6,139 research outputs found
Constraining the quintessence equation of state with SnIa data and CMB peaks
Quintessence has been introduced as an alternative to the cosmological
constant scenario to account for the current acceleration of the universe. This
new dark energy component allows values of the equation of state parameter
, and in principle measurements of cosmological distances to
Type Ia supernovae can be used to distinguish between these two types of
models. Assuming a flat universe, we use the supernovae data and measurements
of the position of the acoustic peaks in the Cosmic Microwave Background (CMB)
spectra to constrain a rather general class of Quintessence potentials,
including inverse power law models and recently proposed Supergravity inspired
potentials. In particular we use a likelihood analysis, marginalizing over the
dark energy density , the physical baryon density
and the scalar spectral index , to constrain the slopes of our Quintessence
potential. Considering only the first Doppler peak the best fit in our range of
models gives . However, including the SnIa data and the three
peaks, we find an upper limit on the present value of the equation of state
parameter, at , a result that appears to
rule out a class of recently proposed potentials.Comment: Accepted for pubblication in PRD, 7 pages, 6 figures. New revised
analysi
Gravitational instantons and internal dimensions
We Study instanton solutions in general relativity with a scalar field. The
metric ansatz we use is composed of a particular warp product of general
Einstein metrics, such as those found in a number of cosmological settings,
including string cosmology, supergravity compactifications and general Kaluza
Klein reductions. Using the Hartle-Hawking prescription the instantons we
obtain determine whether metrics involving extra compact dimensions of this
type are favoured as initial conditions for the universe. Specifically, we find
that these product metric instantons, viewed as constrained instantons, do have
a local minima in the action. These minima are then compared with the higher
dimensional version of the Hawking-Turok instantons, and we argue that the
latter always have lower action than those associated with these product
metrics.Comment: 10 pages, 5 figure
Dynamics of Q-Balls in an expanding universe
We analyse the evolution of light Q-balls in a cosmological background, and
find a number of interesting features. For Q-balls formed with a size
comparable to the Hubble radius, we demonstrate that there is no charge
radiation, and that the Q-ball maintains a constant physical radius. Large
expansion rates cause charge migration to the surface of the Q-ball,
corresponding to a non-homogeneous internal rotation frequency. We argue that
this is an important phenomenon as it leads to a large surface charge and
possible fragmentation of the Q-ball. We also explore the deviation of the
Q-ball profile function from the static case. By introducing a parameter
, which is the ratio of the Hubble parameter to the frequency of
oscillation of the Q-ball field, and using solutions to an analytically
approximated equation for the profile function, we determine the dependence of
the new features on the expansion rate. This allows us to gain an understanding
of when they should be considered and when they can be neglected, thereby
placing restrictions on the existence of homogeneous Q-balls in expanding
backgrounds.Comment: 5 pages, 4 figure
Determination of thermodynamic properties of AeroZINE-50, phase 1
Literature survey of, and test procedure for determination of thermodynamic properties of AeroZINE-5
Scaling Laws for Non-Intercommuting Cosmic String Networks
We study the evolution of non-interacting and entangled cosmic string
networks in the context of the velocity-dependent one-scale model. Such
networks may be formed in several contexts, including brane inflation. We show
that the frozen network solution , although generic, is only a
transient one, and that the asymptotic solution is still as in the
case of ordinary (intercommuting) strings, although in the present context the
universe will usually be string-dominated. Thus the behaviour of two strings
when they cross does not seem to affect their scaling laws, but only their
densities relative to the background.Comment: Phys. Rev. D (in press); v2: final published version (references
added, typos corrected
New type scalar fields for cosmic acceleration
We present a model where a non-conventional scalar field may act like dark
energy leading to cosmic acceleration. The latter is driven by an appropriate
field configuration, which result in an effective cosmological constant. The
potential role of such a scalar in the cosmological constant problem is also
discussed.Comment: 6 page
Study of propellant valve leakage in a vacuum Final summary report
Adverse effects of liquid propellant leakage past control valves in vacuum environmen
Qualitative analysis of a scalar-tensor theory with exponential potential
A qualitative analysis of a scalar-tensor cosmological model, with an
exponential potential for the scalar field, is performed. The phase diagram for
the flat case is constructed. It is shown that solutions with an initial and
final inflationary behaviour appear. The conditions for which the scenario
favored by supernova type Ia observations becomes an attractor in the space of
the solutions are established.Comment: Latex file, 9 pages, 1 figur
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