If we assume that the universe does not expand and we manage to fill the entire universe with 1 atmosphere (atm) of oxygen gas, several things would occur:
Homogeneity: The gas molecules would be distributed evenly throughout the universe. This assumes that the universe is a homogeneous and isotropic space, which is a simplification as the actual distribution of matter in the universe is clumpy and structured.
Pressure and Temperature: The gas would reach an equilibrium state with a pressure of 1 atm. However, the temperature of the gas would depend on its initial conditions and the amount of heat transfer occurring within the universe. In reality, the temperature of the universe varies across different regions and is influenced by various factors such as cosmic microwave background radiation and the presence of galaxies and other celestial objects.
Density and Expansion: Filling the entire universe with gas would increase the overall density of matter. However, if we assume that the universe does not expand, it suggests that the forces countering gravity (such as dark energy) are not present or are balanced out. Without expansion, the gas density would remain relatively constant, assuming no further processes like star formation or other cosmological events.
It's important to note that the assumption of a static universe contradicts our current understanding of cosmology. Observational evidence strongly supports the concept of an expanding universe, as described by the Big Bang theory. The universe's expansion is a fundamental aspect of modern cosmology and is backed by extensive empirical observations.
While contemplating hypothetical scenarios can be intriguing, it's crucial to remember that our understanding of the universe is based on scientific models and observational data, which indicate that the universe is indeed expanding.