Cosmological Inflation

From the summit of Mount Hopkins, Arizona (February 5, 2007)

Tomorrow (Monday) a major press conference is scheduled in the main auditorium of my old institution, the Harvard-Smithsonian Center for Astrophysics. It is widely rumored that the event will concern ground-breaking results from the BICEP2 experiment. Ok, so... what will this be all about?

It is a long story, going back all the way to the birth of the Universe, 13.82 billions years ago. That's the time when the Universe was born, starting from a very dense and hot state where radiation and matter were coupled together in a very thick soup. As the universe aged, it expanded. As it stretched, it cooled. Almost 380,000 years later, the Universe became cold enough to form the first atoms in a process that we call recombination. That was a momentous change: at recombination, when most charged particles condensed into forming hydrogen and helium atoms, light was suddenly free to move around without being continuously absorbed. The Universe suddenly became transparent to the electromagnetic radiation.

After recombination, the Universe entered an era dominated by matter, that still lasts today. The newly formed atoms, helped by other mysterious neutral particles that we call dark matter, were pulled together by the force of gravity to form the first galaxies, then the first stars (let there be light), then planets and life as we know it. The primordial light, liberated by the yoke of matter, started to freely move around the Universe, stretched by the expansion of the Cosmos like a long forgotten echo in an enormous cathedral. This faint echo formed billions of years ago was accidentally found by Arno Penzias and Robert Wilson, two physicists working on a prototype microwave antenna built at the Bell Labs in New Jersey. This echo is what we call the Cosmic Microwave Background Radiation (CMBR).

A nice property of the CMBR is that it is exactly the same, anywhere you look in the sky. That should not be a surprise: the background radiation was generated by the cosmic soup at the time of recombination, when the Universe had expanded to a specific density and temperature. If the Universe was homogeneous and isotropic at that time, the CMBR should reflect that. Small anisotropies (less than 1 part in 100,000) are still found in the CMBR, the consequence of small statistical fluctuations in the distribution of the hot gas that emerged from the primordial soup. These fluctuations, that acted as seeds for the gravitational collapse from which galaxies formed, are the holy grail to understand the details of the Universe expansion and to measure its fundamental parameters. The 13.8 billion years age of the Universe I mentioned above, for example, was determined by the European Space Agency and NASA Planck telescope, precisely by measuring the CMBR fluctuations from space.

Tucson, Arizona

The extreme isotropy of the CMBR, however, is also a huge problem. When we measure the background radiation from opposite directions in the sky, we are observing light that traveled, at the speed of light, for almost 14 billions years (from the time of recombination) until it reached us. Since the CMBR looks identical in these two opposite directions, the thermostat of the primordial soup in these two opposite areas of the Universe must have been set at exactly the same temperature. For this to happen, the two regions must have been able to communicate one with the other, in order to agree on their common temperature set point. This is a problem, because any form of physical communication is limited by the speed of light, and these two regions are now almost 2 x 14 = 28 billion light-years apart. Any communication among them would require a time twice the age of the Universe. In other words, the CMBR coming from opposite regions in the sky was produced by parts of the Cosmos that after the Big Bang somehow managed to get separated from any possible causal connection, incapable of communicating the common temperature that is however apparent in the CMBR. How could that be?

The solution to this paradox was proposed in 1980 by Alan Guth, a physicist at Cornell University (and now at MIT). Guth proposed that shortly after the Big Bang (0.0000000000000000000000000000000001 seconds after the Big Bang, to be precise) the Universe underwent a dramatic expansion, at a rate that left even light behind. This expansion "froze" the conditions in the primordial soup as space was stretched almost instantly by many orders of magnitude. Regions of the Universe that at the Big Bang were next to each other, capable to communicate within the limit of the speed of light, suddenly found themselves separated at distances where communication became impossible. Still they retained the common conditions from before this expansion, leading to identical temperatures when the CMBR was finally released. This process, which is analogous to a sudden phase transition like that of suddenly freezing supercooled water, is called inflation.

Inflation is a pillar of modern cosmology, yet it remains a very plausible, but not fully demonstrated, hypothesis. The detailed processes that caused this sudden expansion are largely undetermined, and many different models of inflation have been proposed to date. A direct proof that inflation existed, and a way to discriminate between these different models has so far eluded science. The most promising place to look for guidance, once more, is the CMBR itself.

Any process involving stretching and compressing space (like banging two very heavy masses together) leads to ripples in the space-time known as gravitational waves. Predicted by Einstein's Theory of General Relativity, gravitational waves have never been directly observed. We have however very strong indirect evidences of their existence. Binary pulsars are pairs of very heavy stellar remnants orbiting each other, that dissipate gravitational energy at the rate corresponding to their predicted production of gravitational waves. If gravitational waves do exists, they must have been produced in droves in such a cataclysmic event as the Big Bang, and must have been amplified even more at the time of inflation when space was stretched at an exponential rate. These primordial gravitational waves, propagating all the way to the time of recombination, must have left their own peculiar imprint in the CMBR, a trace that could have survived until today. This is what the BICEP2 experiment is about.

BICEP2 is a small telescope located at the South Pole, designed to measure with great sensitivity the intensity and polarization of the small fluctuations in the CMBR. The polarization of the CMBR is the key to find the footprints of inflation in the background radiation: the so-called polarization B-mode is a unique orientation in the CMBR light that could have only resulted from the perturbation of the primordial gravitational waves as they crossed the cosmological soup emerging from recombination. It is a unique feature that would prove the occurrence of inflation, at the same time restricting the number of possible inflation models. The detection of a signal in the B-mode polarization of the CMBR would be a Nobel-prize discovery, and open the door for studying the unknown physics that happened at the very first instants after the Big Bang.

For more details, tune-in to the Center for Astrophysics press release web site at 12PM EDT Monday, when the press conference will start. A nice article about the discovery with more details about the CMBR B-mode polarization is also available on the Sky and Telescope magazine.

---

Today's photos have nothing to do with BICEP2 and the CMBR. The telescope responsible for the discovery is at the South Pole, and unfortunately I have never been there. The main photo above, however, is yet another view from the road to the summit of Mount Hopkins, where the MMT telescope is located. The view is looking south-west towards Mexico. The small image on the left, and the image below are instead taken from a plane flying over southern Arizona. It is indeed time to fly north, leaving the mountains of the US southwest, towards the rockies of Colorado. This will be the destination for the next photoblog post.

Snow-capped mountains in southern Arizona (January 19, 2005)