11 September 2023
Gravity rainbows point to ageless Universe
Published online 12 November 2013
The Universe sprung to life about 13.8 billion years ago when an infinitely dense point — known as a singularity — exploded outwards a familiar event known as the Big Bang. But physicists' understanding of the forces of gravity, first proposed by Einstein in his theory of general relativity, cannot describe such hypothetical singularities or what would cause one to form a universe in the first place.
Adel Awad, a physicist at Zewail City of Science and Technology, Egypt, and colleagues contend that if high-energy light rays of different frequencies travel through the cosmos along different paths, then the Universe may not have began with a singularity but rather have been in existence for an infinitely long time. Their work appears in the Journal of Cosmology and Astroparticle Physics.
The universe is pervaded by a four-dimensional fabric that knits together space and time. This fabric is stretched by large objects, such as stars and planets, creating dips and rolls in space-time that define paths for light rays and particles to follow. In Einstein's picture, light rays will follow the same set of paths, no matter what its frequency.
But other physicists have since proposed a set of alternative "gravity rainbow" models,, in which the journey light takes is dependent on its frequency. "Gravity splits light into colours, just like a prism, in these models," explains Awad. Similarly, matter with different energies also travels along different routes to those predicted by Einstein's equations.
Awad and colleagues have calculated that if certain gravity rainbow models are correct, tracing the path of matter and light in the Universe back 13.8 billion years does not arrive at an infinitely small point of origin. Instead, the universe continues to get smaller further back in time, but shrinks at an exponentially slower rate, never reaching an infinitely dense point. "In mathematical terms, the age of the universe is infinite," says Awad.
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