23 research outputs found
The impact of anisotropic sky-sampling on the Hubble constant in numerical relativity
We study the impact of nearby inhomogeneities on an observer's inference of
the Hubble constant. Large-scale structures induce a dependence of cosmological
parameters on observer position as well as an anisotropic variance of those
parameters across an observer's sky. While the former has been explored quite
thoroughly, the latter has not. Incomplete sampling of an anisotropic sky could
introduce a bias in our cosmological inference if we assume an isotropic
expansion law. In this work, we use numerical relativity simulations of
large-scale structure combined with ray tracing to produce synthetic catalogs
mimicking the low-redshift Pantheon supernova dataset. Our data contains all
general-relativistic contributions to fluctuations in the distances and
redshifts along geodesics in the simulation. We use these synthetic
observations to constrain for a set of randomly-positioned observers. We
study both the dependence on observer position as well as the impact of
rotating the sample of supernovae on the observer's sky. We find a 1--2\%
variance in between observers when they use an isotropic sample of
objects. However, we find the inferred value of can vary by up to 4--6\%
when observers simply rotate their Pantheon data set on the sky. While the
variances we find are below the level of the ``Hubble tension'', our results
may suggest a reduction in the significance of the tension if anisotropy of
expansion can be correctly accounted for.Comment: 10 pages, 7 figures, prepared for submission to ApJ, comments welcom
Potential signature of a quadrupolar Hubble expansion in Pantheon+ supernovae
The assumption of isotropy -- that the Universe looks the same in all
directions on large scales -- is fundamental to the standard cosmological
model. This model forms the building blocks of essentially all of our
cosmological knowledge to date. It is therefore critical to empirically test in
which regimes its core assumptions hold. Anisotropies in the cosmic expansion
are expected on small scales due to nonlinear structures in the late Universe,
however, the extent to which these anisotropies might impact our low-redshift
observations remains to be fully tested. In this paper, we use fully general
relativistic simulations to calculate the expected local anisotropic expansion
and identify the dominant multipoles in cosmological parameters to be the
quadrupole in the Hubble parameter and the dipole in the deceleration
parameter. We constrain these multipoles simultaneously in the new Pantheon+
supernova compilation. The fiducial analysis is done in the rest frame of the
CMB with peculiar velocity corrections. Under the fiducial range of redshifts
in the Hubble flow sample, we find a deviation from isotropy. We
constrain the eigenvalues of the quadrupole in the Hubble parameter to be
and
and place a upper limit on its amplitude of . We find no
significant dipole in the deceleration parameter, finding constraints of
. However, in the rest frame of the CMB
without corrections, we find , a
positive amplitude. We also investigate the impact of these anisotropies on the
Hubble tension. We find a maximal shift of km s Mpc in the
monopole of the Hubble parameter and conclude that local anisotropies are
unlikely to fully explain the observed tension.Comment: 12 pages, to be submitted to MNRA
Covariant transverse-traceless projection for secondary gravitational waves
Second-order tensor modes induced by nonlinear gravity are a key component of
the cosmological background of gravitational waves. A detection of this
background would allow us to probe the primordial power spectrum at otherwise
inaccessible scales. Usually, the energy density of these gravitational waves
is studied within perturbation theory in a particular gauge -- a connection
between our physical spacetime and a fictitious background. It is a widely
recognized issue that the second-order, scalar-induced gravitational waves are
gauge dependent. This issue arises because they are not well-defined as tensors
in the physical spacetime at second-order and are thus unphysical. In this
paper, we propose the covariant transverse-traceless projection of the
extrinsic curvature to study cosmological gravitational waves on a spatial
hypersurface. We define a new energy density which is based purely on spacetime
tensors, independent of perturbation theory, and thus is gauge invariant by
definition. We show that, in the context of second-order perturbation theory,
this new energy density contains only propagating modes in the constant-time
hypersurface in the Newtonian gauge. We further show that we can recover the
same gravitational waves after a transformation to the synchronous gauge, so
long as we correctly identify the Newtonian hypersurface.Comment: 14 pages, 2 figure, major revisio
Inhomogeneous Cosmology using General Relativistic Smoothed Particle Hydrodynamics coupled to Numerical Relativity
We perform three-dimensional simulations of homogeneous and inhomogeneous
cosmologies via the coupling of a numerical relativity code for spacetime
evolution and smoothed particle hydrodynamics (SPH) code. Evolution of a flat
dust and radiation dominated Friedmann-Lema\^itre-Roberston-Walker (FLRW)
spacetime shows an agreement of exact solutions with residuals on the order
and respectively, even at low grid resolutions. We
demonstrate evolution of linear perturbations of density, velocity and metric
quantities to the FLRW with residuals of only compared to exact
solutions. Finally, we demonstrate the evolution of non-linear perturbations of
the metric past shell-crossing, such that dark matter halo formation is
possible. We show that numerical relativistic smoothed particle hydrodynamics
is a viable method for understanding non-linear effects in cosmology.Comment: 15 pages, 12 figures, submitted to PR
Binary open clusters in the Milky Way: photometric and spectroscopic analysis of NGC 5617 and Trumpler 22
Using photometry and high resolution spectroscopy we investigate for the first time the physical connection between the open clusters NGC 5617 and Trumpler 22. Based on new CCD photometry we report their spatial proximity and common age of ~70 Myr. Based on high resolution spectra collected using the HERMES and UCLES spectrographs on the Anglo-Australian telescope, we present radial velocities and abundances for Fe, Na, Mg, Al, Si, Ca and Ni. The measured radial velocities are -38.63 ± 2.25 km s⁻¹ for NGC 5617 and -38.46 ± 2.08 km s⁻¹ for Trumpler 22. The mean metallicity of NGC 5617 was found to be [Fe/H] = -0.18 ± 0.02 and for Trumpler 22 was found to be [Fe/H] = -0.17 ± 0.04. The two clusters share similar abundances across the other elements, indicative of a common chemical enrichment history of these clusters. Together with common motions and ages we confirm that NGC 5617 and Trumpler 22 are a primordial binary cluster pair in the Milky Way.Facultad de Ciencias Astronómicas y Geofísica
Genomic investigations of unexplained acute hepatitis in children
Since its first identification in Scotland, over 1,000 cases of unexplained paediatric hepatitis in children have been reported worldwide, including 278 cases in the UK1. Here we report an investigation of 38 cases, 66 age-matched immunocompetent controls and 21 immunocompromised comparator participants, using a combination of genomic, transcriptomic, proteomic and immunohistochemical methods. We detected high levels of adeno-associated virus 2 (AAV2) DNA in the liver, blood, plasma or stool from 27 of 28 cases. We found low levels of adenovirus (HAdV) and human herpesvirus 6B (HHV-6B) in 23 of 31 and 16 of 23, respectively, of the cases tested. By contrast, AAV2 was infrequently detected and at low titre in the blood or the liver from control children with HAdV, even when profoundly immunosuppressed. AAV2, HAdV and HHV-6 phylogeny excluded the emergence of novel strains in cases. Histological analyses of explanted livers showed enrichment for T cells and B lineage cells. Proteomic comparison of liver tissue from cases and healthy controls identified increased expression of HLA class 2, immunoglobulin variable regions and complement proteins. HAdV and AAV2 proteins were not detected in the livers. Instead, we identified AAV2 DNA complexes reflecting both HAdV-mediated and HHV-6B-mediated replication. We hypothesize that high levels of abnormal AAV2 replication products aided by HAdV and, in severe cases, HHV-6B may have triggered immune-mediated hepatic disease in genetically and immunologically predisposed children
Cosmological distances with general-relativistic ray tracing: framework and comparison to cosmographic predictions
Abstract
In this work we present the first results from a new ray-tracing tool to calculate
cosmological distances in the context of fully nonlinear general relativity. We use this tool to
study the ability of the general cosmographic representation of luminosity distance, as truncated
at third order in redshift, to accurately capture anisotropies in the “true” luminosity
distance. We use numerical relativity simulations of cosmological large-scale structure formation
which are free from common simplifying assumptions in cosmology. We find the general, third-order
cosmography is accurate to within 1% for redshifts to z ≈ 0.034 when sampling scales
strictly above 100 h
-1 Mpc, which is in agreement with an earlier prediction. We find the
inclusion of small-scale structure generally spoils the ability of the third-order cosmography to
accurately reproduce the full luminosity distance for wide redshift intervals, as might be
expected. For a simulation sampling small-scale structures, we find a ∼ ±5% variance in
the monopole of the ray-traced luminosity distance at z ≈ 0.02. Further, all 25 observers
we study here see a 9–20% variance in the luminosity distance across their sky at z ≈
0.03, which reduces to 2–5% by z ≈ 0.1. These calculations are based on simulations and
ray tracing which adopt fully nonlinear general relativity, and highlight the potential importance
of fair sky-sampling in low-redshift isotropic cosmological analysis.</jats:p
A prediction for anisotropies in the nearby Hubble flow
International audienceWe assess the dominant low-redshift anisotropic signatures in the distance-redshift relation and redshift drift signals. We adopt general-relativistic irrotational dust models allowing for gravitational radiation — the `quiet universe models' — which are extensions of the silent universe models. Using cosmological simulations evolved with numerical relativity, we confirm that the quiet universe model is a good description on scales larger than those of collapsing structures. With this result, we reduce the number of degrees of freedom in the fully general luminosity distance and redshift drift cosmographies by a factor of ∼ 2 and ∼ 2.5, respectively, for the most simplified case. We predict a dominant dipolar signature in the distance-redshift relation for low-redshift data, with direction along the gradient of the large-scale density field. Further, we predict a dominant quadrupole in the anisotropy of the redshift drift signal, which is sourced by the electric Weyl curvature tensor. The signals we predict in this work should be tested with present and near-future cosmological surveys
Luminosity distance and anisotropic sky-sampling at low redshifts: A numerical relativity study
International audienceMost cosmological data analysis today relies on the Friedmann-Lemaitre-Robertson-Walker (FLRW) metric, providing the basis of the current standard cosmological model. Within this framework, interesting tensions between our increasingly precise data and theoretical predictions are coming to light. It is therefore reasonable to explore the potential for cosmological analysis outside of the exact FLRW cosmological framework. In this work we adopt the general luminosity-distance series expansion in redshift with no assumptions of homogeneity or isotropy. This framework will allow for a full model-independent analysis of near-future low-redshift cosmological surveys. We calculate the effective observational 'Hubble', 'deceleration', 'curvature' and 'jerk' parameters of the luminosity-distance series expansion in numerical relativity simulations of realistic structure formation, for observers located in different environments and with different levels of sky-coverage. With a 'fairly-sampled' sky, we find 0.6% and 4% cosmic variance in the 'Hubble' and 'deceleration' parameters for scales of 200 Mpc/h (corresponding to density contrasts of ~0.05 in the simulated model universe), respectively. On top of this, we find that typical observers measure maximal sky-variance of 2% and 120% in the same parameters, as compared to their analogies in the large scale FLRW model. Our work suggests the inclusion of low-redshift anisotropy in cosmological analysis could be important for drawing correct conclusions about our Universe