224 research outputs found
The Accelerating Universe
In this article we review the discovery of the accelerating universe using
type Ia supernovae. We then outline ways in which dark energy - component that
causes the acceleration - is phenomenologically described. We finally describe
principal cosmological techniques to measure large-scale properties of dark
energy. This chapter complements other articles in this book that describe
theoretical understanding (or lack thereof) of the cause for the accelerating
universe.Comment: Invited review chapter for book "Adventures in Cosmology" (ed. D.
Goodstein) aimed at general scientists; 28 pages, 10 figure
Uncorrelated Estimates of Dark Energy Evolution
Type Ia supernova data have recently become strong enough to enable, for the
first time, constraints on the time variation of the dark energy density and
its equation of state. Most analyses, however, are using simple two or
three-parameter descriptions of the dark energy evolution, since it is well
known that allowing more degrees of freedom introduces serious degeneracies.
Here we present a method to produce uncorrelated and nearly model-independent
band power estimates of the equation of state of dark energy and its density as
a function of redshift. We apply the method to recently compiled supernova
data. Our results are consistent with the cosmological constant scenario, in
agreement with other analyses that use traditional parameterizations, though we
find marginal (2-sigma) evidence for w(z) < -1 at z < 0.2. In addition to easy
interpretation, uncorrelated, localized band powers allow intuitive and
powerful testing of the constancy of either the energy density or equation of
state. While we have used relatively coarse redshift binning suitable for the
current set of about 150 supernovae, this approach should reach its full
potential in the future, when applied to thousands of supernovae found from
ground and space, combined with complementary information from other
cosmological probes.Comment: 5 pages, 2 figure
Importance of Supernovae at z>1.5 to Probe Dark Energy
The accelerating expansion of the universe suggests that an unknown component
with strongly negative pressure, called dark energy, currently dominates the
dynamics of the universe. Such a component makes up ~70% of the energy density
of the universe yet has not been predicted by the standard model of particle
physics. The best method for exploring the nature of this dark energy is to map
the recent expansion history, at which Type Ia supernovae have proved adept. We
examine here the depth of survey necessary to provide a precise and
qualitatively complete description of dark energy. Realistic analysis of
parameter degeneracies, allowance for natural time variation of the dark energy
equation of state, and systematic errors in astrophysical observations all
demonstrate the importance of a survey covering the full range 0<z<2 for
revealing the nature of dark energy.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
Constraining the Properties of Dark Energy
The presence of dark energy in the Universe is inferred directly from the
accelerated expansion of the Universe, and indirectly, from measurements of
cosmic microwave background (CMB) anisotropy. Dark energy contributes about 2/3
of the critical density, is very smoothly distributed, and has large negative
pressure. Its nature is very much unknown. Most of its discernible consequences
follow from its effect on evolution of the expansion rate of the Universe,
which in turn affects the growth of density perturbations and the age of the
Universe, and can be probed by the classical kinematic cosmological tests.
Absent a compelling theoretical model (or even a class of models), we describe
dark energy by an effective equation of state w=p_X/rho_X which is allowed to
vary with time. We describe and compare different approaches for determining
w(t), including magnitude-redshift (Hubble) diagram, number counts of galaxies
and clusters, and CMB anisotropy, focusing particular attention on the use of a
sample of several thousand type Ia supernova with redshifts z < 1.7, as might
be gathered by the proposed SNAP satellite. Among other things, we derive
optimal strategies for constraining cosmological parameters using type Ia
supernovae. While in the near term CMB anisotropy will provide the first
measurements of w, supernovae and number counts appear to have the most
potential to probe dark energy.Comment: 6 pages, 3 figures; proceedings of 20th Texas Symposium on Relavistic
Astrophysic
How many dark energy parameters?
For exploring the physics behind the accelerating universe a crucial question
is how much we can learn about the dynamics through next generation
cosmological experiments. For example, in defining the dark energy behavior
through an effective equation of state, how many parameters can we
realistically expect to tightly constrain? Through both general and specific
examples (including new parametrizations and principal component analysis) we
argue that the answer is 42 - no, wait, two. Cosmological parameter analyses
involving a measure of the equation of state value at some epoch (e.g. w_0) and
a measure of the change in equation of state (e.g. w') are therefore realistic
in projecting dark energy parameter constraints. More elaborate
parametrizations could have some uses (e.g. testing for bias or comparison with
model features), but do not lead to accurately measured dark energy parameters.Comment: 10 pages, 1 figur
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