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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.

We perform a cosmological-model-independent test for the distance-duality (DD) relation η(z) = D L (z)(1 + z) −2 /D A (z), where D L and D A are the luminosity distance and angular diameter distance respectively, with a combination of observational data for D L taken from the latest Union2 SNe Ia and that for D A provided by two galaxy clusters samples compiled by De Filippis et al. and Bonamente et al.. Two parameterizations for η(z), i.e., η(z) = 1 + η 0 z and η(z) = 1 + η 0 z/(1 + z), are used. We find that the DD relation can be accommodated at 1σ confidence level (CL) for the De Filippis et al. sample and at 3σ CL for the Bonamente et al. sample. We also examine the DD relation by postulating two more general parameterizations: η(z) = η 0 + η 1 z and η(z) = η 0 + η 1 z/(1 + z), and find that the DD relation is compatible with the results from the De Filippis et al. and the Bonamente et al. samples at 1σ and 2σ CLs, respectively. Thus, we conclude that the DD relation is compatible with present observations.

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.

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.

Fast radio bursts (FRBs), bright transients with millisecond durations at ∼GHz and typical redshifts probably >0.8, are likely to be gravitationally lensed by intervening galaxies. Since the time delay between images of strongly lensed FRB can be measured to extremely high precision because of the large ratio ∼109 between the typical galaxy-lensing delay time (10 days) and the width of bursts (ms), we propose strongly lensed FRBs as precision probes of the universe. We show that, within the flat ΛCDM model, the Hubble constant H0 can be constrained with a ~0.91% uncertainty from 10 such systems probably observed with the square kilometer array (SKA) in <30 years. More importantly, the cosmic curvature can be model independently constrained to a precision of ∼0.076. This constraint can directly test the validity of the cosmological principle and break the intractable degeneracy between the cosmic curvature and dark energy.

Model-independent estimations for the spatial curvature not only provide a test for the fundamental Copernican principle assumption, but also can effectively break the degeneracy between curvature and dark energy properties. In this paper, we propose to achieve model-independent constraints on the spatial curvature from observations of standard candles and standard clocks, without assuming any fiducial cosmology and other priors. We find that, for the popular Union2.1 type Ia supernovae (SNe Ia ) observations, the spatial curvature is constrained to be Ω K = −0.045 +0.176 −0.172 . For the latest joint light-curve analysis (JLA) of SNe Ia observations, we obtain Ω K = −0.140 +0.161 −0.158 . It is suggested that these results are in excellent agreement with the spatially flat Universe. Moreover, compared to other approaches aiming for model-independent estimations of spatial curvature, this method also achieves constraints with competitive precision.

We present our study on cosmic opacity, which relates to changes in photon number as photons travel from the source to the observer. Cosmic opacity may be caused by absorption/scattering due to matter in the universe, or by extragalactic magnetic fields that can turn photons into unobserved particles (e.g. light axions, chameleons, gravitons, Kaluza-Klein modes), and it is crucial to correctly interpret astronomical photometric measurements like type Ia supernovae observations. On the other hand, the expansion rate at different epochs, i.e. the observational Hubble parameter data H(z), are obtained from differential ageing of passively evolving galaxies or from baryon acoustic oscillations and thus are not affected by cosmic opacity. In this work, we first construct opacity-free luminosity distances from H(z) determinations, taking correlations between different redshifts into consideration for our error analysis. Moreover, we let the light-curve fitting parameters, accounting for distance estimation in type Ia supernovae observations, free to ensure that our analysis is authentically cosmological-model-independent and gives a robust result. Any non-zero residuals between these two kinds of luminosity distances can be deemed as an indication of the existence of cosmic opacity. While a transparent universe is currently consistent with the data, our results show that strong constraints on opacity (and consequently on physical mechanisms that could cause it) can be obtained in a cosmological-model-independent fashion.PACS numbers: 98.80.-k, 98.80.Es

Fast radio bursts (FRBs) are short duration (∼millisecond) radio transients with cosmological origin. The simple sharp features of the FRB signal have been utilized to probe two fundamental laws of physics, namey, testing Einstein's weak equivalence principle and constraining the rest mass of the photon. Recently, Hessels et al. (2018) found that after correcting for dispersive delay, some of the bursts in FRB 121102 have complex time-frequency structures that include sub-pulses with a time-frequency downward drifting property. Using the delay time between sub-pulses in FRB 121102, here we show that the parameterized post-Newtonian parameter γ is the same for photons with different energies to the level of |γ 1 − γ 2 | < 2.5 × 10 −16 , which is 1000 times better than previous constraints from FRBs using similar methods. We also obtain a stringent constraint on the photon mass, m γ < 5.1 × 10 −48 g, which is 10 times smaller than previous best limits on the photon mass derived through the velocity dispersion method.

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