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 3 rd 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… M assive C lustered H alo O bjects 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. W eakly I nteracting M assive P articles = 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).