Expert: Philip Stahl - 7/15/2008

Question
Hi Philip,

Is dark matter and energy in another enviroment?Like the oceans and things in it are in a different enviroment than life on the surface of planet.

If dark matter and energy are in a different enviroment are we able to enter?

with best regards
Riccos

Answer
Hello,

Fritz Zwicky in 1933 actually laid the original, observational basis for dark matter. His measurements of galaxy clusters highlighted a 'missing mass'. He found that the mass needed to bind a cluster of galaxies together gravitationally was at least ten times the (estimated) apparent mass visible.

This mass, because it was inferred but not directly detectable, became the first dark matter. Around the same time there were other confirmations, based on observed stellar motions in the galactic plane, by Dutch astronomer Jan Oort. He determined there had to be at least three times the mass visibly present in order for stars not to escape the galaxy and fly off into space.

By the late 1970s, astronomers realized there were forms of matter that didn’t emit light. Among the most talked about candidates were black holes, marking the end stage of evolution for very massive stars. In the black hole, no light escapes and the mass is typically much greater than that of the Sun. One million black holes in the center of our galaxy (probably a conservative number) represents a lot of dark matter. Multiply that by billions of other galaxies, in similar scenarios, and one has an enormous store of dark matter. In fact, given the number of massive stars in our galaxy, it is likely that eventually, 90 percent or more of the stars will have collapsed into black holes, especially with currently accepted lower mass thresholds for black hole formation.

Dark matter itself occurs in either baryonic ('heavy particle' group - e.g. neturons, protons, etc) or non-baryonic forms, depending on whether it reacts with radiation or not. If not, it's non-baryonic. This non-baryonic matter further breaks down into 'cold dark matter' and 'hot dark matter'.

The terms 'hot' and 'cold' not so much indicative of current temperatures but rather the phase of the early universe at which the particular dark matter 'decoupled'. (An earlier decoupling indicates a higher background temperature - since it's closer in time to the Big Bang).

In other words, the dark matter discovery paved the way to identify and elaborate extreme forms of matter present in the early universe.

The key point here is that dark matter is not in "another environment" but in the midst of our own present universe in 4-D space-time. It's just that for whatever reason, it is not readily visible as luminous matter is. (And, of course, dark matter includes black holes!)

Before 1998, few if any astronomers had heard of “dark energy”. Rather, “dark matter” had come to the fore with a series of articles in various periodicals, journals (e.g. Physics Today, (1992), Vol. 45, No. 2, p. 28 by S. Tremaine)  Dark matter was acceptable to most of us because at least it could be understood easily at some level. Not so with dark energy - which is still being sorted out, as to its particular character and behavior.

Most likely, though we still can't be absolutely certain, dark eenrgy is associated with what is called "vaccum energy" and certain laws apply to its behavior. 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

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 = (r + 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 <  (- r/3)

Some might argue that this shows a "new law" of physics, e.g. repulsive gravity, but in reality it's merely extending the existing concept of gravitation to show it has a repulsive as well as attractive aspect. And it always has been consistent with Einstein's general theory of relativity.

Again, this seems to show that what we are dealing with is native and co-extensive with our own universe and not occurring in any external "different environment" or dimensions, or "parallel universe".

Hope this helps!