The local universe may be expanding more slowly than previously thought, scientists have found. The discovery, made in two separate pieces of research, could relieve one of the most troubling headaches in cosmology, the Hubble tension.
The Hubble constant — named after Edwin Hubble, the astronomer who found in the early 1900s that the universe is expanding — is the rate at which that expansion is occurring.
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The discrepancy has persisted even as the two separate measurement techniques have become more precise. It is troubling because it suggests that some crucial ingredient of physics is missing from our recipe for the cosmos. Hence many astronomers cite the need for a third method to help bridge this disparity, or at least shed some light on why it exists.
Two new studies suggest a new way of measuring 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 dragged apart by the cosmic flow caused by the stretching of the space in which they are embedded.
Both results indicate that the universe is 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 in line to those made using the CMB and the LCDM model, but it also suggests that less dark matter is needed to explain cosmic observations and the dynamics of galaxies.
Halo or no?
The teams reached their conclusions by examining two galaxy groups — the Centaurus A group (one of the nearest to us, barring the Milky Way‘s local group), and the M81 group. Rather than using observations of nearby Type Ia supernovas or the cosmic fossil of the universe’s first light represented by the CMB to measure the Hubble constant, the researchers used the motion of these grouped galaxies under 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 comprise the Centaurus A group are not in fact dominated by the giant elliptical galaxy of the same name. Rather, this galaxy actually forms a binary with the group’s M83 galaxy.
The M81 group was already understood to have binary galaxies (M81 and M82) at its heart. The new research revealed that, though the structure of this group is neatly organized, the inner region of around 1 million light-years is tilted by about 34 degrees with regard to its wider surroundings. Out to a distance of around 10 million light-years, the orientation of the M81 group aligns with that of a vast sheet-like structure of matter that reaches out to the Centaurus A group.
The two teams of scientists also discovered that, in addition to the two galaxy groups sharing a similar environment, the masses of the brightest galaxies in these groupings account for most of the total mass. Thus, the motions of all the galaxies within the groupings can be considered a result of the interplay of the gravitational influence of these bright galaxies and the cosmic flow of the expanding universe.
This means that, in contradiction to the predictions of cosmic simulations, galaxy groups don’t have to be embedded in a vast, all-encompassing dark matter halo exerting 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 being equivalent to around 3.3 million light-years. Currently, when researchers calculate the expansion rate of the universe using local Type Ia supernovas, they obtain a Hubble constant of 73 km/s/Mpc. When the Hubble constant is calculated using the CMB, however, theorists calculate a lower value of 68 km/s/Mpc.
The teams involved in this research arrived at a Hubble constant value of 64 km/s/Mpc. This implied to the researchers that part of the Hubble tension is caused by the methods scientists use to measure the Hubble constant. This could mean that an added, currently unknown element of the cosmos isn’t needed to dispel the Hubble tension; we can complete this cosmic recipe with the ingredients we have at 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 bound 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 wider region of space within our local universe. This could become possible when observations of galaxy groups at larger 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 & Astrophysics.
