39 research outputs found
Explaining LIGO's observations via isolated binary evolution with natal kicks
We compare binary evolution models with different assumptions about
black-hole natal kicks to the first gravitational-wave observations performed
by the LIGO detectors. Our comparisons attempt to reconcile merger rate,
masses, spins, and spin-orbit misalignments of all current observations with
state-of-the-art formation scenarios of binary black holes formed in isolation.
We estimate that black holes (BHs) should receive natal kicks at birth of the
order of (50) km/s if tidal processes do (not) realign
stellar spins. Our estimate is driven by two simple factors. The natal kick
dispersion is bounded from above because large kicks disrupt too many
binaries (reducing the merger rate below the observed value). Conversely, the
natal kick distribution is bounded from below because modest kicks are needed
to produce a range of spin-orbit misalignments. A distribution of misalignments
increases our models' compatibility with LIGO's observations, if all BHs are
likely to have natal spins. Unlike related work which adopts a concrete BH
natal spin prescription, we explore a range of possible BH natal spin
distributions. Within the context of our models, for all of the choices of
used here and within the context of one simple fiducial parameterized
spin distribution, observations favor low BH natal spin.Comment: 19 pages, 14 figures, as published in PR
CMB polarization from secondary vector and tensor modes
We consider a novel contribution to the polarization of the Cosmic Microwave
Background induced by vector and tensor modes generated by the non-linear
evolution of primordial scalar perturbations. Our calculation is based on
relativistic second-order perturbation theory and allows to estimate the
effects of these secondary modes on the polarization angular power-spectra. We
show that a non-vanishing B-mode polarization unavoidably arises from pure
scalar initial perturbations, thus limiting our ability to detect the signature
of primordial gravitational waves generated during inflation. This secondary
effect dominates over that of primordial tensors for an inflationary
tensor-to-scalar ratio . The magnitude of the effect is smaller than
the contamination produced by the conversion of polarization of type E into
type B, by weak gravitational lensing. However the lensing signal can be
cleaned, making the secondary modes discussed here the actual background
limiting the detection of small amplitude primordial gravitational waves.Comment: 14 pages, 3 figures, minor changes matching the version to be
published in Phys. Rev.
A Lensing Reconstruction of Primordial Cosmic Microwave Background Polarization
We discuss a possibility to directly reconstruct the CMB polarization field
at the last scattering surface by accounting for modifications imposed by the
gravitational lensing effect. The suggested method requires a tracer field of
the large scale structure lensing potentials that deflected propagating CMB
photons from the last scattering surface. This required information can come
from a variety of observations on the large scale structure matter
distribution, including convergence reconstructed from lensing shear studies
involving galaxy shapes. In the case of so-called curl, or B,-modes of CMB
polarization, the reconstruction allows one to identify the distinct signature
of inflationary gravitational waves.Comment: 6 pages, 2 figures; PRD submitte
A fully relativistic radial fall
Radial fall has historically played a momentous role. It is one of the most
classical problems, the solutions of which represent the level of understanding
of gravitation in a given epoch. A {\it gedankenexperiment} in a modern frame
is given by a small body, like a compact star or a solar mass black hole,
captured by a supermassive black hole. The mass of the small body itself and
the emission of gravitational radiation cause the departure from the geodesic
path due to the back-action, that is the self-force. For radial fall, as any
other non-adiabatic motion, the instantaneous identity of the radiated energy
and the loss of orbital energy cannot be imposed and provide the perturbed
trajectory. In the first part of this letter, we present the effects due to the
self-force computed on the geodesic trajectory in the background field.
Compared to the latter trajectory, in the Regge-Wheeler, harmonic and all
others smoothly related gauges, a far observer concludes that the self-force
pushes inward (not outward) the falling body, with a strength proportional to
the mass of the small body for a given large mass; further, the same observer
notes an higher value of the maximal coordinate velocity, this value being
reached earlier on during infall. In the second part of this letter, we
implement a self-consistent approach for which the trajectory is iteratively
corrected by the self-force, this time computed on osculating geodesics.
Finally, we compare the motion driven by the self-force without and with
self-consistent orbital evolution. Subtle differences are noticeable, even if
self-force effects have hardly the time to accumulate in such a short orbit.Comment: To appear in Int. J. Geom. Meth. Mod. Phy
Weak Lensing of the CMB: Cumulants of the Probability Distribution Function
We discuss the real-space moments of temperature anisotropies in the cosmic
microwave background (CMB) due to weak gravitational lensing by intervening
large-scale structure. We show that if the probability distribution function of
primordial temperature anisotropies is Gaussian, then it remains unchanged
after gravitational lensing. With finite resolution, however, non-zero
higher-order cumulants are generated both by lensing autocorrelations and by
cross-correlations between the lensing potential and secondary anisotropies in
the CMB such as the Sunayev-Zel'dovich (SZ) effect. Skewness is produced by
these lensing-SZ correlations, while kurtosis receives contributions from both
lensing alone and lensing-SZ correlations. We show that if the projected
lensing potential is Gaussian, all cumulants of higher-order than the kurtosis
vanish. While recent results raise the possibility of detection of the skewness
in upcoming data, the kurtosis will likely remain undetected.Comment: 11 pages, 4 figures, submitted to PR
MYRIAD: A new N-body code for simulations of Star Clusters
We present a new C++ code for collisional N-body simulations of star
clusters. The code uses the Hermite fourth-order scheme with block time steps,
for advancing the particles in time, while the forces and neighboring particles
are computed using the GRAPE-6 board. Special treatment is used for close
encounters, binary and multiple sub-systems that either form dynamically or
exist in the initial configuration. The structure of the code is modular and
allows the appropriate treatment of more physical phenomena, such as stellar
and binary evolution, stellar collisions and evolution of close black-hole
binaries. Moreover, it can be easily modified so that the part of the code that
uses GRAPE-6, could be replaced by another module that uses other
accelerating-hardware like the Graphics Processing Units (GPUs). Appropriate
choice of the free parameters give a good accuracy and speed for simulations of
star clusters up to and beyond core collapse. Simulations of Plummer models
consisting of equal-mass stars reached core collapse at t~17 half-mass
relaxation times, which compares very well with existing results, while the
cumulative relative error in the energy remained below 0.001. Also, comparisons
with published results of other codes for the time of core collapse for
different initial conditions, show excellent agreement. Simulations of King
models with an initial mass-function, similar to those found in the literature,
reached core collapse at t~0.17, which is slightly smaller than the expected
result from previous works. Finally, the code accuracy becomes comparable and
even better than the accuracy of existing codes, when a number of close binary
systems is dynamically created in a simulation. This is due to the high
accuracy of the method that is used for close binary and multiple sub-systems.Comment: 24 pages, 29 figures, accepted for publication to Astronomy &
Astrophysic
The Abell Cluster A586 and the Detection of the Equivalence Principle
We discuss the current bounds on the Equivalence Principle, in particular
from structure formation and, reexamine in this context, the recent claim on
the evidence of the interaction between dark matter and dark energy in the
Abell Cluster A586 and the ensued violation of the Equivalence Principle.Comment: 9 pages, 2 Figures. GRG forma
Scientific optimization of a ground-based CMB polarization experiment
We investigate the science goals achievable with the upcoming generation of
ground-based Cosmic Microwave Background polarization experiments and calculate
the optimal sky coverage for such an experiment including the effects of
foregrounds. We find that with current technology an E-mode measurement will be
sample-limited, while a B-mode measurement will be detector-noise-limited. We
conclude that a 300 sq deg survey is an optimal compromise for a two-year
experiment to measure both E and B-modes, and that ground-based polarization
experiments can make an important contribution to B-mode surveys. Focusing on
one particular experiment, QUaD, a proposed bolometric polarimeter operating
from the South Pole, we find that a ground-based experiment can make a high
significance measurement of the acoustic peaks in the E-mode spectrum, and will
be able to detect the gravitational lensing signal in the B-mode spectrum. Such
an experiment could also directly detect the gravitational wave component of
the B-mode spectrum if the amplitude of the signal is close to current upper
limits. We also investigate how a ground-based experiment can improve
constraints on the cosmological parameters. We estimate that by combining two
years of QUaD data with the four-year WMAP data, an optimized ground-based
polarization experiment can improve constraints on cosmological parameters by a
factor of two. If the foreground contamination can be reduced, the measurement
of the tensor-to-scalar ratio can be improved by up to a factor of six over
that obtainable from WMAP alone.Comment: 17 pages, 11 figures replaced with version accepted by MNRA
Cross-Correlation of the Cosmic Microwave Background with the 2MASS Galaxy Survey: Signatures of Dark Energy, Hot Gas, and Point Sources
We cross-correlate the Cosmic Microwave Background (CMB) temperature
anisotropies observed by the Wilkinson Microwave Anisotropy Probe (WMAP) with
the projected distribution of extended sources in the Two Micron All Sky Survey
(2MASS). By modelling the theoretical expectation for this signal, we extract
the signatures of dark energy (Integrated Sachs-Wolfe effect;ISW), hot gas
(thermal Sunyaev-Zeldovich effect;thermal SZ), and microwave point sources in
the cross-correlation. Our strongest signal is the thermal SZ, at the 3.1-3.7
\sigma level, which is consistent with the theoretical prediction based on
observations of X-ray clusters. We also see the ISW signal at the 2.5 \sigma
level, which is consistent with the expected value for the concordance LCDM
cosmology, and is an independent signature of the presence of dark energy in
the universe. Finally, we see the signature of microwave point sources at the
2.7 \sigma level.Comment: 35 pages (preprint format), 8 figures. In addition to minor revisions
based on referee's comments, after correcting for a bug in the code, the SZ
detection is consistent with the X-ray observations. Accepeted for
publication in Physical Review