A Comparison of Cosmological Models Using Strong Gravitational Lensing Galaxies

TL;DR

This study compares the standard LCDM cosmology and the R_h=ct universe using a catalog of 69 strong gravitational lensing systems, finding current data insufficient to favor one model but estimating future sample sizes needed for decisive tests.

Contribution

It provides a direct comparison of two cosmological models with lensing data and estimates the sample size required to distinguish them at high confidence.

Findings

Both models fit current lensing data well.

Approximately 200 lenses needed to rule out R_h=ct if LCDM is correct.

Approximately 300 lenses needed to rule out LCDM if R_h=ct is correct.

Abstract

Strongly gravitationally lensed quasar-galaxy systems allow us to compare competing cosmologies as long as one can be reasonably sure of the mass distribution within the intervening lens. In this paper, we assemble a catalog of 69 such systems, and carry out a one-on-one comparison between the standard model, LCDM, and the R_h=ct Universe. We find that both models account for the lens observations quite well, though the precision of these measurements does not appear to be good enough to favor one model over the other. Part of the reason is the so-called bulge-halo conspiracy that, on average, results in a baryonic velocity dispersion within a fraction of the optical effective radius virtually identical to that expected for the whole luminous-dark matter distribution. Given the limitations of doing precision cosmological testing using the current sample, we also carry out Monte Carlo simulations based on the current lens measurements to estimate how large the source catalog would have to be in order to rule out either model at a ~99.7% confidence level. We find that if the real cosmology is LCDM, a sample of ~200 strong gravitational lenses would be sufficient to rule out R_h=ct at this level of accuracy, while ~300 strong gravitational lenses would be required to rule out LCDM if the real Universe were instead R_h=ct. The difference in required sample size reflects the greater number of free parameters available to fit the data with LCDM. We point out that, should the R_h=ct Universe eventually emerge as the correct cosmology, its lack of any free parameters for this kind of work will provide a remarkably powerful probe of the mass structure in lensing galaxies, and a means of better understanding the origin of the bulge-halo conspiracy.