651 research outputs found
The effect of pressure gradients on luminosity distance - redshift relations
Inhomogeneous cosmological models have had significant success in explaining
cosmological observations without the need for dark energy. Generally, these
models imply inhomogeneous matter distributions alter the observable relations
that are taken for granted when assuming the Universe evolves according to the
standard Friedmann equations. Moreover, it has recently been shown that both
inhomogeneous matter and pressure distributions are required in both early and
late stages of cosmological evolution. These associated pressure gradients are
required in the early Universe to sufficiently describe void formation, whilst
late-stage pressure gradients stop the appearance of anomalous singularities.
In this paper we investigate the effect of pressure gradients on cosmological
observations by deriving the luminosity distance - redshift relations in
spherically symmetric, inhomogeneous spacetimes endowed with a perfect fluid.
By applying this to a specific example for the energy density distribution and
using various equations of state, we are able to explicitly show that pressure
gradients may have a non-negligble effect on cosmological observations. In
particular, we show that a non-zero pressure gradient can imply significantly
different residual Hubble diagrams for compared to when the
pressure is ignored. This paper therefore highlights the need to properly
consider pressure gradients when interpreting cosmological observations.Comment: Accepted for publication in Classical and Quantum Gravit
Inhomogeneous cosmological models: exact solutions and their applications
Recently, inhomogeneous generalisations of the
Friedmann-Lemaitre-Robertson-Walker cosmological models have gained interest in
the astrophysical community and are more often employed to study cosmological
phenomena. However, in many papers the inhomogeneous cosmological models are
treated as an alternative to the FLRW models. In fact, they are not an
alternative, but an exact perturbation of the latter, and are gradually
becoming a necessity in modern cosmology. The assumption of homogeneity is just
a first approximation introduced to simplify equations. So far this assumption
is commonly believed to have worked well, but future and more precise
observations will not be properly analysed unless inhomogeneities are taken
into account. This paper reviews recent developments in the field and shows the
importance of an inhomogeneous framework in the analysis of cosmological
observations.Comment: 32 pages, 2 figures, contribution to Classical and Quantum Gravity
special issue "Inhomogeneous Cosmological Models and Averaging in Cosmology
Some clarifications about Lema\^itre-Tolman models of the Universe used to deal with the dark energy problem
During the past fifteen years, inhomogeneous cosmological models have been
put forward to explain the observed dimming of the SNIa luminosity without
resorting to dark energy. The simplest models are the spherically symmetric
Lema\^itre-Tolman (LT) solutions with a central observer. Their use must be
considered as a mere first step towards more sophisticated models. Spherical
symmetry is but a mathematical simplification and one must consider spherical
symmetric models as exhibiting an energy density smoothed out over angles
around us. However, they have been taken at face value by some authors who
tried to use them for either irrelevant purposes or to put them to the test as
if they were robust models of our Universe. We wish to clarify how these models
must be used in cosmology. We first use the results obtained by Iguchi and
collaborators to derive the density profiles of the pure growing and decaying
mode LT models. We then discuss the relevance of the different test proposals
in the light of the interpretation given above. We show that decaying-mode
(parabolic) LT models always exhibit an overdensity near their centre and
growing-mode (elliptic or hyperbolic) LT models, a void. This is at variance
with some statements in the literature. We dismiss all previous proposals
merely designed to test the spherical symmetry of the LT models, and we agree
that the value of and the measurement of the redshift drift are valid
tests of the models. However, we suspect that this last test, which is the best
in principle, will be more complicated to implement than usually claimed.Comment: 18 pages, no figure, section 3 modified, results of section 3.2
changed, sections 4.3 and 4.4 added, other minor changes and references adde
One Hundred Years of the Cosmological Constant: from 'Superfluous Stunt' to Dark Energy
We present a centennial review of the history of the term known as the
cosmological constant. First introduced to the general theory of relativity by
Einstein in 1917 in order to describe a universe that was assumed to be static,
the term fell from favour in the wake of the discovery of the expanding
universe, only to make a dramatic return in recent times. We consider
historical and philosophical aspects of the cosmological constant over four
main epochs: (i) the use of the term in static cosmologies (both Newtonian and
relativistic); (ii) the marginalization of the term following the discovery of
cosmic expansion; (iii) the use of the term to address specific cosmic puzzles
such as the timespan of expansion, the formation of galaxies and the redshifts
of the quasars; (iv) the re-emergence of the term in today's Lamda-CDM
cosmology. We find that the cosmological constant was never truly banished from
theoretical models of the universe, but was sidelined by astronomers for
reasons of convenience. We also find that the return of the term to the
forefront of modern cosmology did not occur as an abrupt paradigm shift due to
one particular set of observations, but as the result of a number of empirical
advances such as the measurement of present cosmic expansion using the Hubble
Space Telescope, the measurement of past expansion using type SN 1a supernovae
as standard candles, and the measurement of perturbations in the cosmic
microwave background by balloon and satellite. We give a brief overview of
contemporary interpretations of the physics underlying the cosmic constant and
conclude with a synopsis of the famous cosmological constant problem.Comment: 60 pages, 6 figures. Some corrections, additions and extra
references. Accepted for publication the European Physical Journal (H
Finding and using exact solutions of the Einstein equations
The evolution of the methods used to find solutions of Einstein's field
equations during the last 100 years is described. Early papers used assumptions
on the coordinate forms of the metrics. Since the 1950s more invariant methods
have been deployed in most new papers. The uses to which the solutions found
have been put are discussed, and it is shown that they have played an important
role in the development of many aspects, both mathematical and physical, of
general relativity.Comment: 15 pages, LaTeX2e, aipproc.cls, invited lecture to appear in the
Proceedings of ERE05 (the Spanish Relativity Meeting), Oviedo, September
2005, to be published by the American Institute of Physics. v2: Remarks on
black hole entropy corrected. Other minor change
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