Scientists have proposed a way to find out if there are forces that do not manifest themselves in the interaction of ordinary matter and "emerge" only when it comes to dark matter. It is about the additional attraction or repulsion that is added to gravity.
A team led by Lijing Shao of the Max Planck Institute for Radio Astronomy proposes to study the orbits of binary pulsar systems for this purpose. The method and the first results of observations are described in a scientific article published in the journal Physical Review Letters.
Let us recall that, as far as we know, there are only four fundamental interactions, to which the whole variety of forces acting in nature is reduced. These are strong, weak, electromagnetic and gravitational interactions.
The first two manifest themselves only at distances less than the diameter of the atomic nucleus. Electromagnetic forces act between charged particles. They give rise to seemingly different phenomena such as, for example, the attraction of iron to a magnet, the elasticity of solids and the force of friction. However, such forces do not affect the movement of astronomical objects such as planets, stars or galaxies. Therefore, the only force that an astronomer needs to take into account when calculating the motion of celestial bodies is gravity.
Such results were obtained in the study of all particles discovered by mankind. However, most experts are sure that there is also dark matter, consisting of particles unknown to science, and it accounts for 80% of the mass of matter in the Universe. "Vesti. Nauka" (nauka.vesti.ru) talked in detail about what made scientists come to such extravagant conclusions.
What if dark matter acts on the trajectories of celestial bodies not only through gravity, but also through an unknown fifth force? This possibility cannot be ruled out when it comes to hypothetical particles with unknown properties.
You can check this tempting version like this. The best tested gravity model to date is General Relativity (GR). She gives detailed forecasts of the trajectories of celestial bodies. It is necessary to arrange a test of one of its basic predictions in two situations: when the influence of dark matter can certainly be neglected and when it is significant. If the results coincide, we can say that in both cases only gravity, described by general relativity, is involved. If the second case differs from the first, this can be understood in such a way that not only gravity acts on celestial bodies from the side of dark matter, but also some additional force of attraction or repulsion.
The principle established by Galileo and later confirmed in general relativity is well suited for this role: in a given gravitational field, the acceleration of gravity is the same for all bodies, regardless of their mass, composition and internal structure. This means that the inert mass (which determines what force must be applied to the body in order to give it a given acceleration) is equal to the gravitational mass (which creates the force of gravity). The last statement is known as the weak equivalence principle.
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In 2017, it was verified using an artificial Earth satellite with an error of no more than one trillionth of a percent. In this case, according to most experts, the influence of dark matter could be neglected, since the distance from the Earth to the satellite on an astronomical scale is small, and there is little dark matter between them.
The influence of the mysterious substance could be detected by studying the orbit of the moon. But here the weak principle of equivalence has been tested "only" to within thousandths of a percent, and then only thanks to the mirrors installed on the surface of Selena. The laser beam reflected by them makes it possible to find out the distance between the Earth and the Moon with an error of less than a centimeter.
The new test, proposed by Shao's group, is related to the study of the orbit of a binary system, one of the components of which is a pulsar. Until now, no one has used neutron stars to search for the fifth force from dark matter.
“There are two reasons why binary pulsars open up an entirely new way of testing such a fifth force between ordinary matter and dark matter,” Shao said in a press release from the study. - First, a neutron star consists of matter that cannot be created in a laboratory, many times denser than an atomic nucleus and consisting almost entirely of neutrons. Moreover, the enormous gravitational fields inside a neutron star, a billion times stronger than that of the Sun, could, in principle, significantly enhance the interaction [of a neutron star] with dark matter."
Recall that signals from pulsars arrive with a strict periodicity, sometimes with an accuracy of up to nanoseconds. Due to the motion of the neutron star in its orbit, the time of arrival of the pulses is shifted, which makes it possible to restore the parameters of the trajectory. The orbits of the most stable pulsars can be calculated with an error of less than 30 meters.
Especially suitable in this sense is the neutron star PSR J1713 + 0747, located about 3800 light years from Earth. It is one of the most stable pulsars known to mankind, with a period between pulses of only 4.6 milliseconds. PSR J1713 + 0747 is a binary system with a white dwarf. It is especially fortunate that the period of the pulsar's orbital motion is as much as 68 Earth days.
Let us explain that the longer the orbital period, the more sensitive the system is to violation of the weak equivalence principle. This is the difference with conventional prediction tests in general relativity, which require the tightest possible systems.
The pulsar and the white dwarf have different masses and different internal structures. Gravity, according to general relativity, does not care about this, and the acceleration of free fall in the gravitational field of dark matter for both bodies will be the same. But if from the side of this substance there is still some kind of attraction or repulsion (the same hypothetical fifth force), the additional acceleration given to them may depend on these parameters. In this case, the orbit of the pulsar will gradually change.
To detect such changes, Shao's team processed the results of more than 20 years of observations of the system with radio telescopes included in the European EPTA project and the American NANOGrav. No changes in the orbit could be detected. This means that in the case of a given specific system and the surrounding dark matter, the weak principle of equivalence is fulfilled with approximately the same accuracy as in the "lunar" experiment.
However, the point may be that the density of dark matter here was not high enough. The ideal "testing ground" would be the center of the Galaxy, where dark matter accumulates due to the powerful attraction from ordinary matter. Based on this, the team is looking for a suitable pulsar within 10 parsecs of the center of the Milky Way. Such a finding could increase the accuracy of the experiment by several orders of magnitude.
Let us recall that Vesti. Nauka has already written about a hypothetical non-gravitational interaction of dark matter with ordinary matter and radiation. Only it was not about the influence on the trajectories of celestial bodies, but other effects. Thus, dark matter may be responsible for the excess of positrons near the Earth, strange X-rays from galaxies and cooling of hydrogen in the young universe.
Anatoly Glyantsev