Who ordered our Universe?
The order should be credited as ingenious, since our Universe started as the simplest imaginable on the menu of possibilities. Based on the observed isotropy of the cosmic microwave background, the initial conditions of the Universe shortly after the Big Bang were uniform to within one part in a hundred thousand. This defines a universal cosmic time that is anchored at the mean density of the Universe.
These initial conditions also allow astronomers to figure out statistically what happened in our neighborhood from observing the light emitted a long time ago from distant regions. Without the conformity of initial conditions and the universality of the laws of physics, we would have had a difficult time figuring out our cosmic roots.
Thanks to these fortunate cosmic circumstances, we can imagine filing the Universe with clocks that were all synchronized to show the same initial time at the Big Bang. They would tick at exactly the same rate if the Universe was perfectly uniform.
But there is a rub. According to Albert Einstein’s General Relativity, time is ticking more slowly near a mass concentration and more rapidly in a rarefied region relative to the cosmic mean.
This brings a rarely advertised reason for us to feel happy. Aside from living on a habitable planet near a long-lived star, we live within a mass concentration called the Milky-Way galaxy, where we witness the rest of the Universe aging faster than us. Our circumstances are more fortunate than civilizations which happen to reside in cosmic voids.
How big is the Einstein time shift in the Milky-Way? To calibrate it, we need a measure of the local gravitational potential.
The gravitational potential of the Milky-Way was calibrated most recently by the Gaia sky survey, which identified a deficit in the population of local stars above some speed — because faster stars are not gravitationally bound to the Milky-Way. Based on that, the local escape speed was measured to be about 500 kilometers per second, which is 45 times faster than the escape speed from Earth and an order of magnitude larger than the speed of our fastest chemical rockets.
If other Milky-Way civilizations launched technological objects with chemical rockets over the past billions of years, their interstellar space debris must have accumulated in the Milky-Way like plastics in the ocean. This provides a good motivation to search for technological debris as anomalous interstellar objects, like `Oumuamua or the meteors IM1 and IM2. Hopefully, the next ocean expedition of the Galileo Project will recover larger fragments than the spherules we already analyzed from the wreckage of IM1 and allow us to reveal its nature.
The Einstein time shift equals the gravitational potential in units of the speed of light squared, where the gravitational potential equals half of the square of its escape velocity. Putting this all together gives an Einstein shift of 72.9 minutes over a century in the Milky-Way potential.
In addition, we gain from the relativistic time dilation associated with the motion of the Sun relative to the cosmic frame of reference. This speed was measured most recently by the Planck satellite through the Doppler shift of the cosmic microwave background. Its value of 370 kilometers per second yields an additional time dilation equal to half of the square of this peculiar velocity in units of the speed of light, namely an extra 40 minutes per century. The speed of the Earth around the Sun adds 0.3 minutes per century. All in all, the total Einstein shift from the Milky-Way ends up at 113.2 minutes per century, or 2 hours and 18 minutes for Jeanne Louise Calment who died at the age of 122.
Most humans gain from the Milky-Way more than an extra hour of lifespan relative to the cosmic frame of reference. This means an extra hour to observe the Universe and enjoy its wonders.
Of course, we can be jealous of better locations. The Einstein shift is a million times bigger for a civilization residing near the horizon of a black hole. However, that environment brings the health hazard of spaghettification by its strong gravitational tide or the risk of sliding into the event horizon and being locked forever in a prison of spacetime. Like in Vegas, whatever happens inside of a black hole horizon stays there. There is nothing worse than that risk for Gen-Z members who are addicted to sharing their experiences on Instagram, irrespective of how long they live.
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. His new book, titled “Interstellar”, was published in August 2023.
