The lowest mass stars have 8% of the mass of the Sun and can live for up to 12 trillion years. This lifespan is 870 times longer than the time that elapsed since the Big Bang. The age of the Universe is best estimated as 13.8 billion years based on the Planck satellite map of the temperature fluctuations of the cosmic microwave background, relic from the Big Bang. Are there any relics older than the Universe? If we find such relics, they would call into question the standard cosmological model.
We can get a rough estimate of the age of the Universe with century-old data. The observed Universe is expanding. By reversing the expansion history, we would reach a lookback time when the density of matter and radiation was infinite — the Big Bang. The time that elapsed since the Big Bang is roughly the period over which galaxies in the local Universe receded from us to reach their current distance. As Edwin Hubble and Georges Lemaître discovered a century ago, a galaxy’s recession speed is proportional to its distance. The proportionality coefficient is called the Hubble constant. Combining these facts, we find that the ratio between distance and recession speed equals the inverse of the local Hubble Constant, or about 14 billion years. No older objects would survive the Big Bang event because of its extreme temperature and density, potentially reaching the Planck values of 10^{29} times room temperature and 10^{93} times solid density. The near equivalence of the precise cosmic age and the inverse of the Hubble constant stems from a near cancellation between the early period of cosmic deceleration and the late period of cosmic acceleration, making the constant velocity assumption a good approximation.
The exact value of the Hubble constant is being debated in a controversy labeled as the “Hubble Tension.” Different data sets argue for different values across a range that spans 10% of the Hubble constant, with the Planck satellite data favoring the lowest value. Keeping all other cosmological parameters fixed, an increase by 10% in the value of the Hubble constant would shorten the estimated age of the Universe from 13.8 to just 12.6 billion years. This reduced value is comparable to the estimated lifespan of the Sun, 12.2 billion years, suggesting that the oldest stars can rule out a high value for the Hubble constant and resolve the Hubble tension.
The comparison of stellar ages to the age of the Universe can potentially be done at earlier cosmic times as well. The time it takes light to reach us from very distant galaxies allows us to observe how they looked like at earlier cosmic times. In a 2002 paper that I wrote with Raul Jimenez, we proposed using the ages of distant galaxies to constrain the Hubble constant.
In a more recent paper that I wrote with Sunny Vagnozzi and Fabio Pacucci, we studied the oldest objects in the Universe as a function of lookback time. We used a combination of galaxies and quasars to construct an age-redshift diagram of the oldest objects up to a cosmological redshift of 8, when the age of the Universe was 650 million years — less than 5% of its current value. Our data disfavored the highest values of the Hubble constant reported from the latest observations of Type Ia supernovae. This analysis can be extended farther, as recent observations with the Webb telescope discovered two early galaxies that existed at a redshift of 14, when the Universe was just 300 million years old
Based on the same rationale, it is possible to constrain the Hubble constant by dating the oldest stars in the Milky-Way galaxy. Stars are nuclear reactors with limited fuel. Once Sun-like stars consume their fuel, their envelope is expelled and their core cools and contracts to become a compact metallic remnant the size of the Earth, called a white dwarf. Since the lifespan of stars more massive than the Sun is shorter than the age of the Universe, the Milky-Way graveyard is full of tens of billions of white-dwarf corpses. The older corpses have cooler surface temperature, since they had more time over which they lost their internal heat in the absence of a nuclear engine to compensate for that. Based on the estimated ages of these cooling stellar corpses, it is evident that most stars formed billions of years before the Sun’s birth — 4.6 billion years ago.
The oldest stars and white dwarfs reside in the halo of the Milky-Way. Some the them are located in ancient globular clusters, like M92 which has an age estimate of 14.2 billion years with an uncertainty of 1.2 billion years. Some of the oldest white dwarfs were reported to have an age as old as 13.94 billion years, with an uncertainty of 0.85 billion years.
In a project that I am pursuing with the brilliant Andrew Vanderburg from MIT, we are studying state-of-the-art data on the oldest stars in the Milky-Way galaxy with the goal of setting an upper limit on the Hubble constant.
Remarkably, it is possible to constrain cosmology only from data collected on the surface of planet Earth. As suggested in a paper I wrote with my former undergraduate student Amir Siraj, rocks from planetary systems in the halo of the Milky-Way could collide with Earth and survive as meteorites. One could date their material based on the abundances of its radioactive isotopes analysis. For example, Thorium-232 has a half-life of 14.05 billion years and Uranium-238 has a half-life of 4.468 billion years. By studying the abundance ratio of these isotopes relative to their decay products inside halo meteorites, it would be possible to constrain the Hubble constant.
In June 2023, I led a Galileo Project expedition to retrieve materials from the first recognized interstellar meteor IM1 which was spotted by U.S. government satellites in 2014. The expedition recovered fragments smaller than a millimeter, as reported in our recent publications here and here. The Galileo Project is currently seeking funding for a follow-up expedition to recover larger fragments that would enable isotope dating.
Missions of this type can inform us not only about our immediate cosmic neighborhood in the Milky-Way galaxy, but also about the farthest extent of our cosmic horizon — which is set by how far light travelled since the Big Bang.
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.
