Can Cosmic Inflation be Ruled Out?
hubie writes:
Astrophysicists say that cosmic inflation can in principle be ruled out in an assumption-free way:
"The actual edge of the observable universe is at the distance that any signal could have travelled at the speed-of-light limit over the 13.8 billion years that elapsed since the birth of the Universe," said Loeb. "As a result of the expansion of the universe, this edge is currently located 46.5 billion light years away. The spherical volume within this boundary is like an archaeological dig centred on us: the deeper we probe into it, the earlier is the layer of cosmic history that we uncover, all the way back to the Big Bang which represents our ultimate horizon. What lies beyond the horizon is unknown."
It could be possible to dig even further into the universe's beginnings by studying near-weightless particles known as neutrinos, which are the most abundant particles that have mass in the universe. The Universe allows neutrinos to travel freely without scattering from approximately a second after the Big Bang, when the temperature was ten billion degrees. "The present-day universe must be filled with relic neutrinos from that time," said Vagnozzi.
Vagnozzi and Loeb say we can go even further back, however, by tracing gravitons, particles that mediate the force of gravity.
"The Universe was transparent to gravitons all the way back to the earliest instant traced by known physics, the Planck time: 10 to the power of -43 seconds, when the temperature was the highest conceivable: 10 to the power of 32 degrees," said Loeb. "A proper understanding of what came before that requires a predictive theory of quantum gravity, which we do not possess."
Vagnozzi and Loeb say that once the Universe allowed gravitons to travel freely without scattering, a relic background of thermal gravitational radiation with a temperature of slightly less than one degree above absolute zero should have been generated: the cosmic graviton background (CGB).
However, the Big Bang theory does not allow for the existence of the CGB, as it suggests that the exponential inflation of the newborn universe diluted relics such as the CGB to a point that they are undetectable. This can be turned into a test: if the CGB were detected, clearly this would rule out cosmic inflation, which does not allow for its existence.
[...] However, to claim a definitive detection of the CGB, the 'smoking gun' would be the detection of a background of high-frequency gravitational waves peaking at frequencies around 100 GHz. This would be very hard to detect, and would require tremendous technological advances in gyrotron and superconducting magnets technology. Nevertheless, say the researchers, this signal may be within our reach in future.
Journal Reference:
Sunny Vagnozzi and Abraham Loeb The Challenge of Ruling Out Inflation via the Primordial Graviton Background[open] 2022 ApJL 939 L22 DOI: 10.3847/2041-8213/ac9b0e
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