Dark matter: "aliens" for astrophysicists?


With all our understanding of the laws of physics and the successes of the Standard Model and the general theory of relativity, there are a number of observable phenomena in the universe that can not be explained. The universe is full of mysteries, ranging from star formation to high-energy cosmic rays. Although we are gradually discovering the cosmos, we still do not know everything. For example, we know that dark matter exists, but we do not know what its properties are. Does this mean that we should attribute all unknown effects to the manifestations of dark matter?

Mysteries on the subject of dark matter as well as evidence of its existence. But blaming the dark matter of all the mysterious manifestations of the cosmos is not only myopic, but also wrong. It happens when scientists are running out of good ideas.

Two large, bright galaxies in the center of the Coma cluster, each more than a million light-years in size. The galaxies in the outskirts indicate the existence of a large halo of dark matter along the congestion.

Dark matter is everywhere in the universe. It was first addressed in the 1930s to explain the rapid movement of individual galaxies in galaxy clusters. This happened because all the usual matter, a substance consisting of protons, neutrons and electrons, is not enough to explain the total amount of gravity. This includes stars, planets, gas, dust, interstellar and intergalactic plasma, black holes and everything else we can measure. The lines of evidence that support dark matter are numerous and convincing, as noted by physicist Ethan Siegel.

Dark matter is necessary for an explanation:

  •     rotation properties of individual galaxies,
  •     the formation of galaxies of various sizes, from giant ellipticals to galaxies the size of the Milky Way and small dwarf galaxies close to us,
  •     interactions between pairs of galaxies,
  •     The properties of galaxy clusters and galaxy clusters on a large scale,
  •     The spatial network, including its filamentous structure,
  •     spectrum of fluctuations of the cosmic microwave background,
  •     observed effects of the gravitational lens of distant masses,
  •     the observed separation between the effects of gravity and the presence of ordinary matter in collisions of galactic clusters.


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And on a small scale of individual galaxies, and on the scale of the entire universe, dark matter is necessary.

Putting all this in the context of the rest of cosmology, we believe that each galaxy, including ours, contains a massive diffuse halo of dark matter that surrounds it. Unlike stars, gas and dust in our galaxy, which are mostly in a disk, the halo of dark matter should be spherical, because unlike ordinary matter (based on atoms), matter dark does not "flatten" when you squeeze it. In addition, dark matter should be denser near the galactic center and extend ten times farther than the stars of the galaxy itself. Finally, there should be small groups of dark matter in each halo.

To reproduce the complete set of observations listed above, as well as others, dark matter should have no property, except the following: it must have mass; it must interact gravitationally; must slowly move the relativity of the speed of light; I should not interact much through other forces. All. Any other interaction is severely limited, but not excluded.

Why, whenever an astrophysical observation is made with an excess of an ordinary particle of a certain type, photons, positrons, antiprotons, do people speak first of dark matter?

Earlier this week, a team of scientists who studied sources of gamma radiation around pulsars published their results in Science. In their work they tried to better understand where the observed excess of positrons came from. Positrons, antipodes of electrons, generally occur in several ways: by accelerating ordinary particles to sufficiently high energies, colliding with other particles of matter and producing electron-positron pairs according to Einstein's formula E = mc2. We create such pairs in the course of physical experiments and we can observe the creation of a positron astrophysically, either directly, in the search for cosmic rays, or indirectly, in the search for the energetic signature of the electron-positron annihilation.

These positronic astrophysical signatures are located near the galactic center, focused on point sources such as microquasars and pulsars located in a mysterious region of our galaxy known as the Great Annihilator, and partly of a diffuse background whose origin is unknown. One thing is certain: we see more positrons than we expect to see. And this has been known for a long time. PAMELA measured this, "Fermi" measured it, AMS on board the ISS he measured. More recently, the HAWC observatory measured extremely high energy TeV-level gamma rays and demonstrated that these particles are strongly overclocked from mid-level pulsars. But, unfortunately, this is not enough to explain the observed excess of positrons.

For some reason, with each measurement of positron excess, with each observation of an astrophysical source that does not explain it, the narrative flows toward "we can not explain it, so dark matter is the culprit". And this is bad, because there are many possible astrophysical sources that do not require anything exotic, for example:


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


This list is not definitive, but it represents some examples of what this surplus might create.

Many people working in this field make a choice in favor of dark matter, since it will be a breakthrough if dark matter is destroyed and produces gamma rays and particles of ordinary matter. This would be a dream scenario for astrophysicists: hunters of dark matter. But the illusions never led to great discoveries. Although dark matter is often the explanation for positron excess, it is not more likely than the extraterrestrials that explain the Tabbi star.

When requesting an explanation from Brenda Dingus, the chief investigator of the HAWC, Ethan Siegel, received the following comment:

    "Undoubtedly, there are other sources of positrons, but the positrons do not stray far from their sources, and there are not many sources nearby." The two best candidates were discovered by HAWC, and now we know how many positrons they produce. positrons diffuse from their sources, slower than expected.Although we confirmed nearby positron sources, we discovered that positrons very slowly abandon their place of origin and, therefore, do not create an excess of positrons on Earth. we make other possibilities more likely, however, this does not mean that positrons MUST come from dark matter, we do not mean this. "

It is very remarkable that the positrons in the HAWC data explain only 1% of the positrons observed in other experiments, pointing to something else as the culprit of the celebration. When observing, diverging from our traditional ideas, as with an excess of astrophysical positrons, we should not exclude that dark matter may be involved in the matter. But other astrophysical processes are much more likely to explain these effects. When a mystery arises in science, everyone wants a revolution, but more often they get something ordinary.

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