17,077 research outputs found
Structure formation by cosmic strings with a cosmological constant
Final published version.Comment: 4 Page
Solutions to Cosmological Problems with Energy Conservation and Varying c, G and Lambda
The flatness and cosmological constant problems are solved with varying speed
of light c, gravitational coupling strength G and cosmological parameter
Lambda, by explicitly assuming energy conservation of observed matter. The
present solution to the flatness problem is the same as the previous solution
in which energy conservation was absent.Comment: 5 pages, Replaced with LaTex file with minor change
A Theory of time-varying Constants
We present a flat (K=0) cosmological model, described by a perfect fluid with
the ``constants'' and varying with cosmological time . We
introduce Planck\'s ``constant'' in the field equations through the
equation of state for the energy density of radiation. We then determine the
behaviour of the ``constants'' by using the zero divergence of the second
member of the modified Einstein\'s field equations i.e.
together with the
equation of state and the Einstein cosmological equations. Assuming realistic
physical and mathematical conditions we obtain a consistent result with . In this way we obtain gauge invariance for the Schr\"{o}dinger
equation and the behaviour of the remaining ``constants''Comment: 15 pages, RevTeX
Characterization of nanometer-sized, mechanically exfoliated graphene on the H-passivated Si(100) surface using scanning tunnelling microscopy
We have developed a method for depositing graphene monolayers and bilayers
with minimum lateral dimensions of 2-10 nm by the mechanical exfoliation of
graphite onto the Si(100)-2x1:H surface. Room temperature, ultra-high vacuum
(UHV) tunnelling spectroscopy measurements of nanometer-sized single-layer
graphene reveal a size dependent energy gap ranging from 0.1-1 eV. Furthermore,
the number of graphene layers can be directly determined from scanning
tunnelling microscopy (STM) topographic contours. This atomistic study provides
an experimental basis for probing the electronic structure of nanometer-sized
graphene which can assist the development of graphene-based nanoelectronics.Comment: Accepted for publication in Nanotechnolog
Primordial Adiabatic Fluctuations from Cosmic Defects
We point out that in the context of ``two-metric'' theories of gravity there
is the possibility that cosmic defects will produce a spectrum of primordial
adiabatic density perturbations. This will happen when the speed characterising
the defect-producing scalar field is much larger than the speed characterising
gravity and all standard model particles. This model will exactly mimic the
standard predictions of inflationary models, with the exception of a small
non-Gaussian signal which could be detected by future experiments. We briefly
discuss defect evolution in these scenarios and analyze their cosmological
consequences.Comment: 5 LaTeX pages, no figures; version to appear in Phys. Rev. Let
Cosmic Strings in an Open Universe with Baryonic and Non-Baryonic Dark Matter
We study the effects of cosmic strings on structure formation in open
universes. We calculate the power spectrum of density perturbations for two
class of models: one in which all the dark matter is non baryonic (CDM) and one
in which it is all baryonic (BDM). Our results are compared to the 1 in 6 IRAS
QDOT power spectrum. The best candidates are then used to estimate , the
energy per unit length of the string network. Some comments are made on
mechanisms by which structures are formed in the two theories.Comment: uu-encoded compressed tar of postscript files, Imperial/TP/94-95/0
Quantum-to-classical transition for fluctuations in the early Universe
According to the inflationary scenario for the very early Universe, all
inhomogeneities in the Universe are of genuine quantum origin. On the other
hand, looking at these inhomogeneities and measuring them, clearly no specific
quantum mechanical properties are observed. We show how the transition from
their inherent quantum gravitational nature to classical behaviour comes about
-- a transition whereby none of the successful quantitative predictions of the
inflationary scenario for the present-day universe is changed. This is made
possible by two properties. First, the quantum state for the spacetime metric
perturbations produced by quantum gravitational effects in the early Universe
becomes very special (highly squeezed) as a result of the expansion of the
Universe (as long as the wavelength of the perturbations exceeds the Hubble
radius). Second, decoherence through the environment distinguishes the field
amplitude basis as being the pointer basis. This renders the perturbations
presently indistinguishable from stochastic classical inhomogeneities.Comment: 9 pages, LATE
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