Abstract. We perform a comprehensive cosmological study of the H 0 tension between the direct local measurement and the model-dependent value inferred from the Cosmic Microwave Background. With the recent measurement of H 0 this tension has raised to more than 3 σ. We consider changes in the early time physics without modifying the late time cosmology. We also reconstruct the late time expansion history in a model independent way with minimal assumptions using distance measurements from Baryon Acoustic Oscillations and Type Ia Supernovae, finding that at z < 0.6 the recovered shape of the expansion history is less than 5% different than that of a standard ΛCDM model. These probes also provide a model insensitive constraint on the low-redshift standard ruler, measuring directly the combination r s h where H 0 = h × 100 Mpc −1 km/s and r s is the sound horizon at radiation drag (the standard ruler), traditionally constrained by CMB observations. Thus r s and H 0 provide absolute scales for distance measurements (anchors) at opposite ends of the observable Universe. We calibrate the cosmic distance ladder and obtain a model-independent determination of the standard ruler for acoustic scale, r s . The tension in H 0 reflects a mismatch between our determination of r s and its standard, CMB-inferred value. Without including high-Planck CMB polarization data (i.e., only considering the "recommended baseline" low-polarisation and temperature and the high temperature data), a modification of the early-time physics to include a component of dark radiation with an effective number of species around 0.4 would reconcile the CMB-inferred constraints, and the local H 0 and standard ruler determinations. The inclusion of the "preliminary" high-Planck CMB polarisation data disfavours this solution.arXiv:1607.05617v2 [astro-ph.CO]

We present a detailed overview of the cosmological surveys that we aim to carry out with Phase 1 of the Square Kilometre Array (SKA1) and the science that they will enable. We highlight three main surveys: a medium-deep continuum weak lensing and low-redshift spectroscopic HI galaxy survey over 5 000 deg2; a wide and deep continuum galaxy and HI intensity mapping (IM) survey over 20 000 deg2 from $z = 0.35$ to 3; and a deep, high-redshift HI IM survey over 100 deg2 from $z = 3$ to 6. Taken together, these surveys will achieve an array of important scientific goals: measuring the equation of state of dark energy out to $z \sim 3$ with percent-level precision measurements of the cosmic expansion rate; constraining possible deviations from General Relativity on cosmological scales by measuring the growth rate of structure through multiple independent methods; mapping the structure of the Universe on the largest accessible scales, thus constraining fundamental properties such as isotropy, homogeneity, and non-Gaussianity; and measuring the HI density and bias out to $z = 6$ . These surveys will also provide highly complementary clustering and weak lensing measurements that have independent systematic uncertainties to those of optical and near-infrared (NIR) surveys like Euclid, LSST, and WFIRST leading to a multitude of synergies that can improve constraints significantly beyond what optical or radio surveys can achieve on their own. This document, the 2018 Red Book, provides reference technical specifications, cosmological parameter forecasts, and an overview of relevant systematic effects for the three key surveys and will be regularly updated by the Cosmology Science Working Group in the run up to start of operations and the Key Science Programme of SKA1.

Scattering and decay processes of thermal bath particles involving heavy leptons can dump hot axions in the primordial plasma around the QCD phase transition. We compute their relic density, parameterized by an effective number ∆N eff of additional neutrinos. For couplings allowed by current bounds, production via scattering yields ∆N eff 0.6 and ∆N eff 0.2 for the cases of muon and tau, respectively. Flavor violating tau decays to a lighter lepton plus an axion give ∆N eff 0.3. Such values of ∆N eff can alleviate the tension between the direct local measurement of the Hubble constant H 0 and the inferred value from observations of the Cosmic Microwave Background, assuming ΛCDM. We analyze present cosmological data from the Planck collaboration and baryon acoustic oscillations with priors given in terms of the axion-lepton couplings. For axions coupled to muons, the tension can be alleviated below the 3σ level. Future experiments will measure ∆N eff with higher precision, providing an axion discovery channel and probing the role of hot axions in the H 0 tension.

Abstract. The model in which Primordial Black Holes (PBHs) constitute a non-negligible fraction of the dark matter has (re)gained popularity after the first detections of binary black hole mergers. Most of the observational constraints to date have been derived assuming a single mass for all the PBHs, although some more recent works tried to generalize constraints to the case of extended mass functions. Here we derive a general methodology to obtain constraints for any PBH Extended Mass Distribution (EMD) and any observables in the desired mass range. Starting from those obtained for a monochromatic distribution, we convert them into constraints for EMDs by using an equivalent, effective mass M eq that depends on the specific observable. We highlight how limits of validity of the PBH modelling affect the EMD parameter space. Finally, we present converted constraints on the total abundance of PBH from microlensing, stellar distribution in ultra-faint dwarf galaxies and CMB accretion for Lognormal and Power Law mass distributions, finding that EMD constraints are generally stronger than monochromatic ones.arXiv:1709.07467v2 [astro-ph.CO]

Assuming the existence of standard rulers, standard candles and standard clocks, requiring only the cosmological principle, a metric theory of gravity, a smooth expansion history, and using state-of-the-art observations, we determine the length of the "lowredshift standard ruler". The data we use are a compilation of recent Baryon acoustic oscillation data (relying on the standard ruler), Type 1A supernovae (as standard candles), ages of early type galaxies (as standard clocks) and local determinations of the Hubble constant (as a local anchor of the cosmic distance scale). In a standard ΛCDM cosmology the "low-redshift standard ruler" coincides with the sound horizon at radiation drag, which can also be determined -in a model dependent way-from CMB observations. However, in general, the two quantities need not coincide. We obtain constraints on the length of the low-redshift standard ruler: r h s = 101.0±2.3h −1 Mpc, when using only Type 1A supernovae and Baryon acoustic oscillations, and r s = 150.0 ± 4.7 Mpc when using clocks to set the Hubble normalisation, while r s = 141.0 ± 5.5 Mpc when using the local Hubble constant determination (using both yields r s = 143.9±3.1 Mpc).The low-redshift determination of the standard ruler has an error which is competitive with the model-dependent determination from cosmic microwave background measurements made with the Planck satellite, which assumes it is the sound horizon at the end of baryon drag.