447 research outputs found
Dirac Fields in Loop Quantum Gravity and Big Bang Nucleosynthesis
Big Bang nucleosynthesis requires a fine balance between equations of state
for photons and relativistic fermions. Several corrections to equation of state
parameters arise from classical and quantum physics, which are derived here
from a canonical perspective. In particular, loop quantum gravity allows one to
compute quantum gravity corrections for Maxwell and Dirac fields. Although the
classical actions are very different, quantum corrections to the equation of
state are remarkably similar. To lowest order, these corrections take the form
of an overall expansion-dependent multiplicative factor in the total density.
We use these results, along with the predictions of Big Bang nucleosynthesis,
to place bounds on these corrections.Comment: 15 pages, 2 figures; v2: new discussion of relevance of quantum
gravity corrections (Sec. II) and new estimates (Sec. V
Future Asymptotic Behaviour of Tilted Bianchi models of type IV and VIIh
Using dynamical systems theory and a detailed numerical analysis, the
late-time behaviour of tilting perfect fluid Bianchi models of types IV and
VII are investigated. In particular, vacuum plane-wave spacetimes are
studied and the important result that the only future attracting equilibrium
points for non-inflationary fluids are the plane-wave solutions in Bianchi type
VII models is discussed. A tiny region of parameter space (the loophole) in
the Bianchi type IV model is shown to contain a closed orbit which is found to
act as an attractor (the Mussel attractor). From an extensive numerical
analysis it is found that at late times the normalised energy-density tends to
zero and the normalised variables 'freeze' into their asymptotic values. A
detailed numerical analysis of the type VII models then shows that there is
an open set of parameter space in which solution curves approach a compact
surface that is topologically a torus.Comment: 30 pages, many postscript figure
Models for Little Rip Dark Energy
We examine in more detail specific models which yield a little rip cosmology,
i.e., a universe in which the dark energy density increases without bound but
the universe never reaches a finite-time singularity. We derive the conditions
for the little rip in terms of the inertial force in the expanding universe and
present two representative models to illustrate in more detail the difference
between little rip models and those which are asymptotically de Sitter. We
derive conditions on the equation of state parameter of the dark energy to
distinguish between the two types of models. We show that coupling between dark
matter and dark energy with a little rip equation of state can alter the
evolution, changing the little rip into an asymptotic de Sitter expansion. We
give conditions on minimally-coupled phantom scalar field models and on
scalar-tensor models that indicate whether or not they correspond to a little
rip expansion. We show that, counterintuitively, despite local instability, a
little-rip cosmology has an infinite lifetime.Comment: LaTeX, 10 pages, no figure, version to appear in Phys.Lett
Fleas and smaller fleas: virotherapy for parasite infections
Bacteriophages are viruses of bacteria that are used for controlling bacterial food-borne pathogens and have been proposed for more extensive usage in infection control. Protists are now recognised to harbour viruses and virus-like particles. We propose that investigation of their prevalence in parasites be intensified. We also propose that such viruses might be considered for virotherapy to control certain parasite infections of man and animals
Big Bang Nucleosynthesis Constraints on Brane Cosmologies
We examine constraints from Big Bang nucleosynthesis on type II
Randall-Sundrum brane cosmologies with both a dark radiation component and a
quadratic term that depends on the 5-dimensional Planck mass, M_5. Using limits
on the abundances of deuterium and helium-4, we calculate the allowed region in
the M_5-dark radiation plane and derive the precise BBN bound on M_5 alone with
no dark radiation: M_5 > 13 TeV.Comment: 3 pages, 1 figure, references added, to appear in Phys. Lett.
Gedanken Worlds without Higgs: QCD-Induced Electroweak Symmetry Breaking
To illuminate how electroweak symmetry breaking shapes the physical world, we
investigate toy models in which no Higgs fields or other constructs are
introduced to induce spontaneous symmetry breaking. Two models incorporate the
standard SU(3)_c x SU(2)_L x U(1)_Y gauge symmetry and fermion content similar
to that of the standard model. The first class--like the standard electroweak
theory--contains no bare mass terms, so the spontaneous breaking of chiral
symmetry within quantum chromodynamics is the only source of electroweak
symmetry breaking. The second class adds bare fermion masses sufficiently small
that QCD remains the dominant source of electroweak symmetry breaking and the
model can serve as a well-behaved low-energy effective field theory to energies
somewhat above the hadronic scale. A third class of models is based on the
left-right--symmetric SU(3)_c x SU(2)_L x SU(2)_R x U(1)_{B-L} gauge group. In
a fourth class of models, built on SU(4)_{PS} x SU(2)_L x SU(2)_R gauge
symmetry, lepton number is treated as a fourth color. Many interesting
characteristics of the models stem from the fact that the effective strength of
the weak interactions is much closer to that of the residual strong
interactions than in the real world. The Higgs-free models not only provide
informative contrasts to the real world, but also lead us to consider
intriguing issues in the application of field theory to the real world.Comment: 20 pages, no figures, uses RevTeX; typos correcte
Quark Masses: An Environmental Impact Statement
We investigate worlds that lie on a slice through the parameter space of the
Standard Model over which quark masses vary. We allow as many as three quarks
to participate in nuclei, while fixing the mass of the electron and the average
mass of the lightest baryon flavor multiplet. We classify as "congenial" worlds
that satisfy the environmental constraint that the quark masses allow for
stable nuclei with charges one, six, and eight, making organic chemistry
possible. Whether a congenial world actually produces observers depends on a
multitude of historical contingencies, beginning with primordial
nucleosynthesis, which we do not explore. Such constraints may be independently
superimposed on our results. Environmental constraints such as the ones we
study may be combined with information about the a priori distribution of quark
masses over the landscape of possible universes to determine whether the
measured values of the quark masses are determined environmentally, but our
analysis is independent of such an anthropic approach.
We estimate baryon masses as functions of quark masses and nuclear masses as
functions of baryon masses. We check for the stability of nuclei against
fission, strong particle emission, and weak nucleon emission. For two light
quarks with charges 2/3 and -1/3, we find a band of congeniality roughly 29 MeV
wide in their mass difference. We also find another, less robust region of
congeniality with one light, charge -1/3 quark, and two heavier, approximately
degenerate charge -1/3 and 2/3 quarks. No other assignment of light quark
charges yields congenial worlds with two baryons participating in nuclei. We
identify and discuss the region in quark-mass space where nuclei would be made
from three or more baryon species.Comment: 40 pages, 16 figures (in color), 4 tables. See paper for a more
detailed abstract. v4: Cleaning up minor typo
Correspondence Between Roger J. Traynor, James J. Brosnahan, Robert Russell, Roscoe L. Barrow, Thomas F. Schroeter, and Paul A. Freund, 1974 April - 1974 July
Complete genome sequences of seven Vibrio cholerae phages isolated in China
The complete genome sequences of seven closely related Vibrio cholerae phages isolated from environmental sites in southeastern China are reported here. Phages QH, CJY, H1, H2, H3, J2, and J3 are members of the Podoviridae family and are highly similar to the previously sequenced Vibrio phages VP2, VP5, and phiVC8
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