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 manifestations of dark matter?

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

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

Dark matter is everywhere in the universe. For the first time it was addressed in the 1930s to explain the rapid movement of individual galaxies in galactic 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 that we can measure. The lines of evidence supporting dark matter are numerous and convincing, as physicist Ethan Siegel points out.

Dark matter is necessary for an explanation:

  • rotational properties of individual galaxies,
  • the formation of galaxies of various sizes, from giant ellipticals to galaxies the size of the Milky Way and tiny dwarf galaxies next to us,
  • interactions between pairs of galaxies,
  • The properties of galaxy clusters and galactic clusters on large scales,
  • The space network, including its filamentary structure,
  • spectrum of fluctuations of the cosmic microwave background,
  • observed effects of gravitational lensing of distant masses,
  • the observed separation between the effects of gravity and the presence of ordinary matter in collisions of galactic clusters.

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 the cosmology, we believe that each galaxy, including our own, contains a massive diffuse halo of dark matter surrounding it. Unlike stars, gas and dust in our galaxy, which are for the most part in a disk, the halo of dark matter should be spherical, because unlike the ordinary (on the basis of atoms) matter, dark matter does not “flatten” when you squeeze it . Also, dark matter should be denser near the galactic center and extend ten times farther than the stars of the galaxy itself. Finally, there must be small clumps of dark matter in each halo.

To reproduce the full set of observations listed above, as well as other, dark matter should not have any properties, except the following: it must have mass; it must interact gravitationally; it must slowly move the relativity of the speed of light; it should not interact much through other forces. All. Any other interactions are severely limited, but not excluded.

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

Earlier this week, a team of scientists studying the sources of gamma radiation around pulsars published their results in Science. In their work they tried to understand better where the observed excess of positrons came from. Positrons, electron antipodes, are usually produced in several ways: when accelerating ordinary particles to sufficiently high energies, colliding with other particles of matter and producing electron-positron pairs according to the Einstein formula E = mc 2 . We create such pairs in the course of physical experiments and can observe the creation of a positron astrophysically, either directly, in the search for cosmic rays, or indirectly, in the search for the energy signature of electron-positron annihilation.

These astrophysical positron signatures are found 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 in part of a diffuse background whose origin is unknown. One thing is for certain: we see more positrons than we expect to see. And this is known for a long time. PAMELA measured this, “Fermi” measured it, AMS aboard the ISS it measured. Most recently, the HAWC observatory measured extremely high-energy, TeV-level gamma rays and showed that these are strongly overclocked particles coming from medium-level pulsars. But, unfortunately, this is not enough to explain the observed excess of positrons.

For some reason, with every measurement of the excess of positrons, with each observation of an astrophysical source that does not explain it, the narrative flows into “we can not explain it, so dark matter is to blame.” 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,
  • supernova remnants.

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

Many people working in this field make a choice in favor of dark matter, because it will be a breakthrough if dark matter destroys and produces gamma rays and particles of ordinary matter. This would be a dream scenario for astrophysicists-hunters of dark matter. But the wishful thinking never led to major discoveries. Although dark matter is most often the explanation for the surplus of positrons, it is no more likely than aliens explaining the star Tabbi .

Applying for an explanation to Brenda Dingus, the chief researcher of the HAWC, Ethan Siegel received the following comment:

“Undoubtedly, there are other sources of positrons. But positrons do not go far from their sources, and not so many sources are nearby. The two best candidates were discovered by 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 the sources of positrons nearby, we discovered that the positrons very slowly leave their place of origin, and therefore do 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 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 a surplus of astrophysical positrons, we should not exclude that dark matter can be involved in the matter. But it is much more likely that other astrophysical processes explain these effects. When a mystery arises in science, everyone wants a revolution, but more often they get something ordinary.

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