7,200 research outputs found
Sub-femtosecond electron bunches created by direct laser acceleration in a laser wakefield accelerator with ionization injection
In this work, we will show through three-dimensional particle-in-cell
simulations that direct laser acceleration in laser a wakefield accelerator can
generate sub-femtosecond electron bunches. Two simulations were done with two
laser pulse durations, such that the shortest laser pulse occupies only a
fraction of the first bubble, whereas the longer pulse fills the entire first
bubble. In the latter case, as the trapped electrons moved forward and
interacted with the high intensity region of the laser pulse, micro-bunching
occurred naturally, producing 0.5 fs electron bunches. This is not observed in
the short pulse simulation.Comment: AAC 201
Two-Dimensional Black Holes and Planar General Relativity
The Einstein-Hilbert action with a cosmological term is used to derive a new
action in 1+1 spacetime dimensions. It is shown that the two-dimensional theory
is equivalent to planar symmetry in General Relativity. The two-dimensional
theory admits black holes and free dilatons, and has a structure similar to
two-dimensional string theories. Since by construction these solutions also
solve Einstein's equations, such a theory can bring two-dimensional results
into the four-dimensional real world. In particular the two-dimensional black
hole is also a black hole in General Relativity.Comment: 11 pages, plainte
Effect of Water Content on the Thermal Inactivation Kinetics of Horseradish Peroxidase Freeze-Dried from Alkaline pH
The thermal inactivation of horseradish peroxidase freeze-dried from solutions of different pH (8, 10 and 11.5, measured at 25 C) and equilibrated to different water contents was studied in the temperature range from 110 to 150 C. The water contents studied (0.0, 1.4, 16.2 and 25.6 g water per 100 g of dry enzyme) corresponded to water activities of 0.0, 0.11, 0.76 and 0.88 at 4 C. The kinetics were well described by a double exponential model. The enzyme was generally more stable the lower the pH of the original solution, and for all pH values, the maximum stability was obtained at 1.4 g water/100 g dry enzyme. Values of z were generally independent of water content and of the pH of the original solution, and in the range of 15–25 °C, usually found in neutral conditions, with the exception of the enzyme freeze dried from pH 11.5 and equilibrated with phosphorus pentoxide, where a z-value of the stable fraction close to 10 C was found
The Two-Dimensional Analogue of General Relativity
General Relativity in three or more dimensions can be obtained by taking the
limit in the Brans-Dicke theory. In two dimensions
General Relativity is an unacceptable theory. We show that the two-dimensional
closest analogue of General Relativity is a theory that also arises in the
limit of the two-dimensional Brans-Dicke theory.Comment: 8 pages, LaTeX, preprint DF/IST-17.9
Gravitational magnetic monopoles and Majumdar-Papapetrou stars
A large amount of work has been dedicated to studying general relativity
coupled to non-Abelian Yang-Mills type theories. It has been shown that the
magnetic monopole, a solution of the Yang-Mills-Higgs equations can be coupled
to gravitation. For a low Higgs mass there are regular solutions, and for a
sufficiently massive monopole the system develops an extremal magnetic
Reissner-Nordstrom quasi-horizon. These solutions, called quasi-black holes,
although non-singular, are arbitrarily close to having a horizon. However, at
the critical value the quasi-black hole turns into a degenerate spacetime. On
the other hand, for a high Higgs mass, a sufficiently massive monopole develops
also a quasi-black hole, but it turns into an extremal true horizon, with
matter fields outside. One can also put a small Schwarzschild black hole inside
the magnetic monopole, an example of a non-Abelian black hole. Surprisingly,
Majumdar-Papapetrou systems, Abelian systems constructed from extremal dust,
also show a resembling behavior. Previously, we have reported that one can find
Majumdar-Papapetrou solutions which can be arbitrarily close of being a black
hole, displaying quasi-black hole behavior. With the aim of better
understanding the similarities between gravitational monopoles and
Majumdar-Papapetrou systems, we study a system composed of two extremal
electrically charged spherical shells (or stars, generically) in the
Einstein--Maxwell--Majumdar-Papapetrou theory. We review the gravitational
properties of the monopoles, and compare with the properties of the double
extremal electric shell system. These quasi-black holes can help in the
understanding of true black holes, and can give insight into the nature of the
entropy of black holes in the form of entanglement.Comment: 38 pages,9 Figures, minor change
Electrically charged fluids with pressure in Newtonian gravitation and general relativity in d spacetime dimensions: theorems and results for Weyl type systems
Previous theorems concerning Weyl type systems, including Majumdar-Papapetrou
systems, are generalized in two ways, namely, we take these theorems into d
spacetime dimensions (), and we also consider the very
interesting Weyl-Guilfoyle systems, i.e., general relativistic charged fluids
with nonzero pressure. In particular within Newton-Coulomb theory of charged
gravitating fluids, a theorem by Bonnor (1980) in three-dimensional space is
generalized to arbitrary space dimensions. Then, we prove a new
theorem for charged gravitating fluid systems in which we find the condition
that the charge density and the matter density should obey. Within general
relativity coupled to charged dust fluids, a theorem by De and Raychaudhuri
(1968) in four-dimensional spacetimes in rendered into arbitrary
dimensions. Then a theorem, new in and dimensions, for
Weyl-Guilfoyle systems, is stated and proved, in which we find the condition
that the charge density, the matter density, the pressure, and the
electromagnetic energy density should obey. This theorem comprises, as
particular cases, a theorem by Gautreau and Hoffman (1973) and results in four
dimensions by Guilfoyle (1999). Upon connection of an interior charged solution
to an exterior Tangherlini solution (i.e., a Reissner-Nordstr\"om solution in
d-dimensions), one is able to give a general definition for gravitational mass
for this kind of relativistic systems and find a mass relation with the several
quantities of the interior solution. It is also shown that for sources of
finite extent the mass is identical to the Tolman mass.Comment: 27 page
Charged Lifshitz black holes from general covariance breaking
In this work we use a general covariance breaking method to obtain a class of
topological charged black holes whose background geometry asymptotically
approaches Lifshitz spacetimes. We discuss how this mechanism affects
Einstein's equations and explore the thermodynamics and critical behavior of
the solution found.Comment: 9 pages, 4 figure
Relativistic Static Thin Disks: The Counter-Rotating Model
A detailed study of the Counter-Rotating Model (CRM) for generic finite
static axially symmetric thin disks with nonzero radial pressure is presented.
We find a general constraint over the counter-rotating tangential velocities
needed to cast the surface energy-momentum tensor of the disk as the
superposition of two counter-rotating perfect fluids. We also found expressions
for the energy density and pressure of the counter-rotating fluids. Then we
shown that, in general, there is not possible to take the two counter-rotating
fluids as circulating along geodesics neither take the two counter-rotating
tangential velocities as equal and opposite. An specific example is studied
where we obtain some CRM with well defined counter-rotating tangential
velocities and stable against radial perturbations. The CRM obtained are in
agree with the strong energy condition, but there are regions of the disks with
negative energy density, in violation of the weak energy condition.Comment: 19 pages, 6 figures. Submitted to Physical Review
Quasi-black holes: definition and general properties
Objects that are on the verge of being extremal black holes but actually are
distinct in many ways are called quasi-black holes. Quasi-black holes are
defined here and treated in a unified way through the displaying of their
properties. The main ones are (i) there are infinite redshift whole regions,
(ii) the spacetimes exhibit degenerate, almost singular, features but their
curvature invariants remain perfectly regular everywhere, (iii) in the limit
under discussion, outer and inner regions become mutually impenetrable and
disjoint, although, in contrast to the usual black holes, this separation is of
a dynamical nature, rather than purely causal, (iv) for external far away
observers the spacetime is virtually indistinguishable from that of extremal
black holes. It is shown, in addition, that quasi-black holes must be extremal.
Connections with black hole and wormhole physics are also drawn.Comment: 29 pages, minor change
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