The action of F(R) gravity is replaced by the function of the Ricci scalar instead of the
Einstein-Hilbert action of GR.
The functional degrees of freedom in F(R) gravity is written in terms of the scalar field,
and then, it is known that the F(R) gravity is equivalent to the scalar-tensor theory at the classical
level.
Scalaron as Dark Matter
From the viewpoint of cosmology, the new field corresponding to the modification of gravity is
responsible for the DE.
In the F(R) gravity, the scalar field degree of freedom arises, and it is responsible for the DE at
the classical level.
Moreover, it was proposed that the new field corresponding to the modification can be a DM candidate
when we treat this new field as a new particle.
When the oscillation of this scalar field around the potential minimum is quantized in the F(R)
gravity,
we may get a particle picture of the new scalar field, called the scalaron.
The scalaron is a massive scalar and singlet to the SM, and its coupling to the SM particles are
very weak of its gravitational origin.
In this point, the scalaron can be a CDM candidate.
Unlike the most of DM candidates in beyond-SM, however, the scalaron does not have extra symmetry
and can decay to the SM particles.
We calculated the decay width in the framework of the QFT and discussed the lifetime of scalaron.
Chameleon Mechanism
In the standard sense, the mass of scalar field should be naively at the Hubble scale.
Such a light scalar field is, however, easily detected an excluded by the observation because the
Compton wavelength is very large.
On the other hand, if the scalar field is massive enough for its propagation to be frozen out,
the Compton wavelength is small, and the scalar field can avoid the observational constraints.
However, such a heavy scalar field is irrelevant for the cosmology and gives the inconsequential
effect for the DE problem.
In order to solve this conflict between two different requirements in different scales, the
screening mechanism was proposed.
In the chameleon mechanism which is one of the screening mechanisms,
the scalar field potential depends on the energy-momentum tensor of matter fields.
This results in that the mass of scalar field changes depending on the matter fields surrounding the
scalar field.
If the potential of scalar field could be chosen so that the mass increases in high-density region
and decreases in the low-density region,
the chameleonic scalar field makes the theory relevant in two different regions: the cosmological
scale and Solar-System scale.
Cosmic History
The viable models of F(R) gravity possess the chameleon mechanism for the consistency with the
Solar-System constraints.
And also, it is naively expected that the decay width depends on the mass of the scalaron.
Therefore, taking into account the environment dependence of the chameleonic scalar field is of the
great importance to evaluate the lifetime of scalaron.
It was shown that the scalaron is very light in the current Universe although it is very heavy in
the early Universe
because the energy-momentum tensor decreases according to the expansion of the Universe.
The coincidence problem was also addressed,
to find the observed ratio of DE to DM with respect to energy density is relevant to this framework.
The mainstream of research on DM is to study new particles in the extended framework of SM.
In my work, the new particle, scalaron, is induced from the modification of gravity,
and it has a salient feature that the mass changes according to the chameleon mechanism.
Such properties cannot be found in the ordinary DM candidates in the existing models,
and thus, we proposed the totally new DM candidate as an application of modified gravity.
Reference
-
T. Katsuragawa and S. Matsuzaki,
"Cosmic History of Chameleonic Dark Matter in F(R) Gravity,"
Phys. Rev. D 97, no. 6, 064037 (2018).
-
T. Katsuragawa and S. Matsuzaki,
"Dark matter in modified gravity?,"
Phys. Rev. D 95, no. 4, 044040 (2017).
-
T. Katsuragawa and S. Matsuzaki,
"Modified Gravity Explains Dark Matter?,"
PoS KMI 2013, 032 (2014).