1,640 research outputs found
Combined cosmological tests of a bivalent tachyonic dark energy scalar field model
A recently investigated tachyonic scalar field dark energy dominated universe
exhibits a bivalent future: depending on initial parameters can run either into
a de Sitter exponential expansion or into a traversable future soft singularity
followed by a contraction phase. We also include in the model (i) a tiny amount
of radiation, (ii) baryonic matter (, where the
Hubble constant is fixed as ) and (iii) cold dark matter (CDM). Out of
a variety of six types of evolutions arising in a more subtle classification,
we identify two in which in the past the scalar field effectively degenerates
into a dust (its pressure drops to an insignificantly low negative value).
These are the evolutions of type IIb converging to de Sitter and type III
hitting the future soft singularity. We confront these background evolutions
with various cosmological tests, including the supernova type Ia Union 2.1
data, baryon acoustic oscillation distance ratios, Hubble parameter-redshift
relation and the cosmic microwave background (CMB) acoustic scale. We determine
a subset of the evolutions of both types which at 1 confidence level
are consistent with all of these cosmological tests. At perturbative level we
derive the CMB temperature power spectrum to find the best agreement with the
Planck data for . The fit is as good as for the CDM model at high multipoles, but the power remains slightly overestimated at
low multipoles, for both types of evolutions. The rest of the CDM is
effectively generated by the tachyonic field, which in this sense acts as a
combined dark energy and dark matter model.Comment: 25 pages, 15 figure
Spinning compact binary dynamics and chameleon orbits
We analyse the conservative evolution of spinning compact binaries to second
post-Newtonian (2PN) order accuracy, with leading order spin-orbit, spin-spin
and mass quadrupole-monopole contributions included. As a main result we derive
a closed system of first order differential equations in a compact form, for a
set of dimensionless variables encompassing both orbital elements and spin
angles. These evolutions are constrained by conservation laws holding at 2PN
order. As required by the generic theory of constrained dynamical systems we
perform a consistency check and prove that the constraints are preserved by the
evolution. We apply the formalism to show the existence of chameleon orbits,
whose local, orbital parameters evolve from elliptic (in the Newtonian sense)
near pericenter, towards hyperbolic at large distances. This behavior is
consistent with the picture that General Relativity predicts stronger gravity
at short distances than Newtonian theory does.Comment: to be published in Phys. Rev. D, 19 pages, 3 figure panel
Spin-dominated waveforms for unequal mass compact binaries
We derive spin-dominated waveforms (SDW) for binary systems composed of
spinning black holes with unequal masses (less than 1:30). Such systems could
be formed by an astrophysical black hole with a smaller black hole or a neutron
star companion; and typically arise for supermassive black hole encounters. SDW
characterize the last stages of the inspiral, when the larger spin dominates
over the orbital angular momentum (while the spin of the smaller companion can
be neglected). They emerge as a double expansion in the post-Newtonian
parameter and the ratio of the orbital angular momentum
and dominant spin. The SDW amplitudes are presented to
() orders, while the phase of the gravitational waves to
() orders (omitting the highest order mixed terms). To
this accuracy the amplitude includes the (leading order) spin-orbit
contributions, while the phase the (leading order) spin-orbit, self-spin and
mass quadrupole-monopole contributions. While the SDW hold for any mass ratio
smaller than 1:30, lower bounds for the mass ratios are derived from the best
sensitivity frequency range expected for Advanced LIGO (giving 1:140), the
Einstein Telescope (), the LAGRANGE () and
LISA missions (), respectively.Comment: 14 pages, 2 figures, 5 tables, published versio
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