2,501 research outputs found
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 on arcsecond scales a clumpy
flat universe mimics the angular diameter distance of a smooth
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 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, , and find that it should be easily detectable, (S/N) 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 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 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 , biases in non-\lcdm\
cosmologies that are consistent with current data could reach the 3
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
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