Speaking at the Central Library of Aristotle University

Avi Loeb
5 min readJun 4, 2024


Marble bust of Aristotle (Image credit: Wikimedia)

This morning, I had the privilege of delivering the opening lecture in the conference titled: “Standard Cosmology at the Threshold of Change,” held at the Central Library of Aristotle University in Thessaloniki. This occasion is particularly significant for me, since Aristotle (384–322BC) — arguably the most influential philosopher of all times, argued incorrectly that the Earth is at the center of the Universe, and for a millennium everyone believed him.

Aristotle also claimed that “The horse has forty teeth,” whereas in fact mature stallions have 40–44 and mature mares have 36–40 teeth. A millennium later, the revolution of Nicolaus Copernicus (1473–1543), who as a priest did not intend to “rock the boat,” was to attend to empirical evidence in figuring out reality. He worked out a better model for timing Easter by placing the Sun at the center of the Solar system, and the data favored a heliocentric over a geocentric worldview.

We do not know whether extra dimensions, as advocated by string theorists today, are in the category of unrealistic Aristotelian claims. Without experimental testing that could have proven a physical theory wrong but did not, there is no justification for the belief that it describes reality. If a physical theory is argued to be right irrespective of evidence, then it does not carry new knowledge. Beautiful mathematical ideas cannot be regarded as a description of nature unless they pass successfully a reality check that could have proven their predictions wrong.

A few months ago, I was invited to give a keynote lecture in an official celebration of 550 years to the birth of Copernicus at his birthplace, Toruń, Poland. The title of my lecture was: “The Next Copernican Revolution,” concerning my search for superhuman intelligence in interstellar space.

But this was not the content of my lecture at Aristotle University. There, the title of my lecture was: “Cosmological Insights into Quantum Gravity.” I started by discussing the most fundamental unknown about our Universe: what happened before the Big Bang? Could it be that our Universe was created intentionally in the laboratory of a quantum-gravity scientist? Aristotle would have surely been interested in studying this interpretation of God in his theology.

Next, I discussed my paper with Sunny Vagnozzi on the possibility of ruling-out a beginning from an early period of superluminal cosmic expansion known as cosmic inflation, by detecting a thermal background of gravitational waves with a temperature of about 1 degree Kelvin above absolute zero. Such a background would have been diluted if inflation took place, and so detecting it offers a direct path to falsifying a model that most cosmologists believe in.

Next, I discussed my recent paper with Mark Hertzberg, which showed that the beginning of the Universe, less than a second after the Big Bang, might have had a needle-like geometry rather than a spherically-symmetric configuration. Evidence for this unexpected beginning can be gathered by future observations of neutrinos, dark matter or gravitational waves from early cosmic times.

Subsequently, I discussed quantum mechanical effects for various types of dark matter or forms of modified gravity. It included a recent paper with Mark Hertzberg where we constrained the mass of lightweight dark matter particles based on quantum tunneling from dwarf galaxies, and two papers where we ruled out fluctuating gravity as a substitute for Einstein’s gravity or dark matter, and a paper where I discussed the implications of modified inertia for rocket propulsion.

One of the most puzzling mysteries of cosmology is that the value of the cosmological constant (dark energy) is 122 orders of magnitude smaller than the natural Planck scale. I explained this small value in terms of the electron mass and charge based on a recent Physical Review paper that I published with Mark Hertzberg. If this explanation is correct, then the cosmological constant must not evolve with time. This prediction can be tested by upcoming data from the DESI survey.

Aside from the Big Bang singularity, Einstein’s theory of gravity also breaks down near the center of black holes — where the curvature of spacetime diverges. I discussed the possibility that matter collects there in the form of a Planck density object. The only way to find out the whereabouts of matter near a black hole singularity is to enter into the black hole horizon. When I suggested to string theorists in 2018 that they could test their theory by entering a black hole, Nima Arkani-Hamed blamed me for having ulterior motives for sending them there.

Finally, I considered the possibility of creating a time machine with the aid of negative masses. The lack of visitors from our future can be used to argue that the dark energy cannot be concentrated into an enclosure to make an object with a negative mass. Otherwise, extraterrestrial quantum-gravity engineers could have traveled before the Big Bang and prevented the Universe that contains them from being born — a logical inconsistency.

Too bad. If we only had access to a time machine, I would have loved to bring Aristotle back to life and debate with him on all these cosmological questions today at Thessaloniki.


(Image credit: Chris Michel, October 2023)

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. His new book, titled “Interstellar”, was published in August 2023.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".