The local universe may be expanding more slowly than previously thought, scientists have found. The discovery, made in two separate areas of research, could alleviate one of the most vexing headaches in cosmology, the Hubble tension.
The Hubble constant – named after Edwin Hubblethe astronomer who discovered at the beginning of the 20th century that the universe is expanding – is the rate at which the expansion is occurring.
The Hubble voltage arises from the fact that the observation of the local universe gives a different value for the Hubble constant than that derived using cosmic microwave background (CMB) — the universe’s first light, which shone shortly after Big Bang. Astronomers take CMB measurements and then fast-forward using the standard model of cosmology, the so-called Lambda cold dark matter (LCDM) model.
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The discrepancy has persisted even as the two separate measurement techniques have become more precise. It’s troubling because it suggests that an important ingredient in physics is missing from our recipe for the cosmos. Therefore, many astronomers cite the need for a third method to bridge this disparity, or at least shed some light on why it exists.
Two new studies propose a new way to measure expansion in the immediate cosmos by analyzing the motion of two nearby galaxy groups. Galaxies within these groups are simultaneously bound together by mutual gravity and pulled apart by the cosmic current caused by the stretching of space in which they are embedded.
Both results indicate that the universe expanding more slowly in our vicinity than previously estimated. Not only does this technique bring measurements of the Hubble constant in the nearby universe closer to those made using the CMB and LCDM model, but it also suggests that less dark matter is needed to explain cosmic observations and the dynamics of galaxies.
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The teams reached their conclusions by examining two galaxy groups – the Centaurus A group (one of the closest to us, with the exception of The Milky Wayits local group), and the M81 group. Instead of using observations of nearby Type Ia supernovae or the cosmic fossil of the universe’s first light represented by the CMB to measure the Hubble constant, scientists used the motion of these clustered galaxies during the balancing act of the attractive influence of gravity and the repulsive effect of the expansion of the universe.
The astronomers found that the dozens of small galaxies that make up the Centaurus A group are not actually dominated by the giant elliptical galaxy of the same name. Rather, this galaxy actually forms a binary with the group’s M83 galaxy.
It was already understood that the M81 group had binary galaxies (M81 and M82) at its heart. The new research showed that although the structure of this group is neatly organized, the inner region is around 1 million light years tilted by approx. 34 degrees in relation to its wider surroundings. Out to a distance of about 10 million light-years, the orientation of the M81 group aligns with the orientation of a vast sheet-like structure of matter that reaches out to the Centaurus A group.
The two research teams also discovered that in addition to the two galaxy groups sharing a similar environment, the masses of the brightest galaxies in these groups make up the majority of the total mass. Thus, the motions of all the galaxies in the groupings can be considered as the result of the interaction between the gravitational influence of these bright galaxies and the cosmic flow of the expanding universe.
This means that, contrary to the predictions of cosmic simulations, galaxy clusters do not need to be embedded in a huge, all-encompassing dark matter halo that exerts its gravitational influence.
What does this mean for the Hubble constant?
The Hubble constant is measured in kilometers per second per megaparsec (km/s/Mpc), with 1 megaparsec equaling about 3.3 million light years. Currently, when scientists calculate the universe’s expansion rate using local Type Ia supernovae, they get a Hubble constant of 73 km/s/Mpc. However, when the Hubble constant is calculated using the CMB, theorists calculate a lower value of 68 km/s/Mpc.
The teams involved in this research arrived at a constant Hubble value of 64 km/s/Mpc. This suggested to the researchers that part of the Hubble tension is caused by the methods the researchers use to measure the Hubble constant. This may mean that an additional, currently unknown element of the cosmos is not required to remove the Hubble tension; we can complete this cosmic recipe with the ingredients we have on hand.
Of course, there is still a long way to go before this method overturns existing paradigms. With the technique applied to just two local galaxy groups, the Hubble tension is guaranteed to be a headache for at least a little while longer.
The next step for this investigation will be to apply this galaxy group study technique to a larger region of space in our local universe. This may become possible when observations of galaxy groups at greater distances become available in the next data release from the 4-meter Multi-Object Spectroscopic Telescope (4MOST).
The team’s research was published across two papers in the journal Astronomy and astrophysics.
