Expert: Philip Stahl - 10/13/2004

Question
What if the universe is not expanding but rather being pulled? Do you think it's possible that there may be an enormous blackhole or something similar at the center of the Universe and  all the galaxies that seem to be moving away from us are actually moving towards it? I'll leave out my personal opinions here and hope that you can shed light on whether or not this is remotely possible.  

Answer
Hello.

Not only is there new evidence indicating the universe is expanding, but that this expansion is *accelerating*. This has come with confirmations from Type Ia Supernovae data. (See, e.g. 'Supernovae, Dark Energy and the Accelerating Universe', by Saul Perlmutter, in Physics Today, April, 2003, p. 53. I seem to recall an 'ASTRONOMY' magazine from June, 2003, also had a lengthy article on dark energy)


If a plot of absolute magnitude (vertical axis) against redshfit (z, for the galaxies they occur in )is made then type Ia supernovae will be distributed in a particular way.

Such an plot framework would appear:


M(ABS)         .
!
!
!        .
!
!
!  .
!
!---------------> Z



Visualize a series of scattered points emanating from the origin up to the upper right (I am not sure how the pts. I tried to place will come out in the final text). You also have to try to visualize an imaginary diagonal line going from the joined axes to the upper right. In terms of the real data, the type Ia points all fall to the LEFT of that line, or in what we call the 'accelerating universe' region. On the other side of the diagonal is the "decelerating region".

An additional feature of the accelerating side is 'vacuum energy'. The cosmological "equation of state" (think of something like the equation of state for an ideal gas, e.g. P = nkT) for this vacuum energy is:

w = (Pressure/ energy density) = -1


But, as Perlmutter and others note, whenever w < ( -1/2) we must have a condition of repulsive energy!

This is fully 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

and if: p < (-rho/3) we have gravity that repels.

Looking back to the earlier equation for w, one finds:

p = - rho (pressure = - energy density)

and -rho < (-rho/3)


This is the primary feature of dark energy. That the matter affected is being mutually repelled - in total contradiction to the normal behavior of (attractive) gravitation- and this repulsion is *accelerating the expansion of the cosmos*.

Hence, what one actually has is the furthest thing from "pulling together"!

By the time of the COBE (Cosmic Background Explorer) satellite measurements in the early 1990s, it was estimated that nearly 97% of the matter in the cosmos was 'dark'. (E.g. not visible by optical telescope only by gravitational effects on nearby objects). Dark energy was not yet even on the radar.

The Boomerang and MAXIMA UV measurements to do with type Ia supernovae changed all that. (See, e.g. Physics Today, July, 2000, p. 17). It also caused a re-ordering of the respective contributions of visible and dark matter, and dark energy.

More recent assessments - based on the type Ia data noted earlier, disclose an assay:

7% - ordinary visible matter

and

93% - dark component, of which:

- 70% is DARK (vacuum) energy and
- 23% is dark matter

So, bottom line from all this, not only is the universe "really expanding" but it is evidently accelerating in that expansion!