138 research outputs found
How isotropic is the Universe?
A fundamental assumption in the standard model of cosmology is that the
Universe is isotropic on large scales. Breaking this assumption leads to a set
of solutions to Einstein's field equations, known as Bianchi cosmologies, only
a subset of which have ever been tested against data. For the first time, we
consider all degrees of freedom in these solutions to conduct a general test of
isotropy using cosmic microwave background temperature and polarization data
from Planck. For the vector mode (associated with vorticity), we obtain a limit
on the anisotropic expansion of (95%
CI), which is an order of magnitude tighter than previous Planck results that
used CMB temperature only. We also place upper limits on other modes of
anisotropic expansion, with the weakest limit arising from the regular tensor
mode, (95% CI). Including all
degrees of freedom simultaneously for the first time, anisotropic expansion of
the Universe is strongly disfavoured, with odds of 121,000:1 against.Comment: 6 pages, 1 figure, v2: replaced with version accepted by PR
Spin-SILC: CMB polarisation component separation with spin wavelets
We present Spin-SILC, a new foreground component separation method that
accurately extracts the cosmic microwave background (CMB) polarisation and
modes from raw multifrequency Stokes and measurements of the
microwave sky. Spin-SILC is an internal linear combination method that uses
spin wavelets to analyse the spin-2 polarisation signal . The
wavelets are additionally directional (non-axisymmetric). This allows different
morphologies of signals to be separated and therefore the cleaning algorithm is
localised using an additional domain of information. The advantage of spin
wavelets over standard scalar wavelets is to simultaneously and
self-consistently probe scales and directions in the polarisation signal and in the underlying and modes, therefore providing the ability
to perform component separation and - decomposition concurrently for the
first time. We test Spin-SILC on full-mission Planck simulations and data and
show the capacity to correctly recover the underlying cosmological and
modes. We also demonstrate a strong consistency of our CMB maps with those
derived from existing component separation methods. Spin-SILC can be combined
with the pseudo- and pure - spin wavelet estimators presented in a
companion paper to reliably extract the cosmological signal in the presence of
complicated sky cuts and noise. Therefore, it will provide a
computationally-efficient method to accurately extract the CMB and
modes for future polarisation experiments.Comment: 13 pages, 9 figures. Minor changes to match version published in
MNRAS. Map products available at http://www.silc-cmb.org. Companion paper:
arXiv:1605.01414 "Wavelet reconstruction of pure E and B modes for CMB
polarisation and cosmic shear analyses" (B. Leistedt et al.
SILC: a new Planck Internal Linear Combination CMB temperature map using directional wavelets
We present new clean maps of the CMB temperature anisotropies (as measured by
Planck) constructed with a novel internal linear combination (ILC) algorithm
using directional, scale-discretised wavelets --- Scale-discretised,
directional wavelet ILC or SILC. Directional wavelets, when convolved with
signals on the sphere, can separate the anisotropic filamentary structures
which are characteristic of both the CMB and foregrounds. Extending previous
component separation methods, which use the frequency, spatial and harmonic
signatures of foregrounds to separate them from the cosmological background
signal, SILC can additionally use morphological information in the foregrounds
and CMB to better localise the cleaning algorithm. We test the method on Planck
data and simulations, demonstrating consistency with existing component
separation algorithms, and discuss how to optimise the use of morphological
information by varying the number of directional wavelets as a function of
spatial scale. We find that combining the use of directional and axisymmetric
wavelets depending on scale could yield higher quality CMB temperature maps.
Our results set the stage for the application of SILC to polarisation
anisotropies through an extension to spin wavelets.Comment: 15 pages, 13 figures. Minor changes to match version published in
MNRAS. Map products available at http://www.silc-cmb.or
A framework for testing isotropy with the cosmic microwave background
We present a new framework for testing the isotropy of the Universe using
cosmic microwave background data, building on the nested-sampling ANICOSMO
code. Uniquely, we are able to constrain the scalar, vector and tensor degrees
of freedom alike; previous studies only considered the vector mode (linked to
vorticity). We employ Bianchi type VII cosmologies to model the anisotropic
Universe, from which other types may be obtained by taking suitable limits. In
a separate development, we improve the statistical analysis by including the
effect of Bianchi power in the high-, as well as the low-,
likelihood. To understand the effect of all these changes, we apply our new
techniques to WMAP data. We find no evidence for anisotropy, constraining shear
in the vector mode to (95% CL). For the
first time, we place limits on the tensor mode; unlike other modes, the tensor
shear can grow from a near-isotropic early Universe. The limit on this type of
shear is (95% CL).Comment: 11 pages, 6 figures, v3: minor modifications to match version
accepted by MNRA
EDGE: The origin of scatter in ultra-faint dwarf stellar masses and surface brightnesses
We demonstrate how the least luminous galaxies in the Universe, ultra-faint
dwarf galaxies, are sensitive to their dynamical mass at the time of cosmic
reionization. We select a low-mass () dark matter halo from a cosmological volume, and perform
zoom hydrodynamical simulations with multiple alternative histories using
"genetically modified" initial conditions. Earlier forming ultra-faints have
higher stellar mass today, due to a longer period of star formation before
their quenching by reionization. Our histories all converge to the same final
dynamical mass, demonstrating the existence of extended scatter ( 1 dex)
in stellar masses at fixed halo mass due to the diversity of possible
histories. One of our variants builds less than 2 % of its final dynamical mass
before reionization, rapidly quenching in-situ star formation. The bulk of its
final stellar mass is later grown by dry mergers, depositing stars in the
galaxy's outskirts and hence expanding its effective radius. This mechanism
constitutes a new formation scenario for highly diffuse (, ), metal-poor (), ultra-faint ()
dwarf galaxies within the reach of next-generation low surface brightness
surveys.Comment: Minor edits to match the published ApJL version. Results unchange
Massive neutrinos and degeneracies in Lyman-alpha forest simulations
Using a suite of hydrodynamical simulations with cold dark matter, baryons,
and neutrinos, we present a detailed study of the effect of massive neutrinos
on the 1-D and 3-D flux power spectra of the Lyman- (Ly)
forest. The presence of massive neutrinos in cosmology induces a scale- and
time-dependent suppression of structure formation that is strongest on small
scales. Measuring this suppression is a key method for inferring neutrino
masses from cosmological data, and is one of the main goals of ongoing and
future surveys like eBOSS, DES, LSST, Euclid or DESI. The clustering in the
Ly forest traces the quasi-linear power at late times and on small
scales. In combination with observations of the cosmic microwave background,
the forest therefore provides some of the tightest constraints on the sum of
the neutrino masses. However there is a well-known degeneracy between and the amplitude of perturbations in the linear matter power
spectrum. We study the corresponding degeneracy in the 1-D flux power spectrum
of the Ly forest, and for the first time also study this degeneracy in
the 3-D flux power spectrum. We show that the non-linear effects of massive
neutrinos on the Ly forest, beyond the effect of linear power amplitude
suppression, are negligible, and this degeneracy persists in the Ly
forest observables to a high precision. We discuss the implications of this
degeneracy for choosing parametrisations of the Ly forest for
cosmological analysis.Comment: 18 pages, 7 figure
Inflationary perturbations in anisotropic backgrounds and their imprint on the CMB
We extend the standard theory of cosmological perturbations to homogeneous
but anisotropic universes. We present an exhaustive computation for the case of
a Bianchi I model, with a residual isotropy between two spatial dimensions,
which is undergoing complete isotropization at the onset of inflation; we also
show how the computation can be further extended to more general backgrounds.
In presence of a single inflaton field, there are three physical perturbations
(precisely as in the isotropic case), which are obtained (i) by removing gauge
and nondynamical degrees of freedom, and (ii) by finding the combinations of
the remaining modes in terms of which the quadratic action of the perturbations
is canonical. The three perturbations, which later in the isotropic regime
become a scalar mode and two tensor polarizations (gravitational wave), are
coupled to each other already at the linearized level during the anisotropic
phase. This generates nonvanishing correlations between different modes of the
CMB anisotropies, which can be particularly relevant at large scales (and,
potentially, be related to the large scale anomalies in the WMAP data). As an
example, we compute the spectrum of the perturbations in this Bianchi I
geometry, assuming that the inflaton is in a slow roll regime also in the
anisotropic phase. For this simple set-up, fixing the initial conditions for
the perturbations appears more difficult than in the standard case, and
additional assumptions seem to be needed to provide predictions for the CMB
anisotropies.Comment: 31 pages, 3 figure
EDGE: The shape of dark matter haloes in the faintest galaxies
Collisionless Dark Matter Only (DMO) structure formation simulations predict
that Dark Matter (DM) haloes are prolate in their centres and triaxial towards
their outskirts. The addition of gas condensation transforms the central DM
shape to be rounder and more oblate. It is not clear, however, whether such
shape transformations occur in `ultra-faint' dwarfs, which have extremely low
baryon fractions. We present the first study of the shape and velocity
anisotropy of ultra-faint dwarf galaxies that have gas mass fractions of
. These dwarfs are drawn from the
Engineering Dwarfs at Galaxy formation's Edge (EDGE) project, using high
resolution simulations that allow us to resolve DM halo shapes within the half
light radius (pc). We show that gas-poor ultra-faints (M; ) retain
their pristine prolate DM halo shape even when gas, star formation and feedback
are included. This could provide a new and robust test of DM models. By
contrast, gas-rich ultra-faints (M;
) become rounder and more oblate within half
light radii. Finally, we find that most of our simulated dwarfs have
significant radial velocity anisotropy that rises to at
. The one exception is a dwarf that forms a rotating
gas/stellar disc because of a planar, major merger. Such strong anisotropy
should be taken into account when building mass models of gas-poor
ultra-faints.Comment: 16 pages and 11 figures (excluding appendices), accepted by MNRA
EDGE: the puzzling ellipticity of Eridanus II's star cluster and its implications for dark matter at the heart of an ultra-faint dwarf
The Eridanus II (EriII) 'ultra-faint' dwarf has a large () and
low mass () star cluster (SC) offset from its
centre by in projection. Its size and offset are naturally
explained if EriII has a central dark matter core, but such a core may be
challenging to explain in a CDM cosmology. In this paper, we revisit
the survival and evolution of EriII's SC, focussing for the first time on its
puzzlingly large ellipticity (). We perform a suite of
960 direct -body simulations of SCs, orbiting within a range of spherical
background potentials fit to ultra-faint dwarf (UFD) galaxy simulations. We
find only two scenarios that come close to explaining EriII's SC. In the first,
EriII has a low density dark matter core (of size and
density ). In this
model, the high ellipticity of EriII's SC is set at birth, with the lack of
tidal forces in the core allowing its ellipticity to remain frozen in for long
times. In the second, EriII's SC orbits in a partial core, with its high
ellipticity owing to its imminent tidal destruction. However, this latter model
struggles to reproduce the large size of EriII's SC, and it predicts
substantial tidal tails around EriII's SC that should have already been seen in
the data. This leads us to favour the cored model. We discuss potential caveats
to these findings, and the implications of the cored model for galaxy formation
and the nature of dark matter.Comment: 16 pages, 12 figures + appendices. Published with MNRAS. Comments
welcom
- …