21cm: I’ll see you on the far side of the moon March 28, 2007Posted by dorigo in astronomy, news, physics, science.
While other attendees are sipping coffee and eating cookies, I will post here just a short report of the first three talks given this morning at the Outstanding Questions in Cosmology conference, being held at the Imperial College… After this I need to get running for the Gatwick express.
- Carlo Contaldi spoke on “Relic Anisotropy as a source of all Evil?”. What he meant to be hinting at was the presence of a few non-understood features of the power spectrum of the cosmic microwave background radiation, as measured with precision by WMAP3. In particular, there seems to be a lack of power at low multipoles, or large distance scales, and some intriguing alignment of the features of the temperature differences at small values of l (2,3,4,5). The disagreement with current cosmological models is only at the 2-sigma level, but that seems enough to get cosmologists excited these days, as I commented earlier. Carlo showed that while inflation is a mechanism invoked to make the universe isotropic, it may also be used to create small anisotropies at large distance scales, and some correlations between the low multipoles such as those observed. He considered an axisymmetric inflation model (just for simplicity in calculations) to see what it could result in, and if it would provide a good fit to the data. This idea is interesting also in the context of large extra dimension models, where you can think of a universe in many dimensions originally, all of them microscopic: if just one dimension inflates, it results in dragging a few others, such that the net result is a universe with, say, three macroscopic spatial dimensions and the rest remaining quasi-stable during inflation, and staying microscopic. Numerical calculations show that one can find slightly preferred values for the scale of isotropization, which is his parameter in the fits. The model, most notably, can explain both the lack of large scale power in CMB spectrum, and the observed correlations between temperature anisotropies in low multipole modes.
- Reynald Pain gave a nice account of “Probing Dark Energy with Type 1a Supernovae“. He discussed the problems with using supernovae as “standard candles” for measuring distances – they are not standard! Despite the understood mechanism for SN formation is the reaching of criticality through accretion of the mass, and so a relatively small intrinsic dispersion in their characteristics, there is indeed a wide variation in their light curves as a function of time. Nonetheless, they are very useful to measure the rate of expansion of the universe if corrections are applied and if their spectra are measured well enough. Pain discussed the several projects to amass large amounts of data, and the possible reach in measuring cosmological constants, such as w. A few percent accuracy on that parameter is attainable in the near future, if the new projects will collect the data they promise
- Antony Lewis discussed “CMB and the Dark Ages“, where dark ages are not the beginning of the second millennium, but the time between the end of opacity of the universe (the moment when CMB radiation was released) and the birth of the first stars. He talked in detail about the interest of measuring the 21-cm line of Hydrogen triplet-singlet transitions, and the ability of that radiation of tracking very small scale variations compared with cosmic microwave background data. The spin temperature presents several challenges for a meaningful measurement, because its wavelength is long and scales with z, requiring large detecting structures. The atmosphere of the Earth is opaque to 21cm radiation for z>70, so one needs to get outside of it to make measurements. The moon could be a good place to set out one’s detection array, but terrestrial radio signals would interfere with the detection. So the proposal is to set up the experiment on the dark side of the moon…