221 research outputs found
Strong Gravitational Lensing and Dark Energy Complementarity
In the search for the nature of dark energy most cosmological probes measure
simple functions of the expansion rate. While powerful, these all involve
roughly the same dependence on the dark energy equation of state parameters,
with anticorrelation between its present value w_0 and time variation w_a.
Quantities that have instead positive correlation and so a sensitivity
direction largely orthogonal to, e.g., distance probes offer the hope of
achieving tight constraints through complementarity. Such quantities are found
in strong gravitational lensing observations of image separations and time
delays. While degeneracy between cosmological parameters prevents full
complementarity, strong lensing measurements to 1% accuracy can improve
equation of state characterization by 15-50%. Next generation surveys should
provide data on roughly 10^5 lens systems, though systematic errors will remain
challenging.Comment: 7 pages, 5 figure
Exploring the Expanding Universe and Dark Energy using the Statefinder Diagnostic
The coming few years are likely to witness a dramatic increase in high
quality Sn data as current surveys add more high redshift supernovae to their
inventory and as newer and deeper supernova experiments become operational.
Given the current variety in dark energy models and the expected improvement in
observational data, an accurate and versatile diagnostic of dark energy is the
need of the hour. This paper examines the Statefinder diagnostic in the light
of the proposed SNAP satellite which is expected to observe about 2000
supernovae per year. We show that the Statefinder is versatile enough to
differentiate between dark energy models as varied as the cosmological constant
on the one hand, and quintessence, the Chaplygin gas and braneworld models, on
the other. Using SNAP data, the Statefinder can distinguish a cosmological
constant () from quintessence models with and Chaplygin gas
models with at the level if the value of \om is
known exactly. The Statefinder gives reasonable results even when the value of
\om is known to only accuracy. In this case, marginalizing over
\om and assuming a fiducial LCDM model allows us to rule out quintessence
with and the Chaplygin gas with (both at
). These constraints can be made even tighter if we use the
Statefinders in conjunction with the deceleration parameter. The Statefinder is
very sensitive to the total pressure exerted by all forms of matter and
radiation in the universe. It can therefore differentiate between dark energy
models at moderately high redshifts of z \lleq 10.Comment: 21 pages, 17 figures. Minor typos corrected to agree with version
published in MNRAS. Results unchange
Probing Gravitation, Dark Energy, and Acceleration
The acceleration of the expansion of the universe arises from unknown
physical processes involving either new fields in high energy physics or
modifications of gravitation theory. It is crucial for our understanding to
characterize the properties of the dark energy or gravity through cosmological
observations and compare and distinguish between them. In fact, close
consistencies exist between a dark energy equation of state function w(z) and
changes to the framework of the Friedmann cosmological equations as well as
direct spacetime geometry quantities involving the acceleration, such as
``geometric dark energy'' from the Ricci scalar. We investigate these
interrelationships, including for the case of superacceleration or phantom
energy where the fate of the universe may be more gentle than the Big Rip.Comment: 12 pages, 8 figure
Quinstant Dark Energy Predictions for Structure Formation
We explore the predictions of a class of dark energy models, quinstant dark
energy, concerning the structure formation in the Universe, both in the linear
and non-linear regimes. Quinstant dark energy is considered to be formed by
quintessence and a negative cosmological constant. We conclude that these
models give good predictions for structure formation in the linear regime, but
fail to do so in the non-linear one, for redshifts larger than one.Comment: 9 pages, 14 figures, "Accepted for publication in Astrophysics &
Space Science
Recommended from our members
Importance of Supernovae at z<0.1 for Probing Dark Energy
Supernova experiments to characterize dark energy require a well designed low redshift program; we consider this for both ongoing/near term (e.g. Supernova Legacy Survey) and comprehensive future (e.g., SNAP) experiments. The derived criteria are: a supernova sample centered near z=0.05 comprising 150-500 (in the former case) and 300-900 (in the latter case) well measured supernovae. Low redshift Type Ia supernovae play two important roles for cosmological use of the supernova distance-redshift relation: as an anchor for the Hubble diagram and as an indicator of possible systematics. An innate degeneracy in cosmological distances implies that 300 nearby supernovae nearly saturate their cosmological leverage for the first use, and their optimum central redshift is z=0.05. This conclusion is strengthened upon including velocity flow and magnitude offset systematics. Limiting cosmological parameter bias due to supernova population drift (evolution) systematics plausibly increases the requirement for the second use to less than about 900 supernovae
Induced Gravity and the Attractor Dynamics of Dark Energy/Dark Matter
Attractor solutions that give dynamical reasons for dark energy to act like
the cosmological constant, or behavior close to it, are interesting
possibilities to explain cosmic acceleration. Coupling the scalar field to
matter or to gravity enlarges the dynamical behavior; we consider both
couplings together, which can ameliorate some problems for each individually.
Such theories have also been proposed in a Higgs-like fashion to induce gravity
and unify dark energy and dark matter origins. We explore restrictions on such
theories due to their dynamical behavior compared to observations of the cosmic
expansion. Quartic potentials in particular have viable stability properties
and asymptotically approach general relativity.Comment: 11 pages, 10 figures, accepted in JCAP, results unchanged, an
explanation added on perfect fluids for general spinor Lagrangian
Cosmic Shear with Next Generation Redshift Surveys as a Cosmological Probe
The expansion of the universe causes spacetime curvature, distinguishing
between distances measured along and transverse to the line of sight. The ratio
of these distances, e.g. the cosmic shear distortion of a sphere defined by
observations of large scale structure as suggested by Alcock & Paczynski,
provides a method for exploring the expansion as a function of redshift. The
theoretical sensitivity to cosmological parameters, including the dark energy
equation of state, is presented. Remarkably, sensitivity to the time variation
of the dark energy equation of state is best achieved by observations at
redshifts z<1. While systematic errors greatly degrade the theoretical
sensitivity, this probe may still offer useful parameter estimation, especially
in complementarity with a distance measure like the Type Ia supernova method
implemented by SNAP. Possible future observations of the Alcock-Paczynski
distortion by the KAOS project on a 8 meter ground based telescope are
considered.Comment: 6 pages, 8 figure
Optimal strategies : theoretical approaches to the parametrization of the dark energy equation of state
The absence of compelling theoretical model requires the parameterizing the
dark energy to probe its properties. The parametrization of the equation of
state of the dark energy is a common method. We explore the theoretical
optimization of the parametrization based on the Fisher information matrix. As
a suitable parametrization, it should be stable at high redshift and should
produce the determinant of the Fisher matrix as large as possible. For the
illustration, we propose one parametrization which can satisfy both criteria.
By using the proper parametrization, we can improve the constraints on the dark
energy even for the same data. We also show the weakness of the so-called
principal component analysis method.Comment: 7pages, 11 figures, 2 tables, To match the version accepted by AS
Reducing Zero-point Systematics in Dark Energy Supernova Experiments
We study the effect of filter zero-point uncertainties on future supernova
dark energy missions. Fitting for calibration parameters using simultaneous
analysis of all Type Ia supernova standard candles achieves a significant
improvement over more traditional fit methods. This conclusion is robust under
diverse experimental configurations (number of observed supernovae, maximum
survey redshift, inclusion of additional systematics). This approach to
supernova fitting considerably eases otherwise stringent mission calibration
requirements. As an example we simulate a space-based mission based on the
proposed JDEM satellite; however the method and conclusions are general and
valid for any future supernova dark energy mission, ground or space-based.Comment: 30 pages,8 figures, 5 table, one reference added, submitted to
Astroparticle Physic
Cosmological Models and Latest Observational Data
In this note, we consider the observational constraints on some cosmological
models by using the 307 Union type Ia supernovae (SNIa), the 32 calibrated
Gamma-Ray Bursts (GRBs) at , the updated shift parameter from WMAP
5-year data (WMAP5), and the distance parameter of the measurement of the
baryon acoustic oscillation (BAO) peak in the distribution of SDSS luminous red
galaxies with the updated scalar spectral index from WMAP5. The tighter
constraints obtained here update the ones obtained previously in the
literature.Comment: 10 pages, 5 figures, 1 table, revtex4; v2: discussions added,
accepted by Eur. Phys. J. C; v3: published versio
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