In Defense of an Accelerating Universe: Model Insensitivity of the
  Hubble Diagram
  A recently published paper by Nielsen, Guffanti and Sarkar argues that the
evidence for cosmic acceleration is marginal and that a coasting universe - the
Milne Universe - fits the same supernovae data in a Hubble diagram nearly as
well. The Milne Universe has negative spatial curvature. Nevertheless, we
confirm that the Milne model fits the data just as well as LCDM. We show that
this unexpected result points to a weakness in the Hubble diagram rather than
to a flaw in LCDM. It seems the Hubble diagram is insensitive to spatial
curvature. Here we present the data and both models in a scale factor vs.
cosmological time plot.This plot is exquisitely sensitive to spatial curvature
because one of three unique transformations, for each of curvatures 0,+1,-1, is
applied to transform from the Hubble diagram. Although the Milne negative
curvature did not matter much in the Hubble diagram, it matters critically in
the scale factor plot. Given that space is flat as measured by precise CMB
observations, we find that when the SNe data, the LCDM model and the Milne
model are plotted as scale factor vs. cosmological time the two resulting
curves separate at 2 sigma above the noise- ten times their separation above
the noise in the Hubble diagram. The transformed data fit to the LCDM model
confirms, at a 95% confidence level, that the universe is accelerating and the
Milne coasting universe is ruled out.
