Before we start i’d just like to apologise for our low activity on Rationalising the Universe as of late – given the Christmas period we’ve both been rather busy with family festivities but we’re about to jump back on the science wagon with full force. I hope everybody had a very merry Christmas!
Today we venture to the dark side in order to encounter the two most mysterious and shady characters in our universe: dark matter and dark energy.
Dark matter was first identified as the “missing mass” in galaxies and clusters required to explain their kinematic properties. The reason it is called ‘dark’ is because the matter does not emit or interact at all with electromagnetic radiation (such as light) it is therefore invisible to the entire electromagnetic spectrum. We cannot see it in visible, we cannot see it in infrared, we cannot see it period. Observations of the cosmic microwave background radiation and the large-scale structure of the universe suggest that approximately as much as 85% of the matter in the universe is dark! This makes its nature one of the biggest questions in cosmology and physics today.
The first person thought to concretely theorise the existence of dark matter was astro-physicist Fritz Zwicky, of Caltech in 1933. Though the less famous and first to actually realise there was invisible matter in galaxies was Vera Rubin of Princeton, a maverick female astronomer of her day. Rubin worked on spiral galaxies and noticed that the stars at the edges of the galaxies spun at the same speed at those in the center. This analysis of star velocities in spiral galaxies is called looking at the ‘rotation curve’ of a galaxy. Rubin realised these stars at the edge were travelling far too fast as a result of purely the observable matter and there must be something else tugging on them that we cannot see. Zwicky then went on to estimate the total mass of the Coma Cluster of Galaxies and he then compared that estimate to one based on the number of galaxies in the cluster that could be observed. He found there was about 400 times more estimated mass than was observable. Zwicky inferred that there must be some non-visible form of matter, dark matter, which would provide enough mass, and therefore gravity, to hold the cluster together – just as Rubin had previously done. Vera Rubin died today at the age of 88, and though her contribution was overlooked in her day (many believed she should have been awarded a nobel prize and was not justly treated because she was a woman!) we shall acknowledge her contribution now and include a wonderful and selfless quote by the most excellent lady ‘Fame is fleeting. My numbers mean more to me than my name. If astronomers are still using my data years from now, that’s my greatest compliment.”
On with the explanation. Dark matter is particularly elusive as it does not emit, absorb or reflect light, but makes itself apparent only through gravitational attraction. To locate this mysterious matter a technique called gravitational lensing is used. Gravitational lensing is a phenomenon discovered by Einstein whereby light bends in the presence of matter due to its gravitational warping effect on the surrounding spacetime. The degree of bending of light rays from distant galaxies as they pass through the dark matter’s gravitational field reveals the mass of the dark matter. The greater the bend the greater the mass. By carefully plotting the way that the distant images are distorted, it is possible to quite accurately map the dark matter and estimate its mass.
Dark matter still interacts gravitationally with visible baryonic matter but there is a question as to whether it itself is baryonic. Baryonic matter is the everyday stuff comprised of protons and neutrons – the familiar constituents of matter. Together with electrons, proton and neutrons make up all the different atoms which constitute the building blocks of all objects – you, me, the desk, the table, your cat. One of the primary theories for dark matter was that it is comprised of massive baryonic objects which do not emit light – example candidates were black holes (they do not emit light because light cannot escape) or neutron stars and cool white dwarf stars (they do not emit light because they are too cold to give off any radiation). Together these objects are collectively known as ‘massive compact halo objects’ or MACHOs for short. There were a number of searches along these lines but MACHOs failed to be detected in significant enough numbers to be the responsible culprit. Therefore, a small fraction of the dark matter in the universe may be baryonic but we would have to look elsewhere to explain the majority of the unaccounted for mass in galaxies and clusters.
We must therefore look to more exotic forms of matter for the answer, away from the everyday world of the protons and neutrons: non-baryonic matter. Non-baryonic matter can be broadly classified as either hot or cold. A candidate for dark matter that is hot and non-baryonic has already been identified -the neutrino. Neutrinos come in three known flavours, electron, muon and tau and they interact with baryonic matter only though the gravitational and weak force – making them perfect dark matter candidates. But arguments and observations from cosmology have again ruled them out as the main culprit accounting for the majority of the dark matter mass in our universe.
Therefore that leaves one other type – cold dark matter. Cold dark matter candidates are known as WIMPs – weakly interacting massive particles. Seeing as these particles would only interact via gravity and the weak force they are of course very difficult to detect, however a large number of experiments are on track in the hope to be able to provide a first direct detection. So sadly we’re still at quite a loss when it comes to understanding what the culprit for dark matter is. We are however a lot better off on our quest to understand dark matter than we are to understanding the even more mysterious character – dark energy.
Roughly 68% of the universe is thought to be dark energy making it whoppingly the largest constituent out there. Dark energy is characterised in terms of its energy density and pressure, both negative. Its existence is thought to be cause for the accelerated expansion of the universe. We know how much is out there because we know how it effects the universe’s expansion but apart from that it is shrouded in mystery. There are few strands of thought at which we can grasp, the first is that dark energy is a property of space. As we’ve learn from the post on fields, empty space is not ‘nothing’, space is the gravitational field and can possess energy. If dark energy is a property of space itself and is the cause of expansion, then as the universe expands, more space would come into existence, more energy would appear and as a result this energy would continually cause the universe to expand faster and faster. In this case the dark energy also has a pseudonym – the cosmological constant, a constant energy density filling the space homogeneously which Einstein wrote into his equations for general relativity. (Although the energy increases as space expands, so of course does space, keeping the energy density (energy per unit volume) constant.
Instead, perhaps dark energy is new kind of ‘fluid’ or field that fills all of space but whose effect on the universe is opposite than that of normal energy and matter. Instead of being attractive due to gravity, this fluid or field is repulsive, causing the outward expansion of the universe. This new kind of fluid or field has been given the name ‘quintessence’ a greek name for a fifth element or fundamental force. Fields seem to be at the heart of everything.
So there we have an overview of our current knowledge regarding Dark Matter and Dark Energy which overwhelmingly make up the majority of our universe… not so enlightening I agree, I am sorry to disappoint you. Much ground is yet to be made in this area of physics and we hold hope for future advancements, for now however we largely remain in the dark. (excuse the all too easy puns)