650 research outputs found
Baryogenesis from `electrogenesis' in a scalar field dominated epoch
Scalar fields can play a dominant role in the dynamics of the Universe until
shortly before nucleosynthesis. Examples are provided by domination by a
kinetic mode of a scalar field, which may be both the inflaton and the late
time `quintessence', and also by more conventional models of reheating. The
resultant modification to the pre-nucleosynthesis expansion rate can allow
solely an asymmetry in right handed electrons to produce a net baryon asymmetry
when reprocessed by the anomalous B+L violating processes of the standard
model. The production of such a source asymmetry - what we term
`electrogenesis' - requires no additional B or L violation beyond that in the
standard model. We consider a specific model for its generation, by a simple
perturbative out of equilibrium decay of Higgs like scalar fields with
CP-violating Yukawa couplings to the standard model leptons. We show that,
because of the much enhanced expansion rate, such a mechanism can easily
produce an adequate asymmetry from scalars with masses as low as 1 TeV. Kinetic
mode domination is strongly favoured because it evades large entropy release
which dilutes the asymmetry. We also discuss briefly the effect of the abelian
hypercharge anomaly.Comment: 25 pages, 2 figure
Regulating the infrared by mode matching: A massless scalar in expanding spaces with constant deceleration
In this paper we consider a massless scalar field, with a possible coupling
to the Ricci scalar in a dimensional FLRW spacetime with a constant
deceleration parameter , . Correlation
functions for the Bunch-Davies vacuum of such a theory have long been known to
be infrared divergent for a wide range of values of . We resolve
these divergences by explicitly matching the spacetime under consideration to a
spacetime without infrared divergencies. Such a procedure ensures that all
correlation functions with respect to the vacuum in the spacetime of interest
are infrared finite. In this newly defined vacuum we construct the coincidence
limit of the propagator and as an example calculate the expectation value of
the stress energy tensor. We find that this approach gives both in the
ultraviolet and in the infrared satisfactory results. Moreover, we find that,
unless the effective mass due to the coupling to the Ricci scalar is
negative, quantum contributions to the energy density always dilute away
faster, or just as fast, as the background energy density. Therefore, quantum
backreaction is insignificant at the one loop order, unless is
negative. Finally we compare this approach with known results where the
infrared is regulated by placing the Universe in a finite box. In an
accelerating universe, the results are qualitatively the same, provided one
identifies the size of the Universe with the physical Hubble radius at the time
of the matching. In a decelerating universe however, the two schemes give
different late time behavior for the quantum stress energy tensor. This happens
because in this case the length scale at which one regulates the infrared
becomes sub-Hubble at late times.Comment: 55 pages, 6 figure
The Scalar Field Kernel in Cosmological Spaces
We construct the quantum mechanical evolution operator in the Functional
Schrodinger picture - the kernel - for a scalar field in spatially homogeneous
FLRW spacetimes when the field is a) free and b) coupled to a spacetime
dependent source term. The essential element in the construction is the causal
propagator, linked to the commutator of two Heisenberg picture scalar fields.
We show that the kernels can be expressed solely in terms of the causal
propagator and derivatives of the causal propagator. Furthermore, we show that
our kernel reveals the standard light cone structure in FLRW spacetimes. We
finally apply the result to Minkowski spacetime, to de Sitter spacetime and
calculate the forward time evolution of the vacuum in a general FLRW spacetime.Comment: 13 pages, 1 figur
Problems and hopes in nonsymmetric gravity
We consider the linearized nonsymmetric theory of gravitation (NGT) within
the background of an expanding universe and near a Schwarzschild mass. We show
that the theory always develops instabilities unless the linearized
nonsymmetric lagrangian reduces to a particular simple form. This form contains
a gauge invariant kinetic term, a mass term for the antisymmetric metric-field
and a coupling with the Ricci curvature scalar. This form cannot be obtained
within NGT. Based on the linearized lagrangian we know to be stable, we
consider the generation and evolution of quantum fluctuations of the
antisymmetric gravitational field (B-field) from inflation up to the present
day. We find that a B-field with a mass m ~ 0.03(H_I/10^(13)GeV)^4 eV is an
excellent dark matter candidate.Comment: 9 pages, 1 figure. Based on two talks by the authors at the 2nd
International Conference on Quantum Theories and Renormalization Group in
Gravity and Cosmology (IRGAC) 2006, Barcelon
A Simple Operator Check of the Effective Fermion Mode Function during Inflation
We present a relatively simple operator formalism which reproduces the
leading infrared logarithm of the one loop quantum gravitational correction to
the fermion mode function on a locally de Sitter background. This rule may
serve as the basis for an eventual stochastic formulation of quantum gravity
during inflation. Such a formalism would not only effect a vast simplification
in obtaining the leading powers of at fixed loop orders, it would also
permit us to sum the series of leading logarithms. A potentially important
point is that our rule does not seem to be consistent with any simple infrared
truncation of the fields. Our analysis also highlights the importance of spin
as a gravitational interaction that persists even when kinetic energy has
redshifted to zero.Comment: 39 pages, no figuire.(1) New version has clarified the ultimate
motivation by adding sentences to the abstract and to the penultimate
paragraph of the introduction. (2) By combining a number of references and
equations we have managed to reduce the length by 2 page
The Newtonian Limit of Hermitian Gravity
We construct the gauge invariant potentials of Hermitian Gravity and derive
the linearized equations of motion they obey. A comparison reveals a striking
similarity to the Bardeen potentials of general relativity. We then consider
the response to a point particle source, and discuss in what sense the
solutions of Hermitian Gravity reduce to the Newtonian potentials. In a rather
intriguing way, the Hermitian Gravity solutions exhibit a generalized
reciprocity symmetry originally proposed by Born in the 1930s. Finally, we
consider the trajectories of massive and massless particles under the influence
of a potential. The theory correctly reproduces the Newtonian limit in three
dimensions and the nonrelativistic acceleration equation. However, it differs
from the light deflection calculated in linearized generalrelativity by 25%.
While the specific complexification of general relativity by extension to
Hermitian spaces performed here does not agree with experiment, it does possess
useful properties for quantization and is well-behaved around singularities.
Another form of complex general relativity may very well agree with
experimental data.Comment: The published version in Gen. Rel. Grav. 24 pages, no figure
Instabilities in the nonsymmetric theory of gravitation
We consider the linearized nonsymmetric theory of gravitation (NGT) within
the background of an expanding universe and near a Schwarzschild metric. We
show that the theory always develops instabilities unless the linearized
nonsymmetric lagrangian reduces to a particular simple form. This theory
contains a gauge invariant kinetic term, a mass term for the antisymmetric
metric-field and a coupling with the Ricci curvature scalar. This form cannot
be obtained within NGT. Next we discuss NGT beyond linearized level and
conjecture that the instabilities are not a relic of the linearization, but are
a general feature of the full theory. Finally we show that one cannot add
ad-hoc constraints to remove the instabilities as is possible with the
instabilities found in NGT by Clayton.Comment: 29 page
Vacuum properties of nonsymmetric gravity in de Sitter space
We consider quantum effects of a massive antisymmetric tensor field on the
dynamics of de Sitter space-time. Our starting point is the most general,
stable, linearized Lagrangian arising in nonsymmetric gravitational theories
(NGTs), where part of the antisymmetric field mass is generated by the
cosmological term. We construct a renormalization group (RG) improved effective
action by integrating out one loop vacuum fluctuations of the antisymmetric
tensor field and show that, in the limit when the RG scale goes to zero, the
Hubble parameter -- and thus the effective cosmological constant -- relaxes
rapidly to zero. We thus conclude that quantum loop effects in de Sitter space
can dramatically change the infrared sector of the on-shell gravity, making the
expansion rate insensitive to the original (bare) cosmological constant.Comment: 32 pages, 2 eps figure
Unruh response functions for scalar fields in de Sitter space
We calculate the response functions of a freely falling Unruh detector in de
Sitter space coupled to scalar fields of different coupling to the curvature,
including the minimally coupled massless case. Although the responses differ
strongly in the infrared as a consequence of the amplification of superhorizon
modes, the energy levels of the detector are thermally populated.Comment: 16 pages, 1 figure, accepted for publication by Classical and Quantum
Gravit
Stream Fusion, to Completeness
Stream processing is mainstream (again): Widely-used stream libraries are now
available for virtually all modern OO and functional languages, from Java to C#
to Scala to OCaml to Haskell. Yet expressivity and performance are still
lacking. For instance, the popular, well-optimized Java 8 streams do not
support the zip operator and are still an order of magnitude slower than
hand-written loops. We present the first approach that represents the full
generality of stream processing and eliminates overheads, via the use of
staging. It is based on an unusually rich semantic model of stream interaction.
We support any combination of zipping, nesting (or flat-mapping), sub-ranging,
filtering, mapping-of finite or infinite streams. Our model captures
idiosyncrasies that a programmer uses in optimizing stream pipelines, such as
rate differences and the choice of a "for" vs. "while" loops. Our approach
delivers hand-written-like code, but automatically. It explicitly avoids the
reliance on black-box optimizers and sufficiently-smart compilers, offering
highest, guaranteed and portable performance. Our approach relies on high-level
concepts that are then readily mapped into an implementation. Accordingly, we
have two distinct implementations: an OCaml stream library, staged via
MetaOCaml, and a Scala library for the JVM, staged via LMS. In both cases, we
derive libraries richer and simultaneously many tens of times faster than past
work. We greatly exceed in performance the standard stream libraries available
in Java, Scala and OCaml, including the well-optimized Java 8 streams
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