To Bounce or not to Bounce, that is the Question!
During yesterday’s colloquium at Harvard’s Black Hole Initiative for which I served as the founding director, the brilliant Karim Thebault described a bouncing solution that avoids a black hole singularity. As infalling matter approaches the center of the black hole, it bounces to a white hole and then again collapses to a black hole in a cyclic process that never ends, hidden from view by observers outside the event horizon.
At the end of Karim’s talk, I pointed out that if the bounce occurs on a large enough scale, then the spaghettification of an infalling observer might be avoided in the case of a supermassive black hole, like the four-million solar mass Sagittarius A* at the center of the Milky-Way. This would offer an “acid test” for the conviction of those who proposed this model. If they truly believe the validity of this variant of their model, they should be happy to board a spacecraft that will enter the event horizon of Sagittarius A* and enjoy the bounces that it entails. Admittedly, they will not be able to publish a paper about their experience in a scientific journal outside the event horizon, but bouncing forever inside a black hole should be more fun than bouncing in any amusement park on Earth. In short, my question to these physicists is a paraphrase on Hamlet’s question: “To bounce or not to bounce, that is the question!”
Since we do not have a predictive theory of quantum gravity, nor any data on the interior of a black hole, we do not know if this proposed model is correct. For the same reason, we do not know whether the Big Bang singularity originated from a bounce of a contracting cosmological phase. If it was, then perhaps the universe will bounce between contraction and expansion forever.
Whereas it is impossible to get data on the interior of a black hole horizon, we could potentially observe what happened close to the Big Bang by detecting gravitational radiation that originated at that time. If cosmic inflation did not occur, this radiation would be characterized by frequencies similar to the cosmic microwave background, as I pointed out in a paper with Sunny Vagnozzi.
Figuring out what happened near the Big Bang could be far more challenging than we currently imagine. The commonly adopted simplifications of spherical symmetry, homogeneity and isotropy may have not been satisfied during the early history of the cosmos. An initial anisotropy could have been erased in the later history of the Universe, as we showed in a paper that I wrote with Mark Hertzberg recently.
A few hours after Karim’s colloquium, I gave a lecture to a class of first-year students of Harvard College, during which I explained that what preceded the cycle of chickens and eggs on Earth was not a chicken, nor an egg, but something else. We know that because the Earth has a finite age of 4.54 billion years and the interstellar material that made the solar system was diffuse, with a characteristic mass density that is 24 orders of magnitude smaller than that of a chicken or an egg. The same reasoning, of course, could also apply to the origin of the Universe. Rather than being the result of an infinite cycle of bounces, it might have had an early phase that was completely different.
An hour later, I attended dinner at Harvard’s Society of Fellows, where I was sitting next to a few historians, who were not fully aware of the fundamental differences between cosmological history and documented human history. The period of recent human history that they study constitutes a part in a hundred million of the age of the Universe. In addition, cosmological history is based on measurements by instruments, such as detectors that map to exquisite precision the cosmic microwave background — last scattered 400,000 years after the Big Bang, or the telescopes that map the distribution of visible matter throughout the entire period starting from a few hundred million years after the Big Bang to the present-day. In contrast to history books which are based on reports by people, scientific knowledge is based on data from instruments. Human history depends on who writes it whereas cosmic history is based on detectors’ data that substantiates it. If Hitler would have won World War II, we would have had a different perspective about it because the winning side tends to conceal bad traits and take pride in successes when writing history. On the other hand, studying the Universe brings a sense of humility to human existence because the story spans vast scales of space and time, long before humans, chickens or eggs existed.
The primary privilege of a cosmologist involves the opportunity to observe directly what happened in the early Universe by imaging sources at great distances. This is equivalent to a scholar of human history traveling through a time machine. The secondary privilege of a scientist is the allowance to answer questions that cannot be addressed right now by direct observations, such as “What happened before the Big Bang?” or “What lies inside a black hole?”, with the answer: “We do not know”.
Of course, there is also the human bias of which scientific data to collect and which scientific anomalies to pay attention to. Science is done by humans and humans decide which parts of the unknown they wish to explore.
During a two-hour podcast interview on the same busy day, I explained my dismay at the fact that the scientific mainstream does not invest billions of dollars in the search for technological signatures of intelligent aliens, given the huge significance of this question for the future of humanity. Future historians of science will have to explain this blind spot in our observations of the Universe. There are billions of Earth-Sun analogs in our cosmic neighborhood and the mainstream regards the possibility that they might host residents like us as an extraordinary claim. Just like those who document human history, human scientists decide which parts of cosmic history they wish to study and which parts they choose to conceal. Here’s hoping that alien historians keep a less self-centered record of the cosmos, so that we can learn from them the full history of intelligent civilizations since the Big Bang, 13.8 billion years ago.
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 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. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
