# Cosmology with powerful radio-loud AGNs

## TL;DR

This paper proposes an analytic method to measure cosmic distances using powerful radio-loud AGNs, enabling the construction of standard candles at various redshifts to constrain the universe's curvature and expansion rate.

## Contribution

The paper introduces a novel analytic technique leveraging radio flux, size, and spectral data of FR-II sources to determine distances, independent of traditional methods.

## Key findings

- Distances to Cygnus A are consistent across different lobes within 7%.
- Estimated Hubble constant from Cygnus A is 64 km/s/Mpc with uncertainties.
- Method can be applied to large samples of FR-II sources for cosmological measurements.

## Abstract

Immensely bright quasars and radio-loud active galactic nuclei (AGNs) provide an enticing opportunity to construct standard candles detectable up to the very early universe. An analytic theory is proposed to measure the distance to powerful \citeauthor{FR+1974} type-II radio sources based on their integrated flux density across a broad range of radio frequencies, and the angular size and axis ratio of their synchrotron-emitting lobes. This technique can be used at low-redshift to construct absolute standard candles in conjunction with X-ray observations of the host cluster, or at high-redshift to measure the relative distances of objects and constrain the curvature of our universe. Distances calculated with this method are consistent for dissimilar objects at the same redshift; the two lobes of Cygnus A have flux densities, linear sizes and spectral break frequencies varying by between 15-35\% yet their fitted distances are the same to within 7\%. These distance estimates together yield a transverse comoving distance to Cygnus A of $261_{-55}^{+70}\rm\, Mpc$ corresponding to a Hubble constant of $H_0 = 64_{-13}^{+17}\rm\, km\, s^{-1}\, Mpc^{-1}$. Large samples of suitable FR-II sources could provide a measure of the Hubble constant independent of existing techniques such as the cosmic microwave background, baryon acoustic oscillations, and type 1a supernovae.