Neutrinos may have saved the universe

Print edition : February 28, 2020

A timeline of events from the Big Bang, through phase transition, until the present. Photo: Kavli IPMU

Gravitational waves, which are ripples in space-time, could contain evidence to prove that the universe survived the Big Bang because of a phase transition that allowed neutrino particles to reshuffle matter and anti-matter, according to a new study by an international team of researchers from Japan, United States and Canada. This study was published recently in “Physical Review Letters”.

According to the Big Bang theory, matter was created with an equal amount of anti-matter. If it had stayed that way, matter and anti-matter should have eventually met and annihilated one to one, leading to a complete annihilation. But our existence contradicts this. To overcome a complete annihilation, the universe must have turned a small amount of anti-matter into matter, creating an imbalance between them. The imbalance needed is only a part in a billion. But it has remained a complete mystery when and how the imbalance was created. “The universe becomes opaque to light once we look back to around a million years after its birth. This makes the fundamental question of ‘why are we here?’ difficult to answer,” said co-author Jeff Dror of the University of California, Berkeley, and a researcher at Lawrence Berkeley National Laboratory.

Since matter and anti-matter have the opposite electrical charges, they cannot turn into each other, unless they are electrical neutral. Neutrinos are the only known electrical neutral matter particles. A theory many researchers support is that the universe went through a phase transition so that neutrinos could reshuffle matter and anti-matter.

When the behaviour of matter changes at specific temperatures called critical temperature, it is known to undergo a phase transition, like boiling water to vapour or cooling water to ice or when a certain metal becomes a superconductor when cooled to a very low temperature. “Just like a superconductor, the phase transition in the early universe may have created a very thin tube of magnetic fields called cosmic strings,” explained co-author Hitoshi Murayama of the University of California, Berkeley, and Kavli Institute for the Physics and Mathematics of the Universe, University of Tokyo.

The researchers believe that the cosmic strings then try to simplify themselves, leading up to tiny wobbling of space-time called gravitational waves. These could be detected by future space-borne observatories such as LISA, BBO (European Space Agency) or DECIGO (Japanese Astronautical Exploration Agency) for nearly all possible critical temperatures. “The recent discovery of gravitational waves opens up a new opportunity to look back further to a time, as the universe is transparent to gravity all the way back to the beginning. When the universe might have been a trillion to a quadrillion times hotter than the hottest place in the universe today, neutrinos are likely to have behaved in just the way we require to ensure our survival. We demonstrated that they probably also left behind a background of detectable gravitational ripples to let us know,” said co-author Graham White of TRIUMF, Canada.

“Gravitational waves from cosmic strings have a spectrum very different from astrophysical sources such as merger of black holes. It is quite plausible that we will be completely convinced the source is indeed cosmic strings,” said Kazunori Kohri of the High Energy Accelerator Research Organisation, Japan.

“It would be really exciting to learn why we exist at all,” said Murayama. “This is the ultimate question in science.”

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