2,281 research outputs found
Natural extension of the Generalised Uncertainty Principle
We discuss a gedanken experiment for the simultaneous measurement of the
position and momentum of a particle in de Sitter spacetime. We propose an
extension of the so-called generalized uncertainty principle (GUP) which
implies the existence of a minimum observable momentum. The new GUP is directly
connected to the nonzero cosmological constant, which becomes a necessary
ingredient for a more complete picture of the quantum spacetime.Comment: 4 pages, 1 figure, v2 with added references, revised and extended as
published in CQ
Sub-horizon Perturbation Behavior in Extended Quintessence
In the general context of scalar-tensor theories, we consider a model in
which a scalar field coupled to the Ricci scalar in the gravitational sector of
the Lagrangian, is also playing the role of an ``Extended Quintessence'' field,
dominating the energy content of the Universe at the present time. In this
framework, we study the linear evolution of the perturbations in the
Quintessence energy density, showing that a new phenomenon, named here
``gravitational dragging'', can enhance the scalar field density perturbations
as much as they reach the non-linear regime. The possibility of dark energy
clumps formation is thus discussed.Comment: Proceedings of the 5th International UCLA Symposium on Sources and
Detection of Dark Matter and Dark Energy in the Universe (Dark Matter 2002),
Marina del Rey, California, USA, 20-22 February 200
A generalized linear Hubble law for an inhomogeneous barotropic Universe
In this work, I present a generalized linear Hubble law for a barotropic
spherically symmetric inhomogeneous spacetime, which is in principle compatible
with the acceleration of the cosmic expansion obtained as a result of high
redshift Supernovae data. The new Hubble function, defined by this law, has two
additional terms besides an expansion one, similar to the usual volume
expansion one of the FLRW models, but now due to an angular expansion. The
first additional term is dipolar and is a consequence of the existence of a
kinematic acceleration of the observer, generated by a negative gradient of
pressure or of mass-energy density. The second one is quadrupolar and due to
the shear. Both additional terms are anisotropic for off-centre observers,
because of to their dependence on a telescopic angle of observation. This
generalized linear Hubble law could explain, in a cosmological setting, the
observed large scale flow of matter, without to have recourse to peculiar
velocity-type newtonian models. It is pointed out also, that the matter dipole
direction should coincide with the CBR dipole one.Comment: 9 pages, LaTeX, to be published in Class. Quantum Gra
Cosmological constraints with GRBs: homogeneous medium vs wind density profile
We present the constraints on the cosmological parameters obtained with the
-- correlation found with the most recent sample of
19 GRBs with spectroscopically measured redshift and well determined prompt
emission spectral and afterglow parameters. We compare our results obtained in
the two possible uniform jet scenarios, i.e. assuming a homogeneous density
profile (HM) or a wind density profile (WM) for the circumburst medium. Better
constraints on and are obtained with the
(tighter) -- correlation derived in the wind density
scenario. We explore the improvements to the constraints of the cosmological
parameters that could be reached with a large sample, 150 GRBs, in the
future. We study the possibility to calibrate the slope of these correlations.
Our optimization analysis suggests that GRBs with redshift
can be used to calibrate the -- with
a precision better than 1%. The same precision is expected for the same number
of bursts with . This result suggests that we do not
necessarily need a large sample of low z GRBs for calibrating the slope of
these correlations.Comment: 7 pages, 7 figures, submitted to A&
Making sense of the bizarre behaviour of horizons in the McVittie spacetime
The bizarre behaviour of the apparent (black hole and cosmological) horizons
of the McVittie spacetime is discussed using, as an analogy, the
Schwarzschild-de Sitter-Kottler spacetime (which is a special case of McVittie
anyway). For a dust-dominated "background" universe, a black hole cannot exist
at early times because its (apparent) horizon would be larger than the
cosmological(apparent) horizon. A phantom-dominated "background" universe
causes this situation, and the horizon behaviour, to be time-reversed.Comment: 8 pages, 3 figure
What's Behind Acoustic Peaks in the Cosmic Microwave Background Anisotropies
We give a brief review of the physics of acoustic oscillations in Cosmic
Microwave Background (CMB) anisotropies. As an example of the impact of their
detection in cosmology, we show how the present data on CMB angular power
spectrum on sub-degree scales can be used to constrain dark energy cosmological
models.Comment: 6 pages, proceedings to the TAUP2001 conference, LNGS, Italy, Sept.
200
Averaging inhomogeneities in scalar-tensor cosmology
The backreaction of inhomogeneities on the cosmic dynamics is studied in the
context of scalar-tensor gravity. Due to terms of indefinite sign in the
non-canonical effective energy tensor of the Brans-Dicke-like scalar field,
extra contributions to the cosmic acceleration can arise. Brans-Dicke and
metric f(R) gravity are presented as specific examples. Certain representation
problems of the formalism peculiar to these theories are pointed out.Comment: Comments and references added. 14 page
Can MONDian vector theories explain the cosmic speed up ?
Generalized Einstein - Aether vector field models have been shown to provide,
in the weak field regime, modifications to gravity which can be reconciled with
the successfull MOND proposal. Very little is known, however, on the function
F(K) defining the vector field Lagrangian so that an analysis of the viability
of such theories at the cosmological scales has never been performed. As a
first step along this route, we rely on the relation between F(K) and the MOND
interpolating function to assign the vector field Lagrangian thus
obtaining what we refer to as "MONDian vector models". Since they are able by
construction to recover the MOND successes on galaxy scales, we investigate
whether they can also drive the observed accelerated expansion by fitting the
models to the Type Ia Supernovae data. Should be this the case, we have a
unified framework where both dark energy and dark matter can be seen as
different manifestations of a single vector field. It turns out that both
MONDian vector models are able to well fit the low redshift data on Type Ia
Supernovae, while some tension could be present in the high z regime.Comment: 15 pages, 5 tables, 4 figures, accepted for publication on Physical
Review
ELT Observations of Supernovae at the Edge of the Universe
We discuss the possibility of using Supernovae as tracers of the star
formation history of the Universe for the range of stellar masses
M and possibly beyond. We simulate the observations of 350 SNe, up to
, made with OWL (100m) telescope.Comment: 9 pages, 6 figures. To appear in "Exploring the Cosmic Frontier:
Astrophysical Instruments for the 21st Century", proceedings of the
conference held in Berlin, 18-21 May 200
Model-Independent Reconstruction of the Expansion History of the Universe from Type Ia Supernovae
Based on the largest homogeneously reduced set of Type Ia supernova
luminosity data currently available -- the Union2 sample -- we reconstruct the
expansion history of the Universe in a model-independent approach. Our method
tests the geometry of the Universe directly without reverting to any
assumptions made on its energy content. This allows us to constrain Dark Energy
models and non-standard cosmologies in a straightforward way. The applicability
of the presented method is not restricted to testing cosmological models. It
can be a valuable tool for pointing out systematic errors hidden in the
supernova data and planning future Type Ia supernova cosmology campaigns.Comment: 10 pages, 8 figures, to be published in Montly Notices of the Royal
Astronomical Societ
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