An infant universe may tell us how to resolve the issues with the General Theory of Relativity.
BY DEBDUTTA PAUL
“If you find my cap here, that is a strong hint that I may have been here,” said Tuneer as he sat across my desk. A theoretical physicist here at ICTS-TIFR, he is using similar ideas to propose that gravitation is an inherently quantum mechanical phenomenon.
Tuneer and his colleagues study quantum gravity, which sheds light on the universe’s behaviour across its full extent and cosmic history. One of the signatures of the Big Bang, the universe’s beginning, is the cosmic microwave background radiation. This cosmic background streams across the entire universe and can be mapped across the sky, using its intensity and polarization, which are properties of light. The polarization map may have bumps on it, which hint that gravitation is an inherently quantum mechanical phenomenon — like Tuneer’s cap: not a sure-shot way to tell he was here, but a strong hint nonetheless.
Einstein’s theory has issues
The general theory of relativity was developed by Albert Einstein at the start of the twentieth century to explain gravitation. It has been successful in explaining a vast range of phenomena, like the universe’s evolution, and predicting new ones, like black holes and gravitational waves.
But, it has its problems. “General relativity spells its own demise: it tells you where it fails,” said Tuneer. While it predicts black holes, it also predicts ‘singularities’, or regions of spacetime where it doesn’t work. Two examples are the inside of black holes and the Big Bang.
The singularities appear when microscopic phenomena become important to consider along with gravitation.
Enter quantum physics
Quantum physics is a theoretical framework that successfully explains the world from the microscopic to the macroscopic scales. Physicists have used this mathematical framework as a tool to develop what they call the ‘standard model of particle physics’. It is a mathematical model that explains interactions between different fundamental particles. It also explains the stability of atoms, provides a platform to describe chemistry, and explains why matter has mass.
While a successful theoretical tool, it, too, has its limitations. Moreover, issues like what quantum theory implies about the nature of reality remain unresolved.
The cap on my table
The theoretical framework of quantum physics could, however, explain the bumps in the polarization maps. It could also solve the ‘singularity’ issue of gravitation.
While non-quantum theories could be concocted to explain the polarization bumps, the simplest explanation is quantum gravity. It also satisfies a unified theory of nature: gravitation is an inherently quantum mechanical phenomenon.
Just like the cap on my table: Tuneer’s colleague could have left it on my table, too, but that scenario is less likely. “We are trying to answer a fundamental question: Is gravity even quantum?” said Tuneer.
Quantum cosmology
There are many quantum theories of gravity. String theory — according to which all fundamental particles are strings that vibrate in spacetime — is one such quantum theory.
Tuneer is studying quantum theories that can predict the intensity and polarization distribution of the cosmic microwave radiation across the sky. Observations suggest that the radiation intensity is mostly uniform across the sky, but there are tiny fluctuations.
To explain these observations, physicists rely on a cosmological model, called the ‘inflation model’. It assumes that the universe expanded rapidly for a very small fraction of the very first second, in which its size increased exponentially. According to the inflation model, small deviations of the intensity caused small fluctuations in the nascent universe, which later gave rise to all galaxies, stars, the Sun, and planet Earth.
Such quantum theories attempt to explain the whole universe and its cosmic history.
While there are many inflationary models, there are some basic principles of physics that all of them need to abide by.
One, spacetime is not a passive arena for nature to operate in, but like in the general theory of relativity, it actively participates in the grand theatre. “It can bend and flex due to matter and its motion,” said Tuneer.
Two, the quantum nature of the universe dictates that the universe is but one realisation of many possibilities. The world we observe emerges from the quantum fluctuations among these possibilities.
Three, in the early universe, the inflation accelerated the universe’s expansion before it itself dissipated into matter and energy.
Using these principles, theoretical physicists study the entire universe as a single quantum system. They calculate quantities that are related to the data that experimental cosmologists collect — fluctuations in the intensity profile of the cosmic microwave background. These fluctuations can be explained by a quantum mechanical universe but also by a non-quantum theory of gravitation.
The polarization information of the cosmic microwave background radiation can break the tie. Quantum gravity predicts specific fluctuations in the polarization map. And experimental cosmologists go to the South Pole to see if these fluctuations, signs of the universe being one realisation of plenty of possibilities, actually exist.
However, even if cosmologists find these fluctuations on the polarization map, theoretical physicists won’t be in a position to assert that the nature of gravity is quantum mechanical. “That… requires a different kind of experiment,” said Tuneer. Cosmologists will need to collect data from different moments after the Big Bang, and not only from the time when inflation ended.
But, humanity is taking steps to find the first signatures of quantum gravity.
To know more about the exciting prospects, read the following paper by Tuneer and his colleagues: Holography of information in de Sitter space.
The author thanks Tuneer Chakraborty and Omkar Shetye for discussions.