The seeds of galaxies were planted in the infant Universe as small perturbations in the mean density of matter and radiation. These seeds stemmed, most likely, from quantum mechanical fluctuations in an unknown primordial field, which dominated the cosmic mass budget during a tiny fraction of a second after the Big Bang.
The seeds grew over time thanks to the attractive nature of gravity. On average, the Universe is expanding but regions denser than average are gravitationally bound and their expansion initially slows down, eventually stops and turns around to a local Big Crunch, leading to the formation of stable objects like galaxies.
The region that collapsed to make the Milky-Way contained about a trillion solar masses, 85% of which was dark matter and 15% of which was ordinary matter that we are made of. Since life on Earth revolves around encounters with people and material objects made of ordinary matter, it is common to consider dark matter as esoteric and irrelevant to our daily life. But in reality, the Sun, the Earth and life-as-we-know-it, would never exist without dark matter. Let me explain.
The matter that made the Milky Way originated from a spatial scale of about six million light-years in the present-day Universe. Accounting for cosmic expansion, the region was smaller and the density of matter in it was larger at earlier times. Initially, the deviation of the density within this region relative to the cosmic mean value was small. But over a few billion years, the region turned around and subsequently collapsed to make the Milky-Way galaxy.
This sequence of events would not occur without dark matter. If the Universe contained only ordinary matter, then the initial density perturbation of the Milky-Way would have been washed out. This is because ordinary matter was tightly coupled to radiation for the first 400,000 years after the Big Bang. At much earlier cosmic times, radiation dominated the cosmic mass budget and suppressed the gravitational growth of perturbations since the speed of light greatly exceeds the escape speed from the gravitational potential wells of the perturbations. But as soon as the Universe cooled below 3,000 degrees Kelvin and neutral hydrogen atoms formed out of free electrons and protons, the ordinary matter decoupled from the radiation. During this decoupling process, the density perturbations of ordinary matter were washed out by the diffusion of photons that smoothed out the variation in the density of ordinary matter on the scale of galaxies.
However, dark matter was not coupled to the radiation and so it maintained memory of the primordial density perturbations on the scale of galaxies. These started to grow by their self-gravity as soon as matter dominated the cosmic mass budget, about 50,000 years after the Big Bang. When hydrogen atoms formed, the ordinary matter became electrically neutral and was not chained any more to the radiation, in the same way that the charged electrons and protons were at earlier cosmic times. Subsequently, the neutral ordinary matter was free to fall into the gravitational potential wells of dark matter and get incorporated into the regions that ultimately collapsed into galaxies over the subsequent billions of years.
Let me be clear: if dark matter did not exist, the Milky Way — with stars like the Sun and planets like the Earth, would never form. Terrestrial life blossomed because dark-matter maintained memory of the primordial perturbations on the scale of the Milky-Way. We owe our existence to dark matter.
During dinner this week at the American Academy of Arts and Sciences, the brilliant mathematician Mark Kisin asked me whether the latest discoveries of the first generation of galaxies by the Webb Telescope rule out the standard cosmological model. I responded that the observed discrepancies can be explained away by tweaks in models of early star formation, which are far more uncertain than the details of the background cosmological model. Mark observed: “Physics models are a form of art.” I nodded and added: “Our models often involve assumptions and approximations, as they aim to capture the essence of the physical reality, just like the brush strokes of a painter who cannot compete with the resolution of reality or even that of a photograph.
Even the biggest computers at our disposal have limited resolution and must incorporate simplifying assumptions on sub-grid physics. The only way for us to acquire God’s resolution of the cosmos, is to figure out a recipe for making a baby universe and create it in the laboratory. The experience of giving birth to a Universe will be far richer than all the books in all the libraries on all the planets that exist in our Universe.”
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.
