Will we ever find proof of the elusive dark matter?
Imagine youíre baking a Victoria sandwich cake. Youíve printed out the instructions and have all the ingredients at hand: the flour, the eggs, the butter, baking powder and sugar.
Now imagine you can only see the baking powder. All the other ingredients are there, but invisible to all your senses.
Letís say you could somehow manage to get each item into a bowl. The next conundrum? Whipping the butter and sugar into smooth, light and airy batter would be no mean feat - how would you know you have the right consistency? As a baking novice, Iíve read of the complications that arise from over mixing Ė a chewy, dense cake - and nobody wants granny to lose her dentures over this thought experiment.
All the best bits of cake construction would be decidedly crap. Bowl licking antics turn into choking down baking powder, and when you pop it in the oven at 190oC, it doesnít waft delicious cakey smells. The end result? Well, it would certainly be airy.
Letís face it; if I were to present this on the Great British Bake Off, I wouldnít be glimpsing Mary Berryís iconic side bite.
Yet this is basically equivalent to our understanding of the universe: Of the 4% we can see, less than an eighth of it is visible to our naked eye - the planets, the stars and all things closer to home on Earth - and the other 3.6% is just cold gas floating around space.
The remaining 96% is Ďdarkí. When scientists say dark, it basically means they havenít foggiest as to what it is. Worse still, scientists are not even entirely sure we will ever be able to detect it, although that hasnít stopped them trialling bizarre ways to trick dark matter into revealing itself.
The problem is that this substance is completely unlike anything mankind has ever seen. First off, it doesnít interact with anything. That means light Ė at any wavelength - doesnít bounce off it. It doesnít emit anything either; nor does it interact with atoms, electrons or any of the other subatomic particles. Galaxies smash together, but dark matter just cruises on by. This being the case, how can we even be sure it exists?
The only very subtle clue is that there is something rather than nothing is that dark matter has a mysterious effect on gravity and sound. I hear eyebrows rising at the thought of the latter Ė ďSound in space? But itís a vacuum. If I screamed in space, nobody would hear me.Ē
Itís true. But when the universe was first forming, it was so dense that sound could travel through space - and believe me - you would be screaming, albeit briefly, with temperatures rocketing to a stifling four thousand degrees. Astronomers can see these sound waves today because theyíre imprinted on something called the Cosmic Microwave Background (CMB). These sound waves, despite being billions of years old, cause tiny temperature fluctuations in the CMB and scientists can unpick the origins of our universe, and dark matter, by looking at them.
Sound travels differently depending on what itís travelling through (think water versus air) and because of that, we can infer something about the stuff itís moving through. If we turn back to the skies then, we can see these sound waves are being dampened by something. Ergo, something is affecting these early sound waves.
But thatís not where the first hint of this elusive stuff came from. From his perch on Caltechís Mount Wilson Observatory, Fritz Zwicky photographed a small section of the night sky. If you held your hand at armís length, the section of the sky Zwicky was observing at is about the same size of your thumb nail. Within that thumb nail-sized slice of the sky are a few thousand galaxies, bunched together in gravitational knot called the Coma Cluster.
Under the influence of gravity, all the galaxies within the Coma Cluster orbit one and other, much like how Venus, Earth and Mars all swivel around the Sun.
One of Newtonís brilliant insights was that gravity is a predictable force that acts on all matter in the universe, and the amount of gravitational force on an object is proportional to its mass. With this equation, Zwicky could calculate the mass of the cluster by observing how fast the galaxies were swivelling around.
Except, the maths didnít add up; when he took direct measurements of the total light emitted from the trillion or so stars in the Coma Cluster, there was a huge shortfall. Theyíre simply werenít enough stars and galaxies to account for the speed at which these galaxies were travelling.
Without this extra mass, galaxies should just fly off - like a small child on a roundabout. The galaxies needed to contain about 80-90% more matter for them to stick together. Think of a whale on that same roundabout Ė it isnít going to take off any time soon.
Zwicky speculated it might be called something called Ďdunkle materieí.
And so, the search for dunkle materie, or dark matter, began. At first, scientists of Zwickyís time were still grappling with the Big Bang theory, and so Ďthe dark matter problemí seemed frivolous. However, as the fields of astronomy, technology and particle physics began to make progress, the hunt for dark matter became all the more alluring.
There were numerous theories as to what this excess matter could be: black holes, cold gases and giant planets. These have all been gradually ruled out and the most likely candidate is now weakly interacting particles, or WIMPs. WIMPs are indeed quite wimpy, for they donít really interact with anything around them and thatís why theyíre so tricky to lay our telescopes on.
There are a couple of approaches in which scientists alike have been trying to coax WIMPs from their shadowy existence. On one side, are the particle physicists of the world using things like CERN to create dark matter in the lab or even trying to snatch a WIMP from the ether, right here on Earth.
Believe it or not, a ĎWIMP windí Ė or perhaps gale is more apt Ė is descending on Earth at 220km per second; thus, for every pint of beer you have, theyíll be at least one dark matter particle nestled amongst the bubbles. If there is so much to detect, why canít we find one with a purpose built detector?
Because WIMPs donít interact, these detectors have to be extremely sensitive to radiation. Theyíre so sensitive, even the radioactive decay of potassium from a single banana could set off 2,000 events a minute within the detector. You can never be too safe when searching for these coy constituents; staff arenít allowed to eat bananas.
As a result, these detectors are buried deep beneath the Earthís surface, surrounded in radioactively clean materials and sat in massive water tanks so to avoid any sort of contamination.
Currently, scientists like Dr Chamkur Ghag muse that weíve been unsuccessful because our detectors are just too small. Because interactions with normal matter are so rare, if you increase the size of these detectors, you increase your chances. So scientists from across the world are collaborating to build the biggest detector youíve ever seen. Itís set for completion in 2018 and we should be seeing results by 2021. If not, ďwe might need to rethink our theories,Ē Cham explained to me. A rather daunting prospect I imagine.
The race is on though, because CERN scientist Will Kalderon think we might be able to make a WIMP even sooner! Although Will was quick to quip that there is one condition to that statement: ďif itís made out of the right kind of stuff, then maybe, but if itís made out of the wrong kind of stuff, then who knows.Ē
Willís PhD focuses on this problem. The idea is you should be able to make dark matter in CERN: by smashing protons together at the speed of light, you can create mini Big Bangs and that means you can create dark matter, since dark matter was created during the Big Bang. The problem isnít creating it, itís detecting it. The way Willís hoping to do it is by looking for missing energy.
He can look at all the particles and at what speeds they fly out of the collision, and because he knows the initial energy created when the protons smash, Will can add up all the energy of the particles and check if thereís any unaccounted for. An energy gap.
Further afield are the astronomers like Richard Massey who use telescopes like Hubble to observe how galaxies collide to look for hints of dark matter. Except, the queue is too long for Hubble, so he built a super lightweight telescope and sent it onto the cusp of space using a balloon in September.
With this flurry of activity, the race is on, but by no means does this guarantee that dark matter is going to be found any time soon.
This elusive stuff may continue to evade our detection for decades to come but perhaps thatís OK. One of the wonderful things about physics is that itís very good at problem solving, and with each new problem comes a novel set of ideas for us to ponder. So, in some ways, it might be more interesting if dark matter wasnít really a WIMP, if not a tad frustrating for the scientists at the forefront...
The answer is of course nobody knows. I am wondering whether dark matter exists in the same form of mass as the rest of observable matter. The strong nuclear force is some 10 to the 40 times as strong as gravity. If dark matter is unformatted matter as opposed to atoms which are stable structures, and yet it still carries the same forces, then the matter required is 10 to 40 times less to create the necessary gravity to balance the universe. Also as unformatted matter it would not have the necessary structure to emit energy in quanta to enable us to detect it. dhjdhj, Mon, 18th Jan 2016
There is of course no such thing as Dark Matter.
Of course we will. But shall we?