Expert: Steve Nelson - 4/11/2011

Question
QUESTION: In re "The possibility of recollapse of the universe..." I was doing okay until undefined p's and w's started popping up.  Can you supply?

The Japanese have a term for "thank you for what you are about to do for me":

O-nagai shimas'

ANSWER: No, your question is absolutely too short and without detail to mean anything at all.  What do the "p's" and "w's" refer to?  You've given me no context.  If you were one of my physics students, I'd say you got the letter "p" confused with the Greek letter rho (which looks an awful lot like the same thing), for instance.

---------- FOLLOW-UP ----------

QUESTION: 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).

Thank you.  This is way out of your line, but I'd be grateful for a link to an appropriate expert.  Thank you.

tim

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:

Answer
If you look at the units the author of the answer to your original question gave you, the answers are obvious.  p is density and w is the ratio of two energy densities (pressure has units of force/area, but that's equivalent to units of energy/volume), which is therefore unitless.  You'll have to read his cited work to get more detail.  p is a density and w is a pure number.  And the answer he posted is clearly incomplete, given how it ends where you cut and pasted it.