|
The Question
(Submitted January 04, 2001)
What is the difference between Dark Energy and Dark Matter; aren't they
considered the same thing?
The Answer
Dark Energy appears to be, based on the brightness of the most distant type-Ia
supernovae, a mysterious force that is accelerating the expansion of the
universe. These recent discoveries have provided good evidence that there is
such an outward force on the universe (variously called the cosmological
constant, "quintessence," or "dark energy").
Data about the rotation of galaxies shows us that the outer parts rotate as
fast as the inner parts. This only makes sense if there is a spherical
distribution of matter in each galaxy, which is not what we see. Therefore
we infer that there is a certain amount of Dark Matter in each galaxy. This
could be some exotic particles, or just lots of stars too small to have
ignited.
Aside from this, there is also the Dark Matter that we think is there,
based on theoretical arguments. This is something we can measure by
looking at the cosmic microwave background and distant supernovae.
These are the measurements (recently made) that imply the existence
of both Dark Matter and Dark Energy.
You can read a bit about these recent results at
http://sciencenews.org/20000429/fob1.asp
And more detail than you probably want at
http://panisse.lbl.gov/public/
Hope that helps.
-Kevin Boyce and Martin Still,
for "Ask a High Energy Astronomer"
| 
