# The expanding universe

Hi,
I have read a bit about open, closed, and flat universes, Hubbles Constant, the accellerating universe, big bag, big cruch, etc...

Is it possible that the expansion rate (+ or -) of the universe might be independant of the expansion rate (+ or -) of the matter within it?

Is it possible that the universe might continute to expand while the matter within it might start to converge under the force of gravity?

Have these options been explored?

-Glenn

Homework Helper
Before anyone could explore it you would have to define how you distinguish between "the expansion of the universe" and "the expansion of the matter in it".

Ambitwistor
Cosmological models with expanding space in which the matter isn't just "carried along with the space" aren't very easy to write down. You can't really construct a homogeneous and istotropic universe in which you have some preferred cosmological frame that most of the matter in the universe is moving greatly with respect to.

Think in terms of matter particles moving around on the surface of a balloon: how can the matter have no preferred locations or directions of motion without the average motion across the balloon's surface being zero?

You could construct a solution in which matter is all expanding from or collapsing toward some particular point on the balloon's surface, but that would violate the cosmological principle -- there would be a special location in the universe (that we would have to just happen to be located at). That's the kind of problem that expanding space was supposed to solve in the first place.

In any case, I would guess that the luminosity-redshift relations in such as scenario would differ from conventional Big Bang predictions, but I don't know if anyone has ever bothered to work it out.

Gold Member
Dearly Missed
Originally posted by Glenn

Is it possible that the universe might continute to expand while the matter within it might start to converge under the force of gravity?

local clumping is always a possibility

(you know about the Great Attractor in the hydra-centaurus direction, towards which the virgo cluster and its hangers-on are falling-----the speeds involved are on the ordr of 300 km/s or a THOUSANDTH of c)

however it is not hard to see that any particular local clumping can never involve more than a tiny fraction of the galaxies in the observable universe--just by a practical comparison of speeds

first of all, the great bulk of the observable is at redshift greater than 2----to take a random cutoff for calculation purposes (you probably know that a quasar has been observed with redshift 6.4 and the CMB has redshift 1100, so in the big picture 2 is a small redshift)

anything with cosmological redshift of 2 or more is in a part of space which is receding from us at a speed greater than c

(for an online calculator that gives the recessionspeeds for various redshifts, google "Siobahn Morgan" and find her astronomy course materials, the current Fall 2003 semester)

maybe largescale clumping is not a theoretical impossibility but the usual falling speeds (a thousandth) are just too tiny to
counteract the typical recession speeds (several times c)