For decades, scientists have tried to measure how fast the universe is expanding.
This expansion rate is known as the Hubble constant, and it plays a central role in understanding the history and future of the cosmos.
However, different methods of measuring the Hubble constant have produced conflicting results, creating a major puzzle in modern cosmology.
Now, two new studies suggest that the expansion of the universe in our local cosmic neighborhood may actually be slower than previously estimated.
The research was carried out by an international team of astronomers, including David Benisty from the Leibniz Institute for Astrophysics Potsdam. Their findings were recently published in the journal Astronomy & Astrophysics.
Instead of relying on traditional methods, the scientists used a new approach that examines how nearby groups of galaxies move within the expanding universe.
The Hubble constant describes how quickly galaxies move away from each other as the universe expands. It is usually expressed in kilometers per second per megaparsec. A megaparsec equals about 3.3 million light-years.
Measurements based on the early universe, particularly from radiation known as the cosmic microwave background, suggest the expansion rate is about 68 kilometers per second per megaparsec.
However, measurements based on nearby galaxies and exploding stars known as supernovae have given a higher value of about 73.
This difference, known as the “Hubble tension,” has become one of the biggest unsolved problems in cosmology. If the measurements are both correct, it could mean that scientists do not yet fully understand the physics of the universe.
In the new studies, researchers looked at two nearby galaxy groups: the Centaurus A group and the M81 group. These groups are relatively close to our own Local Group of galaxies, which includes the Milky Way. Instead of measuring the distance to exploding stars, the scientists analyzed the motion of galaxies inside these groups and how they interact with the expanding universe around them.
Within a galaxy group, gravity pulls galaxies toward each other, while cosmic expansion pushes them apart. By studying this balance between gravitational attraction and the overall expansion of space, the researchers were able to estimate both the total mass of the galaxy group and the value of the Hubble constant.
Their analysis produced a value of about 64 kilometers per second per megaparsec, which is noticeably lower than previous measurements based on nearby galaxies. This slower expansion rate is closer to the value calculated from observations of the early universe.
The study also revealed new details about the structure of these galaxy groups. For example, the Centaurus A group appears to be organized around two major galaxies—Centaurus A and M83—rather than being dominated by just one large galaxy as previously thought. In the M81 group, the arrangement of galaxies forms a flattened structure that changes orientation as it extends outward into the surrounding cosmic environment.
Another surprising result is that the motion of galaxies in both groups can largely be explained by the visible mass of the galaxies themselves. This means that less dark matter may be needed to explain their behavior than many theoretical models predict.
The researchers plan to apply this method to larger regions of the universe using new observations from powerful telescopes. These future studies may help resolve the Hubble tension and provide a clearer picture of how the universe is expanding and how much dark matter it truly contains.
Source: KSR.
