Download dark matter - University of Texas Astronomy Home Page

Into the darkness: Dark Matter 101
What is the rest of the
cosmic pie made of?
This is Dark Energy:
⇤
What is the rest of the
cosmic pie made of?
This is Dark Energy:
⇤
What is the rest of the
cosmic pie made of?
This is Dark Energy:
⇤
Ok, so what is dark matter?
This is starting to sound a
little… out there.
Why do astronomers have these bizarre,
unreal sounding things?!?!
Why do astronomers have these bizarre,
unreal sounding things?!?!
To explain what we see!
Why do astronomers have these bizarre,
unreal sounding things?!?!
To explain what we see!
Understanding dark matter is easier compared to dark energy:
Why do astronomers have these bizarre,
unreal sounding things?!?!
To explain what we see!
Understanding dark matter is easier compared to dark energy:
rem
La emb
ws er
an Ke
mo d
p
l
o
tio rb er’s
n? ita
l
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Fact about orbital motion:
r
GM
2⇡ r
=
r
P
the velocity, and period of a body’s
orbital motion depends on the
mass of what it’s orbiting.
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Fact about orbital motion:
r
GM
2⇡ r
=
r
P
the velocity, and period of a body’s
orbital motion depends on the
mass of what it’s orbiting.
Not its shape! As long as that
mass is interior to the orbit.
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Fact about orbital motion:
r
GM
2⇡ r
=
r
P
the velocity, and period of a body’s
orbital motion depends on the
mass of what it’s orbiting.
Not its shape! As long as that
mass is interior to the orbit.
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Fact about orbital motion:
r
GM
2⇡ r
=
r
P
the velocity, and period of a body’s
orbital motion depends on the
mass of what it’s orbiting.
Not its shape! As long as that
mass is interior to the orbit.
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Fact about orbital motion:
r
GM
2⇡ r
=
r
P
the velocity, and period of a body’s
orbital motion depends on the
mass of what it’s orbiting.
Not its shape! As long as that
mass is interior to the orbit.
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Fact about orbital motion:
r
GM
2⇡ r
=
r
P
the velocity, and period of a body’s
orbital motion depends on the
mass of what it’s orbiting.
Not its shape! As long as that
mass is interior to the orbit.
Work through the following with your groups and hand in one sheet per group with answers.
1. Here’s a table listing each planet, the mass
inside each planet’s orbit, and the speed at
which that planet orbits the sun. Where is the
vast majority of mass in the solar system
located?
2. How do the orbital speeds of planets
farther from the Sun compare to the orbital
speeds of planets closer to the Sun?
3. How does the gravitational force on a planet
far from the Sun compare to the gravitational
force on a planet close to the Sun?
PLANET
INTERIOR MASS (Msun)
ORBITAL SPEED (km/s)
Mercury
1.00
47.9
Venus
1.00000017
35.0
Earth
1.0000026
29.8
Mars
1.0000056
24.1
Jupiter
1.0000059
13.1
Saturn
1.00096
9.66
Uranus
1.0012
6.81
Neptune
1.0013
5.43
4. Fill in the blanks (use previous answers):
There are _____ planets inside Neptune’s orbit and _____ planets inside Mercury’s orbit. However, the interior mass for
Neptune is ______ (greater than/approximately the same/less than) the interior mass of Mercury. Neptune is ______ (closer
to/farther from/same distance from) the sun as/than Mercury. Therefore the gravitational force exerted on Neptune is _____
(stronger/weaker/the same) as/than the force exerted on Mercury. As a result, Neptune has an orbital speed that is ____ than
the orbital speed of Mercury.
5. Now imagine you were able to add a very, very large amount of mass distributed evenly between the orbits of
Jupiter and Saturn.
Which planet(s) will experience an increase in gravitational
force and an increase in orbital speed from this added mass?
Check all that apply.
If mass is evenly distributed in a disk, the only mass
that matters in determining the orbit of a given planet
is the interior mass.
The same holds for stars orbiting the center of the galaxy.
If mass is evenly distributed in a disk, the only mass
that matters in determining the orbit of a given planet
is the interior mass.
The same holds for stars orbiting the center of the galaxy.
If mass is evenly distributed in a disk, the only mass
that matters in determining the orbit of a given planet
is the interior mass.
The same holds for stars orbiting the center of the galaxy.
only the stuff inside!
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Meet Spiral Galaxies:
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Meet Spiral Galaxies:
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Meet Spiral Galaxies:
the bulge
mostly old
stars, dense
part of galaxy,
supermassive
black hole at
the center.
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Meet Spiral Galaxies:
the bulge
mostly old
stars, dense
part of galaxy,
supermassive
black hole at
the center.
spiral arms
where most
new stars are
formed
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Meet Spiral Galaxies:
the bulge
mostly old
stars, dense
part of galaxy,
supermassive
black hole at
the center.
spiral arms
where most
new stars are
formed
more dense
along arms
than between!
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Meet Spiral Galaxies:
the bulge
mostly old
stars, dense
part of galaxy,
supermassive
black hole at
the center.
spiral arms
where most
new stars are
formed
roughly composed of:
gas (fuel for star formation)
stars,
dust (product of star formation)
more dense
along arms
than between!
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Meet Spiral Galaxies:
Let’s draw on one observation about orbits, and then
apply it to our solar system, and galaxies on a whole.
Meet Spiral Galaxies:
Work again with your groups…
1. One way to estimate the amount of mass in a spiral galaxy is by
looking at how much light it emits. Where there is more light, there
must be more stars and hence more mass. When we measure
amount of light at different regions in the galaxy, more is emitted at
the center and less on the outskirts. Based on this information,
where do you expect most of the mass to be located in a galaxy?
2. At right is a picture of a spiral galaxy similar to the Milky Way.
The orbits of three stars are labeled. Star A is on the edge of the
bulge. The Sun’s orbit is marked by Star B and Star C is farther
out in the disk than the Sun. Which star do you think is traveling
fastest and which is traveling more slowly?
distance from center distance from center
velocity
velocity
velocity
3. Below are three possible diagrams depicting the orbital speed
of stars different distances from the center of the galaxy. This is
called a rotation curve. Which one best represents your
answer to question 2?
distance from center
A
B
C
Work again with your groups…
1. One way to estimate the amount of mass in a spiral galaxy is by
looking at how much light it emits. Where there is more light, there
must be more stars and hence more mass. When we measure
amount of light at different regions in the galaxy, more is emitted at
the center and less on the outskirts. Based on this information,
where do you expect most of the mass to be located in a galaxy?
2. At right is a picture of a spiral galaxy similar to the Milky Way.
The orbits of three stars are labeled. Star A is on the edge of the
bulge. The Sun’s orbit is marked by Star B and Star C is farther
out in the disk than the Sun. Which star do you think is traveling
fastest and which is traveling more slowly?
distance from center distance from center
velocity
velocity
velocity
3. Below are three possible diagrams depicting the orbital speed
of stars different distances from the center of the galaxy. This is
called a rotation curve. Which one best represents your
answer to question 2?
distance from center
A
B
C
Work again with your groups…
1. One way to estimate the amount of mass in a spiral galaxy is by
looking at how much light it emits. Where there is more light, there
must be more stars and hence more mass. When we measure
amount of light at different regions in the galaxy, more is emitted at
the center and less on the outskirts. Based on this information,
where do you expect most of the mass to be located in a galaxy?
A
B
C
2. At right is a picture of a spiral galaxy similar to the Milky Way.
The orbits of three stars are labeled. Star A is on the edge of the
bulge. The Sun’s orbit is marked by Star B and Star C is farther
out in the disk than the Sun. Which star do you think is traveling
fastest and which is traveling more slowly?
distance from center distance from center
distance from center
4. The Milky Way galaxy’s actual rotation
curve looks something like this.
velocity
velocity
velocity
velocity
3. Below are three possible diagrams depicting the orbital speed
of stars different distances from the center of the galaxy. This is
called a rotation curve. Which one best represents your
answer to question 2?
distance from center
is the gravitational force felt by stars far out in the MW greater than,
less than, or the same as what you expected in question 3?
With your group:
Y
The white line on this diagram starts in
the center of a galaxy and reaches to
its outskirts, 10kpc away.
Distance from the center, D [kpc]
Do your best to sketch the:
(1) density of mass in stars as a function of radius,
(2) the amount of mass interior to a point D from
the center (i.e. a circle of radius D, like the
yellow circle), and
(3)
the expected orbital velocity of stars as a
function of D remembering the equation below.
v=
r
GM
r
Mass density
my sketches
Interior Mass to D
Distance from the center, D [kpc]
Expected Orbital
Velocity
Distance from the center, D [kpc]
Distance from the center, D [kpc]
Mass density
my sketches
Interior Mass to D
Distance from the center, D [kpc]
Expected Orbital
Velocity
Distance from the center, D [kpc]
Distance from the center, D [kpc]
Mass density
my sketches
Interior Mass to D
Distance from the center, D [kpc]
Expected Orbital
Velocity
Distance from the center, D [kpc]
Distance from the center, D [kpc]
Mass density
my sketches
Interior Mass to D
Distance from the center, D [kpc]
Expected Orbital
Velocity
Distance from the center, D [kpc]
Distance from the center, D [kpc]
Mass density
my sketches
Interior Mass to D
Distance from the center, D [kpc]
Expected Orbital
Velocity
Distance from the center, D [kpc]
Distance from the center, D [kpc]
Mass density
my sketches
Interior Mass to D
Distance from the center, D [kpc]
Expected Orbital
Velocity
Distance from the center, D [kpc]
Distance from the center, D [kpc]
Mass density
my sketches
Interior Mass to D
Distance from the center, D [kpc]
Expected Orbital
Velocity
Distance from the center, D [kpc]
Distance from the center, D [kpc]
Mass density
my sketches
Interior Mass to D
Distance from the center, D [kpc]
Expected Orbital
Velocity
Distance from the center, D [kpc]
Orbital Velocity
observed
expe
cted
Distance from the center, D [kpc]
Before dark matter was
widely accepted, this
was referred to as the
galaxy rotation problem.
Distance from the center, D [kpc]
Vera Rubin and the discovery of Dark Matter.
Measured doppler shifts/
velocities for spectral
features in outskirts of
spiral galaxies…
Check out a great piece on Vera Rubin here: https://www.brainpickings.org/2016/04/18/vera-rubin-interview-women-in-science/
Vera Rubin and the discovery of Dark Matter.
Measured doppler shifts/
velocities for spectral
features in outskirts of
spiral galaxies…
Check out a great piece on Vera Rubin here: https://www.brainpickings.org/2016/04/18/vera-rubin-interview-women-in-science/
Vera Rubin and the discovery of Dark Matter.
Measured doppler shifts/
velocities for spectral
features in outskirts of
spiral galaxies…
Check out a great piece on Vera Rubin here: https://www.brainpickings.org/2016/04/18/vera-rubin-interview-women-in-science/
Vera Rubin and the discovery of Dark Matter.
Measured doppler shifts/
velocities for spectral
features in outskirts of
spiral galaxies…
Check out a great piece on Vera Rubin here: https://www.brainpickings.org/2016/04/18/vera-rubin-interview-women-in-science/
Vera Rubin and the discovery of Dark Matter.
Measured doppler shifts/
velocities for spectral
features in outskirts of
spiral galaxies…
Check out a great piece on Vera Rubin here: https://www.brainpickings.org/2016/04/18/vera-rubin-interview-women-in-science/
Vera Rubin and the discovery of Dark Matter.
Measured doppler shifts/
velocities for spectral
features in outskirts of
spiral galaxies…
Check out a great piece on Vera Rubin here: https://www.brainpickings.org/2016/04/18/vera-rubin-interview-women-in-science/
Vera Rubin worked on the galaxy rotation problem:
orb
ital
see motion
me
d to of sta
rs
o fa
st!
Vera Rubin worked on the galaxy rotation problem:
orb
ital
see motion
me
d to of sta
rs
o fa
st!
xy
a
l
a
g
in
s
r
a
t
s
ot
of
n
)
s
s
s
s
a
a
m
m
r
a
l
l
o
e
t
t
s
y
.
x
e
a
.
l
(i
a
g
r
o
f
t
h
a
g
:
u
e
o
r
n
tu
e
c
u
r
t
s
ld
s
u
t
i
o
c
p
e
s
ar
ke
t
s
s
d
y
e
x
e
a
l
p
a
s
g
these scape the
e
)
easily t they don’t
(bu
Vera Rubin worked on the galaxy rotation problem:
orb
ital
see motion
me
d to of sta
rs
o fa
st!
xy
a
l
a
g
in
s
r
a
t
s
ot
of
n
)
s
s
s
s
a
a
m
m
r
a
l
l
o
e
t
t
s
y
.
x
e
a
.
l
(i
a
g
r
o
f
t
h
a
g
:
u
e
o
r
n
tu
e
c
u
r
t
s
ld
s
u
t
i
o
c
p
e
s
ar
ke
t
s
s
d
y
e
x
e
a
l
p
a
s
g
these scape the
e
)
easily t they don’t
(bu
Most widely accepted explanation is that there’s a form of matter
which does NOT interact with light but keeps the motion of stars in
galaxies in check: called DARK MATTER.
Difference between galaxy rotation curve
without and with dark matter:
Difference between galaxy rotation curve
without and with dark matter:
rotational
velocity
What would you conclude about the real
size of the mass distribution in a galaxy that
has this rotation curve? Mark the “edge” of
the galaxy with a vertical line.
radius
rotational
velocity
What would you conclude about the real
size of the mass distribution in a galaxy that
has this rotation curve? Mark the “edge” of
the galaxy with a vertical line.
radius
So far, this is all for spiral galaxies. What about elliptical galaxies?
Elliptical galaxies don’t rotate. Their stars
are moving in all sorts of random directions
(orbits) in a sphere-like distribution.
So far, this is all for spiral galaxies. What about elliptical galaxies?
Spirals
Elliptical galaxies don’t rotate. Their stars
are moving in all sorts of random directions
in a sphere-like distribution.
Ellipticals
So far, this is all for spiral galaxies. What about elliptical galaxies?
Spirals
Elliptical galaxies don’t rotate. Their stars
are moving in all sorts of random directions
in a sphere-like distribution.
Ellipticals
So far, this is all for spiral galaxies. What about elliptical galaxies?
Spirals
Elliptical galaxies don’t rotate. Their stars
are moving in all sorts of random directions
in a sphere-like distribution.
Ellipticals
So far, this is all for spiral galaxies. What about elliptical galaxies?
Spirals
Elliptical galaxies don’t rotate. Their stars
are moving in all sorts of random directions
in a sphere-like distribution.
Ellipticals
So far, this is all for spiral galaxies. What about elliptical galaxies?
Spirals
Elliptical galaxies don’t rotate. Their stars
are moving in all sorts of random directions
in a sphere-like distribution.
Ellipticals
So far, this is all for spiral galaxies. What about elliptical galaxies?
Spirals
Elliptical galaxies don’t rotate. Their stars
are moving in all sorts of random directions
in a sphere-like distribution.
Ellipticals
So far, this is all for spiral galaxies. What about elliptical galaxies?
Spirals
Elliptical galaxies don’t rotate. Their stars
are moving in all sorts of random directions
in a sphere-like distribution.
You can infer their mass by measuring the
width of spectral features: this tells you the
velocity dispersion of stars in the galaxy
2R ⇡
GM
2
Ellipticals
So far, this is all for spiral galaxies. What about elliptical galaxies?
Spirals
Ro
be
llo
r to
Sa
la
glia
el
arc
ro
Ca
M
Does this velocity dispersion for elliptical
galaxies accurately measure dark matter
mass too??
Elliptical galaxies don’t rotate. Their stars
are moving in all sorts of random directions
in a sphere-like distribution.
You can infer their mass by measuring the
width of spectral features: this tells you the
velocity dispersion of stars in the galaxy
2R ⇡
GM
2
Ellipticals
So far, this is all for spiral galaxies. What about elliptical galaxies?
Spirals
Ro
be
llo
r to
Sa
la
glia
el
arc
ro
Ca
M
Does this velocity dispersion for elliptical
galaxies accurately measure dark matter
mass too??
Elliptical galaxies don’t rotate. Their stars
Ellipticals
are moving in all sorts of random directions
UNCLEAR. There are better
in a sphere-like distribution.
methods…
You can infer their mass by measuring the
width of spectral features: this tells you the
velocity dispersion of stars in the galaxy
2R ⇡
GM
2
What do we think the distribution of dark matter looks like
around a galaxy?
What do we think the distribution of dark matter looks like
around a galaxy?
A spherical halo
reaching far beyond the
boundary of the galaxy
itself. We call it the dark
matter halo.
What do we think the distribution of dark matter looks like
around a galaxy?
A spherical halo
reaching far beyond the
boundary of the galaxy
itself. We call it the dark
matter halo.
Astronomers call the
luminous matter
baryonic matter.
What do we think the distribution of dark matter looks like
around a galaxy?
A spherical halo
reaching far beyond the
boundary of the galaxy
itself. We call it the dark
matter halo.
Astronomers call the
luminous matter
baryonic matter.
???
Why is it spherical? Why
does it only affect the motion
of baryonic matter on large
astronomical scales?
What is DARK MATTER?
We don’t know (yet). We’re limited by our observations.
What is DARK MATTER?
We don’t know (yet). We’re limited by our observations.
it does not interact with
electromagnetic forces,
otherwise we would see
its effects via light…
(hence dark matter)
What is DARK MATTER?
We don’t know (yet). We’re limited by our observations.
it does not interact with
electromagnetic forces,
otherwise we would see
its effects via light…
(hence dark matter)
it doesn’t appear to affect
the nuclei of atoms (we
would also see this as
radiated light), so no
strong nuclear force
What is DARK MATTER?
We don’t know (yet). We’re limited by our observations.
it does not interact with
electromagnetic forces,
otherwise we would see
its effects via light…
(hence dark matter)
it DOES interact with
gravity, which is all about
mass, hence it is massive
and a form of matter
it doesn’t appear to affect
the nuclei of atoms (we
would also see this as
radiated light), so no
strong nuclear force
What is DARK MATTER?
We don’t know (yet). We’re limited by our observations.
it does not interact with
electromagnetic forces,
otherwise we would see
its effects via light…
(hence dark matter)
it DOES interact with
gravity, which is all about
mass, hence it is massive
and a form of matter
it doesn’t appear to affect
the nuclei of atoms (we
would also see this as
radiated light), so no
strong nuclear force
it can’t move around too
fast, otherwise it would
escape the gravitational
pull of the galaxy easily
(so neutrinos are not it,
and it’s cold)
What is DARK MATTER?
We don’t know (yet). We’re limited by our observations.
it does not interact with
electromagnetic forces,
otherwise we would see
its effects via light…
(hence dark matter)
it DOES interact with
gravity, which is all about
mass, hence it is massive
and a form of matter
Axions!
A0: not detected, but a
hypothetical particle, could
also solve a problem in
Quantum Chromodynamics.
it doesn’t appear to affect
the nuclei of atoms (we
would also see this as
radiated light), so no
strong nuclear force
it can’t move around too
fast, otherwise it would
escape the gravitational
pull of the galaxy easily
(so neutrinos are not it,
and it’s cold)
What is DARK MATTER?
We don’t know (yet). We’re limited by our observations.
it does not interact with
electromagnetic forces,
otherwise we would see
its effects via light…
(hence dark matter)
it doesn’t appear to affect
the nuclei of atoms (we
would also see this as
radiated light), so no
strong nuclear force
it can’t move around too
fast, otherwise it would
escape the gravitational
pull of the galaxy easily
(so neutrinos are not it,
and it’s cold)
it DOES interact with
gravity, which is all about
mass, hence it is massive
and a form of matter
Axions!
A0: not detected, but a
hypothetical particle, could
also solve a problem in
Quantum Chromodynamics.
MACHOs!
Normal matter that’s just hard
to see: faint stars, black holes,
etc. Observations have
searched and found nothing…
What is DARK MATTER?
We don’t know (yet). We’re limited by our observations.
it does not interact with
electromagnetic forces,
otherwise we would see
its effects via light…
(hence dark matter)
it doesn’t appear to affect
the nuclei of atoms (we
would also see this as
radiated light), so no
strong nuclear force
it can’t move around too
fast, otherwise it would
escape the gravitational
pull of the galaxy easily
(so neutrinos are not it,
and it’s cold)
it DOES interact with
gravity, which is all about
mass, hence it is massive
and a form of matter
Axions!
A0: not detected, but a
hypothetical particle, could
also solve a problem in
Quantum Chromodynamics.
MACHOs!
WIMPs!
Normal matter that’s just hard
to see: faint stars, black holes,
etc. Observations have
searched and found nothing…
Best candidate so far, though
we don’t know what this
would be: the weakly
interacting massive particle.
What is DARK MATTER?
We don’t know (yet). We’re limited by our observations.
it does not interact with
electromagnetic forces,
otherwise we would see
its effects via light…
(hence dark matter)
it DOES interact with
gravity, which is all about
mass, hence it is massive
and a form of matter
Axions!
A0: not detected, but a
hypothetical particle, could
also solve a problem in
Quantum Chromodynamics.
it doesn’t appear to affect
the nuclei of atoms (we
would also see this as
radiated light), so no
strong nuclear force
MOST IMPORTANT
Like all science, we NEED observational
evidence for one of these models.
it can’t move around too
fast, otherwise it would
escape the gravitational
pull of the galaxy easily
(so neutrinos are not it,
and it’s cold)
MACHOs!
WIMPs!
Normal matter that’s just hard
to see: faint stars, black holes,
etc. Observations have
searched and found nothing…
Best candidate so far, though
we don’t know what this
would be: the weakly
interacting massive particle.
What is DARK MATTER?
We don’t know (yet). We’re limited by our observations.
The Lambda-Cold Dark Matter (⇤CDM)
model of the Universe describes our current
thinking about both dark matter (CDM) and
dark energy (⇤)
What is DARK MATTER? And what is it made of?
What is DARK MATTER? And what is it made of?
Major Problems with ⇤CDM:
The Core-Cusp Problem: what is the
distribution (or density) of dark matter
towards the centers of galaxies?
⇤
Major Problems with ⇤CDM:
The Core-Cusp Problem: what is the
distribution (or density) of dark matter
towards the centers of galaxies?
The Missing Satellites Problem:
there should be more satellite
galaxies around the Milky Way if
⇤CDM is right!
Major Problems with ⇤CDM:
The Core-Cusp Problem: what is the
distribution (or density) of dark matter
towards the centers of galaxies?
The Missing Satellites Problem:
there should be more satellite
galaxies around the Milky Way if
⇤CDM is right!
The Satellites
Disk Problem:
the satellites of
galaxies should
be spherically
distributed
around galaxies,
not sit in disks…
Major Strengths of ⇤CDM:
1
3
2
4
It accurately predicts
the cosmic microwave
background, cosmic
expansion of the
Universe, abundances
of hydrogen, helium,
etc., and the largescale structure of the
Universe.