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A Missing Piece in the Big Bang Theory Has ArrivedAbhishek Mehta - Getty Images

Combining different pieces from Big Bang cosmology could help explain an issue we have today.

The Hubble constant, the speed of expansion of our universe, is not observed with consistency.

These scientists suggest that not-well-understood quantum gravity could account for the gap.

In research published earlier this year, physicists from the University of Hyderabad in India say theyre on the path to solving one of the universes biggest outstanding problems. Since Edwin Hubble realized the universe is always expanding nearly 100 years ago, scientists have used the Hubble constant in calculations on virtually every scale in the universe. But today, estimates for the Hubble constant dont always align, with a difference of up to 10 percent between calculations made using different methods. (When someone at NASA mixes up meters and yards and loses an entire spacecraft, thats not even a full 10 percent deviation.)

The paper appears in the peer reviewed journal Classical and Quantum Gravity. The journal has an ongoing, periodically updated focus issue specifically about this measurement tension, and the editors explain the problem therescientists cant say for sure that the different Hubble constants measured are actually different, rather than just observation or calibration issues.

But the authors of the new paper, physicist P.K. Suresh and his research fellow (referred to as just Anupama B.) say that most measurements taken now are reliable. Instrumentation only continues to improveweve all seen those generation-defining, poster-quality photos of the far-out planets, for example. If the measurements on the local and faraway levels are indeed sound, then something is missing.

Its here where they introduce quantum gravity as a possible factor. This variablewhich, to be honest, is another enigmatic placeholder in some wayscould close the gap in Hubble constant observations. Thats because, as the authors propose, quantum gravity could have affected the rate of change at which the universe expanded itself. When a constant can have a variable rate of change, its easy to see why researchers tend to drop the constant label and instead call the fatcor simply H0, H1, and so on to designate which version of the measurement is in play.

The researchers explain that during inflationthe rapid growth of the universe immediately following the Big Bangthere may not have been a single, uniform inflation zone. Instead, more and more scientists are theorizing around the idea of multi field inflation. The idea originated to explain another measurement discrepancy: the number of particles in particular places or times, compared with the massive speed of inflation overall.

If a theory could help explain one gap in our codified equations for how inflation works, it makes sense to try that theory to find other missing pieces. These researchers used what is called the hybrid inflationary model, which describes two fields: one inflating and one rolling over like a waterfall. By accounting for quantum gravity, they found they were able to reconcile H0the current Hubble constantwith both H1 (during inflation) and HT (during phase transition). Just one adjusted equation with a parameter for quantum gravity could draw a curve that includes all three data points.

The researchers say that to resolve the Hubble tension, one must also establish and validate the inflation model linked with it. Cosmology faces unique challenges, including the ongoing question of quantum gravity itself. So, trying to stake out a specific model like this involves choosing and stabilizing other variables that may not be well understood or have a consensus... yet.

But, Suresh told Live Science, that cant stop researchers from pushing forward. Our equation doesnt need to account for everything, Suresh said, but that does not prevent us from testing quantum gravity or its effects experimentally.

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