On Fri, 1 Oct 1999, Robin Hanson wrote:
> This should warm Robert Bradbury's heart. Here is otherwise puzzling
> evidence that seems consistent with galactic aliens. Comments Amara?
>
> http://www.sciencenews.org/sn_arc99/9_18_99/fob1.htm
> "Have Milky Way MACHOs Been Found?" By R. Cowen
>
Saw it a couple of days ago. We really need a good set of photometric brightnesses through various filters to give us the temperature and a spectra to determine if they really are white dwarfs. If these are just at the limit of the Hubble's capabilities we may have to wait a while, though the article indicates some more observations in December.
Richer, et al, seem to be pretty much in the camp of explaining the missing mass as White Dwarfs. It looks like using their models (white dwarfs with hydrogen atmospheres), we are going to need both temperature and spectral data to sort things out. They are careful to qualify the uncertainties [due to the fact that the Hubble Deep Field and Gravitational microlensing observations are in quite different regions.] Making extrapolations about the galaxy mass distribution and composition based on looking through two small but different windows is stretching but it the best we can do at this point.
> Halo populations of white dwarfs pose serious problems, Richer notes.
> Formation of such objects would have thrown into interstellar space far
> more carbon, oxygen, and nitrogen than observations show. In addition,
> the appearance of galaxies today does not indicate that they once had
> enough sunlike stars to form a large population of halo white dwarfs.
> Theorist Bohdan Paczynski of Princeton University says the findings are
> intriguing, but he notes that by invoking the white dwarfs, the researchers
> "are trading one set of difficulties for another that is equally as difficult."
This got a big chuckle out of me. To get all of those white dwarfs you have to trade the "missing mass problem", for the "missing engineering material problem".
Now, what happens if you have a bunch of very large cool, star enveloping astroengineered objects cruising through inter-galactic space? (Because inter-galactic space is has the fewest number of hazards you have to avoid and the lowest background radiation levels that cause you to waste energy repairing nanotech.) Well, then the Universe has more mass than we can "see" (which we know) and thus more gravity, which means you are going to have a force that should slow down the expansion of the universe.
Now, this appears to be in direct conflict with that other astronomical problem (involving an accelerating expansion of the universe). However, if you have all of this missing engineering material, you probably have to significantly rework the abundance of elements as the universe evolves and go back and refigure the supernova brightnesses based on the age of the universe (and element abundances) when they explode. We aren't going to see a solution to these problems anytime soon.
However the one nice thing about Richer's observations is that they seem to be confirming other observations for a Universe that is around 12 billion years old.
Its nice to be getting at least one thing resolved.
Given that you get life/engineering materials very early (in the first few hundreds of million years), especially with more white dwarfs, that makes for a very long time (i.e. lots of chances) for intelligent life to develop and evolve to the SI state. So either (intelligent) life evolving to an SI state is very very difficult *or* they aren't especially interested in "colonization" *or* our "snapshot" of the universe at this time is a very unique timeslice.
Robert
Other references include:
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999astro.ph..8270I&db_key=PRE&high=37740137f609835
and
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999astro.ph..6424R&db_key=PRE&high=37740137f609835