Dark Matter - The Particle Hunt

First, you may want to read my first post that describes the basic problem of dark matter and my second post which talks about an alternative to dark matter.

Also, I mentioned in my second post, 'dark matter' (if it actually exists) should not really be called dark, it should actually be called transparent matter. It's not dark, it just doesn't interact interact with light.

This post is about the search for the types of matter that could be making up the missing mass of the Universe. The short answer is that the particle has not yet been found so there are a lot of proposed candidates. I am only going to present a few of the more likely candidates.

Basically it boils down to two main categories: what humans would call 'normal' matter versus some as yet undiscovered exotic non-interacting matter.

MACHOS

The 'normal' matter category is what a physicist would refer to as baryonic matter or the stuff that is made up of protons and neutrons (i.e. atoms and molecules).

There is a nice acronym for this category which is MACHOs (stands for MAssive Compact Halo Objects).

A MACHO could be any type of familiar matter such as interstellar dust, gases, rocks, comets, asteroids, orphaned planets and brown dwarfs etc.

It is proposed that this type of matter is simply unlit. It's not a glowing star and it is just out there in the dark reaches of interstellar space and we have simply not seen it or accounted for it.

Astronomers therefore have used a couple of techniques to try to find it. The two main ones are micro-lensing events and the other is infrared detectors.

Micro-lensing events: The main method for looking for MACHOs is to look for gravitational lensing events as a MACHO passes in front of a distant star.

Since MACHOs are relatively small the effect would be very small so these are called micro-lensing events. Sadly, results from a ~6 year survey of the Large Magellanic Cloud have indicated that MACHOs do not seem to be a promising candidate to explain the dark matter mystery.

Infrared Detectors: Everything glows no matter how cold it is due to blackbody radiation (unless it is at absolute zero which is impossible to attain) so this technique has been used to try to find if the missing mass would show up in these wavelengths.

Unfortunately the results seem to indicate that if these objects are out there they are small and not that numerous (source). Oh well.

WIMPS

The other candidate for dark matter is fittingly called WIMPS (Weakly Interacting Massive Particles). Neutrinos would be a good candidate as they almost never interact with normal matter.

Neutrinos, by the way, were considered to be candidates for a while but even though they are incredibly incredibly numerous their individual mass is so low that they just can't make up enough of a fraction of the missing matter. Neutrinos also move quite fast (i.e. they are 'hot') and as such do not help out much in cosmological evolution models. So neutrinos even though they are known to exist are basically a no-go as a dark matter candidate.

We are left then with some mystery particle that has not been discovered yet. This leaves us two main ways to determine their existence.

1. Direct Detection

One avenue to find dark matter is via direct detection. This is done via at least two types of experiments: i) large "baths" of some ultra-pure material surrounded by scintillation detectors and ii) particle accelerators.

There are detectors like SNO, AMANDA, ICECUBE etc., that are basically just large quantities of some pure material surrounded by high sensitivity detectors.

The detector just sits there waiting for particles to come in from space, interact with an atom in the large "bath" and emit a burst of identifiable light. The event count rate for these detectors is very low and the background noise from other types of events can be high so no conclusive results have been made yet. No joy here (yet).

2a. Indirect Detection via Particle Accelerator Data (i.e. Inference)

The neutrino was first postulated because it was observed that there was missing momentum in the decay of a free neutron. Pauli inferred that some undetected particle (i.e. the neutrino) carried the momentum away (and thus saving the law of conservation of momentum). He was proved right almost 20 years later.

The idea is similar for inferring the existence of some undiscovered dark matter particle. There are quite a few particle detectors around the world and there are Petabytes of recorded particle collision data.

It is postulated that buried in that data there may be a few repeated instances of particle collisions in which the conservation of momentum is 'violated'. If found, this could provide a decent clue for inferring the existence of a particle that just simply escapes out of the collider without being detected (i.e. dark matter).

2b. Indirect Detection via Cosmic Rays

As per lemouth's helpful comment another good way to infer the existence of dark matter particles is to observe and analyze the gamma ray radiation that is generated near the centre of the Milky Way galaxy.

Researchers analyzed the data from the Fermi Gamma Ray Space Telescope and believe that they have detected a possible signal at an energy of 130 GeV. It is thought that the energy signal could be due to self-annihilating dark matter particles or some other dark matter related particle reaction event chain.

2c. Indirect Detection via Theoretical Cosmological Evolution Models

Another good way to infer the existence of dark matter is to simulate how it would have influenced the evolution of the early Universe after the Big Bang event and compare that to the observations of the cosmic microwave background.

Dark matter is postulated to not interfere with ordinary matter except via the influence of gravity. The perturbations from dark matter versus the perturbations from ordinary matter lead to different tell-tales in the cosmic background radiation.

Basically it boils down to measuring the distributions of the angular sizes of the slight anisotropies in the cosmic background temperature. Running these models with and without the existence of dark matter in the models leads to different power spectra in these deviations.

Hint: the fit is better assuming the existence of dark matter. Even more the constraints on the temperature (average particle velocity) of these particles can even be deduced as well from the models.

Closing Words

The solution for the mystery of Dark Matter has not been found yet. Since it is such an old problem, and since it means discovering a large fraction of the Universe surely there is a Nobel prize in it for the scientists who figure this one out.

Thank you for taking the time to read my post.

_{I, Procrastilearner, donate the above image to the Public Domain.}