Dark matter — "aliens" for astrophysicists?

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2017-12-04 12:00:10

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With all our understanding of the laws of physics and the success of the Standard model and General relativity, in the Universe there are a number of observed phenomena that can't explain. The universe is full of mysteries, ranging from star formation and finishing of high-energy cosmic rays. Although we gradually discover the space, we still don't know. For example, we know that dark matter exists but don't know what are its properties. Does this mean that we have to ascribe the manifestations of dark matter is unknown all the effects?

Riddles on the subject of dark matter the same as evidence of her existence. But to blame dark matter in all the mysterious manifestations of space is not only shortsighted, but also wrong. This is what happens when scientists run out of good ideas.

Two bright and large galaxy in the center of the Coma cluster, each more than a million light years in size. Galaxy on the fringes indicate the existence of a large halo of dark matter around the cluster.

Dark matter there is in the Universe everywhere. For the first time turned to her in the 1930-ies to explain the fast motion of the individual galaxies in galactic clusters. It happened because of all ordinary matter — matter consisting of protons, neutrons, and electrons — are insufficient to explain the total number of gravity. This includes stars, planets, gas, dust, interstellar and intergalactic plasma, black holes and everything else we can measure. Lines of evidence supporting dark matter, numerous and convincing, as noted by physicist Ethan Siegel.

Dark matter needed to explain:

the
    the
  • rotational properties of the individual galaxies,
  • the
  • the formation of galaxies of different sizes, from giant elliptical galaxies to the size of the milky Way and the tiny dwarf galaxies near us
  • the
  • interactions between pairs of galaxies
  • the
  • properties of clusters of galaxies and galaxy clusters on large scales,
  • the
  • space network, including its filamentous structure,
  • the
  • range of fluctuations of the cosmic microwave background
  • the
  • the observed effects of gravitational lensing of distant masses,
  • the
  • the observed separation between the effects of gravity and the presence of normal matter in collisions between galaxy clusters.

And in the small scale of individual galaxies and across the Universe, dark matter is necessary.

If you put all this in the context of the rest of cosmology, we think that every galaxy, including our own, contains diffuse massive dark matter halo surrounding it. Unlike stars, gas and dust in our galaxy, which are mostly in the disk, dark matter halo should be spherical, because in contrast to conventional (based on atoms) of matter, dark matter is not "flattened" when you squeeze it. Also dark matter should be denser at the galactic center and extends ten times further than the stars of the galaxy. Finally, there should be small clumps of dark matter in each halo.

To reproduce the full set of observations listed above, and other, dark matter should not have any properties except the following: it must have a mass; it must interact with gravity; it needs to move slowly of relativity the speed of light; it should not strongly interact through other forces. All. Any other interaction is severely restricted, but not excluded.

Why, whenever produced astrophysical observation with an excess of ordinary particles of a certain type of photons, positrons, antiprotons — the first thing people say about dark matter?

Earlier this week, a team of scientists studying the sources of gamma-radiation around the pulsar, has published their results in Science. In their work they tried to better understand where we observed an excess of positrons. Positrons, and antipodes of the electrons are usually born several ways: the normal acceleration of particles to high enough energies, when colliding with other particles and with the production of electron-positron pairs in the Einstein formula E = mc2. We create such pairs in physical experiments and can observe the creation of a positron astrofizicheskie, directly in the search for cosmic rays, and indirectly, when searching for energy signatures of electron-positron annihilation.

These astrophysical positron signatures occur close to the galactic center, focused on point sources such as microquasars and pulsars, located in the 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 for sure: we're seeing more positrons than expected to see. And this has long been known. PAMELA is measured "Fermi" is measured, AMS aboard the ISS is measured. More recently, the HAWC Observatory has measured an extremely high-energy, TeV-level gamma-rays and showed that this highly dispersed particles coming from the pulsar's average level. But, unfortunately, this is not enough to explain the observed excess of positrons.

For some reason, every measurement of the excess positrons, with each observation of an astrophysical source, which does not explain, narrative spills over into "we can't explain it, why blame dark matter." And this is bad, because there are many possible astrophysical sources that don't require anything more exotic example:

the
    the
  • secondary production of positrons and gamma rays, other particles
  • the
  • microquasars or something else, feeding black holes,
  • the
  • a very young and very old pulsars, Magnetar,
  • the
  • remains of a supernova.

This list is not exhaustive but represents a few examples of what could create this surplus.

Many working in this field make a choice in favor of dark matter, because it will be a breakthrough, if dark matter annihilates and produces a gamma rays and particles of ordinary matter. It would be a dream scenario for astrophysicists hunt for dark matter. But wishful thinking has never led to major discoveries. Although dark matter often seems to be the explanation of the excess positrons, it is not more likely .

Asking for explanations to the Brand Dyngus, chief researcher HAWC, Ethan Siegel has received the following review:

"Undoubtedly, there are other sources of positrons. But positrons don't go far from their sources, and there are not so many sources. The two best candidates were discovered HAWC, and now we know the number of positrons they produce. We also know how these positrons diffuse from their sources; slower than anticipated. Although we confirmed the sources of positrons in the area, we discovered that positrons are very slowly moving away from their places of origin, and therefore does not create a surplus of positrons on Earth. Excluding one possibility, we make other possibilities more likely. However, this does not mean that positrons MUST come from dark matter. We don't mean it."

It was Very remarkable that the positrons in HAWC data explain only 1% of the positrons observed in other experiments, pointing to something else as the culprit of the celebration. When the observation at variance with our traditional ideas as to excess of astrophysical positrons, one should not exclude that in business can be involved with dark matter. But it is much more likely that other astrophysical processes explain these effects. When the science appears a mystery, we all want revolution, but most often you get something mediocre.

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