"We finally got there" probably said an ancient Roman citizen when he saw for the first time that all the seven hills on the east side of the Tiber were finally joined together into the young, but inexorably expanding, city of Rome. Satisfaction and pride for the achieved result were probably in his mind, since it took Romulus and his descendants many decades to unite all the people inside the brand new Roman Walls, starting from the first tiny settlement on the Palatine Hill. It was a step, just a small and mostly obvious step, in the long walk of the history of Rome, but in the mind of our friend who saw that happening, it was still a great moment.
Today, with a proper rescaling, I think that in a similar way a modern cosmologist should feel great excitements and, why not, pride at looking at the Fig.6 of this paper where the seventh peak of the Cosmic Microwave Background power spectrum is finally detected by the Atacama Cosmology Telescope. It took the community ~10 years of experiments to go from the detection of the first peak (remember the old times of Boomerang and Maxima?) to this new measurements that nails, once again, the predictions of the Lambda-CDM Model.
Of course in the paper above and in its companion one there is much more than that. Thanks to the high resolution of the telescope frequency bands, the ACT team managed to measure the CMB fluctuations down to the arcminute scale on a decent portion of clean sky. Basically all the results confirm the current cosmological model. Exploring the intermediate multipoles (500 < l < 3000), it is possible to put limits on the cosmological parameters that affect the small scale CMB power. The ACT seems to deliver what it was built for: in these papers, with the help of the results of WMAP, they report very tight measurements of the abundance of primordial helium and of the number of relativistic species (neutrinos) together with better constraints on the value of the running spectral index and an improvement of the upper limit of the tensor to scalar ratio.
Moreover, at larger multipoles, for l > 3000, the power spectrum becomes more sensitive to a bunch of more local, astrophysical phenomena and it allows to estimate the contribution to the total power coming from different sources: the thermal and the kinetic SZ of the galaxy clusters, both the Poissonian distributed and clustered infrared point sources and the (in this patch) faint diffuse signal of our own Galaxy dust. It's easy to predict that those numbers will be a major benefit in the future high resolution CMB data analysis.
Still, among all these important results, findings and confirmations, to me the most beautiful remains the measurement of the seventh peak, a small step in the CMB walk, that I'm happy to witness today.
I reckon we should not forget about the third paper of the ACT series. The normalization issues, though technical, are of primary importance for putting things together (and no, I will not continue here the Roman analogy ;-)). The new constraints on the spectral index running as derived in one of the two remaining papers, for instance, are crucially dependent on that. And with the 7th peak being indeed a nice addition to the "CMB bestiary" I think that the nearly 2sigma detection of the index running (and from CMB data only) is one of the highlights of this effort and for many a step towards pinning down the inflation as a proper model for the evolution of the very early Universe.
ReplyDelete