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Q & A: dark matter and energy

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Most recent answer: 01/18/2014
Q:
What is the relation between dark energy and dark matter?
- sujan shakya (age 18)
kathmandu
A:
We think that there is no special relationship. Our views may change, since neither dark matter nor dark energy is well-characterized, but here is the standard current story.

Dark matter consists of particles which happen to not interact with our type of particles except by gravity, or perhaps by some other very weak effects. Dark matter clumps in the same galaxies as other matter, and generally seems not too mysterious. Perhaps it's not surprising that some of the constituents of the universe don't have much interaction with us. There are some experiments underway to look for particular particles suspected of being part of dark matter.

Dark energy is more peculiar. It shows up as a gravitational effect driving an acceleration of the expansion of the universe. That's what one gets if there's some form of mass/energy which is not present in fixed amounts but rather in fixed density. In other words, whatever space forms automatically fills up with the dark energy.

There are a variety of ideas as to its origin, all speculative. One possibility is that space is stuck in a state with a little more energy than the lowest possible energy state. At some point in the very distant future, a collapse to the lowest energy state would occur, essentially wiping out whatever was around beforehand. There are some indirect tests underway for various physical theories which might shed light on the where the dark energy effect comes from.

Mike W.

(published on 08/23/2010)

Follow-Up #1: dark matter, dark energy, and other possibilities

Q:
Why is it reasonable to assume stuff like dark matter or dark energy? For example, galaxies are rotating too fast compared to the gravity created by its visible matters but what if the apparent missing mass in a galaxy comes from all kinds of "potential/kinetic energy" stored between/within the visible matters? If this is the case, maybe there is no need to assume dark matter for missing mass? Is this the case or not? If not, why?
- Anonymous
A:

As we explain above, there's little reason to think that "stuff like dark matter or dark energy" is a reasonable category. The best current guess is that we're more like dark matter than either of us is like dark energy.

What you describe, various other types of energy hanging out near ordinary matter, is essentially the standard guess for what dark matter is. So what you're proposing is really just another name for dark matter. However, the gravitational effects indicate that this extra energy is not located right near the ordinary matter but forms more of a ball around a disk-shaped galaxy- assuming this general type of model is correct. Furthermore, one observed pair of colliding galaxy clusters looks like it shows the ordinary matter slowed by the collision while the dark matter zoomed right through, giving a bigger separation of the dark and visible components. () 

The goal at this point is to pin down just what the other properties are of this hypothetical dark stuff. If none of the suggested other types of particles can be found, then maybe a whole different model is needed.

Mike W.


(published on 01/18/2014)

Follow-up on this answer.