There are lots of times in science when more than one theory could explain what we are observing, and there aren’t always tests to determine which of the two theories matches reality. 

Take gravity. We know Newtonian equations work at human scale velocities and masses. We know relativity works at extremely high masses and velocities. What we don’t know is if Space-Time is loopy or stringy at the smallest scales. While string theory so far hasn’t won anyone over with its testability, it has captured people’s imaginations thanks to popular books by Stephen Hawkings and Brian Green, among others. 

The theory that has been trying to make predictions, however, is loop quantum cosmology. In a new paper in Physical Review Letters, a team led by Abhay Ashtekar is able to explain both the large-scale power spectrum’s anomaly at low values and the anomaly in lensing amplitude that shows inconsistency with Lambda-Cold Dark Matter cosmologies. 

According to Ashtekar: The standard inflationary paradigm based on general relativity treats space-time as a smooth continuum. Consider a shirt that appears like a two-dimensional surface, but on closer inspection, you can see that it is woven by densely packed one-dimensional threads. In this way, the fabric of space-time is really woven by quantum threads. In accounting for these threads, loop quantum cosmology allows us to go beyond the continuum described by general relativity where Einstein’s physics breaks down—for example beyond the Big Bang. 

This paper is the first to consistently explain both these known anomalies. Co-author Donghui Jeong adds: A recent study in the journal Nature Astronomy proposed an explanation for one of these anomalies that raised so many additional concerns, they flagged a ‘possible crisis in cosmology.’ Using quantum loop cosmology, however, we have resolved two of these anomalies naturally, avoiding that potential crisis.

In addition to solving these two known anomalies, this paper also makes new predictions about a fundamental cosmological parameter and primordial gravitational waves that might be tested by future, planned space missions. 

This is not definitive proof that Loop Quantum Gravity is real or a nail in the coffin of string theory, but it is definitely a paper that made me sit up and take note. I now have a small hope that we’ll have a unifying theory that brings gravity into a quantum context, and sometimes, a small hope is enough.

More Information

Penn State Eberly College of Science press release 

“Alleviating the Tension in the Cosmic Microwave Background Using Planck-Scale Physics,” Abhay Ashtekar, Brajesh Gupt, Donghui Jeong, and V. Sreenath, 2020 July 29, Physical Review Letters (Preprint on arxiv.org)