What Would Have Happened to the Dinosaurs if the Chicxulub Impactor Was a Black Hole?
Sixty-six million years ago, a 10-kilometer asteroid, the size of Manhattan Island, impacted the Earth and raised a cloud of dust that changed the climate and killed the dinosaurs. The scar left as the Chicxulub crater under the Yucatán Peninsula in Mexico, measuring 180 kilometers in diameter and 20 kilometers in depth, provides the `smoking gun’ for this catastrophic impact. Recent evidence suggests a second impact related to the dinosaur extinction event at the Nadir Crater offshore West Africa. This new finding is consistent with the breakup of a bigger comet, proposed a year earlier in a Nature paper that I wrote with Amir Siraj.
The Chicxulub blast lifted material weighing tens of trillions of tons, igniting wildfires and triggering death and burial of many lifeforms under the debris. Sunlight was blocked from reaching the Earth’s surface for over a decade, and the excessive production of carbon dioxide through the destruction of carbonate rocks triggered a sudden greenhouse effect, akin to the futuristic horror of climate change activists.
The Chicxulub impactor was no doubt made of solar system materials. But we also know that most of the matter in the Universe is composed of a different substance. Our ignorance about its nature stems from the fact that it only interacts gravitationally with ordinary matter. Hence, it is labeled as dark matter. It was first recognized by the astronomer Fritz Zwicky, who noticed that galaxies are moving too fast relative to each other to be bound by visible matter in clusters, implying that invisible matter must bind them.
The simplest form of dark matter is primordial black holes, produced in the very early universe. This association is ruled out for all black hole masses except a window of a few orders of magnitude around the mass of the Chicxulub impactor. Interestingly, if the dark matter is made of primordial black holes comparable in mass to the Chicxulub impactor, ten trillion tons, then the Earth could be impacted once during its lifespan by one of them. And if primordial black holes were the mass of a kilometer-sized asteroid, then the last impact by a dark-matter black-hole would have been around the same time as the Chicxulub event.
What would have been the consequence of an impact by a Chicxulub-mass primordial black hole? Most importantly, we should keep in mind that the characteristic speed of the dark matter in the Milky Way halo is ten times higher than the typical speed of space rocks near Earth.
As a result of its high speed, the black hole would have traversed the entire Earth in less than a minute. Because of the short time available, its gravitational pull would have shifted terrestrial material by the thickness of a human hair — a minuscule amount too small to trigger a noticeable Earthquake in the ground or a tsunami in the oceans. The total gravitational energy deposited in the Earth’s atmosphere would have been equivalent to 10 kilogram of TNT, similar to the energy deposited by a hundred gram asteroid which is 17 orders of magnitude less massive than Chicxulub. Moreover, Hawking evaporation of the black hole would have released less energy than a ten-Watt light bulb. All dinosaurs, except an unlucky creature along the path of the black hole, would have ignored the impact and moved on with their daily routines.
This brings home the sober realization that dark matter has very limited impact on terrestrial life because it interacts with ordinary matter only gravitationally.
However, we should also keep in mind the often-ignored fact that life-as-we-know-it would not be possible without dark matter. The primordial density perturbations on the scales of galaxies would have been washed out for ordinary matter. This is the inevitable result of the coupling of ordinary matter to the cosmic radiation in the first 400,000 years after the Big Bang. The cosmic radiation diffused on small scales and smoothed out the galaxy-sized inhomogeneities in the ordinary matter that coupled to it.
The only reason that galaxies actually formed later on is that the dark matter, constituting 84% of the matter in the Universe, did not couple to the radiation and maintained memory of the primordial inhomogeneities on a small scale. Its gravity eventually pulled together matter on galactic scales and resulted in the formation of the first galaxies and stars hundreds of millions of years after the Big Bang, as explained in two textbooks that I published with Princeton University Press in 2010 and 2013. Traces of the earliest galaxies are now observed in the deepest images obtained from the Webb Space Telescope, which tentatively reached a redshift of 17 when the Universe was merely 230 million years old.
Eliminating dark matter but keeping all other cosmological parameters unchanged, would have made our Universe incapable of producing galaxies like the Milky Way inside of which stars like the Sun and planets like the Earth form. Life-as-we-know-it would not be possible since the synthesis of elements lasted only a few minutes during the expanding hot phase following the Big Bang, an insufficient period of time to produce significant amounts of carbon or oxygen on which organic chemistry in liquid water is based.
Altogether, we should feel like children who lost contact with their parents after leaving home a long time ago. We owe our existence to dark matter but its impact on our daily life is negligible.
If we ever encounter extraterrestrial siblings, we could ask them whether they figured out the nature of dark matter. We might be surprised by the answer, especially if their vehicles are propelled by this abundant cosmic fuel without leaving any visible trace on the environment. This would constitute the ideal embodiment of clean energy, akin to the desires of climate change activists.
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. His new book, titled “Interstellar”, is scheduled for publication in August 2023.
