2,539 research outputs found
New Vistas in Braneworld Cosmology
Traditionally, higher-dimensional cosmological models have sought to provide
a description of the fundamental forces in terms of a unifying geometrical
construction. In this essay we discuss how, in their present incarnation,
higher-dimensional `braneworld' models might provide answers to a number of
cosmological puzzles including the issue of dark energy and the nature of the
big-bang singularity.Comment: Honorable mention in the 2002 Essay Competition of the Gravity
Research Foundation. 10 pages, 2 figure
Theoretical study of atomic and molecular gases and their reactions in the upper atmosphere Summary report, 1 May - 31 Oct. 1965
Wave functions and total energy of energy states of diatomic molecules - Hartree-Fock calculatio
Age problem in holographic dark energy
We study the age problem of the universe with the holographic DE model
introduced in [21], and test the model with some known old high redshift
objects (OHRO). The parameters of the model have been constrained using the
SNIa, CMB and BAO data set. We found that the age of the old quasar APM 08
279+5255 at z = 3.91 can be described by the model.Comment: 13 page
CMB acoustic scale in the entropic-like accelerating universe
We consider generalizations of the entropic accelerating universe recently
proposed in Ref. [4,5] and show that their background equations can be made
equivalent to a model with a dark energy component with constant parameter of
state , where is related to the
coefficients of the new terms in the Friedman equations. After discussing all
the Friedman equations for an arbitrary , we show how to recover the
standard scalings for dust and radiation. The acoustic scale , related
to the peak positions in the pattern of the angular power spectrum of the
Cosmic Microwave Background anisotropies, is also computed and yields the
stringent bound . We then argue that future data might be able
to distinguish this model from pure CDM (corresponding to ).Comment: 6 pages, 2 figures. Accepted for publication in Physical Review
Anisotropic Cosmological Constant and the CMB Quadrupole Anomaly
There are evidences that the cosmic microwave background (CMB) large-angle
anomalies imply a departure from statistical isotropy and hence from the
standard cosmological model. We propose a LCDM model extension whose dark
energy component preserves its nondynamical character but wield anisotropic
vacuum pressure. Exact solutions for the cosmological scale factors are
presented, upper bounds for the deformation parameter are evaluated and its
value is estimated considering the elliptical universe proposal to solve the
quadrupole anomaly. This model can be constructed from a Bianchi I cosmology
with cosmological constant from two different ways: i) a straightforward
anisotropic modification of the vacuum pressure consistently with
energy-momentum conservation; ii) a Poisson structure deformation between
canonical momenta such that the dynamics remain invariant under scale factors
rescalings.Comment: 8 pages, 2 columns, 1 figure. v2: figure improved, added comments on
higher eccentricity powers and references. v3: typos corrected, version to
appear in PR
A new null diagnostic customized for reconstructing the properties of dark energy from BAO data
Baryon Acoustic Oscillations (BAO) provide an important standard ruler which
can be used to probe the recent expansion history of our universe. We show how
a simple extension of the Om diagnostic, which we call Om3, can combine
standard ruler information from BAO with standard candle information from type
Ia supernovae (SNIa) to yield a powerful novel null diagnostic of the
cosmological constant hypothesis. A unique feature of Om3 is that it requires
minimal cosmological assumptions since its determination does not rely upon
prior knowledge of either the current value of the matter density and the
Hubble constant, or the distance to the last scattering surface. Observational
uncertainties in these quantities therefore do not affect the reconstruction of
Om3. We reconstruct Om3 using the Union 2.1 SNIa data set and BAO data from
SDSS, WiggleZ and 6dFGS. Our results are consistent with dark energy being the
cosmological constant. We show how Om and Om3 can be used to obtain accurate
model independent constraints on the properties of dark energy from future data
sets such as BigBOSS.Comment: 9 pages, 4 figures, discussions extended, results unchanged, matches
the final version published in PR
Quantum mechanical study of molecules transition probabilities, Einstein A coefficients and oscillator strengths of some band systems of diatomic molecules
Quantum mechanical study of transition probabilities, Einstein A coefficients, and oscillator strengths of band systems of diatomic molecule
Quantum effects, soft singularities and the fate of the universe in a braneworld cosmology
We examine a class of braneworld models in which the expanding universe
encounters a "quiescent" future singularity. At a quiescent singularity, the
energy density and pressure of the cosmic fluid as well as the Hubble parameter
remain finite while all derivatives of the Hubble parameter diverge (i.e.,
, , etc. ). Since the Kretschmann invariant
diverges () at the singularity, one expects
quantum effects to play an important role as the quiescent singularity is
approached. We explore the effects of vacuum polarization due to massless
conformally coupled fields near the singularity and show that these can either
cause the universe to recollapse or, else, lead to a softer singularity at
which , , and remain finite while {\dddot H} and
higher derivatives of the Hubble parameter diverge. An important aspect of the
quiescent singularity is that it is encountered in regions of low density,
which has obvious implications for a universe consisting of a cosmic web of
high and low density regions -- superclusters and voids. In addition to vacuum
polarization, the effects of quantum particle production of non-conformal
fields are also likely to be important. A preliminary examination shows that
intense particle production can lead to an accelerating universe whose Hubble
parameter shows oscillations about a constant value.Comment: 19 pages, 3 figures, text slightly improved and references added.
Accepted for publication in Classical and Quantum Gravit
From de Sitter to de Sitter: A non-singular inflationary universe driven by vacuum
A semi-classical analysis of vacuum energy in the expanding spacetime
suggests that the cosmological term decays with time, with a concomitant matter
production. For early times we find, in Planck units, ,
where H is the Hubble parameter. The corresponding cosmological solution has no
initial singularity, existing since an infinite past. During an infinitely long
period we have a quasi-de Sitter, inflationary universe, with .
However, at a given time, the expansion undertakes a phase transition, with H
and decreasing to nearly zero in a few Planck times, producing a huge
amount of radiation. On the other hand, the late-time scenario is similar to
the standard model, with the radiation phase followed by a dust era, which
tends asymptotically to a de Sitter universe, with vacuum dominating again.Comment: This essay received an "honorable mention" in the 2006 Essay
Competition of the Gravity Research Foundatio
Type I singularities and the Phantom Menace
We consider the future dynamics of a transient phantom dominated phase of the
universe in LQC and in the RS braneworld, which both have a non-standard
Friedmann equation. We find that for a certain class of potentials, the Hubble
parameter oscillates with simple harmonic motion in the LQC case and therefore
avoids any future singularity. For more general potentials we find that damping
effects eventually lead to the Hubble parameter becoming constant. On the other
hand in the braneworld case we find that although the type I singularity can be
avoided, the scale factor still diverges at late times.Comment: More references added. Final PRD versio
- …