On the Dark Side?

A few weeks ago, I took part in a “Big Questions” debate with Subir Sarkar, a colleague from Oxford, on Dark Energy and the Fate of the Universe. For those of you who couldn’t attend, a related podcast is available, you can download my meagre slides, and it’s been mentioned on Physics World, as well as by a fellow blogger who referred to me as a “Dark Side proponent”. Unlike the perhaps more contentious previous debate on the origin of the Universe (i.e., the existence of god), we decided to allow the audience to vote on the outcome, usually not the way scientific questions are decided. Of course I take such a petulant tone since, in fact, I lost…

I am amused and actually a little disturbed that my position is seen to be simultaneously radical — I am advocating the idea that the universe is dominated by an all-pervasive repulsive fluid — and conservative — just jumping on the same bandwagon as my colleagues.

In fact, I (we) are just doing science: we’ve made some measurements of the Universe and its constituents. Our simplest theories, that the Universe is dominated by what we could call “normal” matter, simply don’t fit the data, since normal matter requrires that the expansion of the Universe be slowing down (decelerating) over time.

Indeed, several different lines of observational argument all lead separately to this contradiction with the simpler theories: the Universe as a whole would be older than the objects in it; distant objects are dimmer; and present-day structures are growing more slowly. These problems and other related ones can be solved if we open up our theories to allow the expansion of the Universe to be accelerating. And how can we implement that idea? What is the physics behind acceleration? Well, the simplest possibility is just to reinstate Einstein’s own “cosmological constant”. Other possibilities are a so-called “scalar field” or even some modifications to Einstein’s theory itself. All of these nowadays fall under the rubric of “dark energy”, originally coined by Mike Turner of the University of Chicago in the 1990s when the evidence for such a concordance model was beginning to grow. I don’t know which of these possibilities is true, nor even whether these ideas will stand the test of time. But despite a decade of attempts to find other explanations for the observations without resorting to dark energy, none have so far succeeded.

So that’s why I plumped for Dark Energy — it’s the simplest, perhaps only, explanation of our cosmological observations.