Is the Observed Jitter of the Sun Induced by Asteroids and Not Gravitational Waves?
I woke up at 2AM last night with a new scientific idea and cold sweat on my forehead. What if the detection of a gravitational wave background from the Universe, which was announced by large international teams of scientists recently, is something else?
These teams used a collection of pulsars, which are rapidly spinning neutron stars, as precise clocks to monitor the motion of the solar system and detect a signal of such motion. The variation in pulsar arrival times on a timescale of a few years was correlated among the pulsars and therefore unrelated to any noise source near one of them. The signal was interpreted in terms of a cosmic background of overlapping gravitational waves that were generated by many pairs of supermassive black holes that coalesced as a result of galaxy mergers throughout the Universe.
Neutron stars carry roughly the mass of the Sun within the size of a large city of about 12 kilometers. Pulsars are spinning neutron stars which emit opposing beams of radio waves along their poles. As their beams are usually tilted relative to the spin axis, they appear as Galactic lighthouses. Their radio beams intercept Earth periodically and show as pulses. The collection of pulsars used to measure the tantalizing signal are the fastest rotators in the Milky Way galaxy. They spin roughly a thousand times per second and are called millisecond pulsars.
Pulsars are very stable rotators and their pulse arrival times serve as the best clocks beyond Earth. The international collaborations used Pulsar Timing Arrays (PTAs) to measure correlated variations in the pulse arrival times to Earth. The key for such a measurement is an accurate ephemeris model for the entire Solar system, where planets, Moons and asteroids tug the Sun back and forth as they move around it.
What I realized that morning is that the standard ephemeris model for the Solar system includes only the 343 most massive asteroids in addition to the known planets and moons of the Solar system. However, there are many more known asteroids of radii below 20 kilometers, roughly the size of the rock that killed the non-avian dinosaurs on Earth 66 million years ago. These asteroids are not being modeled and their motion around the Sun generates an unmodeled jitter. The Sun follows a random walk, also termed Brownian motion, akin to a grain of a grain of dust kicked around by air molecules.
Physicists often solve a problem by realizing that it is similar to another problem for which they already have a solution. I realized that the Sun’s jitter is similar to the random motion of a supermassive black hole embedded in a cluster of stars, a common occurrence in galactic nuclei. Two decades ago, I wrote papers with my student Pinaki Chatterjee and my colleague, Lars Hernquist, in which we developed an analytic model backed by computer simulations for the Brownian motion of a massive black hole surrounded by a cluster of much lighter stars. I now realized that I can use this model to calculate the jitter of the Sun as a result of the thousands of rocks orbiting around it.
Consider two hemispheres of equal volume in the swarm of asteroids around the Sun. Because of random statistics in the number of asteroids, one hemisphere will have a small excess, roughly equal to the square root of its number of asteroids. This excess pulls the Sun towards that hemisphere until the asteroids change position and move to the other hemisphere over half of their orbital time. Since the main belt of asteroids has orbital periods of a few years, this jitter will appear on the same timescale for which the PTAs reported their stochastic signal.
Remarkably, when I calculated the expected motion of the Sun as a result of these random fluctuations in the number of unmodeled rocks around it, I found a level that resembles the signal reported by PTAs. Within a few hours, I wrote a paper that summarizes these results and submitted it for publication.
The Rubin Observatory in Chile will map these unmodeled asteroids and can help in distinguishing between a true cosmic background of gravitational waves and the unmodeled jitter of the Sun as a result of the swarm of unmodeled asteroids around it. But no matter what future analysis will show, one thing is clear. Doing creative science is a lot of fun.
Today, I am hosting a conference at Harvard, titled `To Our Cosmic Horizon and Beyond’, in celebration of 20 years of the Institute for Theory & Computation, for which I serve as director. Spring has arrived and the flowers and trees will be blossoming along my morning jog at sunrise. Similarly, many of our former postdoctoral fellows blossomed into leadership positions at major universities worldwide. I look forward to enjoying their magnificent colors through their talks at the conference.
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.
