65 research outputs found
Is the Universe Inflating? Dark Energy and the Future of the Universe
We consider the fate of the observable universe in the light of the discovery
of a dark energy component to the cosmic energy budget. We extend results for a
cosmological constant to a general dark energy component and examine the
constraints on phenomena that may prevent the eternal acceleration of our patch
of the universe. We find that the period of accelerated cosmic expansion has
not lasted long enough for observations to confirm that we are undergoing
inflation; such an observation will be possible when the dark energy density
has risen to between 90% and 95% of the critical. The best we can do is make
cosmological observations in order to verify the continued presence of dark
energy to some high redshift. Having done that, the only possibility that could
spoil the conclusion that we are inflating would be the existence of a
disturbance (the surface of a true vacuum bubble, for example) that is moving
toward us with sufficiently high velocity, but is too far away to be currently
observable. Such a disturbance would have to move toward us with speed greater
than about 0.8c in order to spoil the late-time inflation of our patch of the
universe and yet avoid being detectable.Comment: 7 pages, 7 figure
Cosmic Mimicry: Is LCDM a Braneworld in Disguise ?
For a broad range of parameter values, braneworld models display a remarkable
property which we call cosmic mimicry. Cosmic mimicry is characterized by the
fact that, at low redshifts, the Hubble parameter in the braneworld model is
virtually indistinguishable from that in the LCDM cosmology. An important point
to note is that the \Omega_m parameters in the braneworld model and in the LCDM
cosmology can nevertheless be quite different. Thus, at high redshifts (early
times), the braneworld asymptotically expands like a matter-dominated universe
with the value of \Omega_m inferred from the observations of the local matter
density. At low redshifts (late times), the braneworld model behaves almost
exactly like the LCDM model but with a renormalized value of the cosmological
density parameter \Omega_m^{LCDM}. The redshift which characterizes cosmic
mimicry is related to the parameters in the higher-dimensional braneworld
Lagrangian. Cosmic mimicry is a natural consequence of the scale-dependence of
gravity in braneworld models. The change in the value of the cosmological
density parameter is shown to be related to the spatial dependence of the
effective gravitational constant in braneworld theory. A subclass of mimicry
models lead to an older age of the universe and also predict a redshift of
reionization which is lower than z_{reion} \simeq 17 in the LCDM cosmology.
These models might therefore provide a background cosmology which is in better
agreement both with the observed quasar abundance at z \gsim 4 and with the
large optical depth to reionization measured by the Wilkinson Microwave
Anisotropy Probe.Comment: 22 pages, 4 figures. A subsection and references added; main results
remain unchanged. Accepted for publication in JCA
Asymptotic behavior of w in general quintom model
For the quintom models with arbitrary potential , the
asymptotic value of equation of state parameter w is obtained by a new method.
In this method, w of stable attractors are calculated by using the ratio (d ln
V)/(d ln a) in asymptotic region. All the known results, have been obtained by
other methods, are reproduced by this method as specific examples.Comment: 8 pages, one example is added, accepted for publication in Gen. Rel.
Gra
Linear and non-linear perturbations in dark energy models
I review the linear and second-order perturbation theory in dark energy
models with explicit interaction to matter in view of applications to N-body
simulations and non-linear phenomena. Several new or generalized results are
obtained: the general equations for the linear perturbation growth; an
analytical expression for the bias induced by a species-dependent interaction;
the Yukawa correction to the gravitational potential due to dark energy
interaction; the second-order perturbation equations in coupled dark energy and
their Newtonian limit. I also show that a density-dependent effective dark
energy mass arises if the dark energy coupling is varying.Comment: 12 pages, submitted to Phys. Rev; v2: added a ref. and corrected a
typ
Bianchi Type III Anisotropic Dark Energy Models with Constant Deceleration Parameter
The Bianchi type III dark energy models with constant deceleration parameter
are investigated. The equation of state parameter is found to be time
dependent and its existing range for this model is consistent with the recent
observations of SN Ia data, SN Ia data (with CMBR anisotropy) and galaxy
clustering statistics. The physical aspect of the dark energy models are
discussed.Comment: 12 pages, 2 figures, Accepted version of IJT
The Evolution of Inverse Power Law Quintessence at Low Redshift
Quintessence models based on a scalar field, phi, with an inverse power law
potential display simple tracking behavior at early times, when the
quintessence energy density, rho_phi, is sub-dominant. At late times, when
rho_phi becomes comparable to the matter density, the evolution of phi diverges
from its scaling behavior. We calculate the first order departure of phi from
its tracker solution at low redshift. Our results for the evolution of phi,
rho_phi, Omega_phi, and w are suprisingly accurate even down to z=0. We find
that w and Omega_phi are related linearly to first order. We also derive a
semi-analytic expression for w(z) which is accurate to within a few percent.
Our analytic techniques are potentially applicable to any quintessence model in
which the quintessence component comes to dominate at late times.Comment: 6 pages, 6 figures, new figure added, numerous clarification
Constraining the dark energy with galaxy clusters X-ray data
The equation of state characterizing the dark energy component is constrained
by combining Chandra observations of the X-ray luminosity of galaxy clusters
with independent measurements of the baryonic matter density and the latest
measurements of the Hubble parameter as given by the HST key project. By
assuming a spatially flat scenario driven by a "quintessence" component with an
equation of state we place the following limits on the
cosmological parameters and : (i) and (1) if the
equation of state of the dark energy is restricted to the interval (\emph{usual} quintessence) and (ii) and
() if violates the null energy condition and assume values (\emph{extended} quintessence or ``phantom'' energy). These results are in
good agreement with independent studies based on supernovae observations,
large-scale structure and the anisotropies of the cosmic background radiation.Comment: 6 pages, 4 figures, LaTe
Born-Infeld Type Phantom Model in the Plane
In this paper, we investigate the dynamics of Born-Infeld(B-I) phantom model
in the plane, which is defined by the equation of state
parameter for the dark energy and its derivative with respect to (the
logarithm of the scale factor ). We find the scalar field equation of motion
in plane, and show mathematically the property of attractor
solutions which correspond to , , which avoid
the "Big rip" problem and meets the current observations well.Comment: 6 pages, 3 figures, some references adde
Further evidence for a variable fine-structure constant from Keck/HIRES QSO absorption spectra
[Abridged] We previously presented evidence for a varying fine-structure
constant, alpha, in two independent samples of Keck/HIRES QSO spectra. Here we
present a detailed many-multiplet analysis of a third Keck/HIRES sample
containing 78 absorption systems. We also re-analyse the previous samples,
providing a total of 128 absorption systems over the redshift range
0.2<z_abs<3.7. All three samples separately yield consistent, significant
values of da/a. The analyses of low- and high-z systems rely on different
ions/transitions with very different dependencies on alpha, yet they also give
consistent results. We identify additional random errors in 22 high-z systems
characterized by transitions with a large dynamic range in apparent optical
depth. Increasing the statistical errors on da/a for these systems gives our
fiducial result, a weighted mean da/a=(-0.543+/-0.116)x10^-5, representing
4.7-sigma evidence for a smaller weighted mean alpha in the absorption clouds.
Assuming that da/a=0 at z_abs=0, the data marginally prefer a linear increase
in alpha with time: dota/a=(6.40+/-1.35)x10^-16 yr^-1. The two-point
correlation function for alpha is consistent with zero over 0.2-13 Gpc comoving
scales and the angular distribution of da/a shows no significant dipolar
anisotropy. We therefore have no evidence for spatial variations in da/a. We
extend our previous searches for possible systematic errors, identifying
atmospheric dispersion and isotopic structure effects as potentially the most
significant. However, overall, known systematic errors do not explain the
results. Future many-multiplet analyses of QSO spectra from different
telescopes and spectrographs will provide a now crucial check on our Keck/HIRES
results.Comment: 31 pages, 25 figures (29 EPS files), 8 tables. Accepted by MNRAS.
Colour versions of Figs. 6, 8 & 10 and text version of Table 3 available at
http://www.ast.cam.ac.uk/~mim/pub.htm
The fundamental constants and their variation: observational status and theoretical motivations
This article describes the various experimental bounds on the variation of
the fundamental constants of nature. After a discussion on the role of
fundamental constants, of their definition and link with metrology, the various
constraints on the variation of the fine structure constant, the gravitational,
weak and strong interactions couplings and the electron to proton mass ratio
are reviewed. This review aims (1) to provide the basics of each measurement,
(2) to show as clearly as possible why it constrains a given constant and (3)
to point out the underlying hypotheses. Such an investigation is of importance
to compare the different results, particularly in view of understanding the
recent claims of the detections of a variation of the fine structure constant
and of the electron to proton mass ratio in quasar absorption spectra. The
theoretical models leading to the prediction of such variation are also
reviewed, including Kaluza-Klein theories, string theories and other
alternative theories and cosmological implications of these results are
discussed. The links with the tests of general relativity are emphasized.Comment: 56 pages, l7 figures, submitted to Rev. Mod. Phy
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