It’s one of the most alluring but hard to prove (and unlikely) theories: a Rainbow Universe. A Universe in which time simply stretches back indefinitely, without a big bang to start things off, simply with no beginning – something pretty hard to fathom.

Image credits: cherrychill

The name comes from the so-called Rainbow Gravity; basically gravity’s effects on spacetime are felt differently by different wavelengths of light (all the colors in the rainbow). Rainbow gravity was first proposed 10 years ago in an attempt to unify quantum mechanics and general relativity. However, it can’t explain all the quantum effects on gravity, and it gained little popularity since it was developed.

According to Einstein’s general relativity, gravity is not really a force, like in Newtonian physics. Basically, objects with a large mass warp spacetime so that anything traveling through it, including light, takes a curving path – and this is gravity. In standard physics, the path shouldn’t depend on the energy of the particles, but in a Rainbow Universe, it does.

“Particles with different energies will actually see different spacetimes, different gravitational fields,” says Adel Awad of the Center for Theoretical Physics at Zewail City of Science and Technology in Egypt, who led the new research, published in October in the Journal of Cosmology and Astroparticle Physics.

The color of the light is determined by its frequency; different frequencies have different energies, and therefore will be affected differently by gravity. The effects would be very small on a cosmic scale, but they become quite important when dealing with particles emitted by stellar explosions called gamma-ray bursts, for instance. But this theory is still largely disregarded.

“So far we have no conclusive evidence that this is going on,” says Giovanni Amelino-Camelia, a physicist at the Sapienza University of Rome who has researched the possibility of such signals.

It has to be kept in mind that one of the biggest supporters of a Rainbow Universe is Lee Smolin. He’s one of the most “creative” theoretical physicists of our time, but many physicists disregard many of his theories because they’re usually scant on details and provide little opportunities to falsify his claims.

“It’s a model that I do not believe has anything to do with reality,” says Sabine Hossenfelder of the Nordic Institute for Theoretical Physics. This idea is not the only way to do away with the big bang singularity, she adds. “The problem isn’t to remove the singularity, the problem is to modify general relativity in a consistent way, so that one still reproduces all its achievements and that of the Standard Model [of particle physics] in addition.”

Smolin says that in his mind, the idea rainbow gravity has been subsumed in a larger idea called relative locality. The mind boggling idea of relative locality is that observers in different locations across spacetime will see it happening in different places — in other words, location is relative.

“Relative locality is a deeper way of understanding the same idea” as rainbow gravity, Smolin says. The new paper by Awad and his colleagues “is interesting,” he adds, “but before really believing the result, I would want to redo it within the framework of relative locality. There are going to be problems with locality the way it’s written that the authors might not be aware of.”

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