Download General Astronomy Dark Matter

General Astronomy
Dark Matter
Most 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 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.
In Our Solar System
Kepler's 3rd Law holds
Orbital velocity
(km/s)
60
50
40
30
20
10
0
0
10
20
30
40
Distance from the Sun
(AU)
50
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.
Around Galaxies
Orbital velocity (km/s)
Looks like Kepler's 3rd Law isn't doing so well
300
250
200
150
100
50
0
0
20
40
60
Distance from the Galaxy Centre (kpc)
80
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 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.
True Picture
What 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
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.
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
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
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:
Baryonic Dark Matter:
Non-Baryonic Dark Matter:
Dark Energy:
Wstars~ 0.005
Wbaryon~ 0.04
WDM~ 0.225
WL~ 0.73
(0.5%)
(4%)
(22.5%)
(73%)
A Baryon is the term for 'normal' matter: protons, neutrons,
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...
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
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)
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.
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.
Gas
The 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.
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.
Massive neutrinos
• Unlike all the other candidates for nonbaryonic 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.
• 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).
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.
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…
Cosmic rays
from the Sun
When it is midday in Japan, they
detect these
muon 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
So when it is
midnight in
Japan, they
detect fewer
muon neutrinos
- as they are
not sensitive to
tau neutrinos.
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:
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
Galaxy
Clusters
Galaxies
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.
Density
Galactic center
Position
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 centers of galaxies.
• The results are very new, but seem to suggest that
there are no density cusps in the centers of galaxies.
Density
CDM prediction
Observations
Position
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 nonbaryonic 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.