6,558 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
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
Relationally Aggressive Media Exposure and Children’s Normative Beliefs: Does Parental Mediation Matter?
Research indicates that relationally aggressive media exposure is positively associated with relational aggression in children. Theories of media effects suggest that these associations may be mediated by aggressive cognitions. Although parental mediation can attenuate the effects of violent media, it is unknown whether there are similar benefits of parental mediation of relationally aggressive media. The current study examined concurrent and longitudinal associations between relationally aggressive television and movie exposure and normative beliefs about relational aggression, and whether parental mediation moderates these associations. Participants were 103 children (50% female) in grades 3-6 and their parents. The following year, 48 children (52% female) were again assessed. Relationally aggressive media exposure predicted concurrent relational aggression norms, even after controlling for physically aggressive media exposure and physical aggression norms. Relationally aggressive television and movie exposure predicted greater subsequent approval of relational aggression only among children whose parents engaged in low levels of active mediation
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
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
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
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
Physics of SNeIa and Cosmology
We give an overview of the current understanding of Type Ia supernovae
relevant for their use as cosmological distance indicators. We present the
physical basis to understand their homogeneity of the observed light curves and
spectra and the observed correlations. This provides a robust method to
determine the Hubble constant, 67 +- 8 (2 sigma) km/Mpc/sec, independently from
primary distance indicators.
We discuss the uncertainties and tests which include SNe Ia based distance
determinations prior to delta-Ceph. measurements for the host galaxies. Based
on detailed models, we study the small variations from homogeneities and their
observable consequences. In combination with future data, this underlines the
suitability and promises the refinements needed to determine accurate relative
distances within 2 to 3 % and to use SNe Ia for high precision cosmology.Comment: to be published in "Stellar Candles", eds. Gieren et al. Lecture
Notes in Physics (http://link.springer.de/series/lnpp
Competition between Pressure and Gravity Confinement in Lyman-Alpha Forest Observations
A break in the distribution function of Ly clouds (at a typical
redshift of ) has been reported by Petitjean et al. (1993). This feature
is what would be expected from a transition between pressure confinement and
gravity confinement (as predicted in Charlton, Salpeter, and Hogan (1993)). The
column density at which the feature occurs has been used to determine the
external confining pressure, , which could be
due to a hot, intergalactic medium. For models that provide a good fit to the
data, the contribution of the gas in clouds to is small. The specific
shape of the distribution function at the transition (predicted by models to
have a non-monotonic slope) can serve as a diagnostic of the distribution of
dark matter around Ly forest clouds, and the present data already
eliminate certain models.Comment: 10 pages plain TeX, 2 figures available upon request, submitted to
ApJ Letters, PSU-jc-
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
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