412 research outputs found
Non-relativistic Extended Gravity and its applications across different astrophysical scales
Using dimensional analysis techniques we present an extension of Newton's
gravitational theory built under the assumption that Milgrom's acceleration
constant is a fundamental quantity of nature. The gravitational force converges
to Newton's gravity and to a MOND-like description in two different mass and
length regimes. It is shown that a modification on the force sector (and not in
the dynamical one as MOND does) is more convenient and can reproduce and
predict different phenomena usually ascribed to dark matter at the
non-relativistic level.Comment: 4 pages, 2 figures. To appear in the proceedings of the 2011 Spanish
Relativity Meeting (ERE2011) held in Madrid, Spai
A cosmological dust model with extended f(chi) gravity
Introducing a fundamental constant of nature with dimensions of acceleration
into the theory of gravity makes it possible to extend gravity in a very
consistent manner. At the non-relativistic level a MOND-like theory with a
modification in the force sector is obtained, which is the limit of a very
general metric relativistic theory of gravity. Since the mass and length scales
involved in the dynamics of the whole universe require small accelerations of
the order of Milgrom's acceleration constant a_0, it turns out that the
relativistic theory of gravity can be used to explain the expansion of the
universe. In this work it is explained how to use that relativistic theory of
gravity in such a way that the overall large-scale dynamics of the universe can
be treated in a pure metric approach without the need to introduce dark matter
and/or dark energy components.Comment: 7 pages, 1 figure. Accepted for publication in the European Physical
Journal
Gravitational anomalies signaling the breakdown of classical gravity
Recent observations for three types of astrophysical systems severely
challenge the GR plus dark matter scenario, showing a phenomenology which is
what modified gravity theories predict. Stellar kinematics in the outskirts of
globular clusters show the appearance of MOND type dynamics on crossing the
threshold. Analysis shows a ``Tully-Fisher'' relation in these systems,
a scaling of dispersion velocities with the fourth root of their masses.
Secondly, an anomaly has been found at the unexpected scales of wide binaries
in the solar neighbourhood. Binary orbital velocities cease to fall along
Keplerian expectations, and settle at a constant value, exactly on crossing the
threshold. Finally, the inferred infall velocity of the bullet cluster
is inconsistent with the standard cosmological scenario, where much smaller
limit encounter velocities appear. This stems from the escape velocity limit
present in standard gravity; the ``bullet'' should not hit the ``target'' at
more than the escape velocity of the joint system, as it very clearly did.
These results are consistent with extended gravity, but would require rather
contrived explanations under GR, each. Thus, observations now put us in a
situation where modifications to gravity at low acceleration scales cease to be
a matter of choice, to now become inevitable.Comment: 10 pages, 5 figures, Astrophysics and Space Science Proceedings 38,
4
Wide binaries as a critical test of Classical Gravity
Modified gravity scenarios where a change of regime appears at acceleration
scales have been proposed. Since for systems the
acceleration drops below at scales of around 7000 AU, a statistical
survey of wide binaries with relative velocities and separations reaching
AU and beyond should prove useful to the above debate. We apply the
proposed test to the best currently available data. Results show a constant
upper limit to the relative velocities in wide binaries which is independent of
separation for over three orders of magnitude, in analogy with galactic flat
rotation curves in the same acceleration regime. Our results are
suggestive of a breakdown of Kepler's third law beyond
scales, in accordance with generic predictions of modified gravity theories
designed not to require any dark matter at galactic scales and beyond.Comment: accepted for publication in EPJ
Flattened velocity dispersion profiles in Globular Clusters: Newtonian tides or modified gravity?
Over the past couple of years, a number of observational studies have
confirmed the flattening of the radial velocity dispersion profiles for stars
in various nearby globular clusters. As the projected radial coordinate is
increased, a radius appears beyond which, the measured velocity dispersion
ceases to drop and settles at a fixed value, . Under Newtonian
gravity, this is explained by invoking tidal heating from the overall Milky Way
potential on the outer, more loosely bound stars, of the globular clusters in
question. From the point of view of modified gravity theories, such an outer
flattening is expected on crossing the critical acceleration threshold ,
beyond which, a transition to MONDian dynamics is expected, were equilibrium
velocities cease to be a function of distance. In this paper we attempt to sort
out between the above competing explanations, by looking at their plausibility
in terms of an strictly empirical approach. We determine Newtonian tidal radii
using masses accurately calculated through stellar population modelling, and
hence independent of any dynamical assumptions, distances, size and orbital
determinations for a sample of 16 globular clusters. We show that their
Newtonian tidal radii at perigalacticon are generally larger that the radii at
which the flattening in the velocity dispersion profiles occurs, by large
factors of 4, on average. While this point makes the Newtonian tidal
explanation suspect, it is found that the radii at which the flattening is
observed on average correlate with the radii where the threshold is
crossed, and that values scale with the fourth root of the
total masses, all features predicted under modified gravity theories.Comment: Accepted for publication in MNRAS. Includes much enlarged GC sample
confirming MONDian TF for GCs. 11 pages, 7 figure
Galactic rotation curves in brane world models
In the braneworld scenario the four dimensional effective Einstein equation
has extra source terms, which arise from the embedding of the 3-brane in the
bulk. These non-local effects, generated by the free gravitational field of the
bulk, may provide an explanation for the dynamics of the neutral hydrogen
clouds at large distances from the galactic center, which is usually explained
by postulating the existence of the dark matter. In the present paper we
consider the asymptotic behavior of the galactic rotation curves in the brane
world models, and we compare the theoretical results with observations of both
High Surface Brightness and Low Surface Brightness galaxies. For the chosen
sample of galaxies we determine first the baryonic parameters by fitting the
photometric data to the adopted galaxy model; then we test the hypothesis of
the Weyl fluid acting as dark matter on the chosen sample of spiral galaxies by
fitting the tangential velocity equation of the combined baryonic-Weyl model to
the rotation curves. We give an analytical expression for the rotational
velocity of a test particle on a stable circular orbit in the exterior region
to a galaxy, with Weyl fluid contributions included. The model parameter ranges
for which the test provides agreement (within 1 confidence
level) with observations on the velocity fields of the chosen galaxy sample are
then determined. There is a good agreement between the theoretical predictions
and observations, showing that extra-dimensional models can be effectively used
as a viable alternative to the standard dark matter paradigm.Comment: to be published in MNRAS, 17 pages, 31 figures, version including
corrections on the proo
Daily monitoring of TeV gamma-ray emission from Mrk 421, Mrk 501, and the Crab Nebula with HAWC
We present results from daily monitoring of gamma rays in the energy range
to TeV with the first 17 months of data from the High
Altitude Water Cherenkov (HAWC) Observatory. Its wide field of view of 2
steradians and duty cycle of % are unique features compared to other TeV
observatories that allow us to observe every source that transits over HAWC for
up to hours each sidereal day. This regular sampling yields
unprecedented light curves from unbiased measurements that are independent of
seasons or weather conditions. For the Crab Nebula as a reference source we
find no variability in the TeV band. Our main focus is the study of the TeV
blazars Markarian (Mrk) 421 and Mrk 501. A spectral fit for Mrk 421 yields a
power law index and
an exponential cut-off
TeV. For Mrk 501, we find an index and exponential cut-off TeV. The light curves for both sources show clear
variability and a Bayesian analysis is applied to identify changes between flux
states. The highest per-transit fluxes observed from Mrk 421 exceed the Crab
Nebula flux by a factor of approximately five. For Mrk 501, several transits
show fluxes in excess of three times the Crab Nebula flux. In a comparison to
lower energy gamma-ray and X-ray monitoring data with comparable sampling we
cannot identify clear counterparts for the most significant flaring features
observed by HAWC.Comment: 18 pages, 10 figures, accepted for publication in The Astrophysical
Journa
The 2HWC HAWC Observatory Gamma Ray Catalog
We present the first catalog of TeV gamma-ray sources realized with the
recently completed High Altitude Water Cherenkov Observatory (HAWC). It is the
most sensitive wide field-of-view TeV telescope currently in operation, with a
1-year survey sensitivity of ~5-10% of the flux of the Crab Nebula. With an
instantaneous field of view >1.5 sr and >90% duty cycle, it continuously
surveys and monitors the sky for gamma ray energies between hundreds GeV and
tens of TeV.
HAWC is located in Mexico at a latitude of 19 degree North and was completed
in March 2015. Here, we present the 2HWC catalog, which is the result of the
first source search realized with the complete HAWC detector. Realized with 507
days of data and represents the most sensitive TeV survey to date for such a
large fraction of the sky. A total of 39 sources were detected, with an
expected contamination of 0.5 due to background fluctuation. Out of these
sources, 16 are more than one degree away from any previously reported TeV
source. The source list, including the position measurement, spectrum
measurement, and uncertainties, is reported. Seven of the detected sources may
be associated with pulsar wind nebulae, two with supernova remnants, two with
blazars, and the remaining 23 have no firm identification yet.Comment: Submitted 2017/02/09 to the Astrophysical Journa
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