[extropy-chat] Dark matter and ET

[Hal Finney]

As I understand it, astronomers have pretty much ruled out the possibility
that dark matter is composed of massive, star and planet sized objects.
Of course they were not thinking in terms of stellar engineering but
in more prosaic possibilities like brown dwarfs and rogue planets.
But still I think the evidence against them would apply to ETI as well.

The debate was framed as MACHOs, Massive Compact Halo Objects, vs WIMPs,
Weakly Interacting Massive Particles.  MACHOs are astronomical sized
bodies, while WIMPs are subatomic particles, possibly ones based on
supersymmetry such as the hypothetical neutralino.

There were a number of observational efforts mounted in the late 1990s to
try to observe MACHOs via their eclipsing effects against remote galaxies,
but they generally failed.  There were not nearly enough MACHOs observed
to account for dark matter.  This has pretty much ruled out the MACHO
theory and now WIMPs prevail.  The absence of experimental evidence for
WIMPs has so far not managed to defeat that theory.

I have also seen claims that early universe simulations do not produce
enough baryonic (conventional) matter to account for all the dark matter,
hence it can't be MACHOs.

Another problem with the ETI theory is that if we looked back into the
past, older galaxies would presumably not yet have been transformed by
life, so their "dark matter" would still be light.  But we don't see that.

All in all I think this theory is pretty much a non starter, as is any
theory IMO that assumes that the universe we see is full of massive
stellar engineering activities.  The ancients looked out on nature and
saw a world full of intelligence, tree sprites and sun gods.  It's a
natural instinct we have, to abhor a world empty of thought.  But so
far it's all we've found, and at this point we should continue to assume
that is how things are until we see good evidence to the contrary.

Hal