Pedagoguery

Cosmic inflation does a very good job of explaining the universe we see around us. Specifically, the fact that is appears substantially the same in all directions, and the amazing uniformity of the cosmic microwave background radiation. But physicists have a difficult time explaining why inflation should exist. String theory may be able to bridge that gap.

We do know that it takes a special kind of energy to produce inflation. It must have positive pressure, but it cannot be diluted by the expansion of space itself. Such characteristics are those of a scalar field. A scalar field is one that is characterized by a single value at all points in space. Fields like the magnetic field, by contrast, are vector fields, characterized by a value and a direction (toward the local north magnetic pole). String theory produces a wide range of potential scalar fields. The question becomes, do any of them have the right characteristics to produce inflation.

The scalar field that produces inflation, called the inflaton field, must have a large value, but it must be slightly unstable. At then end of the inflationary era, the field collapsed, converting its large energy into more conventional forms of matter and energy. String theory can conceivably produce such fields when branes are taken into account.

String theory postulates more dimensions than the three we see. Specifically, it requires at least 9 physical dimensions, all twisted together in a complex shape called a Calabi-Yau shape. Now there are two reasons why these extra dimensions are not visible to us. The first possibility is that they are very small, which is fairly self-explanatory. The second is more complicated and hinges on the nature of strings themselves. Strings can either be closed loops, like a rubber band, or open. Physicists believe that most particles that comprise matter are made of open strings. In addition, higher dimensional structures called branes can exists in Calabi-Yau space. Such branes can have any number of dimensions, but they have a special interaction with open strings. The ends of open strings become bound to the brane – they can move within the brane but not off of it. If our universe were such a brane, then the mechanisms by which we observe the universe would themselves be confined to our brane, and thus prevent us from observing the other dimensions.

It is in the interactions between branes that a scalar field can arise. If two branes are close together, there are forces between them. To inhabitants of one of those branes, those forces appear as scalar fields, since the directionality of the force points outside the brane and is thus unobservable. However, such fields are not strong enough to create inflation.

If you have branes and anti-branes, however, things change. Branes and anti-branes attract each other, in much the same way that matter and anti-matter does, and with much the same result. As the brane and anti-brane approach each other, they inflate, and upon their collision, they annihilate, producing enormous amounts of energy. The inflation effect can even spill over to other nearby branes, and the energy released in the annihilation can be deposited in such branes. Thus, branes can affect other branes in such a way as to produce inflation in them.

There is one potential issue, however. The main criticism of string theory is that it does not produce predictions that can be tested. However, there is a way that the string version of inflation can be tested. Most inflationary theories predict that inflation itself produced gravitation radiation. The gravitational radiation produced by string inflation, however, would be unobservably weak. So if the Planck satellite scheduled to be launched in October of this year cannot detect any effects of gravitational radiation in the signature of the cosmic microwave background, it would be a boost to string inflation.

Next time, the Large Hadron Collider.