Science Society Lecture: Dark Matter

Professor Nick Evans is a professor of theoretical physics at Southampton University and researches into forces within the nuclei of atoms and quantum chromodynamics.  He has also written a novel The Newtonian Legacy available from the Southampton university website in which a particle physicist becomes detective.  A  "Sexy novel set to put the spark back in Physics" said the Times Higher Educational Supplement.  His lecture was about dark energy which is the core problem at the centre of Physics at the moment.

He started the evening with the start of everything, the Big Bang, when the universe began with an enormous explosion. We have measured the speed of galaxies and their distance from us which shows that everything is flying apart. Running time backwards we discover that all of matter started as a tiny volume.  Imagine compressing the entire universe into the space of a golf ball. There are no atoms just the constituent parts of atoms crushed together. It exploded outwards 14 billion years ago and is still expanding. The remnant of the heat of that explosion is also left as background radiation which is detected at almost identical levels throughout the universe.  But what is the fate of the universe? Things are flying apart but gravity is pulling it all together. The gravity gets weaker with distance but will the pull result in stopping the expansion and then the collapse of the universe? The answer to that comes from knowing how much mass there is in the universe, but that turns out to be complicated. 

We know about 'normal stuff' which is matter as we know it, made of atoms with a structure of a nucleus and orbiting electrons.  Matter like this is also responsible for light, electricity, magnetism, chemistry etc. But there is other stuff called 'dark matter'.  It has no charge and doesn't interact with other charges or with nuclear forces. It is therefore difficult to detect. We know of one type of dark matter, the neutrino, produced in beta decay as a product of a neutron turning into a proton plus to an electron. It has no charge, has no nuclear force and hardly interacts with normal matter at all. Billions of neutrinos pass through us every second, released by the sun. There could be other similar particles that make up dark energy but we do not know what they are.  Looking for dark matter is very hard because it does not interact with light, so we have to detect its mass by gravitational forces. When we analyse the rotation of galaxies we discover much more mass than can be seen and than is accounted for by stars, planets or dust. The bending of light by gravitational lensing also reveals more hidden mass. The mass discovered was however not enough to stop the universe from expanding.

Then there is dark energy, which was proposed mathematically by Einstein with his cosmological constant and is causing the universe to expand ever faster rather than slow down. Dark energy is a very difficult concept to grasp. Imagine a flexible fish tank in a fish farm floating in a Scottish loch. The fish are like normal matter and the water like the dark matter. Make the tank bigger, then the fish spread out and the density decreases. But you have more water of the same density filling the tank. The expanding universe has a growing amount dark matter of constant density while the density of normal matter is decreasing as it is spread out through a greater volume. Dark energy has mass (all energy has mass, as Einstein demonstrated) and so it also adds to the mass of the Universe calculation. The search for dark energy begins by looking at the rate of expansion of the universe and measuring its acceleration. We now also look for dark energy by studying ripples in the background radiation showing up as tiny temperature variations. These variations are due to tiny variations in the density of the very early universe. These higher density regions create a red or blue doppler shift in the background radiation which is what we detect. These higher density regions evolved to be the galaxy clusters that are the largest structures in the Universe.

We can use measurements now to work out how much of the universe is made up of normal matter and of other stuff. It turns out that the universe is made up of more dark stuff than normal matter. The proportions are 73% dark energy, 23% dark matter, 4.2% normal matter. So over 95% of the universe is made of stuff that we cannot easily detect. Worse than that, we not only don’t know what makes up dark matter, we have even less of an idea as to what dark energy actually is.

What do particle physicists know about dark energy? Einstein's work showed us that the mass of a particle is a reflection of the energy in a part of space. Accelerators like the Large Hadron Collider at CERN collide particles with a huge amount of energy so that they produce new matter. Particle physicists have built up the energy of the collisions making more massive particles. So what has been found? Copies of electrons were found called muons, which are over 200 times heavier. We found heavier copies of the particles that make up our everyday matter, heavier quarks. Having made a heavier particle it breaks down to produce the matter we are familiar with very quickly so they are not a part of our everyday lives and they have no purpose of which we are aware.

The puzzle was how do particles that interact identically with each other have different masses?  Fundamentally, what is mass?  Peter Higgs considered a space filling material which interacts with matter to cause mass. This is the Higgs field. An analogy is someone trying to move through a crowd of people that gather round them impeding their progress. As matter goes through the Higgs field it is impeded and this is what we detect as mass, making it harder to accelerate a greater mass (something Isaac Newton knew a long time ago).  Discovering whether Higgs’ theory was true meant putting a huge amount of energy into a small space to disrupt the Higgs field, which is done at the Large Hadron Collider at CERN. The collisions between nuclei at near the speed of light have enough energy to produce 14000 protons. The particles collide (billions of them in a bunch) producing 600 million collisions per second. The right products are produced very rarely so there have to be many collisions. The data produced is so vast that only unusual collisions can be studied. The Higgs particle was detected because particles at the predicted energy were produced in sufficient numbers over years of testing to support the theory.

So a conclusion is that the Higgs field is dark energy. Even the vacuum of space is full of Higgs particles generating particle masses and it's energy density is 100 times that of normal matter. Unfortunately observed dark energy is 10 to the power of 44 times nuclear energy density and so the two figures do not come close. In fact, it is probably the greatest mismatch in the history of science. The problem may be our understanding of gravity. We have no data for what happens when objects are very close together because electrical forces dominate at close range and gravity is swamped. So perhaps gravity theory is wrong at a small scale and hence our particle physics may be wrong.  

Clearly there is much work to be done and Prof Evans finished with some of the more outlandish possible solutions to the problem but exhorted the big crowd of students listening to take the work further and pursue this most fundamental and puzzling aspect of modern science.

Our next Science Society Lecture will take place on Friday 12 February at 7.30pm where Dr David Martill will discuss 'Tetrapodophis: The Four Legged Snake'