The authors argue in a paper published in The Astrophysical Journal Letters that a signal - known as cosmic graviton background (CGB) - is feasibly detectable, and could disprove the Big Bang.
Cosmic inflation is the exponential expansion of space-time in the infancy of the universe. It is what scientists mean when they speak of the Big Bang. But what if we were to rule it out? What if we could do this without even making assumptions? A clear, unambiguous signal in the cosmos could eliminate inflation as a possibility. This is according to astrophysicists at the University of Cambridge, the University of Trento, and Harvard University. The authors argue in a paper published in The Astrophysical Journal Letters that this signal – known as cosmic graviton background (CGB) – is feasibly detectable, yet technical and scientific challenges remain.
According to the paper’s first author, Dr. Sunny Vagnozzi, of Cambridge’s Kavli Institute for Cosmology, now at Trento University, inflation explains various fine-tuning challenges of the so-called hot Big Bang model. Furthermore, quantum fluctuations explain the origins of structure in our Universe. “However, the large flexibility displayed by possible models for cosmic inflation, which span an unlimited landscape of cosmological outcomes, raises concerns that cosmic inflation is not falsifiable. Even if individual inflationary models can be ruled out. Is it possible in principle to test cosmic inflation in a model-independent way?”
Is Our Universe The Result of a Previous One?
As the Planck satellite first measured the cosmic microwave background (CMB), known as the oldest light source in the universe, some scientists questioned the possibility of cosmic inflation. The results of Planck were held up as proof of cosmic inflation at the time of their announcement, according to Professor Avi Loeb, Vagnozzi’s co-author. There is a possibility, however, that the results may, in fact, prove the opposite.
In addition to Anna Ijjas and Paul Steinhardt, Loeb believed Planck’s results showed that inflation posed more problems than it solved. This meant that a radically different theory about the origin of the universe had to be considered. For example, perhaps the universe began not as a result of a bang. Perhaps it began as a result of a bounce from an earlier contracting universe. Planck released maps of the CMB representing 100 million years before the first stars formed, the earliest time in the universe we are able to see. It is impossible for us to see further.
According to Loeb, the edge of the universe is at the distance at which a signal traveling at the speed of light could have traveled in 13.8 billion years since the universe was born. Currently, the edge of the universe is located 46.5 billion light years away because of the expansion of the universe. Our spherical volume within this boundary is like an archaeological dig in which we discover layers of cosmic history that extend all the way back to the Big Bang. This is the ultimate horizon of our universe. It is impossible to predict what lies beyond the horizon,” explained Loeb.
The universe’s origins
It may be possible to gain new insight into the universe’s origins by studying neutrinos, which account for the majority of the universe’s mass. The temperature of the universe at ten billion degrees allowed neutrinos to travel freely without scattering shortly after the Big Bang. “The present-day universe must be filled with relics of neutrinos from that time,” said Vagnozzi. By tracing gravitons, particles that mediate the force of gravity, Vagnozzi, and Loeb say we can go even further back.
“The Universe was transparent to gravitons all the way back to the earliest instant traced by known physics. It is called 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.” When gravitons could travel freely through the Universe without scattering, Vagnozzi and Loeb think that a relic background of thermal gravitational radiation with a temperature a little less than one degree above absolute zero should have formed: the cosmic graviton background (CGB).
The Big Bang and OGB
The Big Bang theory, however, does not concur with the CGB’s existence. Due to exponential inflation, relics like the CGB have been diluted to the point that they are undetectable. By detecting the CGB, it could be proven that cosmic inflation does not exist, therefore ruling it out. It is possible to do such a test in the future, and the CGB can, in theory, be detected, according to Vagnozzi and Loeb. In addition to microwave and neutrino radiation backgrounds, the CGB contributes to the cosmic radiation budget. Next-generation cosmological probes could provide the first indirect measurement of the CGB. This would be done by detecting its effect on the cosmic expansion rate of the early Universe.
If a background of high-frequency gravitational waves peaks at frequencies around 100 GHz is detected, it would be the ‘smoking gun’ for detecting the CGB. The detection of this would be extremely difficult and would require substantial technological advances in gyrotron and superconducting magnets. Even so, future research may be able to identify this signal, say the researchers.