New observational constraints on f(T)f(T) cosmology from radio quasars

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 $\Omega_m$ obtained by quasars is much lager than that derived from other observations. For two of the models considered ($f_1$CDM and $f_2$CDM) a small but noticeable deviation from $\Lambda$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 intermediate-luminosity quasars could provide an effective observational probe comparable to SN Ia at much higher redsifts, and $f(T)$ gravity is a reasonable candidate for the modified gravity theory

Ultra-compact structure in intermediate-luminosity radio quasars: building a sample of standard cosmological rulers and improving the dark energy constraints up to z∼3z\sim 3

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 $l_m= 11.03\pm0.25$ pc, which is the typical radius at which AGN jets become opaque at the observed frequency $\nu\sim 2$ GHz. In the framework of flat $\Lambda$CDM model, we find a high value of the matter density parameter, $\Omega_m=0.322^{+0.244}_{-0.141}$, and a low value of the Hubble constant, $H_0=67.6^{+7.8}_{-7.4}\; \rm{kms}^{-1}\rm{Mpc}^{-1}$, which is in excellent agreement with the CMB anisotropy measurements by \textit{Planck}. We obtain ${\Omega_m}=0.309^{+0.215}_{-0.151}$, $w=-0.970^{+0.500}_{-1.730}$ at 68.3% CL for the constant $w$ of a dynamical dark-energy model, which demonstrates no significant deviation from the concordance $\Lambda$CDM model. Consistent fitting results are also obtained for other cosmological models explaining the cosmic acceleration, like Ricci dark energy (RDE) or Dvali-Gabadadze-Porrati (DGP) brane-world scenario. While no significant change in $w$ with redshift is detected, there is still considerable room for evolution in $w$ and the transition redshift at which $w$ departing from -1 is located at $z\sim 2.0$. Our results demonstrate that the method extensively investigated in our work on observational radio quasar data can be used to effectively derive cosmological information. Finally, we find the combination of high-redshift quasars and low-redshift clusters may provide an important source of angular diameter distances, considering the redshift coverage of these two astrophysical probes.Comment: 36 pages, 5 tables, 16 figures, A&A, in pres

Testing and selecting cosmological models with ultra-compact radio quasars

In this paper, we place constraints on four alternative cosmological models under the assumption of the spatial flatness of the Universe: CPL, EDE, GCG and MPC. A new compilation of 120 compact radio quasars observed by very-long-baseline interferometry, which represents a type of new cosmological standard rulers, are used to test these cosmological models. Our results show that the fits on CPL obtained from the quasar sample are well consistent with those obtained from BAO. For other cosmological models considered, quasars provide constraints in agreement with those derived with other standard probes at $1\sigma$ confidence level. Moreover, the results obtained from other statistical methods including Figure of Merit, $Om(z)$ and statefinder diagnostics indicate that: (1) Radio quasar standard ruler could provide better statistical constraints than BAO for all cosmological models considered, which suggests its potential to act as a powerful complementary probe to BAO and galaxy clusters. (2) Turning to $Om(z)$ diagnostics, CPL, GCG and EDE models can not be distinguished from each other at the present epoch. (3) In the framework of statefinder diagnostics, MPC and EDE will deviate from $\rm{\Lambda}$CDM model in the near future, while GCG model cannot be distinguished from $\rm{\Lambda}$CDM model unless much higher precision observations are available.Comment: 12 pages, 8 figures, 1 tabl