Sergei D. Odintsov
Use Axion modified gravity cosmologyAbstract
We present a model of F(R) gravity in the presence of a string theory
motivated misalignment axion like particle materialized in terms of a
canonical scalar field minimally coupled with gravity, and we study the
cosmological phenomenology of the model, emphasizing mainly on the
late-time era. The main result of the paper is that inflation and the
dark energy era may be realized in a geometric way by an F(R) gravity,
while the axion is the dark matter constituent of the Universe. The F
(R) gravity model consists of an R2 term, which as we show dominates the
evolution during the early time, thus producing a viable inflationary
phenomenology, and a power law term ∼ Rδ with δ ≪
1 and positive, which eventually controls the late-time era. The axion
field remains frozen during the inflationary era, which is an effect
known for misalignment axions, but as the Universe expands, the axion
starts to oscillate, and its energy density scales eventually as we
show, as ρa ∼ a−3. After appropriately rewriting the
gravitational equations in terms of the redshift z, we study in detail
the late-time phenomenology of the model, and we compare the results
with the ΛCDM model and the latest Planck 2018 data. As we show,
the model for small redshifts 0 < z < 5 is phenomenologically similar to
the ΛCDM model, however at large redshifts and deeply in the matter
domination era, the results are different from those of the ΛCDM
model due to the dark energy oscillations. For the late-time study we
investigate the behavior of several well-known statefinder quantities,
like the deceleration parameter, the jerk and Om(z), and we demonstrate
that the statefinders which contain lower derivatives of the Hubble rate
have similar behavior for both the ΛCDM and the axion F(R) gravity
model. We conclude that the axion F(R) gravity model can unify in a
geometric way the inflationary epoch with the dark energy era, and with
the axion being the main dark matter constituent.
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