Re: Dark matter/energy - is it real?

Tue Sorensen wrote:

Let's say I was sceptical about the existence of dark matter and dark
energy - how would you convince me that it's there?

It is mentioned here, for instance:
http://physicsweb.org/articles/news/10/3/22/1
"we are living in a flat universe comprising just 4% ordinary matter,
22% dark matter and 74% dark energy -- in agreement with the standard
model of cosmology."

Dark matter is apparently supposed to be some form of non-luminescent
matter, but what is dark energy supposed to be?

The existence of dark matter is inferred by, among other things, looking at galactic rotation curves. We can see galaxies spinning, and we can measure how fast the matter in them is moving (due to Doppler shift). By doing this, we see that they're moving much faster than they would be if only the glowing matter we see were there. That is, if our theories of gravity are anything like correct, there must be a lot of matter there we can't see.

This really isn't all that surprising; the fundamental concept of dark matter is not all that mysterious. Most of the matter around you consists of dark matter: It does not emit its own light. (It may be warm now compared to interstellar space, but that's only because it's in equilibrium with its surroundings; if it were put in deep space it would rapidly cool far below direct detectability.) In some frequencies, even _you_ are dark matter.

Dark energy is a more recent discovery (and is named to be similar to dark matter even though it's not necessarily a related phenomenon). Careful measurements of the rate of expansion of the Universe show that instead of the expansion slowing as you'd expect from gravity, it is in fact accelerating. Thus there must be some force that is causing it to expand; this force is named dark energy. It acts like a cosmological constant -- the thing Einstein added to his cosmology to try to make the Universe steady state, but which later regretted. Dark energy is certainly weirder than dark matter, but here too it is not totally unprecedented: We're pretty sure that the Universe looks as flat as it does because early on it went through a period of inflation, which was also equivalent to having a nonzero cosmological constant.

Dark matter and energy are really only the names we give to phenomenon that we can logically conclude must be taking place if our understanding of gravity and matter (and so on) are remotely correct. So far they are extremely good theories, and so we are confident that _something_ is going on which we're not seeing directly. This is perfectly valid science and happens all the time; the existence of the neutrino was inferred since some radioactive decays appeared to be violating conservation of energy. Rather than conclude that conservation of energy is wrong after all, it makes much more scientific sense to conclude that a particle that wasn't being detected was carrying away that energy.

What would happen to the
calculations if the speed of light was variable over vast distances
(which we are currently unable to measure)? If the force of gravity was
variable on the large scale?

The speed of light somehow changing would result in all sorts of detectable effects which we don't see -- after all, this would affect spectra significantly. To explain away dark matter _and_ energy, the force of gravity wouldn't have to just change, it would have to reverse at large scales in bizarre ways that would probably be hard to make consistent. (You'd also probably have to explain why our current understanding of gravity coincidentally works so well despite being so wrong.)

--
Erik Max Francis && max@xxxxxxxxxxx && http://www.alcyone.com/max/
San Jose, CA, USA && 37 20 N 121 53 W && AIM erikmaxfrancis
The people are to be taken in very small doses.
-- Ralph Waldo Emerson
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