All Experts Redux: The Cosmological Basis Of Dark Matter, Dark Energy

 Question: I am curious about dark matter  and dark energy and whether these are novel insights in cosmology or based on existing physical laws

Answer:

It is more true to say that cosmology generalizes and extends the basis of existing physical laws. That is, it discloses their extent and applicability to conditions that can't be duplicated in any earthly physics laboratory. Hence, cosmology's findings and insights help us to build and expand our particle physics. As well as expand the purview and applicability of *existing* physical laws.

For example, 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.

 

Cosmology's forays continued with :

a)The COBE (Cosmic Background Explorer) satellite measurements in the early 1990s,

 and

 b)The Boomerang and MAXIMA UV measurements to do with type Ia supernovae. (

 The preceding, along with other analyses now shows that the breakdown of the universe is:

7% - ordinary visible matter

93% - dark component, of which:

- 70% is DARK (vacuum) energy and

- 23% is dark matter

Note, that the dark vacuum energy is nowhere observed in any earth physics lab, and hence only cosmology had shown it to be consistent with the observations made (e.g. on Type 1a supernovae in hundreds of distant galaxies).

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 generally expressed:

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

If we set:  0 = (r + 3p) then the pressure p:

p =  -r/3

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

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

p = - r  (pressure =  - energy density)

and - r <  (- r/3)

In other words, without cosmology, we'd never have known that repulsive gravity  (already allowed by general relativity) actually exists. Not on earth, but in the accelerating expansion of the universe!

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.

See Also:

• Dark Energy - Evidence for a New Law of Physics?