In a Physics Review Letters paper published on Dec 15th, an international research team, led by Cai Yifu, Professor of the University of Science and Technology of China, and his collaborators discovered the hypothetical possibility of resonantly generating primordial gravitational waves within the high energy physics when the universe was in the babyhood.
The originally invisible gravitational wave signals can be amplified by parametric resonance by 4 to 6 orders of magnitude or even larger through this phenomenon, and then become likely to be probed by primordial gravitational wave detectors, hence, validating some theoretical models of the very early universe that are “inaccesible” in traditional observational windows.

In the baby universe, all matters once existed as extremely tiny elementary particles. The temperature of the baby universe was far beyound the highest temperature (energy scale) that human can reach in any high-energy experiments. Therefore, the new physics of this period is called the desert zone of high-energy physics.

At present, the major method for exploring the origin of the universe is to search for primordial gravitatiaonal waves, whose magnitude is directly determined by the energy scale of the baby universe. Therefore, capturing primordial gravitational waves has become almost the only chance for humans to reach new physics at high-energy scales beyond the standard model of particle physics [note 1]. However, if inflation [note 2] occurred in the desert zone of high-energy physics, the amplitude of primordial gravitational waves will be too small to be detected. Thus, the traditional academic perspective considers it almost as a “mission impossible” to search for primordial gravitational waves and the related new physics in this energy scale.

In this work, the research team led by Cai Yifu and Misao Sasaki from University of Tokyo, introduced a heavy field with a behavior of parametric resonance to nonlinearly couple with primordial gravitational waves, thereby providing an adequate energy source for the resonant amplification of primordial gravitational waves. In addition, the special dynamical properties of the background evolution of inflation can ensure that the newly introduced heavy field can hardly interfere with the observed primordial density perturbations, and thus the new theory can perfectly fit to current cosmological observations.

To be specific, by constructing a concrete example of the background model, the researchers accurately demonstrated that, even if inflation occurred in the desert zone of high-energy scale that exceeds the standard model of particle physics, primordial gravitational waves can be resonantly generated with a sufficiently large magnitude that is of observable interests. From the theoretical perspective, this result explicitly showed that, even in the dune of high-energy physics there exists some oasis sustaining the life of new physics.

This research provides an important scientific goal for the present and upcoming primordial gravitational wave experiments worldwide, and also opens a novel window for searching for high-energy new physics beyond the standard model of particle physics.

Note 1: Strictly speaking, gravitational-wave detection is merely one of many scientific approaches in seeking for new physics beyond the standard model of particle physics, as well as traditional collider science, high-energy cosmic rays, and quantum measurements, etc. Moreover, regarding the scientific targets of gravitational-wave detection, except for primordial gravitational waves, there are signals from phase transitions, those induced by primordial black holes, etc. However, these technologies are either far from the desert zone of high-energy physics, or their underlying theories remain unclear. As long as the big bang theory is well established, primordial gravitational waves exist. In this regard, the scientific exploration of primordial gravitational waves has become the key clue for mankind to walk out of the dune of the baby universe and to find out new physics.

Note 2: Inflation is a prevailing theory about the origin of the universe, and a representative description of new physics in high-energy scales as well. It describes that the volume of the univser was enlarged sharply by about 10^{80} times within about 10^{-30} seconds after the big bang. The sky map of the cosmic microwave background radiation partly validates this theory. But primordial gravitational waves have not yet been found. Once humans have successfully observed primordial gravitational waves, inflation models can be tested, and the energy scale of new physics can be determined.

Beating the Lyth Bound by Parametric Resonance during Inflation, Physics Review Letters