Summary of the summary talks March 31, 2007Posted by dorigo in astronomy, news, physics, science.
A day has passed from the end of the “Outstanding Questions in Cosmology” conference, but only now I find the time to write down my final report, centered on the summary talks given on Thursday afternoon. I was busy enjoying London with my wife… But let’s keep this about Cosmology.
So. Al Stebbins gave the first of the two summary talks.
He started by noting that the conference had not created too much “back and forth”, and so he hoped he would manage to say something that made somebody mad. But that did not happen, unfortunately: his talk was quite un-controversial…
Stebbins tried to put the current “precision cosmology measurements” in a bit of perspective, by noting that if you asked cosmologists in the mid 70s what was the universe made of, they would reply baryons 40% and curvature 60%, end of the story. The universe, that is, did not look closed back then. A decade later, the general view was thet dark matter would be 85%, and baryons 15%. Moreover, cosmologists would agree that by symmetry, the universe must be flat: zero curvature. In more recent years, all of a sudden dark matter went down to 24%, baryons to 4%, and dark energy up to a whooping 72%. So the universe is made of repulsive stuff, mostly. You could extrapolate what could happen in some more years…
The Fermilab scientist did not go as far as to compute it, but I guess I can do the math myself: baryons will cease to exist in five more years. So cosmologists really have to work hard since there is not much time left before pencils start to vanish… If that were so, Krauss’ worries about the miserable future of Cosmology would have been far too optimistic (and he in fact denied being a pessimist during the question time following his talk!) Anyway, seriously: the point made by Stebbins was rather that theorists must remain vigilant. We could indeed be declaring victory too soon.
Stebbins then reminded the audience of a few of the Big Questions that were dealt with this week: questions of content, of kinematics… Where are things, what is the interclusters medium, where are the baryons… And other questions of dynamics: what are the forces that cause things to move. Also, questions about initial conditions. He noted that inflation was never talked very much at the conference. And indeed, inflation -as many had noted in their talks- is not a testable theory. No matter what you say that you measure, you cannot really test it.
Later, Einstein was quoted: “The most incomprehensible thing about the world is that it is at all comprehensible“. Stebbins noted that scientists tend to really believe in their model. The models are not perceived as just good descriptions, but just plain real. But there are problems with Physics: it is difficult to explain everything. And the universe seems to be “too explainable” rather than “at all comprehensible” (another Albert quote, but Stebbins this time, not Einstein).
Model selection, a topic discussed widely at the conference, was then mentioned. Stebbins noted that in Cosmology there seem to be brief periods of time when you learn a lot about a specific topic. The most recent trends in Cosmo-statistics see Bayesian statistical tests, Fisher matrices, MCMC (Gibbs sampling), Bayesian Evidence/Model selection. Many people at the conference have computed “Bayesian evidence”. It has some sort of sociological content. But the relevant question remains, Do we really believe in our model ? Or are we forced into it by the data ? If that is the case, soon a lot of people might have to stop doing what they are doing.
Then Stebbins said a few things about dark energy. Why do not we stop at a cosmological constant ? Is it because it does not fit the data, or is it because some do not like the whole idea ? If you believed in the degrees of freedom that are actually there until you take a stand on a particular model, you would not be able to constrain anything with the experiments. Here Stebbins quoted Krauss: even in the case of just two parameters, it is practically impossible to put constraints on anything. You learn very little even by putting together all the data from different experiments and tests. Stebbins however also quoted Pahud, who said that the Planck experiment will probably be able to do something to move us from the impasse, especially in combination with low-z observations.
About dark matter and modified gravity models, new particles or wimps: Stebbins said dark matter appears to be not controversial, but it should be. The thing is, we prefer to add parameters to somebody else’s standard model to make our fit work, rather than adding parameters to our own. The highlight is mine, because this is really a witty remark which we should think over. And another remark by Stebbins I subscribe to was that globular clusters and dwarfs galaxies are simple systems, so we should study those and try to understand them. And in fact, another comment I venture to insert here is that the new evidence of black holes sitting in star clusters, and the apparent constancy of the ratio of black hole to total host mass for these and other much larger systems are compelling reasons to get back to the blackboard to get an explanation for these things.
In any case, even if presently proposed alternatives to dark energy do not work, one can be assured that there is an alternative which is indistinguishable from the standard cosmological model. So the question really is, Stebbins noted: do we have to wait for the data to require an alternative ? The real truth is that these missions that measure dark energy are also measuring a lot of other things. And, Stebbins said, there will always be anomalies: Situation Normal, All Fucked Up: SNAFU. Which, by the way, is what makes things interesting to me: as long as there are disagreements with the most fashionable theoretical model, there is a chance that advances can be made…. Compare the situation in cosmology with what happens with the standard model of particle physics and you get the picture.
Talking about anomalies, Stebbins mentioned the wide exposure that the CMBR anomaly got at the conference. It is an effect of low statistical significance, but worth investigating. Spending few words on the issue, Stebbins mentioned different views on the matter: according to Andrew Jaffe, a particular topology of the universe can expain it. Carlo Contaldi instead favors the possibility that the anomaly is a result of anisotropic inflation. Others gave their own interpretations, but my own pitch is that this particular anomaly is really not even that: a two-sigma effect is too little to cause so much interest, in my opinion. It is good to be prepared for a possible future when those two sigma become four or five, but spending too much time discussing the alternatives now seems a bit overkill to me.
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After Stebbins, Richard Lieu gave a more detailed summary, in a style complementary to that of the former presentation. He started by showing a table of different elements of the present-day cosmology, with crosses on two columns, signalling whether there is strong evidence or established observation of each effect, or insufficient or controversial evidence. Then he went on to discuss each line, by showing the most interesting plots presented by the contributors, and discussing them in turn. I did not take detailed notes of his talk, because of its more strict “review” nature, and because I had gotten fed up with typing!
However, I did notice one point he made when he discussed the plot of Delta(M-m) versus redshift (z) of supernovae in far galaxies. Lieu clarified that the supernovae data is actually spread vertically quite a bit for every value of z. What one usually gets to see in the supernova plots is the combination of many different Delta(M-m) values measured from different supernovae for each z. And the combination is done by taking the median of the Delta(M-m) values, not the means! Now, there surely is a good reason for doing that – but I had not noticed it, and I will investigate the matter for my own education as soon as I have a chance.
In fact, in the conference I also learned that Ia supernovae – those that appear to be caused by accretion from a companion until criticality is reached, and could thus be thought of as close copies of each other (for, if mass is by far the most important variable in determining the dynamics of the explosion, one might expect that a critical mass is equal to any other) are far from being “standard candles”: they show a wide variety of time evolution in their luminosity, such that their use for distance measurements is not completely straightforward. Enough to doubt about the results of the analyses ? Probably not, but worth keeping in mind.
By reading back the last paragraph, I notice that one sentence is 95 words long. It is a defect – ok let’s call it a feature – in my writing style which I inherited from my father. Was the sentence too hard to follow ? Probably yes. I need to restrain myself a bit in that respect. And with this totally irrelevant comment, dear reader, I leave you and get a deserved sleep.