2,346 research outputs found

### A classification of scalar field potentials with cosmological scaling solutions

An attractive method of obtaining an effective cosmological constant at the
present epoch is through the potential energy of a scalar field. Considering
models with a perfect fluid and a scalar field, we classify all potentials for
which the scalar field energy density scales as a power-law of the scale factor
when the perfect fluid density dominates. There are three possibilities. The
first two are well known; the much-investigated exponential potentials have the
scalar field mimicking the evolution of the perfect fluid, while for negative
power-laws, introduced by Ratra and Peebles, the scalar field density grows
relative to that of the fluid. The third possibility is a new one, where the
potential is a positive power-law and the scalar field energy density decays
relative to the perfect fluid. We provide a complete analysis of exact
solutions and their stability properties, and investigate a range of possible
cosmological applications.Comment: 8 pages RevTeX file with four figures incorporated (uses RevTeX and
epsf

### User's Guide for ERB 7 SEFDT. Volume 1: User's Guide. Volume 2: Quality Control Report, Year 1

The Nimbus-7 ERB SEFDT Data User's Guide is presented. The guide consists of four subsections which describe: (1) the scope of the data User's Guide; (2) the background on Nimbus-7 Spacecraft and the ERB experiment; (3) the SEFDT data product and processing scenario; and (4) other related products and documents

### Thawing quintessence with a nearly flat potential

The thawing quintessence model with a nearly flat potential provides a
natural mechanism to produce an equation of state parameter, w, close to -1
today. We examine the behavior of such models for the case in which the
potential satisfies the slow roll conditions: [(1/V)(dV/dphi)]^2 << 1 and
(1/V)(d^2 V/dphi^2) << 1, and we derive the analog of the slow-roll
approximation for the case in which both matter and a scalar field contribute
to the density. We show that in this limit, all such models converge to a
unique relation between 1+w, Omega_phi, and the initial value of
(1/V)(dV/dphi). We derive this relation, and use it to determine the
corresponding expression for w(a), which depends only on the present-day values
for w and Omega_phi. For a variety of potentials, our limiting expression for
w(a) is typically accurate to within delta w < 0.005 for w<-0.9. For redshift z
< 1, w(a) is well-fit by the Chevallier-Polarski-Linder parametrization, in
which w(a) is a linear function of a.Comment: 8 pages, 5 figures, discussion added, references updated, typos
corrected, to appear in Phys. Rev.

### Predicting Big Bang Deuterium

We present new upper and lower bounds to the primordial abundances of
deuterium and helium-3 based on observational data from the solar system and
the interstellar medium. Independent of any model for the primordial production
of the elements we find (at the 95\% C.L.): $1.5 \times 10^{-5} \le (D/H)_P \le
10.0 \times 10^{-5}$ and $(^3He/H)_P \le 2.6\times 10^{-5}$. When combined with
the predictions of standard big bang nucleosynthesis, these constraints lead to
a 95\% C.L. bound on the primordial abundance of deuterium: $(D/H)_{best} =
(3.5^{+2.7}_{-1.8})\times 10^{-5}$. Measurements of deuterium absorption in the
spectra of high redshift QSOs will directly test this prediction. The
implications of this prediction for the primordial abundances of helium-4 and
lithium-7 are discussed, as well as those for the universal density of baryons.Comment: Revised version of paper to reflect comments of the referee and reply
to suggestions of Copi, Schramm, and Turner regarding the overall analysis
and treatment of chemical evolution of D and He-3. Best-fit D/H abundance
changes from (2.3 + 3.0 - 1.0)x10^{-5} to (3.5 +2.7 - 1.8) x10^{-5}. See also
hep-ph/950531

### The sun's magnetic sector structure

The synoptic appearance of solar magnetic sectors is studied using 454 sector boundaries observed at earth during 1959-1973. The sectors are clearly visible in the photospheric magnetic field. Sector boundaries can be clearly identified as north-south running demarcation lines between regions of persistent magnetic polarity imbalances. These regions extend up to about 35 deg of latitude on both sides of the equator. They generally do not extend into the polar caps. The polar cap boundary can be identified as an east-west demarcation line marking the poleward limit of the sectors. The typical flux imbalance for a magnetic sector is about 4 x 10 to the 21st power Maxwells

### On Random Bubble Lattices

We study random bubble lattices which can be produced by processes such as
first order phase transitions, and derive characteristics that are important
for understanding the percolation of distinct varieties of bubbles. The results
are relevant to the formation of topological defects as they show that infinite
domain walls and strings will be produced during appropriate first order
transitions, and that the most suitable regular lattice to study defect
formation in three dimensions is a face centered cubic lattice. Another
application of our work is to the distribution of voids in the large-scale
structure of the universe. We argue that the present universe is more akin to a
system undergoing a first-order phase transition than to one that is
crystallizing, as is implicit in the Voronoi foam description. Based on the
picture of a bubbly universe, we predict a mean coordination number for the
voids of 13.4. The mean coordination number may also be used as a tool to
distinguish between different scenarios for structure formation.Comment: several modifications including new abstract, comparison with froth
models, asymptotics of coordination number distribution, further discussion
of biased defects, and relevance to large-scale structur

### Constraints on the Variation of G from Primordial Nucleosynthesis

We study here the effect of a varying G on the evolution of the early
Universe and, in particular, on primordial nucleosynthesis. This variation of G
is modelled using the Brans-Dicke theory as well as a more general class of
scalar-tensor theories. Modified nucleosynthesis codes are used to investigate
this effect and the results obtained are used to constrain the parameters of
the theories. We extend previous studies of primordial nucleosynthesis in
scalar-tensor theories by including effects which can cause a slow variation of
G during radiation domination and by including a late-time accelerating phase
to the Universe's history. We include a brief discussion on the epoch of
matter-radiation equality in Brans-Dicke theory, which is also of interest for
determining the positions of the cosmic microwave background power-spectrum
peaks.Comment: 10 pages, 7 figures. Published versio

### Primordial nucleosynthesis as a probe of fundamental physics parameters

We analyze the effect of variation of fundamental couplings and mass scales
on primordial nucleosynthesis in a systematic way. The first step establishes
the response of primordial element abundances to the variation of a large
number of nuclear physics parameters, including nuclear binding energies. We
find a strong influence of the n-p mass difference (for the 4He abundance), of
the nucleon mass (for deuterium) and of A=3,4,7 binding energies (for 3He, 6Li
and 7Li). A second step relates the nuclear parameters to the parameters of the
Standard Model of particle physics. The deuterium, and, above all, 7Li
abundances depend strongly on the average light quark mass hat{m} \equiv
(m_u+m_d)/2. We calculate the behaviour of abundances when variations of
fundamental parameters obey relations arising from grand unification. We also
discuss the possibility of a substantial shift in the lithium abundance while
the deuterium and 4He abundances are only weakly affected.Comment: v2: 34 pages, 2 figures, typo in last GUT scenario corrected, added
discussion and graph of nonlinear behaviour in GUT scenarios, added short
section discussing binding of dineutron and 8Be, refs added, conclusions
unaltered. Accepted for publication, Phys. Rev.

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