Cosmological QUOKKAS: Proof of concept and early results on the Hubble Constant tension

Distances are one of the most important yet difficult to get quantities in astronomy. Normally, astronomers use redshifts (which are relatively easy to measure) as distance measure, but this only works if one assumes that there is a relationship between redshift and distance. The relationship between redshift and distance was first observationally verified in the 1920s by Edwin Hubble. He found that the further away a galaxy was, the galaxy would be moving away from us (i.e. redshifted) at ever faster rates. This was interpreted as being due to the expansion of the universe and the rate of this expansion is governed by the Hubble Constant. In more recent years, the Hubble Constant has been measured quite precisely by both direct distance measurements ("the distance ladder") and via model assumptions in observations of the Cosmic Microwave Background (CMB). There is currently a tension between these two measurements at the >4\sigma level. This suggests either new physics or systematics in the observations.

In this context, it is clear that a new distance measure is needed in order to resolve this tension. In this talk, I will present the 'standard speed-gun' method of measuring distances. The core assumption of the method is that the variability seen in quasars is constrained by the speed of light and therefore allowing us to use the variability timescale to create a standardizable ruler. We applied this method in a proof-of-concept to the famous radio source 3C 84 and found that our results were consistent with other measures. In preliminary results, we have calibrated the systematics of the method on a galactic microquasar with parallax measurements. Applying these corrections to 3C 84 lowers our estimate of the Hubble Constant and makes us more consistent with the CMB measurements. While our results are very preliminary, if our results hold up, this would suggest that there are systematic errors in the "distance ladder" measurement of the Hubble Constant.

25/01/2022 - 12:30
Dr. Jeff Hodgson
Sejong University, Korea