Echoes of the Big Bang, University of Birmingham astrophysicists discuss world-leading research into gravitational wave theory

Cosmologists, who believe the universe emerged from an initial “explosion” 14 billion years ago, have this week announced a new discovery that may go a long way to confirming their theory.

The BICEP2 Collaboration has reported the detection of a characteristic “twisting pattern” in the Cosmic Microwave Background using a new telescope at the South Pole - the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) telescope.

The pattern is believed to be due to gravitational waves generated in the very early universe during an inflationary period. If confirmed, this result would represent a landmark discovery: it is a glimpse of the status of the universe when it was a fraction of a second old.

The standard Hot Big Bang scenario that describes our universe as expanding from an initial “explosion” 14 billion years ago is extremely successful. However, it does not address some fundamental aspects. For example, the Hot Big Bang model does not provide a mechanism for producing “seeds” that ultimately grow into structures such as galaxies that we observe today, nor does it explains why very distant regions of the universe, despite apparently never "talking to each other", have the same temperature.

In a bid to cure the shortcomings of the standard theory, a new paradigm was proposed in the 1980's: inflation. Inflation, as the word suggests, introduces an early period of extremely rapid expansion followed by the standard Hot Big Bang expansion. According to this theory, inflation took place when the universe was a fraction of a second old, and the energies were well over a billion times higher than those reached at the Large Hadron Collider. This is a regime in which the physics of the very small (quantum mechanics) and the physics of very strong gravity (general relativity) are both hard at work, and we do not have a theory that describes this situation. Inflation therefore requires an enormous extrapolation of the laws of physics from the regimes in which they have been tested.

Regardless of the unknown details of this very early epoch, inflation robustly predicts that the quantisation of the gravitational field coupled to super-rapid expansion produces a “whisper” in the form of waves of space-time: a primordial gravitational wave stochastic background. Gravitational waves are a fundamental prediction of Einstein's general theory of relativity. They are ripples in the fabric of space-time produced by high concentrations of mass and energy moving at relativistic speed, like black holes and the very early universe. Their existence was confirmed indirectly over two decades ago.

At present, gravitational waves produced during an inflationary epoch are not directly observable. However, they leave a distinct "curl signature” in another cosmic echo from the Big Bang: the Cosmic Microwave Background. The BICEP2 team has reported the detection of this twisty pattern consistent with what is considered to be a smoking gun signature of inflation. If this result will stand the surely intense scrutiny of the international science community, the consequences for cosmology and fundamental physics will be far reaching, to say the least.

This result also highlights the unique role that gravitational-wave observations will play in the coming years in unveiling some of the best preserved secrets of the universe that are not accessible using electro-magnetic radiation. The direct observation of gravitational waves is at the centre of a large international effort with the goal of opening a new observational window on the universe.

Laser interferometers such as Advanced LIGO, in which the UK is heavily involved, are coming on-line next year. These instruments are L-shaped interferometers that can detect a change of less than a thousandth the diameter of an atomic nucleus in the lengths of the km-long arms relative to each other: this is the tiny effect of gravitational waves. These instruments will directly measure gravitational waves created by the most violent compact astrophysical sources in the universe, such colliding neutron stars and black holes. In the future they may also provide constraints on cosmic gravitational waves at frequencies totally different from those probed by experiments such as BICEP2, which will be essential to fully understand the physics of the very early universe.

Professor Alberto Vecchio is Professor of Astrophysics and Head of the Astrophysics and Space Research Group at the School of Physics and Astronomy, University of Birmingham