What we see in the mirror represents the way we looked in the past. For a mirror situated 30 centimeters away from us, the roundtrip travel time for light bouncing off our face to the mirror and back to our eyes is 2 billionths of a second, given the finite speed of light. We can ignore this delay because it is shorter by 17 orders of magnitude than the period over which we age, measured in years. But if a large mirror had been located at a distance of 1013 light years away from Earth, then we would have been able to see what happened on Earth 2026 years ago, a year before Jesus was born. This privilege of seeing back in time is offered to us on the grand scale of the Universe. Because that scale is so vast, we detect light that was emitted long ago, allowing us to observe how distant regions looked like at earlier times.
There is another cosmic privilege that we should not take for granted. The initial conditions after the Big Bang were statistically the same to one part in a hundred thousand throughout the entire observable volume traversed by light since the Big Bang. In other words, the Bang happened everywhere at the same time to that precision. This synchronization may reflect special initial conditions or perhaps an early phase of faster-than-light expansion, called cosmic inflation. But irrespective of its origin, our cosmic fortune allows us to infer statistically what happened in our neck of the woods based on what we observe far away from us. Sharing initial conditions with distant regions enables us to infer where we came from, in the same way that witnessing the birth of babies with our human DNA allows us to figure out how we were born.
Cosmological times are measured in billions of years. The age of the Universe is a hundred million times longer than the lifespan of Jeanne Calment, the oldest person in human history whose age has been verified. As a result, we are used to observing a snapshot of the Universe without noticing that it may have evolved during our lifespan. In reality, the Hubble constant was different when we were born than it is today, but its evolution is extremely slow and hence ignored in textbooks. If AI-assisted medicine will make humans immortal, we would be able to witness the Universe age. Before then, we can seek extraterrestrial astronomers who collected data over billions of years before humans came along. But if we stay down to Earth and accept our current limitations, can we observe the evolution of the Universe-at-large in real time?
In 1998, I came up with a novel idea on how to accomplish this task. In a single-authored paper, I suggested a method to infer the drift of cosmological redshift in real time. The Lyman-alpha forest is a series of absorption features imprinted by concentrations of hydrogen atoms along the line-of-sight to distant quasars. Denser-than-average regions absorb the quasar light at a wavelength corresponding to an electronic transition from the ground state to the first excited level of hydrogen, the so-called the Lyman-alpha transition discovered by Theodore Lyman IV at Harvard University in 1906. In the course of cosmic expansion, the recession speed and hence the redshift (namely, the cosmological shift to longer wavelengths) increase with increasing distance, as realized by Edwin Hubble in 1925. As a result, hydrogen absorption at different distances translates to Lyman-alpha absorption features at different wavelengths in the observed spectrum of the quasar. The resulting Lyman-alpha forest of absorption features represents a snapshot of cosmic history. In my paper, I suggested that over a period of several decades, our best spectrographs might notice a systematic redshift drift in the Lyman-alpha forest imprinted on the spectrum of many quasars. The predicted drift provides a novel probe of universal expansion and cosmology. Its magnitude is of order the speed of light over the age of the Universe. Its measurement can be used to constrain the nature of dark energy and dark matter that fill the Universe. After writing my paper, I found out that the cosmologist Alan Sandage published a paper in 1962, my birth year, which suggested a less practical method for measuring the redshift drift for galaxies. By now, the redshift drift is often referred to in the scientific literature as the Sandage-Loeb test.
A few days ago, an attempt to measure the redshift drift of the Lyman-alpha forest was reported in a new paper. The authors performed the first steps towards a measurement of the redshift drift in the Lyman-alpha forest of a bright quasar as a real-time tracer of cosmological expansion. They used 12 observing hours of the state-of-the-art ESPRESSO spectrograph on the Very Large Telescope (VLT) in Chile over 0.875 years of the brightest quasar known, J052915.80–435152.0 at a redshift of 3.962. As expected, the authors measured a velocity drift of the Lyman-alpha forest consistent with zero but they estimate that reaching a detection with a 99% confidence of the cosmic drift requires a monitoring campaign of 5400 hours of integration time over 54 years with the next generation European Extremely-Large-Telescope and its planned high-resolution spectrograph ANDES. This challenging measurement will take half a century. Patience is mandatory for witnessing the evolution of the cosmos.
What happens beyond the distance that light travels after the Big-Bang is unknown. Based on the uniformity of the relic radiation from the early hot cosmos, the cosmic microwave background, we can infer that similar conditions exist out to a distance that is 4,000 larger than our cosmic horizon. Beyond that, all bets are off.
Assuming that “anything that can happen will happen an infinite number of times” in the multiverse that lies beyond our cosmic horizon, might trigger irresponsibility in the way we live our life, because it gives legitimacy for some of us to argue that somewhere else in the multiverse there is a better version of us behaving properly and so our bad behavior here and now is not significant in a global cosmic perspective. Instead, we should accept the finiteness of the speed of light as limiting our view in the same way that blinkers limit the side view of a horse to keep its focus on the road ahead.
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.
