Cambridge and Harvard scientists detail a scenario in which our universe, as we observe it, is simply the result of an earlier cosmological phase ending and a new one beginning.
An international team of astrophysicists says there is a clear and unequivocal signal in the cosmos that could rule out inflation – a theory of the exponential expansion of space in the early universe – as a possibility, suggesting that our understanding of the origins of our universe may need some updates.
In their article, published in The Astrophysical Journal Letters, the European astrophysicist Sunny Vagnozzi, from the University of Trento and the University of Cambridge, together with the Israeli-American researcher Avi Loeb, from Harvard University, argue that this signal – known as the cosmic graviton background (CGB) – can be detected, although it will pose a huge technical and scientific challenge.
“Big Bounce” instead of a Big Bang
Eliminating inflation could mean, according to the researchers, that the universe could have started with a “Big Bounce” instead of a Big Bang. In other words, the cosmos could have come into being after the end of an earlier cosmological phase – a rebound – and not the result of space-time exponentially inflating into existence.
“Inflation was theorized to explain various fine-tuning problems in the so-called hot Big Bang model,” explains Vagnozzi, the paper’s first author. “It also explains the origin of structure in our universe as a result of quantum fluctuations,” he added.
But, as Vagnozzi argues, there is still a chance to prove the theory wrong, despite having been able to rule out “individual inflationary models.” “The great flexibility shown by possible cosmic inflation models, encompassing an unlimited panorama of cosmological results, 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 independently of the model?” Vagnozzi asks.
Confirmation of cosmic inflation?
Concerns about cosmic inflation were raised by some scientists in 2013, when the Planck satellite published its first measurements of the cosmic microwave background (CMB), the oldest light in the universe, according to the Cambridge University press release.
“When the results from the Planck satellite were announced, they were presented as confirmation of cosmic inflation,” Harvard University astronomer Avi Loeb, who also worked on the article, said in the statement. “However, some of us argue that the results could be showing just the opposite,” he added.
Studying the universe moments after the Big Bang
But until we see the universe as it was right after the supposed Big Bang, we won’t know for sure. “The actual edge of the observable universe is as far away as any signal could have traveled at the limiting speed of light in the 13.8 billion years since the birth of the universe,” Loeb said. “As a result of the expansion of the universe, this edge is currently 46.5 billion light-years away,” he added.
“The spherical volume within this boundary is like an archaeological dig centered on us: the deeper we go into it, the earlier the layer of cosmic history we discover, until we reach the Big Bang that represents our final horizon,” he added. “What lies beyond the horizon is unknown.”
In short, we have to dig deeper to study the nature of the universe just after its creation. But even if we did get a glimpse of it, we’d have a hard time predicting what came before. “A proper understanding of what came before requires a predictive theory of quantum gravity, which we don’t possess,” Loeb said.
Neutrinos, almost weightless particles
Despite the difficulties, this reality has not deterred researchers, who believe it might be possible to delve further into the early universe by studying the nearly weightless particles known as neutrinos, which are the most abundant particles with mass in the universe.
This is because the universe has allowed neutrinos to travel freely without scattering since about a second after the Big Bang, when the temperature was ten billion degrees, according to the statement. “The current universe must be full of relict neutrinos from that time,” Vagnozzi said.
Detect CGB and completely rule out the Big Bang theory?
However, Vagnozzi and Loeb say we can go back even further in time by tracking gravitons, hypothetical free-traveling elementary particles that could explain gravitational interactions.
The researchers thus suggest that the cosmic graviton background, or CGB, could have existed just around the creation of the universe. According to the Big Bang theory, the CGB cannot exist, since it would have been diluted –due to the exponential inflation of the universe– to the point of ceasing to be detectable. Therefore, if researchers were to detect it, they could completely rule out the Big Bang theory.
Even so, detecting the CGB would require extremely sophisticated technology of superconducting gyroscopes and magnets that does not yet exist. However, according to the researchers, this signal could be within our reach in the future.
