accelerating universe, fundamental observations

On the bus (yes, the flammable Chinatown bus) to Boston and in Burles's office, I spent time thinking about fundamental observed properties of our Universe, inspired by our transparency discussions of the last few weeks. I was also inspired by the non-observation that the Universe is accelerating. The world seems to take as gospel that acceleration has been demonstrated, but the data are in fact fully consistent with positive q₀ at the present day. What really has been shown is that if you restrict yourself to the two-dimensional space of matter plus cosmological constant, the best-fit model has present-day acceleration. That does not say that there is sufficient weight in the low-redshift data to see the acceleration locally; indeed there is not. It may be a subtle distinction, but it is an important one, because there can easily be new components emerging at the present day that are important but don't have much impact on the current observations.

That said, I was trying to list the truly fundamental observations, the ones that will never go away. Here are some:

• The night sky is dark (Olbers's paradox).
• We are not being shredded by gravitational radiation (the gravitational radiation equivalent of Olbers's paradox).
• Spacetime is locally flat (related to the above but stronger, really).
• The Universe is transparent at most wavelengths.
• The Universe is isotropic.
• The Universe is expanding.
• The laws of physics look very similar at enormous distances from us (and therefore in the distant past).
• The Universe was hotter in the past.

Those are sort-of in the order of most certain to least.