# 10 things that still puzzle me

1. Jan 4, 2009

### mysearch

As an interested hobbyist only, I have mainly been trying to understand some of the key ideas that underpin modern cosmology. While I may have made some relative progress, there are a number of really basic issues that still puzzle me. As it’s the New Year, I thought I start by trying to clear up what might be some obvious confusion on my part:

1. What caused the universe to expand for the 1st 7 billion years?

2. Why is Friedmann’s equation thought to define an expansive velocity as implicit within the Hubble constant (H)?

3. If the universe is homogeneous, at the very large scale, and for most of its existence has a very low energy density per unit volume of space, wouldn’t this approximate to a weak gravitational field model, with non-relativistic (local) speeds, and as a result the metric tensor of general relativity approaches the metric tensor of special relativity?

4. What additional information is GR telling us about the geometry within a large-scale model with no explicit centre of gravity, i.e. the point-mass implications of the Schwarzschild solution to Einstein’s field equations doesn’t seem to apply?

5. While accepting that mathematics can describe a 3D spatial curvature in terms of a higher dimension, is there any compelling data that suggests that spatial curvature [k] is not zero for all practical purposes?

6. Is there any sense that space is thought to have structure, which then explains how "matter tells space how to curve, and space tells matter how to move”, especially if (k=0)?

7. If (k=0), doesn’t the FLRW metric become a lot simpler with spacetime curvature essentially corresponding to the expansion of space, i.e. the path of 2 photons initially travelling in parallel will eventually be described by a geodesic due to the expansion of space rather than its spatial curvature. Is the metric suggesting more?

8. What is meant by the expansion of space that results in distance between galaxies increasing, i.e. if gravity is no longer described as a force, what geodesic curvature causes the galaxies to roll away from each other?

9. How is the increase in energy explained for any given volume of space in an expanding model with dark energy?

10. In a large-scale homogeneous model with no centre of gravity, does the concept of an overall change in gravitational potential energy, in an expanding universe, have any meaning?

That probably more than enough!