantly to the final recoil, we are only able to make crude estimates of its magnitude. We find that the recoil can easily reach ~ 100-200 km/s, but most likely does not exceed ~ 500 km/s. Though much lower than previous estimates, this recoil is large enough to have important astrophysical consequences. These include the ejection of black holes from globular clusters, dwarf galaxies, and high-redshift dark matter halos.

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arXiv2004

Gravitational Waves from Stellar Collapse: Correlations to Explosion Asymmetries

The collapse of massive stars not only produces observable outbursts across the entire electromagnetic spectrum but, for Galactic (or near-Galactic) supernovae, detectable signals for ground-based neutrino and gravitational wave detectors. Gravitational waves and neutrinos provide the only means to study the actual engine behind the optical outbursts: the collapsed stellar core. While the neutrinos are most sensitive to details of the equation of state, gravitational waves provide a means to study the mass asymmetries in this central core. We present gravitational wave signals from a series of 3-dimensional core-collapse simulations with asymmetries derived from initial perturbations caused by pre-collapse convection, core rotation, and low-mode convection in the explosion engine itself. A Galactic supernovae will allow us to differentiate these different sources of asymmetry. Combining this signal with other observations of the supernova, from neutrinos to gamma-rays to the compact remnant, dramatically increases the predictive power of the gravitational wave signal. We conclude with a discussion of the gravitational wave signal arising from collapsars, the leading engine for long-duration gamma-ray bursts.

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arXiv2002

Stellar collapse and gravitational waves

The new generation of gravitational wave (GW) detectors have the potential to open a novel window onto the violent dynamics of core collapse. Although it is certain that core collapse events generate gravitational radiation, understanding the characteristics of the radiation -- whether it can be measured with these detectors, and the best way to go about doing so -- is a challenging problem. In this chapter we review the promise of GWs as observational probes, including a discussion of the current state of GW detectors, and discuss the status of work to understand the waves generated by stellar core collapse.

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arXiv2002

On the Remarkable Spectrum of a Non-Hermitean Random Matrix Model

A non-Hermitean random matrix model proposed a few years ago has a remarkably intricate spectrum. Various attempts have been made to understand the spectrum, but even its dimension is not known. Using the Dyson-Schmidt equation, we show that the spectrum consists of a non-denumerable set of lines in the complex plane. Each line is the support of the spectrum of a periodic Hamiltonian, obtained by the infinite repetition of any finite sequence of the disorder variables. Our approach is based on the ``theory of words.'' We make a complete study of all 4-letter words. The spectrum is complicated because our matrix contains everything that will ever be written in the history of the universe, including this particular paper.

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