33,018 research outputs found
Gravitational energy
Observers at rest in a stationary spacetime flat at infinity can measure
small amounts of rest-mass+internal energies+kinetic energies+pressure energy
in a small volume of fluid attached to a local inertial frame. The sum of these
small amounts is the total "matter energy" for those observers. The total
mass-energy minus the matter energy is the binding gravitational energy.
Misner, Thorne and Wheeler evaluated the gravitational energy of a
spherically symmetric static spacetime. Here we show how to calculate
gravitational energy in any static and stationary spacetime for isolated
sources with a set of observers at rest.
The result of MTW is recovered and we find that electromagnetic and
gravitational 3-covariant energy densities in conformastatic spacetimes are of
opposite signs. Various examples suggest that gravitational energy is negative
in spacetimes with special symmetries or when the energy-momentum tensor
satisfies usual energy conditions.Comment: 12 pages. Accepted for publication in Class. Quantum Gra
Mesoscopic Mechanical Resonators as Quantum Non-Inertial Reference Frames
An atom attached to a micrometer-scale wire that is vibrating at a frequency
of 100 MHz and with displacement amplitude 1 nm experiences an acceleration
magnitude 10^9 ms^-2, approaching the surface gravity of a neutron star. As one
application of such extreme non-inertial forces in a mesoscopic setting, we
consider a model two-path atom interferometer with one path consisting of the
100 MHz vibrating wire atom guide. The vibrating wire guide serves as a
non-inertial reference frame and induces an in principle measurable phase shift
in the wave function of an atom traversing the wire frame. We furthermore
consider the effect on the two-path atom wave interference when the vibrating
wire is modeled as a quantum object, hence functioning as a quantum
non-inertial reference frame. We outline a possible realization of the
vibrating wire, atom interferometer using a superfluid helium quantum
interference setup.Comment: Published versio
Polynomial Interpretation of Multipole Vectors
Copi, Huterer, Starkman and Schwarz introduced multipole vectors in a tensor
context and used them to demonstrate that the first-year WMAP quadrupole and
octopole planes align at roughly the 99.9% confidence level. In the present
article the language of polynomials provides a new and independent derivation
of the multipole vector concept. Bezout's Theorem supports an elementary proof
that the multipole vectors exist and are unique (up to rescaling). The
constructive nature of the proof leads to a fast, practical algorithm for
computing multipole vectors. We illustrate the algorithm by finding exact
solutions for some simple toy examples, and numerical solutions for the
first-year WMAP quadrupole and octopole. We then apply our algorithm to Monte
Carlo skies to independently re-confirm the estimate that the WMAP quadrupole
and octopole planes align at the 99.9% level.Comment: Version 1: 6 pages. Version 2: added uniqueness proof to Corollary 2;
added proper citation (to Starkman et al.) for Open Question; other minor
improvement
On the mass of a Kerr-anti-de Sitter spacetime in D dimensions
We show how to compute the mass of a Kerr-anti-de Sitter spacetime with
respect to the anti-de Sitter background in any dimension, using a
superpotential which has been derived from standard Noether identities. The
calculation takes no account of the source of the curvature and confirms
results obtained for black holes via the first law of thermodynamics.Comment: minor changes; accepted by CQ
Some Aspects of New CDM Models and CDM Detection Methods
We briefly review some recent Cold Dark Matter (CDM) models. Our main focus
are charge symmetric models of WIMPs which are not the standard SUSY LSP's
(Lightest Supersymmetric Partners). We indicate which experiments are most
sensitive to certain aspects of the models. In particular we discuss the
manifestations of the new models in neutrino telescopes and other set-ups. We
also discuss some direct detection experiments and comment on measuring the
direction of recoil ions--which is correlated with the direction of the
incoming WIMP. This could yield daily variations providing along with the
annual modulation signatures for CDM.Comment: 14 page
Black holes, cuspy atmospheres, and galaxy formation
In cuspy atmospheres, jets driven by supermassive black holes (BHs) offset
radiative cooling. The jets fire episodically, but often enough that the cuspy
atmosphere does not move very far towards a cooling catastrophe in the
intervals of jet inactivity. The ability of energy released on the sub-parsec
scale of the BH to balance cooling on scales of several tens of kiloparsecs
arises through a combination of the temperature sensitivity of the accretion
rate and the way in which the radius of jet disruption varies with ambient
density. Accretion of hot gas does not significantly increase BH masses, which
are determined by periods of rapid BH growth and star formation when cold gas
is briefly abundant at the galactic centre. Hot gas does not accumulate in
shallow potential wells. As the Universe ages, deeper wells form, and
eventually hot gas accumulates. This gas soon prevents the formation of further
stars, since jets powered by the BH prevent it from cooling, and it mops up
most cold infalling gas before many stars can form. Thus BHs set the upper
limit to the masses of galaxies. The formation of low-mass galaxies is
inhibited by a combination of photo-heating and supernova-driven galactic
winds. Working in tandem these mechanisms can probably explain the profound
difference between the galaxy luminosity function and the mass function of dark
halos expected in the cold dark matter cosmology.Comment: To appear in Phil Trans Roy So
Implications of the Visible and X-Ray Counterparts to GRB970228
The gamma-ray burst source GRB970228 has been observed after a delay of 8--12
hours in X-rays and after one day in visible and near infrared light. This
marks the first detection of emission at lower frequencies following the
gamma-ray observation of a GRB and the first detection of any visible
counterpart to a GRB. We consider possible delayed visible and X-ray emission
mechanisms, and conclude that the intrinsic gamma-ray activity continued at a
much reduced intensity for at least a day. There are hints of such continued
activity in other GRB, and future observations can decide if this is true of
GRB in general. The observed multi-band spectrum of GRB970228 agrees with the
predictions of relativistic shock theory when the flux is integrated over a
time longer than that required for a radiating electron to lose its energy.Comment: 5 pp., tex, 1 figur
Beyond Counting: New Perspectives on the Active IPv4 Address Space
In this study, we report on techniques and analyses that enable us to capture
Internet-wide activity at individual IP address-level granularity by relying on
server logs of a large commercial content delivery network (CDN) that serves
close to 3 trillion HTTP requests on a daily basis. Across the whole of 2015,
these logs recorded client activity involving 1.2 billion unique IPv4
addresses, the highest ever measured, in agreement with recent estimates.
Monthly client IPv4 address counts showed constant growth for years prior, but
since 2014, the IPv4 count has stagnated while IPv6 counts have grown. Thus, it
seems we have entered an era marked by increased complexity, one in which the
sole enumeration of active IPv4 addresses is of little use to characterize
recent growth of the Internet as a whole.
With this observation in mind, we consider new points of view in the study of
global IPv4 address activity. Our analysis shows significant churn in active
IPv4 addresses: the set of active IPv4 addresses varies by as much as 25% over
the course of a year. Second, by looking across the active addresses in a
prefix, we are able to identify and attribute activity patterns to network
restructurings, user behaviors, and, in particular, various address assignment
practices. Third, by combining spatio-temporal measures of address utilization
with measures of traffic volume, and sampling-based estimates of relative host
counts, we present novel perspectives on worldwide IPv4 address activity,
including empirical observation of under-utilization in some areas, and
complete utilization, or exhaustion, in others.Comment: in Proceedings of ACM IMC 201
A new look at the kinematics of the bulge from an N-body model
(Abridged) By using an N-body simulation of a bulge that was formed via a bar
instability mechanism, we analyse the imprints of the initial (i.e. before bar
formation) location of stars on the bulge kinematics, in particular on the
heliocentric radial velocity distribution of bulge stars. Four different
latitudes were considered: , , , and
, along the bulge minor axis as well as outside it, at
and . The bulge X-shaped structure comprises
stars that formed in the disk at different locations. Stars formed in the outer
disk, beyond the end of the bar, which are part of the boxy peanut-bulge
structure may show peaks in the velocity distributions at positive and negative
heliocentric radial velocities with high absolute values that can be larger
than 100 , depending on the observed direction. In some
cases the structure of the velocity field is more complex and several peaks are
observed. Stars formed in the inner disk, the most numerous, contribute
predominantly to the X-shaped structure and present different kinematic
characteristics. Our results may enable us to interpret the cold high-velocity
peak observed in the APOGEE commissioning data, as well as the excess of
high-velocity stars in the near and far arms of the X-shaped structure at
= and =. When compared with real data, the kinematic
picture becomes more complex due to the possible presence in the observed
samples of classical bulge and/or thick disk stars. Overall, our results point
to the existence of complex patterns and structures in the bulge velocity
fields, which are generated by the bar. This suggests that caution should be
used when interpreting the bulge kinematics: the presence of substructures,
peaks and clumps in the velocity fields is not necessarily a sign of past
accretion events.Comment: 21 pages, 18 figures. Accepted for publication in A&
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