Dark matter - "aliens" for astrophysicists?
With all our understanding of physical laws and the success of the Standard Model and general relativity theory, the universe has a number of observed phenomena, which it is impossible to explain. The universe is full of mysteries, from the star and ending with high-energy cosmic rays. Although we are gradually discovering space, we still do not know everything. For example, we know that dark matter exists, but does not know what its properties. Does this mean that we must attribute the manifestations of the dark matter all the unknown effects?
Riddles on dark matter as much, and the evidence of its existence. But to blame all the dark matter in the cosmos mysterious manifestations not only shortsighted, but also wrong. This is what happens when scientists are drying up good ideas.
Two bright large galaxies in the Coma cluster's center, each more than a million light-years in size. On the outskirts of the galaxy indicates the existence of a large halo of dark matter around clusters.
Dark matter has everywhere in the universe. For the first time it appealed to the 1930s to explain the rapid motion of the individual galaxies in galaxy clusters. This happened because all ordinary matter - matter consisting of protons, neutrons and electrons - are not enough to explain the total number of gravity. This includes stars, planets, gas and dust, interstellar and intergalactic plasma, black holes and anything else that we can measure. lines of evidence supporting dark matter, are numerous and convincing, as a physicist Ethan Siegel notes.
The dark matter needed to explain:
- rotational properties of individual galaxies
- the formation of galaxies of various sizes, from the giant elliptical galaxy to the size of the Milky Way and the tiny dwarf galaxies near us,
- interaction between pairs of galaxies,
- properties of clusters of galaxies and clusters of galaxies on large scales,
- Space Network, including the filamentary structure,
- range of fluctuations of the cosmic microwave background,
- observed effects of gravitational lensing of distant masses,
- observed separation between the effects of gravity and the presence of ordinary matter in collisions of galaxy clusters.
And on a small scale of individual galaxies, and the scale of the entire universe of dark matter is needed.
If you put all this in the context of the rest of cosmology, we believe that every galaxy, including our own, contain massive diffuse halo of dark matter that surrounds it. Unlike the stars, gas and dust in our galaxy, which are for the most part in the disc, the halo of dark matter should be spherical, as opposed to the usual (by atoms) of matter, dark matter is not "flattened" when you squeeze it . Also, dark matter should be densely near the galactic center and extend to ten times more than the stars of the galaxy itself. Finally, should be small clumps of dark matter in the halo of each.
To play a full set of observations listed above, as well as other, dark matter does not have any properties, except for the following: it should have a mass; it must interact gravitationally; it should slowly move relative to the speed of light; it should not interact strongly by other forces. All. Any other interaction strongly limited, but not eliminated. Why is it whenever astrophysical observations made with an excess of a certain type of ordinary particles - photons, positrons, antiprotons - the first thing people talk about dark matter?
Earlier this week, a team of scientists studying the gamma-ray sources around the pulsar, published their results in Science. In their work they tried to better understand where we observed an excess of positrons. The positrons antipodes electrons are usually produced in several ways: with conventional disperse particles to a sufficiently high energy, upon collision with other particles and substances from the production of electron-positron pair Einstein formula E = mc 2. We create such pairs in the physical experiments and can observe the creation of a positron astrophysical both directly, in the search for cosmic rays, and indirectly, when searching the energy signatures of electron-positron annihilation.
These astrophysical positronic signatures are found near the galactic center, focused on point sources, such as pulsars and microquasars located in a mysterious region of our galaxy, known as the Great Annihilator, and part of the diffuse background, the origin of which is unknown. One thing is certain: we see more positrons than expected. And it has long been known. PAMELA is measured, "Fermi" is measured, the AMS on board the ISS is measured. Most recently HAWC Observatory measured the extremely high energy, the TeV level gamma rays and shown that it is highly accelerated particles coming from pulsars average. But, unfortunately, this is not enough to explain the observed excess of positrons. For some reason, every measurement of excess positrons, with each observation of astrophysical source that it does not explain the narrative flows into the "we can not explain it, so to blame the dark matter." And this is bad, because there are many possible astrophysical sources that do not require anything exotic, such as:
- secondary production of positrons and gamma rays other particles,
- microquasars or something else, nursing a black hole,
- very young or very old pulsars, magnetars,
- remains of supernovae.
This list is not final, but it represents a few examples of what could be creating this surplus.
Many people working in this area opt for dark matter, because it will be a breakthrough, if dark matter destroys and produces gamma rays and particles of ordinary matter. It would be a dream scenario for astrophysicists hunters dark matter. But wishful thinking never led to major discoveries. Although dark matter often seems explanation excess positrons, it is no more likely than the aliens explaining star Tabby.
Referring to the explanations for Brenda Dingus, principal investigator HAWC, Ethan Siegel received the following comment:
"Sure, there are other sources of positrons. But positrons do not move far from their sources, and around not so many sources. The two best candidates were found HAWC, and now we know the number of positrons they produce. We also know how these positrons diffuse from their sources; slower than expected. Although we confirmed by sources close to the positron, we discovered that the positrons are very slowly moving away from their places of origin, and therefore do not create a surplus of positrons in the world. Excluding one possibility, we do other possibilities more likely. However, this does not mean that the positrons must come from dark matter. We do not mean it. "
It is quite remarkable that the positrons in these HAWC account for only 1% of positrons observed in other experiments, pointing to something else as the culprit celebration. When the observation is at variance with our traditional ideas of how a surplus of astrophysical positrons, can not be excluded that the case may be implicated dark matter. But it is much more likely that other astrophysical processes explain these effects. When the puzzle appears in science, everyone wants the revolution, but more often get something mediocre.