Recent content by Steve Crow

All these comments have been most helpful. Here is a summary of my current understanding. 1. An expanding empty space-time is flat. 2. Because of diverging world lines, the 3-space component of an expanding empty space-time is curved. 3. The curvature parameter of the three-space component...

That is a compelling argument. The Big Bang itself induces curvature. "Only by introducing space-time curvature can you have such a congruence where the surface of common proper time from the beginning is spatially flat". Bravo! Of course I still have to think a lot about it, but I shall not...

I think Orodruin's answer is elegant, not crass. An equally elegant answer would be: "Because otherwise the Einstein field equations would be wrong as applied to cosmology." That answer would spare us dark matter and dark energy but would entail a lot of thought. PALLen's comment is...

Yes, and well stated. Yet the field equations support a flat universe that is not expanding, or at least I think they do. Why does expansion create curvature as implied by your simplified Friedmann equation? And how do you write such a nice equation in this environment? :)

I'm new to this impressive forum and have a question that may have been addressed a thousand times, but here goes. A FLRW metric is happy with a time-varying scale factor a(t) and zero curvature parameter k and could care less about density. The combination of a FLRW metric and the Einstein...

I'm Steve Crow, presently retired in Boulder, Colorado. I attended Caltech as both an undergraduate and graduate and got my PhD in 1966. The degree is in "Aeronautics and Applied Mathematics", but I have always been interested in physics, especially cosmology. I spent my career in aerospace...