For the last post I looked up Adam Riess’s website – he shared the 2011 Nobel Prize in Physics for co-discovering the remarkable fact that the rate of expansion of the universe is actually increasing. From that I followed a link to a page with a scan of the page in his logbook where he did the first simple calculation on the data his team had gathered. This was the calculation that first showed that the universe is expanding faster and faster.

The finding is at the bottom right where he gets an effective mass (for which he uses the Greek letter Ω) which is negative. For those of you studying physics – note the error bars, when you are taught to always produce error bars you should listen. These error bars exclude Ω = 0 – which would be a constant rate of expansion.

Mass produces gravitational attraction, and this attraction slows down relative motion, and hence expansion. We have all seen gravity slow down relative motion. When we through a ball in the air, the gravitational attraction between it and the Earth causes the upward motion of the ball away from the Earth to slow down, and eventually reverse.

The idea is the same on the vast scales Riess studied – gravity pulls galaxies (and everything else with mass) together, slowing down the rate at which they move apart, and this acts to slow down the rate at which the universe expands. This was well known when Riess started this page of his logbook. And so people would have expected that as there is a fair amount of mass in the universe that this would cause the rate of expansion of the universe to decrease. This would correspond to a positive effective mass Ω.

But at the bottom of this page in his logbook, Riess found a negative effective mass. The universe looked as though it has negative mass, which because it is negative causes a gravitational repulsion, not attraction. A startling result that earned him and two others a trip to Stockholm.

It is inspiring seeing the page in the logbook. I also think it is interesting that the observation was so exciting, so prize-worthy, because it was the opposite to what scientists expected. They expected the rate of expansion to be decreasing not increasing. It can be a privileged life being a scientist, you can be paid to surprise yourself.