From: Robert J. Bradbury (email@example.com)
Date: Sat Jan 12 2002 - 23:19:48 MST
On Sun, 13 Jan 2002, Damien Broderick wrote:
> At 11:27 AM 1/12/02 -0800, Spike wrote:
> >Could be that we evolved just in time to see the final stages of
> >our own galaxy going dark.
> >Counter evidence: all galaxies are mostly dark matter? We couldn't
> >have caught *all* of them in that stage.
I'd question the use of the term "all". I think the rotation curves
have been measured for a number of galaxies. Its probably more than
50, but I doubt its more than a few thousand. I also don't know
how "wide" the rotational velocity distribution is. If its
moderately wide that could be signs of galaxies in various
stages of going dark. If its narrow, that seems to suggest
some universal form of dark matter clustering with the visible
If it went dark a long time ago we probably don't see it as a galaxy.
What's the difference between a "dwarf galaxy" and a non-dwarf one that
has mostly dark stars? The entire field of study of "diffuse" galaxies
has only opened up recently because if its nearby, you need to study
a fairly large patch of sky and if its distant you need a really
big telescope to identify individual stars and classify them as
being gravitationally bound to one another.
> This might be old stuff to Robert, but has anyone run off a detailed range
> of possibilities for how such a history might affect the observable cosmos?
I don't think so -- everyone is pretty much trying to prove the
standard model. The only real competitor is the steady state
approach (which I think is Hoyle et al.).
If you want to check you could poke around in the ADS search
engine looking for reviews. Unfortunately Astronomers don't
seem to do as much "review" production as medical types seem
to, so its difficult for a non-astronomer to get a handle on it.
I think there is an "Annual Review of Astronomy" however.
> Assuming that life couldn't emerge until, say, five billion years ago, but
> gets statistically more likely with each gigayear since (and hence
> proximity to YOU ARE HERE <= ), can we chart corrections to luminosity
> factors taken for granted by astronomers who assume an unengineered
Of course specific galactic history is going to vary by whether one
has collided with other galaxies -- that stirs up the pot (probably
for the worse). IMO, almost everyone is looking at the universe like
the glass is full (unengineered). There hasn't been much thought
devoted to the glass being "half-full" (half-engineered).
> If closer galaxies are stochastically dimmer or red-downshifted,
> mustn't this do something interesting to the canonical calculated
> recession rate and hence projected closure vs. openness?
The only thing I've seen is the recent paper (perhaps from Amara)
discussing how Axions could explain the apparent acceleration of
the expansion of the Universe by precisely such a "dimming" effect
(as photons oscillate into Axions).
We may also want to be wary of over simplification. There may
be two different explanations for dark matter. Galactic halos
may be ATC while dark matter effects in galactic clusters may
be intergalactic hydrogen. Think about how long it has taken
to resolve the GRB observations into sub-classes such as colliding
neutron stars and hypernovae. But everyone started out looking
for a one-size fits all explanation.
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