The universe might be younger than we thought, galaxies' motion suggests

The universe could be younger than we think, based on the motions of satellite galaxies that reveal how recently they have fallen into a galaxy grouping.

According to measurements of the cosmic microwave background radiation (CMB) by the European Space Agency's Planck mission, the universe is about 13.8 billion years old. This calculation is based on what's known as the Standard Model of cosmology, which describes a flat universe dominated by dark energy and dark matter and which is expanding at an accelerating rate.

The Standard Model is then used as a basis for supercomputer simulations that can depict the growth of large-scale structure in the universe — galaxies, galaxy clusters and huge chains and walls of galaxies.

However, these models have now run afoul of new measurements of the motions of pairs of galaxies that don't tally with what the simulations are telling us.

In a new study, astronomers led by Guo Qi from the National Astronomical Observatories of the Chinese Academy of Sciences studied pairs of satellites in galaxy groups.

Galaxy groups are small collections of galaxies, such as our own Local Group, in which a few large galaxies are joined by a swarm of smaller ones. Like larger galaxy clusters, these galaxy groups form where filaments in the cosmic web of matter that spans the universe meet, with smaller galaxies moving along the filaments before falling into a group.

Using observations made by the Sloan Digital Sky Survey (SDSS) of 813 galaxy groups within about 600 million light-years from Earth, Qi's team focused on the most massive galaxy in each group and measured how pairs of satellites on opposite sides of that galaxy moved.

They found that the fraction of satellite galaxies that were counter-rotating with respect to each other — in other words, orbiting the large galaxy in opposite directions — is higher than predicted by computer simulations of large-scale structure, such as the Millennium Simulation and the Illustris TNG300 model, which are both based on the Standard Model as described by the Planck mission.

This is a natural state of affairs if the satellites have just fallen into orbit around the larger galaxy of the group. But over time, galaxy groups and clusters should reach a dynamically relaxed state, with most satellites co-rotating. If galaxy groups and clusters coalesced when the Standard Model suggests they should have, then the fraction of counter-rotating satellites should be smaller. The fact that they are a greater fraction of satellites is a problem for the Standard Model.

"We found in the SDSS data that satellite galaxies are just accreting/falling into the massive groups, with a stronger signal of ongoing assembly compared to simulations with Planck parameters," Qi told Space.com in an email.

In other words, it seems that the satellite galaxies have only recently fallen into their respective groups.

"This suggests that the universe is younger than that suggested by the Planck observations of the CMB," said Qi. "Unfortunately, this work cannot estimate the age of the universe in a quantitative manner."

This is because there is still too much leeway in the motions of the satellite pairs and models of how groups form to be able to place a firm figure on how much younger than 13.8 billion years these results suggest that the universe is.

If correct, then the new findings imply that something is amiss in the Standard Model, and that some of our assumptions about the universe must be wrong. In fact, one cosmic paradox that scientists are currently investigating could be the answer.

The expansion rate of the universe is defined by a number called the Hubble constant. Planck measured the Hubble constant to be 67.8 kilometers per second per megaparsec — in other words, every megaparsec volume of space is expanding by 67.8 kilometers (42.1 miles) every second. (One megaparsec is about (3.26 million light-years.) Based on this expansion rate, cosmologists are able to calculate the universe's age as 13.8 billion years by rewinding the clock.

However, observations of the redshift of Type Ia supernovae, which are exploding white dwarfs, give the value of the Hubble constant as 73.2 kilometers (45.5 miles) per second per megaparsec. With this expansion rate, rewinding the clock would give a younger age of 12.6 billion years.

Both measurements of the Hubble constant are considered to be unimpeachable, and yet they differ drastically. This paradox has become known as the "Hubble tension."

"This of course could be related to the Hubble tension problem," said Qi when asked whether the younger age suggested by satellite pairs in galaxy groups is support for the faster rate of expansion from the supernova measurements.

However, there are other hurdles to overcome. If we lower the age of the universe too much, then astronomers will find themselves in the awkward position of having stars that are known to be older than the universe itself.

Perhaps the explanation lies with other aspects of the Standard Model. For example, the model is heavily dependent upon dark matter, but so far scientists do not know what dark matter is. Other researchers argue that dark matter does not exist at all, and that its gravitational effects can be explained by a modification of the laws of gravity at low accelerations, such as those experienced by satellite galaxies orbiting at greater distances. Qi's team did find that satellite pairs at larger orbital radii are more likely to be counter-rotating.

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