# Ultra-compact structure in radio quasars as a cosmological probe: a revised study of the interaction between cosmic dark sectors

## TL;DR

This study uses high-redshift radio quasars as standard rulers to constrain interacting dark energy models, finding minimal interaction and potential to address the Hubble tension, with model ranking favoring simpler scenarios.

## Contribution

It introduces a revised analysis of dark sector interactions using a new quasar dataset, extending constraints to higher redshifts than previous probes.

## Key findings

- Radio quasars provide constraints comparable to traditional probes.
- Dark energy-dark matter interaction is negligible or slightly negative.
- Simpler dark energy models are statistically favored over more complex ones.

## Abstract

A new compilation of $120$ angular-size/redshift data for compact radio quasars from very-long-baseline interferometry (VLBI) surveys motivates us to revisit the interaction between dark energy and dark matter with these probes reaching high redshifts $z\sim 3.0$. In this paper, we investigate observational constraints on different phenomenological interacting dark energy (IDE) models with the intermediate-luminosity radio quasars acting as individual standard rulers, combined with the newest BAO and CMB observation from Planck results acting as statistical rulers. The results obtained from the MCMC method and other statistical methods including Figure of Merit and Information Criteria show that: (1) Compared with the current standard candle data and standard clock data, the intermediate-luminosity radio quasar standard rulers , probing much higher redshifts, could provide comparable constraints on different IDE scenarios. (2) The interaction between dark energy and dark matter seems to be vanishing or slightly smaller than zero. At the 68.3\% confidence level, the energy is seen transferred from dark matter to dark energy, which implies that those IDE models can not alleviate the coincidence problem or even more sever. However, the strong degeneracies between the interaction term and Hubble constant may contribute to alleviate the tension of $H_0$ between the recent Planck and HST measurements. (3) Concerning the ranking of competing dark energy models, IDE with more free parameters are substantially penalized by the BIC criterion, which agrees very well with the previous results derived from other cosmological probes.