In this work, we review the field perturbations in inflationary models with multi-sound speeds. To be able to do explicit calculation, we consider only two fields. We obtain the evolution equations of perturbations, then we project the field perturbations along (adiabatic) and perpendicular (entropy) to the background fields’ trajectory. Direct computation, we obtain the equation of the projections evolution, these equations cannot be written in the form of an evolution equation of a perturbed field with specified field equations, (in the evolution equation of a perturbed field, the coefficient of second order spatial derivative is proportion to square of propagation or sound speed), therefore the common formalism in which the adiabatic and entropy perturbations are defined and with their evolution equations, the influence on the observational parameters is investigated, cannot be applied to this model.
khoeini moghaddam,S and kheirkhah gheh,A . (2026). How to define adiabatic and entropy perturbations in the
multi-speed inflationary model. New Research in Physics, 2(1), 25-38. doi: 10.52547/jmrph.2.1.25
MLA
khoeini moghaddam,S , and kheirkhah gheh,A . "How to define adiabatic and entropy perturbations in the
multi-speed inflationary model", New Research in Physics, 2, 1, 2026, 25-38. doi: 10.52547/jmrph.2.1.25
HARVARD
khoeini moghaddam S, kheirkhah gheh A. (2026). 'How to define adiabatic and entropy perturbations in the
multi-speed inflationary model', New Research in Physics, 2(1), pp. 25-38. doi: 10.52547/jmrph.2.1.25
CHICAGO
S khoeini moghaddam and A kheirkhah gheh, "How to define adiabatic and entropy perturbations in the
multi-speed inflationary model," New Research in Physics, 2 1 (2026): 25-38, doi: 10.52547/jmrph.2.1.25
VANCOUVER
khoeini moghaddam S, kheirkhah gheh A. How to define adiabatic and entropy perturbations in the
multi-speed inflationary model. New Research in Physics. 2026;2(1):25-38 (In Persian). doi: 10.52547/jmrph.2.1.25
