I am indebted to Professor Furton of GVSU for presenting the Carl Sagan Day Lecture in Grand Rapids. It was presented in a gentle and friendly spirit.
He was also humble in his presentation of cosmological data and analysis. As I must be humble but only in part, since I am not a master of coordinate based calculations when the premises are unphysical, namely the cosmological constant. (But I am a master of diagrammatical solutions of curved space. Take my tutorial!) The lecture was an incentive for me to do more study.
None the less, I must comment on a bias in the discussion by physicists of cosmological data. Here is a link to supernova Ia data and analysis, which shows a very mild departure from empty and flat space - about 5%. This is a philosophically important comparison. Evolution of supernovae and accumulation of graphene dust (a nanotube form recently observed) might yet account for this small departure of 10% from the expected brightness.
http://www.astro.ucla.edu/~wright/sne_cosmology.html
Physicists habitually use Friedmann coordinates for discussion of cosmology. But a point mostly forgotten is that this coordinate system is highly noninertial and artificially curved. Such coordinates invoke fictitious acceleration. By the weak principle of equivalence, fictitious acceleration is the equivalent of fictitious gravity, and thereby the equivalent of fictitious sources of gravity. So declarations of cosmological constants and acceleration of the universe must be adjusted for this bias. (Analysis from data on the cosmological microwave background must also be adjusted.)
So the challenge for me is to analysis the data in inertial coordinates.
The cosmological constant breaks the principle of conservation from mechanics. It also violates the Bianchi identities from differential geometry. (Confusion here comes from another habitual error of physicists - using the wrong tensor rank, or diagram type, to represent physical objects.)
Lets speculate that spacetime is indeed flat, which is fairly close to the evidence. That would require a background source of negative gravity, which is conserved in agreement with the Bianchi identities, and which is sufficient to counterbalance the positive gravity from matter, radiation and dark matter. (Differential geometry has no prohibition on negative gravity.) By the evidence as taken in an inertial coordinate system, the universe fairly closely resembles one which is flying apart with neither acceleration nor deceleration.
The cosmological constant is not needed now, only a conserved background source of gravity. If anything, the universe is decelerating. A simple kinetic understanding of supernovae distances also indicates this deceleration if it is real.
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Michael J. Burns