6 research outputs found
Should we doubt the cosmological constant?
While Bayesian model selection is a useful tool to discriminate between
competing cosmological models, it only gives a relative rather than an absolute
measure of how good a model is. Bayesian doubt introduces an unknown benchmark
model against which the known models are compared, thereby obtaining an
absolute measure of model performance in a Bayesian framework. We apply this
new methodology to the problem of the dark energy equation of state, comparing
an absolute upper bound on the Bayesian evidence for a presently unknown dark
energy model against a collection of known models including a flat LambdaCDM
scenario. We find a strong absolute upper bound to the Bayes factor B between
the unknown model and LambdaCDM, giving B < 3. The posterior probability for
doubt is found to be less than 6% (with a 1% prior doubt) while the probability
for LambdaCDM rises from an initial 25% to just over 50% in light of the data.
We conclude that LambdaCDM remains a sufficient phenomenological description of
currently available observations and that there is little statistical room for
model improvement.Comment: 10 pages, 2 figure
Cosmological Constant Problems and Renormalization Group
The Cosmological Constant Problem emerges when Quantum Field Theory is
applied to the gravitational theory, due to the enormous magnitude of the
induced energy of the vacuum. The unique known solution of this problem
involves an extremely precise fine-tuning of the vacuum counterpart. We review
a few of the existing approaches to this problem based on the account of the
quantum (loop) effects and pay special attention to the ones involving the
renormalization group.Comment: 12 pages, LaTeX, based on the on the talk at IRGAC-2006 (Barcelona,
July 11-15, 2006), misprints corrected, comment on anthropic approach
modified, some references added, accepted in Journal of Physics
Sinks in the Landscape, Boltzmann Brains, and the Cosmological Constant Problem
This paper extends the recent investigation of the string theory landscape in
hep-th/0605266, where it was found that the decay rate of dS vacua to a
collapsing space with a negative vacuum energy can be quite large. The parts of
space that experience a decay to a collapsing space, or to a Minkowski vacuum,
never return back to dS space. The channels of irreversible vacuum decay serve
as sinks for the probability flow. The existence of such sinks is a
distinguishing feature of the string theory landscape. We describe relations
between several different probability measures for eternal inflation taking
into account the existence of the sinks. The local (comoving) description of
the inflationary multiverse suffers from the so-called Boltzmann brain (BB)
problem unless the probability of the decay to the sinks is sufficiently large.
We show that some versions of the global (volume-weighted) description do not
have this problem even if one ignores the existence of the sinks. We argue that
if the number of different vacua in the landscape is large enough, the
anthropic solution of the cosmological constant problem in the string landscape
scenario should be valid for a broad class of the probability measures which
solve the BB problem. If this is correct, the solution of the cosmological
constant problem may be essentially measure-independent. Finally, we describe a
simplified approach to the calculations of anthropic probabilities in the
landscape, which is less ambitious but also less ambiguous than other methods.Comment: 42 pages, 5 figures, the paper is substantially extended, a section
on the cosmological constant is addeed; the version published in JCA
New varying speed of light theories
We review recent work on the possibility of a varying speed of light (VSL).
We start by discussing the physical meaning of a varying , dispelling the
myth that the constancy of is a matter of logical consistency. We then
summarize the main VSL mechanisms proposed so far: hard breaking of Lorentz
invariance; bimetric theories (where the speeds of gravity and light are not
the same); locally Lorentz invariant VSL theories; theories exhibiting a color
dependent speed of light; varying induced by extra dimensions (e.g. in the
brane-world scenario); and field theories where VSL results from vacuum
polarization or CPT violation. We show how VSL scenarios may solve the
cosmological problems usually tackled by inflation, and also how they may
produce a scale-invariant spectrum of Gaussian fluctuations, capable of
explaining the WMAP data. We then review the connection between VSL and
theories of quantum gravity, showing how ``doubly special'' relativity has
emerged as a VSL effective model of quantum space-time, with observational
implications for ultra high energy cosmic rays and gamma ray bursts. Some
recent work on the physics of ``black'' holes and other compact objects in VSL
theories is also described, highlighting phenomena associated with spatial (as
opposed to temporal) variations in . Finally we describe the observational
status of the theory. The evidence is currently slim -- redshift dependence in
the atomic fine structure, anomalies with ultra high energy cosmic rays, and
(to a much lesser extent) the acceleration of the universe and the WMAP data.
The constraints (e.g. those arising from nucleosynthesis or geological bounds)
are tight, but not insurmountable. We conclude with the observational
predictions of the theory, and the prospects for its refutation or vindication.Comment: Final versio
Improved constraints on cosmological parameters from SNIa data
We present a new method based on a Bayesian hierarchical model to extract
constraints on cosmological parameters from SNIa data obtained with the SALT-II
lightcurve fitter. We demonstrate with simulated data sets that our method
delivers tighter statistical constraints on the cosmological parameters over
90% of the time, that it reduces statistical bias typically by a factor ~ 2-3
and that it has better coverage properties than the usual chi-squared approach.
As a further benefit, a full posterior probability distribution for the
dispersion of the intrinsic magnitude of SNe is obtained. We apply this method
to recent SNIa data, and by combining them with CMB and BAO data we obtain
Omega_m=0.28 +/- 0.02, Omega_Lambda=0.73 +/- 0.01 (assuming w=-1) and
Omega_m=0.28 +/- 0.01, w=-0.90 +/- 0.05 (assuming flatness; statistical
uncertainties only). We constrain the intrinsic dispersion of the B-band
magnitude of the SNIa population, obtaining sigma_mu^int = 0.13 +/- 0.01 [mag].
Applications to systematic uncertainties will be discussed in a forthcoming
paper.Comment: Enlarged prior on w (down to w>-4), resulting in better coverage for
this parameter. Main results unchanged. Matched version accepted by MNRA