Astronomers are closing in on a solution to one of cosmology’s biggest puzzles: the discrepancy in how fast the universe is expanding. Two recent studies suggest the expansion rate in our cosmic neighborhood may be slower than previously thought, potentially easing the so-called “Hubble tension.” This tension arises because different measurement methods yield conflicting values for the Hubble constant—the rate at which the universe expands—and resolving it could mean our understanding of the cosmos is incomplete.
The Hubble Constant: A Cosmic Yardstick
The Hubble constant, named after Edwin Hubble, quantifies the universe’s expansion. However, observations of the nearby universe using methods like Type Ia supernovas give a higher value (around 73 km/s/Mpc) than those derived from studying the cosmic microwave background (CMB) – the afterglow of the Big Bang (around 68 km/s/Mpc). This mismatch isn’t just a minor disagreement; it suggests a fundamental gap in our understanding of the universe’s composition and evolution.
A New Approach: Galaxy Group Dynamics
The latest research offers a third, independent way to measure expansion. Instead of relying on supernovas or the CMB, scientists analyzed the motion of galaxies within two nearby groups: Centaurus A and M81. These groups are caught in a tug-of-war between gravity (pulling galaxies together) and the expansion of space (pushing them apart). By studying these movements, researchers can infer the local expansion rate.
Both studies found a Hubble constant of approximately 64 km/s/Mpc—closer to the CMB-derived value than previous local measurements. This suggests the tension may stem from measurement biases rather than missing physics.
Dark Matter and Haloes: Rethinking Cosmic Structures
The findings also challenge assumptions about dark matter distribution. Simulations predict that galaxy groups are embedded in massive dark matter haloes, exerting strong gravitational influence. However, the new data implies that these haloes may not be as dominant as previously thought. The observed motions suggest the bright, central galaxies within these groups account for most of the gravitational effect, rather than a surrounding dark matter halo.
What This Means for the Future
The research team discovered that the Centaurus A group’s two largest galaxies, Centaurus A and M83, behave as a binary system. The M81 group was already known to have a binary structure (M81 and M82). The orientation of these groups also plays a role, with the M81 group tilted at 34 degrees relative to its surroundings.
“This method could mean that we don’t need to add new ingredients to our cosmic recipe. We may be able to resolve the tension with the tools we already have.”
While promising, this technique has been applied to only two galaxy groups. Further investigation is needed. Future observations from telescopes like the 4-meter Multi-Object Spectroscopic Telescope (4MOST) will expand this study to a larger region of the universe, potentially solidifying these findings and refining our understanding of the Hubble constant.
In conclusion, these new measurements offer a potentially simpler solution to the Hubble tension—one that doesn’t necessarily require invoking exotic new physics. The universe may be expanding more slowly than we thought, and our current models might be closer to complete than previously believed.
