Scientists have redefined gravity to explain the Big Bang and perhaps change our picture of the earliest moments of the cosmos. This new framework of "quantum gravity" may explain aspects of the Big Bang that Albert Einstein's 1915 theory of gravity, general relativity, fails to account for — maybe even doing away with the challenging concept of a singularity existing prior to the dawn of the universe.
Proving the concept of quantum gravity is something of a holy grail for physicists, as it would bridge the gap between the explanation we have of the universe on vast cosmic scales (general relativity) and on tiny scales (quantum physics).
"General relativity works extraordinarily well in many settings, but when we run it back to the Big Bang, and apply it to the inside of black holes, it predicts a singularity: a moment where density, curvature and temperature formally become infinite. That is usually a sign that the theory is being pushed beyond where it can be trusted," Afshordi told Space.com. "In other words, general relativity is likely incomplete for describing the very first moments of the universe, when quantum effects should also matter."
Expanding the standard picture of general relativity
Afshordi explained that in the standard picture of the Big Bang, scientists usually start with Einstein's theory of gravity, then add extra ingredients to explain the earliest moments of the universe, most notably a hypothetical "inflation field" to account for the initial rapid expansion of the cosmos."
Our approach asks whether some of that early-universe behavior could come directly from gravity itself, once gravity is extended in a way that remains better behaved at extremely high energies," he said. "So, instead of treating the Big Bang as a point where our equations fail and then patching over that with additional assumptions, we study a theory in which gravity already contains the ingredients needed to describe that ultra-early phase more consistently. This is what physicists call an ultraviolet completion: a theory that remains complete and self-consistent even at arbitrarily high energies."
The team's quantum-consistent extension of gravity recovers a model of early cosmic inflation, while also potentially removing the troubling concept of an initial singularity.
"Our model provides a very good fit to current data, in some cases better than many standard inflationary models," Afshordi said. "What surprised me most was how naturally an inflation-like phase emerged once the theory was treated in a consistent high-energy, or ultraviolet complete, framework. We often think of inflation as something that must be added on top of gravity, so it is striking that it may instead arise from gravity itself. More broadly, it was encouraging to see that a relatively minimal extension of Einstein's theory could already go a long way toward resolving the deep problem of our cosmic origins."
The researcher added that the next step for the team is to sharpen the model’s observational predictions and compare them carefully with future data.
"There are two main directions. The first is theoretical: we want to understand the framework more fully and test how robust the conclusions are beyond the simplified setting we studied," Afshordi continued. "The second is observational: we want to work out clearer predictions for primordial gravitational waves and other relics from the early universe. That will help determine whether this idea can be distinguished from more conventional models of inflation."
Observational evidence to help confirm the team's theory could come from some of the oldest observable signals in the universe, especially tiny ripples in space and time called primordial gravitational waves as well as subtle imprints in a cosmic fossil called the cosmic microwave background (CMB), a remnant of the universe's first light. "These are among the few probes that can tell us directly about physics at extremely early times," Afshordi said. "If future observations detect the right pattern of primordial gravitational waves, or other distinctive imprints in the CMB, that could provide a way to test whether this picture of the early universe is correct, or whether a more conventional explanation is needed."
The team's research was published in the journal Physical Review Letters.
