Inflation: Solving puzzles from the Big Bang model


The Big Bang is a great model for describing almost all of the history of our universe, but for the Big Bang to work, the universe has to start off as extremely uniform with tiny variations in the distributions of matter and energy, and it has to be extremely flat in order to match our observations. However, there is no a priori reason for the universe to begin that way. Inflation sets out to solve these problems; Kimmy Wu sat down to discuss them with Eva Silverstein (Stanford) after the final session at KIPAC@10.



Inflation refers to a time right after the Big Bang during which the universe expanded very quickly. To give a comparable scale, it is like going from the size of a hydrogen atom to that of the Milky Way galaxy in less than 10^{-33} seconds (that's a one divided by 10 with 33 zeroes after it)! Because of this exponential expansion, quantum fluctuations were stretched to become seeds of the stars and galaxies that we see today, and space was stretched so much that regardless of how curved it started out to be, it is observed as flat today.

Inflation is a theory under active scrutiny by observations. As part of that scrutiny, we ask:

  1. Inflation is great on paper, but does it stand the test of observations?
  2. What are important signatures of inflation that we can look for?
  3. What alternatives do we have besides inflation?

One challenge of studying inflation is how one could "observe" something that happened over 13.8 billion years ago. The universe is kind to us in that it leaves traces for us to follow and figure out what happened then. One of the great resources for probing the early universe is the cosmic microwave background (CMB). It is the earliest light detectable and it carries imprints from the time before it was formed, i.e., inflation would have changed the way the CMB looks. The temperature data released by the Planck experiment - a satellite successor to NASA’s WMAP to map the CMB - earlier this year gave some significant support to inflation. One of the important signatures to detect is the B-mode polarization from primordial gravitational waves. The Planck satellite might be able to detect it, but it is likely to be muddied by noise.

To go after this miniscule signal, ground-based and balloon-borne experiments like ACTPol, SPTPol, Spider, and the Keck Array, all gear up with high sensitivity detectors and observe from the driest places on earth like the Atacama desert and the South Pole. The CMB polarization signals are very important not only for learning about inflationary physics, but also about physics frontiers like neutrinos. The first detection of B-mode polarization was published just two months ago. A next-generation ground-based experiment called CMB-S4 will not only give us new insights into high energy physics, but we'll also be able to answer important astrophysics questions by measuring galaxy cluster masses with high accuracy.

We're not dependent on the CMB alone, though. The simplest model of inflation predicts that the fluctuations in the CMB and in the large scale structures are gaussian – they are described by a bell-curve. Deviations from this are characterized by how much non-gaussianity there is in the fluctuations. One of the ways to distinguish between inflationary models is by measuring the non-gaussianity in the CMB and comparing it to that in the large scale structures of the universe – the web of galaxy clusters that we've been able to map.

One important point to note is that inflation may have happened at an energy scale so high that everything behaves quantum mechanically within the fabric of gravity. This means that inflation could provide an arena in which mechanisms in quantum gravity plays out – thus a place for physicists to study quantum gravity.

Efforts are also underway to study a period called reheating, which was the very end of inflation, when elementary particles started forming. Different models are being developed that describe the dynamics during reheating – which can be described by mathematics similar to those that describes a child on a swing. The clips we watched of these fields evolving through time were a real treat.

Another open question is whether there are theoretically well defined alternatives for inflation, and if so what their observational signatures would be and how they would be fundamentally different from inflation.

Even though we are far from solving all the puzzles of inflation, with all these activities in both the experimental and theory fronts, we are closer than ever to figuring out the physics of the early universe. The incredible progress in our understanding of cosmology in just the last 10 years, from both top-down and bottom-up approaches, is an encouraging hint of what to expect in the coming few years.


You can watch all the talks in this session on the KIPAC youtube channel.

You can also read more about KIPAC@10 on the conference blog home page.