Archive for critical density

Cranks Anonymous

Posted in Biographical, Books, Talks and Reviews, The Universe and Stuff with tags , , , , on September 22, 2009 by telescoper

Sean Carroll, blogger-in-chief at Cosmic Variance, has ventured abroad from his palatial Californian residence and is currently slumming it in a little town called Oxford where he is attending a small conference in celebration of the 70th birthday of George Ellis. In fact he’s been posting regular live commentaries on the proceedings which I’ve been following with great interest. It looks an interesting and unusual meeting because it involves both physicists and philosophers and it is based around a series of debates on topics of current interest. See Sean’s posts here, here and here for expert summaries of the three days of the meeting.

Today’s dispatches included an account of George’s own talk which appears to have involved delivering a polemic against the multiverse, something he has been known to do from time to time. I posted something on it myself, in fact. I don’t think I’m as fundamentally opposed as Geroge to the idea that we might live in a bit of space-time that may belong to some sort of larger collection in which other bits have different properties, but it does bother me how many physicists talk about the multiverse as if it were an established fact. There certainly isn’t any observational evidence that this is true and the theoretical arguments usually advanced are far from rigorous.The multiverse certainly is  a fun thing to think about, I just don’t think it’s really needed.

There is one red herring that regularly floats into arguments about the multiverse, and that concerns testability. Different bits of the multiverse can’t be observed directly by an observer in a particular place, so it is often said that the idea isn’t testable. I don’t think that’s the right way to look at it. If there is a compelling physical theory that can account convincingly for a realised multiverse then that theory really should have other necessary consequences that are testable, otherwise there’s no point. Test the theory in some other way and you test whether the  multiverse emanating from it is sound too.

However, that fairly obvious statement isn’t really the point of this piece. As I was reading Sean’s blog post for today you could have knocked me down with a feather when I saw my name crop up:

Orthodoxy is based on the beliefs held by elites. Consider the story of Peter Coles, who tried to claim back in the 1990’s that the matter density was only 30% of the critical density. He was threatened by a cosmological bigwig, who told him he’d be regarded as a crank if he kept it up. On a related note, we have to admit that even scientists base beliefs on philosophical agendas and rationalize after the fact. That’s often what’s going on when scientists invoke “beauty” as a criterion.

George was actually talking about a paper we co-wrote for Nature in which we went through the different arguments that had been used to estimate the average density of matter in the Universe, tried to weigh up which were the more reliable, and came to the conclusion that the answer was in the range 20 to 40 percent of the critical density. There was a considerable theoretical prejudice at the time, especially from adherents of  inflation, that the density should be very close to the critical value, so we were running against the crowd to some extent. I remember we got quite a lot of press coverage at the time and I was invited to go on Radio 4 to talk about it, so it was an interesting period for me. Working with George was a tremendous experience too.

I won’t name the “bigwig” George referred to, although I will say it was a theorist; it’s more fun for those working in the field to guess for themselves! Opinions among other astronomers and physicists were divided. One prominent observational cosmologist was furious that we had criticized his work (which had yielded a high value of the density). On the other hand, Martin Rees (now “Lord” but then just plain “Sir”) said that he thought we were pushing at an open door and was surprised at the fuss.

Later on, in 1996, we expanded the article into a book in which we covered the ground more deeply but came to the same conclusion as before.  The book and the article it was based on are now both very dated because of the huge advances in observational cosmology over the last decade. However, the intervening years have shown that we were right in our assessment: the standard cosmology has about 30% of the critical density.

Of course there was one major thing we didn’t anticipate which was the discovery in the late 1990s of dark energy which, to be fair, had been suggested by others more prescient than us as early as 1990. You can’t win ’em all.

So that’s the story of my emergence as a crank, a title to which I’ve tried my utmost to do justice since then. Actually, I would have liked to have had the chance to go to George’s meeting in Oxford, primarily to greet my ertswhile collaborator whom I haven’t seen for ages. But it was invitation-only. I can’t work out whether these days I’m too cranky or not cranky enough to get to go to such things. Looking at the reports of the talks, I rather think it could be the latter.

Now, anyone care to risk the libel laws and guess who Professor BigWig was?