In 1992, the Vatican admitted that Galileo Galilei was right and the Earth is not at the center of the Universe. This was a bit late, 350 years after Galileo died and 33 years after humans landed on the Moon. We now know based on data extending across 13.8 billion years of cosmic history that not the Sun, nor the Milky Way, are at the center of the Universe.
Gravity controls our cosmic motion. On much smaller scales than the distance that light travelled since the Big-Bang, we find that we are glued by gravity to Earth. The Earth is circling the Sun at a separation of 8.3 light-minutes. The Sun is circling the center of the Milky-Way galaxy at a separation of 26,000 light-years. The Milky-Way is falling towards its sister galaxy, Andromeda, at a separation of 2.5 million light-years. The two galaxies are the primary constituents of the Local Group of galaxies, which is moving with a velocity of 630 kilometers per second — about 22,000 times larger than the 65 miles-per-hour speed limit on a highway — relative to the cosmic rest frame. That cosmic frame is represented by the relic radiation from the Big Bang.
What is the source of the excess velocity of the Local Group relative to the cosmic frame?
In 2008, I wrote a paper with my colleague, Ramesh Narayan, in which we compared the motion of the Local Group relative to the cosmic microwave background to the expected velocity from the large scale structure mapped by the deepest survey of galaxies at that time, the Two-Micron All-Sky Redshift Survey. Our analysis was limited by the lack of surveyed galaxies behind the dusty core of the Milky Way, the so-called Zone of Avoidance. We inferred the existence of an unknown concentration of mass behind the Zone of Avoidance, hidden from view by the Galactic Center’s veil of dust. This inference was based on the need for an excess gravitational pull from large scale structures in the direction of the Galactic Center.
Last year, the missing mass concentration was found. A 2024 paper discovered a radio signal from atomic hydrogen in a mass concentration which was largely obscured by Galactic dust. Indeed, this concentration of galaxies appeared behind the Zone of Avoidance.
Altogether, the Local Group is moving towards the so-called Great Attractor at the center of the Laniakea Supercluster of galaxies which contains about 100,000 galaxies, including the Milky Way, with a total mass of about 10^{16} solar masses. The Great Attractor is partly obscured by the Milky Way’s plane. The center of the Great Attractor is defined by the Norma cluster of galaxies.
But this is not the end of the story about the primary movers in the local Universe. The Great Attractor itself appears to be moving towards the Shapley Supercluster at a distance of about 650 million light years. Another recent study suggests that the Local Group is also repelled by a mass deficit from the opposite direction relative to the Shapley Supercluster.
The main gravitational attraction in the Local Universe is centered on the Shapley supercluster across a region of order a tenth of the cosmic horizon scale.
A Vatican-like institution at the center of mass of the Shapley supercluster would be justified in claiming to be located at the gravity center of the Local Universe. It would not be arrogant for clergy in that region to think unapologetically that they are situated near the primary mover of matter in the local Universe.
However, as the Latin phrase goes: Sic transit gloria mundi. The glory of that world is doomed to pass, because the Shapley supercluster will be ripped apart by the accelerated expansion of the Universe. As I showed in a paper with my former postdoc, Ken Nagamine, all galaxies beyond the Local Group will be carried away out of our cosmic horizon by the cosmic expansion within a hundred billion years, if the cosmological constant will remain constant.
At that future time, the Local Group will be surrounded by the darkness of empty space. Within our cosmic horizon, future life will only be possible in the Local Group. At long last, our descendants will be able to argue that they are privileged observers of the cosmos.
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.
