Cosmic Inflation and Primordial Gravitational Waves

bicep2

Two days ago the BICEP2 collaboration has made an astonishing announcement: the first direct evidence of cosmic inflation has been found.

Cosmic inflation is a theory that traces back to the ’80s and that predicts that the very early Universe has expanded at a tremendous rate for a fraction of a second (about one billionth of a trillionth of a trillionth of a second). We know that today our Universe is still expanding, but the current rate of expansion is tiny when compared to what happened during the cosmic inflation. Such theory solves a large number of otherwise puzzling features of our Universe, for example it explains why the Universe is homogeneous and isotropic (on large scales it looks the same in every direction) or why it is flat (the geometry of the Universe is Euclidean).

The BICEP2 team has discovered some signatures of primordial gravitational waves in the cosmic microwave background, a relic of radiation coming from the Universe when its age was only about 300.000 years. It is the first time that primordial gravitational waves are observed and so far we were not even sure about their existence.

There has been a bit of controversy about whether this measurement 
constitutes the first direct detection of gravitational waves or 
not. I will not discuss this here, since I believe this is a 
marginal aspect of the discovery and quite beside the point.  
Indeed no physicist really believes that the BICEP2 observation 
represents the first discovery of gravitational waves. We have 
already discovered gravitational waves, through observation of 
radio pulsars, and the 1993 Nobel prize in physics to Joseph Taylor
 and Russell Hulse clearly witnesses this. It is certainly true, 
however, that primordial gravitational waves are something new and 
this is what the BICEP2 team has discovered: an imprint of 
primordial gravity waves on the cosmic microwave background

This said, the primordial gravitational waves discovered by BICEP2 have two incredibly exciting implications. The first is that we can rule out alternative cosmological models.   On December 2013, I attended the 27th Texas Symposium of Relativistic Astrophysics, in Dallas. There I heard a talk of P. Steinhardt, one of the greatest cosmologists of our time, who discussed our current knowledge of the very early Universe.  One claim he made that really struck me was that “any claim that the cosmic microwave background and the large scale structure supports inflation is misleading”. Alternatives to the inflation exist indeed (or at least existed before the BICEP2 discovery) like the ekpyrotic universe that requires no inflation and that predicts that no initial singularity ever existed. In that model, the Universe was created by the collision of two “branes” (exotic entities described by string theory) that gave rise to the Big Bang and the Universe as we know it. In this model the Universe is cyclic, and is destroyed and regenerated periodically. However, such model predicts no primordial gravitational wave perturbations on cosmic scales. But such waves have now been observed by the BICEP2 experiment ! So we can confidently rule out such alternative cosmological scenarios. 

The second great implication of the BICEP2 experiment is that we have now a clear idea about what is the energy scale of inflation. This means at which energies did inflation occur. During that immensely small amount of time the inflation of the Universe was driven by something that goes beyond the four fundamental forces that rule Nature (i.e., gravitational force, weak and strong interaction and electromagnetic force). There has been large speculation about what kind of new physics we might require to explain the inflation, and the detection reported by the BICEP2 team on Monday suggests that inflation is indeed related with the grand unification of fundamental forces (all but gravity), which is one of the most important unsolved problems of modern physics.

Finally, one fascinating aspect of this discovery is that primordial gravitational waves are generated in inflationary models by quantum-mechanical processes which are analogous to those generating Hawking radiation in evaporating black holes. The Hawking radiation around black holes is determined by the curvature of space-time, which is given by the black hole mass. In inflationary models, the strength of the primordial gravitational waves is determined by the curvature of space-time of the Universe, which is given by the energy of inflation. The BICEP2 discovery represents therefore a first detection of an Hawking process.

We will now wait to see what other implications this discovery will have on modern physics, but it is certainly true that it represents a revolution for our understanding of Nature. There are very exciting times ahead for fundamental physics !

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