Most of the Universe’s baryon mass lies in the intergalactic medium, is unaccreted, and can be treated as an ideal monatomic gas with a uniform comoving density. The thermal loss of such a gas is herein shown to partition into one-third gravity gain and two-thirds entropic gain. The entropic gain of an unbound gas with uniform comoving density is differentially expressed as instant pressure. This instant pressure is proposed as the principal cause of the Hubble parameter. Using gas laws, a three-term expression of the gas’s Hubble parameter can be derived from estimated present-day energy density values, which are extrapolated to the time of last scattering, when baryon content was all unaccreted atoms having a uniform comoving density. This "GCDM thermal model" gives an exclusive dependence of the Hubble parameter on baryon mass density. The ΛCDM model is similarly covariant but uses total energy as the central variable instead of baryon mass. Instant pressure at last scatter gives a Hubble parameter that is 25% larger than the value found from baryon acoustic oscillation calculations, which rely on ΛCDM’s Friedmann and fluid equations as accurate. These two equations appear to be inaccurate for that time period. Relativistic energy at last scatter gave increased instant pressure, as it made the baryons more dense. These two models’ opposing predictions of the effect of relativistic energy density on the Hubble parameter is proposed as the cause of the Hubble tension. Today, the intergalactic medium consists of a plasma. The plasma’s instant pressure is fed by Compton scattering and covariant with photon flux. Much of this pressure doesn’t obey the gas laws, as it arises from high-energy suprathermal electrons. This suprathermal pressure in the late Universe is expressed within the ΛCDM model by giving Einstein’s time-invariant Λ a nonzero value. The effects attributed to Λ are proposed to instead arise from the ratio of suprathermal to thermal energies in the intergalactic medium. Both the presently held time-invariance of Λ, and the Hubble tension, are proposed to arise from neglect of entropic gain. Entropic gain is not found within the ΛCDM model because the Friedmann and fluid equations which comprise the model are developed isoentropically.