**Summary***:*

*◉ Recai Erdem and colleagues check the possibility of cosmic accelerated expansion by considering a case where matter is converted to radiation (or vice versa by particle physics processes).*

*◉ They found that cosmic accelerated expansion can be obtained in this way only if an intermediate state with negative equation of state forms during the conversion.*

*◉ They said it is difficult to obtain present cosmic accelerated expansion wholly through the usual particle physics interactions in this way since the localization scales of corresponding ρR’s for the usual particle physics processes are at the order of atomic scales i.e. at scales much smaller than the cosmological scales. *

◉ *Even when they have such an effect, these interactions will first accelerate the universe and then decelerate it in the time scale of the interaction time (which is smaller than ∼ 10^–8 sec), hence the net effect would be zero.*

*◉ Present cosmic accelerated expansion may be obtained in this way only if the life time of the resonance condensate has a cosmologically relevant time scale.*

Observations showed that the universe is undergoing accelerated expansion at present, and many theoretical arguments and observational evidence suggest that the universe must have undergone an accelerated expansion period at the early times as well. Although the standard explanations for these accelerated expansions are cosmological constant at present era and inflationary models at early times there are many alternative ways; for example, quintessence, f(R) models, and gravitational particle production. However all these models have some problems. There is a problem associated with cosmological constant called the cosmological constant problem, and it seems that the best way may be the use of some symmetry to make it cancel and seek another method for late time cosmic acceleration. Inflationary models usually employ at least one new postulated scalar, and need special initial conditions, a similar situation (although less severe) is true for quintessence models.

f(R) type modified gravity models use an extension of general relativity, in gravitational particle production the energy density of the universe is an open thermodynamical system that is assumed to acquire energy from gravitational field while the question of if the universe is a closed system in this case is not clear enough. Therefore it is useful to seek the possibility of additional alternative ways for accelerated expansion. In particular, it would be desirable to have a model where the accelerated expansion is achieved with a minimal extension of the standard models of particle physics and cosmology. Recai Erdem and colleagues in their paper, in the light of the fact that, coupling an energy density to another one, modifies its equation of state, they seek if an energy density transfer due to elementary particle processes may have the potential of providing a source for cosmic accelerated expansion.

Although the analysis in their paper, in principle, is applicable to all types of particle physics processes, they specified it to the case of conversion of heavy particles to light particles i.e. to the conversion of matter to radiation. In fact there must be an era of the creation of matter and radiation not only because the ordinary matter and radiation must be produced anyway but also to have a well defined model that may serve at all eras of the universe. Moreover in the standard lore of cosmology the ordinary matter and radiation are assumed to be produced by the decays or the collisions of some other particles such as Higgs particle, curvaton etc. at early times. Particle physics processes ranging from high energies to atomic physics have an important role at present as well. Therefore the possibility of using just matter and radiation (as in this paper) interacting through the particle physics processes for cosmic acceleration with minimal need for exotic matter is interesting. The results of the following analysis shows that obtaining cosmic acceleration through conversion of matter to radiation (or vice versa) seems impossible except through formation of an intermediate state with negative equation of state (e.g. a QCD-like condensate formed by intermediate particles produced in the particle physics processes).

They considered the Robertson-Walker metric:

and for simplicity they take k = 0 which is in agreement with observations of Patrignani and colleagues. For the illustration of the method they considered a simple case; a universe that consists of matter and radiation. They assume that, at some time t1, the energy density of either of matter or radiation starts to be transferred to the other through some particle physics processes such as those given in Figure 1 below.

It seems difficult to obtain the present cosmic accelerated expansion wholly through the usual particle physics interactions in this way since the localization scales of corresponding ρR’s for the usual particle physics processes are at the order of atomic scales i.e. at scales much smaller than the cosmological scales. Even when they have such an effect, these interactions will first accelerate the universe and then decelerate it in the time scale of the interaction time (which is smaller than ∼ 10^–8 sec), hence the net effect would be zero.– said Edrem.

This type of interactions may be relevant cosmologically only at early times (if they involve the usual particles) provided that a significant redshift takes place during their interaction time e.g. during the lifetime of the resonance particle. A very early time acceleration may be induced by fast out of equilibrium processes as those given in Figure 1 provided an intermediate state with ω < 0 forms. Present cosmic accelerated expansion may be obtained in this way only if the life time of the resonance condensate has a cosmologically relevant time scale.

Although they have considered such a toy model in this study, in order to entertain these possibilities in detail one needs to study different specific models in more detail along the lines given in this paper and confront it with observational data which is beyond the scope of this study that aims to seek the degree of possibility of obtaining the late time and the early time accelerated expansions of the universe in this way.

Specific models along these lines where different options for ρR and ρ'(t) / dt (where, ρ'(t) / dt is the rate of the energy density transfer from matter to radiation) are specified and their theoretical origins discussed and whose the results are confronted with observational data may be considered in future.– Concluded authors of the study

**Reference***: Recai Erdem, “Is it possible to obtain cosmic accelerated expansion through energy transfer between different energy densities?,” Physics of the Dark Universe, Volume 15, 2017, Pages 57-71, ISSN 2212-6864,**https://doi.org/10.1016/j.dark.2016.12.001.**(http://www.sciencedirect.com/science/article/pii/S2212686416300711)*

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