Chapter 22 ± Dark Matter, Dark Energy, and the Fate of the Universe
Dark Matter and Dark Energy
x Dark matter is simply a name given to whatever unseen influence is causing the
observed gravitational effects.
x Dark energy is whatever it is that may be causing the expansion to accelerate. It is
AKA quintessence or cosmological constant.
Distribution of Mass in the Milky Way
x A rotation curve plots the rotational velocity of stars or gas clouds against their
distance from the center of the galaxy, summarizing the results of orbital velocity
x Objects farther from the center move at faster speeds
x The rotation curve for our solar system drops off with distance from the Sun, because
inner planets orbit at faster speeds than outer planets.
x The drop-off in speed with distance occurs because all the mass of the solar system is
concentrated in the Sun.
x The gravitational force holding a planet in its orbit decreases with distance from the
Sun, and a smaller force means a lower orbital speed.
x Most of the mass of the Milky Way must not be concentrated at its center, but instead,
the orbits of progressively more distant gas clouds must encircle more and more gas.
surrounds twice as much mass, and so on.
lies well beyond our Sun.
Dark Matter in Other Spiral Galaxies
x The rotation curves of most spiral galaxies turn out to be remarkably flat as far out as
we can see.
x These flat rotation curves imply that a great deal of matter lies far out in the halos of
these other spiral galaxies.
x Typical galaxies are made of 90% or more dark matter, and 10% or less matter in
Dark Matter in Elliptical Galaxies
x We generally weigh the inner parts of elliptical galaxies by observing the motions of
the stars themselves.
x The motions of stars in an elliptical galaxy are disorganized, so we cannot assemble
their velocities into a sensible rotation curve.
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x Some stars are moving toward us and some are moving away from us.
x As a result, every star has a slightly different Doppler shift.
x Like spirals, most of the matter in elliptical galaxies must lie beyond the distance
where the light trails off and hence must be dark matter.
x Measurements of the speeds of globular star clusters orbiting at large distances from
the center of an elliptical galaxy suggest that elliptical galaxies contain far more matter
than we can see in the form of stars.