When I started my career in cosmology forty years ago, I was told by my early mentor John Bahcall that the cosmological model describing the early Universe is simple due to lack of data.
Over the past four decades, cosmologists collected vast amounts of data on the anisotropies of the relic radiation from the Big Bang, the cosmic microwave background, which indicate that the early universe indeed started simple. Statistically, the initial conditions can be summarized on a single sheet of paper.
After Albert Einstein derived in November 1915 the equations describing the gravitational evolution of the Universe, it was realized that the simplest solution to these equations is for a homogeneous and isotropic distribution of matter and radiation. Homogeneity means that the densities of matter and radiation are the same everywhere. Isotropy means that the Universe appears to have the same properties in all directions.
This simple cosmological solution of Einstein’s equations was derived by the Soviet mathematician Alexander Friedmann a century ago in 1922, and independently in 1927 by the Belgian priest and astronomer Georges Lemaître and in 1935–1937 by the American physicist Howard P. Robertson and the British mathematician Arthur Geoffrey Walker. This solution is currently recognized as the Friedmann–Lemaître–Robertson–Walker (FLRW) cosmological model.
The reason this simple model is celebrated today is because it describes the data on the expanding Universe to exquisite precision. On large scales, the universe appears to follow the simple FRLW model, while on small scales inhomogeneities grow by their self-gravity and collapse to make galaxies and clusters of galaxies. The formation of structures out of the primordial inhomogeneities imprinted on the microwave background is not isotropic. The collapse first proceeds along one axis, making a sheet, then a second axis, making a filament and finally along the third axis, making an object like our Milky Way galaxy.
The success of the FLRW model means that cosmic time is well defined and clocks can be synchronized to a high precision throughout the observable universe. In fact, time is ticking at the same rate everywhere to within one part in a hundred thousand. This is why we can learn about our own history by studying galaxies far away with the Webb telescope. What we see at great distances is how the infant universe looked like when light from these ancient galaxies was emitted long ago. Given that the cosmos evolved statistically in the same way everywhere, this educates us about our own history. By peering deep into space we observe the cosmic roots of the Milky Way, inside of which the Sun formed, leaving behind debris that made the Earth, on which life-as-we-know-it evolved. Cosmology resembles a spherical archaeological dig from the inside out, for which the outermost layers are the most ancient. By studying light that was emitted early on from far away, we can recreate the scientific version of the story of genesis: “Let there be light.” For more details, see my textbooks “How Did the First Stars and Galaxies Form?” and “The First Galaxies in the Universe”, as well as “Life in the Cosmos”.
This resulting homogeneous and isotropic FLRW model is consistent with all observational data on the horizon scale of the Universe to within one part in a hundred thousand.
In the first class of my cosmology course at Harvard, I often ask the students: “What would constitute an isotropic but inhomogeneous universe?” The answer is spherically-symmetric onion-shells of variable density centered on us. This requires us to be at the center, violating the Copernican Principle which posits that we do not occupy a privileged location.
My second question to the students is: “What would constitute a homogeneous but not isotropic universe?” The answer involves a uniform distribution of matter or radiation in a spacetime that expands at different rates in different directions. This introduces preferred directions into our cosmic landscape.
Did our Universe start isotropic? The answer is unclear. In collaboration with the brilliant cosmologist Mark Hertzberg from Tufts University, we solved Einstein’s equations for the evolution of an anisotropic universe with the known composition of radiation, matter and a cosmological constant, and showed that generically — the level of anisotropy declines with increasing cosmic time. This implies that even if the cosmic expansion rate was much faster along a preferred axis early on — so that the infant Universe resembled a needle just after the Big Bang, the expansion would become nearly isotropic to an exquisite precision by the present time. In other words, the isotropy limits set by our best data today cannot constrain the level of global anisotropy of the expansion geometry at early cosmic times.
If we were able to detect gravitational waves on tiny (centimeter-wavelength) scales, generated less than a quecto-second (ten to the power of -30 of a second) after the Big Bang, then their anisotropy on the sky could have informed us about the level of anisotropy that existed at the beginning, around the Big Bang. But for now, we can only detect spacetime fluctuations on much larger scales that entered the horizon of the Universe when it closely resembled the FLRW model. Hence, we cannot say much about whether the infant universe resembled a needle or a sphere.
Despite the huge progress in our understanding of cosmology since the beginning of my career in cosmology, the nearly isotropic FLRW model for the early Universe may be simple because of lack of data. Here we go again, John.
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”, was published in August 2023.
