General AstronomyDark Matter

Many of these slides are adapted from a lecture from the Australian National University

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.

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.

The Start of the MysteryFor 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.

For example, suppose this is ALL we could see of the Earth.Assuming each speck of light is, on the average, a ‘City Mass’ we could estimate the total city mass and think that this is all of the Earth.But, in reality, this is how much there is:

In Our Solar SystemKepler's 3rd Law holds

0 5 10 15 20 25 30 35 40 450

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Distance from the Sun (AU)

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Rotation CurvesSo - 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.

Looks like Kepler's 3rd Law isn't doing so well

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Distance from the Galaxy Centre (kpc)

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In the 1980’s, Vera Rubin began to measure the rotational speeds of the galaxies. The curve was NOT what was expected!

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!

Dark MatterAccording 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 associated with a galaxy is about ten times more massive than all the visible matter in the galaxy, and spread over a much larger region.

True PictureWhat we see

What is really there.

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

ResistanceThis 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.

So what is this dark stuff?1. It doesn’t emit light, down to the sensitivity limit of

current telescopes (around 29th magnitude).2. It doesn’t block the light from background objects

to any appreciable extent, at any wavelength.3. 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.

4. The average density of dark matter in the universe is around 50-100 times that in things we can see

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

So what is this dark stuff?

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

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).

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.

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.

• The problem with this idea is Hawking Radiation. Black Holes of this microscopic size should have evaporated by now

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.

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 since the time it would take a white dwarf to cool is greater than the age of the Universe.

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.

Large Magellenic Cloud

Great Melbourne Telescope, Mt. Stromlo(destroyed by bushfire)

A MACHO

MACHO’s gravity focuses the light of the background star on the Earth

So the background star briefly appears brighter

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.

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.

GasThe other two leading contenders for the Baryonic

matter are both made of gas.1. 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.

2. 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.

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.

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

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.

• 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).

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

Destroyed• Alas Super Kamiokande was 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…

Cosmic rays from the Sun

Hit the atmosphere and produce muon neutrinos

When it is mid-day in Japan, they detect these muon neutrinos

But as the neutrinos fly through the Earth, they have time to change to a different, undetectable (perhaps tau neutrinos) type

So when it is midnight in Japan, they detect fewer muon neutrinos - as they are not sensitive to tau neutrinos.

WIMPSWIMP stands for Weakly Interacting Massive Particle.

This is an entire class of new fundamental particles that emerge from supersymmetry. Supersymmetry is a theoretical notion by which known elementary particles have supersymmetric partner particles. This notion is not part of the highly successful standard model of particle physics, but might exist since the Higgs boson exists. In the so-called Minimal Supersymmetric Standard Model (MSSM), which was hypothesized to explain the hierarchy problem (i.e., why do the elementary particles have the various masses that they do), the lightest stable supersymmetric particle is the neutralino. This is the WIMP that presumably makes up the dark matter.

The WIMP Miracle• From a particle physics perspective, the early universe was a high energy place

where energy and mass could switch from one form to the other. • Pairs of particles and their antiparticles could come and go. However, as the

universe expands, it cools. As it cools, it loses the energy necessary to create particle pairs. When this happens for a particular particle depends on the mass of the particle - the more mass, the more energy is required, and the earlier that particle-antiparticle pair "freeze out."

• After freeze-out, the remaining particle-antiparticle pairs can mutually annihilate, leaving only energy. To avoid this fate, there must either be some asymmetry (apparently there was about one extra proton for every billion proton-antiproton pairs - an asymmetry on which our existence depends even if we don't yet understand it) or the probability for interacting must be so low that particles and their antiparticles go their separate ways without meeting often enough to annihilate completely.

• The WIMPs hypothesized by supersymmetry fit the bill. This coincidence is sometimes referred to as the "WIMP miracle" and was part of the motivation to adopt the WIMP as the leading candidate for cosmological dark matter.

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.

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.

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.

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.

Neutrino Dominated Universe

Zel’dovich Pancake

Galaxies

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

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.

Cold Dark Matter Dominated Universe

Galaxies Galaxy Clusters

Bottom-up: primordial gas collapses into galaxies, which then drift together to form galaxy clusters and superclusters.

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.

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.

Dens

ity

Position

Galactic Center

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 center of galaxies.

• The results are very new, but seem to suggest that there are no density cusps in the center of galaxies.

Dens

ity

Position

CDM predictionObservations

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...

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

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.

Jim Brau