What is causing space to expand?
Is it due to some kind of innate property or is it due to some force?
I did a google search on this and found lots of articles explaining that space is expanding but nothing as to why it is expanding.
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If we look around the distribution of matter in the universe seems to be roughly isotropic and homogeneous (if we go to large enough scales). General relativity tells us that matter distributed in this fashion must evolve with time in a way described by the FLRW metric.
To work out exactly what happens we need some initial conditions, so we look around and estimate the densities of the various types of energy, and starting from these we can describe how the universe changes with time.
All this relies on the fact that GR tells us how the spacetime geometry is related to the matter distribution. But if you're asking why spacetime geometry is related to the matter distribution then there is no answer. All we can say is that's the way the universe is. Maybe some more fundamental theory will give us more insight into why GR works, but maybe we'll never know.
The observation that galaxies and clusters of galaxies are moving away from earth in three dimensional space was established observationally by Hubble
Objects observed in deep space (extragalactic space, 10 megaparsecs (Mpc) or more) are found to have a Doppler shift interpretable as relative velocity away from Earth;
This Doppler-shift-measured velocity, of various galaxies receding from the Earth, is approximately proportional to their distance from the Earth for galaxies up to a few hundred megaparsecs away .
The simplest interpretation was that space was expanding, similar to the raisin bread model.
Thus expansion is an observational fact. Similar that an apple falls is an observational fact.
Searching for how this could happen lead to the Big Bang model , a general relativity model, that expansion started about fourteen million years ago and is continuing now. So it is within the Big Bang model that one understands the expansion of space time.
Edit after comment:
are you saying the cause of the expansion is general relativity
No, I am saying that in the framework of General relativity, i.e. four dimensional space described by an energy momentum tensor, a mathematical model has been built that incorporates data from astrophysical observations and uses elementary particle physics knowledge to model the beginning of the universe.
The model started like a four dimensional explosion at a beginning of time =0 but has developed to the undefined region at the left, rapid expansion, and then slower expansion as the universe we know is generated.
hisrory of universe click for original