Exploring the Expansion History of the Universe

Exploring the recent expansion history of the universe promises insights into the cosmological model, the nature of dark energy, and potentially clues to high energy physics theories and gravitation. We examine the extent to which precision distance-redshift observations can map out the history, including the acceleration-deceleration transition, and the components and equations of state of the energy density. We consider the ability to distinguish between various dynamical scalar field models for the dark energy, as well as higher dimension and alternate gravity theories. Finally, we present a new, advantageous parametrization for the study of dark energy.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Letter

Transition from Clumpy to Smooth Angular Diameter Distances

Distance relations in a locally inhomogeneous universe are expected to behave like the Dyer-Roeder solution on small angular scales and the Friedmann-Robertson-Walker solution on large angular scales. Within a simple compact clump model the transition between these asymptotic behaviors is demonstrated and quantified. The redshift dependent transition scale is of order a few arcseconds; this implies it should have little influence on large angular scale cosmological tests such as the volume-redshift relation but possibly significant effects on arcsecond angular diameter measurements of radio galaxies and AGNs. For example, at $z=2$ on arcsecond scales a clumpy flat universe mimics the angular diameter distance of a smooth $\Omega=0.27$ model.Comment: 10 pages, 3 figures, AAS LaTeX (aaspp4.sty), accepted by ApJ. Revised version contains additional text and figure on observational consequences for cosmological test

Cross-Correlating Cosmic Microwave Background Radiation Fluctuations with Redshift Surveys: Detecting the Signature of Gravitational Lensing

Density inhomogeneities along the line-of-sight distort fluctuations in the cosmic microwave background. Usually, this effect is thought of as a small second-order effect that mildly alters the statistics of the microwave background fluctuations. We show that there is a first-order effect that is potentially observable if we combine microwave background maps with large redshift surveys. We introduce a new quantity that measures this lensing effect, $$, where T is the microwave background temperature and $\delta \theta$ is the lensing due to matter in the region probed by the redshift survey. We show that the expected signal is first order in the gravitational lensing bending angle, $< (\delta \theta)^2 >^{1/2}$, and find that it should be easily detectable, (S/N) $\sim$ 15-35, if we combine the Microwave Anisotropy Probe satellite and Sloan Digital Sky Survey data. Measurements of this cross-correlation will directly probe the ``bias'' factor, the relationship between fluctuations in mass and fluctuations in galaxy counts.Comment: 13 pages, 4 postscript figures included; Uses aaspp4.sty (AASTeX v4.0); Accepted for publication in Astrophysical Journal, Part

The Paths of Quintessence

The structure of the dark energy equation of state phase plane holds important information on the nature of the physics. We explain the bounds of the freezing and thawing models of scalar field dark energy in terms of the tension between the steepness of the potential vs. the Hubble drag. Additionally, we extend the phase plane structure to modified gravity theories, examine trajectories of models with certain properties, and categorize regions in terms of scalar field hierarchical parameters, showing that dark energy is generically not a slow roll phenomenon.Comment: 12 pages, 7 figures; matches PRD versio

Extending the Gravitational Growth Framework

The gravitational growth index formalism provides a model independent way to look for deviations from general relativity by testing dark energy physics distinct from its effects on the cosmic expansion history. Here we extend the approach to incorporate an early time parameter g_star in addition to the growth index in describing the growth of large scale structure. We illustrate its utility for models with modified gravity at high redshift, early acceleration, or early dark energy. Future data will have the capability to constrain the dark energy equation of state, the growth index gamma, and g_star simultaneously, with no degradation in the equation of state determination.Comment: 8 pages, 5 figure

Seeing Darkness: the New Cosmology

We present some useful ways to visualize the nature of dark energy and the effects of the accelerating expansion on cosmological quantities. Expansion probes such as Type Ia supernovae distances and growth probes such as weak gravitational lensing and the evolution of large scale structure provide powerful tests in complementarity. We present a ``ladder'' diagram, showing that in addition to dramatic improvements in precision, next generation probes will provide insight through an increasing ability to test assumptions of the cosmological framework, including gravity beyond general relativity.Comment: plenary talk at TAUP2005; to appear in Journal of Physics; 7 page

Gravitational Wave Sirens as a Triple Probe of Dark Energy

Gravitational wave standard sirens have been considered as precision distance indicators to high redshift; however, at high redshift standard sirens or standard candles such as supernovae suffer from lensing noise. We investigate lensing noise as a signal instead and show how measurements of the maximum demagnification (minimum convergence) probe cosmology in a highly complementary manner to the distance itself. Revisiting the original form for minimum convergence we quantify the bias arising from the commonly used approximation. Furthermore, after presenting a new lensing probability function we discuss how the width of the lensed standard siren amplitude distribution also probes growth of structure. Thus standard sirens and candles can serve as triple probes of dark energy, measuring both the cosmic expansion history and growth history.Comment: 7 pages, 5 figures; v2 minor changes matching published versio

Will Gravitational Wave Sirens Determine the Hubble Constant?

Lack of knowledge about the background expansion history of the Universe from independent observations makes it problematic to obtain a precise and accurate estimation of the Hubble constant $H_0$ from gravitational wave standard sirens, even with electromagnetic counterpart redshifts. Simply fitting simultaneously for the matter density in a flat \lcdm\ model can reduce the precision on $H_0$ from 1\% to 5\%, while not knowing the actual background expansion model of the universe (e.g.\ form of dark energy) can introduce substantial bias in estimation of the Hubble constant. When the statistical precision is at the level of 1\% uncertainty on $H_0$, biases in non-\lcdm\ cosmologies that are consistent with current data could reach the 3$\sigma$ level. To avoid model-dependent biases, statistical techniques that are appropriately agnostic about model assumptions need to be employed.Comment: 7 pages, 7 figure

Cosmic Growth History and Expansion History

The cosmic expansion history tests the dynamics of the global evolution of the universe and its energy density contents, while the cosmic growth history tests the evolution of the inhomogeneous part of the energy density. Precision comparison of the two histories can distinguish the nature of the physics responsible for the accelerating cosmic expansion: an additional smooth component - dark energy - or a modification of the gravitational field equations. With the aid of a new fitting formula for linear perturbation growth accurate to 0.05-0.2%, we separate out the growth dependence on the expansion history and introduce a new growth index parameter \gamma that quantifies the gravitational modification.Comment: 8 pages, 3 figures; minor changes to match version accepted to PR