arXiv subjects

Adam Moss

Publications and source records attributed to Adam Moss.

At least 55 records · Page 3Linked to original sources

arXiv2017

Planck confronts large scale structure: methods to quantify discordance

Discordance in the $Λ$CDM cosmological model can be seen by comparing parameters constrained by CMB measurements to those inferred by probes of large scale structure. Recent improvements in observations, including final data releases from both Planck and SDSS-III BOSS, as well as improved astrophysical uncertainty analysis of CFHTLenS, allows for an update in the quantification of any tension between large and small scales. This paper is intended, primarily, as a discussion on the quantifications of discordance when comparing the parameter constraints of a model when given two different data sets. We consider KL-divergence, comparison of Bayesian evidences and other statistics which are sensitive to the mean, variance and shape of the distributions. However, as a by-product, we present an update to the similar analysis in (Battye, Charnock and Moss; 2015) where we find that, considering new data and treatment of priors, the constraints from the CMB and from a combination of LSS probes are in greater agreement and any tension only persists to a minor degree. In particular, we find the parameter constraints from the combination of LSS probes which are most discrepant with the Planck2015+Pol+BAO parameter distributions can be quantified at a 2.55$σ$ tension using the method introduced in (Battye, Charnock and Moss; 2015). If instead we use the distributions constrained by the combination of LSS probes which are in greatest agreement with those from Planck2015+Pol+BAO this tension is only 0.76$σ$.

astro-ph.CO↗

arXiv2016

CMB constraints on cosmic strings and superstrings

We present the first complete Markov chain Monte Carlo analysis of cosmological models with evolving cosmic (super)string networks, using the unconnected segment model in the unequal-time correlator formalism. For ordinary cosmic string networks, we derive joint constraints on Lambda cold dark matter (CDM) and string network parameters, namely the string tension Gmu, the loop-chopping efficiency c_r and the string wiggliness α. For cosmic superstrings, we obtain joint constraints on the fundamental string tension Gmu_F, the string coupling g_s, the self-interaction coefficient c_s, and the volume of compact extra dimensions w. This constitutes the most comprehensive CMB analysis of LambdaCDM cosmology + strings to date. For ordinary cosmic string networks our updated constraint on the string tension is, in relativistic units, Gmu<1.1x10^-7, while for cosmic superstrings our constraint on the fundamental string tension is Gmu_F<2.8x10^-8, both obtained using Planck2015 temperature and polarisation data.

astro-ph.CO↗

arXiv2015

Constraining dark sector perturbations I: cosmic shear and CMB lensing

We present current and future constraints on equations of state for dark sector perturbations. The equations of state considered are those corresponding to a generalized scalar field model and time-diffeomorphism invariant $L(g)$ theories that are equivalent to models of a relativistic elastic medium and also Lorentz violating massive gravity. We develop a theoretical understanding of the observable impact of these models. In order to constrain these models we use CMB temperature data from Planck, BAO measurements, CMB lensing data from Planck and the South Pole Telescope, and weak galaxy lensing data from CFHTLenS. We find non-trivial exclusions on the range of parameters, although the data remains compatible with $w=-1$. We gauge how future experiments will help to constrain the parameters. This is done via a likelihood analysis for CMB experiments such as CoRE and PRISM, and tomographic galaxy weak lensing surveys, focussing in on the potential discriminatory power of Euclid on mildly non-linear scales.

astro-ph.CO↗

arXiv2014

Nowhere to hide: closing in on cosmological homogeneity

Homogeneity is a crucial, but poorly tested, assumption in cosmology. We introduce a new approach which allows us to place limits on the presence of localized structures within essentially our entire observable volume, using cosmic microwave background secondary anisotropies. We find that structures cannot exceed roughly 20 times their expected amplitude over most of our observable volume. Similarly, we place tight constraints on statistical inhomogeneity within our volume, performing the first power spectrum reconstruction using secondary anisotropies alone. We find that the standard model passes this important new consistency test. Our approach probes homogeneity over vastly larger volumes and scales than previous studies based on surveys.

astro-ph.CO↗

arXiv2014

Tension between the power spectrum of density perturbations measured on large and small scales

There is a tension between measurements of the amplitude of the power spectrum of density perturbations inferred using the Cosmic Microwave Background (CMB) and directly measured by Large-Scale Structure (LSS) on smaller scales. We show that this tension exists, and is robust, for a range of LSS indicators including clusters, lensing and redshift space distortions and using CMB data from either $Planck$ or WMAP+SPT/ACT. One obvious way to try to reconcile this is the inclusion of a massive neutrino which could be either active or sterile. Using $Planck$ and a combination of all the LSS data we find that (i) for an active neutrino $\sum m_ν= (0.357\pm0.099)\,{\rm eV}$ and (ii) for a sterile neutrino $m_{\rm sterile}^{\rm eff}= (0.67\pm0.18)\,{\rm eV}$ and $ΔN_{\rm eff}= 0.32\pm0.20$. This is, however, at the expense of a degraded fit to $Planck$ temperature data, and we quantify the residual tension at $2.5σ$ and $1.6 σ$ for massive and sterile neutrinos respectively. We also consider alternative explanations including a lower redshift for reionization that would be in conflict with polarisation measurements made by WMAP and $ad$-$hoc$ modifications to primordial power spectrum.

astro-ph.CO↗

arXiv2014

Did BICEP2 see vector modes? First B-mode constraints on cosmic defects

Scaling networks of cosmic defects, such as strings and textures, actively generate scalar, vector and tensor metric perturbations throughout the history of the universe. In particular, {\em vector} modes sourced by defects are an efficient source of the CMB B-mode polarization. We use the recently released BICEP2 and POLARBEAR B-mode polarization spectra to constrain properties of a wide range of different types of cosmic strings networks. We find that in order for strings to provide a satisfactory fit on their own, the effective inter-string distance needs to be extremely large -- spectra that fit the data best are more representative of global strings and textures. When a local string contribution is considered together with the inflationary B-mode spectrum, the fit is improved. We discuss implications of these results for theories that predict cosmic defects.

astro-ph.CO↗

arXiv2014

Evidence for massive neutrinos from CMB and lensing observations

We discuss whether massive neutrinos (either active or sterile) can reconcile some of the tensions within cosmological data that have been brought into focus by the recently released {\it Planck} data. We point out that a discrepancy is present when comparing the primary CMB and lensing measurements both from the CMB and galaxy lensing data using CFHTLenS, similar to that which arises when comparing CMB measurements and SZ cluster counts. A consistent picture emerges and including a prior for the cluster constraints and BAOs we find that: for an active neutrino model with 3 degenerate neutrinos, $\sum m_ν= (0.320 \pm 0.081)\,{\rm eV}$, whereas for a sterile neutrino, in addition to 3 neutrinos with a standard hierarchy and $\sum m_ν= 0.06\,{\rm eV}$, $m_{ν, \, \rm sterile}^{\rm eff}= (0.450 \pm 0.124)\,{\rm eV}$ and $ΔN_{\rm eff} = 0.45 \pm 0.23$. In both cases there is a significant detection of modification to the neutrino sector from the standard model and in the case of the sterile neutrino it is possible to reconcile the BAO and local $H_0$ measurements. However, a caveat to our result is some internal tension between the CMB and lensing/cluster observations, and the masses are in excess of those estimated from the shape of the matter power spectrum from galaxy surveys.

astro-ph.CO↗

arXiv2013

Fast Analytic Computation of Cosmic String Power Spectra

We present analytic expressions for the cosmic string unequal time correlator (UETC) in the context of the Unconnected Segment Model (USM). This eliminates the need to simulate the many thousands of network realisations needed to estimate the UETC numerically. With our approach we can compute the UETC very accurately, over all scales of interest, in the order of ~20-30 seconds on a single CPU. Our formalism facilitates an efficient approach to computing Cosmic Microwave Background (CMB) anisotropies for cosmic strings. Discretising the UETC and performing an eigen-decomposition to act as sources in the CAMB CMB code, the power spectrum can be calculated by summing over a finite number of eigenmodes. A much smaller number of eigenmodes are required compared to the conventional approach of averaging power spectra over a finite number of realisations of the string network. With the additional efficiency and performance improvements offered by the OpenMP CAMB code, the time required to compute string power spectra is significantly reduced compared to the standard serial CMBACT code. The latter takes ~30 hours on a modern single threaded CPU for 2000 network realisations. Similar percent level accuracy can be achieved with our approach on a moderately threaded CPU (8 threads) in only ~15 minutes. If accuracy is only required at the 10 percent level and the CPU is more highly threaded, cosmic string power spectra are now possible in ~2-3 minutes. This makes exploration of the string parameter space now possible for Markov-Chain-Monte-Carlo (MCMC) analysis.

astro-ph.CO↗

arXiv2012

Origin of Delta N_eff as a Result of an Interaction between Dark Radiation and Dark Matter

Results from the Wilkinson Microwave Anisotropy Probe (WMAP), Atacama Cosmology Telescope (ACT) and recently from the South Pole Telescope (SPT) have indicated the possible existence of an extra radiation component in addition to the well known three neutrino species predicted by the Standard Model of particle physics. In this paper, we explore the possibility of the apparent extra dark radiation being linked directly to the physics of cold dark matter (CDM). In particular, we consider a generic scenario where dark radiation, as a result of an interaction, is produced directly by a fraction of the dark matter density effectively decaying into dark radiation. At an early epoch when the dark matter density is negligible, as an obvious consequence, the density of dark radiation is also very small. As the Universe approaches matter radiation equality, the dark matter density starts to dominate thereby increasing the content of dark radiation and changing the expansion rate of the Universe. As this increase in dark radiation content happens naturally after Big Bang Nucleosynthesis (BBN), it can relax the possible tension with lower values of radiation degrees of freedom measured from light element abundances compared to that of the CMB. We numerically confront this scenario with WMAP+ACT and WMAP+SPT data and derive an upper limit on the allowed fraction of dark matter decaying into dark radiation.

astro-ph.CO↗

arXiv2012

Dimensionless cosmology

Although it is well known that any consideration of the variations of fundamental constants should be restricted to their dimensionless combinations, the literature on variations of the gravitational constant $G$ is entirely dimensionful. To illustrate applications of this to cosmology, we explicitly give a dimensionless version of the parameters of the standard cosmological model, and describe the physics of Big Bang Neucleosynthesis and recombination in a dimensionless manner. The issue that appears to have been missed in many studies is that in cosmology the strength of gravity is bound up in the cosmological equations, and the epoch at which we live is a crucial part of the model. We argue that it is useful to consider the hypothetical situation of communicating with another civilization (with entirely different units), comparing only dimensionless constants, in order to decide if we live in a Universe governed by precisely the same physical laws. In this thought experiment, we would also have to compare epochs, which can be defined by giving the value of any {\it one} of the evolving cosmological parameters. By setting things up carefully in this way one can avoid inconsistent results when considering variable constants, caused by effectively fixing more than one parameter today. We show examples of this effect by considering microwave background anisotropies, being careful to maintain dimensionlessness throughout. We present Fisher matrix calculations to estimate how well the fine structure constants for electromagnetism and gravity can be determined with future microwave background experiments. We highlight how one can be misled by simply adding $G$ to the usual cosmological parameter set.

astro-ph.CO↗

arXiv2012

Cosmological surveys with the Australian Square Kilometre Array Pathfinder

This is a design study into the capabilities of the Australian Square Kilometre Array Pathfinder in performing a full-sky low redshift neutral hydrogen survey, termed WALLABY, and the potential cosmological constraints one can attain from measurement of the galaxy power spectrum. We find that the full sky survey will likely attain 0.6 million redshifts which, when combined with expected Planck CMB data, will constrain the Dark Energy equation of state to 20%, representing a coming of age for radio observations in creating cosmological constraints.

astro-ph.CO↗

arXiv2012

X-type and Y-type junction stability in domain wall networks

We develop an analytic formalism that allows one to quantify the stability properties of X-type and Y-type junctions in domain wall networks in two dimensions. A similar approach might be applicable to more general defect systems involving junctions that appear in a range of physical situations, for example, in the context of F- and D-type strings in string theory. We apply this formalism to a particular field theory, Carter's pentavac model, where the strength of the symmetry breaking is governed by the parameter $|ε|< 1$. We find that for low values of the symmetry breaking parameter X-type junctions will be stable, whereas for higher values an X-type junction will separate into two Y-type junctions. The critical angle separating the two regimes is given by α_c = 293^{\circ}\sqrt{|ε|}$ and this is confirmed using simple numerical experiments. We go on to simulate the pentavac model from random initial conditions and we find that the dominant junction is of \ytype for $|ε| \geq 0.02$ and is of \xtype for $|ε| \leq 0.02$. We also find that for small $ε$ the evolution of the number of domain walls $\qsubrm{N}{dw}$ in Minkowski space does not follow the standard $\propto t^{-1}$ scaling law with the deviation from the standard lore being more pronounced as $ε$ is decreased. The presence of dissipation appears to restore the $t^{-1}$ lore.

hep-th↗

arXiv2011

Constraints on the fundamental string coupling from B-mode experiments

We study signatures of cosmic superstring networks containing strings of multiple tensions and Y-junctions, on the cosmic microwave background (CMB) temperature and polarisation spectra. Focusing on the crucial role of the string coupling constant $g_s$, we show that the number density and energy density of the scaling network are dominated by different types of string in the $g_s \sim 1$ and $g_s \ll 1$ limits. This can lead to an observable shift in the position of the B-mode peak --- a distinct signal leading to a direct constraint on $g_s$. We forecast the joint bounds on $g_s$ and the fundamental string tension $μ_F$ from upcoming and future CMB polarisation experiments, as well as the signal to noise in detecting the difference between B-mode signals in the limiting cases of large and small $g_s$. We show that such a detectable shift is within reach of planned experiments.

astro-ph.CO↗

arXiv2011

Predicted Constraints on Cosmic String Tension from Planck and Future CMB Polarization Measurements

We perform a Fisher matrix calculation of the predicted uncertainties on estimates of the cosmic string tension Gmu from upcoming observational data (namely, cosmic microwave background power spectra from the Planck satellite and an idealized future polarization experiment). We employ simulations that are more general than others commonly used in the literature, leaving the mean velocity of strings, correlation length of the string network, and "wiggliness" (which parametrizes smaller-scale structure along the strings) as free parameters that can be observationally measured. In a new code, StringFast, we implement a method for efficient computation of the C_l spectra induced by a network of strings, which is fast enough to be used in Markov Chain Monte Carlo analyses of future data. Performing a calculation with the string parameters left free results in projected constraints on Gmu that are larger than those obtained by fixing their values a priori, typically by a factor of ~2-7. We also find that if Gmu is equal to the current observational maximum, Planck will be able to make a confident detection of strings. However, if Gmu is two orders of magnitude smaller, even a perfect, lensing-free measurement of polarization power spectra will not be able to detect a nonzero string tension at better than 2 sigma confidence.

astro-ph.CO↗

arXiv2011

Linear kinetic Sunyaev-Zel'dovich effect and void models for acceleration

There has been considerable recent interest in cosmological models in which the current apparent acceleration is due to a very large local underdensity, or void, instead of some form of dark energy. Here we examine a new proposal to constrain such models using the linear kinetic Sunyaev-Zel'dovich (kSZ) effect due to structure within the void. The simplified "Hubble bubble" models previously studied appeared to predict far more kSZ power than is actually observed, independently of the details of the initial conditions and evolution of perturbations in such models. We show that the constraining power of the kSZ effect is considerably weakened (though still impressive) under a fully relativistic treatment of the problem, and point out several theoretical ambiguities and observational shortcomings which further qualify the results. Nevertheless, we conclude that a very large class of void models is ruled out by the combination of kSZ and other methods.

astro-ph.CO↗

arXiv2011

No evidence for anomalously low variance circles on the sky

In a recent paper, Gurzadyan & Penrose claim to have found directions on the sky centred on which are circles of anomalously low variance in the cosmic microwave background (CMB). These features are presented as evidence for a particular picture of the very early Universe. We attempted to repeat the analysis of these authors, and we can indeed confirm that such variations do exist in the temperature variance for annuli around points in the data. However, we find that this variation is entirely expected in a sky which contains the usual CMB anisotropies. In other words, properly simulated Gaussian CMB data contain just the sorts of variations claimed. Gurzadyan & Penrose have not found evidence for pre-Big Bang phenomena, but have simply re-discovered that the CMB contains structure.

astro-ph.CO↗

arXiv2011

Precision cosmology defeats void models for acceleration

The suggestion that we occupy a privileged position near the centre of a large, nonlinear, and nearly spherical void has recently attracted much attention as an alternative to dark energy. Putting aside the philosophical problems with this scenario, we perform the most complete and up-to-date comparison with cosmological data. We use supernovae and the full cosmic microwave background spectrum as the basis of our analysis. We also include constraints from radial baryonic acoustic oscillations, the local Hubble rate, age, big bang nucleosynthesis, the Compton y-distortion, and for the first time include the local amplitude of matter fluctuations, σ_8. These all paint a consistent picture in which voids are in severe tension with the data. In particular, void models predict a very low local Hubble rate, suffer from an "old age problem", and predict much less local structure than is observed.

astro-ph.CO↗

arXiv2010

Tilted Physics: A Cosmologically Dipole-Modulated Sky

Physical constants and cosmological parameters could vary with position. On the largest scales such variations would manifest themselves as gradients across our Hubble volume, leading to dipole-modulation of the cosmic microwave anisotropies. This generically leads to a correlation between adjacent multipoles in the spherical harmonics expansion of the sky, a distinctive signal which should be searched for in future data sets.

astro-ph.CO↗