In Pursuit of the Unknown (46 page)

Scientific knowledge, however, is always provisional. New discoveries can change it. The Big Bang has been the accepted cosmological paradigm for the last 30 years, but it is beginning to show some cracks. Several discoveries either cast serious doubt on the theory, or require new physical particles and forces that have been inferred but not observed. There are three main sources of difficulty. I'll summarise them first, and then discuss each in more detail. The first is galactic rotation curves, which suggest that most of the matter in the universe is missing. The current proposal is that this is a sign of a new kind of matter, dark matter, which constitutes about 90% of the matter in the universe, and is different from any matter yet observed directly on Earth. The second is an acceleration in the expansion of the universe, which requires a new force, dark energy, of unknown origin but modelled using Einstein's cosmological constant. The third is a collection of theoretical issues related to the popular theory of inflation, which explains why the observable universe is so uniform. The theory fits observations, but its internal logic is looking shaky.

Dark matter first. In 1938 the Doppler effect was used to measure the speeds of galaxies in clusters, and the results were inconsistent with Newtonian gravitation. Because galaxies are separated by large distances, space-time is almost flat and Newtonian gravity is a good model. Fritz Zwicky suggested that there must be some unobserved matter to account for the discrepancy, and it was named dark matter because it could not be seen in photographs. In 1959, using the Doppler effect to measure the speed of rotation of stars in the galaxy M33, Louise Volders discovered that the observed rotation curve – a plot of speed against distance from the centre – was also inconsistent with Newtonian gravitation, which again is a good model. Instead of the speed falling off at greater distances, it remained almost constant,
Figure 52
. The same problem arises for many other galaxies.

If it exists, dark matter must be different from ordinary ‘baryonic' matter, the particles observed in experiments on Earth. Its existence is accepted by most cosmologists, who argue that dark matter explains several different anomalies in observations, not just rotation curves. Candidate particles have been suggested, such as WIMPs (weakly interacting massive particles), but so far these particles have not been detected in experiments. The distribution of dark matter around galaxies has been plotted by assuming dark matter exists and working out where it has to be to make the rotation curves flat. It generally seems to form two globes of galactic proportions, one above the plane of the galaxy and the other below it, like a giant dumb-bell. This is a bit like predicting the existence of Neptune from discrepancies in the orbit of Uranus, but such predictions require confirmation: Neptune had to be found.

Fig 52
Galactic rotation curves for M33: theory and observations.

Dark energy is similarly proposed to explain the results of the 1998 High-z Supernova Search Team, which expected to find evidence that the expansion of the universe is slowing down as the initial impulse from the Big Bang dies away. Instead, the observations indicated that the expansion of the universe is speeding up, a finding confirmed by the Supernova Cosmology Project in 1999. It is as though some antigravity force pervades space, pushing galaxies apart at an ever-increasing rate. This force is not any of the four basic forces of physics: gravity, electromagnetism, strong nuclear force, weak nuclear force. It was named dark energy. Again, its existence seemed to solve some other cosmological problems.

Inflation was proposed by the American physicist Alan Guth in 1980 to explain why the universe is extremely uniform in its physical properties on very large scales. Theory showed that the Big Bang ought to have produced a universe that was far more curved. Guth suggested that an ‘inflaton field' (that's right, no second i: it's thought to be a scalar quantum field corresponding to a hypothetical particle, the inflaton) caused the early universe to expand with extreme rapidity. Between 10
^−36
and 10
^−32
seconds after the Big Bang, the volume of the universe grew by a mindboggling factor of 10
⁷⁸
. The inflaton field has not been observed (this would require unfeasibly high energies) but inflation explains so many features of the universe, and fits observations so closely, that most cosmologists are convinced it happened.

It's not surprising that dark matter, dark energy, and inflation were popular among cosmologists, because they let them continue to use their favourite physical models, and the results agreed with observations. But things are starting to fall apart.

The distributions of dark matter don't provide a satisfactory explanation of rotation curves. Enormous amounts of dark matter are needed to keep the rotation curve flat out to the large distances observed. The dark matter has to have unrealistically large angular momentum, which is inconsistent with the usual theories of galaxy formation. The same rather special initial distribution of dark matter is required in every galaxy, which seems unlikely. The dumb-bell shape is unstable because it places the additional mass on the outside of the galaxy.

Dark energy fares better, and it is thought to be some kind of quantum-mechanical vacuum energy, arising from fluctuations in the vacuum. However, current calculations of the size of the vacuum energy are too big by a factor of 10
¹²²
, which is bad news even by the standards of cosmology.
⁶

The main problems affecting inflation are not observations – it fits those amazingly well – but its logical foundations. Most inflationary scenarios would lead to a universe that differs considerably from ours; what counts is the initial conditions at the time of the Big Bang. In order to match observations, inflation requires the early state of the universe to be very special. However, there are also very special initial conditions that produce a universe just like ours without invoking inflation. Although both sets of conditions are extremely rare, calculations performed by Roger Penrose
⁷
show that the initial conditions that do not require inflation outnumber those that produce inflation by a factor of one googolplex – ten to the power ten to the power 100. So explaining the current state of the universe without inflation would be much more convincing than explaining it with inflation.

Penrose's calculation relies on thermodynamics, which might not be an appropriate model, but an alternative approach, carried out by Gary Gibbons and Neil Turok, leads to the same conclusion. This is to ‘unwind' the universe back to its initial state. It turns out that almost all of the potential initial states do not involve a period of inflation, and those that do require it are an exceedingly small proportion. But the biggest problem of all is that when inflation is wedded to quantum mechanics, it predicts that quantum fluctuations will occasionally trigger inflation in a small region of an apparently settled universe. Although such fluctuations are rare, inflation is so rapid and so gigantic that the net result is tiny islands of
normal space-time surrounded by ever-growing regions of runaway inflation. In those regions, the fundamental constants of physics can be different from their values in our universe. In effect, anything is possible. Can a theory that predicts
anything
be testable scientifically?

There are alternatives, and it is starting to look as though they need to be taken seriously. Dark matter might not be another Neptune, but another Vulcan – an attempt to explain a gravitational anomaly by invoking new matter, when what really needs changing is the law of gravitation.

The main well-developed proposal is MOND, modified Newtonian dynamics, proposed by Israeli physicist Mordehai Milgrom in 1983. This modifies not the law of gravity, in fact, but Newton's second law of motion. It assumes that acceleration is not proportional to force when the acceleration is very small. There is a tendency among cosmologists to assume that the only viable alternative theories are dark matter or MOND – so if MOND disagrees with observations, that leaves only dark matter. However, there are many potential ways to modify the law of gravity, and we are unlikely to find the right one straight away. The demise of MOND has been proclaimed several times, but on further investigation no decisive flaw has yet been found. The main problem with MOND, to my mind, is that it puts into its equations what it hopes to get out; it's like Einstein modifying Newton's law to change the formula near a large mass. Instead, he found a radically new way to think of gravity, the curvature of space-time.

Even if we retain general relativity and its Newtonian approximation, there may be no need for dark energy. In 2009, using the mathematics of shock waves, American mathematicians Joel Smoller and Blake Temple showed that there are solutions of Einstein's field equations in which the metric expands at an accelerating rate.
⁸
These solutions show that small changes to the Standard Model could account for the observed acceleration of galaxies without invoking dark energy.

General relativity models of the universe assume that it forms a manifold; that is, on very large scales the structure smoothes out. However, the observed distribution of matter in the universe is clumpy on very big scales, such as the Sloan Great Wall, a filament composed of galaxies 1.37 billion light years long,
Figure 53
. Cosmologists believe that on even larger scales the smoothness will become apparent – but to date, every time the range of observations has been extended, the clumpiness has persisted.

Fig 53
The clumpiness of the universe.

Robert MacKay and Colin Rourke, two British mathematicians, have argued that a clumpy universe in which there are many local sources of large curvature could explain all of the cosmological puzzles.
⁹
Such a structure is closer to what is observed than some large-scale smoothing, and is consistent with the general principle that the universe ought to be much the same everywhere. In such a universe there need be no Big Bang; in fact, the whole thing could be in a steady state, and be far, far older than the current figure of 13.8 billion years. Individual galaxies would go through a life cycle, surviving relatively unchanged for around 10
¹⁶
years. They would have a very massive central black hole. Galactic rotation curves would be flat because of inertial drag, a consequence of general relativity in which a rotating massive body drags space-time round with it in its vicinity. The red shift observed in quasars would be caused by a large gravitational field, not by the Doppler effect, and would not be indicative of an expanding universe – this theory has long been advanced by American astronomer Halton Arp, and never satisfactorily disproved. The alternative model even indicates a temperature of 5°K for the cosmological microwave background, the main evidence (aside from red shift interpreted as expansion) for the Big Bang.

MacKay and Rourke say that their proposal ‘overturns virtually every tenet of current cosmology. It does not, however, contradict any observational evidence.' It may well be wrong, but the fascinating point is that you can retain Einstein's field equations unchanged, dispense with dark matter, dark energy, and inflation, and
still
get behaviour reasonably like all of those puzzling observations. So whatever the theory's fate, it suggests that cosmologists should consider more imaginative mathematical models before resorting to new and otherwise unsupported
physics. Dark matter, dark energy, inflation, each requiring radically new physics that no one has observed. . . In science, even one
deus ex machina
raises eyebrows. Three would be considered intolerable in anything other than cosmology. To be fair, it's difficult to experiment on the entire universe, so speculatively fitting theories to observations is about all that can be done. But imagine what would happen if a biologist explained life by some unobservable ‘life field', let alone suggesting that a new kind of ‘vital matter' and a new kind of ‘vital energy' were also necessary – while providing no evidence that any of them existed.