If the universe is constantly expanding, where or what is it expanding into?
Or, was the Big Bang not the creation of space (the Universe itself) but the occurrence of matter in it; ie space already existed?
How did that happen?
My theory is that someone made a nice, clean and tidy space and then someone else came along and put something in it. A bit like that Jones wretch at the office who insists on filling any empty pigeon hole even though he’s already got his own and they’re being saved for new people.
The Big Bang is the creation of space. Space itself if expanding, not into any pre-existing space. The Big Bang is also the creation of time, so there’s no sense in talking about what happened before the Big Bang either.
In convential big bang theory it’s not expanding in to anything as such, expansion just means that the distance between objects is increasing with time.
The basic idea of Big bang theory is Friedmann-Lemaitre-Robertson-Walker cosmology (Friedmann and Lemaitre arrived at the nascent big bang theory independently of each other and Robertson and Walker refined the theory), though it has naturally seen many refinements and additions since it was first theorized 85 years ago.
The basic idea of FLRW cosmology is the application of Einstein’s theory of general relativity to cosmology with the key assumption of the Copernican cosmological principle. The Copernican cosmological principle states that the universe is homogenous (the same at all points) and isotropic (the same in all directions). Using this assumption the FLRW metric was arrived at, the FLRW metric describes spacetimes where space appears homogenous and istotropic.
The Copernican cosmological principle can only apply to certain set of observers (as a simple local Lorentz transformation can transfrom an isotropic observer in to a non-isotropic observer). These observers are called co-moving FLRW observers and they are used to create a cooridnate system on FLRW spacetime called FLRW coordinates.
In FLRW cooridinates the cooridnate distance between any two FLRW observers is a function of coordinate time, it is pretty much impossible except under unphysically idealized conditions for the coordinate distance to be constant function of coordinate time. I.e. the distance must either be increasing or decreasing and when it is increasing the universe is said to be expanding.
Currently the universe is observed to be expanding and infact observations suggest it will continue to expand forever. If we rewind that expansion back in time we get to a point where the distance between some (or all) FLRW observers goes to zero and at this point in a matter-filled uinverse the density goes to infinity. This intial point is called the big bang singularity.