What Lies Beyond Our Cosmic Horizon?
There is a limit to how far we can see into the universe. Our cosmic horizon is set by the distance traveled by light since the big bang. More distant objects cannot be observed, because their light has not yet reached the Earth. But of course the universe does not end at the horizon, and the question is what lies beyond. Is it more of the same — more galaxies, more stars, or could it be that remote parts of the universe are very different from what we see around here? I am optimistic that we will be able to answer this question and understand the structure of the universe as a whole, even though we can observe only a small part of it.
Until recently cosmologists made the simplest assumption — that the universe is homogeneous, i.e. looks everywhere more or less the same. (It was glorified under the name of "Cosmological Principle", but it was still only an assumption.) Now, recent developments in cosmology and particle physics have led to a drastic revision of this view and to a heated debate about the future of our science. According to the new worldview, most of the universe is in the state of explosive, accelerated expansion, called "inflation". In our local region, inflation ended 14 billion years ago, and the energy that drove the expansion went to ignite a hot fireball of elementary particles. This is what we call the big bang. Other big bangs constantly go off in remote parts of the universe, producing regions with diverse properties. Some of these regions are similar to ours, while others are very different.
The properties of any given region are determined by the quantities we call "constants of nature". These include particle masses, Newton's constant, which controls the strength of gravity, and so on. We do not know why the constants in our region have their observed values. Some physicists believe that these values are unique and will eventually be derived from some fundamental theory. However, string theory, which is at present our best candidate for the fundamental theory of nature, suggests that the constants can take a wide range of possible values. Regions of all possible types are then produced in the course of eternal inflation. This picture of the universe, or multiverse, as it is called, explains the long-standing mystery of why the constants of nature appear to be fine-tuned for the emergence of life. The reason is that life evolves only in those rare regions where the constants happen to yield suitable chemistry and physics. The values of the constants in our own region are then determined partly by chance and partly by how suitable they are for the evolution of life.
Many of my colleagues find this multiverse picture very alarming. Since all those regions with different values of the constants are beyond our horizon, how can we verify that they really exist? Is this science — to talk about things that can never be observed? In my view, it is science, and there are good reasons to be optimistic about the new picture. If the constants vary from one part of the universe to another, their local values cannot be predicted with certainty, but we can still make statistical predictions. We can try to predict what values of the constants are most likely to be observed. One such prediction, that the vacuum should have a small nonzero energy, has already been confirmed. We have only started along this path, and formidable challenges lie ahead. I believe, however, that what we are facing now is not the end of cosmology, as some people fear, but the beginning of a new era — the exploration of the multiverse.