With all our understanding of the laws of physics and the success of the Standard Model and general relativity, there are a number of observable phenomena in the Universe that cannot be explained. The universe is full of mysteries, from star formation to high-energy cosmic rays. Although we are gradually discovering space for ourselves, 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?
There are as many mysteries about dark matter as there is evidence of its existence. But to blame dark matter for all the mysterious manifestations of space is not only myopic, but also wrong. This is what happens when scientists run out of good ideas.
Two bright large galaxies at the center of the Coma Cluster, each over a million light years in size. The galaxies on the outskirts indicate the existence of a large halo of dark matter throughout the cluster.
Dark matter is everywhere in the universe. It was first consulted in the 1930s to explain the rapid movement of individual galaxies in galaxy clusters. This happened because all of ordinary matter - matter made up 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 supporting dark matter are numerous and compelling, as noted by physicist Ethan Siegel.
Dark matter is needed to explain:
- rotational properties of individual galaxies, - the formation of galaxies of different sizes, from giant elliptical to - galaxies the size of the Milky Way and tiny dwarf galaxies near us, Promotional video:
- interactions between pairs of galaxies, - properties of clusters of galaxies and galaxy clusters on a large scale, - the space network, including its threadlike structure, - spectrum of fluctuations of the cosmic microwave background, - the 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.
Both on a small scale of individual galaxies and on a scale of the entire Universe, dark matter is needed.
Putting all of this in the context of the rest of cosmology, we believe that every galaxy, including our own, contains a massive, diffuse dark matter halo surrounding it. Unlike the stars, gas and dust in our galaxy, which are mostly in the disk, the dark matter halo should be spherical because, unlike ordinary (atom-based) matter, dark matter does not "flatten" when you squeeze it … Also, dark matter should be denser at the galactic center and extend ten times farther than the stars of the galaxy itself. Finally, there should be small lumps of dark matter in each halo.
To reproduce the full set of observations listed above, as well as others, dark matter should not have any properties other than the following: it should have a mass; it must interact gravitationally; it must move slowly relative to the speed of light; it should not interact strongly through other forces. All. Any other interactions are highly restricted but not excluded.
Why, then, whenever an astrophysical observation is made with an excess of an ordinary particle of a certain type - photons, positrons, antiprotons - people first of all talk about dark matter?
Earlier this week, a team of scientists studying gamma-ray sources around pulsars published their findings in Science. In their work, they tried to better understand where the excess of positrons we observed came from. Positrons, antipodes of electrons, are usually born in several ways: when ordinary particles are accelerated to sufficiently high energies, when they collide with other particles of matter, and with the production of electron-positron pairs according to the Einstein formula E = mc2. We create such pairs in the course of physical experiments and can observe the creation of a positron astrophysically, both directly, in the search for cosmic rays, and indirectly, in the search for the energy signature of electron-positron annihilation.
These astrophysical positron signatures occur near the galactic center, targeting point sources such as microquasars and pulsars located in a mysterious region of our galaxy known as the Great Annihilator, and in a portion 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 it, Fermi measured it, AMS onboard the ISS measured it. More recently, the HAWC observatory measured extremely high-energy, TeV-level gamma rays and showed that they are highly accelerated particles coming from mid-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 every observation of an astrophysical source that does not explain it, the narrative flows into "we cannot 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:
- 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 definitive, but provides several examples of what could create this surplus.
Many people working in this field opt for dark matter because it would 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 for dark matter. But wishful thinking has never led to major discoveries. And while dark matter is most often presented as an explanation for the positron surplus, it is no more likely than the aliens explaining the star Tabby.
After asking Brenda Dingus, Principal Investigator for HAWC, for an explanation, Ethan Siegel received the following comment:
“There are undoubtedly other sources of positrons. But positrons don't stray far from their sources, and there aren't many sources nearby. The two best candidates were discovered by HAWC and we now know the number of positrons they produce. We also know how these positrons diffuse from their sources; slower than expected. Although we have confirmed the sources of positrons nearby, we have discovered that positrons are very slow to move away from their place of origin, and therefore do not create an excess of positrons on Earth. By eliminating 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."
Remarkably, the positrons in the HAWC data account for only 1% of the positrons seen in other experiments, pointing to something else as the hero of the day. When an observation is made that is at odds with our traditional ideas, as with a surplus of astrophysical positrons, it should not be ruled out that dark matter may be involved. But it is much more likely that other astrophysical processes explain these effects. When a mystery appears in science, everyone wants a revolution, but more often than not they get something ordinary.
Ilya Khel