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Q & A: dark matter and energy

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Most recent answer: 06/16/2013
Q:
What is Dark Matter and Dark Energy?
- Dhrubajyoti (age 21)
New Delhi, Delhi,India
A:
The astronmer Fritz Zwicky first proposed the idea of dark matter in 1933 when he observed that the relative motion of galaxies within glalactic clusters could not be explained solely by the presence of luminous matter, i.e. stars.  There had to be something else contributing to the gravitational forces.   Since that time more and more evidence has piled up in favor of this idea.  According to modern astrophysical observations the amount of the stuff exeeds that of luminous matter by a factor of ten!  There are various theories about the composition of dark matter, for example, a kind of elementary particle that interacts very weakly with ordinary matter.   Sensitive earth-based experiments are being carried out to try to detect the individual constituents of dark matter but up to now no convincing evidence has been found. 

The idea of dark energy is a relatively new, although Einstein had a term in his famous general relativiy equations that could mimic the effect.   The recent observational evidence comes partly from looking at the excessive dimming of supernovae stars as a function of their distance, or red-shift, from the earth.  The effect  can be explained as an increase in the Hubble expansion rate as a function of time.  This can, possibly, be explained by ascribing to space itself a property called dark energy.   It's pretty weird.  The idea is fairly new and many astronomical observations must be made in order to confirm it. 

Check out the Wikipedia articles on dark matter and dark energy for more information.

Lee H

There are also many excellent Scientific American articles in the last few years on these topics. Several other lines of evidence, in addition to the red-shifts, support the idea that some sort of dark energy is driving an accelerating expansion. The pattern of random wiggles in the intensity of cosmic microwave radiation from different directions provides one such line of supporting evidence.
Mike W.

(published on 10/22/2007)

Follow-Up #1: light, dark matter, and dark energy

Q:
i have been using your answers all day and was wondering if dark matter/energy could be related to light. light has mass and although it has no rest mass would that even be necessary for it to fill the dark matter/energy requirements. i know that what we know to be light can't hold the universe together but would it be possible that there could be another electromagnetic wave on the wave spectrum that could fill the position of dark matter/energy?
- Kylie (age 14)
Austin, TX, USA
A:

When physicsts use "light" in this general topic, we usually mean any type of electromagnetic radiation, not just the little visible slice of the spectrum. It turns out that no form of electromagnetic radiation can account for either dark matter or dark energy.

Dark matter is probably not all that different from more familiar forms of matter.  It certainly isn't electromagnetic radiation itself, since that doesn't clump up gravitationally with other matter, and dark matter does. It's called dark precisely because it has no electromagnetic interactions. That's why a cloud of it can move right through another galaxy, while the ordinary matter interacts via electromagnetic forces.

Electromagnetic radiation also can't be dark energy. Classical EM radiation doesn't cause an accelerating expansion, but that's what dark energy was invented to try to explain. In principle the zero-temperature quantum EM background would cause an accelerating expansion, but the effect would be way too big.  (see the discussion of Casimir in Cosmology on ) So we need some other sort of understanding of the whole acceleration issue.

Mike W.


(published on 06/16/2013)

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