Expert: Philip A. Stahl - 10/16/2007

Question
The theory of the big rip explains that all matter in the universe is moving away from each other, and that eventually everything will rip apart.  Starting with galaxies, then to solar systems, all the way down to atoms being split.
I'm in no way an expert on the matter, just an enthusiast.  I was wondering if it was logical for everything to be moving apart from the thrust of the big bang, and eventually gravity will take hold and pull everything back together.  I picture everything having a bungee cord (gravity) attached to it and once it reaches the maximum stretch it pulls back.  Larger masses attract smaller masses until everything is one infinite mass and weight piece of matter.  Then maybe another big bang, as if it were an endless process just happening countless times over.
Could this be a possible scenario?

Answer
Hello,

The possibility of recollapse of the universe actually hinges on a very critical quantity which we call the "cosmic density parameter" - identified by the capital Greek letter 'OMEGA'.   This is defined:

Omega = rho / rho_c

where the denominator refers to the critical mass density of the cosmos, and the numerator is the measured density. Thus if:

rho >  rho_c

(c = critical)

then the cosmic density is able to reverse the expansion (e.g. decelerate it) and conceivably usher in a new cycle. (New Big bang etc.) The observations that help determine how large rho is, come mainly from observing galaxy clusters in different directions in space and obtaining a density estimate from them.

For the past dozen years or more there has been an enormous amount of work done to determine this parameter to see if, indeed, the cosmos will recollapse, or if it is geometrically "flat" (in 4D space-time) meaning it will not recollapse.

All our current data (e.g. from the 'Boomerang' balloon measurements, See, e.g. Physics Today, July, 2000, p. 17), show that:

OMEGA ~  0.3

In other words (from the previous relation):

rho = 0.3 (rho_C)

I.e. that  rho <  rho_c so there is no danger of the cosmos decelerating and hence re-collapsing.

Precision measurements of the cosmic microwave background (CMB), including data from the Wilkinson Microwave Anisotropy Probe (WMAP), have recently provided further evidence for dark energy as the "culprit" in this piece. The agent which ultimately will prevent any recollapse. The same is true of data from two extensive projects charting the large-scale distribution of galaxies - the Two-Degree Field (2DF) and Sloan Digital Sky Survey (SDSS).

Dark energy has been further corroborated from plots of Type Ia Supernovae data. (See, e.g. 'Supernovae, Dark Energy and the Accelerating Universe', by Saul Perlmutter, in Physics Today, April, 2003, p. 53.) This leads directly to the inference of an equation of state (op. cit.):

w = (Pressure/ energy density) = -1

And here is where dark energy enters – not by choice! -  but by the data and observations being forced on us to accept it!

This is consistent with Einstein's general theory of relativity - which one could say approaches the status of a 'basic law of physics'. In this case, the existence of a negative pressure is consistent with general relativity's allowance for a "repulsive gravity" - since any negative pressure has associated with it gravity that repels rather than attracts.


Specifically the term (rho + 3p) acts as a source of gravity in general relativity, (where rho = energy density).

If we set:  0 = (rho + 3p) then:

p =  -rho /3   (or  rho  = - 3p)

and if:  p <  (rho /3)

we have gravity that repels!

When the predictions of the different theoretical models are combined with the best measurements of the cosmic microwave background, galaxy clustering and supernova distances, we find that:

0.62 < OMEGA_dark < 0.76,

where OMEGA_dark = rho_dark/ rho_c,

and -1.3 < w < -0.9

Thus, the dark energy component of the cosmos can comprise up to 76% of the total mass-energy. This leaves too little ordinary matter (and ordinary gravity) to halt the "rip" and precipitate a new collapse and Big Bang.

Alas, the cosmos will continue to go on accelerating its expansion forever.