Light meson gas in the QCD vacuum and oscillating universe

Abstract

We have developed a phenomenological effective quantum-field theoretical model describing the "hadron gas" of the lightest pseudoscalar mesons, scalar σ-meson and σ-vacuum, i.e. the expectation value of the σ-field, at finite temperatures. The corresponding thermodynamic approach was formulated in terms of the generating functional derived from the effective Lagrangian providing the basic thermodynamic information about the "meson plasma + QCD condensate" system. This formalism enables us to study the QCD transition from the hadron phase with direct implications for cosmological evolution. Using the hypothesis about a positively-definite QCD vacuum contribution stochastically produced in early universe, we show that the universe could undergo a series of oscillations during the QCD epoch before resuming unbounded expansion.

Cosmology from Strong Interactions

“…The hadronic vacuum term is negative had vac < 0 and appears to have a relatively small magnitude, i.e. had vac / QCD vac ≈ 0.15 [258]. The current u, d, s quark masses break the global chiral SU L (3) × SU R (3) symmetry explicitly, while it also gets broken spontaneously by means of a σ-field expectation value,…”

“…To shed more light on the microscopic theory in the canonical picture, the dynamical aspects of relativistic gluon and hot meson plasmas, both coupled to the homogeneous condensate, have been discussed in detail in Refs. [255][256][257] and [258], respectively.…”

“…Shortly after the confinement phase transition, the Universe enters the state of hot meson plasma whose thermal evolution has been thoroughly explored in the framework of the Linear Sigma Model (LσM) in Ref. [258]. This analysis exploits the non-perturbative method of generating functional derived from the effective Lagrangian at finite temperatures (for further references on this method, see Refs.…”

“…The effective LσM chiral Lagrangian accounting for the lightest scalar and pseudoscalar degrees of freedom π ± , π 0 , K ± , K 0 , K0 , η, η , and the σ-meson, in the hadron gas approximation at T = 0 reads [258],…”

“…Using the above formalism, in Ref. [258] the properties of different phases and transitions between them have been explored in detail. For instance, at temperatures T > T c , hadrons de-confine into quarks and gluons, while the condensates melts away, yielding a deconfined (or zero-condensate) phase of the QCD matter.…”