"Fixed universe" appears to be a term used contrary to
"fixed earth", which is the very odd belief that planet
Earth isn't moving in space, and, separately, isn't
rotating.
It also appears in some discussion of "determinism",
a belief that whatever happens couldn't have not happened,
versus "free will", in which you, personally, make choices.
It has been used to describe Sir Isaac Newton's cosmology,
in which material bodies have relative motions relative
to other physical bodies but light and other electromagnetic
radiation travel at constant speed in vacuum as waves in
an intangible gel called "ether", to Albert Einstein's,
in which light waves appear to have constant speed even
when theoretically you are passing rapidly through the
ether while observing the waves.
It could be applied to "Steady State" scientific cosmology,
an abandoned theory in which the universe always will
exist in the overall state that it is now, and, to some,
always has existed. This would require that the increase
in entropy in natural processes is somehow compensated for.
This perhaps best fits with mentioning "panta rei".