Study uses thermodynamics to describe expansion of the universe

The idea that the universe is expanding dates from almost a century ago. It was first put forward by Belgian cosmologist Georges Lemaître (1894–1966) in 1927 and confirmed observationally by American astronomer Edwin Hubble (1889-1953) two years later. Hubble observed that the redshift in the electromagnetic spectrum of the light received from celestial objects was directly proportional to their distance from Earth, which meant that bodies farther away from Earth were moving away faster and the universe must be expanding.

A surprising new ingredient was added to the model in 1998 when observations of very distant supernovae by the Supernova Cosmology Project and the High-Z Supernova Search Team showed that the universe is accelerating as it expands, rather than being slowed down by gravitational forces, as had been supposed. This discovery led to the concept of dark energy, which is thought to account for more than 68% of all the energy in the currently observable universe, while dark matter and ordinary matter account for about 27% and 5% respectively.

“Measurements of redshift suggest that the accelerating expansion is adiabatic [without heat transfer] and anisotropic [varying in magnitude when measured in different directions],” said Mariano de Souza, a professor in the Department of Physics at São Paulo State University (UNESP) in Rio Claro, Brazil. “Fundamental concepts in thermodynamics allow us to infer that adiabatic expansion is always accompanied by cooling due to the barocaloric effect [pressure-induced thermal change], which is quantified by the Grüneisen ratio [Γ, gamma].”

In 1908, German physicist Eduard August Grüneisen (1877–1949) proposed a mathematical expression for Γ_eff, the effective Grüneisen parameter, an important quantity in geophysics that often occurs in equations describing the thermoelastic behavior of material. It combines three physical properties: expansion coefficient, specific heat, and isothermal compressibility.

Almost a century later, in 2003, Lijun Zhu and collaborators demonstrated that a specific part of the Grüneisen parameter called the Grüneisen ratio, defined as the ratio of thermal expansion to specific heat, increases significantly in the vicinity of a quantum critical point owing to the accumulation of entropy. In 2010, Souza and two German collaborators showed that the same thing happens near a finite-temperature critical point.

Now Souza and fellow researchers at UNESP have used the Grüneisen parameter to describe intricate aspects of the expansion of the universe in an article published in the journal Results in Physics, presenting part of the Ph.D. research of first author Lucas Squillante, currently a postdoctoral fellow under Souza’s supervision.

“The dynamics associated with the expansion of the universe are generally modeled as a perfect fluid whose equation of state is ω = p/ρ, where ω [omega] is the equation of state parameter, p is pressure, and ρ [rho] is energy density. Although ω is widely used, its physical meaning hadn’t yet been appropriately discussed. It was treated as merely a constant for each era of the universe. One of the important results of our research is the identification of ω with the effective Grüneisen parameter by means of the Mie-Grüneisen equation of state,” Souza said.