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.

4 responses to “On the Dark Side?”

  1. Kea avatar

    LOL, you’re a sport.
    What do you think then of Starkman’s recent work on an extra subtraction of WMAP III data in the galactic plane, which (a) removes the dubious alignments and (b) sends the angular correlation function to zero beyond an angle of roughly 60deg, in stark disagreement with the dark energy (LCDM) curve, and in agreement with some radical alternatives, such as the Riofrio cosmology.

  2. Jason Dick avatar
    Jason Dick

    I wouldn’t worry about losing too much, Andrew! I get the feeling that in discussions like this, the “underdog” obtains a distinct advantage, just due to being the “underdog”.
    I actually had the opportunity of discussing this very topic with Subir over dinner. Being the argumentative person that I am, I jumped right into it, and had a bit of fun. My impression of his stance from that discussion is that he’s far too hung up on this idea that the value of the cosmological constant that fits the observations is too small to be real. If it’s that small, according to him, it must be zero.
    He did, however, seem to be rather reluctant to actually put forward his stance in the discussion, which I think I teased out as him believing that the observed accelerated expansion is just a feature of improperly taking into account the effects of the anisotropies. From everything I’ve read, it seems that has been pretty-well ruled out as a possibility.
    P.S. Kea, it looks to me like the most likely possibility is that they either made some significant error in their analysis, or worse. This is a pretty good description of the paper, though the really telling part is near the bottom of the post:
    As for my own impressions, let me say that the paper was *really* sloppily-written, and that alone raises my skepticism. If they’re not taking the time to put the effort into the paper, I tend to worry that they have used a similar lack of care in putting the effort into the research. It didn’t help that the paper was extremely light on details.

  3. Kea avatar

    Er … I never referred to that paper. Sheeesh. Nobody actually reads anymore.

  4. Robin Booth avatar
    Robin Booth

    As one of the audience at your Big Questions debate the other week, I though that you put up a good argument for an essentially untenable position – trying to defend the modern day equivalent of epicycles. I am still not quite sure whether you were merely playing ‘devil’s advocate’ for the purposes of educating and entertaining the general public, or whether you do indeed believe in the Lambda-CDM ‘Concordance Model’. Presumably you are aware of the number of alternative theoretical cosmological models that are out there, which explain the apparent cosmic acceleration (and galactic rotation curves and the rest), in an elegant and economical manner without the need for recourse to the Voodoo physics of dark energy and dark matter? The Conformal Gravity model (Mannheim et al), is a case in point.
    You are in the fortunate position, as a member of one of the Planck consortia, of very soon being in possession of the data that could validate (or refute) some of these alternative models. This might be a more fruitful avenue of research than merely fine tuning the parameters of those epicycles!