I have read this question:
Hence it is not possible that photons generated by stars is contributing to dark energy.
The expansion of the universe is the increase of the distance between two distant parts of the universe with time.[1] It is an intrinsic expansion whereby the scale of space itself changes. The universe does not expand "into" anything and does not require space to exist "outside" it. Technically, neither space nor objects in space move. Instead it is the metric governing the size and geometry of spacetime itself that changes in scale.
https://en.wikipedia.org/wiki/Expansion_of_the_universe
It's not possible for the universe to be in a non-expanding phase and then enter an inflationary phase.
Was the universe already expanding before inflation occured?
So basically, in other words, does space expansion require the presence of matter of radiation in any way, or is space expansion independent of what exists in space?
The first answer says that radiation does not contribute to dark energy. The wiki article says that technically, neither space nor objects in space move, it is rather the metric that changes in scale.
So space expansion does not require radiation energy, it does not require matter to move, and creating more space does not require energy at all (in fact to our knowledge, space expansion just creates more dark energy). Does this mean that an empty universe, with no matter and radiation in it would expand too? The last answer says that the universe probably was always expanding.
Now my question is about this, if the universe was always expanding, then does this require the presence of any radiation or matter, or could a void universe expand without the presence of any matter or radiation? In other words, do we need the presence of matter and radiation to explain how the universe started expanding, or is expansion independent of those?
As far as I understand, space expansion is a widely accepted fact, that is independent of everything else, but I do not know whether explaining it needs the presence of matter and radiation.
Question:
- Did the universe need the presence of matter and radiation to start expanding?
Best Answer
The expansion of the universe is the relative motion of the stuff in it. It's meaningless to talk about expansion of a vacuum.
There are vacuum FLRW solutions: they are Minkowski space (for $Λ=0$), de Sitter space ($Λ>0$) and anti de Sitter space ($Λ<0$). All of them are maximally symmetric—every spacetime point looks the same as every other—and so there's no generally covariant way to define a time-varying scale factor, or any other quantity that varies with spacetime position. There are many different FLRW parametrizations of those spacetimes, or portions of them, that assign different cosmological times and scale factors to each point. They're just coordinate systems, no one more correct than any other, like inertial reference frames in special relativity.
During inflation, spacetime is not actually de Sitter. The inflaton field has a time-varying energy density (the slow roll) which you can use to define a time coordinate and a scale factor. If you don't count that as matter, then the universe doesn't need matter to expand. What it needs, regardless of nomenclature, is something that resembles a Hubble flow, i.e., something that has the defining symmetry of FLRW geometries and no larger symmetry. A random spacetime is not symmetric enough, and a FLRW vacuum is too symmetric, to be unambiguously expanding.