In a recent interview, I was presented with a hypothetical dilemma of choosing between only two options: communicate science or do science. Which option would I choose? I promptly responded: “do science.” I get pleasure from finding things out, in the words of Richard Feynman. Communicating my scientific work to others is the icing on the cake. You can easily tell if a science communicator is authentically doing science by checking whether they published a single original scientific paper over the past decade. Today, for example, I completed three new scientific papers. I enjoy being in the trenches of science and expanding the frontiers of knowledge. I do not care how many “likes” I get.
The first, and perhaps most significant paper among today’s three papers, was written in collaboration with the brilliant Harvard postdoc, Fabio Pacucci. Our new paper explains the compact galaxies discovered in recent years by the Webb telescope, named Little Red Dots (LRDs). These are early galaxies, observed at cosmological redshifts of 4–8, which are characterized by small sizes of order a few hundred light years. Their abundances are intermediate between typical galaxies and quasars. In 2024, I published a research note explaining LRDs based on an idea for the origin of quasar black holes that I proposed thirty years earlier in a paper with my first PhD student, Daniel Eisenstein. Following that, Fabio and I present a comprehensive theoretical model in which LRDs descend from dark matter halos in the extreme low-spin tail of the distribution of galaxies. As galaxies collapse out of over-dense regions in the Universe, their matter gets spun up by the torque exerted on it by its large-scale environment. Galaxies born in low-torque regions spin less. As the gas in them cools, it forms a compact disk which could be as small as the size of LRDs. Within this framework, Fabio and I explain three key observational signatures of LRDs, namely their abundance, their compactness, and their redshift distribution. Our model focuses on observed, not modeled, properties, and is therefore independent of whether LRDs are powered primarily by a central black hole or stars. The inference that the prototypical LRD at redshift 5 originates from halos in the lowest percentile of the spin distribution is sufficient to reproduce both their observed number per unit volume and their physical sizes. The redshift evolution of their observability is driven by the interplay between the evolving fraction of compact disks and the cosmological dimming of their surface brightness. This leads to a well-defined “LRDs Era” at redshifts between 4 and 8, during which the LRDs are common and detectable. At late cosmic time and redshifts below 4 LRDs are bright but rare, whereas at early cosmic times with redshift above 8, they are common but faint. Comparing the predicted redshift trend with observational data yields excellent agreement. Additional observational support comes from the excess small-scale clustering of LRDs and the spectral signatures of their extreme core densities, both of which are expected outcomes of galaxy formation in low-spin halos. Our findings suggest that LRDs are not a fundamentally distinct population but the natural manifestation of galaxies forming in the rarest, lowest spin environments.
The second paper, with the amazing Vanderbilt graduate student, Oem Trivedi, explores the possibility that dark matter is composed of Planck-mass remnants, each with the mass of a dust particle, about 10 micrograms. Oem and I studied the end state of gravitational collapse taking account of quantum gravity effects and proposed that Planck Star Remnants (PSR), formed through nonsingular bounces, could serve as viable dark matter candidates. Within the framework of Loop Quantum Cosmology, Oem and I modeled the collapse of a homogeneous matter distribution and show that the classical singularity is replaced by a quantum bounce at the Planck density of 10^{93} grams per cubic centimeter, where quantum mechanics is expected to affect gravity. By analytically matching the collapsing interior to the exterior spacetime using the so-called Israel junction conditions after the physicist Werner Israel, we demonstrated that the bounce remains causally hidden from external observers, avoiding any observable re-expansion. This naturally leads to the formation of stable, non-radiating PSR, whose radius coincides with the Schwarzschild radius when the black hole mass approaches the Planck mass as a result of Hawking evaporation. We suggest that such remnants may originate from evaporating primordial black holes in the early universe, and estimate the relic abundance needed for PSR to account for the observed dark matter density. The scenario is shown to be consistent with existing astrophysical and cosmological constraints, offering a unified framework connecting quantum gravitational collapse, primordial black hole evaporation, and the nature of dark matter.
Finally, my third paper today, with the outstanding Harvard College student, Shokhruz Kakharov, explores extragalactic dark matter. Shokhruz and I calculated the contribution of extragalactic dark matter to the local dark matter density and flux in the Milky Way. By analyzing the Galactic escape velocity as a function of direction, we established the criterion for separating dark matter particles bound to the Milky Way from those originating outside the Milky Way. Our analysis reveals that approximately a quarter of the dark matter particles in the Solar neighborhood have an extragalactic origin, contributing nearly 38% of the total mass flux of dark matter. The directional dependence of this extragalactic component shows significant anisotropy across the sky, with implications for direct detection experiments of dark matter. We provide quantitative predictions for detection rates and signatures that could help identify the extragalactic dark matter component in current and future experiments.
In addition to these three paper, a new paper of the Galileo Project research team under my leadership was posted publicly today. It describes the architecture of the three Galileo Observatories under construction.
There is no greater joy than expanding the territories of knowledge from the trenches of science. The hard-to-concur frontiers of science expand the land of the territories of knowledge in the vast jungle of ignorance that surrounds it. Communicating this mission to the public is as rewarding as writing letters to loved ones at home from the trenches of a battlefield. The pouring support and supplies that we scientists get, are received with much gratitude, especially in these days of budget cuts. Science funding is invaluable for securing our success at the front. And of course, there are always news reporters who are engaged in talking about what active scientists do, while at the same time monitoring the number of “likes” they get on social media. They are the birds that sing and thrive in the new territories of knowledge that scientists work so hard to expand.
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.
