[Cidermaker]

I think VitaEstBrevis' post (the one before last, talking about cosmology) is a bit wide of the mark.

The talk of edges is misleading. Granted, we cannot see the whole of the Universe, but we can make local observations, and propose models based on these. None of these models includes an edge in the sense of a boundary, beyond which there is something that is not the Universe. Some of these models say that space is curved back on itself, analagously to the way the surface of a sphere (e.g. the Earth) is curved, and some of them say it goes on forever, like a plane.

What VEB is talking about is usually called a horizon of one sort or another, since it is the limit of what an observer can observe, but of course, another observer situated on the first observer's horizon will see something pretty similar to what the first observer sees: no edges, just horizons. Again, the analogy with the surface of the Earth is a good one.

This brings me on to the founding principal of (scientific) cosmology: that the Universe is homogeneous (i.e. there are no special places), and that the Universe is isotropic (i.e. there are no special directions). This is often called a statement of cosmic humility: we do not occupy a special place in the Universe. This principal is borne out by observations on large scales (much bigger than galaxies).

The other foundation of cosmology is the observation that distant objects are receding from us at a speed that increases with distance. We should be careful: the observation is that light from distant objects is red-shifted, and while the standard interpretation is that this is caused by recession, there are some theories that explain it differently (e.g. "tired light" theories). The appeal of the standard approach is that it uses laws of physics that can be observed directly here on Earth.

In the decades following the discovery of the red-shift (by Hubble, in the 1920s, I think), there were two competing models: The Big Bang; and the Steady-state model of Hoyle et al.

The Big Bang model very naturally incorporates the recession of distant objects, but hand-in-hand with that is the view that if the Universe is expanding now, at sometime in the past, it must've been a lot (infinitely) smaller. The Steady-state model tries to avoid this "beginning" of the Universe by saying the Universe is infinitely old, and that although it is expanding, matter is being spontaneously created to fill up the gaps, as it were.

The Big Bang model "won" with the discovery of the cosmic microwave background radiation: the Universe was filled with very hot radiation shortly after the Big Bang, which has now cooled to 2.7K. While this is a natural prediction of the BB model, the Steady-state model struggles to explain this observation.

The question then remains: will the Universe continue to expand indefinitely, or will gravity slow down the expansion, turn it around, and return the Universe to a Big Crunch? In the late 1990s, observations of distant supernovae (exploding stars, very bright) showed that the expansion is accelerating, which frankly, came as a bit of a surprise. This then begs the question: why on Earth is it expanding? I don't think anyone really knows (yet).

Kind regards,

Cidermaker.