Did the Big Bang exist?
Published online the first results of the satellite ESA polarized dust emission at high galactic latitudes. Results, which put a heavy question mark on the detected signal to the South Pole from BICEP2, too contaminated to be the unmistakable signature of primordial gravitational waves.
It was to be the signal oldest of the universe, the echo of cosmic inflation, the signature of primordial gravitational waves. But it looks like it is not nothing but a shadow on a dusty window. Or rather, whatever is out there, the glass is too dirty to say what it is. To put a tombstone on the enthusiasm generated by the announcement – declaimed loudly six months ago by researchers from Harvard BICEP2, during a press conference planetary – the detection of primordial B modes in the signal in the polarization of the cosmic microwave background , is a long article on the net tonight loaded by the team of the European Space Agency’s Planck satellite.
An article greatly anticipated by cosmologists, because if it is true that from March 17 to today’s enthusiasm has been gradually diminishing as they emerged uncertainties and doubts, only in the data to nine frequencies Planck could discern with reasonable certainty the ‘origin of the signal: the dawn of the universe, just moments after the Big Bang, or more prosaically the galactic dust that surrounds us?
A doubt, this, that cosmologists have had from the very first minutes of the presentation of the results of BICEP2. Results transposed immediately with amazement, when not just with skepticism. The concerns were due to the enormous difficulty that involves removing the contamination due to the galactic foreground, first of all the dust and the synchrotron. Already, galactic dust and synchrotron radiation: these are the two black beasts of BICEP2 and similar experiments. The first at frequencies a bit ‘higher, the second at lower frequencies, both imprinted on the maps of the polarization of the microwave sky a signature damn similar to that which would have to leave, if the inflationary model is correct, gravitational waves immediately following the Big Bang. To minimize the possibility of confusion, there are basically two methods: the first is equipped with tools to recognize the signatures “false”, as does Planck, while the second is to observe the sky only through the “windows” cleaner. And it is this second method that adopted by BICEP2.
There is dust on the windows, even on the upper floors
The cleaner, among the hundreds of windows of the palace of glass that is the Milky Way, the galaxy in which we stand, had been identified by the team of BICEP2 in a hole visible from the South Pole. Why that? In part, and here the story is tinged with yellow, thanks to a map in polarization Planck leaking prematurely: not being able to wait for the official data, in fact, researchers BICEP2, according to the reconstruction made by Adrian Cho in Science, they had to make do with what little they had: a pdf file presented at a conference. To further reduce the contamination by dust and synchrotron, researchers BICEP2 had also designed their experiment so as to be sensitive to the frequency less disturbed by the two signals foreground: with detectors to 150 GHz, a window, “clean” from the point of view of the electromagnetic band, above the peak of the synchrotron and below that of the powder.
Of course, the ideal would be not be forced to a choice. We should be able to look through all the windows of the palace of glass and many frequencies. And that’s exactly what he did the Planck mission, with its all-sky survey in nine frequencies, from 30 to 857 GHz. But Planck has its weak point: its sensitivity to polarized radiation is much lower than that of BICEP2 . In short, as polarization, BICEP2 is designed to see what’s out, Planck to characterize the dirt that is in the window. The result of this second venture is published today, and in fact says that the “glass over the South Pole” was not as clean as it was hoped, at least with regard to the dust.
“The publication of data on the powder to 353 GHz polarized emphasized that all or part of the signal observed by BICEP2 to 150 GHz, and attributed by the team to the polarization of the cosmic background radiation in the ways B, could be due to dust galactic alas, pervades the whole sky, even regions at high galactic latitudes, first considered free of dust, “says N. Mandolesi, responsible for the product at low frequency on board Planck (LFI) and associated INAF. “It is however confirmed that the contribution of the output from the synchrotron, the other active galactic contaminant at frequencies below 150 GHz, does not contaminate the signal BICEP2 for more than 2 percent.”
We must therefore conclude that BICEP2 has not seen the polarized signal? No, not at all, the Planck data do not affect in any way the detection of the signal at 150 GHz. It is its origin which is questioned: it is not insignificant contribution of dust, the probability that it is due to primordial gravitational waves is reduced significantly.
BICEP and Planck together to hunt for ‘r’
But the most important conclusion, at least for the moment, does not have much to do with cosmology as with the relationship between the teams of the two experiments. If BICEP2 is more sensitive to the polarization and Planck better able to distinguish between background and foreground, it looks like to find ways primordial B – if they really exist – the two teams will have to sacrifice at least in part while stimulating the competitive spirit that he probed them so far and start collaborating. As already have begun to do.
“Since August team tight Planck and BICEP2 are working together as part of a MOU (memorandum of understanding) signed by the Planck Science Team and the leader of BICEP2, with the main purpose to obtain an estimate of the ratio scientifically convincing tensor-scalar primordial, the so-called “factor r”, based on measures of BICEP2 and Planck in the region of the sky observed by BICEP2. Then get to write a common work, possibly coinciding with the release of data Planck in 2014, “anticipates Mandolesi.
Concretely, this means that access to data will be mutual, and neither team will have more recourse to ruses such as extraction from a pdf or a slide presented at a seminar. And if all goes well later this year, probably before or during the international conference on the polarization microwave scheduled in Ferrara in early December, we will have the first results of this unprecedented joint work.