109 research outputs found
The Dyer-Roeder distance-redshift relation in inhomogeneous universes
Using Monte-Carlo methods, we determine the best-fit value of the homogeneity
parameter alpha in the Dyer-Roeder distance-redshift relation for a variety of
redshifts, inhomogeneity models and cosmological parameter values. The relation
between alpha and the fraction of compact objects, f_p, is found to be
approximately linear. This relation can be parametrized with reasonable
accuracy for all cases treated in this paper by alpha = a*f_p, where a = 0.6.Comment: 5 pages, 10 figures, submitted to Phys.Rev.
Type Ia supernova constraints on compact object dark matter
The nature of dark matter (DM) is an open question in cosmology, despite its
abundance in the universe. While elementary particles have been posited to
explain DM, compact astrophysical objects such as black holes formed in the
early universe offer a theoretically appealing alternate route. Here, we
constrain the fraction of DM that can be made up of primordial black holes
(PBHs) with masses , using the Type Ia supernova Hubble
diagram. Utilizing the Dyer-Roeder distance relation, where the homogeneous
matter fraction is parameterized with , we find a maximum fractional
amount of DM in compact objects () of 0.50 at 95\% confidence level
(C.L.), in the flat CDM model and 0.49 when marginalising over a
constant dark energy equation of state. These limits do not change when
marginalising over cosmic curvature, demonstrating the robustness to the
cosmological model. When allowing for the prior on to include , we derive at 95 C.L., showing that the prior assumption of
gives a conservative upper limit on . When including Cepheid
calibrated supernovae, the 95\% C.L. constraints improve to . We
find that the estimate for the Hubble constant in our inference is consistent
with the homogeneous case, showing that inhomogeneities in the form of compact
dark matter cannot account for the observed Hubble tension. In conclusion, we
strongly exclude the possibility that PBHs with stellar masses and above form a
dominant fraction of the dark matter.Comment: to be submitted to MNRAS Letter
Measuring the properties of extragalactic dust and implications for the Hubble diagram
Scattering and absorption of light by a homogeneous distribution of
intergalactic large dust grains has been proposed as an alternative,
non-cosmological explanation for the faintness of Type Ia supernovae at z\s im
0.5. We investigate the differential extinction for high-redshift sources
caused by extragalactic dust along the line of sight.
Future observations of Type Ia supernovae up to , e.g. by the
proposed SNAP satellite, will allow the measurement of the properties of dust
over cosmological distances. We show that 1% {\em relative} spectrophotometric
accuracy (or broadband photometry) in the wavelength interval 0.7--1.5 m
is required to measure the extinction caused by ``grey'' dust down to magnitudes.
We also argue that the presence of grey dust is not necessarily inconsistent
with the recent measurement of the brightness of a supernova at (SN
1997ff), in the absence of accurate spectrophotometric information of the
supernova.Comment: Accepted by A&
Limiting the dimming of distant type Ia supernovae
Distant supernovae have been observed to be fainter than what is expected in
a matter dominated universe. The most likely explanation is that the universe
is dominated by an energy component with negative pressure -- dark energy.
However, there are several astrophysical processes that could, in principle,
affect the measurements and in order to be able to take advantage of the
growing supernova statistics, the control of systematic effects is crucial. We
discuss two of these; extinction due to intergalactic grey dust and dimming due
to photon-axion oscillations and show how their effect on supernova
observations can be constrained using observed quasar colours and spectra. For
a wide range of intergalactic dust models, we are able to rule out any dimming
larger than 0.2 magnitudes for a type Ia supernova at z=1. The corresponding
limit for intergalactic Milky Way type dust is 0.03 mag. For the more
speculative model of photons mixing with axions, we find that the effect is
independent of photon energy for certain combinations of parameter values and a
dimming as large as 0.6 magnitudes cannot be ruled out. These effects can have
profound implications for the possibility of constraining dark energy
properties using supernova observations.Comment: 19 pages, 11 figures Matches version accepted in JCAP. Some
corrections due to minor bug in simulations, major conclusions unchange
Is Dark Energy Dynamical? Prospects for an Answer
Recent data advances offer the exciting prospect of a first look at whether
dark energy has a dynamical equation of state or not. While formally theories
exist with a constant equation of state, they are nongeneric -- Einstein's
cosmological constant is a notable exception. So limits on the time variation,
w', directly tell us crucial physics. Two recent improvements in supernova data
from the Hubble Space Telescope allow important steps forward in constraining
the dynamics of dark energy, possessing the ability to exclude models with
w'\ga 1, if the universe truly has a cosmological constant. These data bring us
much closer to the ``systematics'' era, where further improvements will come
predominantly from more accurate, not merely more, observations. We examine the
possible gains and point out the complementary roles of space and ground based
observations in the near future. To achieve the leap to precision understanding
of dark energy in the next generation will require specially designed space
based measurements; we estimate the confidence level of detection of dynamics
(e.g. distinguishing between and ) will be ~1.8\sigma after the
ongoing generation, improving to more than 6.5\sigma in the dedicated space
generation.Comment: 6 pages, 2 figures; version accepted to Phys. Rev.
SNOC: a Monte-Carlo simulation package for high-z supernova observations
We present a Monte-Carlo package for simulation of high-redshift supernova
data, SNOC. Optical and near-infrared photons from supernovae are ray-traced
over cosmological distances from the simulated host galaxy to the observer at
Earth. The distances to the sources are calculated from user provided
cosmological parameters in a Friedmann-Lemaitre universe, allowing for
arbitrary forms of ``dark energy''. The code takes into account gravitational
interactions (lensing) and extinction by dust, both in the host galaxy and in
the line-of-sight. The user can also choose to include exotic effects like a
hypothetical attenuation due to photon-axion oscillations. SNOC is primarily
useful for estimations of cosmological parameter uncertainties from studies of
apparent brightness of Type Ia supernovae vs redshift, with special emphasis on
potential systematic effects. It can also be used to compute standard
cosmological quantities like luminosity distance, lookback time and age of the
universe in any Friedmann-Lemaitre model with or without quintessence.Comment: 16 pages, 3 figure
Cosmic distance-duality as probe of exotic physics and acceleration
In cosmology, distances based on standard candles (e.g. supernovae) and
standard rulers (e.g. baryon oscillations) agree as long as three conditions
are met: (1) photon number is conserved, (2) gravity is described by a metric
theory with (3) photons travelling on unique null geodesics. This is the
content of distance-duality (the reciprocity relation) which can be violated by
exotic physics. Here we analyse the implications of the latest cosmological
data sets for distance-duality. While broadly in agreement and confirming
acceleration we find a 2-sigma violation caused by excess brightening of SN-Ia
at z > 0.5, perhaps due to lensing magnification bias. This brightening has
been interpreted as evidence for a late-time transition in the dark energy but
because it is not seen in the d_A data we argue against such an interpretation.
Our results do, however, rule out significant SN-Ia evolution and extinction:
the "replenishing" grey-dust model with no cosmic acceleration is excluded at
more than 4-sigma despite this being the best-fit to SN-Ia data alone, thereby
illustrating the power of distance-duality even with current data sets.Comment: 6 pages, 4 colour figures. Version accepted as a Rapid Communication
in PR
Near-IR search for lensed supernovae behind galaxy clusters: III. Implications for cluster modeling and cosmology
Massive galaxy clusters at intermediate redshifts act as gravitational lenses
that can magnify supernovae (SNe) occurring in background galaxies. We assess
the possibility to use lensed SNe to put constraints on the mass models of
galaxy clusters and the Hubble parameter at high redshift. Due to the standard
candle nature of Type Ia supernovae (SNe Ia), observational information on the
lensing magnification from an intervening galaxy cluster can be used to
constrain the model for the cluster mass distribution. A statistical analysis
using parametric cluster models was performed to investigate the possible
improvements from lensed SNe Ia for the accurately modeled galaxy cluster A1689
and the less well constrained cluster A2204. Time delay measurements obtained
from SNe lensed by accurately modeled galaxy clusters can be used to measure
the Hubble parameter. For a survey of A1689 we estimate the expected rate of
detectable SNe Ia and of multiply imaged SNe. The velocity dispersion and core
radius of the main cluster potential show strong correlations with the
predicted magnifications and can therefore be constrained by observations of
SNe Ia in background galaxies. This technique proves especially powerful for
galaxy clusters with only few known multiple image systems. The main
uncertainty for measurements of the Hubble parameter from the time delay of
strongly lensed SNe is due to cluster model uncertainties. For the extremely
well modeled cluster A1689, a single time delay measurement could be used to
determine the Hubble parameter with a precision of ~ 10%. We conclude that
observations of SNe Ia behind galaxy clusters can be used to improve the mass
modeling of the large scale component of galaxy clusters and thus the
distribution of dark matter. Time delays from SNe strongly lensed by accurately
modeled galaxy clusters can be used to measure the Hubble constant at high
redshifts.Comment: 10 pages, 8 figures, 3 tables. Accepted for publication in A&
Constraining dark energy fluctuations with supernova correlations
We investigate constraints on dark energy fluctuations using type Ia
supernovae. If dark energy is not in the form of a cosmological constant, that
is if the equation of state is not equal to -1, we expect not only temporal,
but also spatial variations in the energy density. Such fluctuations would
cause local variations in the universal expansion rate and directional
dependences in the redshift-distance relation. We present a scheme for relating
a power spectrum of dark energy fluctuations to an angular covariance function
of standard candle magnitude fluctuations. The predictions for a
phenomenological model of dark energy fluctuations are compared to
observational data in the form of the measured angular covariance of Hubble
diagram magnitude residuals for type Ia supernovae in the Union2 compilation.
The observational result is consistent with zero dark energy fluctuations.
However, due to the limitations in statistics, current data still allow for
quite general dark energy fluctuations as long as they are in the linear
regime.Comment: 18 pages, 6 figures, matches the published versio
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