A new twenty page review article recaps the latest efforts to explain phenomena often attributed to dark matter and dark energy.
Dark sector, constituting about 95 percent of the Universe, remains the subject of numerous studies. There are lots of models dealing with the cause of the effects assigned to dark matter and dark energy. This brief review is devoted to the very recent theoretical advances in these areas: only to the advances achieved in the last few years.
For example, in section devoted to particle dark matter we overview recent publications on sterile neutrinos, self-interacting dark matter, dibarions, dark matter from primordial bubbles, primordial black holes as dark matter, axions escaping from neutron stars, and dark and usual matter interacting via the fifth dimension.
We also overview the second flavor of hydrogen atoms: their existence was proven by analyzing atomic experiments and is also evidenced by the latest astrophysical observations of the 21 cm spectral line from the early Universe.
While discussing non-particle models of the cause of dark matter effects, we refer to modified Newtonian dynamics and modifications of the strong equivalence principles.
We also consider exotic compact objects, primordial black holes, and retardation effects.
Finally, we review recent studies on the cause of dark energy effects. Specifically, we cover two disputes that arose in 2019 and 2020 on whether the observations of supernovas, previously interpreted as the proof of the existence of dark energy, could have alternative explanations.
Besides, we note a study of 2021, where dark energy is substituted by a new hypothetical type of dark matter having a magnetic-type interaction. We also refer to the recent model of a system of nonrelativistic neutral gravitating particles providing an alternative explanation of the entire dynamics of the Universe expansion. without introducing dark energy or new gravitational degrees of freedom.
The second flavor of hydrogen atomsBowman et al (2018) published a perplexing observation of the redshifted 21 cm spectral line from the early Universe. The amplitude of the absorption profile of the 21 cm line, calculated by the standard cosmology, was by a factor of two smaller than it was actually observed. The consequence of thus striking discrepancy was that the gas temperature of the hydrogen clouds was actually significantly smaller than predicted by the standard cosmology.The first attempt to explain this perplexing observation was undertaken by Barkana (2018). His hypothesis involved some unspecified dark matter colliding with the hydrogen gas and making it cooler compared to the standard cosmology. For fitting the observations by Bowman et al (2018), the mass of these dark matter particles should not exceed 4.3 GeV, as estimated by Barkana (2018).Thereafter McGaugh (2018) examined the results by Bowman et al (2018) and Barkana (2018) and came to an important conclusion. Namely, the observations by Bowman et al (2018) constitute an unambiguous proof that dark matter is baryonic, so that models introducing nonbaryonic nature of dark matter have to be rejected.What if dark matter baryons, which provided an additional cooling to the primordial hydrogen gas, whose existence has been already proven – rather than being some yet unknown, unspecified particles, suggested by Barkana (2018)?
This question has been posed and positively answered by Oks (2020a), as follows. . . .
At the end of this section, we note the following. As McGaugh (2018) demonstrated that the observations by Bowman et al (2018) clearly prove that dark matter is baryonic, Tatum (2020) hypothesized “cold atomic hydrogen in its ground state” might represent dark matter required for explaining the observations by Bowman et al (2018). However, Tatum (2020) disregarded the following. Ordinary hydrogen atoms, to which he referred, interact with the electric dipole radiation and thus can be excited to higher states (from the ground state) by absorbing a quantum of the electric dipole radiation. For this reason, the ordinary hydrogen atoms decouple from the cosmic microwave background at the stage predicted by the standard cosmology and therefore cannot justify the cooling to the temperature required for explaining the observations by Bowman et al (2018).3. Non-particle dark matterModified gravityModified gravity is also known as “modified Newtonian dynamics”. The motivation behind this hypothesis was to explain the observed “rotation curves”, i.e., why the stars in most galaxies are observed to move at similar speeds regardless of their distance from the center of the galaxy. These observations are contrary to the prediction by Newtonian dynamics, according to which stars at the edge of a galaxy should move with smaller velocities compared to stars that are closer to the galactic center. For removing the discrepancy between the theory and the observed rotation curves, Milgrom (1983) proposed that at the edge of galaxies the gravitational force is weaker than in Newton gravity.In later versions the modifications concerned the inertial mass. In the modified Newtonian dynamics, it was proposed that the inertial mass is the emergent (rather than inherent, as by Newton and Einstein) property of the object. Presumably, the net gravitational pull from the rest of the Universe (i.e., the external gravitational force) affects the inertial mass.For the very latest developments, tests, and restrictions on various modified gravity hypotheses we refer to the following two reviews published in 2021 and references therein. The first review is by Baker et al (2021). It discusses, in particular, which modify gravity models survive the (so far) available astrophysical tests. Another review is by Chen et al (2021). They discuss modified gravity in the context of quasinormal modes of perturbed black holes.The primary problem of modified gravity hypotheses is caused by the fact that galaxy clusters, even after being processed by means of the modified Newtonian dynamics, still show a residual mass discrepancy (McGaugh (2015). So, these hypotheses cannot completely eliminate the need for dark matter to exist.To conclude this brief discussion of the modified gravity we note the paper by Chae et al (2020) where they analyzed 175 galaxies as follows. If the modified gravity corresponds to reality, then the external field, caused by the distribution of nearby galaxies, should change the rotation curve of the galaxy under consideration. The authors found that such change was confirmed for galaxies, experiencing the strongest external field. However, there was no change in the rotation curve for galaxies experiencing a weaker external field. Thus, Chae et al (2020) did not disprove dark matter: a further study would be in order. . . .
Retardation effectsYahalom (2020) engaged retardation effects within general relativity for explaining the observed rotation curves of galaxies – without resorting to dark matter. This is an interesting idea from the theoretical point of view. However, it leaves without any explanation the gravitational lensing effect. So, the latter would still require introducing dark matter. . . .
There is a garden variety of hypotheses that, while attempting to explain dark matter, go beyond the Standard Model. Some of them propose never-discovered subatomic particles, while others propose to significantly change the known physical laws. Out of these hypotheses, the one that came relatively close to having an experimental confirmation (by Adlarson et al (2014)), is the dibaryon hypothesis. However, Bugg (2014) pointed out logical flaws in the interpretation of that experiment as the discovery of the dibaryon and provided an alternative explanation to the experimental results of Adlarson et al (2014).
There is only one explanation of dark matter that has the following four features simultaneously:
1) it has the experimental confirmation, namely from the analysis of atomic experiments (Oks 2001);
2) it does not go beyond the Standard Model (and thus is favored by the Occam razor principle);
3) it is based on the standard quantum mechanics, namely on the Dirac equation – without any change to the physical laws (and thus is favored by the Occam razor principle);
4) it explains the perplexing astrophysical observations by Bowman et al (2018), as well as the puzzling astrophysical observation by Jeffrey et al (2021).
This is the explanation based on the existing second flavor of hydrogen atoms (Oks 2020a, 2020b, 2021b). This explanation belongs to the category of baryonic dark matter. The results by Bowman et al (2018) and Barkana (2018), constituted a clear proof – according to the analysis by McGaugh (2018) – that dark matter is baryonic and hypotheses on non-baryonic nature of dark matter have to be excluded.
The situation with dark energy is less certain. The least exotic and the most popular hypothesis is the fifth fundamental force – quintessence, proposed by Peebles & Ratra (1988), which is a kind of a scalar field. Within the Standard Model, scalar fields must have relatively large masses, so that viable quintessence models should go beyond the Standard Model, as noted above. This hypothesis is popular, but whether the dark energy is quintessence is not decided by the popularity contest.
Besides, there are hypotheses that dark energy does not exist. These hypotheses – such as modified gravity etc, or the one by Oks (2021b) based on the application of Dirac’s Generalized Hamiltonian Dynamics – provide alternative explanations to the entire time evolution of the Universe.