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The Standard Model is under attack!

The term ’Standard Model’ is confusing, because there is a standard model of particle physics and a standard model of cosmology. Between them, they seem to offer the nearest thing scientists have to a theory of everything.

The standard model of particle physics encompasses The Big Bang and everything that has happened since then to produce the Universe we know (or think we know?) today. If this seems odd - a leap from the micro to the macro world - we need to remember that the Universe and everything in it, including ourselves, is composed of tiny subatomic particles. In fact, as I have discovered from Professor Jim Al-Khalili’s book Quantum: a guide for the perplexed and discussed in the page that is growing from my reading of the book, this should perhaps read ’ composed of tiny subatomic particles and a vast amount of absolutely nothing’. So planets and their inhabitants (if any), the stars around which they orbit, the galaxies in which the stars are grouped - all consist of nothing but particles, and only a small subset of the particles that are known, or believed, to exist.

The standard model of cosmology is a bit more elusive on Wikipedia. The most relevant page is called Lambda-CDM model, and says ’ΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter. It is frequently referred to as the standard model of big bang cosmology, since it attempts to explain the existence and structure of the cosmic microwave background, the large scale structure of galaxy clusters and the distribution of hydrogen, helium, lithium, oxygen and also the accelerating expansion of the universe observed in the light from distant galaxies and supernovae. It is the simplest model that is in general agreement with observed phenomena.

Anyway, the standard models are not static: they are constantly evolving to adapt to new discoveries and theories. Unless I have misunderstood a recent Horizon programme, Is Everything We Know About The Universe Wrong?, however, the standard model of cosmology still depends on Newton’s laws of gravitation.

According to this programme, three ’dark entities’ (my phrase) have had to be invented (sorry - I should probably say ’postulated’) to account for relatively recent astronomical discoveries.

Galaxies don’t behave like our Solar System

Newton’s laws explain the observed behaviour of the planets, their moons and the asteroids in our Solar System perfectly (provided you accept Einstein?s assurance that the gravity of very massive bodies such as the sun bends light-waves). One of the most obvious points is that, for their orbits to be stable, the planets that are more remote from the sun must be orbiting more slowly than those that are closer. And they are. If they weren’t, they would not be in orbit - they would simply fly away into interstellar space under centrifugal force.

It seems quite reasonable to assume that the same rules apply to the stars that form galaxies. They are all quite clearly in orbit around the centres of their galaxies (which are now believed to be black holes - see my page on another recent Horizon programme), so it follows that those most remote from the centres should be moving much more slowly than those close to the centres.

Unfortunately this doesn’t seem to be true. Apparently, the stars in the outer reaches of galaxies are moving at the same speeds as those nearer the centres. So, according to strictly Newtonian principles, galaxies should be coming apart at the seams. But they are not.

Thankfully, nobody has suggested abandoning Newton’s laws. So there must be something else exercising a powerful gravitational attraction to hold the speeding outer stars in their orbits. But what? There is nothing else emitting light or any other frequency of electromagnetic radiation which we can detect. Whatever it is, the source of this extra gravity is, in astronomical terms, dark. As the only thing known to exercise gravitational attraction is matter, then - until we learn different - the mystery substance must be a new form of matter.

Dark matter.

I’m not clear why Horizon has presented this as ’hot news’, because it is not a new idea. In fact, according to Wikipedia, dark matter was postulated by Fritz Zwicky. Earlier I said ’relatively recent astronomical discoveries’, but Zwicky’s postulation happened in 1934! I also said that nobody had suggested abandoning dear old Newton - wrong again: a bit further down the Wikipedia page cited here, it says ’Though the theory of dark matter remains the most widely accepted theory to explain the anomalies in observed galactic rotation, some alternative theories such as modified Newtonian dynamics and tensor-vector-scalar gravity have been proposed.’ For the moment, though, Sir Isaac can sleep easy in his tomb: ’None of these alternatives, however, has garnered equally widespread support in the scientific community.

After 76 years, scientists are still searching for the first clear evidence of what dark matter actually is. It cannot be detected directly, let alone analysed. All we have are its apparent effects.

Is the expansion of the Universe slowing down?

Newtonian laws dictate that, if the expansion of the universe is a consequence of The Big Bang - if all the matter in the universe was accelerated to enormous velocity by a single cataclysmic explosion and has not had any other forces applied to it, apart from the gravitational attraction of its own constituents, in the ensuing 13.7 billion years - it would gradually slow down. The mutual gravitational attraction of all the stars, planets and other bits and pieces would ensure this. And until recently this seems to have been an essential part of the standard model of cosmology.

Not any more, it seems. Wikipedia tells us that dark energy ’is the most popular way to explain recent observations and experiments that the universe appears to be expanding at an accelerating rate.

The phrase ’recent observations’ suggests that this is relatively hot news: ’In 1998 observations of Type Ia supernovae suggested that the expansion of the universe is accelerating’ (Wikipedia again).

Going back to the first of the two pages I have cited, it appears that the theory of dark energy, postulated to explain this anomaly, has already been incorporated into the standard model of cosmology. Moreover, it is thought to account for 74% of the total mass-energy of the universe. It’s a bit scary to read that three-quarters of the entire universe consists of something we cannot observe directly, and which scientists only believe exists because it is the ’most popular’ way of explaining otherwise anomalous behaviour on the part of the entire cosmos!

(The term ’mass-energy’ refers to Einstein’s idea of mass-energy equivalence, expressed in the famous equation E=mc2. In all the cataclysmic reactions that have taken place and are taking place throughout the Universe, mass is constantly being converted into energy and vice-versa, but the total mass-energy of the Universe remains constant.)

So it looks as if Horizon’s second ’attack’ on the standard model is also fairly old news.

Three’s a crowd

Nevertheless, the observations that led to the postulation of dark matter and dark energy have still not been explained, and will not be explained until we can establish what these two dark entities actually are.

And now they have been joined by a third!

This time, it was anomalies in the motion of entire galactic clusters that caused a re-think, and this led to the postulation of dark flow. If your head hasn’t exploded by now, have a look at the dark flow entry in Wikipedia.

One thing is certain: the more scientists learn, the more stuff they find that they can’t (yet) understand or explain.

Personal site for Paul Marsden: frustrated writer; experimental cook and all-round foodie; amateur wine-importer; former copywriter and press-officer; former teacher, teacher-trainer, educational software developer and documenter; still a professional web-developer but mostly retired.

This site was transferred in June 2005 to the Sites4Doctors Site Management System, and has been developed and maintained there ever since.