Eric Flesch wrote:
Yes, 30 years ago, our FLRW was clear: While a "critical density"
(omega=1) flat space-time manifold was theorized, still it was held
that a low-density (omega<1) hyperbolic metric was possible, for which
space would expand forever. All was well in the Garden of FLRW.
Already 30 years ago there were hints that the Universe is (at least
almost) spatially flat. Your description is good for the time before
that. Another way to phrase it: cosmologists had so little data that a
large number of cosmological models were compatible with the data.
Also, while some favoured a flat Universe because they believed in
inflation, what tipped the scales were purely observational reasons for believing in flatness.
In 1998, new SNIa results showed that "expansion" was "accelerating"!
Panic! The new physics of "dark energy" was whipped up to fit the new observations while maintaining flat space-time.
This is just wrong. First, while some might have been surprised, for
others it was essentially measuring a parameter which until then hadn't
been well constrained. No new physics was whipped up; the cosmological constant, introduced in the first paper on relativistic cosmology almost
a hundred years before, fit the data just fine.
But the old FLRW could have accomodated the new observations just
fine.
That is just wrong.
All that was needed was to accept that, in a forever-expanding
universe, the space-time metric was indeed omega<1 hyperbolic, as FLRW
had prescribed. In that metric, distant objects are fainter, as is
observed. There is no need for acceleration, nor "dark energy".
That is just wrong. Yes, in a non-accelerating hyperbolic universe,
objects are fainter than in the Einstein--de Sitter universe. Just read
any paper about supernova cosmology. Usually quite a wide range of cosmological models are considered, including spatially hyperbolic ones
without acceleration. They are ruled out by the data. It's not just a question of whether something is fainter than in some fiducial model,
it's a question of how much fainter and how the brightness depends on
redshift. Precision cosmology.
I'm not aware that in 1998 it was even considered that the metric
could be hyperbolic.
Again, it is in essentially every paper on supernova cosmology.
This is because all our cosmological theory is
premised on a flat metric.
Again, simply not true. While it is true that, somewhat later, some
started assuming flatness because observations had indicated that the
Universe is at least almost flat, there is no sense in which all
cosmological theorty depends on a flat metric. Yes, inflation strongly
hints that the Universe must be close to flat, but these days it is an observational fact. Traditional observational cosmology also doesn't
depend on inflation.
Nobody wanted to ditch decades of work.
Which decades? It was only after 1980 or so that belief in inflation
led to the prejudice of a flat Universe. Yes, Einsten and de Sitter had suggested their flat model back in the 1930s, but read the last
sentence, in which they make it clear that since the curvature is
essentially measurable, one should try to measure it, but until better
data are available, why not use a simple model as an approximation?
But, of course, the universe does not care what kind of work we are
doing.
Of course not. But who claims that it does?
So it seems that we were not serious about the FLRW of olde, we only pretended that the metric might be hyperbolic. When the chips came
down, the flat metric was clung to at all costs. That's not how
science should work.
That's not what happened. That the Universe is at least very close to
being flat is an observational fact. Other types of universe were
considered and are ruled out by the data.
So I think we need some astrophysicists to research how well current observations conform to an open hyperbolic universal metric, without
any need for dark energy.
Such models have been ruled out based solely on observations for at
least a quarter of a century.
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