Our universe, the poor man's accelerator

The Hindu
March 25, 2014

On March 17, radio astronomers from the Harvard-Smithsonian Center for Astrophysics, Massachusetts, announced a remarkable discovery. They found evidence of primordial gravitational waves imprinted on the cosmic microwave background (CMB), a field of energy pervading the universe.

A confirmation that these waves exist is the validation of a theory called cosmic inflation. It describes the universe’s behaviour less than one-billionth of a second after it was born in the Big Bang, about 14 billion years ago, when it witnessed a brief but tremendous growth spurt. The residual energy of the Bang is the CMB, and the effect of gravitational waves on it is like the sonorous clang of a bell (the CMB) that was struck powerfully by an effect of cosmic inflation. Thanks to the announcement, now we know the bell was struck.

Detecting these waves is difficult. In fact, astrophysicists used to think this day was many more years into the future. If it has come now, we must be thankful to human ingenuity. There is more work to be done, of course, because the results hold only for a small patch of the sky surveyed, and there is also data due from studies done until 2012 on the CMB. Should any disagreement with the recent findings arise, scientists will have to rework their theories.

Remarkable in other ways

The astronomers from the Harvard-Smithsonian used a telescope called BICEP2, situated at the South Pole, to make their observations of the CMB. In turn, BICEP2’s readings of the CMB imply that when cosmic inflation occurred about 14 billion years ago, it happened at a tremendous amount of energy of 10¹⁶ GeV (GeV is a unit of energy used in particle physics). Astrophysicists didn’t think it would be so high.

Even the Large Hadron Collider (LHC), the world’s most powerful particle accelerator, manages a puny 10⁴ GeV. The words of the physicist Yakov Zel’dovich, “The universe is the poor man’s accelerator”— written in the 1970s — prove timeless.

This energy at which inflation has occurred has drawn the attention of physicists studying various issues because here, finally, is a window that allows humankind to naturally study high-energy physics by observing the cosmos. Such a view holds many possibilities, too, from the trivial to the grand.

For example, consider the four naturally occurring fundamental forces: gravitation, strong and weak-nuclear force, and electromagnetic force. Normally, the strong-nuclear, weak-nuclear and electromagnetic forces act at very different energies and distances.

However, as we traverse higher and higher energies, these forces start to behave differently, as they might have in the early universe. This gives physicists probing the fundamental texture of nature an opportunity to explore the forces’ behaviours by studying astronomical data — such as from BICEP2 — instead of relying solely on particle accelerators like the LHC.

In fact, at energies around 10¹⁹ GeV, some physicists think gravity might become unified with the non-gravitational forces. However, this isn’t a well-defined goal of science, and doesn’t command as much consensus as it submits to rich veins of speculation. Theories like quantum gravity operate at this level, finding support from frameworks like string theory and loop quantum gravity.

Another perspective on cosmic inflation opens another window. Even though we now know that gravitational waves were sent rippling through the universe by cosmic inflation, we don’t know what caused them. An answer to this question has to come from high-energy physics — a journey that has taken diverse paths over the years.

Consider this: cosmic inflation is an effect associated with quantum field theory, which accommodates the three non-gravitational forces. Gravitational waves are an effect of the theories of relativity, which explain gravity. Because we may now have proof that the two effects are related, we know that quantum mechanics and relativity are also capable of being combined at a fundamental level. This means a theory unifying all the four forces could exist, although that doesn’t mean we’re on the right track.

At present, the Standard Model of particle physics, a paradigm of quantum field theory, is proving to be a mostly valid theory of particle physics, explaining interactions between various fundamental particles. The questions it does not have answers for could be answered by even more comprehensive theories that can use the Standard Model as a springboard to reach for solutions.

Physicists refer to such springboarders as “new physics”— a set of laws and principles capable of answering questions for which “old physics” has no answers; a set of ideas that can make seamless our understanding of nature at different energies.

Supersymmetry

One leading candidate of new physics is a theory called supersymmetry. It is an extension of the Standard Model, especially at higher energies. Finding symptoms of supersymmetry is one of the goals of the LHC, but in over three years of experimentation it has failed. This isn’t the end of the road, however, because supersymmetry holds much promise to solve certain pressing issues in physics which the Standard Model can’t, such as what dark matter is.

Thus, by finding evidence of cosmic inflation at very high energy, radio-astronomers from the Harvard-Smithsonian Center have twanged at one strand of a complex web connecting multiple theories. The help physicists have received from such astronomers is significant and will only mount as we look deeper into our skies.