Dark Matter

One of the biggest conundrums in modern astronomy is the fact that over 90% of the Universe is invisible. This mysterious missing stuff is known as 'dark matter'.The problem started when astronomers tried to weigh galaxies. There are two methods of doing this. Firstly, we can tell how much a galaxy weighs just by looking at how bright it is and then converting this into mass.The second way is to look at the way stars move. Everything in the Universe rotates. The Earth spins on its axis. The whole planet orbits around our parent star, the Sun. The Sun rotates around the centre of the Milky Way, along with the billions of other stars in the Galaxy, forming a huge cosmic dance. This rotation provides another way of weighing a galaxy. Studying how fast stars at the very edge move reveals the mass of the whole galaxy. The faster the Galaxy rotates, the more mass there is inside it.But when astronomers such as Jan Oort and Fritz Zwicky did the two sets of sums in the early 1930s they hit a big problem. For every galaxy they studied the two answers didn't match. They were very confident that both methods were sound as they'd been tried and tested for many years. So they came to a startling conclusion - there must be stuff out there that we just can't see - and so they called it 'dark matter'. This dark matter was really important, as if it wasn't there then galaxies would fly apart as they spun round.This might seem like a strange conclusion, but it's not really that bizarre. Imagine looking at a tower block at night. Although you can only see lights coming from some of the rooms, that doesn't mean that there aren't any more rooms in the tower. Just like these unlit rooms, dark matter can't be seen, because it doesn't shine.Astronomers are currently hunting for this missing matter. It may consist of lots of strange sounding things like MACHOs, WIMPs and neutrinos. Or there may be new solutions involving dark energy or superstring theory. But whatever it is, finding it will help to answer one of the most fundamental questions in astronomy - what is the fate of the Universe?

WIMPS :
Why are physicists looking underground to try to find the missing mass of the Universe? Well, they're searching for WIMPs. A WIMP is a Weakly Interacting Massive Particle. This is a type of 'exotic' particle that is one of the candidates for the mysterious dark matter that's missing from the Universe. Although they were formed in distant places, 'exotic' just means that they are different to the ordinary particles that make up the world around us. WIMPs have never been found, but physicists think that there could be loads of them flying straight through us all the time. In fact about a million of them could be passing through your fingernail this very second!If there are so many flying around, how come we haven't spotted one yet? Although they are called 'weak', they are amazingly strong - they can pass through solids without stopping. And they are not the only things that are zooming down from space - we are also being bombarded by cosmic rays. Both these factors make them extremely difficult to detect.But what makes WIMPs special is their ability to travel straight through solid objects. So one of the best ways to catch them is to go deep underground. The UKDMC project is housed 1100m down the deepest mine in Europe and the GENIUS team work from inside the Gran Sasso mountain in Italy. Seemingly strange locations to study the Universe, but the feature that both these places have in common is rock, tons of it. The thick rock walls act as a natural filter, so as the space particles travel downwards, the cosmic rays collide with the atoms in the rock and are halted. WIMPs should be able to pass straight through the rock and out into the caverns below where they can be detected.Teams from all around the world are currently racing to find out whether these elusive particles exist. If they do, they'll form just one of the missing pieces in the jigsaw puzzle of dark matter. It's estimated that they could contribute up to 90% of dark matter. The other main contenders for the rest are MACHOs and neutrinos.

MACHOS :
Some of the invisible dark matter that is missing from the Universe may be massive dark bodies such as planets, black holes, asteroids or failed stars (brown dwarfs). These don't produce light and so cannot be seen over great distances. [We can see planets and asteroids that lie within the Solar System because they reflect light from the Sun, but they don't make their own light like stars].All these heavyweight candidates for the missing mass are collectively known as 'MACHOs', Massive Compact Halo Objects. This is because they are large bodies that live mostly in the 'halo' of galaxies. This is like the 'outer suburbs' of the galaxy, extending from outside the 'inner city' of the galaxy's core right to the wastelands at the edge.Another possibility is that 'dark galaxies' may exist - mini-galaxies that roam undetected in intergalactic space. Around a thousand times smaller than the Milky Way, these galaxies would be too faint to see from Earth. Instead of harbouring billions of stars like our own Galaxy, they would be full of brown and black dwarfs.As with all dark matter, dark galaxies and MACHOs are both hard to detect, as we can't see them. But a surprising solution comes from the amazing effect that gravity has on the fabric of space. Everything with mass has a gravitational field, even you and me. The bigger you are, the more gravity you wield. According to Einstein's theory of General Relativity, gravity actually bends the fabric of space and the more gravity an object has, the more it warps space. This was proved by Arthur Eddington on an expedition to the Amazonian forest in 1919.This effect can be used to detect MACHOs. When a really massive object [such as a MACHO] moves in front of a distant star, its gravity bends the space around it. When light from the distant star travels through this distorted region of space it bends and becomes magnified. So while the MACHO is crossing in front of the star, it seems to move and grow in size. This is known as gravitational lensing and allows astronomers to weigh the invisible MACHO causing this phenomenon.There is no doubt that MACHOs account for some of the missing dark matter in the Universe. But there is a limit to the amount that can be attributed to them alone. This is governed by the amount of atoms formed in the Big Bang, putting a limit on the amount of ordinary matter there can be in the Universe. All in all, MACHOs could account for around 20% of the missing dark matter needed to glue the Universe together. To solve the rest of the story we have to start looking for matter that isn't ordinary, but 'exotic', such as WIMPs or neutrinos.

NEUTRINOS:
Early last century, a physicist called Wolfgang Pauli invented a particle to help his sums balance. It was a tiny elementary particle, smaller than an atom with no electric charge and no mass. All this particle did was carry energy as it zipped along at the speed of light. The problem was that even if they existed, being so small and placid these particles would be very tricky to find. Pauli himself confessed, "I have done a terrible thing. I have postulated a particle that cannot be detected."Although Pauli's particle was just made up, people took it seriously enough to integrate it into new theories in particle physics. Enrico Fermi called this enigmatic character the 'neutrino', meaning 'little neutron' in Italian. After this scientific baptism, the neutrino was accepted, even though it was still absent. The hunt was on. It took over 20 years before a particle with all the right features was spotted and the neutrino was declared alive and well.A huge amount of neutrinos was created in the Big Bang. But they are still being created in the heart of the hottest, most energetic regions in the Universe, such as supernovae, gamma ray bursts, quasars and even stars like our Sun. They even filter down to the Earth - by the time you have read this sentence an estimated 10 million neutrinos born inside the Sun will have passed through your body. When you have finished this paragraph, the same neutrinos will already be further away than the Moon.But what does this mean for dark matter, the missing mass of the Universe? Neutrinos can be considered missing because they're so difficult to detect, as they don't interact much with ordinary matter. But if they have no mass then how can they contribute to dark matter? he dark matter mystery was uncovered in the same year as Pauli 'invented' neutrinos, and being mass-less they were quickly crossed off the list of potential candidates. But a groundbreaking discovery in a Japanese zinc mine in 1998 proved everyone wrong. The Super-Kamiokande project announced that they had found that a certain type of neutrino did actually have a mass, albeit a small one, placing the particle firmly in the running as a dark matter contender.Neutrinos are known as 'hot dark matter' as they travel at the speed of light. However, they may be plentiful, but because of their extremely low mass it is estimated that only make up around 25% of the missing dark matter. So where's the rest? It's a contest between MACHOs and WIMPs.

HIGHLY STRUNG :
Currently there are two 'golden rules' of physics - General Relativity, which governs the large-scale Universe and quantum mechanics, which governs the nanoworld of atoms. The problem is that the two laws don't really agree with each other. What is needed to bridge the gap is an extra clause that links the two, a 'theory of everything', known as quantum gravity, that has yet to be discovered.Superstring theory is a contender for this prize. The idea is that the zoo of thousands of tiny 'elementary' particles that exist are not disparate entities but all originate from the same source - a vibrating string. The easiest way to imagine this is to think of a guitar string. Pressing on the fretboard alters the length of the vibrating string, producing a new note. Similarly, in superstring theory, elementary particles can be thought of as different notes played on the same string. Each string is unimaginably small, about 10²⁰ (100 billion billion) times smaller than a proton. Vibrating the string at different frequencies generates all the different types of oddly-named elementary particles, such as 'gluons', 'weakons' and 'strange quarks'.But in order to vibrate, strings need lots of room. In fact they need more room than is available in the four-dimensional world in which we live (made up of height, width, depth and time). Superstring theory requires the presence of ten dimensions! But where have the other six gone? Physicists have suggested that during the Big Bang these other dimensions were folded away, or 'compactified' leaving only four to expand and evolve.But what does this mean for dark matter, the missing mass of the Universe? If superstring theory is right then it could provide an unusual answer to this cosmic mystery. Although these hidden dimensions remain too small to be measured, gravity can travel in between them. Hence the extra mass that is missing from our Universe may just be fallout from these unseen dimensions.First we'll have to wait to see whether superstring theory is accepted as the crucial 'theory of everything'. If it is, then astronomers might have finally discovered where dark matter has been hiding out.

DARK ENERGY :
Over the past five years, the mysterious tale of dark matter has been taken yet another bizarre twist. Just as cosmologists decided that the Universe is full of a strange invisible matter, they then realised that space was even weirder than they had thought.After a full census of all the galaxies visible in the Universe, astronomers calculated that their total mass (including their hidden dark matter) only made up about one-third of the critical density needed to satisfy the best current theory about the early Universe [known as inflation].At first, cosmologists thought that inflation must be wrong. But then further data from measurements of the cosmic microwave background showed that the total density of the Universe did add up to this special critical density, so inflation could be reinstated and another solution had to be found. So even after their best attempts to renovate our cosmic home, most of the Universe still remained elusive.The problem is that this new component seems even stranger than dark matter. Not only is it invisible just like dark matter, but it must have a repulsive force, otherwise it would get sucked into galaxies and affect their motion. So this mysterious stuff has been labelled 'dark energy' and is a kind of cosmic antigravity force that counteracts the attractive force of gravity. This means that instead of the expansion of the Universe slowing down, in fact, it is speeding up. Recent measurements of distant supernovae agreed with this conclusion, finding that the Universe was indeed expanding with increasing pace.Interestingly, this had already been foreseen in a botched sum. But this wasn't the sloppy homework of a spotty student, this was a calculation by Albert Einstein no less! It was 1917 and he was trying to reduce the dynamics of the whole Universe into a set of mathematical formulae. His results insinuated that the Universe was expanding. However like the vast majority of people at the time, Einstein assumed that the Universe was static. So to stop his model Universe from growing, he invented a number he called the 'cosmological constant' to insert into his equations, restoring the safe, steady nature of his static Universe.After Edwin Hubble found that the Universe was indeed expanding, Einstein quickly retracted this number, calling it his 'biggest blunder'. But he needn't have been so hasty because now, nearly 80 years later, it has been reinstated to account for this mysterious dark energy that is overtaking gravity. These new findings are helping cosmologists look into the future and foresee the destiny of the whole Universe.

DOES DARK MATTER?

Imagine holding a black velvet bag full of ball bearings. You can feel their shape and put them on the scales to weigh them. But if you couldn't see or reach inside the bag, how would you be able to know for sure how many ball-bearings were in there?As if that wasn't difficult enough, imagine how confused you'd be if you did finally manage to examine them, only to find that there were far fewer than you originally thought. But not only that, but when you put them on the scales again they only weigh a tiny fraction of their former mass!This is similar to the conundrum facing physicists looking for dark matter - they are faced with two conflicting pieces of evidence when it comes to studying galaxies. There are two scientifically sound methods of weighing a galaxy. The problem is that they both give completely conflicting answers.Firstly, if we consider a spiral galaxy, such as our own Milky Way, or our neighbour, Andromeda, we can measure its mass by measuring how fast it spins. The faster the galaxy turns, the more mass it contains. The second way is to calculate the mass of all the observable luminous parts within the galaxy.But when physicists first made these measurements for spiral galaxies in the 1930s, they discovered that the two answers were not the same. And it wasn't a matter of a slight difference - the Milky Way was behaving as if its mass was TEN times that of the luminous component. The startling conclusion was that there must be stuff inside these galaxies that we couldn't see - 'dark matter'.The race was on to discover what this elusive dark matter actually was. If we consider the ball bearings in the bag again, the analogy of the dark matter quest is like trying to measure the ball bearings inside the bag without being able to see them directly. So how do we know what we're looking for?It's thought that the majority of dark matter is sub-atomic particles. This prediction is based on a particle physics theory called the Standard Model. This is a new theory that is attempting to unify the four fundamental forces in the Universe - the electromagnetic force (which keeps electrons bound to nuclei), the weak force (causing radioactive decay), the strong force (binding the atomic nucleus together) and gravity (which sticks everything that has mass together).The Standard Model predicts that a group of particles exists called WIMPs or Weakly Interacting Massive Particles. They tend not to interact with everyday objects, which is why we haven't spotted them so far. The rest of the dark matter could be made up of MACHOs (Massive Astronomical Compact Halo Objects). Recent data indicates that MACHOs constitute no more than 20% of the missing mass.A small percentage of the mass could also be made up from neutrinos which are now known to have mass. Or another candidate is an even more exotic particle called the axion. At the moment though, WIMPS, in particular a certain type called 'neutralinos', seem to have the best chance of filling the mass deficit.How do we look for dark matter? There are several searches taking place around the world for neutralinos using different techniques. An experiment in a laboratory near Rome, called CRESST has a small crystal of sappire supercooled to milliKelvin temperatures. When an interaction between the neutralino and the crystal takes place there should be a measurable increase in temperature. An experiment based in the United States, called CDMS, uses a combination of temperature rise and ionisation of silicon or germanium. Another experiment, DAMA in the same Gran Sasso laboratory as CRESST uses scintillation detectors. It is this type of detector that we are currently using here in the UK, as part of the UK Dark Matter Collaboration, UKDMC.All these experiments have one thing in common - they are all based underground! Our facility is based 1100m under the surface in Boulby Mine, approximately 10 miles north of Whitby on the North Yorkshire coast. The reason we conduct our experiments inside the deepest mine in Europe is to stop the detectors being bombarded by cosmic rays. These would completely mask the dark matter signal. The detectors are also constructed from the purest materials possible, as again any radiation emitted from the detector materials would swamp the signal.The scintillation detector, in this case a crystal of sodium iodide doped with thallium, emits a number of photons in direct proportion to the amount of energy deposited in the crystal by the dark matter particle. Light is also emitted when background radiation such as gamma rays and neutrons interact with the crystal. Some noise signals can be rejected online whilst others are rejected during offline analysis.Identification of the different energetic particles, including dark matter, is done by pulse shape discrimination. This is possible due to the different way the pulses interact with the target. Gamma rays will give rise to electron recoil whereas neutrons and dark matter particles will cause the nucleus to recoil. Each of the interactions gives different pulse shapes.Another way to look for neutralinos is by measuring the annual modulation of the signals from the detectors. As the earth rotates around the Sun, the Earth is either moving in the same direction as the dark matter flux (in December) or against it (in June). This is the technique utilised by the DAMA group. Preliminary results have been published which claim to show this effect, revealing the presence of neutralinos, but greater statistics are necessary to convince the scientific community that it is not just due to errors within the experiment.Here at UKDMC we have recently installed a new type of scintillation detector based on liquid xenon, called ZEPLIN I. This should improve our results considerably, as liquid xenon has a higher nuclear mass than sodium iodide (which is used inside our current detectors), making it better suited as a target for neutralinos. The problem with using xenon as a target material is that it is only liquid over a very small temperature range of four degrees, which has provided us with some interesting challenges. But controlling the target between these temperatures has been achieved and a working detector is now operational in Boulby Mine.The next stage in the ZEPLIN development is to improve the discrimination between signal (the neutralinos) and noise (other radiation). Two new detectors have been proposed and we are awaiting the birth of ZEPLIN II and III. Another detector currently under construction for deployment at Boulby uses a low-pressure gas and is called DRIFT. The beauty of this system is that it has almost 100% background noise discrimination and will be able to tell from which direction the particles are coming - the background noise should be homogeneous whilst the dark matter signal should show the annual modulation. With these advances in detector development we should soon be able to tell exactly what this elusive dark matter is and, in doing so, help solve one of the most perplexing problems in the Universe.