This month’s issue of Physics Today has an interesting article by Robert Brandenberger of McGill University, entitled Alternatives to Cosmological Inflation. As a refresher, cosmological inflation is the theory that sets up the Big Bang: it takes whatever was in the Universe prior to inflation and expands it away, leaving you with a Universe that has roughly (to a few parts in 100,000) the same properties everywhere, and is spatially flat.
There are many models of inflation which give a Universe like ours, although we have to fine-tune the parameters of it. For instance, if we treat inflation like any other field theory, we need to specify the shape of its potential very precisely (and very much unlike real potentials we observe), we need to ensure that the tiny fluctuations are close to, but not exactly the same on all scales, and we need to ignore the fact that simple models of inflation still give you a singularity to start your Universe.
But the biggest problems with inflation are the fact that we don’t know how to make a realistic particle physics model of it. Another problem is that if inflation lasts a very long time (and almost all models of it do), then the scales that are the size of the Universe today were once really, really small. Smaller than an atom, smaller than a proton, smaller than the Planck length, and this is a problem.
Why? Because when you go smaller than the Planck scale, your laws of physics, like quantum mechanics and gravity, don’t make any sense anymore. We don’t know what the fluctuations should look like (see the image at right). So we have no idea what to make of the fact that there ought to be sub-Planck-scale physics signals littered throughout the Universe; this is known as the Trans-Planckian problem. With a view to addressing some of these concerns, Robert goes over three alternatives (and none of them are very appealing).
1. Add defects and vary the Speed of Light. This one’s out. Why? Because we would see defects in the Cosmic Microwave Background, and we don’t. The constancy of the speed of light is highly supported by experiments, but there is a theoretical disaster if it turns out that either c (the speed of light), G (the Gravitational constant), or h (Planck’s constant) changes: energy is no longer conserved in the Universe! It’s possible, but… yeesh!
2. Bouncing Cosmologies. These theories have the Universe go through cycles, where they have big bangs followed by expansions, turnarounds, and contractions, followed by a big crunch. There are models out there that have this happen in a manner consistent with what we observe, but they’re extremely difficult to reconcile with dark energy in the Universe, which we have. If these scenarios are correct, it means that this dark energy is a temporary, transient thing. Again, these models need to be very finely tuned to mimic the observations that have already been made.
3. String gas cosmology. At least it doesn’t have a singularity. Unfortunately, it also doesn’t really have the properties that our Universe does, like homogeneity; in other words, this scenario doesn’t solve the horizon problem. It also doesn’t explain why our Universe is so large, which is what inflation does.
Now to be fair, despite having a vested interest in string gas cosmology, Robert actually states the pros and cons of these various ideas, and the conclusion I draw from this, really, is to pick your poison. Nothing solves all the problems yet, but at least they are, in principle, distinguishable from one another through methods like measuring the spectral index (whether fluctuations are larger at small or large scales) of both matter and gravitational waves. Actually, PLANCK has a good chance of making some of these measurements, and it launches later this year! I’ll give you the update when we figure it out!