Download Dark Matter: What is it?

Dark Matter: What is it?
Dark Matter: What is it?
Key Concepts
1) Some dark matter consists of MACHOs
(MAssive Compact Halo Objects).
2) Some dark matter may consist of WIMPs
(Weakly Interacting Massive Particles).
3) The universe contains five times as much
dark matter as “ordinary” luminous matter.
Dark matter could also be called “invisible matter”.
The properties of invisible objects
are rather difficult to determine.
We know dark matter exists because of its
gravitational pull on luminous matter;
otherwise, information is lacking.
Some of the dark matter in galaxy “halos” consists of
Massive Compact Halo Objects (MACHOs, for short).
MACHOs can be “failed stars”;
balls of gas smaller than a star
but bigger than Jupiter.
These failed stars are called
brown dwarfs.
Strictly speaking, brown dwarfs are “dim matter”,
not “dark matter”.
A cold
brown dwarf
(T = 300 K)
Brown dwarfs are warm enough to glow
feebly at infrared wavelengths.
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MACHOs can be “ex-stars”; very dense
compact objects that once were stars.
White Dwarfs
• Remnants of low-mass stars:
These ex-stars can be
white dwarfs (dense),
neutron stars (denser),
or black holes (densest).
– M* < 4 Msun
– Mass < 1.4 Msun
– Radius ~ 0.01R sun (size of the Earth)
White dwarfs are initially bright, but
eventually they cool and fade away.
Neutron Stars
• Remnants of massive stars:
– 8 < M* <18 Msun (??)
– Mass ~ 1.2 - 2 Msun (???)
– Radius ~ 10 km (size of a small city)
– Density ~ 1014 g/cc
• “With all reserve, we advance the view
that supernovae represent the transition
from ordinary stars into neutron stars,
which in their final stages consist of
extremely closely packed neutrons”
• -F. Zwicky & W. Baade 1930s
• A sugar-cube of neutron star stuff
weighs as much as all of humanity!
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Black Holes
Seeing what can’t be seen
• Close to a black hole:
– Gravity is so strong that nothing, not even
light, can escape.
– Infalling matter is shredded by powerful
tides and crushed to infinite density.
• Becomes a Black Hole
– “Black”: neither emits nor reflects light
– “Hole”: nothing entering can ever escape
Less than 20% of the dark matter is MACHOs:
Most of the dark matter in galaxy “halos”
consists of exotic matter.
• Q: If no light gets out of a black hole, how
can we ever hope to find one?
• A: Look for the effects of their gravity on
their surroundings:
– A star orbiting around an unseen massive
companion.
Particle Physics for Dummies
Astronomers
Electron: low mass, negative charge
Suppose there existed a type of massive elementary
particle that didn’t absorb, emit, or scatter photons.
Proton: higher mass, positive charge
Neutron: ≈ proton mass, no charge
We’d detect such a particle only by its
gravitational pull on luminous matter.
↑ “ordinary”
↓ exotic
Neutrino: VERY low mass, no charge
What are Neutrinos?
• Weakly interacting neutral subatomic
particles.
– Mass-less (or very nearly mass-less).
– Travel at (or very near) the speed of light.
– Interact with matter via the weak nuclear
force.
– Can pass through lead 1 parsec thick!
Cosmic Gall (John Updike)
Neutrinos, they are very small.
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids down a drafty hall
Or photons through a pane of glass.
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Neutrinos make up part of the exotic dark matter.
Neutrinos: Observed!
• Detection of neutrinos is very difficult:
– Need massive amounts of detector materials.
– Work deep underground to shield out other
radiation.
• We detect neutrinos from the Sun and
from other sources.
Although detecting neutrinos
is difficult, it has been done!
Although we don’t know the mass of
neutrinos exactly, we know it’s tiny…
Most of the dark matter must be particles
other than neutrinos.
neutrinos
electron
One candidate for the position of
“dark matter”: the WIMP.
WIMP = Weakly Interacting Massive Particle
Neutrinos provide < 10% of the dark matter.
According to particle physics theory, WIMPs
should be much like neutrinos only more massive.
Dark matter is also found on larger scales,
in clusters of galaxies.
Neutrinos have already been detected:
particle physicists are still trying to detect WIMPs.
I predict a Nobel Prize
for the 1st to succeed!
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Consider the Coma Cluster of galaxies;
in the constellation Coma Berenices,
about 100 million parsecs away.
It contains at least 10,000 galaxies, moving
relative to each other at roughly 1000 km/sec.
Mass of stars in the Coma Cluster
≈ 1013 solar masses
Mass of ICM (hot gas) in the Coma Cluster
≈ 1014 solar masses
The Coma Cluster contains an intracluster medium
(ICM) of very hot gas (T > 10 million Kelvin).
↑ Visible light from stars
in galaxies
↑ X-ray light from the
very hot ICM
In the universe, there must be roughly five times
as much dark matter as “ordinary” luminous matter
(made from protons, neutrons, and electrons).
1/6 luminous
Mass of dark matter in the Coma Cluster
≈ 1015 solar masses
5/6 dark
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