The Science of Discworld Revised Edition

by onboard nuclear generators, and they radiate a small amount of surplus heat into space. The pressure of that radiation might slow the craft down by the observed amount. The other possible explanation is that the Pioneers may be venting tiny quantities of fuel into space. Anderson thought about these explanations and found problems with them both.
    The strangest feature of the observed slowing down is that it is precisely what would be predicted by an unorthodox theory suggested in 1983 by Mordehai Milgrom. This theory changes not the law of gravity, but Newton’s law of motion: force equals mass times acceleration. Milgrom’s modification applies when the acceleration is very small, and it was introduced in order to explain another gravitational puzzle, the fact that galaxies do not rotate at the speeds predicted by either Newton or Einstein. This discrepancy is usually put down to the existence of ‘cold dark matter’ which exerts a gravitational pull but can’t be seen in telescopes. If galaxies have a halo of cold dark matter then they will rotate at a speed that is inconsistent with the matter in the visible portions. A lot of theorists dislike cold dark matter (because you can’t observe it directly – that’s what ‘cold dark’ means) and Milgrom’s theory has slowly gained in popularity. Further studies of the Pioneers may help decide.
    The other discovery is about the expansion of the universe. The universe is getting bigger, but it now seems that the very distant universe is expanding faster than it ought to. This startling result – confirmed by later, more detailed studies – comes from the Supernova Cosmology project headed by Saul Perlmutter and its arch-rival High-Z Supernova Search Team headed by Brian Schmidt. It shows up as a slight bend in a graph of how a distant supernova’s apparent brightness varies with its red shift. According to General Relativity, that graph ought to be straight, but it’s not. It behaves as if there is some repulsive component to gravity which only shows up at extremely long distances – say half the radius of the universe. A form of antigravity, in fact.
    Recent work seems to have confirmed this remarkable discovery. But – as always – ingenious scientists have come up with alternative explanations. In 2001 Csaba Csáki, John Terning, and Nemanja Kaloper put forward a totally different theory to explain the observations. They suggest that the light from distant supernovas is dimmer than expected because some of the particles of light – photons – are changing into something else. Specifically, they are changing into ‘axions’, hypothetical particles predicted by several of the currently fashionable quantum-mechanical theories of particle physics. Axions are not expected to interact much with other matter, which makes it hard to detect them; but if they have a very small but non-zero mass, about one sextillionth of that of an electron, then they will interact with intergalactic magnetic fields. This interaction would convert a small fraction of photons into axions, and that would account for the missing light. In fact, the most distant supernovas could lose one third of their photons this way.
    It is a sobering thought that such a tiny a modification of known physics, by introducing a particle whose mass ought to be negligible, could have such a big effect. At any rate, either gravity is not as we thought, or axions exist (as expected) and have mass (not as expected). Or there’s a third reason for the observations, which no one has yet thought of.
    One theory of the repulsive force is an exotic form of matter, ‘quintessence’. 2 This is a form of vacuum energy that pervades all of space, and exerts negative pressure. (As we write this, we can picture Ridcully’s expression. We shall have to ignore it. This isn’t something sensible, like magic. This is science. Empty space can be full of interest.) Curiously, Einstein originally included a repulsive force of this kind in his relativistic equations for gravity: he called it the cosmological constant. Later he changed his mind and threw the cosmological constant out, complaining that he’d been foolish to include it in the first place. He died thinking it was a blemish on his record, but maybe his original intuition was spot on after all.
    Unless axions exist and have mass, of course.
    In Einstein’s approach to the cosmological constant, quintessence is effectively spread uniformly