Transcript Dark Matter!

Dark Matter!
• First clue came way back in the 1930’s.
Fritz Zwicky studied the motions of
galaxies in the Coma Cluster of galaxies –
the nearest “relaxed” rich cluster of
galaxies.
• How do we measure the mass of clusters of
stars or galaxies…?
Coma cluster
• These objects are moving randomly
around and past each other like a
swarm of bees.
• Higher average velocity must mean
higher gravity in order to keep them all
corral’ed together: higher mass
• A generalization of Kepler’s 3rd Law can
be derived
• Bottom line – you measure the velocity
dispersion of the stars or galaxies in
the cluster
Zwicky found…
Mass of Coma Cluster; ~100
times the mass of the visible
stars! So what is the rest, if not
stars? That’s a hard question!!
Let’s just look at more evidence
of Dark Matter…
Vera Rubin in 1970’s Measured
Rotation Curves of Spiral Galaxies
• We had our first electronic detectors in the
1970’s, now could measure spectra of very
faint things – like the outskirts of distant
spiral galaxies
• If mass follows light, here’s what you’d
expect…follow the reasoning on the
whiteboard.
There’s 2 other means to
measure the mass of Galaxy
Clusters…
• 1. X-rays from the gas in the cluster. Idea
is this – high mass means high speeds of gas
particles means HOT means X-rays given
off
• 2. Gravitational lensing of background
objects. Gravity bends light!
Xray+optical 9blyr cluster
Abell arcs closeup
Abell arcs
Even our own
Milky Way Galaxy
is Dominated by
Dark Matter…
Flat rotation curve
Conclusion - The Evidence is
Strong…
Our Galaxy is deep inside
a huge spherical cloud
of… Dark Matter
DM halo diagram
What Could it Be?????
• Now it’s your turn…
• Come up with some possibilities. Like good
scientists, let’s not reach for the bizarre until
we’ve ruled out the well-known things such
as we’ve already discussed in this class
• Your list?....
• Gas? No, we’d see new absorption lines when we
look at stars from the LMC. Not there!
• Dust? No, we’d see reddening of background
stars and galaxies. Not there!
• Rocks? Asteroids? Comets? Tiny planets? No,
they’d have to be made mostly of heavy elements.
How can 90% of the halo be made up of heavy
elements yet the stars, the most gravitating things
out there, are pristine hydrogen and helium Pop II
stars with no heavy elements? Makes no sense.
• What about bigger things?...
All Such Self-Gravitating
Objects can be Detected by a
particularly clever test…
• First- these things need a name:
• They’re massive, they’re clumped into a big
thing, they’re in the halo of our galaxy…
• Massive Clustered Halo Objects
• MACHO’s!
• Look for gravitational lensing of distant stars
as a MACHO passes in front of it.
LMC star field
MACHO lensing, photo
MACHO microlens photo
What did the MACHO
Collaboration Find?
• A few gravitational lensing MACHO’s have been
found, but not nearly enough to account for the
dark matter.
• It’s NOT MACHO’s (stars, planets, black holes…)
• So, we’ve now RULED OUT all forms of
ordinary matter, from elementary particles all the
way up to self-gravitating things like stars and
planets.
NOW (and only now) is it
time to GET CRAZY!
• It must be some form of matter that only
interacts by the weak force and gravity.
• Weakly Interacting Massive Particles = WIMP’s!
• Yeah, it would’ve been much Cooler if the
dark matter had been MACHO’s, but looks like it
may instead by WIMP’s. Bummer!
• What WIMPs do we know for sure exist? Only
one – the neutrino
Hot Dark Matter and Cold Dark
Matter
• Hot Dark Matter – particles which are
moving rapidly and randomly within the
expanding universe
• Cold Dark Matter (CDM) – particles
born without rapid random motion, and
only acquire velocity within the
expanding universe by gravity as the
universe evolves
What WIMPs Do We Know for
Sure Exist?
• Neutrino – Hot Dark Matter
• Cold Dark Matter – none discovered
yet, but there’s possibilities from
theories which are still in the running…
axions, quark nuggets, super symmetric
partners of ordinary matter particles.
…But alas…
• Pure neutrino dark matter won’t work.
• To see why, we need to talk about another
strategy for figuring out the Dark Matter.
And that strategy is…
How Dark Matter affects Large
Scale Structure of the Universe
• For this purpose, it’s useful to divide Dark Matter
candidates into either “Hot” or “Cold”
• Hot Dark Matter – particles that have a high
random velocity in addition to their normal
expansion with the rest of the universe, or any
tugging supplied by gravity. Just like the
molecules of air in this room!
• Cold Dark Matter – particles that have very low
or zero random velocity. All their velocities were
acquired by gravity and by the expanding universe
after it was created.
Hot Dark Matter Candidates?
• There’s only one, but it’s a good one because we know
these actually exist!
• It’s – the neutrino!
• ALAS! The Dark Matter can’t be mainly neutrinos…
• Neutrinos are zipping around way too fast, especially in
the early universe. Only supercluster sized objects could
gravitate together early on
• Later to be followed by clusters, then galaxies, then stars.
• This is a “top down” scenario. Exactly opposite to what we
in fact see: Stars are in fact seen to be the oldest things,
and galaxy clusters are very young, and super clusters are
barely starting to separate themselves out.
Cold Dark Matter Candidates?
• There’s a bunch – axions, super-symmetric
partners to ordinary matter particles – photinos,
gravitinos… bunched together we call them
“neutralinos” (little neutral ones).
• If the DM is pure CDM, then structure forms
“bottom up” – first stars form, then galaxies, then
galaxy groups and clusters, and finally
superclusters. This IS what we see!
Conclusion – the Dark Matter is
Cold Dark Matter
• Probably some form of neutral sub-atomic
elementary particle such as predicted by some
grand unification theories involving symmetry –
“neutralinos”.
• If so, we’ll probably detect them in the next
generation of particle physics experiments at big
particle accelerators, such as at CERN or in Italy
(The U.S. killed it’s big project – superconducting
supercollider - half completed, after the Bush
administration came to power).