MACHOs and WIMPs

Outline

the critical density and Omega
the production of matter
limits on the baryon density
dark matter possibilities and detection

The Critical Density and Omega

to keep track of the densities of differ types of "stuff" in the expanding Universe

define
where is the critical density

when , the Universe is just balanced between recollapsing and expanding forever:

It is also convenient to speak of:

, the baryon density and
, the dark matter density

measurements of dark matter in galaxies and clusters give

~ 0.2
but could be larger due to matter in distant parts of galaxies and clusters

The Production of Matter in the Hot Big Bang

At early times, T is large and particles of all types can be created and destroyed:

or:

There are two was that particles can stop interacting with other particles

annihilation: temperature drops below T = mc² and there's too little energy for particle production: stops,

but

continue

annihilation happens for particles which feel electric or strong nuclear forces
almost all protons, neutrons, electrons and their antiparticles have annihilated
except there was an excess of particles over antiparticles by a factor of about

1.000000001 = 1 + 10^-9

so for every proton around now, there used to be a billion more
there are still a billion times more photons than protons and neutrons

Freeze out: for particles like neutrinos that feel only the weak nuclear force

both pair production and annihilation stop:

because the density becomes too low
this happens to neutrinos when t = 1 sec
there are about the same number of neutrinos than photons
It could happen to other particles, too.

is constrained by Big Bang Nucleosythesis

recall that BBN starts when T is low enough for p + n -> D (deuterium)

deuterium survives long enough to make helium-3, tritium (hydogen-3)

Almost all the deuterium ends up as Helium-4
but how much Deuterium is left over?

It depends on the baryon density, or

if is low, then it is harder to for each D nucleus to find another to combine with
low or => a higher D fraction!
measurements of the D fraction tell us about

Here is the nuclear production history

recent measurements of D at high redshift give a D fraction of ~3 * 10^-5

=> ~ 0.05
total mass in stars is only ~ 0.007

Here's our best guess at the mass budget of the Universe:

Dark Matter Candidates

What is that stuff?

baryonic

NOT stars

too bright

NOT gas

can be seen in radio waves
or in absorption lines with bright sources outside our Galaxy

NOT dust

dust absorbs starlight and radiates in the infrared

brown dwarfs

like stars, but have M < 0.1M_sun
they never have core temperatures high enough to burn Hydrogen to Helium

white dwarfs

stars like the Sun that have used up their nuclear fuel
known formation mechanism involves ejection of outer layers

where did that stuff go?

black holes

baryons can collapse into black holes
stars with M > 40M_sun may collapse directly to black holes with no supernova explosion
once a black hole forms, we can't tell if it is made of baryons

referred to as MAssive Compact Halo Objects or MACHOs
constrained by Big Bang Nucleosythesis

elementary particles

massive neutrinos

neutrinos exist!
but they may not have mass
they are 10⁹ times more abundant than protons
they need to have m ~ 10^-8m_p to be the dark matter

Weakly Interacting Massive Particles or WIMPs

non-baryonic dark matter
predicted by some particle physics theories
may not exist
preferred theoretically

Axions

predicted by some particle physics theories
may not exist

Weakly Interacting Massive Particles

detect WIMPs as they pass through the lab as they orbit takes them through the Earth
look for rare collision depositing a small amount of energy:

detectors are very cold (< 0.1 K) and must be shielded from radiation

experiments may still not be sensitive enough

planned experiments can find only some of the predicted types of WIMPs

MAssive Compact Halo Objects or MACHOs

Prof. Bennett's project is the MACHO Project

other microlensing projects include:

EROS
OGLE
AGAPE

dark objects in the mass range of planets to stars can be found via gravitational lensing:

the gravity of the MACHO bends the light from the source star
with perfect alignment, we have an "Einstein ring":

since the MACHO is oribiting in our Galaxy, the lens moves in front of the source star

the image changes in time:

the circle shows where the Einstein ring would be

The angular size of the Einstein ring is about 0.001 arc sec for a typical Galactic lens

that's why it's called microlensing
on the gound, the atmosphere limits our resolution to 1 arc sec
HST has angular resolution of 0.1 arc sec
we can't see the separate images, but lensed images cover more area on the sky
so they are brighter

We look for time variations in brightness:

The main difficulty

the alignment we need is very rare - only 1 star in 2 million are microlensed at any given time
we must observe ~ 10 million stars
we can see more than this in the Large Magellanic Cloud (LMC):

about 10% of the size of the Milky Way
visible only from the Southern Hemisphere
the MACHO Project also observes towards the Galactic Center

but the LMC observations have the most sensitivity to dark matter

Here's the MACHO Telescope:

MACHO (& EROS) results to date:

less than 10% of the dark matter is in MACHOs with 10^-7M_sun < M < 0.1M_sun
an unexpected number of 3 month long events are seen

~50% of the dark matter in ~0.5M_sun objects?

white dwarfs?
black holes?

much more lensing by LMC stars than expected?

more details here