Dark Matter
Dark Matter: Types
- We will name two possibilities for dark matter: ordinary and
extraordinary matter. Ordinary matter is matter that we already know about.
For example, anything that is made of protons, neutrons and electrons would
be considered ordinary matter. Extraordinary matter would be matter that we
don't know about yet (or, at least, don't know much about).
- We normally call these BARYONIC and NONBARYONIC matter. Protons and neutrons
are made up of baryons, so ordinary matter is baryonic. There are strong
reasons to believe that dark matter is non-baryonic.
Dark Matter: Candidates
- There have been many searches for baryonic dark matter (mainly from using
gravitational lensing), and the observations show strong evidence that
there is not enough baryonic dark matter to explain what we need. The most
that baryons could contribute to the dark matter content in the halo of the
Milky Way (for example), is about 20%.
- For nonbaryonic matter, WIMP is the biggest category which consists of 100s
of suggested particles. WIMP stands for weakly interaction massive particle.
The most promising WIMPs are neutralinos.
Axions are another candidate but these are not WIMPs.
- The problem with WIMPs is that since they are weakly interacting they are very
difficult to detect. Right now there are trillions of neutrinos passing through
our bodies and we don't care because they don't do anything to us.
- WIMPS are favored as the dark matter particle due to the "WIMP miracle". That is,
from theory, there is believed to be a stable particle with mass around 100 Gev
(which is the scale of the weak force)
that will have just the right amount of numbers to explain dark matter.
Dark Matter: Searches for Non-Baryonic Matter
- The gold standard for understanding the dark matter particle is to see it. From
astronomy studies, this will likely not happen, since we so for can only indirectly
measure the gravitational effect of the dark matter. We can get the total mass,
and possibly how it is distributed around a galaxy. We find that the dark matter
is significantly more extended than the light that we see.
- There is a potential for astronomical observations to see the collision and
annihilation of dark matter particles. If two dark matter particles hit head on
with the proper speed they can annihiliate each other and produce gamma rays.
The telescope FERMI observes gamma rays.
FERMI does see gamma-rays coming from the center of galaxies, and
some have
claimed that this is evidence for dark matter annhiliation; however, a recent
result
shows that the gamma-ray emission coming from our Galaxy is likely not from dark matter.
- The most likely technique that will discover the dark matter particle will come from
physics experiments. These direct detection technique rely on the dark matter particle
interacting with baryonic matter. The dark matter particle can scatter of a
nucleus and cause it to recoil (which is measurable). The particle can excited bound
electrons in an atom, which emits radiation. Depending on the type of dark matter
particle, it can hit a nucleus and create other particles which can be detected.
Thus, we still never "see" the dark matter particle yet, but should detect its
presence directly.
- WIMPs are hard to find, but recently the detector technology has increased to allow
for the search for neutrinos. There are many searches ongoing now and the initial
motivation was to explain the lack of neutrinos coming from the Sun.
- We know that the nuclear reactions in the Sun produce copious amounts of neutrinos.
We can detect some of the them but not the numbers that we expect (and we
understand the Sun very well).
- A theory was proposed in the mid 80s that suggested
that the neutrinos on their journey from the Sun to here undergo a shift from
one type of neutrino to another type. But the only way for this to happen would be
if the neutrino had mass. Thus, the search to measure the neutrino mass started.
- For neutrinos, all experiments rely on trying to detect reactions from the neutrinos
as they travel through a large body (the ocean in some cases). When the neutrino
strikes something head-on, it creates a particle shower that are recorded by
various detectors laid throughout the body.
- The neutrino experiments so far can only detect a mass difference between
the two types of neutrinos, and not their absolute mass. They have found that
the mass difference is very small. On probabilistic grounds, if the mass difference
is small, then the actual mass is likely to be of that order. And at this mass,
the neutrinos may contribute to some of the dark matter, but probably not all of it.
- This website list many
experiments that are trying to detect various particles.
- A controversial results comes from
DAMA . Since the Earth is moving through the Milky Way halo at different angle throughout
the year, the interaction of the dark matter in the detector to modulate during the
year. Below is their data:
- One of the more important dark matter detection experiments is
CDMS , the
Cryogenic Dark Matter Search.
- See summary here
for dark matter particles.
Dark Matter: Expectations
- The consensus is that dark matter is a particle. An important aspect is to determine
the mass. This will best come from the particle physics experiments, but we can
learn a lot from astronomical observations.
- One of the strongest arguments for dark matter being a particle comes from
observations of the Bullet Cluster .
- The mass of the particle is related to the temperature that the particle has,
which is related to the velocity.
- By far, the most popular had been cold dark matter (or CDM). There are still many
models where the dark matter is warm or hot, but these models have a hard time
making the structure that we see in the Universe.
- For some time now
we have understood the basic properties for making structure. There has to
be enough time and mass for systems to come together.
- For example, we start the simulation with an expanding Universe filled with
hot gas and let it evolve. There are spots in the Universe that have an
over-density by random. These spots eventually come together if there is
enough mass.
- If there is not enough mass, the structure will never be able to overcome
the expansion. This result was a theoretical reason as to why we needed
dark matter: without it there was not enough structure
- The other requirement is that there has to be enough time for the structure
to form that we see today. This is way the age of the Universe is so important
for the modelers since they have to tie the age with the amount of structure.
- The history of cosmological simulations tracks very well with the improvement
of computers. Since we are trying to simulate the Universe, we need as many
particles as possible (this requires memory) and then we must be able
to track them (this requires speed).
- What is probably the most important aspect of the cosmological simulations
are the initial conditions. I.e., just how do we start the Universe model?
We said that we start with a smooth distribution of gas, and some parts of
if have over-densities. But the important question is just how over-dense
and how many of these regions are there.
- The answers lie in the cosmic microwave background.