Transcript Chapter 16 Dark Matter, Dark Energy, & The Fate of the Universe

Chapter 16
Dark Matter, Dark Energy,
&
The Fate of the Universe
What do we mean by dark matter
and dark energy?
16.1 Unseen Influences in the Cosmos
Dark Matter: An undetected form of mass that
emits little or no light but whose existence we
infer from its gravitational influence
Dark Energy: An unknown form of energy thought
to be the source of a repulsive force causing the
expansion of the universe to accelerate
Contents of Universe (% of total Energy)
• “Normal” Matter: ~ 4.4%
– Normal Matter inside stars: ~ 0.6%
– Normal Matter outside stars: ~ 3.8%
• Dark Matter:
• Dark Energy
~ 25%
~ 71%
What have we learned?
• What do we mean by dark matter and dark energy?
• Dark matter and dark energy have never been directly
observed, but each has been proposed to exist because
it seems the simplest way to explain a set of observed
motions in the universe.
• Dark matter is the name given to the unseen mass
whose gravity governs the observed motions of stars
and gas clouds.
• Dark energy is the name given to whatever may be
causing the expansion of the universe to accelerate.
16.2 Evidence for Dark Matter
Our Goals for Learning
• What is the evidence for dark matter in galaxies and
in clusters of galaxies?
• Does dark matter really exist?
• What might dark matter be made of?
What is the evidence for dark
matter in galaxies?
We measure the
mass of the solar
system using the
orbits of planets
• Orb. Period
• Avg. Distance
Or for circles:
• Orb. Velocity
• Orbital Radius
Rotation curve
A plot of orbital
velocity versus
orbital radius
Solar system’s
rotation curve
declines because
Sun has almost
all the mass
Which person has
the largest rotational
velocity on this
merry-go-round?
That is, who moves
fastest?
A (close to center),
B (halfway to edge),
C (close to edge),
or D (all the same)?
Get your vote
ready…
Which person moves the fastest on
this merry-go-round?
40%
A (close to center)
B (halfway to edge)
C (close to edge)
D (all the same)
30%
29%
m
e)
sa
ll
th
e
D
(a
e
C
(c
lo
s
ay
al
fw
(h
B
e)
to
ed
to
ce
n
to
e
lo
s
(c
ed
g
ge
)
te
r)
1%
A
1.
2.
3.
4.
Rotation
curve of
merry-goround rises
with radius
Rotation curve
of Milky Way
stays flat with
distance
Milky Way is
not a solid body,
but its mass
must be more
spread out than
mass in the solar
system
(spiral anim.)
Mass in
Milky Way is
spread out
over a larger
region than
the stars
(last anim.)
Mass in
Milky Way is
spread out
over a larger
region than
the stars
Most of the
Milky Way’s
mass seems to
be dark
matter!
Mass within Sun’s
orbit = 100 billion
Suns:
1.0 x 1011 MSun
Total mass = 1,000
billion Suns
(one trillion):
~1012 MSun
The visible
portion of a
galaxy lies
deep in the
heart of a
large halo of
dark matter
We can
measure
rotation
curves of
other spiral
galaxies
using the
Doppler
shift of the
21-cm line
of atomic H
Is it possible to measure rotation curves for
spiral galaxies that we see face-on?
1. Sure. They’re just
another spiral galaxy.
2. Yes. In fact, they’re the
easiest kind to measure,
because you can see the
spiral arms move.
3. No. Their rotation
doesn’t cause a Doppler
shift in our direction.
49%
40%
..
d.
ro
ta
tio
n
ei
r
N
o.
Th
In
s.
Ye
Su
re
.
Th
ey
fa
c
’re
t,
th
e
ju
s
ta
y’
re
n.
..
t..
12%
Spiral galaxies all tend to have flat rotation curves
indicating large amounts of dark matter
Broadening of
spectral lines in
elliptical galaxies
tells us how fast
the stars are
orbiting
These galaxies also
have dark matter
What would you conclude about a
spiral galaxy whose rotational velocity
rises steadily with distance beyond the
visible part of its disk?
27%
6%
62%
6%
1.
2.
3.
4.
Its mass is concentrated at the center
It rotates like the solar system
It’s especially rich in dark matter
It’s just like the Milky Way
What is the evidence for dark
matter in clusters of galaxies?
We can
measure the
velocities of
galaxies in a
cluster from
their Doppler
shifts
The mass we
find from
galaxy
motions in a
cluster is
about
50 times
larger than
the mass in
stars!
Clusters contain
large amounts of Xray emitting hot gas
Temperature of hot
gas (particle
motions) tells us
cluster mass:
85% dark matter
13% hot gas
2% stars
Gravitational lensing, the bending of light rays by
gravity, can also tell us a cluster’s mass
A gravitational lens distorts our view of things behind it
A gravitational lens distorts our view of things behind it
All three methods of measuring cluster mass indicate
similar amounts of dark matter
What kind of measurement does NOT
tell us the mass of a cluster of galaxies?
9%
28%
41%
22%
1. Measure velocities of cluster galaxies
2. Measure total mass of cluster’s stars
3. Measure temperature of cluster’s hot gas
4. Measure distorted images of background galaxies
Does dark matter really exist?
Our Options
1. Dark matter really exists, and we are observing
the effects of its gravitational attraction
2. Something is wrong with our understanding of
gravity, causing us to mistakenly infer the
existence of dark matter
Our Options
1. Dark matter really exists, and we are observing
the effects of its gravitational attraction
2. Something is wrong with our understanding of
gravity, causing us to mistakenly infer the
existence of dark matter
Because gravity is so well tested on small scales,
and because dark matter explains galaxy
formation, most astronomers prefer option #1.
But option #2 can't be completely ruled out!
Of what might dark matter be made?
How dark is dark matter?
How dark is dark matter?
… not as bright as a star.
Two Basic Options
• 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
MACHOs
occasionally
make other
stars appear
brighter
through
lensing
MACHOs
occasionally
make other
stars appear
brighter
through
lensing
… but not
enough
lensing
events to
explain dark
matter
Two Basic Options
• 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
Why Believe in WIMPs?
• There’s not enough ordinary matter for it to be the
dark matter; dark matter must be something new
• WIMPs could be left over from the Big Bang
• Models involving WIMPs explain pretty well how the
galaxies we see today formed and developed
What have we learned?
• What is the evidence for dark matter in galaxies?
• The orbital velocities of stars and gas clouds in galaxies do not
change much with distance from the center of the galaxy. Applying
Newton’s laws of gravitation and motion to these orbits leads to the
conclusion that the total mass of a galaxy is far larger than the mass
of its stars. Because no detectable visible light is coming from this
matter, we call it dark matter.
What have we learned?
• What is the evidence for dark
matter in clusters of galaxies?
• We have three different ways of
measuring the amount of dark
matter in clusters of galaxies:
from galaxy orbits, from the
temperature of the hot gas in
clusters, and from the
gravitational lensing predicted
by Einstein. All of these
methods agree that the total
mass of a cluster is about 50
times the mass of its stars,
implying huge amounts of dark
matter.
What have we learned?
• Does dark matter really exist?
• We infer that dark matter exists from its gravitational
influence on the matter we can see, leaving two
possibilities: Either dark matter exists, or there is
something wrong with our understanding of gravity.
We cannot rule out the latter possibility, but we have
good reason to be confident of our current
understanding of gravity and the idea that dark matter
is real.
What have we learned?
• What might dark matter be
made of?
• Some of the dark matter could
be ordinary or baryonic matter
in the form of dim stars or
planetlike objects, but there
does not appear to be enough
ordinary matter to account for
all the dark matter. Most of it is
probably extraordinary or
nonbaryonic matter consisting
of undiscovered particles that
we call WIMPs.