The internal energy and thermodynamic behaviour of a boson gas below the Bose–Einstein temperature

We have examined the issue of the kinetic energy of particles in the ground state of an ideal boson gas. By assuming that the particles, on dropping into the ground state, retain the kinetic energy they possess at the Bose–Einstein temperature TB, we obtain new expressions for the pressure and internal energy of the gas below TB, that are free of the difficulties associated with the corresponding expressions in current theory. Furthermore, these new equations yield a value for the maximum density temperature in liquid 4He that is very close to the measured value. ► A new equation of state for an ideal boson gas that is anomaly-free. ► A prediction of the existence of a density maximum in all ideal boson gases. ► Calculation of the temperature at which a density maximum will occur in liquid 4He.