128 research outputs found
Cosmological implications of baryon acoustic oscillation measurements
We derive constraints on cosmological parameters and tests of dark energy models from the combination of baryon acoustic oscillation (BAO) measurements with cosmic microwave background (CMB) data and a recent reanalysis of Type Ia supernova (SN) data. In particular, we take advantage of high-precision BAO measurements from galaxy clustering and the Lyman-α forest (LyaF) in the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS). Treating the BAO scale as an uncalibrated standard ruler, BAO data alone yield a high confidence detection of dark energy; in combination with the CMB angular acoustic scale they further imply a nearly flat universe. Adding the CMB-calibrated physical scale of the sound horizon, the combination of BAO and SN data into an 'inverse distance ladder' yields a measurement of H0=67.3 ±1.1 km s-1 Mpc-1 , with 1.7% precision. This measurement assumes standard prerecombination physics but is insensitive to assumptions about dark energy or space curvature, so agreement with CMB-based estimates that assume a flat Λ CDM cosmology is an important corroboration of this minimal cosmological model. For constant dark energy (Λ ), our BAO +SN +CMB combination yields matter density Ωm=0.301 ±0.008 and curvature Ωk=-0.003 ±0.003 . When we allow more general forms of evolving dark energy, the BAO +SN +CMB parameter constraints are always consistent with flat Λ CDM values at ≈1 σ . While the overall χ2 of model fits is satisfactory, the LyaF BAO measurements are in moderate (2 - 2.5 σ ) tension with model predictions. Models with early dark energy that tracks the dominant energy component at high redshift remain consistent with our expansion history constraints, and they yield a higher H0 and lower matter clustering amplitude, improving agreement with some low redshift observations. Expansion history alone yields an upper limit on the summed mass of neutrino species, ∑mν<0.56 eV (95% confidence), improving to ∑mν<0.25 eV if we include the lensing signal in the Planck CMB power spectrum. In a flat Λ CDM model that allows extra relativistic species, our data combination yields Neff=3.43 ±0.26 ; while the LyaF BAO data prefer higher Neff when excluding galaxy BAO, the galaxy BAO alone favor Neff≈3 . When structure growth is extrapolated forward from the CMB to low redshift, standard dark energy models constrained by our data predict a level of matter clustering that is high compared to most, but not all, observational estimates
On the capture of dark matter by neutron stars
We calculate the number of dark matter particles that a neutron star
accumulates over its lifetime as it rotates around the center of a galaxy, when
the dark matter particle is a self-interacting boson but does not
self-annihilate. We take into account dark matter interactions with baryonic
matter and the time evolution of the dark matter sphere as it collapses within
the neutron star. We show that dark matter self-interactions play an important
role in the rapid accumulation of dark matter in the core of the neutron star.
We consider the possibility of determining an exclusion region of the parameter
space for dark matter mass and dark matter interaction cross sections based on
the observation of old neutron stars with strong dark matter self-interactions.
We show that for a dark matter density of GeV/cm and dark matter
mass less than approximately 10 GeV, there is a potential exclusion
region for dark matter interactions with nucleons that is three orders of
magnitude more stringent than without self-interactions. The potential
exclusion region for dark matter self-interaction cross sections is many orders
of magnitude stronger than the current Bullet Cluster limit. For example, for
high dark matter density regions, we find that for GeV when the
dark matter interaction cross section with the nucleons ranges from
cm to cm, the dark matter
self-interaction cross section limit is cm,
which is about ten orders of magnitude stronger than the Bullet Cluster limit.Comment: 12 pages, 9 figures, v2. change in treatment of dark matter collapse
in neutron star, conclusions changed; v3. minor revisions of text for
clarification, added references, v4. version accepted for publication in JCA
Revisiting cosmological bounds on sterile neutrinos
We employ state-of-the art cosmological observables including supernova surveys and BAO information to provide constraints on the mass and mixing angle of a non-resonantly produced sterile neutrino species, showing that cosmology can effectively rule out sterile neutrinos which decay between BBN and the present day. The decoupling of an additional heavy neutrino species can modify the time dependence of the Universe's expansion between BBN and recombination and, in extreme cases, lead to an additional matter-dominated period; while this could naively lead to a younger Universe with a larger Hubble parameter, it could later be compensated by the extra radiation expected in the form of neutrinos from sterile decay. However, recombination-era observables including the Cosmic Microwave Background (CMB), the shift parameter R-CMB and the sound horizon r(s) from Baryon Acoustic Oscillations (BAO) severely constrain this scenario. We self-consistently include the full time-evolution of the coupled sterile neutrino and standard model sectors in an MCMC, showing that if decay occurs after BBN, the sterile neutrino is essentially bounded by the constraint sin(2) theta less than or similar to 0.026(m(s)/eV)(-2)
Observational constraints on conformal time symmetry, missing matter and double dark energy
The current concordance model of cosmology is dominated by two mysterious
ingredients: dark matter and dark energy. In this paper, we explore the
possibility that, in fact, there exist two dark-energy components: the
cosmological constant , with equation-of-state parameter
, and a `missing matter' component with , which we
introduce here to allow the evolution of the universal scale factor as a
function of conformal time to exhibit a symmetry that relates the big bang to
the future conformal singularity, such as in Penrose's conformal cyclic
cosmology. Using recent cosmological observations, we constrain the present-day
energy density of missing matter to be . This is
consistent with the standard CDM model, but constraints on the energy
densities of all the components are considerably broadened by the introduction
of missing matter; significant relative probability exists even for
, and so the presence of a missing matter component
cannot be ruled out. As a result, a Bayesian model selection analysis only
slightly disfavours its introduction by 1.1 log-units of evidence. Foregoing
our symmetry requirement on the conformal time evolution of the universe, we
extend our analysis by allowing to be a free parameter. For this more
generic `double dark energy' model, we find and
, which is again consistent with the standard
CDM model, although once more the posterior distributions are
sufficiently broad that the existence of a second dark-energy component cannot
be ruled out. The model including the second dark energy component also has an
equivalent Bayesian evidence to CDM, within the estimation error, and
is indistinguishable according to the Jeffreys guideline.Comment: Revised version emphasising a different version of the underlying
symmetry, as published in JCA
A 6% measurement of the Hubble parameter at z~0.45 : direct evidence of the epoch of cosmic re-acceleration
MM, LP and AC acknowledge financial contributions by grants ASI/INAF I/023/12/0 and PRIN MIUR 2010-2011 "The dark Universe and the cosmic evolution of baryons: from current surveys to Euclid". RJ and LV thank the Royal Society for financial support and the ICIC at Imperial College for hospitality while this work was being completed. LV is supported by the European Research Council under the European Community's Seventh Framework Programme FP7-IDEAS-Phys.LSS 240117. Funding for this work was partially provided by the Spanish MINECO under projects AYA2014-58747-P and MDM-2014-0369 of ICCUB (Unidad de Excelencia "Maria de Maeztu") Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science.Deriving the expansion history of the Universe is a major goal of modern cosmology. To date, the most accurate measurements have been obtained with Type Ia Supernovae (SNe) and Baryon Acoustic Oscillations (BAO), providing evidence for the existence of a transition epoch at which the expansion rate changes from decelerated to accelerated. However, these results have been obtained within the framework of specific cosmological models that must be implicitly or explicitly assumed in the measurement. It is therefore crucial to obtain measurements of the accelerated expansion of the Universe independently of assumptions on cosmological models. Here we exploit the unprecedented statistics provided by the Baryon Oscillation Spectroscopic Survey (BOSS, [1-3]) Data Release 9 to provide new constraints on the Hubble parameter H(z) using the cosmic chronometers approach. We extract a sample of more than 130000 of the most massive and passively evolving galaxies, obtaining five new cosmology-independent H(z) measurements in the redshift range 0.3 < z < 0.5, with an accuracy of ~11–16% incorporating both statistical and systematic errors. Once combined, these measurements yield a 6% accuracy constraint of H(z = 0.4293) = 91.8 ± 5.3 km/s/Mpc. The new data are crucial to provide the first cosmology-independent determination of the transition redshift at high statistical significance, measuring zt = 0.4 ± 0.1, and to significantly disfavor the null hypothesis of no transition between decelerated and accelerated expansion at 99.9% confidence level. This analysis highlights the wide potential of the cosmic chronometers approach: it permits to derive constraints on the expansion history of the Universe with results competitive with standard probes, and most importantly, being the estimates independent of the cosmological model, it can constrain cosmologies beyond—and including—the ΛCDM model.PostprintPeer reviewe
PEXO : a global modeling framework for nanosecond timing, microsecond astrometry, and μm/s radial velocities
54 pages, 2 tables, 19 figures, accepted for publication in ApJS, PEXO is available at https://github.com/phillippro/pexoThe ability to make independent detections of the signatures of exoplanets with complementary telescopes and instruments brings a new potential for robust identification of exoplanets and precision characterization. We introduce PEXO, a package for Precise EXOplanetology to facilitate the efficient modeling of timing, astrometry, and radial velocity data, which will benefit not only exoplanet science but also various astrophysical studies in general. PEXO is general enough to account for binary motion and stellar reflex motions induced by planetary companions and is precise enough to treat various relativistic effects both in the solar system and in the target system. We also model the post-Newtonian barycentric motion for future tests of general relativity in extrasolar systems. We benchmark PEXO with the pulsar timing package TEMPO2 and find that PEXO produces numerically similar results with timing precision of about 1 ns, space-based astrometry to a precision of 1{\mu}as, and radial velocity of 1 {\mu}m/s and improves on TEMPO2 for decade-long timing data of nearby targets, due to its consideration of third-order terms of Roemer delay. PEXO is able to avoid the bias introduced by decoupling the target system and the solar system and to account for the atmospheric effects which set a practical limit for ground-based radial velocities close to 1 cm/s. Considering the various caveats in barycentric correction and ancillary data required to realize cm/s modeling, we recommend the preservation of original observational data. The PEXO modeling package is available at GitHub (https://github.com/phillippro/pexo).Peer reviewe
AB Aurigae::Possible evidence of planet formation through the gravitational instability
Recent observations of the protoplanetary disc surrounding AB Aurigae have
revealed the possible presence of two giant planets in the process of forming.
The young measured age of Myr for this system allows us to place strict
time constraints on the formation histories of the observed planets. Hence we
may be able to make a crucial distinction between formation through core
accretion (CA) or the gravitational instability (GI), as CA formation
timescales are typically Myrs whilst formation through GI will occur within the
first yrs of disc evolution. We focus our analysis on the
M planet observed at AU. We find CA formation
timescales for such a massive planet typically exceed the system's age. The
planet's high mass and wide orbit may instead be indicative of formation
through GI. We use smoothed particle hydrodynamic simulations to determine the
system's critical disc mass for fragmentation, finding M. Viscous evolution models of the disc's mass history
indicate that it was likely massive enough to exceed in the
recent past, thus it is possible that a young AB Aurigae disc may have
fragmented to form multiple giant gaseous protoplanets. Calculations of the
Jeans mass in an AB Aurigae-like disc find that fragments may initially form
with masses M, consistent with the planets which have
been observed. We therefore propose that the inferred planets in the disc
surrounding AB Aurigae may be evidence of planet formation through GI.Comment: 12 pages, 5 figure
KiDS-450: the tomographic weak lensing power spectrum and constraints on cosmological parameters
We present measurements of the weak gravitational lensing shear power
spectrum based on sq. deg. of imaging data from the Kilo Degree Survey.
We employ a quadratic estimator in two and three redshift bins and extract band
powers of redshift auto-correlation and cross-correlation spectra in the
multipole range . The cosmological interpretation of
the measured shear power spectra is performed in a Bayesian framework assuming
a CDM model with spatially flat geometry, while accounting for small
residual uncertainties in the shear calibration and redshift distributions as
well as marginalising over intrinsic alignments, baryon feedback and an
excess-noise power model. Moreover, massive neutrinos are included in the
modelling. The cosmological main result is expressed in terms of the parameter
combination yielding $S_8 = \
0.651 \pm 0.0583.2\sigma$ (3 z-bins). We
cross-check the results of the 3 z-bin analysis with the weaker constraints
from the 2 z-bin analysis and find them to be consistent. The high-level data
products of this analysis, such as the band power measurements, covariance
matrices, redshift distributions, and likelihood evaluation chains are
available at http://kids.strw.leidenuniv.nl/Comment: 23 pages, 18 figures, 5 tables; results unchanged, version accepted
for publication by MNRAS. Data products available at
http://kids.strw.leidenuniv.nl
The Hawaii Infrared Parallax Program. III. 2MASS J0249-0557 c:A Wide Planetary-mass Companion to a Low-mass Binary in the β Pic Moving Group
We have discovered a wide planetary-mass companion to the Pic moving
group member 2MASSJ02495639-0557352 (M6 VL-G) using CFHT/WIRCam astrometry from
the Hawaii Infrared Parallax Program. In addition, Keck laser guide star
adaptive optics aperture-masking interferometry shows that the host is itself a
tight binary. Altogether, 2MASSJ0249-0557ABc is a bound triple system with an
object separated by AU (40")
from a relatively close ( AU, 0.04") pair of
and objects. 2MASSJ0249-0557AB is
one of the few ultracool binaries to be discovered in a young moving group and
the first confirmed in the Pic moving group ( Myr). The mass,
absolute magnitudes, and spectral type of 2MASSJ0249-0557 c (L2 VL-G) are
remarkably similar to those of the planet Pic b (L2,
). We also find that the free-floating object
2MASSJ2208+2921 (L3 VL-G) is another possible Pic moving group member
with colors and absolute magnitudes similar to Pic b and
2MASSJ0249-0557 c. Pic b is the first directly imaged planet to have a
"twin," namely an object of comparable properties in the same stellar
association. Such directly imaged objects provide a unique opportunity to
measure atmospheric composition, variability, and rotation across different
pathways of assembling planetary-mass objects from the same natal material.Comment: Accepted to AJ, only change is color scheme of figure
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