If the inflationary theory of cosmology is right, it means that the universe is far larger than we had thought. Most likely the universe is also much older than we thought, and it includes not just one, but an infinity of big bangs.
The inflationary theory was developed to explain observable features of the universe, and it has been remarkably successful. One example is the nature of the cosmic background radiation, which cosmologists interpret as the afterglow of the heat of the big bang itself.
Astronomers have made high precision measurements of this radiation, finding that it arrives at Earth with the same intensity from all directions, to the extraordinary accuracy of about 1/1000 of a percent.
Tracing the history of this radiation backwards in time, cosmologists conclude that the temperature and the density of matter in the universe must have been uniform to this accuracy when the cosmic background radiation was released, about 300,000 years after the big bang. Without inflation, this extreme uniformity of the early universe must be assumed, but cannot be explained.
Calculations show that without inflation there would not have been nearly enough time for this uniformity to come about, so one is forced to assume, without explanation, that the universe was uniform from its very beginning.
Despite its name, the classical form of the big bang theory is not really a theory of a bang at all. It really describes only the aftermath of the bang. It describes how the early, hot, dense universe expanded and cooled; it describes how the light chemical elements were synthesized during this expansion, and how the matter coagulated to form galaxies and stars. But it says nothing about what banged or what caused it to bang, and therefore it makes no predictions about the uniformity of the universe just after the bang.
Inflation, on the other hand, can explain the “bang” of the big bang. It relies on a proposal, originating in modern particle physics, that extraordinarily high densities can lead to a form of matter that would turn gravity on its head, causing it to become repulsive rather than attractive.
For reasons that are not important here, this form of matter is called a “false vacuum.” Inflation is the proposal that the expansion of the universe that we see today is the result of the gravitational repulsion of a false vacuum that filled the universe during a small fraction of a second of its early history.
In the inflationary theory the extreme uniformity of the universe was established early, before inflation began. At this time the region destined to become the presently observed universe was tiny—more than a billion times smaller than the size of a single proton. For such a small region, there was plenty enough time for uniformity to arise by the same kind of mundane processes by which the air in a room spreads out to uniformly fill the volume.
After this uniformity was established, inflation took over to stretch the region to become large enough to include all the stars and galaxies that we see today.
Inflation not only explains the uniformity that we see in the cosmic background radiation, but it also explains the statistical properties of the very faint nonuniformities that have been observed with instruments so sensitive that they can measure minute variations of less than 1/1000 of a percent.
While inflation must be tested and judged on the basis of what it says about observable features of the universe, curiosity leads us to ask what inflation says about the universe as a whole. The answer is bizarre.
The gravitational repulsion of the false vacuum that is believed to have driven inflation is so strong that it would have launched a period of incredibly rapid expansion. The region would have doubled in size in about 10-37 (i.e., a decimal point followed by 36 zeroes and a 1) second. In the next 10-37 second it would have doubled again, and it would have kept doubling every 10-37 seconds for as long as the false vacuum survived.
The false vacuum is unstable, however, so at some point it “decayed,” converting its energy to a hot soup of ordinary particles. From this point onward the scenario would coincide with the standard hot big bang picture. The dramatic expansion, however, strongly suggests that the universe would be far larger than one would have otherwise imagined, so the observed part of the universe would be merely a speck in a much larger space.
But the whole story is much more complicated. The false vacuum is unstable, but in most versions of the theory it decays like a radioactive substance, such as radium. The decay is described by a half-life: half of the false vacuum will remain after one half-life, a quarter will remain after two half-lives, etc. However, unlike a radioactive material, the false vacuum would expand as it decayed, and the expansion would be faster than the decay. Although only half of the false vacuum would remain after one half-life, it would be larger than the initial region. The false vacuum would never disappear, but instead would continue increasing in volume indefinitely. Pieces of the false vacuum region would randomly decay, producing new “bubble” universes at an ever-increasing rate. Our universe would be just one of the universes on this infinite tree of bubbles.
The process would go on forever. The bubbles would collide so rarely that any observer would have no more than a negligible probability of seeing any sign of the other bubbles’ existence. Nonetheless, an understanding of the infinite tree of universes seems to be needed in order to make statistical predictions about the properties of our own universe, which is assumed to be a typical “branch” on the tree.
In studying a scenario such as this, cosmologists generally assume that the laws of physics are the same throughout this multi-bubble universe. We don’t really have any way of knowing, but our goal is to understand the consequences of the laws of physics as we know them, and not to idly speculate about other mythical worlds.
Nonetheless, there is a possibility that the other bubbles could be very different from our own.
While empty space appears to be devoid of properties, to a modern particle physicist empty space, also called the vacuum, is an enormously complicated substance.
Particle-antiparticle pairs are incessantly appearing and disappearing, and space itself is believed to break up into a poorly understood “quantum foam” when magnified enough so that distances as short as 10-33 centimeter become visible. Because of this complexity, physicists do not know whether only one kind of empty space is stable, or whether there are many kinds. Other kinds of space might not be three-dimensional, and they might alter the masses of elementary particles, or the forces that govern their behavior. If there are many kinds of space, the infinite tree of bubble universes would sample all the possibilities.>/b>