Effective equation of state oscillations at matter–radiation equality and primordial gravitational waves
Odintsov S.D. Oikonomou V.K.
September 2025Elsevier B.V.
Physics of the Dark Universe
2025#49
The theory controlling the Universes evolution in the classical regime has to be motivated by particle physics reasoning and should also generate inflation and dark energy eras in a unified way. One such framework is F(R) gravity. In this work we examine a class of exponential deformations of R2 gravity motivated by fundamental physics of scalaron evolution in a de Sitter background. As we show this class of models describe both inflation and the dark energy era in a viable way compatible with the Planck constraints on inflation and the cosmological parameters. Regarding the inflationary era, the exponentially deformed R2 model also yields a rescaled Einstein–Hilbert term which remarkably does not affect the dynamics and the inflationary evolution is identical to that of an R2 model. The dark energy era is also found to be viable and mimics the Λ-Cold-Dark-Matter model. More importantly, this class of F(R) gravity exponential R2 deformations also has an important characteristic, and specifically it yields total equation of state oscillations deeply in the matter domination era, for redshifts z∼3400, so near the matter–radiation equality. These total equation of state deformations at such a large redshift may directly affect the energy spectrum of the primordial gravitational waves. Indeed as we show, the effect is measurable and it leads to an enhancement of the tensor perturbations energy spectrum for low frequencies probed by the future LiteBIRD mission. This enhancement might have a measurable effect on the B-modes of the Cosmic Microwave Background radiation and thus may be detectable by the LiteBIRD mission. Only a handful of theoretical frameworks can generate the gravitational wave pattern generated by the class of exponentially deformed R2 models we presented.
Dark Energy , Equation of State , F(R) gravity , Primordial gravitational waves
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Institute of Space Sciences (ICE, CSIC), C. Can Magrans s/n, Barcelona, 08193, Spain
ICREA, Passeig Luis Companys, 23, Barcelona, 08010, Spain
Laboratory for Theoretical Cosmology, International Center of Gravity and Cosmos, Tomsk State University of Control Systems and Radioelectronics (TUSUR), Tomsk, 634050, Russian Federation
Department of Physics, Aristotle University of Thessaloniki, Thessaloniki, 54124, Greece
L.N. Gumilyov Eurasian National University - Astana, 010008, Kazakhstan
Institute of Space Sciences (ICE
ICREA
Laboratory for Theoretical Cosmology
Department of Physics
L.N. Gumilyov Eurasian National University - Astana
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