Did the universe need the presence of matter and radiation to start expanding?

Question

I have read this question:

Hence it is not possible that photons generated by stars is contributing to dark energy.

Could photons generated from the many trillions of stars be some how contributing energy to dark energy? Where do they go and what becomes of them?

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:

1. Did the universe need the presence of matter and radiation to start expanding?

Answer 1

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.

Answer 2

Lets look at the data the General Relativity (GR) model is describing. GR is a theory as far as mathematics goes. In order to be relevant to physics it has to model the data of the universe(in this case).

The data of the universe consist of numbers and observations of radiation and matter, if these are not there , there is no universe, only mathematics and science fiction.

Lets see the basic GR relativity mathematical relation

On the left hand side we have the space variables defining a tensor, on the right hand side is the energy momentum tensor. For a specific problem with energy and radiation on the right hand side, it is a complicated equation that has to be calculated for the specific problem of matter and radiation. Because of the mathematical equality between space and energy/momentum the mathematical solutions create what is called curvature of space.

If the right hand side is zero, there can be no solutions that show curvature of the space coordinates . Here this is mentioned

In general relativity, the symmetric stress–energy tensor acts as the source of spacetime curvature,..

I found this elaborate answer in Quora that may interest you.

In my opinion, mathematical equations can have any number of solutions, the job of physics analysis is to establish where those solutions describe the data and observations, and, important, are predictive . When there is no matter and energy one should pick to describe the physical situation those solution that are compatible with that, which have no curvature.

Answer 3

It could be that the initial virtual matter fields were constrained to our 3d space, while our space was embedded in a highly negatively curved 4d space (somewhat as described in pyrotechnical approaches) The virtual character could induce this negative curvature. It pulled (or pushed) the virtual stuff into real existence in a bang. Thereby imparting positive curvature and stopping the acceleration. We see the universe expanding in the 3d space to which matter is confined, but this 3d space expands in a 4d space. So it took only virtual matter.

If the matter in our universe, all distances between charged particles expanded at the same rate, the moving away from each other wouldn't be noticeable. All photons would maintain their wavelength, when measured to distances between interacting particles. But particles do interact. So thanks to this interaction we notice expansion. But for the initial speed spurt, inflation, only virtual matter was needed.

Answer 4

Your question addresses deep issues about the beginning of the universe as described by 'Concordance Cosmology' and you are right that there are contradictions.

The answer is that Concordance Cosmology is wrong.

General Relativity and Big Bang are correct, but the extra additions that have been added since about 1990, inflation and dark energy are wrong.

It would make the answer too long and subjective to go into further details about how the current cosmology could be improved.