On 11/23/15 12:37 PM, jacobnavia wrote:
Le 22/11/2015 20:38, Phillip Helbig (undress to reply) a ecrit :
If I go into a cafe and see three people more than 2 metres tall, I
would be surprised. If I surveyed all the people in a large country, I
wouldn't be.


There are LIMITS imposed on the growth of galaxies by physical laws that
make galaxies of just 750My and that enormous size IMPOSSIBLE you see?

jacob

[[Mod. note -- Physical laws do indeed place limits on how fast galaxies
can form and grow. If we had solid and well-validated theoretical models
of galaxy formation/growth we could probably compute those limits.

But I don't think we have this level of understanding (yet). We sort
understand the main ingredients that go into galaxy formation/growth,
but I don't think we know all the parameters (density? temperature?
pressure? chemical composition? magnetic fields? turbulence? ambient
generation-I stellar population environment?) well enough to put
reliable limits on just how fast/slow formation and growth occur.
Nor do we have good observational data (yet) on very young galaxies
to help constrain these theoretical models.

So how do we know that 750 My isn't enough time?
-- jt]]

Based on theoretical, laboratory and industrial experience
the most unknown contributor to galactic formation
is surely "turbulence?".
The Navier Stokes equations describing this viscous contribution
have never been solved.
"The Clay Mathematics Institute has called
Navier--Stokes existence and smoothness
one of the seven most important open problems
in mathematics and has offered a US$1,000,000 prize
for a solution or a counter-example."
RDS


[[Mod. note -- Not to disagree with anything the poster wrote, but
note that in *some* (not all) contexts, we can model the large-scale
effects of turbulence quite well. For example, models of stellar
structure for solar-type stars include a large (turbulent) "convection
zone". For the sun these models agree very nicely with the measured
speed of sound determined from helioseismology. (To answer a frequent
concern addressed in other discussions, I think the number of measured
helioseismology speed-of-sound data points is much larger than the
number of free parameters in the solar models.)
-- jt]]