Under very general assumptions of metric theory of spacetime, photons traveling along null geodesics and photon number conservation, two observable concepts of cosmic distance, i.e. the angular diameter and the luminosity distances are related to each other by the so-called distance duality relation (DDR)Observational validation of this relation is quite important because any evidence of its violation could be a signal of new physics. In this paper we introduce a new method to test DDR based on strong gravitational lensing systems and type Ia supernovae under a flat universe. The method itself is worth attention, because unlike previously proposed techniques, it does not depend on all other prior assumptions concerning the details of cosmological model. We tested it using a new compilation of strong lensing systems and JLA compilation of type Ia supernovae and found no evidence of DDR violation. For completeness, we also combined it with previous cluster data and showed its power on constraining DDR. It could become a promising new probe in the future in light of forthcoming massive strong lensing surveys and because of expected advances in galaxy cluster modlelling.

Recently, Sahni, Shafieloo & Starobinsky (2014) combined two independent measurements of H(z) from BAO data with the value of the Hubble constant H 0 = H(z = 0), in order to test the cosmological constant hypothesis by means of an improved version of the Om diagnostic. Their result indicated a considerable tension between observations and predictions of the ΛCDM model. However, such strong conclusion was based only on three measurements of H(z). This motivated us to repeat similar work on a larger sample. By using a comprehensive data set of 29 H(z), we find that discrepancy indeed exists. Even though the value of Ω m,0 h 2 inferred from Omh 2 diagnostic depends on the way one chooses to make a summary statistics (weighted mean or the median), the persisting discrepancy supports the claims of Sahni, Shafieloo & Starobinsky (2014) that ΛCDM model may not be the best description of our Universe.

The spectral lags of gamma-ray bursts (GRBs) have been viewed as the most promising probes of the possible violations of Lorentz invariance (LIV). However, these constraints usually depend on the assumption of the unknown intrinsic time lag in different energy bands and the use of a single highest-energy photon. A new approach to test the LIV effects has been proposed by directly fitting the spectral-lag behavior of a GRB with a well-defined transition from positive lags to negative lags. This method simultaneously provides a reasonable formulation of the intrinsic time lag and robust lower limits on the quantum-gravity energy scales (E QG). In this work, we perform a global fitting to the spectral-lag data of GRB 190114C by considering the possible LIV effects based on a Bayesian approach. We then derive limits on E QG and the coefficients of the standard model extension. The Bayes factor output in our analysis shows very strong evidence for the spectral-lag transition in GRB 190114C. Our constraints on a variety of isotropic and anisotropic coefficients for LIV are somewhat weaker than existing bounds, but they can be viewed as comparatively robust and have the promise to complement existing LIV constraints. The observations of GRBs with higher-energy emissions and higher temporal resolutions will contribute to a better formulation of the intrinsic time lag and more rigorous LIV constraints in the dispersive photon sector.

We use the newly published 28 observational Hubble parameter data (H(z)) and current largest SNe Ia samples (Union2.1) to test whether the universe is transparent. Three cosmologicalmodel-independent methods (nearby SNe Ia method, interpolation method and smoothing method) are proposed through comparing opacity-free distance modulus from Hubble parameter data and opacity-dependent distance modulus from SNe Ia . Two parameterizations, τ (z) = 2ǫz and τ (z) = (1 + z) 2ǫ − 1 are adopted for the optical depth associated to the cosmic absorption. We find that the results are not sensitive to the methods and parameterizations. Our results support a transparent universe.

Relic gravitational waves (RGWs) generated in the early Universe form a stochastic GW background, which can be directly probed by measuring the timing residuals of millisecond pulsars. In this paper, we investigate the constraints on the RGWs and on the inflationary parameters by the observations of current and potential future pulsar timing arrays. In particular, we focus on effects of various cosmic phase transitions (e.g. e + e − annihilation, QCD transition and SUSY breaking) and relativistic free-streaming gases (neutrinos and dark fluids) in the general scenario of the early Universe, which have been neglected in the previous works. We find that the phase transitions can significantly damp the RGWs in the sensitive frequency range of pulsar timing arrays, and the upper limits of tensor-to-scalar ratio r increase by a factor ∼ 2 for both current and future observations. However, the effects of free-steaming neutrinos and dark fluids are all too small to be detected. Meanwhile, we find that, if the effective equation of state w in the early Universe is larger than 1/3, i.e. deviating from the standard hot big bang scenario, the detection of RGWs by pulsar timing arrays becomes much more promising.