Modern Cosmology sometimes appears to concentrate on two rather arcane subjects – Dark Matter and Dark Energy. So what are these weird things and why do we need to include them in our theories of how the cosmos works?
Dark Matter
The phrase Dark Matter refers to something we have no knowledge of (same as in Dark Ages) that acts in a matter-like way – ie adds to the overall mass of an object. The effects of Dark matter are most obvious in large scale galactic motions.
In 1933, Fritz Zwicky of Caltech made observations of the motions of galaxies in the Coma cluster. He realised that the orbits of the galaxies were unsustainable if the galaxies were of the then assumed masses. The galaxies were orbiting each other so fast the cluster should tear apart. In total, the amount of missing mass was equivalent to around 400 times the assumed mass of the cluster.
In 1959, Louise Volders investigated the rotation rates of stars around the centre of M33. It was expected from Kepler’s laws that the rotational velocity of stars in the galactic disk should drop as the square of the distance from the centre of mass (see dotted line on the graph below), however this was not so. The rotation rate of the outer stars stayed fairly constant until quite far out (see solid line on the graph below). In the 1970’s, Vera Rubin applied spectroscopy to measuring the Doppler shift of stars in galactic disks – another way of measuring how fast they were spinning around the centre. She confirmed the earlier works and lent more credence to the idea that there was some kind of missing mass in galaxies.
Galactic rotation rates against distance from bulge
Further investigations in the 1990’s showed that Dwarf Elliptical galaxies were pretty much dominated by dark matter. Globular clusters, which surround spiral galaxies in a rough sphere, didn’t seem to have too much, but they moved as if they were embedded in a spherical Dark Matter halo centred on the Galactic centre. So not only were the stars in the disk dragged round the Galaxy at the same time as the centre rotated, apparently totally separate clusters of stars outside the disk were spinning in the same way too.
New methods of observing brought new evidence of Dark Matter. Observations of how much galaxies bend space and time due to their mass – known as gravitational lensing because the bending of space and time affects how light passes through it, causing a lens effect – confirmed there was a significant amount of missing mass. It also provided an interesting observation of how Dark Matter and matter interact with each other. The image below shows the Bullet Cluster, two colliding clusters of galaxies. Gravitational lensing studies have mapped out the distribution of mass within the cluster, pink identifies where visible, normal baryonic matter is and blue identifies where missing mass appears to be (but can’t be seen).
Dark Matter in the Bullet Cluster
In fact, models of the formation of the Bullet cluster require Dark matter to be successful. The cluster is the end result of two initial clusters that collided. The two original clusters were composed of matter and Dark Matter. As they collided, the matter in one cluster interacted with the matter from the other – bouncing off, orbiting, all the rest that stars and gas can do. The Dark matter of each cluster, however, passed ghost like through the collision, failing to interact with the matter, failing to interact with the other clump of Dark matter too, except gravitationally. As a result, the matter is scrunched up in the middle and the Dark matter has passed through to either side.
So this gives us a bit of insight into what Dark matter is and what it isn’t. It isn’t in general normal baryonic matter like the stuff making up you, me and the computer I’m writing on. If it was, it would be scrunched up with the rest of the baryons in the pink. There may well be some baryonic dark matter – brown dwarfs, cold gas, planets and asteroids we cannot see – but not too much. The blue stuff in the picture above has ignored all electromagnetic or other interactions with the matter in pink, it is something quite different.
The existence of huge blobs of dark matter – galactic halos, even those blue blobs above, also mean Dark matter can’t be anything like neutrinos – ghost like particles we know about that interact very little with matter, but do get produced during nuclear fusion on the heart of stars in massive amounts. These particles do carry some of the missing mass of the universe, but they don’t hang around enough to keep it in a nice halo or any of the other blobs we see. However, neutrinos do point out something to us – there is no reason why a particle in the universe has to easily interact with other particles in the universe. Neutrinos will happily pass through the entire body of the planet without noticing a thing, it is only because there are so many of them and particle decay theories told us where to look that we are able to detect any (a very few) at all.
As a result, our search for the light in Dark matter seems to be honing in on something called a WIMP – a Weakly Interacting Massive Particle – something that has some level of mass and will act gravitationally, but which ignores other particles in all other situations. But it does hang around a bit.
Alternatives to Dark Matter?
The two main alternatives to Dark Matter come from a rewriting of Newton’s Laws of gravitation and quantum physics. The first of those two concentrates on rewriting the laws of gravitation so they are compatible with the rotation curves of galaxies. However as this MOdified Newtonian Dynamics, or MOND, theory must also be compatible with gravitational lensing and explain all the various observations that have agreed with relativity since Sir Arthur Eddington first observed gravitational lensing in images taken in the 1919 eclipse at Principe, the theory seems stalled.
The second hypothesis postulates the existence of extra dimensions beyond the four evident around us. I state hypothesis as despite all the work that goes into this line of reasoning, experimental observations haven’t yet been available to provide the evidence required to designate M-Theory or SuperString-Theory a theory. The basic idea here is that in other dimensions there are other galaxies in existence that can interact gravitationally with our matter through one or more of these dimensions. This is supposedly why gravity follows the inverse square law rather than having a cubic relation to distance like electromagnetism – some part of gravity is feeding elsewhere, making it a very weak force. There are a multitude of problems with dealing with such a tenuous framework and as well as this the Bullet Cluster observations would require at least three separate occurrences to be gravitationally bound – the two clusters in our own point of this other dimension and two other clusters, which must be at two other points in order not to have collided themselves.
At the moment, strange though it may seem, the evidence points towards Dark Matter as an integral part of our Universe.
Dark Energy
As with Dark Matter, which is a thing we know nothing about that acts like matter, Dark energy is something we know nothing about that acts as an energy. The energy that drives the ever increasing expansion of the Universe.
In 1915, Albert Einstein published the theory of General Relativity, bringing the ideas of Special Relativity and introducing them to acceleration rather than just concentrating on constant motion. This included bringing about ideas on the nature of gravity and producing and solving the gravitational field equation. The initial solutions Einstein got to his equations shocked and dismayed him – they suggested that the Universe was expanding. It was very much accepted at the time that the Universe wasn’t, so Einstein popped in a cosmological constant, which would oppose the expansion of the Universe and create a static universe. Observations by Edwin Hubble soon convinced Einstein to dump the cosmological constant and accept that the expansion of the Universe followed from the gravitational field theories.
Many decades later and the Big Bang theory had acquired a new facet – inflation, the idea that soon after the formation of the Universe it expanded rapidly before slowing down to more manageable levels. Alan Guth suggested in the 1970’s that a negative pressure field could do something like this. In essence the field would slowly drive apart the early universe, which was gravitationally bound to every other part, until a point was reached when the matter was at a low enough density for the expansion to no longer be inhibited by this gravity. There was a sudden rapid expansion, which then slowed down as the pent up energy dissipated.
In 1998, Michael Turner brought the term Dark Energy into being as observations of Type 1a supernovae seemed to indicate that the expansion of the universe was increasing. Like the field Guth suggested, this would require negative energy, unlike Guth’s suggestion, this one was speeding up.
There are two suggestions for what Dark Energy is – either a form of negative Cosmological Constant (ie it increases rather than opposes expansion of the Universe) or something called Quintessence, which is a cosmological constant that changes with time. Either way, the suggestion is that space itself has a form of energy with negative pressure that increases and increasingly increases the size of the Universe. Determining the exact nature of either of these two has proven difficult as although both can be derived from quantum physics, they are massively out of proportion to what is observed.
Alternatives to Dark Energy?
As with Dark Matter, the two main bodies of alternate thought to Dark Energy are altering the gravitational equations themselves or turning to string theory for answers. The first tends to either fail or lead inevitably to the same equations available in quintessence, the second is considered to be nothing more than wild speculation.
The current idea
The present cosmological model is known as Lambda-CDM. Lambda is the Greek letter that represents the Cosmological Constant and CDM stands for Cold Dark Matter, which are the chosen favourates from the various forms of the two Darks, though some models also use quintessence.
It is believed that there are three possible fates for the Universe – one that the expansion of the Universe will be stopped by gravity and everything returned to one point – the Big Crunch. One that gravity will fail to hold anything in and the galaxies will part company, vanishing off alone into the cold void at an ever expanding rate – the Big Rip. One that the Universe has just the right balance that the expansion will be slowed but will never stop – the flat universe. This latter version of events is believed to be the likely fate of our Universe as determined through various observations. However, this also gives us an idea of the Universe’s total mass density – if it has to be similar to the critical mass density, then we can assume it is about that. We know roughly the density of baryonic matter, as we can see and measure it. We have an idea of the rough density of Dark Matter as we can measure its effects too. Taking these off the critical density leaves almost three quarters to be filled by a different component – Dark Energy. The normal matter around us doesn’t make up most of the Universe (see pie chart below), the Dark Matter we infer doesn’t even have much of a stab. It is Dark Energy’s realm.
It seems the Darkest horse wins the race to be the largest component of our Universe.
Sources of Mass Density in our Universe
Where the Sun hits the sky 