Can cosmic inflation be ruled out? -SpaceRef

Astrophysicists say cosmic inflation – a point in the infancy of the Universe where spacetime expanded exponentially, and what physicists are really referring to when they talk about the “Big Bang ” – can in principle be excluded without any assumptions.

Astrophysicists from the University of Cambridge, the University of Trento and Harvard University say there is a clear and unambiguous signal in the cosmos that could rule out inflation as a possibility. Their paper, published in The Astrophysical Journal Letters, argues that this signal – known as the cosmic graviton background (CGB) – can be feasibly detected, although it will be a huge technical and scientific challenge. .

“Inflation has been theorized to explain various fine-tuning challenges of the so-called hot Big Bang model,” said the paper’s first author, Dr Sunny Vagnozzi, of the Kavli Institute for Cosmology in Cambridge, and is now based at the University of Trento. “It also explains the origin of the structure of our Universe as a result of quantum fluctuations.

“However, the great flexibility displayed by possible models of cosmic inflation that cover an unlimited landscape of cosmological outcomes raises concerns that cosmic inflation is not falsifiable, even though individual inflationary models can be ruled out. Is it possible in principle to test cosmic inflation model-independently?

Some scientists raised concerns about cosmic inflation in 2013, when the Planck satellite released its first measurements of the cosmic microwave background (CMB), the oldest light in the universe.

“When the results from the Planck satellite were announced, they were presented as confirmation of cosmic inflation,” said Professor Avi Loeb of Harvard University, Vagnozzi’s co-author on the current paper. “However, some of us argued that the results might show just the opposite.”

Along with Anna Ijjas and Paul Steinhardt, Loeb was one of those who argued that Planck’s results showed that inflation posed more puzzles than it solved, and that it was time to consider new ones. ideas about the beginnings of the universe, which, for example. may have started not with a bang but with a rebound of a previously contracting cosmos.

The CMB maps published by Planck represent the earliest time in the universe that we can “see”, 100 million years before the first stars were formed. We can’t see any further.

“The real edge of the observable universe is the distance any signal could have traveled at the limit of the speed of light in the 13.8 billion years since the birth of the universe” , Loeb said. “Due to 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 centered on us: the further we Let’s explore deeply, the earlier the layer of cosmic history we discover, down to the Big Bang which represents our ultimate horizon.What lies beyond the horizon is unknown.

It might be possible to dig even deeper into the early universe by studying nearly weightless particles called neutrinos, which are the most abundant particles that have mass in the universe. The Universe allows neutrinos to travel freely without scattering about a second after the Big Bang, when the temperature was ten billion degrees. “The current universe must be filled with relic neutrinos from that time,” Vagnozzi said.

Vagnozzi and Loeb say we can go back even further, however, by tracing gravitons, particles that mediate the force of gravity.

“The Universe was transparent to gravitons from the first instant traced by known physics, Planck time: 10 to the power of -43 seconds, when the temperature was highest conceivable: 10 to the power of 32 degrees”, said Loeb. “A proper understanding of what preceded this requires a predictive theory of quantum gravity, which we don’t have.”

Vagnozzi and Loeb say that once the Universe allowed gravitons to move freely without scattering, a relict background of thermal gravitational radiation with a temperature just under a degree above absolute zero would have had to be generated: the Cosmic Graviton Background (CGB).

However, the Big Bang theory does not allow for the existence of CGB, as it suggests that the exponential inflation of the newborn universe has diluted relics such as CGB to such an extent that they are undetectable. This can be turned into a test: if CGB were detected, this would clearly exclude cosmic inflation, which does not allow its existence.

Vagnozzi and Loeb argue that such a test is possible and that CGB could in principle be detected in the future. CGB adds to the cosmic radiation budget, which otherwise includes microwave and neutrino backgrounds. It therefore affects the cosmic expansion rate of the early Universe to a level detectable by next-generation cosmological probes, which could provide the first indirect detection of CGB.

However, to claim definitive detection of 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 difficult to detect and would require huge technological advances in gyrotron and superconducting magnet technology. Nevertheless, say the researchers, this signal could be within reach in the future.

The Challenge of Excluding Inflation via the Primordial Graviton Background, The Astrophysical Journal Letters