Researchers have discovered that in the exotic conditions of the early universe, waves of gravity may have shaken space-time so hard that they spontaneously created radiation.

The physical concept of resonance surrounds us in everyday life. When you’re sitting on a swing and want to go higher, you naturally start pumping your legs back and forth. You very quickly find the exact right rhythm to make the swing go higher. If you go off rhythm then the swing stops going higher. This particular kind of phenomenon is known in physics as a parametric resonance.

Your legs act as an external pumping mechanism. When they match the resonant frequency of the system, in this case your body sitting on a swing, they are able to transfer energy to the system making the swing go higher.

These kinds of resonances happen all over the place, and a team of researchers have discovered that an exotic form of parametric resonance may have even occurred in the extremely early universe.

Perhaps the most dramatic event to occur in the entire history of the universe was inflation. This is a hypothetical event that took place when our universe was less than a second old. During inflation our cosmos swelled to dramatic proportions, becoming many orders of magnitude larger than it was before. The end of inflation was a very messy business, as gravitational waves sloshed back and forth throughout the cosmos.

Normally gravitational waves are exceedingly weak. We have to build detectors that are capable of measuring distances less than the width of an atomic nucleus to find gravitational waves passing through the Earth. But researchers have pointed out that in the extremely early universe these gravitational waves may have become very strong.

And they may have even created standing wave patterns where the gravitational waves weren’t traveling but the waves stood still, almost frozen in place throughout the cosmos. Since gravitational waves are literally waves of gravity, the places where the waves are the strongest represent an exceptional amount of gravitational energy.

The researchers found that this could have major consequences for the electromagnetic field existing in the early universe at that time. The regions of intense gravity may have excited the electromagnetic field enough to release some of its energy in the form of radiation, creating light.

This result gives rise to an entirely new phenomenon: the production of light from gravity alone. There’s no situation in the present-day universe that could allow this process to happen, but the researchers have shown that the early universe was a far stranger place than we could possibly imagine.

This is assuming gravitational waves were strong during the end of inflation, which has become less likely the more observations are made.

“In principle, the CMB and dust can be distinguished because they differ spectrally. Over the years, researchers from the BICEP experiment and a joint project called the Keck Array have developed dust models using data at different frequencies. By removing the estimated dust contribution, the BICEP/Keck Collaboration has been able to place a progressively tightening bound on the gravitational-wave contribution. This bound is typically given in terms of the tensor-to-scalar ratio r , which characterizes the amplitude of gravitational waves relative to that of density waves. Before BICEP/Keck, the tensor-to-scalar ratio was known to be less than 0.11, based on CMB observations by the Planck satellite [3]. With BICEP/Keck data, the bound dropped to r<0.09 in 2016 [4] and then to r<0.07 in 2018 [5]. Earlier this year, an analysis of Planck data pushed the bound lower [6], and now the latest results from BICEP/Keck provide the tightest constraint yet, r<0.036 [1]. The BICEP/Keck team have shrunk this limit by combining data at three frequencies (96 GHz, 150 GHz, and 220 GHz) from their own experiment, complemented by archival data from the WMAP and Planck satellites."

["Squeezing down the Theory Space for Cosmic Inflation", Daniel Meerburg, October 4, 2021• Physics 14, 135.]

One interesting question which the demonstrated theory gives is how the reverse process appears, since a quantum field process is in principle reversible. But the paper gives an interesting prediction in a footnote, assuming their theory is correct:

"Note that the inverse process, namely the production of gravitational waves via parametric resonance from an oscillating scalar field, does not occur in a Minkowski background in a vacuum since the scalar field only enters the source term in the gravitational wave equation and not in the mass term (see e.e. Eq. 44 of [25]). However, in an expanding background, there is the possibility of parametric resonance of gravitational waves if the oscillating scalar fields lead to small amplitude periodic fluctuations of the Hubble expansion rate H(t) superimposed on the regular decrease of H (see e.g. [26])."

Seems we can continue to count out gravity generators for Star Trek-ian spaceships. 🙁