Knocking on Heaven's Door - Lisa Randall [173]
Both types of researchers investigate basic structure and employ fundamental physical laws. Each needs to take into account the results of the other. The content of the universe that is studied by particle physicists is an important research subject for cosmologists too. Furthermore, the laws of nature that incorporate both general relativity and particle physics describe the universe’s evolution, as they must if both theories are correct and apply to a single cosmos. At the same time, the known evolution of the universe constrains what properties matter can have if it is to avoid disrupting the observed history. The universe was in some respects the first and most powerful particle accelerator. Energies and temperatures were very high in the early stages of its evolution, and the high energies that accelerators currently achieve aim to reproduce some aspects of those conditions today on Earth.
Recent attention to this convergence of interests has led to many fruitful investigations and major insights and will hopefully continue to do so. This chapter considers some of the big open questions in cosmology that particle physicists and cosmologists both explore. The overlapping arenas include cosmological inflation, dark matter, and dark energy. We’ll consider aspects we understand about each of these phenomena and—more important for active research—those that we don’t.
COSMOLOGICAL INFLATION
Even though we can’t yet say what happened at the very beginning of the universe, since we would need a comprehensive theory that incorporates both quantum mechanics and gravity, we can assert with reasonable certainty that at some time very early on (perhaps as early as 10−39 seconds into the universe’s evolution), a phenomenon called cosmological inflation occurred.
In 1980, Alan Guth first suggested this scenario, which says that the very early universe essentially exploded outward. Interestingly, he was initially trying to solve a problem for particle physics involving the cosmological consequences of Grand Unified Theories. Coming from a particle background, he used methods rooted in field theory—the theory combining special relativity and quantum mechanics that particle physicists employ for our calculations. But he ended up deriving a theory that revolutionized our thinking about cosmology. How and when inflation occurred is still a matter of speculation. But a universe that underwent this explosive expansion would leave clear evidence, and much of it has now been found.
In the standard Big Bang scenario, the early universe grew calmly and steadily—for example, doubling in size when its age increased by a factor of four. But in an inflationary epoch, a patch of the sky underwent a phase of incredibly rapid expansion, growing exponentially with time. The universe doubled in size in a fixed time and then doubled again in that same time and then kept doubling at least 90 times in a row until the inflationary epoch ended and the universe was as smooth as we see it today. This exponential expansion means, for example, that when the universe’s age had multiplied by 60 times, the size of the universe would have increased by more than a trillion trillion trillions in size. Without inflation, it would have increased by a mere factor of eight. In some sense, inflation was the beginning of our story of evolving from the small to the large—at least the part that we can potentially understand through observations. The initial enormous inflationary expansion would have diluted the matter and radiation content of the universe to practically nothing. Everything we observe today in the universe must therefore have arisen right after inflation, when the energy that drove the inflationary explosion converted into matter and radiation. At this point in time, conventional Big Bang evolution took over—and the universe began its further expansion into the huge structure we see today.
We can think of the inflationary explosion as the “bang” that was the precursor to the universe’s evolving according to the standard Big Bang theory. It’s not truly