General Astronomy Dark Matter

1. General AstronomyDark Matter Most of these slides are adapted from a lecture from the Australian National University

2. The Biggest Embarrassment in Astronomy So far in this course, we’ve talked about stars, planets and galaxies. Unfortunately, it is now clear that these are absolutely irrelevant contributers to the universe as a whole. 99.5% of the universe is made of something else - something we cannot see, because it doesn’t emit electromagnetic radiation. What is this “Dark Matter”? We don’t know. It’s arguably the biggest unsolved problem in astrophysics today.

3. Cosmological Significance • Dark matter is also crucial to the shape and fate of our universe. • If there is enough of it, it will curve the universe into a finite, spherical shape. • It could also have so much gravity that it will stop and then reverse the expansion of space, causing our universe to collapse into a “Big Crunch” - the opposite of the Big Bang.

4. The Start of the Mystery For centuries, astronomers believed that the things they could see in space were all that there was in space. A few astronomers speculated that there could be invisible objects out there. After all, most things do not shine. You could put people in space and they wouldn’t be detectable at any distance - we are examples of dark matter, astrophysically thinking. But most people brushed the whole issue under the carpet, and kept on studying stars and galaxies, in ignorance of the truth. The first cracks in this complacency occurred with measurements of orbital speeds around galaxies.

5. In Our Solar SystemKepler's 3rd Law holds Orbital velocity (km/s) Distance from the Sun (AU)

6. Rotation Curves So - as you would expect, the rotational velocities fall as you move further from the central mass (in this case, the Sun). But what about galaxies? If you take random gas clouds, halo stars, globular clusters and dwarf galaxies that orbit around a galaxy like our Milky Way, you would expect to see the same thing - the more distant the orbit, the slower it should move.

7. Around GalaxiesLooks like Kepler's 3rd Law isn't doing so well Orbital velocity (km/s) Distance from the Galaxy Centre (kpc)

8. Flat Rotation Curves • The rotation curve is flat, as far out as it is currently possible to measure. • What’s going on? Why don’t things get slower as they go further out? This makes no sense!

9. Dark Matter According to this idea, rotation curves are flat because galaxies are embedded in vast clouds of something invisible and undetectable - dark matter. The dark matter can only be detected by its gravitational effect. The dark matter is about ten times more massive than all the visible matter in the galaxy, and spread over a much larger region.

10. True Picture What we see What is really there.

11. Larger Scales • On larger scales, the discrepancy is worse. • We can look, for example, on how normal galaxies orbit around galaxy clusters: • The clusters turn out to be around 50 times more massive than the stars within them • Everywhere we look, on scales bigger than galaxies, we see motions that are too fast • Much too fast

12. Resistance This curious tendency was originally pointed out by the mad Swiss astronomer Fritz Zwicky in the 1930s. He was ignored. After all, it was both ridiculous and insulting to suggest that all the work of all his colleagues had missed 98% of the mass in the universe… But more and more troubling data kept coming in. It wasn’t until the 1970s, however, that astronomers really started to take dark matter seriously.

13. So what is this dark stuff? • It doesn’t emit light, down to the sensitivity limit of current telescopes (around 29th magnitude). • It doesn’t block the light from background objects to any appreciable extent, at any wavelength. • It is more spread-out than stars. Stars are mostly found in galaxy disks: the dark matter extends far beyond the disks, and into the space between galaxies. • The average density of dark matter in the universe is around 50-100 times that in things we can see

14. So what is this dark stuff? More constraints come from primordial nucleosynthesis and the microwave background. These constraints tell us that around 20% of the dark matter is made of normal particles, like protons and electrons. The rest must be made of something else - something that has no electric charge and which does not interact via the force that hold nuclei together

15. The Dark Matter Budget • It has recently become clear that we actually need at least three types of dark matter. • We write the average density of each type as a fraction of the density (W0) needed to give the universe a flat geometry. • Stars: Wstars~ 0.005 (0.5%) • Baryonic Dark Matter: Wbaryon~ 0.04 (4%) • Non-Baryonic Dark Matter: WDM~ 0.225 (22.5%) • Dark Energy: WL~ 0.73 (73%) A Baryon is the term for 'normal' matter: protons, neutrons, electrons

16. Baryonic Dark Matter • This should be the easiest - after all, we are made of Baryons and we think we understand them. • Many candidates have been put forward over the years, and most have rapidly been disposed of (because they shine too much, or block light, or disturb the matter we can see in some way). • 3 leading contenders remain at present...

17. MACHOs • Massive Compact Halo Objects – i.e. lumpy things that orbit in the halos of galaxies. • Once a very popular candidate for dark matter, their nature (if they exist) has now been VERY strongly constrained by gravitational lensing experiments like the MACHO project. • If there are “dark lumps” in the halo of our galaxy, they should occasionally get directly in front of stars in the Magellanic Clouds. Their gravity will focus the light of the stars on the Earth, making them appear momentarily brighter.

18. Large Magellenic Cloud MACHO’s gravity focuses the light of the background star on the Earth A MACHO So the background star briefly appears brighter Great Melbourne Telescope, Mt. Stromlo (recently destroyed by bushfire)

19. The Odds • The odds of a MACHO moving precisely enough in front of a given star in the LMC is tiny - but if you look at enough stars, it will happen regularly. • The Great Melbourne Telescope at Mt Stromlo was used for the MACHO project - monitoring 16 million stars in the LMC every clear night for five years. • It detected dozens of “Microlensing events”. • From the length of the event, it could deduce the mass of the lens. • The average mass was curious: around 0.5 solar masses.

20. MACHOs Detected • Most people had expected that MACHOs would be brown dwarfs - as they are very faint. • The MACHO result found rather larger objects • the lensing had to be done by something with a mass around half that of the Sun. • It is possible that they could be stars within the LMC, but if they are MACHOs, they would have to be either white dwarfs or primordial black holes, to have such a large mass. • Whatever they are, they can only account for part of the dark matter - though quite possibly all of the Baryonic dark matter.

21. Primordial White Dwarfs • White dwarfs would normally be quite bright enough to see directly. Unless, that is, if they were VERY old. • Imagine that the very first objects to form in the universe were an incredible number of medium mass stars. • These would have formed about 15 billion years ago, out of the primordial gas. • Quite rapidly they would have died and turned into white dwarfs. • For 14 or so billion years, they have been just sitting there, gently cooling down and getting fainter and fainter.

22. Primordial White Dwarfs Observed? • People were pretty dubious about the white dwarf theory. But two years ago, it was announced that they may have been seen! • The deepest image ever taken in the Hubble Deep Field. • Recently, the field was re-observed • two very faint blue things appear to have slightly moved between the two exposures. • Does this mean they are nearby white dwarfs, and not distant dwarf galaxies? • This has yet to be resolved, but it is regarded with considerable scepticism.

23. Gas The other two leading contenders for the Baryonic matter are both made of gas. • Hot diffuse gas. Take loads of gas, spread it uniformly in the spaces between galaxies, and heat it to one million degrees or so. It will be almost completely undetectable by any means at our disposal. • Cool gas clouds. Take loads of gas, cool down to only a few K, and assemble it into small dense clouds. It too will be almost completely undetectable by any means at our disposal. Both theories are very much in contention at present.

24. Non-Baryonic Dark Matter • What of the next 22.5% of the critical density? • This cannot be made of normal matter, or it would have affected the formation of chemical elements in the Big Bang • So what can it be? Many suggestions have been made, including primordial black holes, massive neutrinos and a host of other exotic particles.

25. Primordial Black Holes • The early universe was ever so slightly lumpy • these lumps were the nuclei around which stars, galaxies and galaxy clusters formed • They are thought to come from quantum mechanical fluctuations when the universe was very young (10-40 s) • There is no good theory for predicting how many and how big they should be • What if there were lots of very strong, very small fluctuations • They might have turned into trillions of microscopic black holes.

26. Atom-sized Black Holes • Such a black hole might have the mass of a person, yet be smaller than the nucleus of at atom. • Or there could be even smaller black holes - with masses of a proton or less. • Or bigger - perhaps with half a solar mass, to explain the MACHO results. • They wouldn’t be very destructive because they are too small to eat very much.

27. Massive neutrinos • Unlike all the other candidates for non-baryonic dark matter, we actually know that neutrinos exist. • They are about the closest a particle can be to non-existence, while still being detectable. • A neutrino can fly through the Earth without noticing. • Millions (emitted by nuclear reactions in the Sun) are flying through your body every second

28. But do they have mass? • We know that neutrinos have very little mass. • But is it exactly zero, or could they have really tiny masses? If so, there are so many of them that they could contribute significantly to the density of the universe. • Recently, a Japanese group (Super-Kamiokande) presented evidence that they do indeed have a very small mass. • According to unified theories, if they have mass, neutrinos should constantly be changing between at least three different types of neutrinos. • The Japanese detector was only sensitive to two of these three types (electron and muon neutrinos).

29. Super-Kamiokande, run by the Institute for Cosmic Ray Research of the University of Tokyo, consists of a 50,000 tonne tank of water, buried 1km under a mountain, and lined with detectors.

30. Destroyed • Alas Super Kamiokande was recently destroyed. • One of the thousands of detectors lining the inside of the tank exploded. This was a known problem, and the researchers expected to regularly replace a few detectors. • Unfortunately, this explosion triggered a chain reaction. It caused the neighbouring detectors to explode, and they in turn destroyed their neighbours, and so on…

31. Cosmic rays from the Sun When it is mid-day in Japan, they detect these muon neutrinos So when it is midnight in Japan, they detect fewer muon neutrinos - as they are not sensitive to tau neutrinos. Hit the atmosphere and produce muon neutrinos But as the neutrinos fly through the Earth, they have time to change to a different, undetectable (perhaps tau neutrinos) type

32. Massive Neutrinos • This result is still tentative - but seems to suggest that neutrinos do have a very small mass. • This means that they must be significant contributors to the dark matter in the universe. • But they cannot account for the bulk of the non-baryonic dark matter. Here’s why:

33. Collisions • Today, neutrinos, and any other weakly interacting particles, barely interact with normal matter. • Very early in the history of the universe (say 10-12 s after the Big Bang), on the other hand, the density of every part of the universe was so large that even weakly interacting particles were constantly colliding with more normal particles. • When particles are constantly colliding, they exchange energy. Particles with more energy constantly whack the less energetic particles.

34. Equipartition • In the process, the more energetic particles loose energy and the less energetic gain it • Before long, all particles have, on average, the same kinetic energy. This state is known as equipartition • Since all particles have the same average energy, the more massive they are, the slower they must be moving.

35. Hot Dark Matter (HDM) • This means that if dark matter is made of low mass particles, they must be moving very fast. This sort of dark matter is called ‘Hot Dark Matter” or HDM, because hot particles move fast. • When the universe was young, HDM would move around so fast that it would tend to smooth out small scale density fluctuations. • Any small region which is denser than average will get smoothed out as its neutrinos fly into adjacent, less dense parts of the universe. • These small density fluctuations are the ones that should form galaxies. • Thus HDM slows down galaxy formation.

36. Top Down • Big density fluctuations, on the other hand, will not get smoothed out • even the fast moving neutrinos cannot get out of them quickly enough. • These big fluctuations will turn into galaxy clusters and superclusters. • Thus in an HDM universe, clusters and superclusters form first. Only later, as the universe expands and the neutrinos slow down, can galaxies form. This is called “Top Down Structure Formation”. • This clashes with observations. Our universe appears too lumpy on small scales to be consistent with HDM domination.

37. Neutrino Dominated Universe Top-down: primordial gas collapses into supercluster sized gas sheets called Zel’dovich pancakes. These finally fragment into galaxies.

38. Cold Dark Matter (CDM) • So - we are left with Cold Dark Matter. Weakly interacting particles that are so massive that they move slowly and do not slow down the formation of galaxies and similarly small things. • No such particles are currently known, but various unified theories predict the existence of many such particles. • There are currently over 30 candidates, predicted by various unified theories. • These particles are generically called WIMPS (Weakly Interacting Massive Particles). • Primordial black holes might also (depending on the mass) count as CDM.

39. Cold Dark Matter Dominated Universe Bottom-up: primordial gas collapses into galaxies, which then drift together to form galaxy clusters and superclusters.

40. CDM Triumphant! • CDM is the most popular dark matter theory to date. It seems consistent with nearly all observations. • Of course, this doesn’t tell us what the dark matter is - only that it consists of massive particles that do not interact strongly with normal matter. • If CDM is correct, WIMPS should be streaming through our bodies all the time. • A variety of groups are currently trying to detect them as they fly through our labs, with no success to date.

41. Or is it? • Within the last year or two, however, problems have started to emerge with CDM. • Computer simulations of CDM dominated universes suggest that the WIMPs should congregate in the centres of galaxies, forming dense massive “Cusps” of dark matter. Galactic center Density Position

42. No Cusps • By measuring the orbits of stars and gas, it is possible (but very difficult) to measure the distribution of dark matter in the centersof galaxies. • The results are very new, but seem to suggest that there are no density cusps in the centersof galaxies. CDM prediction Observations Density Position

43. Ways Out? • This problem has only just been realised - and some dark matter theorists still hope that the inconvenient observations will go away. • Others are starting to try and save CDM. • One possibility is that WIMPs can interact with each other, via some unknown force. Where they are densest (in the cores of galaxies) this interaction might remove some of them, or scatter them to other locations. • Nobody knows...

44. Dark Energy • And so, on to the remaining 70% of the universe: The “Dark Energy” that probably dominates even the non-baryonic dark matter • This dark energy was discovered by measurements of Type Ia Supernovae • It has two properties that make it quite different from normal matter or dark matter • It repels itself - so it tends to make the universe expand faster • The bigger the universe, the more dark energy

45. Summary • We actually know remarkably little of what our universe is made of. • Astronomers spend their lives studying the 0.5% that is bright and shiny • Ten times more massive is the unknown baryonic dark matter. • More mysterious still, and a further five times more massive, is the non-baryonic dark matter. • And yet more mysterious, and twice as big again is the dark energy.