Dark matter is an incredibly elusive force which is necessary for many modern theories that describe the movement of galaxies and – effectively – all of the major elements of the universe. One of the reasons that dark matter has arisen as a theory is the fact that when we observe certain galaxies, they have much more kinetic energy then the masses in them would suggest, therefore, a separate force must be at play. However, we have not yet been able to detect it because it does not interact with electromagnetic radiation, and is also not found on the electromagnetic spectrum. Because of how crucial it is to a huge variety of astrophysical concepts – it is critical that we pinpoint exactly what dark matter is in order to build other theories off of that. Due to the fact that it is undetectable by many modern astronomical instruments (this is because dark matter does not interact electromagnetically, this means that it does not absorb or emit light – making detection difficult), there are a vast array of methods and equipment that we can utilise to find this particle. However, almost all of these methods are expensive and extremely time consuming, this means that not all of them can be properly carried out to completion.
Therefore, we need to pinpoint some specific avenues down which we can take the search for dark matter in order to achieve a relatively quick and effective solution to this problem. Therefore, we need to weigh up the different detection methods and state what level of investment they are deserving of. Dark matter is not just useful for proving theoretical concepts about things that have no bearing on our day to day lives. If we could study dark matter intensely it could unlock many useful principles about the nature of gravity which we could manipulate for our own gain. As can be seen by the diagram below a huge chunk of all of the matter in the universe is not easily detectable, this simply emphasises how much further we need to go to push the boundaries of our knowledge about the universe as a whole, therefore experiments such as these should not be cast aside, but instead be given paramount government funding attention. Sadly this is often not the case. This lack of funding is another one of the main reasons that it is important for us to evaluate the most likely method to turn up with hard evidence of dark matters existence. This year the US government only pledged $24 million towards dark matter experiments over a four year period. If we cannot effectively evaluate the most probable way of detecting dark matter, therefore, our scientific development as a society could be hampered for years to come.
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Firstly, one of the most popular methods of dark matter detection is ADMX (The Axion Dark Matter Experiment). Basically, all methods of dark matter detection isolate certain properties of groups of particles and test based upon this. In the case of ADMX, experiments aim to take advantage of its unique ability to turn into photons of light when they are exposed to a magnetic field, it does this by using a resonant microwave cavity and a large superconductive magnet. One of the main pros of this experiment is that if it runs its course it will have completely ruled out axions for contention if they are not detected. ADMX has gained notoriety in recent years after the lack of detection of WIMP particles after many years of testing. As well as this, unlike many methods of detection, ADMX does not need to be located underground as the experiment is not massively affected by products of the outside world. This means that it is cheaper to construct because laboratories miles underneath the ground do not need to be constructed. In general, because of the relative cost effectiveness of this method, I think that it should run its course in order to fully decide whether or not dark matter particles are axions.
However, the most popular potential particle that could be behind dark matter has long been WIMPs, these are weakly interacting massive particles which are predicted by nuclear theory. WIMPs would only interact with other particles with a weak nuclear force or gravitational force and would be quite large when compared to the average particle. So, if WIMPs have been one of the top suspects for dark matter for decades, how come we’ve never detected them? Well, because of their aforementioned weak gravitational and nuclear force, it is rather difficult to pinpoint them through testing. The most likely way that we could possibly detect a WIMP particle would be to see if it rebounds elastically when it comes into contact with the nucleus of an atom. Therefore, one could test to and measure the energy change in an impacted nucleus and, having ensured that no other particles had caused this result, could say that this recoil of the nucleus was caused by a WIMP particle. However, in order to achieve a state in which it can be proven beyond reasonable doubt that the recoiling nucleus was not caused by any external matter.
A good analogy for this is if one were trying to detect a mosquito crashing into the windshield of a car. You could see the stain of what was once a mosquito but in fact many bugs are flying around all of the time, you could not prove that this specific one was a mosquito over a gnat etc. Therefore, we need to create an environment in which only WIMPS could interact with these nuclei rather than other subatomic particles, or mosquitos with the windshield if you will. For example, in the large underground xenon experiment (LUX) the actual equipment was surrounded by earth – as it was 1510 metres below the ground – and water, in an attempt to block background particles from interfering with the experiment. However, as previously stated, this comes at a cost. Whilst there are now figures specifically available for ADMX, due to the fact that it is independently run it is estimated that it falls into the price range of $2-15 million. The LUX-ZEPLIN WIMP detection project had an estimated $70 million apparatus cost according to wired. This is significantly more expensive than any Axion detection project, which means that we need to critically evaluate what it actually brings to the table to justify this cost.
Now, having laid out the basics of each of the experiments and some simple positives and negatives surrounding them, I am going to look at the history of both projects to see how they have failed in the past, and how this failure impacts their future. The figure below demonstrates what potential spin rate and WIMP mass combinations that we have tested negatively over the course of multiple experiments, the edges of the box represent the limits of the physical properties of a WIMP particle which would make it viable under our current model of the Universe. The way to interpret this diagram is that everything above the lines has been ruled out. This leaves a rather small area within which WIMP particles could lie, leading some to ask the question: is it worth moving on?
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