132 research outputs found
Designer Cosmology
We highlight the flexibility of the IPSO experiment-design framework by
contrasting its application to CMB, weak lensing and redshift surveys. We
illustrate the latter with a 10 parameter MCMC D-optimisation of a dark energy
redshift survey. When averaged over a standard dark energy model space the
resulting optimal survey typically has only one or two redshift bins, located
at z<2. By exploiting optimisation we show how the statistical power of such
surveys is significantly enhanced. Experiment design is aided by the richness
of the figure of merit landscape which means one can impose secondary
optimisation criteria at little cost. For example, one may choose either to
maximally test a single model (such as \Lambda CDM) or to get the most general
model-independent constraints possible (e.g. on a whole space of dark energy
models). Such freedom points to a future where cosmological experiments become
increasingly specialised and optimisation increasingly important.Comment: 5 pages, 5 colour figures, ApJ Styl
Optimizing baryon acoustic oscillation surveys – I. Testing the concordance ΛCDM cosmology
We optimize the design of future spectroscopic redshift surveys for
constraining the dark energy via precision measurements of the baryon acoustic
oscillations (BAO), with particular emphasis on the design of the Wide-Field
Multi-Object Spectrograph (WFMOS). We develop a model that predicts the number
density of possible target galaxies as a function of exposure time and
redshift. We use this number counts model together with fitting formulae for
the accuracy of the BAO measurements to determine the effectiveness of
different surveys and instrument designs. We search through the available
survey parameter space to find the optimal survey with respect to the dark
energy equation-of-state parameters according to the Dark Energy Task Force
Figure-of-Merit, including predictions of future measurements from the Planck
satellite. We optimize the survey to test the LambdaCDM model, assuming that
galaxies are pre-selected using photometric redshifts to have a constant number
density with redshift, and using a non-linear cut-off for the matter power
spectrum that evolves with redshift. We find that line-emission galaxies are
strongly preferred as targets over continuum emission galaxies. The optimal
survey covers a redshift range 0.8 < z < 1.4, over the widest possible area
(6000 sq. degs from 1500 hours observing time). The most efficient number of
fibres for the spectrograph is 2,000, and the survey performance continues to
improve with the addition of extra fibres until a plateau is reached at 10,000
fibres. The optimal point in the survey parameter space is not highly peaked
and is not significantly affected by including constraints from upcoming
supernovae surveys and other BAO experiments.Comment: 15 pages, 9 figure
WFMOS - Sounding the Dark Cosmos
Vast sound waves traveling through the relativistic plasma during the first
million years of the universe imprint a preferred scale in the density of
matter. We now have the ability to detect this characteristic fingerprint in
the clustering of galaxies at various redshifts and use it to measure the
acceleration of the expansion of the Universe. The Wide-Field Multi-Object
Spectrograph (WFMOS) would use this test to shed significant light on the true
nature of dark energy, the mysterious source of this cosmic acceleration. WFMOS
would also revolutionise studies of the kinematics of the Milky Way and provide
deep insights into the clustering of galaxies at redshifts up to z~4. In this
article we discuss the recent progress in large galaxy redshift surveys and
detail how WFMOS will help unravel the mystery of dark energy.Comment: 6 pages, pure pdf. An introduction to WFMOS and Baryon Acoustic
Oscillations for a general audienc
Searching for modified gravity with baryon oscillations: from SDSS to wide field multiobject spectroscopy (WFMOS)
We discuss how the baryon acoustic oscillation (BAO) signatures in the galaxy
power spectrum can distinguish between modified gravity and the cosmological
constant as the source of cosmic acceleration. To this end we consider a model
characterized by a parameter , which corresponds to the
Dvali-Gabadadze-Porrati (DGP) model if and reduces to the standard
spatially flat cosmological constant concordance model for equal to
infinity. We find that the different expansion histories of the modified
gravity models systematically shifts the peak positions of BAO. A preliminary
analysis using the current SDSS LRG sample indicates that the original DGP
model is disfavored unless the matter density parameter exceeds 0.3. The
constraints will be strongly tightened with future spectroscopic samples of
galaxies at high redshifts. We demonstrate that WFMOS, in collaboration with
other surveys such as Planck, will powerfully constrain modified gravity
alternatives to dark energy as the explanation of cosmic acceleration.Comment: Physical Review D, in pres
Optimizing baryon acoustic oscillation surveys - I. Testing the concordance ΛCDM cosmology
We optimize the design of future spectroscopic redshift surveys for constraining the dark energy via precision measurements of the baryon acoustic oscillations (BAOs), with particular emphasis on the design of the Wide-Field Multi-Object Spectrograph. We develop a model that predicts the number density of possible target galaxies as a function of exposure time and redshift. We use this number counts model together with fitting formulae for the accuracy of the BAO measurements to determine the effectiveness of different surveys and instrument designs. We search through the available survey parameter space to find the optimal survey with respect to the dark energy equation-of-state parameters according to the Dark Energy Task Force Figure-of-Merit, including predictions of future measurements from the Planck satellite. We optimize the survey to test the Lambda cold dark matter model, assuming that galaxies are pre-selected using photometric redshifts to have a constant number density with redshift, and using a non-linear cut-off for the matter power spectrum that evolves with redshift. We find that line-emission galaxies are strongly preferred as targets over continuum emission galaxies. The optimal survey covers a redshift range 0.8 < z < 1.4, over the widest possible area (6000 deg2 from 1500 h observing time). The most efficient number of fibres for the spectrograph is 2000, and the survey performance continues to improve with the addition of extra fibres until a plateau is reached at 10 000 fibres. The optimal point in the survey parameter space is not highly peaked and is not significantly affected by including constraints from upcoming supernovae surveys and other BAO experiment
Optimizing baryon acoustic oscillation surveys II: curvature, redshifts, and external datasets
We extend our study of the optimization of large baryon acoustic oscillation
(BAO) surveys to return the best constraints on the dark energy, building on
Paper I of this series (Parkinson et al. 2007). The survey galaxies are assumed
to be pre-selected active, star-forming galaxies observed by their line
emission with a constant number density across the redshift bin. Star-forming
galaxies have a redshift desert in the region 1.6 < z < 2, and so this redshift
range was excluded from the analysis. We use the Seo & Eisenstein (2007)
fitting formula for the accuracies of the BAO measurements, using only the
information for the oscillatory part of the power spectrum as distance and
expansion rate rulers. We go beyond our earlier analysis by examining the
effect of including curvature on the optimal survey configuration and updating
the expected `prior' constraints from Planck and SDSS. We once again find that
the optimal survey strategy involves minimizing the exposure time and
maximizing the survey area (within the instrumental constraints), and that all
time should be spent observing in the low-redshift range (z<1.6) rather than
beyond the redshift desert, z>2. We find that when assuming a flat universe the
optimal survey makes measurements in the redshift range 0.1 < z <0.7, but that
including curvature as a nuisance parameter requires us to push the maximum
redshift to 1.35, to remove the degeneracy between curvature and evolving dark
energy. The inclusion of expected other data sets (such as WiggleZ, BOSS and a
stage III SN-Ia survey) removes the necessity of measurements below redshift
0.9, and pushes the maximum redshift up to 1.5. We discuss considerations in
determining the best survey strategy in light of uncertainty in the true
underlying cosmological model.Comment: 15 pages, revised in response to referees remarks, accepted for
publication in MNRAS. 2nd paper in a series. Paper 1 is at
http://arxiv.org/abs/astro-ph/070204
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
Dark energy reflections in the redshift-space quadrupole
We show that the redshift-space quadrupole will be a powerful tool for
constraining dark energy even if the baryon oscillations are missing from the
usual monopole power spectrum and even if bias is scale- and time-dependent. We
calculate the accuracy with which a next-generation galaxy survey based on KAOS
will measure the quadrupole power spectrum, which gives the leading
anisotropies in the power spectrum in redshift space due to the linear
velocity, Finger of God and Alcock-Paczynski effects. Combining the monopole
and quadrupole power spectra breaks the degeneracies between the multiple bias
parameters and dark energy both in the linear and nonlinear regimes and, in the
complete absence of baryon oscillations (), leads to a roughly 500%
improvement in constraints on dark energy compared with those from the monopole
spectrum alone. As a result the worst case - with no baryon oscillations - has
dark energy errors only mildly degraded relative to the ideal case, providing
insurance on the robustness of next-generation galaxy survey constraints on
dark energy.Comment: 6 pages, 3 colour figure
Fisher Matrix Preloaded -- Fisher4Cast
The Fisher Matrix is the backbone of modern cosmological forecasting. We
describe the Fisher4Cast software: a general-purpose, easy-to-use, Fisher
Matrix framework. It is open source, rigorously designed and tested and
includes a Graphical User Interface (GUI) with automated LATEX file creation
capability and point-and-click Fisher ellipse generation. Fisher4Cast was
designed for ease of extension and, although written in Matlab, is easily
portable to open-source alternatives such as Octave and Scilab. Here we use
Fisher4Cast to present new 3-D and 4-D visualisations of the forecasting
landscape and to investigate the effects of growth and curvature on future
cosmological surveys. Early releases have been available at
http://www.cosmology.org.za since May 2008 with 750 downloads in the first
year. Version 2.2 is made public with this paper and includes a Quick Start
guide and the code used to produce the figures in this paper, in the hope that
it will be useful to the cosmology and wider scientific communities.Comment: 30 Pages, 15 figures. Minor revisions to match published version,
with some additional functionality described to match the current version
(2.2) of the code. Software available at http://www.cosmology.org.za. Usage,
structure and flow of the software, as well as tests performed are described
in the accompanying Users' Manua
Dynamical Dark Energy or Simply Cosmic Curvature?
We show that the assumption of a flat universe induces critically large
errors in reconstructing the dark energy equation of state at z>~0.9 even if
the true cosmic curvature is very small, O(1%) or less. The spuriously
reconstructed w(z) shows a range of unusual behaviour, including crossing of
the phantom divide and mimicking of standard tracking quintessence models. For
1% curvature and LCDM, the error in w grows rapidly above z~0.9 reaching
(50%,100%) by redshifts of (2.5,2.9) respectively, due to the long cosmological
lever arm. Interestingly, the w(z) reconstructed from distance data and Hubble
rate measurements have opposite trends due to the asymmetric influence of the
curved geodesics. These results show that including curvature as a free
parameter is imperative in any future analyses attempting to pin down the
dynamics of dark energy, especially at moderate or high redshifts.Comment: 5 pages, 2 figures. To appear in JCA
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