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Thermodynamics used to describe expansion of the cosmos

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

A startling new ingredient was added to the model in 1998 when observations of very far away supernovae by the Supernova Cosmology Project and the High-Z Supernova Search Team revealed that the expansion of universe is speeding up, rather than slowing down due to gravitational forces, as had been supposed. This discovery led to the idea of dark energy, which is believed to account for more than 68% of all the energy in the presently observable universe, while ordinary matter anddark matter account for about 5% and 27% 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) offered a mathematical expression for Γeff, the effective Grüneisen parameter, a key quantity in geophysics that often occurs in equations describing the thermoelastic conduct of material. It combines three physical properties: expansion coefficient, isothermal compressibilityand specific heat.

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

Now Souza and fellow scientists at UNESP have used the Grüneisen parameter to describe tricky aspects of the expansion of the cosmos in an article published in the journal Results in Physics, presenting part of the Ph.D. research of first author Lucas Squillante, presently 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 stated.

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