So our current overall picture of the Universe is as follows: only around 5 per cent of the energy in the Universe is in the form of normal matter; about 25 per cent is thought to be in the form of the gravitationally attractive dark matter; and the remaining 70 per cent is thought to be in the form of the gravitationally repulsive dark energy. These proportions, give or take a few percentage points here and there, seem suicient to explain all astronomical observations that have been made to date. The total of all three of these types of energy, added together, also seems to be just the right amount to make space flat (rather than positively or negatively curved, like a sphere or a saddle, as illustrated in Figure 10). The flat Universe, filled with mostly dark energy and dark matter, is usually referred to as the Concordance Model of the Universe. Among astronomers, it is now the consensus view that this is the model of the Universe that best fits their data.

The Concordance Model, and all of the observations that have led to it, is undoubtedly a great achievement of 21st-century physics. However, it is certainly not the end of the story. Not with regard to our understanding of the history of the Universe, nor with regards to the way we understand its contents, or the gravitational fields within it. To be blunt, the Concordance Model has a number of shortcomings. First, it appears to have started of in a particularly special configuration. For space to be so close to flat, and to have the background radiation and the distribution of galaxies look so evenly spread, the early Universe needs to have been extremely close to perfectly uniform in density. Second, some of the ripples we see in the CMB appear to be larger than the distance light could have travelled since the Big Bang. Nothing should travel faster than light, in Einstein’s theory, so this is genuinely puzzling. Third, we have no idea what dark matter really is. We only know that it should gravitate, and that it should not interact with light. It doesn’t show up in the Standard Model of particle physics, which has a place for every other known type of particle, and it hasn’t yet been seen in any particle physics experiment. Fourth, the existence of dark energy, and its repulsive gravitational field, seems to require enormous fine-tuning in order to have the efect that we see today. A bit more of it and galaxies would never have formed. A bit less and we wouldn’t have ever noticed it at all.

These four problems are the focus of much attention for physicists. The first two are thought to be solved by a period of very rapid expansion in the early Universe, called cosmic in’ation. I’ll describe cosmic in’ation in Chapter 6. It is hoped that the third will be solved by extending the Standard Model of particle physics, and there are a number of proposals on how this could be done. At the time of writing, it is thought that the properties of dark matter particles can be investigated directly by using the Large Hadron Collider (LHC) in Geneva. Whether or not nature will have been kind enough to allow them to fall into the range of energy levels that the LHC can probe remains to be seen. But the last of these problems is probably the most mystifying of all. The lengths that some physicists have gone to try to explain dark energy are truly extraordinary. Again, I’ll describe this further in Chapter 6.

Of course, there remain a few physicists that are sceptical that dark energy and dark matter exist at all. They maintain that we need to understand in more detail how gravity works on the scale of the Universe before we can be sure that they are really there. After all, it’s only through their gravitational interaction that we know about these substances at all. If we’ve misunderstood gravity, we may therefore have misidentified them. Future astronomical observations will be used to investigate this possibility and to further explore the properties of dark matter and dark energy.

The future of cosmology

It often turns out to be folly to try to predict the future in science, but it seems reasonably clear that the 21st century will see significant advances in cosmology. We now know a lot about the way the Universe is expanding and the way that structures in the Universe have formed, but our current knowledge will be dwarfed by observations that will happen over the next couple of decades. Much of the motivation for this work comes from dark matter and dark energy. The search for these dark materials will shed further light on gravity.

Let’s start with the CMB. To date, most observations of this radiation have focused on measuring its temperature in diferent directions on the sky, and trying to infer what the ripples in the early Universe must have looked like. The cutting edge in this work has so far been the Planck satellite. This mission was so successful, however, that it is almost impossible to do any better with future space-based missions. What can be done, however, is build bigger telescopes on the surface of the Earth. This is currently being done in the Atacama Desert in Chile, and at the South Pole. These are two of the lowest-humidity locations on the planet, and the thin dry air makes them ideal places for looking into space. These telescopes will map the CMB to very high resolution, and will supply a wealth of information about the structures that exist in the Universe.

As well as temperature, there are other things that can be observed in the CMB data. Astronomers can also measure its polarization (the orientation of a set of electromagnetic waves, as illustrated in Figure 11). The polarization of the background radiation carries additional information about what happened in the early Universe, and by looking for particular patterns astronomers can infer what the gravitational field looked like very early on in the Universe’s history.

Some of this information duplicates what can be deduced from the temperature, but some of it is entirely new. In particular, by looking for a characteristic curl pattern in the polarization, it is possible to deduce whether there were gravitational waves travelling around the early Universe. You will recall, from Chapter 4, that considerable efort has been expended to directly detect the gravitational waves that travel through the Earth. The polarization of the CMB provides the scope for similar experiments in an entirely diferent environment.

11. An illustration of (a) polarized light; and (b) unpolarized light. The orientation of the waves in the unpolarized light is random, while the waves in the polarized case are aligned. Arrows denote the direction in which the light is travelling.

In March 2014, scientists working on the BICEP2 experiment at the South Pole announced that they had used this method to discover gravitational waves in the early Universe. At the time of writing, it appears that the excitement generated by this announcement was premature. While the scientists had seen the curl pattern in the polarization of the CMB, it now seems that this was generated by sources closer to us and not from gravitational waves at all. This doesn’t mean that gravitational waves aren’t out there, in the early Universe. Future experiments will measure the polarization of the background radiation to even higher accuracy, and in more frequency bands. If gravitational waves were present in the early Universe, to any considerable level, then it’s likely we’ll know about them in the next decade or so. The successor to BICEP2 has already been built, and will give its first scientific results soon.

Also of great promise are the next generation of galaxy survey missions. We discussed the 2dF and SDSS surveys earlier, which were very ambitious attempts to record the positions of the galaxies in the Universe we see around us. Future surveys will be much, much bigger. Three of the grandest of these will be the Large Synoptic Survey Telescope (LSST), which is already under construction in Chile; the Square Kilometre Array (SKA), which will begin construction in 2018; and the European Space Agency’s Euclid satellite, which is expected to launch around 2020. These missions will measure billions of astronomical sources, which will be used to construct maps of the Universe on unprecedented scales.

The LSST, the SKA, and the Euclid satellite will turn cosmology into a precision science. When they are in operation, we will know much more about the nature of the dark matter and dark energy: through the efects they have on the structure in the Universe; from the way in which light travels to us from those structures; and from the way that they evolve in time. In fact, it is expected that this information will be so precise that, for the first time, we will be able to start performing tests of gravity that will rival those we can already perform in the Solar System and in binary pulsar systems. This will open up a whole new window on gravity, and will allow us to test it in new ways and on new distance scales.