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Posted by Phillip Helbig---undress to re on February 6, 2010, 3:49 pm
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> About half a century ago the expansion of the universe was
> widely accepted and there were only small islands of
> disbelief e.g. around Sir Fred Hoyle.
Fred Hoyle doubted many things, but not the expansion of the universe.
He doubted the big bang, and instead preferred a steady-state universe,
one which always looked the same. If this expands (as Hoyle---and most
cosmologists---believe on account of the cosmological redshift), then it
has to expand exponentially, with new matter being continuously created
to keep the mean density constant. (The latter is not in conflict with
observations, since the rate of creation is below detection thresholds.)
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> It was expected that
> the expansion velocity should decrease due to the masses in
> the universe. The only question discussed further was,
> whether the expansion would nevertheless continue infinitely
> or would come to a halt, followed by a collapse.
This applies a) for a big-bang universe and b) if one neglects the
cosmological constant. Note that models with the cosmological constant
were discussed already during the 1920s. The main reason for
discounting them for a while was that calculations in them are much more
complicated and the observations weren't good enough to distinguish
between models anyway.
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> Nowadays many (or most?) cosmologists believe in a
> possibility not seen in those old times: the expansion is
> accelerated, the expansion velocity is increasing.
As mentioned above, this was already discussed by Friedmann in the
1920s.
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> AFAIK
> this is based on the luminosities of far distant supernovae,
> leading to the conclusion that their home galaxies are
> farther distant from us than anticipated from their
> redshift.
To calculate the distance from the redshift, one has to assume a
cosmological model. It would be more correct to say that the galaxies
are fainter than would be expected from their redshift if one assumes no
cosmological constant.
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> How reliable are these newly derived distances?
The distance is calculated, not derived. The error in the measurement
of the redshift is negligible. The problem is the luminosity. While
the apparent luminosity can be measured, there is some scatter because
there is some scatter in absolute luminosity. Since having a good idea
about the absolute luminosity is essential for this line of research, a
lot of work has gone into establishing that the supernovae are standard
candles.
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> The age of the compared supernovae is dependent on their
> distance. Maybe supernovae in the infant times of our
> universe had - for some unknown reasons - a higher
> luminosity.
Maybe. But there is no reason to believe this, neither theoretically
nor observationally.
- I remind on the correction of galactic
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> distances that was necessary when the difference between
> population I and population II Cepheids became apparent.
Yes, science is a self-correcting process, but that doesn't mean that
everything must be incorrect.
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> widely accepted and there were only small islands of
> disbelief e.g. around Sir Fred Hoyle.