> [and Loren Haarsma responded...]
> The neutrino mass measurement should cause a revised estimate of
>"omega," the mass density of the universe. But so far, I haven't seen
> anything to suggest that this measurement will either make, or break,
> inflation theory. (There are other, independent measurements which set
> limits on the mass density and the "cosmological constant.") Neutrino
> mass measurements give additional data in helping to construct a "Grand
> Unified Theory" of physics -- which may in turn help resolve the
> question of inflation -- but that could still take many years or decades
> to work out.
The discovery of non-zero neutrino mass is indeed an important discovery
in particle physics, and it will influence how unified theories are
approached in the future, but I do not think inflationary theories will be
much influenced by the finding.
The reason is simple: inflation has become too sophisticated. In the
early days of inflationary theory (the early 80s), it was a definite
prediction of inflation that the density of the universe should be exactly
the critical density ie. 'omega = 1'. This was an exciting turn of events,
for cosmology hardly ever makes definite predictions about anything. When
astronomers looked at the universe, however, they found 'omega < 1' or even
'omega << 1', which might make one suspect that inflation would fall into
disrepute. This did not happen. Instead, theorists just worked harder,
and finally they were able to come up with inflationary models that predict
'omega < 1' or even 'omega << 1'. These are called 'open inflation' or
'bubble inflation'.
The basic idea is that the universe began inflating in the usual way, but
then the inflaton (that is the thing in the theory that drives inflation -
it is a quantum field) tunneled into a more stable state at some particular
place. The result of this tunneling is that inflation begins all over
again from the tunneling point, and the secondary inflating universe looks
like a bubble inside the old one - that is why it is called 'bubble
inflation'. The remarkable thing is that to someone inside the bubble, it
looks just like an ordinary inflating universe except - here it comes - the
density need no longer be 'omega = 1'.
The finding of neutrino mass will undoubtedly push up the measured value
of omega. We don't know how much it will increase because (a) we aren't
totally sure how many neutrinos are in the universe, and (b) we don't know
what the neutrino masses are! The experiment at SuperKamiokande only
measured a mass _difference_ between two types of neutrinos, not the masses
themselves. Most inflationary cosmologists would probably be happy if they
could measure 'omega = 1' because the simplest inflationary models predict
that. But even if 'omega < 1', there are models like 'bubble inflation'
that survive.
A more interesting experimental test of inflation will take place soon
when the Planck (http://astro.estec.esa.nl/Planck/) and MAP
(http://map.gsfc.nasa.gov/) satellites are launched in 2007 and 2000,
respectively. These experiments will make precise measurements of
anisotropies in the angular spectrum of the cosmic microwave radiation.
This data is expected to eliminate many competing cosmological theories,
and perhaps vindicate one. Who knows?
[Loren continued with a good exposition of the relic, horizon, and flatness
problems - all of which are solved by inflation. Then he said...]
>A prediction of inflation theory is the "many different universes."
A minor quibble: ordinary inflation does not so much predict 'many
different universes' as it does 'one *really* big universe'. As far as
observation goes, the two ideas are probably indistinguishable, but there
is a conceptual distinction. Inflation says that an infinitesimally small
bit of the early universe was inflated to be our entire observable
universe. That is why it makes sense for our whole universe to be in
thermal equilibrium - it was once small enough to have causal contact. The
size of our observable universe is a tiny fraction of the size of the
actual universe, just like the part that inflated into our universe was a
tiny part of what was originally there.
You are right in saying we don't know whether the part of the universe
unobservable to us has different laws of physics, and so anthropic
arguments are not very well motivated in this scenario. There are some
proposed mechanisms for explaining why physical laws might vary from place
to place, but these generally come from very speculative branches of
'physics' like string theory. As far as I know, all observations confirm
that the laws of physics have not changed in _time_ in our observable
universe, and that is about all we can say.
Sincerely,
Craig Burrell