What if the universe had no beginning, and time stretched back infinitely without a big bang to start with.
That’s one possible consequence of an idea called “rainbow gravity,” named because it supposes that gravity’s effects on spacetime are felt differently by different wavelengths of light, in other words different colours in the rainbow.
The idea is not a complete theory for describing quantum effects on gravity, and is not widely accepted. Nevertheless, physicists have now applied the concept to the question of how the universe began, and found that if rainbow gravity is correct, spacetime may have a drastically different origin story than the widely documented and accepted story of the big bang.
According to Einstein’s general relativity, massive objects warp spacetime so that anything traveling through it, including light, takes a curved path.
Standard physics says this path shouldn’t depend on the energy of the particles moving through spacetime, however in rainbow gravity, 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.
The colour of light is determined by its frequency, and because different frequencies agree to different energies, photons of different colours would journey on slightly different paths through spacetime, according to their energy.
The effects would usually be extremely small, so that we wouldn’t notice the difference in most observations of stars, galaxies and also other cosmic phenomena. But with extreme energies, in the case of particles emitted by stellar explosions called gamma-ray bursts, for example, the change might be detectable. In such situations photons of different wavelengths released by the same gamma-ray burst would reach Earth at slightly different times, after traveling somewhat altered courses through billions of light-years of time and space.
The extreme energies needed to bring out strong results from rainbow gravity, although rare now, were dominant in the dense early universe, and could mean things got started in a completely different fashion than we think.