In the early stages of his life, the Universe was very hot and very dense. High energy photons prevented the formation of stable atoms, preventing electrons from binding to protons. But as the primordial Universe expanded, density and temperature decreased to the point where the first atoms finally formed and the radiation became free to propagate. The Universe, which for 380,000 years had been a dense and opaque cloud, became transparent and the primordial radiation began to propagate. Matter and energy had decoupled. This is the cosmic background radiation and is one of the decisive evidence in favor of the Big Bang and inflation.
Today, about 13.7 billion years after the primordial explosion, we are still able to observe that radiation, which propagates in an almost isotropic way throughout the Universe. Its spectrum is that of a black body at about 2.725 K. Because of the expansion of the Universe, the background cosmic radiation temperature has decreased to present value, and will continue to decline in the future.
As we said, the background cosmic radiation is almost isotropic, and has fluctuations of one part in one hundred thousand. These fluctuations, originating from quantum fluctuations of primordial matter, would be the seeds of future galaxies. By now you will have understood that mapping the cosmic background radiation is absolutely a priority in cosmology.
This is why astronomers have designed different, increasingly sensitive tools to get the most accurate map possible. The last in terms of time, and of course the most precise, is the ESA Planck satellite. Launched in 2009, Planck collected data until 2013 and in that year the first version of the map was released, obtained after only two observation steps. But Planck could do more. In fact, in 2015, a second version of the map was released, obtained from all the eight envisaged observation steps, which in addition to the temperature also included the polarization of the radiation. The latter, measuring if the radiation oscillates along a preferential direction, is fundamental to understand how energy and primordial matter interacted. However, in this second version it also included warnings, stating that the data were not accurate enough yet to be used in cosmology.
But scientists continued to work on it, and after three years of processing, they released the final version of the map a few days ago, which contains all the temperature and polarization data, with finally enough precision.
In this regard, said Reno Mandolesi, of the University of Ferrara, an INAF associate and principal investigator of Planck’s LFI (Low Frequency Instrument):
Finally we can elaborate a cosmological model based exclusively on temperature, or exclusively on polarization, or finally on both temperature and polarization. And all three correspond.
he increasingly precise measurements of the cosmic microwave background have allowed us to confirm the correctness of the inflationary model of the Universe and the existence of dark matter and dark energy (although we still do not know what they are), but it has also raised new questions, in particular on the Hubble constant.
The latter is a measure of the expansion rate of the Universe and also allows us to derive the age of the Universe with a simple calculation. Thanks to increasingly precise observations of cosmic distances, obtained also thanks to the Hubble space telescope, the constant should have a value of about 73.5 Km/s/Mpc with an error of just 2%, while from Planck’s observations the the correct value would be about 67.4 Km/s/Mpc with a lower error, equal to 1%. Such a discrepancy is not justifiable as a measurement error, so there must surely be some new physical phenomenon that we are currently ignoring, perhaps new particles or new forces responsible for this discrepancy.
This is what Planck has left us: many confirmations, lots of data, many new questions that will keep scientists busy for the next few decades.