Re: Anthropic predictions for neutrino masses

From: Hal Finney <>
Date: Thu, 1 May 2003 09:02:01 -0700

Saibal Mitra forwards:
> Anthropic predictions for neutrino masses
> Authors: Max Tegmark (Penn), Alexander Vilenkin (Tufts)
> Comments: 4 pages. Color figs and links at this http URL
> It is argued that small values of the neutrino masses may be due to
> anthropic selection effects. If this is the case, then the combined mass
> of the three neutrino species is expected to be ~1eV, neutrinos causing
> a non-negligible suppression of galaxy formation.

Tegmark's has a brief page on this at,
where he adds, "The taboo of the ``A-word'' seem to be weakening in
the physics community, as you'll see if you click
to search for anthropic on My last search gave as many as
77 hits, mostly recent work, providing a good entry point into the

The idea of the paper is that if neutrinos are too massive, they will
carry away energy during galaxy formation and keep it from happening.
Then universes with too-heavy neutrinos won't have many galaxies, hence
less chance for life.

Looking at it in the context of what he called the "level 2" multiverse in
his SciAm paper (multiple bubbles separated during the inflationary era),
he says that string theory offers the possibility that neutrino masses
derive from a field F_a, which will take on different values in different
level 2 universes. This would suggest that the different universe-bubbles
would have relatively random neutrino masses. Universes with too-high
neutrino masses would form few galaxies, so we don't live in one of those.

Applying the galaxy-formation limit implies that the sum of the masses
of the 3 types of neutrinos must be less than approximately 3 eV. Then
he assumes that the masses are otherwise more or less randomly distributed
within the range from 0 to 3 eV, and (taking some other factors into
account) he comes up with a 90% confidence prediction that the sum of
the masses are > 0.07 eV.

Current observations of solar neutrinos have been interpreted to
imply that the sum of the masses are > 0.04 eV. So the new prediction
is consistent with observations and is somewhat stronger.

It seems to me that Tegmark is applying anthropic reasoning in a rather
bold way which actually weakens his conclusion. If I am understanding his
charts, a neutrino mass sum of as high as 3 eV would exert considerable
suppression on galaxy formation, to the point that only 1/10 as many
galaxies would form as if the masses were less than about 0.1 eV.
He still considers this possible in his predictions, because life would
still have a good chance of forming in such a universe, and there are
particle-physics reasons to believe that higher neutrino masses are
more likely.

So he's applying the AP from the perspective of all possible forms of
life throughout the universe. He's saying, given that we are alive,
what are the possible neutrino masses? And then they could be as high
as 3 eV.

But we know more than this. We can look out and see galaxies. From this
we can derive some estimates on their formation rates. Is it possible
that in our universe only 1/10 as many galaxies have formed as would
have happened at lower neutrino masses? I'm not sure, but I am doubtful
of that. I would think that this would be a well known and recognized
puzzle in cosmology - the "missing galaxies problem" - and I've never
heard of such a thing. (Keep in mind that I am not a physicist.)

Assuming this is true, Tegmark could have applied a more stringent form
of anthropic reasoning, asking not just what parameters are consistent
with the formation of life, but asking which ones are consistent with
our specific observations. If I'm right about the lack of observational
evidence for suppression of galaxies, then he could lower his upper
bound on neutrino masses by perhaps an order of magnitude, and tighten up
his predictions. (However, due to his methodology, lowering the upper
bound would correspondingly reduce his lower bound, perhaps eliminating
the advance over the already-known 0.04 eV.)

Although his approach leads to a weaker prediction, it is more consistent
with the multiverse philosophy, where we anticipate living observers
arising in many different universes. In that context it is natural to
seek limitations which would apply as broadly as possible.

Hal Finney
Received on Thu May 01 2003 - 12:04:53 PDT

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