Armed with data on the first gravitational waves recorded last year and theoretical analysis, physicists have shown that gravitational waves can oscillate between two different forms, the g- and f-types of gravitational waves. Physicists explain that this phenomenon is analogous to how neutrinos oscillate between three different flavors – electronic, muonic and tau. Oscillating gravitational waves appear in a modified gravity theory called bimetric gravity, or “bi-gravity,” and physicists show that oscillations can be detected in future experiments.

The work of Kevin Max, Moritz Platscher and Yuri Smirnov was published in a recent issue of Physical Review Letters.

Physicists note that this work can help to find the answer to the question “what 95% of the Universe consists of.” The fact is that the answer may lie in the modifications of gravity, and not in the new particles.

“Only 5% of the substance refers to one that we think we understand well,” Smirnov told Phys.org. “Trying to answer the question of what our universe (dark energy and dark matter) consists of, most authors are discussing alternative models of particle physics with new particles. However, experiments like those conducted at the Large Hadron Collider have so far not found any exotic particles. The question arises: maybe it is necessary to reconsider the gravitational side? “.

“In our work, we are asking ourselves what signals we might expect from modified gravity. It turns out that bi-gravitation has such a unique signal and can therefore be distinguished from the background of other theories. The recent discovery of gravitational waves opened a new window for us in the dark sectors of the universe. Regardless of whether nature has chosen the general theory of relativity, bi-gravity, or some other theory, it remains for us to study only concrete signals. ”

Two gravitons instead of one

Currently, the best theory of gravity is Einstein’s general theory of relativity, which uses a single metric to describe space-time. As a result, gravitational interactions are mediated by a single hypothetical particle – a graviton – which has no mass and therefore moves with the speed of light.

The main difference between the general theory of relativity and bi-gravity is that bi-gravity uses two metrics, g and f. While g is a physical metric and is associated with matter, f is a sterile metric and does not bind to matter. In bi-gravitation, gravitational interactions are mediated by two gravitons, one of which has mass and the other does not. Both gravitons consist of different combinations (or superpositions) of metrics g and f, and therefore bind to the surrounding matter in different ways. The existence of two metrics (and two gravitons) within the framework of bi-gravity ultimately leads to the phenomenon of oscillations.

As physicists explain, the idea that a graviton with a mass can exist was born almost simultaneously with the general theory of relativity.

“The general theory of relativity of Einstein predicts one mediator of gravitational interactions (graviton), which moves at the speed of light (is massless),” says Max. “Already in the 1930s, people were trying to find a theory in which the mediator will have a mass and, consequently, move at a speed less than light. The task was extremely difficult and was completed only in 2010. Bigravitation became a variation of this development in 2010, only it already contained not one, but two dynamic metrics. One of them is associated with matter, and the other is not; their linear combination becomes massive (slower than the speed of light), and the other is massless (at the speed of light).

Oscillations

Physicists have shown that within the framework of bi-gravity, when gravitational waves are produced and propagated in space, they oscillate between g- and f-types – although only the g-type can be detected. Although previous studies have shown that such oscillations can exist, they led to far from physics results, for example, violated the law of conservation of energy. A new study showed that oscillations can theoretically arise in a realistic physical scenario, involving a massive graviton that is large enough to be detected during the experiment.

So that we can understand these oscillations, scientists compare them with neutrino oscillations. Although neutrinos are of three flavors (electron, muon and tau), electron neutrinos (or electron antineutrinos) are usually produced in the course of nuclear reactions, because others are too heavy to form a stable substance. Similarly, in bi-gravitation, only one metric is associated with matter, therefore gravitational waves, which are born by astrophysical events like fusion of black holes, g-type – the f-type of gravitational waves is simply not associated with matter.

“The key to understanding the phenomenon of oscillations is that electronic neutrinos do not have a certain mass: they represent a superposition of three neutrino mass states,” explains Platscher. “The wave equation that describes their motion through space, mixes them up and leads to oscillations.”

“The same is true for bi-gravity: g is a mixture of massive and massless gravitons and therefore, when the gravitational wave passes through the universe, it will oscillate between the g- and f-types of gravitational waves. Nevertheless, we can measure only the first with the help of detectors (consisting of matter), and the second will go unnoticed. And this, if bi-gravitation is a correct description of Nature, will leave an important trace on the signal of the gravitational wave, which we showed. ”

The similarity between neutrinos and gravitational waves is maintained even though neutrino oscillations are a phenomenon of quantum mechanics described by the Schrodinger wave equation, and the oscillation of the gravitational wave is not a quantum effect and is described by a classical wave equation.

One of the specific effects predicted by physicists is that oscillations of the gravitational wave lead to larger modulations than predicted by general relativity. These results mark a path for experimental detection of oscillations of gravitational waves and search for support for bi-gravity.

“Because bi-gravitation is a very young theory, much remains to be done and studied. In this direction, some work has been done, but we hope to make our contribution in the future, “say the scientists.