3,046 research outputs found
Proton translocation in proteins
The active transport of protons across the low dielectric barrier imposed by biological membranes is accomplished by a plethora of proteins that span the ca. 40 Ă… of the phospholipid bilayer. The free energy derived from the proton electrochemical potential established by the translocation of these protons can subsequently be used to drive vital chemical reactions of the cell, such as ATP synthesis and cell locomotion. Membrane-bound proton translocating proteins have now been found for a variety of organisms and tissues (1). The driving force for proton pumping in these proteins is supplied by numerous mechanisms, including light absorption (e.g. bacteriorhodopsin) (2a,b), ligand binding (e.g. ATPase) (3), and electrochemistry (e.g. electron transfer through cytochrome c oxidase) (4). Thus nature has devised a variety of methods for supplying the energy required for proton pumping by these proteins. Such diversity notwithstanding, the proteins most likely share some common elements of structure and mechanism that allow them to function as proton pumps. A number of theoretical mechanisms have been put forth for both general proton translocation (5-7) and for energy coupling in specific proton pumps. However, despite almost three decades of intensive research, the details of the mechanism(s) and structural requirements for proton pumping remain largely unresolved. To some extent this is the result of the paucity of structural information available for integral membrane proteins. This situation may soon improve as a result of advances in protein methodologies that have allowed several integral membrane proteins to be successfully crystalized (8), and the increased use of genetic engineering to obtain recombinant proton translocating proteins that will offer an opportunity to assess the importance of specific amino acids for the proton translocation process (9)
On the reliability of inflaton potential reconstruction
If primordial scalar and tensor perturbation spectra can be inferred from
observations of the cosmic background radiation and large-scale structure, then
one might hope to reconstruct a unique single-field inflaton potential capable
of generating the observed spectra. In this paper we examine conditions under
which such a potential can be reliably reconstructed. For it to be possible at
all, the spectra must be well fit by a Taylor series expansion. A complete
reconstruction requires a statistically-significant tensor mode to be measured
in the microwave background. We find that the observational uncertainties
dominate the theoretical error from use of the slow-roll approximation, and
conclude that the reconstruction procedure will never insidiously lead to an
irrelevant potential.Comment: 16 page LaTeX file with eight postscript figures embedded with epsf;
no special macros neede
Low energy effective string cosmology
We give the general analytic solutions derived from the low energy string
effective action for four dimensional Friedmann-Robertson-Walker models with
dilaton and antisymmetric tensor field, considering both long and short
wavelength modes of the -field. The presence of a homogeneous -field
significantly modifies the evolution of the scale factor and dilaton. In
particular it places a lower bound on the allowed value of the dilaton. The
scale factor also has a lower bound but our solutions remain singular as they
all contain regions where the spacetime curvature diverges signalling a
breakdown in the validity of the effective action. We extend our results to the
simplest Bianchi I metric in higher dimensions with only two scale factors. We
again give the general analytic solutions for long and short wavelength modes
for the field restricted to the three dimensional space, which produces an
anisotropic expansion. In the case of field radiation (wavelengths within
the Hubble length) we obtain the usual four dimensional radiation dominated FRW
model as the unique late time attractor.Comment: 22 pages, LaTeX, SUSX-TH-94/37, SUSSEX-AST-94/6-2. (Some terminology
and figure captions corrected, references added.
Universality and Critical Phenomena in String Defect Statistics
The idea of biased symmetries to avoid or alleviate cosmological problems
caused by the appearance of some topological defects is familiar in the context
of domain walls, where the defect statistics lend themselves naturally to a
percolation theory description, and for cosmic strings, where the proportion of
infinite strings can be varied or disappear entirely depending on the bias in
the symmetry. In this paper we measure the initial configurational statistics
of a network of string defects after a symmetry-breaking phase transition with
initial bias in the symmetry of the ground state. Using an improved algorithm,
which is useful for a more general class of self-interacting walks on an
infinite lattice, we extend the work in \cite{MHKS} to better statistics and a
different ground state manifold, namely , and explore various different
discretisations. Within the statistical errors, the critical exponents of the
Hagedorn transition are found to be quite possibly universal and identical to
the critical exponents of three-dimensional bond or site percolation. This
improves our understanding of the percolation theory description of defect
statistics after a biased phase transition, as proposed in \cite{MHKS}. We also
find strong evidence that the existence of infinite strings in the Vachaspati
Vilenkin algorithm is generic to all (string-bearing) vacuum manifolds, all
discretisations thereof, and all regular three-dimensional lattices.Comment: 62 pages, plain LaTeX, macro mathsymb.sty included, figures included.
also available on
http://starsky.pcss.maps.susx.ac.uk/groups/pt/preprints/96/96011.ps.g
Reconstructing the Equation of State of Tachyon
Recent progress in theoretical physics suggests that the dark energy in the
universe might be resulted from the rolling tachyon field of string theory.
Measurements to SNe Ia can be helpful to reconstruct the equation of state of
the rolling tachyon which is a possible candidate of dark energy. We present a
numerical analysis for the evolution of the equation of state of the rolling
tachyon and derive the reconstruction equations for the equation of state as
well as the potential.Comment: 6 pages, 3 figures, to appear Phys. Rev.
Anisotropic String Cosmology at Large Curvatures
We study the effect of the antisymmetric tensor field on the
large curvature phase of string cosmology. It is well-known that a
non-vanishing value of leads to an anisotropic expansion of the spatial
dimensions. Correspondingly, in the string phase of the model, including
corrections, we find anisotropic fixed points of the evolution,
which act as regularizing attractors of the lowest order solutions. The
attraction basin can also include isotropic initial conditions for the scale
factors. We present explicit examples at order for different values
of the number of spatial dimensions and for different ans\"{a}tze for .Comment: 16 pages, Latex, 2 figure
A new view of k-essence
K-essence models, relying on scalar fields with non-canonical kinetic terms,
have been proposed as an alternative to quintessence in explaining the observed
acceleration of the Universe. We consider the use of field redefinitions to
cast k-essence in a more familiar form. While k-essence models cannot in
general be rewritten in the form of quintessence models, we show that in
certain dynamical regimes an equivalence can be made, which in particular can
shed light on the tracking behaviour of k-essence. In several cases, k-essence
cannot be observationally distinguished from quintessence using the homogeneous
evolution, though there may be small effects on the perturbation spectrum. We
make a detailed analysis of two k-essence models from the literature and
comment on the nature of the fine tuning arising in the models.Comment: 7 pages RevTeX4 file with four figures incorporate
Symmetries for generating string cosmologies
We discuss the symmetry properties of the low-energy effective action of the
type IIB superstring that may be employed to derive four-dimensional solutions.
A truncated effective action, compactified on a six-torus, but including both
Neveu/Schwarz-Neveu/Schwarz and Ramond-Ramond field strengths, can be expressed
as a non-linear sigma model which is invariant under global SL(3,R)
transformations. This group contains as a sub-group the SL(2,R) symmetry of the
ten-dimensional theory and a discrete Z2 reflection symmetry which leads to a
further SL(2,R) sub-group. The symmetries are employed to analyse a general
class of spatially homogeneous cosmological solutions with non-trivial
Ramond-Ramond fields.Comment: Substantially extended version with new sections on further
symmetries and anisotropic cosmological solutions. New title. To appear in
Physical Review D. 13 pages, LaTeX, no figure
Oscillons: Resonant Configurations During Bubble Collapse
Oscillons are localized, non-singular, time-dependent, spherically-symmetric
solutions of nonlinear scalar field theories which, although unstable, are
extremely long-lived. We show that they naturally appear during the collapse of
subcritical bubbles in models with symmetric and asymmetric double-well
potentials. By a combination of analytical and numerical work we explain
several of their properties, including the conditions for their existence,
their longevity, and their final demise. We discuss several contexts in which
we expect oscillons to be relevant. In particular, their nucleation during
cosmological phase transitions may have wide-ranging consequences.Comment: 31 pages Revtex, 20 uufiles-encoded figures. Section "Possible
Applications of Oscillons" slightly expande
On the Realization of Assisted Inflation
We consider conditions necessary for a successful implementation of so-called
assisted inflation. We generalize the applicability of assisted inflation
beyond exponential potentials as originally proposed to include standard
chaotic (\lambda \phi^4 or m^2 \phi^2) models as well. We also demonstrate that
in a purely 4-dimensional theory, unless the assisted sector is in fact
decoupled, the additional fields of the assisted sector actually impede
inflation. As a specific example of an assisted sector, we consider a
5-dimensional KK model for which the extra dimension may be somewhat or much
larger than the inverse Planck scale. In this case, the assisted sector (coming
from a KK compactification) eliminates the need for a fine-tuned quartic
coupling to drive chaotic inflation. This is a general result of models with
one or more "large" extra dimensions.Comment: 25 pages, LaTeX fil
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