Using supermassive black holes to measure cosmic distances

One of the major problems in astronomy is measuring very large distances in the universe. The current most common methods measure relative distances, but now research from the Dark Cosmology Centre demonstrates that precise distances can be measured using supermassive black holes. The results are published in the scientific journal, Nature.

Magical research process

The research is primarily a collaboration between Darach Watson and Sebastian Hönig, who led the study and now works at the University of Southampton, but was then working at the Dark Cosmology Centre. Darach Watson says that they were both thrilled with the results. “The process was almost magical. The most important thing about measuring distance is high precision – how accurate is the method. We knew that if we could get the uncertainty down to about 10 percent, it would be significant, but we had no idea that it was possible. When we first realised that we could carry out this measurement, we knew that the precision of the measurements of the anglular size using interferometry and the physical size based on the time delay were both only about 30 percent. Normally, when you combine two such numbers, the accuracy of the ratio is worse, so we expected an overall accuracy of 40 percent or so. But that was not what happened. It turned out that the greatest uncertainty in both measurements was the distribution of the brightness across the dust ring. And it was the same in both measurements, so when we took the ratio, the uncertainties cancelled – simply disappeared. Sebastian Hönig, after making the first calculation, came to me and said: “You’ll never believe what the precision is, guess! Usually in science you fight so hard to get something to fit or work properly. But every so often – very rarely, something magical happens – it’s like a gift and everything just falls into place. That is what happened here,” explains Darach Watson.