Most cosmologists would say the answer is "inflation," and, until recently, I would have been among them. But "facts have changed my mind" — and I now feel compelled to seek a new explanation that may or may not incorporate inflation.
The idea always seemed incredibly simple. Inflation is a period of rapid accelerated expansion that can transform the chaos emerging from the big bang into the smooth, flat homogeny observed by astronomers. If one likens the conditions following the bang to a wrinkled and twisted sheet of perfectly elastic rubber, then inflation corresponds to stretching the sheet at faster-than-light speeds until no vestige of its initial state remains. The "inflationary energy" driving the accelerated expansion then decays into the matter and radiation seen today and the stretching slows to a modest pace that allows the matter to condense into atoms, molecules, dust, planets, stars and galaxies.
I would describe this version as the "classical view" of inflation in two senses. First, this is the historic picture of inflation first introduced and now appearing in most popular descriptions. Second, this picture is founded on the laws of classical physics, assuming quantum physics plays a minor role. Unfortunately, this classical view is dead wrong. Quantum physics turns out to play an absolutely dominant role in shaping the inflationary universe. In fact, inflation amplifies the randomness inherent in quantum physics to produce an universe that is random and unpredictable.
This realization has come slowly. Ironically, the role of quantum physics was believed to be a boon to the inflationary paradigm when it was first considered twenty-five years ago by several theorists, including myself. The classical picture of inflation could not be strictly true, we recognized, or else the universe would be so smooth after inflation that galaxies and other large-scale structures would never form. However, inflation ends through the quantum decay of inflationary energy into matter and radiation. The quantum decay is analogous to the decay of radioactive uranium, in which there is some mean rate of decay but inherent unpredictability as to when any particular uranium nucleus will decay. Long after most uranium nuclei have decayed, there remain some nuclei that have yet to fission.
Similarly, inflationary energy decays at slightly different times in different places, leading to spatial variations in the temperature and matter density after inflation ends. The "average" statistical pattern appears to agree beautifully with the pattern of microwave background radiation emanating from the earliest stages of the universe and to produce just the pattern of non-uniformities needed to explain the evolution and distribution of galaxies. The agreement between theoretical calculation and observations is a celebrated triumph of the inflationary picture.
But is this really a triumph? Only if the classical view were correct. In the quantum view, it makes no sense to talk about an "average" pattern. The problem is that, as in the case of uranium nuclei, there always remain some regions of space in which the inflationary energy has not yet decayed into matter and radiation at all. Although one might have guessed the undecayed regions are rare, they expand so much faster than those that have decayed that they soon overtake the volume of the universe. The patches where inflationary energy has decayed and galaxies and stars have evolved become the oddity — rare pockets surrounded by space that continues to inflate away.
The process repeats itself over and over, with the number of pockets and the volume of surrounding space increasing from moment to moment. Due to random quantum fluctuations, pockets with all kinds of properties are produced — some flat, but some curved; some with variations in temperature and density like what we observe, but some not; some with forces and physical laws like those we experience, but some with different laws. The alarming result is that there are an infinite number of pockets of each type and, despite over a decade of attempts to avoid the situation, no mathematical way of deciding which is more probable has been shown to exist.
Curiously, this unpredictable "quantum view" of inflation has not yet found its way into the consciousness of many astronomers working in the field, let alone the greater scientific community or the public at large.
One often reads that recent measurements of the cosmic microwave background or the large-scale structure of the universe have verified a prediction of inflation. This invariably refers to a prediction based on the naïve classical view. But if the measurements ever come out differently, this could not rule out inflation. According to the quantum view, there are invariably pockets with matching properties.
And what of the theorists who have been developing the inflationary theory for the last twenty-five years? Some, like me, have been in denial, harboring the hope that a way can be found to tame the quantum effects and restore the classical view. Others have embraced the idea that cosmology may be inherently unpredictable, although this group is also vociferous in pointing how observations agree with the (classical) predictions of inflation.
Speaking for myself, it may have taken me longer to accept its quantum nature than it should have, but, now that facts have changed my mind, I cannot go back again. Inflation does not explain the structure of the universe. Perhaps some enhancement can explain why the classical view works so well, but then it will be that enhancement rather than inflation itself that explains the structure of the universe. Or maybe the answer lies beyond the big bang. Some of us are considering the possibility that the evolution of the universe is cyclic and that the structure was set by events that occurred before the big bang. One of the draws of this picture is that quantum physics does not play the same dominant role, and there is no escaping its predictions of the uniformity, flatness and structure of the universe.