In this paper, we present a new compiled milliarcsecond compact radio data set of 120 intermediate-luminosity quasars in the redshift range 0.46 < z < 2.76. These quasars show negligible dependence on redshifts and intrinsic luminosity, and thus represents, in the standard model of cosmology, a fixed comoving-length of standard ruler. We implement a new cosmology-independent technique to calibrate the linear size of of this standard ruler as l m = 11.03 ± 0.25 pc, which is the typical radius at which AGN

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.

In this paper, based on a 2.29 GHz VLBI all-sky survey of 613 milliarcsecond ultracompact radio sources with 0.0035 < z < 3.787, we describe a method of identifying the sub-sample which can serve as individual standard rulers in cosmology. If the linear size of the compact structure is assumed to depend on source luminosity and redshift as l m = lL β (1 + z) n , only intermediate-luminosity quasars (10 27 W/Hz< L < 10 28 W/Hz) show negligible dependence (|n| ≃ 10 −3 , |β| ≃ 10 −4 ), and thus represent a population of such rulers with fixed characteristic length l = 11.42 pc. With a sample of 120 such sources covering the redshift range 0.46 < z < 2.80, we confirm the existence of dark energy in the Universe with high significance under the assumption of a flat universe, and obtain stringent constraints on both the matter density Ω m = 0.323 Mpc. Similar reconstruction of the expansion rate function H(z) based on the data from cosmic chronometers and BAO gives us H(z m ) = 176.77 ± 6.11 km sec −1 Mpc −1 . These measurements are used to estimate the speed of light: c = 3.039(±0.180) × 10 5 km/s. This is the first measurement of the speed of light in a cosmological setting referring to the distant past.

The two-point diagnostics Om(z i , z j ) and Omh 2(z i , z j ) have been introduced as an interesting tool for testing the validity of the Λ cold dark matter (ΛCDM) model. Recently, Sahni et al. combined two independent measurements of H(z) from baryon acoustic oscillation (BAO) data with the value of the Hubble constant H 0, and used the second of these diagnostics to test the ΛCDM (a constant equation-of-state parameter for dark energy) model. Their result indicated a considerable tension between observations and predictions of the ΛCDM model. Since reliable data concerning the expansion rates of the universe at different redshifts H(z) are crucial for the successful application of this method, we investigate both two-point diagnostics on the most comprehensive set of N = 36 measurements of H(z) from BAOs and the differential ages (DAs) of passively evolving galaxies. We discuss the uncertainties of the two-point diagnostics and find that they are strongly non-Gaussian and follow the patterns deeply rooted in their very construction. Therefore we propose that non-parametric median statistics is the most appropriate way of treating this problem. Our results support the claims that ΛCDM is in tension with H(z) data according to the two-point diagnostics developed by Shafieloo, Sahni, and Starobinsky. However, other alternatives to the ΛCDM model, such as the wCDM or Chevalier–Polarski–Linder models, perform even worse. We also note that there are serious systematic differences between the BAO and DA methods that ought to be better understood before H(z) measurements can compete with other probes methods.

Cosmological applications of the "redshift -angular size" test require knowledge of the linear size of the "standard rod" used. In this paper, we study the properties of a large sample of 140 milliarcsecond compact radio sources with flux densities measured at 6 cm and 20 cm, compiled by . Using the best-fitted cosmological parameters given by Planck/WMAP9 observations, we investigate the characteristic length l m as well as its dependence on the source luminosity L and redshift l m = lL β (1 + z) n . For the full sample, measurements of the angular size θ provide a tight constraint on the linear size parameters. We find that cosmological evolution of the linear size is small (|n| ≃ 10 −2 ) and consistent with previous analysis. However, a substantial evolution of linear sizes with luminosity is still required (β ≃ 0.17). Furthermore, similar analysis done on sub-samples defined by different source optical counterparts and different redshift ranges, seems to support the scheme of treating radio galaxies and quasars with distinct strategies. Finally, a cosmological-model-independent method is discussed to probe the properties of angular size of milliarcsecond radio quasars. Using the corrected redshift -angular size relation for quasar sample, we obtained a value of the matter density parameter, Ω m = 0.292 +0.065 −0.090 , in the spatially flat ΛCDM cosmology.

Using a new recently compiled milliarcsecond compact radio data set of 120 intermediate-luminosity quasars in the redshift range 0.46 < z < 2.76, whose statistical linear sizes show negligible dependence on redshifts and intrinsic luminosity and thus represent standard rulers in cosmology, we constrain three viable and most popular f (T ) gravity models, where T is the torsion scalar in teleparallel gravity. Our analysis reveals that constraining power of the quasars data (N = 120) is comparable to the Union2.1 SN Ia data (N = 580) for all three f (T ) models. Together with other standard ruler probes such as cosmic microwave background and baryon acoustic oscillation distance measurements, the present value of the matter density parameter m obtained by quasars is much larger than that derived from other observations. For one of the models considered ( f 1 CDM) a small but noticeable deviation from CDM cosmology is present, while in the framework of f 3 CDM the effective equation of state may cross the phantom divide line at lower redshifts. These results indicate that intermediateluminosity quasars could provide an effective observational probe comparable to SN Ia at much higher redshifts, and f (T ) gravity is a reasonable candidate for the modified gravity theory.

Probing the speed of light is as an important test of General Relativity but the measurements of c using objects in the distant universe have been almost completely unexplored. In this letter, we propose an idea to use the multiple measurements of galactic-scale strong gravitational lensing systems with type Ia supernova acting as background sources to estimate the speed of light. This provides an original method to measure the speed of light using objects located at different redshifts which emitted their light in a distant past. Moreover, we predict that strongly lensed SNe Ia observed by the LSST would produce robust constraints on ∆c/c at the level of 10 −3 . We also discuss whether the future surveys such as LSST may succeed in detecting any hypothetical variation of c predicted by theories in which fundamental constants have dynamical nature.

In this paper, by using the recently compiled set of 120 intermediate-luminosity quasars (ILQSO) observed in a single-frequency VLBI survey, we propose an improved model-independent method to probe cosmic curvature parameter Ω k and make the first measurement of the cosmic curvature referring to a distant past, with redshifts up to z ∼ 3.0. Compared with other methods, the proposed one involving the quasar data achieves constraints with higher precision in this redshift range. More importantly, our results indicate that the measured Ω k is in good agreement with zero cosmic curvature, implying that there is no significant deviation from a flat Universe. Finally, we investigate the possibility of testing Ω k with a much higher accuracy using quasars observed in the future VLBI surveys. It is shown that our method could provide a reliable and tight constraint on the prior Ω k and one can expect the zero cosmic curvature to be established at the precision of ∆Ω k ∼ 10 −2 with 250 well-observed radio quasars.