5,904 research outputs found
Towards the Theory of Stationary Universe
This talk presents some progress achieved in collaboration with A.Linde and
D.Linde towards understanding the true nature of the global spatial structure
of the Universe as well as the most general stationary characteristics of its
time-dependent state with eternally growing total volume.Comment: Talk, presented at the TEXAS/PASCOS conference, Berkeley, December
1992. To appear in the proceedings. 9 p., LaTeX, Stanford preprint
SU-ITP-93-
Hard Art of the Universe Creation
We develop a stochastic approach to the theory of tunneling with the baby
universe formation. This method is applied also to the theory of creation of
the universe in a laboratory.Comment: 20 page
Vacuum fluctuations and topological Casimir effect in Friedmann-Robertson-Walker cosmologies with compact dimensions
We investigate the Wightman function, the vacuum expectation values of the
field squared and the energy-momentum tensor for a massless scalar field with
general curvature coupling parameter in spatially flat
Friedmann-Robertson-Walker universes with an arbitrary number of toroidally
compactified dimensions. The topological parts in the expectation values are
explicitly extracted and in this way the renormalization is reduced to that for
the model with trivial topology. In the limit when the comoving lengths of the
compact dimensions are very short compared to the Hubble length, the
topological parts coincide with those for a conformal coupling and they are
related to the corresponding quantities in the flat spacetime by standard
conformal transformation. In the opposite limit of large comoving lengths of
the compact dimensions, in dependence of the curvature coupling parameter, two
regimes are realized with monotonic or oscillatory behavior of the vacuum
expectation values. In the monotonic regime and for nonconformally and
nonminimally coupled fields the vacuum stresses are isotropic and the equation
of state for the topological parts in the energy density and pressures is of
barotropic type. In the oscillatory regime, the amplitude of the oscillations
for the topological part in the expectation value of the field squared can be
either decreasing or increasing with time, whereas for the energy-momentum
tensor the oscillations are damping.Comment: 20 pages, 2 figure
Extended Inflation with an Exponential Potential
In this paper we investigate extended inflation with an exponential potential
, which provides a simple cosmological
scenario where the distribution of the constants of Nature is mostly determined
by . In particular, we show that this theory predicts a uniform
distribution for the Planck mass at the end of inflation, for the entire
ensemble of universes that undergo stochastic inflation. Eternal inflation
takes place in this scenario for a broad family of initial conditions, all of
which lead up to the same value of the Planck mass at the end of inflation. The
predicted value of the Planck mass is consistent with the observed value within
a comfortable range of values of the parameters involved.Comment: 6 pages, 2 figure
From the Big Bang Theory to the Theory of a Stationary Universe
We consider chaotic inflation in the theories with the effective potentials
phi^n and e^{\alpha\phi}. In such theories inflationary domains containing
sufficiently large and homogeneous scalar field \phi permanently produce new
inflationary domains of a similar type. We show that under certain conditions
this process of the self-reproduction of the Universe can be described by a
stationary distribution of probability, which means that the fraction of the
physical volume of the Universe in a state with given properties (with given
values of fields, with a given density of matter, etc.) does not depend on
time, both at the stage of inflation and after it. This represents a strong
deviation of inflationary cosmology from the standard Big Bang paradigm. We
compare our approach with other approaches to quantum cosmology, and illustrate
some of the general conclusions mentioned above with the results of a computer
simulation of stochastic processes in the inflationary Universe.Comment: No changes to the file, but original figures are included. They
substantially help to understand this paper, as well as eternal inflation in
general, and what is now called the "multiverse" and the "string theory
landscape." High quality figures can be found at
http://www.stanford.edu/~alinde/LLMbigfigs
Applications of the Mellin-Barnes integral representation
We apply the Mellin-Barnes integral representation to several situations of
interest in mathematical-physics. At the purely mathematical level, we derive
useful asymptotic expansions of different zeta-functions and partition
functions. These results are then employed in different topics of quantum field
theory, which include the high-temperature expansion of the free energy of a
scalar field in ultrastatic curved spacetime, the asymptotics of the -brane
density of states, and an explicit approach to the asymptotics of the
determinants that appear in string theory.Comment: 20 pages, LaTe
Aluminium substitution induced superstructures in Mg1-xAlxB2 (x = 0.0 to 0.50): An X-ray diffraction study
The physical property characterization of Al doped Mg1-xAlxB2 system with x =
0.0 to 0.50 is reported. The results related to phase formation, structural
transition, resistivity R(T) and magnetization M(T) measurements are discussed
in detail. It is shown that the addition of electrons to MgB2 through Al
results in loss of superconductivity. Also seen is a structural transition
associated with the collapse of boron layers reflected by the continuous
decrease in the c parameter. The main emphasis in this paper is on slow scan
X-ray diffraction (XRD) results, which confirm the existence of a
superstructure along the c-direction for the x = 0.50 sample. The appearance of
some additional peaks, viz. [103], [004], [104] and [112], results in doubling
of the lattice parameter along the c-axis. This possibly indicates the
alternative ordering of Al and Mg in MgAlB4 separated by hexagonal boron layers
but still maintaining the same hexagonal AlB2 type structure.Comment: 17 Pages Text + Figs. To appear in Physica
The Structure of n-Point One-Loop Open Superstring Amplitudes
In this article we present the worldsheet integrand for one-loop amplitudes
in maximally supersymmetric superstring theory involving any number n of
massless open string states. The polarization dependence is organized into the
same BRST invariant kinematic combinations which also govern the leading string
correction to tree level amplitudes. The dimensions of the bases for both the
kinematics and the associated worldsheet integrals is found to be the unsigned
Stirling number S_3^{n-1} of first kind. We explain why the same combinatorial
structures govern on the one hand finite one-loop amplitudes of equal helicity
states in pure Yang Mills theory and on the other hand the color tensors at
quadratic alpha prime order of the color dressed tree amplitude.Comment: 75 pp, 8 figs, harvmac TeX, v2: published versio
A cryogenic beam of refractory, chemically reactive molecules with expansion cooling
Cryogenically cooled buffer gas beam sources of the molecule thorium monoxide
(ThO) are optimized and characterized. Both helium and neon buffer gas sources
are shown to produce ThO beams with high flux, low divergence, low forward
velocity, and cold internal temperature for a variety of stagnation densities
and nozzle diameters. The beam operates with a buffer gas stagnation density of
~10^15-10^16 cm^-3 (Reynolds number ~1-100), resulting in expansion cooling of
the internal temperature of the ThO to as low as 2 K. For the neon (helium)
based source, this represents cooling by a factor of about 10 (2) from the
initial nozzle temperature of about 20 K (4 K). These sources deliver ~10^11
ThO molecules in a single quantum state within a 1-3 ms long pulse at 10 Hz
repetition rate. Under conditions optimized for a future precision spectroscopy
application [A C Vutha et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 074007],
the neon-based beam has the following characteristics: forward velocity of 170
m/s, internal temperature of 3.4 K, and brightness of 3x10^11 ground state
molecules per steradian per pulse. Compared to typical supersonic sources, the
relatively low stagnation density of this source, and the fact that the cooling
mechanism relies only on collisions with an inert buffer gas, make it widely
applicable to many atomic and molecular species, including those which are
chemically reactive, such as ThO
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