The Fermi Space Telescope team has found dark galaxies in the sky with no stars, but dark matter smoldering. The discoverers do not yet believe in the find, and it is impossible to independently verify their results - scientists do not disclose where the candidates are
In addition to the large conflict of two cultures - "physicists and lyricists", introduced into circulation by Briton Charles Snow exactly 50 years ago, for centuries there has also been a small conflict concerning exclusively "physicists". It is a clash between theory and experiment, in which the former usually play the role of reckless liberals and the latter as responsible conservatives.
In the last couple of years in astrophysics, this conflict has nowhere manifested itself so clearly as in the history of dark matter particles, the transformation of which into the substance we are accustomed to, some scientists see, while others do not. Both beliefs are based on the same data.
Paparazzi and PAMELA
Decay and annihilation
Decay is called the spontaneous decay of particles, like the decay of a uranium nucleus or a neutron that leaves any atomic nucleus. Annihilation is the mutual destruction of particles when they meet each other, for example, the annihilation of an electron and a positron, and, in general, matter and antimatter.
The decay rate depends only on the number of unstable particles, and the annihilation signal determines the frequency of particle collisions with each other. Therefore, the decay rate is proportional to the density, and the annihilation rate is proportional to the square of this quantity. This is how astronomers hope to distinguish annihilation from decay in observational data.
The greatest tension of passion reached in mid-2008, when the international scientific experiment PAMELA aboard the Russian satellite Resurs-DK discovered an excess of high-energy positrons in the vicinity of the Sun. They could well have been born during the spontaneous decay or mutual annihilation of exotic particles, which are supposed to make up dark matter.
Of course, other explanations are possible, but the prospect of "seeing" invisible matter was so attractive that in order to obtain unpublished PAMELA data, rumors about which were circulating in the astrophysical environment, many young theorists went all out. Some even photographed unpublished PAMELA charts on mobile phones during the reports of the project participants at conferences and based on these data they wrote theoretical articles. Such brave souls, violating the unwritten ethical rules of the scientific community, have even been nicknamed "scientific paparazzi."
As a result, the PAMELA data were nevertheless formally published, but they still do not have an unambiguous interpretation. Someone thinks that these are traces of dark particles, someone blames neutron stars in the vicinity of the Sun for their appearance, someone generally believes that we are talking about unaccounted systematic errors in the operation of the PAMELA equipment.
Fog, fog
Many hoped that the situation would be clarified by the launch of the Fermi Space Observatory, which detects photons of very high energies. They may well be produced by the interaction of ordinary light with high-energy charged particles (this is the so-called Compton backscattering). And that is how the scientists hoped to clarify the situation with the PAMELA data.
WMAP Haze WMAP
found an excess of microwave radiation from the galactic center - the so-called "WMAP haze", which remains in the data after subtracting all known microwave sources from them. One of the most probable explanations for it is the synchrotron radiation of energetic electrons winding on the induction line of interstellar magnetic fields. Exactly on the same electrons, with the help of the inverse Compton effect, high-energy photons can be produced, which Fermi is able to see.
If dark matter particles really become a source of energetic positrons and electrons, then they should be born more often exactly where there is more dark matter. According to modern concepts, such places are considered to be the centers of galaxies. So astronomers were looking forward to seeing Fermi heading towards the heart of our own Milky Way. Moreover, astronomers received a hint of a large number of electrons here several years ago from the WMAP spacecraft.
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Fermi entered orbit in June 2008 and began collecting scientific data a few months later. According to the group's rules, the data from the telescope appears in public space only a year after they are received - in order to enable "their" theoreticians to skim the main scientific cream from them. The year was running out in early autumn, but one group of theoreticians did not wait and almost repeated the story with the "scientific paparazzi." According to an article that appeared in July, Fermi does see an excess of radiation towards the galactic center. Moreover, preliminary analysis showed that this radiation can be generated on exactly the same particles that the PAMELA project caught.
When Fermi's data were nevertheless published, the scientists repeated their analysis and more confidently stated: in addition to the "WMAP haze" there is also a "Fermi haze", into which the theory of decay or annihilation of dark matter fits well. This work, led by Gregory Dobler of the Harvard Center for Astrophysics, is no longer ashamed to refer even to serious scientists, although its results are not too different from the conclusions from the previous work of the same group.
Mister No
There is, however, one important caveat. Even if a large number of high-energy electrons and positrons exist in the region of the galactic center (and there are less and less doubts about this), then their origin from dark matter particles still needs to be proved. In principle, they may have other sources - for example, shock waves from supernova explosions or all the same neutron stars that remain at the site of such explosions. The center of the Galaxy should be teeming with both - simply because there are so many stars, some of which sooner or later explode. And even though the alternative models should be pretty "far-fetched", for many it is still a more acceptable explanation than some kind of dark matter.
"Dobler and company have stepped on thin ice," warned Elliot Bloom, one of the few pure theorists on the Fermi experiment team, after their article was published. In his heart, this person probably has to struggle with himself - a theorist who devoted half his life to the prospects of an indirect explanation of the nature of dark matter, he recently became "Mr. No" of the Fermi collaboration. It is he who most often has to comment on works like Dobler's article and convince colleagues and journalists that the conclusions of the "upstarts" are at least premature.
Ironically, it is Bloom's work (pdf file), presented on behalf of the collaboration in the form of a poster at the Fermi 2009 symposium in Washington, that another episode in the story of the observational detection of dark matter may begin. The results of this work drew attention to a well-known hotbed of physical rumors - the blog Resonaances, which is maintained by the Polish physicist Adam Falkowski from the American Rutgers University.
Dark galaxies
The Substructure Problem The
discrepancy between the predicted theory and the actual number of dwarf satellites in the Milky Way and other galaxies is called the substructure problem. Its standard solution is that there are dwarf galaxies around us, but stars do not form in them.
Recent evidence suggests that such an explanation may indeed work: the smallest of the recently discovered satellites of our galaxy really consist of only a few hundred stars. But their mass (it can be estimated by the motion of the stars) is much larger. Most of it is assumed to be contained in dark matter.
Bloom reasoned rightly: in order to exclude an alternative with the acceleration of electrons on shock waves, one must look where supernovae do not explode. Ideally - where there are no stars at all, and dark matter should be. According to the theory, such starless dark matter halos should indeed surround our Galaxy - the theory predicts a dozen or two times more dwarf galaxies than is actually observed.
To find what the stars do not illuminate, Bloom and his colleague Pin Wan had to shovel through the entire Fermi data archive in search of extended objects, the gamma radiation of which matches the model of decay or annihilation of dark matter particles. Moreover, such objects should not coincide with known sources, and the flux of photons from them should not change over time.
Bloom and Wang found 54 extended sources that stood out at least four standard deviations above the background. After examining each of them sequentially, the scientists rejected 50 potential "starless galaxies" as not meeting the chosen criteria. There are four left that meet the criteria. Above the background, they all stand out not even by four, but by at least five standard deviations.
Nonetheless, Bloom donned the "Mr. No" mask again and concluded that no new dark dwarfs had been detected in the Fermi data for the first ten months. The main argument given by the scientist is the discrepancy between the spectra of these sources to the chosen theoretical models of the decay of dark matter.
Subtle question
But this is ridiculous, Falkovsky believes, - give a normal theorist almost any spectrum in his hands, and he will come up with a model for you in 15 minutes that will describe this spectrum. About 15 minutes is, of course, an artistic exaggeration, but the models of decay and annihilation so far indeed provide a very wide room for theoretical maneuver.
Perhaps that is why Bloom does not give spectra. He does not give in his work the coordinates of the candidates or any other data about them.
All this is very intriguing, believes Falkovsky. Bloom does not say that there are no dark galaxies, he only claims that "they were not found in Fermi's data for the first ten months." Nobody knows what will happen with the data for the next years. What is clear is that Bloom, as a member of the Fermi collaboration, will have access to them before anyone else.
