A Test of Cosmic Inflation and Quantum Gravity
The cosmic microwave background, relic from the Big Bang, accounts for a percent of the static noise visible as “snow” in old-fashioned television sets. The photosphere that last-scattered this radiation 400,000 years after the Big Bang, had a temperature of 3000 degrees Kelvin, half of the Sun’s effective temperature. The spherical last-scattering surface around us marks the boundary of the transparent volume of the observable Universe. We cannot see farther. Gladly, the first stars formed about a hundred million years later. The Webb telescope will search for them through its longest exposures.
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 Big Bang. 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 centered 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.
Within the thin spherical shell between the Big Bang and the microwave background photosphere, the Universe was opaque to light. Nevertheless, we can probe into this layer. Neutrinos have a weak cross-section for interactions, and so the Universe was transparent to them back to 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.
The expansion of the Universe cooled the neutrino background to a present-time temperature of 1.95 degrees above absolute zero, comparable to the 2.73 degrees of the cosmic microwave background.
Can we probe even deeper into our cosmic archaeological dig? In principle, yes. Gravitational radiation has weaker interaction than neutrinos. So much so, that 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. A proper understanding of the Planck epoch requires a predictive theory of quantum gravity, which we do not possess.
If gravitational radiation was thermalized at the Planck time, should there be a relic background of thermal gravitational radiation with a temperature of about one degree above absolute zero?
Not so, according to the popular theory of cosmic inflation, which suggests that the Universe went through a subsequent phase of exponential expansion that diluted all earlier relics to undetectable levels. Inflation was theorized to explain various fine-tuning challenges of the Big Bang model. It also explains the origin of structure in our Universe as a result of quantum fluctuations. However, the large flexibility displayed by numerous possible inflationary models raises concerns that the inflationary paradigm as a whole is not falsifiable, even if individual models of it can be ruled out. Is it possible in principle to test the entire inflationary paradigm in a model-independent way?
In our new paper, Sunny Vagnozzi, a brilliant postdoc from the University of Cambridge in the UK, and I showed that future detectors could potentially discover the one-degree gravitational wave background, if it exists. This cosmic graviton background adds to the cosmic radiation budget, which otherwise includes microwave and neutrino backgrounds. It therefore affects the cosmic expansion rate of the early Universe at a level that might be detectable by the next generation of cosmological probes.
A discovery of the thermal graviton background holds the potential of ruling out the inflationary paradigm and bringing us back to the drawing board of how the Universe began. Given that the interiors of black holes are hidden from view and are risky to venture into, the early Universe might represent our best opportunity for testing predictive theories of quantum gravity.
There is no reason to assume that our cosmic roots started at the Planck time. Albert Einstein was inclined to think that our past timeline should have no beginning. But to his dismay, he later realized that the equations of the General Theory of Relativity do not admit a stable static solution, and moreover — the actual Universe appears to be expanding. His philosophical preference for no beginning might be validated by the ultimate theory to combine General Relativity with Quantum Mechanics, which could explain what predated the Big Bang.
A predictive theory of quantum gravity could rid us of the Big Bang singularity. But just as with the development of quantum mechanics, we might need guidance from experimental data or else we will have too many possible theoretical scenarios.
Here’s hoping that the new paper I wrote with Sunny will lead to progress by eliminating theoretical possibilities. Scientific knowledge encapsulates what actually exists in the cosmos out of the many possibilities that could have existed. Sometimes, the most beautiful possibilities are ruled-out, even if they were conceived by the most brilliant scientists on planet Earth, like Albert Einstein. As known from social media or politics, beauty and truth are not necessarily the same.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021.