Dark Matter, Dark Energy, and
the Fate of the Universe
Mass within Sun’s
orbit:
1011 MSun
Observable stars
and gas clouds:
~few 109 MSun
Dark matter and dark energy
Dark Matter: An undetected form of mass that emits little or
no photons, but we know it must exist because we observe
the effects of its gravity
Dark Energy: An unknown form of energy that is causing
the universe to expand faster over time
What is the Universe made of?
• “Normal” Matter:
~ 4.4%
– Normal Matter inside stars:
– Normal Matter outside stars:
• Dark Matter:
• Dark Energy
~ 0.6%
~ 3.8%
~ 25%
~ 71%
Rotation curve for the solar system
12
orbital speed
10
8
6
Series
4
2
0
0
10
20
30
distance from sun
40
50
Spiral galaxies all tend to have flat rotation curves
indicating large amounts of dark matter
The visible
portion of a
galaxy lies
deep in the
heart of a
large halo of
dark matter
measure the
velocities of
galaxies in a
cluster from
their Doppler
shifts
Mass is 50 x
larger than
the mass in
stars!
Clusters contain
large amounts hot
gas: emits x rays
Temperature of hot
gas tells us cluster
mass:
85% dark matter
13% hot gas
2% stars
Gravitational lensing of background galaxies also tells us
the mass
What is dark matter made of?
• Ordinary Dark Matter (MACHOS)
– Massive Compact Halo Objects:
dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
– Weakly Interacting Massive Particles:
mysterious neutrino-like particles
The
Best
Bet
MACHOs do not
cause enough
lensing events to
explain all the
dark matter
Why Believe in WIMPs?
• There’s not enough ordinary matter
• WIMPs could be left over from Big Bang
• Models involving WIMPs explain how galaxy
formation works

Gravity of dark matter is what caused protogalactic clouds
to contract early in time
WIMPs don’t
contract to
center because
they don’t emit
photons, so
they can not
radiate away
their orbital
energy
Maps of galaxy positions reveal extremely large
structures: superclusters and voids
WIMP models agree better with observations
Fate of
universe
depends
on the
amount
of dark
matter
Lots of
dark matter
Critical
density of
matter
Not enough
dark matter
Amount of dark matter
is ~25% of the critical
density suggesting fate
is eternal expansion
Not enough
dark matter
But expansion
appears to be
speeding up!
Dark
Energy?
Not enough
dark matter
Brightness of distant white-dwarf supernovae tells us how
much universe has expanded since they exploded
Accelerating universe is best fit to supernova data