A Big Day for Dark Energy

For the second time this decade, the Nobel Prize in Physics has been awarded for cosmology, to Saul Perlmutter, Adam Riess and Brian Schmidt. They are among the leaders of the teams that used the properties of supernovae — exploding stars — to measure the rate of expansion of the Universe over time. In so doing, they found that the expansion has been speeding up for the last few billion years. This is difficult to accommodate in a Universe with matter that experiences gravity in the attractive way to which we are accustomed; instead it seems to require that the Universe today be dominated by an exotic form of matter given the purposely uninformative name “Dark Energy”. This is exemplified by Cosmological Constant, a term Einstein originally included in his equation of General Relativity but abandoned when it did not fit the available data — Einstein’s motivation was not to have an accelerating Universe, but a static one, with the attraction exactly balanced by the acceleration. In the late 1990s, those two groups began to see evidence of acceleration on larger scales than Einstein envisaged, evidence that has only got better over time (especially, I should say, when combined with evidence from the Cosmic Microwave Background on the flat overall geometry of the Universe).

I was impressed to see the Guardian liveblogging the announcement of the Nobel Prize in Physics, something that usually happens for Apple product announcements and high-profile sporting events. In the blog, Martin Rees makes the excellent point that, like much physics nowadays, these discoveries were made by teams of people, with excellent leadership by the prizewinners, absolutely, but that there should be a mechanism to recognise the full scope of highly expert scientists involved. (Indeed, the Gruber Cosmology prize, which was awarded for the same research in 2007, officially recognises “Saul Perlmutter & the Supernova Cosmology Project” and “Brian Schmidt & the High-z Supernova Search Team”.)

The big problem with Dark Energy isn’t the observations, however, but the underlying theory — there is no good particle physics model which allows a cosmological constant anything like we see today. The simplest ideas say that it is just zero, and the next simplest give something that is about 10 to the power 122 or so too large.

Luckily, cosmologists and astrophysicists have ideas to solidify the supernova results and hopefully get a handle on the underlying nature of whatever is causing the acceleration, by mapping the expansion of the Universe in space and time in even more detail. There are a plethora of ground-based telescopes making observations already, but the next step will be to go to space. And it turns out that there is another reason why this is a great day for scientists studying Dark Energy: we have just had word that ESA has decided that one of its next M-class (“M” for “medium”) will be Euclid, a satellite explicitly designed to measure the properties of the accelerating Universe.