938 research outputs found
Dark-energy dependent test of general relativity at cosmological scales
The CDM framework offers a remarkably good description of our
universe with a very small number of free parameters, which can be determined
with high accuracy from currently available data. However, this does not mean
that the associated physical quantities, such as the curvature of the universe,
have been directly measured. Similarly, general relativity is assumed, but not
tested. Testing the relevance of general relativity for cosmology at the
background level includes a verification of the relation between its energy
contents and the curvature of space. Using an extended Newtonian formulation,
we propose an approach where this relation can be tested. Using the recent
measurements on cosmic microwave background, baryonic acoustic oscillations and
the supernova Hubble diagram, we show that the prediction of general relativity
is well verified in the framework of standard CDM assumptions, i.e. an
energy content only composed of matter and dark energy, in the form of a
cosmological constant or equivalently a vacuum contribution.
However, the actual equation of state of dark fluids cannot be directly
obtained from cosmological observations. We found that relaxing the equation of
state of dark energy opens a large region of possibilities, revealing a new
type of degeneracy between the curvature and the total energy content of the
universe.Comment: 5 pages, 3 figures. Accepted for publication in Physical Review
On the inevitability of reionization: implications for cosmic microwave background fluctuations
Early photoionization of the intergalactic medium is discussed in a nearly
model-independent way, in order to investigate whether early structures
corresponding to rare Gaussian peaks in a CDM model can photoionize the
intergalactic medium sufficiently early to appreciably smooth out the microwave
background fluctuations. We conclude that this is indeed possible for a broad
range of CDM normalizations and is almost inevitable for unbiased CDM, provided
that the bulk of these early structures are quite small, no more massive than
about 10^8 solar masses. Typical parameter values predict that reionization
occurs around z=50, thereby suppressing fluctuations on degree scales while
leaving the larger angular scales probed by COBE reasonably unaffected.
However, for non-standard CDM, incorporating mixed dark matter, vacuum density
or a tilted primordial power spectrum, early reionization plays no significant
role.Comment: 32 pages, incl 10 figures. Stone-age postscript replaced by LATeX.
Latest version at http://www.sns.ias.edu/~max/reion.html (faster from the
US), from http://www.mpa-garching.mpg.de/~max/reion.html (faster from Europe)
or from [email protected]
Can Dark Energy emerge from quantum effects in compact extra dimension ?
The origin of the observed acceleration of the expansion of the universe is a
major problem of modern cosmology and theoretical physics. Simple estimations
of the contribution of vacuum to the density energy of the universe in quantum
field theory are known to lead to catastrophic large values compared to
observations. Such a contribution is therefore generally not regarded as a
viable source for the acceleration of the expansion. In this letter we propose
that the vacuum contribution actually provides a small positive value to the
density energy of the universe. The underlying mechanism is a manifestation of
the quantum nature of the gravitational field, through a Casimir-like effect
from an additional compact dimension of space. A key ingredient is to assume
that only modes with wavelength shorter than the Hubble length contribute to
the vacuum. Such a contribution gives a positive energy density, has a Lorentz
invariant equation of state in the usual 4D spacetime and hence can be
interpreted as a cosmological constant. Its value agrees with observations for
a radius of a 5th extra dimension given by m. This implies a
modification of the gravitational inverse square law around this scale, close
but below existing limits from experiments testing gravity at short range.Comment: To be published in A\&
Cosmological Interpretation from High Redshift Clusters Observed Within the XMM-Newton -Project
During the last ten years astrophysical cosmology has brought three
remarkable results of deep impact for fundamental physics: the existence of
non-baryonic dark matter, the (nearly) flatness of space, the domination of the
density of the universe by some gravitationally repulsive fluid. This last
result is probably the most revolutionizing one: the scientific review Sciences
has considered twice results on this question as Breakthrough of the Year (for
1998 and 2003). However, direct evidence of dark energy are still rather weak,
and the strength of the standard scenario relies more on the "concordance"
argument rather than on the robustness of direct evidences. Furthermore, a
scenario can be build in an Einstein-de Sitter universe, which reproduces as
well as the concordance model the following various data relevant to cosmology:
WMAP results, large scale structure of the universe, local abundance of massive
clusters, weak lensing measurements, most Hubble constant measurements not
based on stellar indicators. Furthermore, recent data on distant x-ray clusters
obtained from XMM and Chandra indicates that the observed abundances of
clusters at high redshift taken at face value favors an Einstein de Sitter
model and are hard to reconcile with the concordance model. It seems wise
therefore to consider that the actual existence of the dark energy is still an
open question.Comment: Proceedings of DARK 2004, the Fifth International Heidelberg
Conference, October 3-9, 2004, Texas A&M Universit
Shaping Economic Practices in China’s post-Command Economy Period: the interaction of politics, economics and institutional constraints
Although much has been published on China\u27s economic transition, less research has focused on how Chinese culture and the Communist political system have interacted to shape the new Chinese economy. In this paper, we argue that China\u27s post-command economy period reflects not only the \u27new\u27 infusion of neo-liberal ideology into the country, but also the consistent filtering of economic practices through a historical and complex institutional arrangement of cultural and political norms. The tensions between neo-classical free market principles and the overarching authority of the Communist state explain the variety of institutional constraints on actual economic practices in China. While the economic reforms initiated a change in the traditional patriarchal society, leading women to further integrate themselves into the labour market, in many cases this has become localised in the informal sectors of the economy. We shed light on this phenomenon by discussing how longstanding paternalist norms have been translated into institutional constraints on economic activity
Dark sectors of the Universe: A Euclid survey approach
In this paper we study the consequences of relaxing the hypothesis of the
pressureless nature of the dark matter component when determining constraints
on dark energy. To this aim we consider simple generalized dark matter models
with constant equation of state parameter. We find that present-day
low-redshift probes (type-Ia supernovae and baryonic acoustic oscillations)
lead to a complete degeneracy between the dark energy and the dark matter
sectors. However, adding the cosmic microwave background (CMB) high-redshift
probe restores constraints similar to those on the standard CDM model.
We then examine the anticipated constraints from the galaxy clustering probe of
the future Euclid survey on the same class of models, using a Fisher forecast
estimation. We show that the Euclid survey allows us to break the degeneracy
between the dark sectors, although the constraints on dark energy are much
weaker than with standard dark matter. The use of CMB in combination allows us
to restore the high precision on the dark energy sector constraints.Comment: 10 pages, 6 figure
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