1,235 research outputs found
Non-monotonic orbital velocity profiles around rapidly rotating Kerr-(anti-)de Sitter black holes
It has been recently demonstrated that the orbital velocity profile around
Kerr black holes in the equatorial plane as observed in the locally
non-rotating frame exhibits a non-monotonic radial behaviour. We show here that
this unexpected minimum-maximum feature of the orbital velocity remains if the
Kerr vacuum is generalized to the Kerr-de Sitter or Kerr-anti-de Sitter metric.
This is a new general relativity effect in Kerr spacetimes with non-vanishing
cosmological constant. Assuming that the profile of the orbital velocity is
known, this effect constrains the spacetime parameters.Comment: 9 pages, 4 figures, accepted for Class. Quant. Gra
Dark Viscous Fluid coupled with Dark Matter and future singularity
We study effects of viscous fluid coupled with dark matter in our universe.
We consider bulk viscosity in the cosmic fluid and we suppose the existence of
a coupling between fluid and dark matter, in order to reproduce a stable de
Sitter universe protected against future-time singularities. More general
inhomogeneous fluids are studied related to future singularities.Comment: 11 page
Cosmologies with variable parameters and dynamical cosmon: implications on the cosmic coincidence problem
Dynamical dark energy (DE) has been proposed to explain various aspects of
the cosmological constant (CC) problem(s). For example, it is very difficult to
accept that a strictly constant Lambda-term constitutes the ultimate
explanation for the DE in our Universe. It is also hard to acquiesce in the
idea that we accidentally happen to live in an epoch where the CC contributes
an energy density value right in the ballpark of the rapidly diluting matter
density. It should perhaps be more plausible to conceive that the vacuum
energy, is actually a dynamical quantity as the Universe itself. More
generally, we could even entertain the possibility that the total DE is in fact
a mixture of vacuum energy and other dynamical components (e.g. fields, higher
order terms in the effective action etc) which can be represented collectively
by an effective entity X (dubbed the ``cosmon''). The ``cosmon'', therefore,
acts as a dynamical DE component different from the vacuum energy. While it can
actually behave phantom-like by itself, the overall DE fluid may effectively
appear as standard quintessence, or even mimic at present an almost exact CC
behavior. Thanks to the versatility of such cosmic fluid we can show that a
composite DE system of this sort (``LXCDM'') may have a key to resolving the
mysterious coincidence problem.Comment: LaTeX, 13 pages, 5 figure
Massive Black Holes in Dwarf Spheroidal Galaxy Haloes?
It is now established that several of the Local Group dwarf Spheroidal
galaxies (dSphs) have large mass-to-light ratios. We consider the possibility
that the dark matter in the halos of dSphs is composed of massive black holes
with masses in the range 10^5 to 10^7 solar masses. We use direct N-body
simulations to determine the long term evolution of a two-component dSph
composed of a pressure-supported stellar population in a black hole halo. The
black holes are initially distributed according to a Navarro, Frenk & White
profile. For black hole masses between 10^5 and 10^6 solar masses, the dark
matter halo evolves towards a shallower inner profile in less than a Hubble
time. This suggests that black holes in this mass range might provide an
explanation for the origin of the dark matter cores inferred from observations
of Low Surface Brightness galaxy rotation curves. We compare the simulated
evolution of the stellar population with observed data for the Draco dSph. The
dependence of the heating rate on the black hole mass is determined, and an
upper limit of 10^5 solar masses is placed on the individual black holes
comprising the dark matter halo of Draco, if its present stellar distribution
is representative of the initial one. We also present a simple scaling argument
which demonstrates that the dynamical heating of an initially compact, though
not self-gravitating, stellar distribution might produce a remnant distribution
similar in extent to Draco after 10 Gyr, even for black hole masses somewhat in
excess of 10^5 solar masses.Comment: 15 pages, 13 figures; accepted for publication in MNRAS. With
augmented discussion, and revised to match accepted versio
Constraining the evolutionary history of Newton's constant with gravitational wave observations
Space-borne gravitational wave detectors, such as the proposed Laser
Interferometer Space Antenna, are expected to observe black hole coalescences
to high redshift and with large signal-to-noise ratios, rendering their
gravitational waves ideal probes of fundamental physics. The promotion of
Newton's constant to a time-function introduces modifications to the binary's
binding energy and the gravitational wave luminosity, leading to corrections in
the chirping frequency. Such corrections propagate into the response function
and, given a gravitational wave observation, they allow for constraints on the
first time-derivative of Newton's constant at the time of merger. We find that
space-borne detectors could indeed place interesting constraints on this
quantity as a function of sky position and redshift, providing a
{\emph{constraint map}} over the entire range of redshifts where binary black
hole mergers are expected to occur. A LISA observation of an equal-mass
inspiral event with total redshifted mass of 10^5 solar masses for three years
should be able to measure at the time of merger to better than
10^(-11)/yr.Comment: 11 pages, 2 figures, replaced with version accepted for publication
in Phys. Rev. D
A New Cosmological Model of Quintessence and Dark Matter
We propose a new class of quintessence models in which late times
oscillations of a scalar field give rise to an effective equation of state
which can be negative and hence drive the observed acceleration of the
universe. Our ansatz provides a unified picture of quintessence and a new form
of dark matter we call "Frustrated Cold Dark Matter" (FCDM). FCDM inhibits
gravitational clustering on small scales and could provide a natural resolution
to the core density problem for disc galaxy halos. Since the quintessence field
rolls towards a small value, constraints on slow-roll quintessence models are
safely circumvented in our model.Comment: Revised. Important new results added in response to referees comment
Planck Sunyaev-Zel'dovich Cluster Mass Calibration using Hyper Suprime-Cam Weak Lensing
Using 140 deg Subaru Hyper Suprime-Cam (HSC) survey data, we stack
the weak lensing (WL) signal around five Planck clusters found within the
footprint. This yields a 15 detection of the mean Planck cluster mass
density profile. The five Planck clusters span a relatively wide mass range,
with a mean mass of . The ratio of the stacked
Planck Sunyaev-Zel'dovich (SZ) mass to the stacked WL mass is . This mass bias is
consistent with previous WL mass calibrations of Planck clusters within the
errors. We discuss the implications of our findings for the calibration of SZ
cluster counts and the much discussed tension between Planck SZ cluster counts
and Planck CDM cosmology.Comment: 12 pages, 2 tables, 7 figures, accepted to PASJ special issu
XMM-{\em Newton} and FUSE Tentative Evidence for a WHIM filament along the Line of Sight to PKS~0558-504
We present a possible OVIII X-ray absorption line at
which, if confirmed, will be the first one associated with a broad HI Ly
(BLB: FWHM= km s) absorber. The absorber lies along the
line of sight to the nearby () Seyfert 1 galaxy PKS~0558-504,
consistent with being a WHIM filament. The X-ray absorber is marginally
detected in two independent XMM-Newton spectra of PKS~0558-504, a long ks Guest-Observer observation and a shorter, ks total,
calibration observation, with a combined single line statistical significance
of 2.8 (2.7 and 1.2 in the two spectra, respectively).
When fitted with our self-consistent hybrid-photoionization WHIM models, the
combined XMM-{\em Newton} spectrum is consistent with the presence of OVIII
K at . This model gives best fitting temperature
and equivalent H column density of the absorber of log
K, and logN cm. The
statistical sigificance of this single X-ray detection is increased by the
detection of broad and complex HI Ly absorption in archival FUSE spectra
of PKS~0558-504, at redshifts consistent with the
best-fitting redshift of the X-ray absorber. The single line statistical
significance of this line is 4.1 (3.7 if systematics are
considered), and thus the combined (HI+OVIII) statistical significance of the
detection is of 5.0.
The detection of both metal and H lines at a consistent redshift, in this hot
absorbing system, allows us to speculate on its metallicity. By associating the
bulk of the X-ray absorber with the BLB line detected in the FUSE spectrum at
, we obtain a metallicity of 1-4\% Solar.Comment: 37 pages, 12 figures, 3 Tables. Accepted for publication by the ApJ
Dark matter from SU(4) model
The left-right symmetric Pati-Salam model of the unification of quarks and
leptons is based on SU(4) and SU(2)xSU(2) groups. These groups are naturally
extended to include the classification of families of quarks and leptons. We
assume that the family group (the group which unites the families) is also the
SU(4) group. The properties of the 4-th generation of fermions are the same as
that of the ordinary-matter fermions in first three generations except for the
family charge of the SU(4)_F group: F=(1/3,1/3,1/3,-1), where F=1/3 for
fermions of ordinary matter and F=-1 for the 4-th generation. The difference in
F does not allow the mixing between ordinary and fourth-generation fermions.
Because of the conservation of the F charge, the creation of baryons and
leptons in the process of electroweak baryogenesis must be accompanied by the
creation of fermions of the 4-th generation. As a result the excess n_B of
baryons over antibaryons leads to the excess n_{\nu 4}=N-\bar N=n_B of
neutrinos over antineutrinos in the 4-th generation. This massive
fourth-generation neutrino may form the non-baryonic dark matter. In principle
their mass density n_{\nu 4}m_N in the Universe can give the main contribution
to the dark matter, since the lower bound on neutrino mass m_N from the data on
decay of the Z-bosons is m_N > m_Z/2. The straightforward prediction of this
model leads to the amount of cold dark matter relative to baryons, which is an
order of magnitude bigger than allowed by observations. This inconsistency may
be avoided by non-conservation of the F-charge.Comment: 9 pages, 2 figures, version accepted in JETP Letters, corrected after
referee reports, references are adde
Testing models of inflation with CMB non-gaussianity
Two different predictions for the primordial curvature fluctuation bispectrum
are compared through their effects on the Cosmic Microwave Background
temperature fluctuations. The first has a local form described by a single
parameter f_{NL}. The second is based on a prediction from the warm
inflationary scenario, with a different dependence on wavenumber and a
parameter f_{WI}. New expressions are obtained for the angular bispectra of the
temperature fluctuations and for the estimators used to determine and
f_{WI}. The standard deviation of the estimators in an ideal experiment is
roughly 5 times larger for f_{WI} than for f_{NL}. Using 3 year WMAP data gives
limits -375<f_{WI}<36.8, but there is a possibility of detecting a signal for
f_{WI} from the Planck satellite.Comment: 13 pages, 5 figures in ReVTe
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