The researchers have found a new second type of gravitational waves which tend to oscillate in space. The physicists explain that this phenomenon is similar to the way that neutrinos oscillate between three distinct flavors electron, muon, and tau
Using data physicists have shown that gravitational waves may oscillate between two different forms called “g” and “f”-type gravitational waves. The oscillating gravitational waves arise in a modified theory of gravity called bimetric gravity, or “bigravity,” and the physicists show that the oscillations may be detectable in future experiments.
The researchers, Kevin Max, a PhD student at Scuola Normale Superiore di Pisa and INFN Pisa, Italy; Moritz Platscher, a PhD student at the Max Planck Institute for Nuclear Physics, Germany; and Juri Smirnov, a postdoc at the University of Florence, Italy explained the work to help and answer the question of what the other part of universe is made of. By suggesting that the answer may lie in modifications to gravity rather than new particles.
Laser Interferometer Gravitational-Wave Observatory
“In our work, we ask what signals we could expect from a modification of gravity. It turns out that bigravity features a unique such signal and can therefore discriminated from other theories. The recent detection of gravitational waves by LIGO [Laser Interferometer Gravitational-Wave Observatory] has opened a new window on the dark sectors of the universe for us. Whether Nature has chosen general relativity, bigravity, or any other theory is a different question in the end. We can only study possible signals for experimentalists to look for.”
Currently, the best theory of gravity is Einstein’s theory of general relativity, which uses a single metric to describe space-time. As a result, gravitational interactions mediated by a single hypothetical particle called Graviton. A mass less and so travels at the speed of light.
The main difference between general relativity and bigravity is that bigravity uses two metrics, g and f. Whereas g is a physical metric and couples to matter. And f is a sterile metric and does not couple to matter. In bigravity gravitational interactions are mediated by two gravitons. One of which has mass and the other of which is massless. The two gravitons are composed of different combinations or superposition of the g and f metrics. So they couple to the surrounding matter in different ways. The existence of two metrics n the bigravity framework eventually leads to the oscillation phenomenon.
As the physicists explain, the idea that there might exist a graviton with mass has been around since almost as long general relativity itself.
The physicists show framework of bigravity as gravitational waves produced and propagate through space. They oscillate between the g- and f-types but only g-type detected. Although previous research has suggested that these oscillations might exist. It appeared to lead to unphysical results, such as a violation of energy conservation. The new study shows that the oscillations can theoretically emerge in a realistic physical scenario when considering graviton masses. These are large enough detected by current astrophysical tests.
In order to understand these oscillations, the scientists explain that in many ways they resemble neutrino oscillations. Although neutrinos come in three flavors (electron, muon, and tau), typically the neutrinos produces in nuclear reactions are electron neutrinos (or electron anti-neutrinos) because the others are too heavy to form stable matter. In a similar way, in bigravity only the g metric couples to matter, so the gravitational waves produced by astrophysical events, such as black hole mergers, are g-type since f-type gravitational waves do not couple to matter.
“The key to understand oscillation phenomenon is electron neutrinos do not have a definite mass they are superposition of the three neutrino mass eigenstates,” Platscher explained. “More mathematically speaking, the mass matrix is not diagonal in the flavor (electron-muon-tau) basis. Therefore, the wave equation that describes how they move through space will mix them up and therefore they ‘oscillate.’
Schrödinger wave equation
The same is true in bigravity g is a mixture of the massive and the massless graviton. Therefore as the gravitational wave travels through the Universe, it will oscillate between g- and f-type gravitational waves. However,only measure the former with our detectors the latter would pass through us unseen. if bigravity is a correct description of Nature, leave an important imprint in the gravitational wave signal, as we have shown.
The similarity between neutrinos and gravitational waves hold even though neutrino oscillation is quantum mechanical phenomenon. Described by the Schrödinger wave equation, whereas gravitational wave oscillation is not a quantum effect. Instead described by a classical wave equation.
One particular effect that the physicists predict is that gravitational wave oscillations lead to larger strain modulations compared to those predicted by general relativity. These results suggest a path toward experimentally detecting gravitational wave oscillations and finding support for bigravity.