A morning full of interesting talks March 26, 2007Posted by dorigo in astronomy, physics, science.
This was the first morning of the “Outstanding Questions” conference on Cosmology I am attending in London, and it was already quite exciting. The session was named “CMB and the Standard cosmological Model”.
CMB stands for cosmic microwave background, the relic radiation released when the universe became transparent to light, 380,000 years after the Big Bang. Discovered by Penzias and Wilson by accident in 1965, it was investigated by the COBE experiment and more recently by the WMAP probe with extreme accuracy. The tiny fluctuations in the temperature of this radiation as you probe it at different positions in the sky tell a lot about the history of our universe in its early days.
For the benefit of the few readers of this blog I am now skipping lunch in order to report on what I heard this morning. I warn the less physics-inclined ones of you that the discussion will not be entirely at an accessible level…
- The session started with a talk by Roger Blandford (pictured above), ”Origins and Tests of the Standard Model“. This is not the standard model I often talk about here, but the standard Cosmological model, the real focus of the conference. Blandford gave a nice introductory talk, explaining what most cosmologists now think the Universe is: flat, accelerating, and light-weight. The recent data – from WMAP, from supernovae, from analysis of clusters of galaxies, and much more – favors an hubble constant of about 0.07/Gyr -or an age of the Universe of about 14 billion years, and a total mass of atoms only making up for 4% of the total mass-energy of the universe. The rest are two entities for which there still is no explanation: dark matter – 23% – and dark energy – whose historical origin is the controversial “cosmological constant” invoked by Einstein in his static model of the Universe. The picture above shows him dealing with “radical alternatives” to the standard cosmology.
- After Blandford, Gary Hinshaw spoke on ”Three-Year Results from WMAP“. He gave a nice account of the experiment, and its successful mapping of the sky from a lagrangian point a million miles from Earth. WMAP measured the microwave temperature variations in all directions in space at five different frequencies. The data is used to construct a function of the angular scale of variations in the temperature, which is then fit to provide an estimate of the composition of the Universe in terms of baryonic matter and the other components. Hinshaw concluded his description of the experiment and the data by pointing at an intriguing cold spot in the southern emisphere, which appears to stand out at 2-3 standard deviations from expected range of fluctuations in the temperature. Not much to get drooling about, though – but cosmologists appear to have a smaller excitation threshold to small-significance signals (see below).
- Ofer Lahav then spoke on “Surveys of Dark Energy: Challenges and Prospects“. His talk was interesting because he discussed some sociological aspects of the recent trends in Cosmology. He argued that globalization had an impact in the concordance of cosmologists on several aspects which could actually be more controversial than they appear to be on the surface. He made the remark that during the cold war, the West and the East had two different interpretations on dark matter, cold and hot respectively. Nowadays things are much less divided, and after all, hot dark matter is the only one that is proven to exist by direct observation, namely – that provided by the mass of neutrinos. What I found particularly amusing was that, while seriously discussing a “signal” of something not going according to a fit line in a plot (forgot which) he quoted the disagreement at the level of 1-sigma, adding “the history of cosmology is full of 1-sigma results later becoming 3-sigma“, as if 3-sigma were a definitive observation! To me this is hilarious because in particle physics people are quite accustomed to seeing 3-sigma results evaporating as more data are added or more careful treatment of systematic errors are accounted for. Hell, in HEP we do not believe in 5-sigma signals! (A 5-sigma signal is one that happens by chance less than once in a million experiments, while 3-sigma can be observed by chance three times every thousand. One-sigma excesses are there more than thirty percent of the time!!!).
Then coffee break followed. A long line to grab a cup and two cookies. But I had the pleasure to meet in person Jenny Hogan, a charming lady and the reporter of Nature who contacted me some months ago to get some information on the indirect information on the Higgs mass from global fits to electroweak observables and how the new W mass result by CDF was going to change the picture. Back to the talks.
- Lloyd Knox presented “Future CMB Observations“. He spoke of the chances of constraining inflation models by future new experiments aiming at a better determination of the cosmic microwave background and other observations. Inflation is a mechanism hypothesized to reconcile the observational data with the Big Bang theory: a phase of exponential acceleration driven by a negative value of the vacuum energy. The Planck experiment should allow a more model-independent determination of the two parameters Ns and r which distinguish different models of inflation. Knox discussed also how these new experiments could find evidence of the “Landscape” – a large spectrum of values for the cosmological constant, most of which would not allow life to form. The issue is connected to the existence of an enormous number (“10 to the 500 to 10 to the 1000″ is what he quoted, maybe forgetting that the two numbers are maybe the farthest apart ever considered by science) of metastable vacua. He claimed that a measurement of the residual curvature of the universe of 0.0004 would constrain the probability of the anthropic bound to 10%, by performing an integral on the number of e-foldings… Oh well, too much for me to grasp. Read a recent paper by Freivogel if you need more information, and leave me alone.
- Bob Nichol talked about “Observing Dark Energy“. He discussed, among other things, a study of the correlation between the temperature anisotropies in the microwave background and the presence of foreground galaxies (determined by surveys such as the Sloan Digital Sky Survey). The correlation between WMAP data on the cosmic microwave background and the SDSS galaxy data is there at 5-sigma level, and it is achromatic (independent on the frequency probed by WMAP): this is an important test of the theory. Also quasars can be tested for correlation with WMAP anisotropies, and Nichol claimed the test allows to detect an effect of dark energy at high redshift (z>1).
- Subir Sarkar gave a very entertaining talk titled “Does WMAP require Dark Energy?” . He was quick to note that any talk with a question mark in the title imply that the answer is NO. Indeed, he gave convincing arguments. He started by undermining the strongest piece of evidence, the fit of acoustic peaks in the WMAP data, mentioning that the probability of the fit is only 7%. He then went on to explain that the standard model of particle physics describes matter only up to some cutoff energy, and adding terms to the lagrangian which get suppressed by powers of the cutoff do not change the theory. But dark energy does change it, and it requires the introduction of supersymmetry and a fine tuning of the order of 10^60 to be consistent with the standard model lagrangian (see slide below). His talk was interesting but I admit I could not grasp everything…
That is all for now.
Update: sorry, I intended to attach a picture of his slide on the SM, but I lost a bunch of pictures during transfer… “Smart media” ? Grunt!