76 research outputs found
Radiative stability and observational constraints on dark energy and modified gravity
Radiative stability places strong constraints on general dark energy and
modified gravity theories. We consider Horndeski scalar-tensor theories with
luminally propagating gravitational waves (as extensively discussed in the wake
of GW170817) and show that generically there is a tension between obtaining
observable deviations from General Relativity (GR) in cosmology and the
requirement of radiative stability. Using this as a constraint, we discuss the
subsets of theories that are capable of yielding observable, radiatively stable
departures from GR. A key consequence are significantly tightened cosmological
parameter constraints on dark energy and modified gravity parameters, which we
explicitly compute using data from the Planck, SDSS/BOSS and 6dF surveys.Comment: 7 pages, 2 figure
Cosmological parameter constraints for Horndeski scalar-tensor gravity
We present new cosmological parameter constraints for general Horndeski
scalar-tensor theories, using CMB, redshift space distortion, matter power
spectrum and BAO measurements from the Planck, SDSS/BOSS and 6dF surveys. We
focus on theories with cosmological gravitational waves propagating at the
speed of light, , implementing and discussing several
previously unaccounted for aspects in the constraint derivation for such
theories, that qualitatively affect the resulting constraints. In order to
ensure our conclusions are robust, we compare results for three different
parametrisations of the free functions in Horndeski scalar-tensor theories,
identifying several parametrisation-independent features of the constraints. We
also consider models, where in cosmological settings (still
allowed after GW170817 for frequency-dependent ) and show how this
affects cosmological parameter constraints.Comment: 30 pages, 9 figures, 3 table
Integrated cosmological probes: Concordance quantified
Assessing the consistency of parameter constraints derived from different
cosmological probes is an important way to test the validity of the underlying
cosmological model. In an earlier work [Nicola et al., 2017], we computed
constraints on cosmological parameters for CDM from an integrated
analysis of CMB temperature anisotropies and CMB lensing from Planck, galaxy
clustering and weak lensing from SDSS, weak lensing from DES SV as well as Type
Ia supernovae and Hubble parameter measurements. In this work, we extend this
analysis and quantify the concordance between the derived constraints and those
derived by the Planck Collaboration as well as WMAP9, SPT and ACT. As a measure
for consistency, we use the Surprise statistic [Seehars et al., 2014], which is
based on the relative entropy. In the framework of a flat CDM
cosmological model, we find all data sets to be consistent with one another at
a level of less than 1. We highlight that the relative entropy is
sensitive to inconsistencies in the models that are used in different parts of
the analysis. In particular, inconsistent assumptions for the neutrino mass
break its invariance on the parameter choice. When consistent model assumptions
are used, the data sets considered in this work all agree with each other and
CDM, without evidence for tensions.Comment: 17 pages, 4 figures, 2 tables, updated following referee's comments,
now includes discussion of the Riess et al., 2016 Hubble parameter
measurement, matches version accepted by JCA
Integrated approach to cosmology: Combining CMB, large-scale structure and weak lensing
Recent observational progress has led to the establishment of the standard
CDM model for cosmology. This development is based on different
cosmological probes that are usually combined through their likelihoods at the
latest stage in the analysis. We implement here an integrated scheme for
cosmological probes, which are combined in a common framework starting at the
map level. This treatment is necessary as the probes are generally derived from
overlapping maps and are thus not independent. It also allows for a thorough
test of the cosmological model and of systematics through the consistency of
different physical tracers. As a first application, we combine current
measurements of the Cosmic Microwave Background (CMB) from the Planck
satellite, and galaxy clustering and weak lensing from SDSS. We consider the
spherical harmonic power spectra of these probes including all six auto- and
cross-correlations along with the associated full Gaussian covariance matrix.
This provides an integrated treatment of different analyses usually performed
separately including CMB anisotropies, cosmic shear, galaxy clustering,
galaxy-galaxy lensing and the Integrated Sachs-Wolfe (ISW) effect with galaxy
and shear tracers. We derive constraints on CDM parameters that are
compatible with existing constraints and highlight tensions between data sets,
which become apparent in this integrated treatment. We discuss how this
approach provides a complete and powerful integrated framework for probe
combination and how it can be extended to include other tracers in the context
of current and future wide field cosmological surveys.Comment: 29 pages, 19 figures, 3 tables, to appear in PRD, updated following
referee's comments including small changes in result
Consistency tests in cosmology using relative entropy
With the high-precision data from current and upcoming experiments, it
becomes increasingly important to perform consistency tests of the standard
cosmological model. In this work, we focus on consistency measures between
different data sets and methods that allow us to assess the goodness of fit of
different models. We address both of these questions using the relative entropy
or Kullback-Leibler (KL) divergence [Kullback et al., 1951]. First, we revisit
the relative entropy as a consistency measure between data sets and further
investigate some of its key properties, such as asymmetry and path dependence.
We then introduce a novel model rejection framework, which is based on the
relative entropy and the posterior predictive distribution. We validate the
method on several toy models and apply it to Type Ia supernovae data from the
JLA and CMB constraints from Planck 2015, testing the consistency of the data
with six different cosmological models.Comment: 31 pages, 10 figures, 4 tables, updated following referee's comments,
matches version accepted by JCA
Growing galaxies via superbubble-driven accretion flows
We use a suite of cooling halo simulations to study a new mechanism for rapid accretion of hot halo gas on to star-forming galaxies. Correlated supernova (SN) events create converging ‘superbubbles' in the halo gas. Where these collide, the density increases, driving cooling filaments of low-metallicity gas that feed the disc. At our current numerical resolution (∼20pc; mgas = 4 × 104 M⊙) we are only able to resolve the most dramatic events; however, as we increase the numerical resolution, we find that the filaments persist for longer, driving continued late-time star formation. This suggests that SN-driven accretion could act as an efficient mechanism for extracting cold gas from the hot halo, driving late-time star formation in disc galaxies. We show that such filament feeding leads to a peak star formation rate of ∼3 M⊙ yr−1, consistent with estimates for the Milky Way (MW). The filaments we resolve extend to ∼50 kpc, reaching column densities of N ∼ 1018cm−2. We show that such structures can plausibly explain the broad dispersion in Mgii absorption seen along sightlines to quasars. Our results suggest a dual role for stellar feedback in galaxy formation, suppressing hot-mode accretion while promoting cold-mode accretion along filaments. Finally, since the filamentary gas has higher angular momentum than that coming from hot-mode accretion, we show that this leads to the formation of substantially larger gas disc
- …