Juan Casado said:
Another explanation for SNe Ia faintness
This explosions would seem farther away than they really are (were) because of a small negative curvature of space. In a slightly hyperbolic Universe, the wave front of light is spreading out faster than in a flat one (the light cone resembling a horn) so that luminosity distances would appear longer than they are.
In such scenario no dark energy would be needed.
Juan as
selfAdjoint said in the
dark matter, dark energy & gravity thread you have posted this text several times, however its rightful place is here, so that is where I shall answer it!
You are correct, hyperbolic space will cause initially parallel light rays to diverge and objects will
appear smaller, further away and fainter than in flat space, similar to a concave lens effect.
This effect is convoluted with the expansion rate of the universe, because an accelerating universe will also result in objects that will
actually be further away than at the same red shift z in a non-accelerating or decelerating universe.
There is always a degeneracy in the interpretation of cosmological effects, however in this case both the curvature and expansion effects have already been taken into account in the models that then are compared to the observations of these distant Type Ia supernovae.
The standard model fits several parameters to the data, not only that of these standard candles but also the WMAP data, quasar lensing data and other cosmological constraints.
That mainstream fit
does require DE.
However as an example of your suggestion see Figure 2 in the primary paper on the subject of high red-shift Type Ia supernovae: Perlmuter et al's paper:
Measurements of Omega and Lambda from 42 High-Redshift Supernovae (page 23)
The middle solid curve is for (\Omega_M,\Omega_{\Lambda}) = (0,0). Note that this plot is practically identical to the magnitude residual plot for the best-fit unconstrained cosmology of Fit C, with(\Omega_M,\Omega_{\Lambda}) = (0.73,1.32).
The (\Omega_M,\Omega_{\Lambda}) = (0,0) plot being the empty, [URL=[PLAIN]http://arxiv.org/abs/astro-ph/0306448[/URL] model universe which has hyperbolic space and no DE. (It also doesn't require undiscovered non-baryonic DM either, but that is another story!)
As a caveat I must also add that this (\Omega_M,\Omega_{\Lambda}) = (0,0) plot is not such a good fit at higher red-shifts where the supernovae begin to become brighter than expected.
Notice also that this cosmic acceleration interpretation depends on the assumption that Type Ia supernovae are standard candles. They are standard candles in our own galaxy, which is why this type of supernova is used as such, but that is no guarantee that they remain of the same intrinsic luminosity over cosmological time, especially if physical constants actually vary over such time scales.
There may be several alternative explanations for these observations.
Garth