WHEN SITTING ALONE at night, my thoughts often turn to dark topics. By this, I mean dark matter and dark energy. In the 30 years since I completed my PhD in physics, the world has changed a great deal, for better and for worse. On the negative front, I never would have imagined we would still be battling ignorant religious fundamentalism within my own country.
On the plus side, we have obtained more information about the cosmos in my lifetime than I would have believed possible. Our picture of reality has dramatically altered from what it was when I was a graduate student. In some cases, things we were virtually certain of turned out to be wrong; in others, the boldest and wildest extrapolations have sometimes turned out to be right on the money. In all cases, the universe has surprised us.
One of the things we don’t emphasise enough when writing about science is that most theoretical proposals about nature, made in advance of experiment, are wrong. Nature has a way of surprising us, so it’s vitally important to realise that science cannot proceed by pure thought and logic alone. Without the guidance of experiment, scientists are like those who choose to immerse themselves in sensory deprivation tanks. They inevitably tend to veer off into hallucination. So it is all the more telling when new unexpected clues about nature come from observations, or when some of our creative imaginings turn out to reflect the way nature really works.
ONE OF THE EXOTIC surprises physics has thrown at us in the past 30 years is the concept of dark matter. Dark matter was first inferred by Swiss astronomer Fritz Zwicky in 1933 when trying to figure out what stopped galaxies in massive clusters from flying apart. When I was a student, in the 1970s, the case for dark matter was suggestive, but not compelling. As a particle physicist, I became fascinated by the idea that particles could naturally be produced in the early universe with the necessary abundance and characteristics of dark matter today. But the game was wide open.
Neutrinos, which after all are known to exist, still seemed like a good bet to be dark matter particles, but it was still possible that our estimates about the amount of dark matter were wrong, and maybe it could be made up of boring things that simply don’t shine as brightly as stars or hot gas, such as planets.
Now, there is compelling evidence that cold dark matter is real; from direct measures of mass in galaxies and clusters, calculations of the abundance of light elements produced in the Big Bang, measurements of large-scale structure coming from millions of galaxies, and remarkable observations of the primordial seeds of structure observed in the cosmic microwave background (CMB) radiation from the Big Bang.
Observations of colliding galaxies suggest very convincingly not only that dark matter exists in profusion; but also that it cannot be made of normal matter. Moreover, candidates for dark matter may be on the verge of detection by direct detectors of the sort that I and others proposed more than 25 years ago, or perhaps may be produced in the coming years at the Large Hadron Collider.
Another satisfying, if initially perplexing, discovery that took me by surprise is that the universe is flat (rather than curved, or ‘open’. This rather complex idea essentially says we can follow a triangular path from point a to b to c and back to a, with the angles adding, as would be expected, to 180 degrees). I remember when I was a young professor at Yale University in Connecticut, one of my senior colleagues, an observational astronomer, told me of a theorem he was pretty certain was true: the universe would conspire to ensure that we couldn’t accurately measure any fundamental cosmological parameter.
After decades of false claims and false starts he had reason to be jaded. All of that has changed. Twenty-five years ago, observers were certain that there was not enough matter in the universe to produce a geometrically flat 3-D space – space must be ‘open’ or negatively curved. Theorists, on the other hand, were certain the universe was flat, because mathematically that was the only possibility that made sense. I wouldn’t have dreamed we would directly measure the geometry of the universe in my lifetime. But, once again thanks to new observations of the CMB we have done so, to an accuracy of 1%.
We theorists cannot pat ourselves on the back too much for our correct guess about geometry, however, because we were completely wrong about what it is that makes the universe flat. We thought it was lots of dark matter. Instead, in the biggest surprise in a century, it was discovered that empty space apparently accounts for more than 70% of all of the energy in the universe. And we don’t have the slightest idea why!
Our earlier calculations had suggested that if the energy of space wasn’t zero, it had to be 120 orders of magnitude larger than the energy of everything we see, which was so ridiculous (and observationally impossible) that we figured it must be zero. So we could sleep at night. But we were wrong.
The fact that empty space has energy has changed everything: not only our understanding of the current universe, but also its future. Nothing prepared us for this shock, and it could take us a long time before we understand the origin of this energy of nothing.
ONE OF THE MOST FUNDAMENTAL questions in cosmology is: where do the primordial lumps that form galaxies, stars, planets, and eventually us, come from? Thirty years ago, there wasn’t any theoretical clue, and observationally we were also in the dark, at least metaphorically. In the intervening period, a fundamental idea called inflation not only allowed a possible explanation of a flat universe, but also predicted that quantum mechanics in the early universe could have resulted in all the structures we now observe.
Then, in 1992, we discovered that nature had been unexpectedly kind to us. Against all odds, the CMB radiation turned out to be unpolluted by other astrophysical sources so that we could observe its fine structure and get direct information about the early universe. These observations have confirmed our ideas about dark matter, dark energy and the formation of structure. Additionally, the primordial hot and cold spots we see have precisely the distribution that inflationary models predicted. Have we proved inflation yet? No, but things are looking very good.
Things look good also for the belated appearance of the Higgs boson, predicted by several theorists, including British physicist Peter Higgs, in 1964. I admit I was betting against this. The idea that an otherwise invisible background field exists throughout space, whose interactions with most elementary particles moving through the field gives them their observed masses, just seemed too slippery, and too simple to be true.
But, if you slap space hard enough at one point, quantum mechanics says you should produce particles associated with the Higgs field, and that is apparently what has happened at the Large Hadron Collider at CERN. If this is confirmed, it will not only cap the greatest intellectual adventure humans have ever undertaken, but also confirm another fascinating feature of the universe: our existence is essentially an accidental by-product of the state the universe cooled into, and the key determinants of our present and future reside in what otherwise appears to be empty space.
Science fiction writers couldn’t imagine a universe more remarkable than the one we live in. What surprises await in the next 30 years? If I knew, they wouldn’t be surprises.