I went to an excellent colloquium yesterday at the SUSY 2005 conference here in Durham, just before the singing and dancing I mentioned in a previous post. It was by Carlos Frenk, of the Institute for Computational Cosmology, also based here (in fact, both it and the IPPP I mentioned earlier are in the same building, the Ogden Centre for Fundamental Physics.)

The title of the talk was “The Emergence of a Cosmological Paradigm”, and he was talking about what readers of Sean’s previous writing might know is called the “Lambda-CDM” model of the universe. What is that? Well, the title shows a refreshing honesty and emphasizes two remarkable things about our universe about which we know very little, in some sense. We don’t know what the vast majority of our universe is made of, but the unknown components are currently believed to be an unknown form of matter, and an unknown form of energy. We can’t directly see either of these forms and so they are called, helpfully, “Dark Matter” and “Dark Energy”. While we do not know exactly what these missing forms are, we’ve been clever enough to give them names after the sort of behaviour that they have that characterizes what they do to make us believe they are there.
(I say “we”, but I mean this to represent something like “humankind”, or something equally non-specific. I’m not personally a cutting-edge cosmologist naming anything. I should be doing the string theory beat, but our cutting-edge cosmologist who usually takes this as his beat is busy….Doing the string theory beat. Excellent.)

How bad is the situation? Well, the rough numbers are as follows:

First, know that we have just enough matter and energy in the universe to (more…)

Before our right-wing commenters get too excited by the title, I should point out that the “w” is lower-case, and most certainly is not to be pronounced “dubya“. Rather, this post is all about dark energy.

Tomorrow morning, my Ph.D. student, Antonio De Felice, defends his thesis. Assuming all goes well, he will become Dr. De Felice, some time shortly after noon, and I will then take him out to a slap-up lunch at Mrs. Miggins’ Pie Shop, or some reasonably nice restaurant at any rate (if the preceding sentence is puzzling to you, immediately go out and rent the entire Blackaddder series).

The Cosmology group here in Syracuse has already pre-celebrated with Antonio, because he’s leaving town pretty soon (so long as we pass him tomorrow) to take a short vacation and then head off to his postdoc at Sussex in England. We went out for a few beers and snacks the other night (no expense spared here) and a good time was had by all.

Antonio’s thesis contains a number of different pieces of work that he has carried out during his time at Syracuse. I thought it might be interesting to sketch one of the ideas here.

As has been discussed a number of times both by Sean and by me, the observed acceleration of the universe is one of the most puzzling problems facing physicists today. Many different cosmological observations, including those of the cosmic microwave background and large scale structure, imply that, within the context of General Relativity, the majority of mass-energy in the universe is in the form of a smoothly-distributed, negligibly-clustering component referred to as dark energy. If the results obtained from observing the light-curves of type-Ia supernovae are included, one is forced, at a high level of confidence, to also conclude that this dark energy is causing the expansion of the universe to speed up at present times – cosmic acceleration. This is a weird effect to be sure, and presents a formidable obstacle for theorists. Nevertheless, putting aside the ridiculous fine-tunings required, there are some candidates for what this dark energy might be; the best know of which is Einstein’s cosmological constant.

Hard as it is to imagine, the situation could be even worse for theorists. It is sometimes convenient to parameterize dark energy by what is called the equation of state parameter, w (not “dubya” remember). This parameter tells us how, if we treat dark energy as a perfect fluid, the pressure is related to the energy density of the fluid. Current observational bounds place w in the range (very roughly, I’m not trying to quote precise numbers here) -0.8 to -1.2 (it’s important to note that acceleration will occur for any w<-1/3). What I want to focus on for the rest of this post is the possibility that w might be less than -1.

This possibility makes physicists shudder, because various stability proofs in General Relativity require so-called energy conditions, all of which are violated by a source of energy-momentum with w<-1. Thus, this type of dark energy runs the risk of being unstable. In fact, as (among others) Sean, his student Mark Hoffman and I showed, specific models satisfying w<-1 exhibit a catastrophic decay under most circumstances.

Given this state of affairs, Sean, Antonio and I asked the question; is there a way that observers could infer w<-1 in a theory within which all energy-momentum sources obey sensible energy conditions (i.e. no matter has an equation of state parameter that is less than -1)? Our idea was to investigate what happens to how one interprets the data in a theory that is not quite Einstein’s gravity.

The concept is extremely simple. Einstein’s equations for cosmology relate the expansion rate of the universe, and its time derivatives, to the amounts and equations of state of the various components of the cosmic energy budget. If, for simplicity, we discuss only constant equations of state, and approximately instantaneous measurements, then those equations can be rewritten to express the equation of state of dark energy as a function of the various amounts of different types of matter (e.g. dark matter and dark energy) and derivatives of the expansion rate of the universe. This is how one infers w from the data.

Now imagine that gravity at large distances is a little different from Einstein’s theory. One would still write down the same relationship to extract w from the data but, because the way this relationship was originally derived is no longer valid, the value extracted is now, in general, just some function of the expansion rate and other variables and, most importantly, does not represent the equation of state parameter of any matter content. Thus, under certain circumstances, one might infer w<-1 from this relationship without any unstable matter being present!

Thus far this is just a general statement. What Antonio and his Obi-Wan-like mentors did, was to consider how this might work within a simple model of non-Einstein gravity – Brans-Dicke theories with a potential. This isn’t the place to go into the details of such theories; rather for now it’s enough to think of them as theories in which Newton’s constant is replaced by a field, so that, on cosmological scales, it might vary with time.

The dynamics of this field become important in the relationship between w and the various observable cosmological parameters. Part of our work therefore involved us playing around with possible potentials for the field, and identifying what is required to make the relationship yield w<-1 today. Obviously, this isn’t too hard to do if satisfying the data on the equation of state parameter is all one has to achieve. Unfortunately, nature isn’t so kind.

Whenever one modifies gravity, there are all kinds of trouble one can get into. There are strong cosmological constraints on such theories from measurements within the solar system, for example from careful measurements of the time delay of communications with spacecraft far from Earth. In fact, in the last couple of years these constraints have been tightened by an order of magnitude due to measurements made on communications with the Cassini mission (and you thought it was only useful for looking at Saturn and Titan). Applying these constraints to the kind of potentials we constructed yields some pretty odd-looking results.

The upshot is that any potential one constructs to achieve a value for w that is measurably less than -1 and which simultaneously satisfies all other constraints, is exceptionally fine tuned. There are two well-known fine tunings associated with dark energy. The first is the ridiculously small energy scale associated with this component (120 orders of magnitude less than the Planck scale). The second is the coincidence problem – why has dark energy begun to dominate only recently in cosmic history. What Sean, Antonio and I showed was that, if Brans-Dicke theories (with a potential) are to explain an observed w<-1 then there is a third fine-tuning – a significant feature in the potential (essentially a local maximum), occurring precisely where the field is at the present cosmological epoch. Obviously, we find this less than compelling and a strike against such theories. Nevertheless, it is important to underscore that our interpretations of cosmological datasets can be complicated by the possibility of a modification of gravity at cosmological scales.

Hangers-on from my days at Preposterous Universe will recall how (with encouragement from Mark) I managed to avoid a tempting apple offered by a slick-talking serpent … okay, that’s probably a bad metaphor. What I actually did was decide not to go to a conference sponsored by the Templeton Foundation, so as to not give even implicit support to that organization’s attempt to encourage reconciliation between science and religion.

I mentioned this incident to NPR science correspondent David Kestenbaum, who became interested in the entanglement between Templeton and the physics community. He’s been working on a piece about the story for a while now, and it’s finally scheduled to air on Morning Edition tomorrow. Details will vary, but in many places it will air between 6 and 6:30 a.m. Eastern time, and be repeated two and four hours later. It should eventually appear on the web site, and I’ll put up a link when it does. I haven’t heard the piece myself, so if I’m quoted saying anything especially silly — well, I’m sure I’ll come up with some excuse.

Tomorrow’s news today — all in a day’s work here at Cosmic Variance.

Update: Here’s the story; audio not available yet, but it will be soon. I didn’t say anything I’d take back; in fact, I think David chose not to use some of my more confrontational statements. The story brought out one aspect of the Templeton rhetorical strategy that hadn’t been clear before: rather than explicitly promoting “religious” themes in a scientific context, they try to promote discussion of “foundational” issues, the “big questions” that get lost in ordinary scientific discourse.

You may recall my recollection in an earlier post about the dancing of the string theorist participants of the Strings 1998 meeting in Santa Barbara, and you may have listened to the singing of the youth of the field at Strings 2005. A good time was had by all in each case, I understand.

Well, the good times continue. Last night, the participants of SUSY 2005, which is taking place in Durham (see some remarks in an earlier post) showed that they can holler and shake their collective booty too. At a barbecue in the sunny long evening, (more…)

I am often surprised at the level of disdain and resentment with which string theory is viewed by non-string-theorists. I’m thinking not so much of people on the street, but of physicists, other scientists, and even other academics. As a physicist who is not personally identified as a string theorist, I get to hear all sorts of disparaging remarks about the field from experimental particle physicists, condensed matter physicists, astrophysicists, chemists, philosophers, and so on. I sometimes wonder whether most string theorists understand all the suspicion directed against them.

It shouldn’t be like this. String theory, with all of its difficulties, is by far the most promising route to one of the most long-lasting and ambitious goals of natural science: a complete understanding of the microscopic laws of nature. In particular, it is by far the most promising way to reconcile gravity and quantum mechanics, the most important unsolved problem in fundamental physics. At the moment, it’s a notably incomplete and frustrating theory, but not without genuinely astonishing successes to its credit.

The basic idea is incredibly simple: instead of imagining that elementary particles are really fundamentally pointlike, imagine that they are one-dimensional loops or line segments — strings. Now just take that idea and try to make it consistent with the rules of relativity and quantum mechanics. Once you set off down this road, you are are inevitably led to a remarkably rich structure: extra dimensions, gauge theories, supersymmetry, new extended objects, dualities, holography, and who knows what else. Most impressively of all, you are led to gravity: one of the modes of a vibrating string corresponds to a massless spin-two particle, whose properties turn out to be that of a graviton. It’s really this feature that separates string theory from any other route to quantum gravity. In other approaches, you generally start with some way of representing curved spacetime and try to quantize it, soon getting more or less stuck. In string theory, you just say the word “strings,” and gravity leaps out at you whether you like it or not.

(more…)

Good news. There’s at least one other person (a physicist, to boot) I know of who went to see Rize. Hurrah! Peter Steinberg, a scientist at the Brookhaven Lab who is part of the Quantumdiaries blog project has a post on his impressions (see below**) (he also went to see Murderball- I have not seen that), and also notes wisely that it is great to be able to see documentaries in mainstream movie theatres.

[**update: Wait - oh no! He gives away one of the suspense details....don't go there if you're planning to see the movie!]

-cvj

Well, we’ll talk a lot about science journalism soon. A lot. It’s one of my passions. But I’ll hold off for another time. In the meantime, you in the USA should know that your belief (because you’re often being told this again and again by Brits) that the UK might have vastly superior serious daily newspaper journalism (as well as superior tabliod journalism) ought not to extend neccesarily to science coverage. For a long time, the Science Times -which appears in the New York Times on Tuesdays-, even with all of its flaws, was a model of enlightenment compared to what is regularly produced over here in the dailies.

Well, not long ago the Guardian tried to address this (bless them) with their Thursday regular tabloid-sized science pullout section called “Life”. I hope that by now several of the other newspapers have followed suit, but I do not know since I don’t live here anymore. Someone please let me know. So for all the incredible amount of news, sport and celebrity coverage that is going on in any number of national newspapers over here, this is the pinnacle of science coverage in a country that claims to be full of educated people (too many of whom look down on the US on the matter of education).

Anyway, this week’s Life is out today. Following in the tradition of doing big science stories by focusing on a personality, instead of the science, the front cover page has a giant closeup of the face of the loveable cuddly Craig Venter. There are three huge pages on the guy (with some science in there, I’ll grant you), but this cover story turns out to just be an extract from a book on him and other biotech folk. Oh dear, the big science cover story is a book extract. This is great journalism folks! (Anyway, it might be interesting actually, so here’s the info: “The Geneticist Who Played Hoops with My DNA : . . . And Other Masterminds from the Frontiers of Biotech”, by David Ewing Duncan.)

To be fair, there are a few other pages filled with science reporting, which have to fight for space with stuff about medicine (why not, I suppose) and technology (not all about gadgets either). It’s a pretty good effort, and on some days can hold its own against the Science Times, I’m pleased to note. You cannot easily tell this from the Guardian Life website, as far as I can see, which seems to have gone to some effort to hide many of the stories and so you see at first only a couple of the stories, one of them being the extract. But I did note that a regular feature called “Bad Science” by Ben Goldacre does show up on the site too. I note his nice comment (the third paragraph) on a misunderstanding a woman he met at a party has about the role of women in science.

If you’re in the uk and care about science, you should be getting this every Thursday, whether or not you’re a sandal-wearing, salad-eating, whiny left-wing Guardian reader.

-cvj

So the Ashes start today! “What’s are those?”, I hear you cry. Well, it’s the regular grudge match between England and Australia. A battle of the titans, good vs evil (either way), red vs blue, that sort of thing. The weapons of choice? A hard round bludgeoning object, and a long hard wooden other bludgeoning object….oh yeah! No kidding.

Ok. That was an attempt to have you reading a whole paragraph of this post before I mention that it’s a cricket tournament, and then you remember you’ve got to go and ipod a whole bunch of cds of your (now cheekily ironic) ’80′s Britpop.

The country (England!) is all excited. Well, some people are anyway. I don’t really follow cricket much these days (but my sister -cmj to my cvj- does!) but I love the idea that people are still excited about it. I spent part of my youth in the Caribbean, where when the big cricket matches were on, everybody -everybody- needed to know the score, and every other person had a little transistor radio switched on that they carried around with them, so that you could get from one place to another hopping between these crackling sonic stepping stones without every losing touch with the action.

On a side note, I recall the look on JoAnne’s face many years ago when she took me to a baseball game, when after she explained patiently about the game, and apologized that it seemed to go on a bit long to the uninitiated, I calmly reassured her that we had a similar game in the UK, and they go on for…. five days.

-cvj

Links:

You can listen live(!) courtesy of Radio 5. Be aware of the timezone differences of course.
Have a look at the site 334notout for some Ashes information and fun.
There’s a Wikipedia entry on Lord’s Cricket ground, where the first match of the series will be played.
For those of you who think this is just boys’ silliness, on Radio 4 -arguably the best (non-music) radio station on the planet- on today’s “Woman’s Hour” program (I kid you not) there is a special today entitled The Women Behind the Ashes. Listen to (or record) the download fast before they take it down within 24 hours and replace it by the next show (a really annoying feature of the otherwise excellent website).

I have to say that the response here to the first couple of days at Cosmic Variance has been wonderful. We especially appreciate everyone who mentioned our new existence on their own blogs. My, it sure is easier to get a running start these days than it was back in the Wild West pioneering era of the blogosphere (last year). As time goes on and we get into a groove, hopefully a fuller picture of who we are and what we have to say will begin to emerge.

Which leads me to return the favor: please welcome another new group blog, The UnCapitalist Journal, dedicated to economic and political issues from a fettered-free-market perspective. (I’m presuming that “fettered” is the opposite of “unfettered.” We need some better names here.) One of the contributors is Lindsay Beyerstein of Majikthise, who seems to be aiming to contribute to every blog in existence. You go, Lindsay.

Shakespeare’s Sister, kind enough to give us a shout-out (well, she’s from Chicago, we stick together), also points to Our Word, a community site for women’s voices. Hopefully some men will read it, too. Perhaps they will learn, for example, that abandoning popular liberal stances, like support for abortion rights, is not the way to win more elections.

Everyone knows that the Bush administration moved up the timing of its Supreme Court nomination to push chatter about Karl Rove off the front pages. No reason we should go along with the plan.

Part of the Republican strategy, of course, has been to shift the focus away from Rove and onto Joseph Wilson and his wife, CIA agent Valerie Plame. (The mind absolutely boggles at what these exact same people would be saying if a Democratic political operative had blown the cover of a CIA agent — flogging wouldn’t be good enough for them.) They want to give the impression that Plame wasn’t really undercover, so it was no big deal to give a few reporters her identity in order to settle a political score.

Actual CIA agents disagree, and they’ve written an open letter to Congress to make their stance clear.

We, the undersigned former U.S. intelligence officers are concerned with the tone and substance of the public debate over the ongoing Department of Justice investigation into who leaked the name of Valerie Plame, wife of former U.S. Ambassador Joseph Wilson IV, to syndicated columnist Robert Novak and other members of the media, which exposed her status as an undercover CIA officer. The disclosure of Ms. Plame’s name was a shameful event in American history and, in our professional judgment, may have damaged U.S. national security and poses a threat to the ability of U.S. intelligence gathering using human sources. Any breach of the code of confidentiality and cover weakens the overall fabric of intelligence, and, directly or indirectly, jeopardizes the work and safety of intelligence workers and their sources.

The Republican National Committee has circulated talking points to supporters to use as part of a coordinated strategy to discredit Ambassador Joseph Wilson and his wife. As part of this campaign a common theme is the idea that Ambassador Wilson’s wife, Valerie Plame was not undercover and deserved no protection. [...] These comments reveal an astonishing ignorance of the intelligence community and the role of cover. The fact is that there are thousands of U.S. intelligence officers who “work at a desk” in the Washington, D.C. area every day who are undercover. Some have official cover, and some have non-official cover. Both classes of cover must and should be protected.

Via Dynamics of Cats and AMERICAblog.