Cosmologists have struggled to understand an obvious tension in their measurements of the current rate of expansion of the universe known as Hubble’s constant. Observations of the early cosmos – mainly the cosmic microwave background – suggest a significantly lower Hubble constant than the value obtained from observations of the late universe, mainly of supernovae. A team of astronomers examined the data to find that one possible way to relieve this tension is for Hubble’s constant to paradoxically evolve over time. This result could either indicate new physics … or just a misunderstanding of the data.
“The point is, there seems to be a tension between the larger values for universe late observations and the lower values for universe early observations,” said Enrico Rinaldi, research fellow at the Department of Physics at the University of Michigan and co-author of the study. “The question we asked in this article is, what if Hubble’s constant is not constant? What if it actually changes? “
Something is rotten in the state of the universe
Cosmologists use a variety of probes and observations to determine the basic properties of our universe. They are trying to measure its age, content, rate of expansion, and more. After nearly a century of scrutiny, these cosmologists have developed a coherent, consistent model of the universe. In short, our cosmos is approximately 13.77 billion years old, is constantly expanding, and consists primarily of dark energy and dark matter – with normal matter such as stars and planets and gas clouds being a brightly lit minority of the constituent parts of the universe.
Aside from the gigantic secrets of the true nature of dark energy and dark matter, cosmologists have come across another frustrating puzzle in recent years: various probes disagree on today’s rate of expansion, known as the Hubble constant.
Measurements of the young universe, such as the cosmic microwave background (the afterglow pattern released when the universe cooled from a plasma state when it was 380,000 years old) show that Hubble’s constant is somewhere at 68 km / s /. Mpc (which means that for every million parsecs removed from our point of view, the rate of expansion of the universe increases by 68 kilometers per second).
However, more local measurements in the late Universe, such as observations of supernovae, tend to have a different answer: a Hubble constant greater than 74 km / s / Mpc.
It is
A team of astronomers led by Maria Dainotti, Assistant Professor at the National Astronomical Observatory of Japan and the Graduate University for Advanced Studies, SOKENDAI in Japan, and an affiliate scientist at the US Space Science Institute have studied this discrepancy in more depth. The work was published in the Astrophysical Journal in May.
The team focused their work on type 1a supernovae, a special type of explosion that occurs when white dwarf stars accumulate too much mass from a companion star, triggering a runaway nuclear fusion event. This fusion event has roughly the same brightness each time, so astronomers can use these supernovae as “standard candles”. Since you know how bright the supernovae should be, you can compare them with their brightness and calculate a distance. By combining many such measurements over a wide range of distances, astronomers can calculate the expansion history of the universe.
The team used a catalog of over 1,000 supernovae observations and divided them into different distance ranges, with each container representing the same number of supernovae. They then used each container to measure Hubble’s constant. In the standard cosmological picture, the rate of expansion of the universe is constantly changing as the cosmos evolves, but Hubble’s constant is a fixed number – it is the rate of expansion of the universe at the moment.
Any container with supernovae was supposed to give the same Hubble constant, but in their analysis, the researchers admitted the Hubble constant was not as constant – they allowed the possibility that it could change over time. By using different bins, they could test whether the Hubble constant stayed fixed across the different bins, or whether it actually varied.
“If it’s a constant, it shouldn’t be any different when we extract it from containers at different distances. However, our main finding is that it actually changes with distance, ”said Rinaldi. “The voltage of the Hubble constant can be explained by a certain dependence of this constant on the distance of the objects you are using.”
Not very consistent
Ultimately, the astronomers in the study found that by adding a little flexibility to the standard cosmological models – by letting the Hubble constant change over time – they could release almost all of the tension between the supernovae and the cosmic microwave background measurements. The researchers were able to calculate their developing Hubble constant back to the time of the cosmic microwave background and compare it with these results.
“The extracted parameters are still compatible with the standard cosmological understanding we have,” he said. “But this time they shift just a little when we change the distance, and that little shift is enough to explain why we have this tension.”
Overall, the new results are not surprising. It is always possible to agree on different observations as models become more complex. In this case, the researchers added a new variable – how quickly the Hubble constant changes over time – and were able to find a way to connect the early and late measurements of the Hubble constant. The work also did not find a statistically significant measure of this varying Hubble constant. Although they were able to release the tension in cosmological observations, they could not conclusively say that Hubble’s constant changes over time.
These results could offer theorists a way to introduce new physics into the universe to explain the constant Hubble stress, if it persists. Or it could also mean that supernovae are not as “standard” as we believe, and that some bias may creep into the observations to spoil these Hubble constant measurements.
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