>From: "******, Joe"
>Date: Tue, 3 Apr 2001 15:08:07 -0400
>
>This article in cnn suggests that dark matter is repulsive and according to
>Einstein, the reason that the universe is now expanding at an accelerated
>rate. Your page indicates that gravity is universally an attractive force
>for all matter. This cnn article isn't written by scientists for scientists
>are may be misinterpreted. Could you address this contradiction and tell me
>how dark matter is the instigator in the acceleration of an expanding
>universe rather (as suggested in the article) than the opposite.
>
>http://www.cnn.com/2001/TECH/space/04/02/hubble.images/index.html?s=10


	Ah, very good question, Joe.  The universe is
getting crazier by the minute.
	So, first there is the dark matter problem.  Many pieces of
evidence suggest that the mass of the universe is constituted primarily
of a yet-unidentified form of matter.  The atoms which form galaxies,
stars, interstellar and intergalactic dust and you and me are a cosmic
minority, constituting only about 5% of the mass of the universe.  There
seems to be much more mass out there for sure.
	Now, on top of that, the expansion of the universe is
accelerating as the article you cited suggests.  This does seem to
contradict the first problem.
If there is even MORE mass than we think, shouldn't that make
the universe's gravity even stronger and slow the expansion
not speed it up?  Yes -- but there is something else throwing off the
equation.  Einstein found in his equations of general relativity
that there could be an extra term floating about called the
cosmological constant.  It turns out that this constant would
have the strange property of pushing on space-time, not attracting
like gravity.  Recent observations of super novae (which actually
happened billions of years ago) tell us that this constant is not
just a mathematical possibility but a real thing.
	So, EVEN given the fact that there is much more mass out
there than we thought, the cosmological constant (dark energy,
quintessence, what have you...) is dominating the expansion
and causing the acceleration -- despite all of the extra
matter.  In fact, if the value of the cosmological constant
were converted to units of mass, there would be twice as much
dark energy as dark matter.
	Furthermore, the mass density of the universe decreases as the
universe expands.  This occurs just in the same way as the density,
meaning mass per volume, of the atoms of gas in a box decreases as the
size of the box increases.  As the size of the universe increases with
the expansion, the density decreases.  It turns out that just now we
are in an era in which the density has decreased enough to allow the
dark energy to be noticed (this is what I mean by "twice as much
dark energy as dark matter" in the preceding paragraph).  Billions
of years ago, the universe was smaller, so all of the mass was in a smaller
box so its density was greater, so we might not have noticed the dark
energy.  The dark energy density, according to our best guess, doesn't
change with time -- it is constant, which is why we call it the "Cosmological
Constant".  It is simply a property of the space in the box itself regardless
of the size of the box.

	Two big problems!  Stay tuned to the news as we
look for an aswer....

Michael Scott Armel
Center for Particle Astrophysics, Berkeley