Hubble constant: Entirely new measurement method fails to resolve mysterious discrepancy

The debate about the mysterious discrepancy in the so-called Hubble constant, which has been going on for years, has now been enriched by another entry. With a completely new approach, a value has now been determined that is closer to one of the contradicting results, but also does not represent a solution. The research group led by Patrick Kelly from the University of Minnesota determined the expansion rate of the universe based on a star explosion, which was observed for the first time in 2014 and then again in 2015 thanks to a gravitational lens.

In their work, the group took advantage of a theory developed by Norwegian astrophysicist Sjur Refsdal in 1964. So far it has not been possible to verify it. The opportunity then arose in 2014, when a distant supernova appeared several times on astronomical images thanks to a gravitational lens. So the light was deflected by massive objects in the foreground in such a way that it came to us from different directions at the same time. However, models had predicted that the explosion should be visible again in 2015 thanks to another gravitational lens.

When that actually happened, it was possible to make a completely independent calculation of the Hubble constant based on the time difference and Refsdal’s theory. This fundamental quantity for understanding the universe indicates the speed at which the universe is currently expanding. It means that an object one megaparsec (3.26 million light-years) away is moving away from us at that rate simply because of the expansion of the universe. Depending on the model used, the result was a value of 64.8 or 66.6 km/sec/Mpc.



Overview of the values ​​determined so far, including the error widths

(Image: Patrick L. Kelly et.al)

Overall, the results are closer to the value determined with the Planck space telescope, but the other cannot be ruled out. This adds a new value to the debate, but it is probably no closer to an answer to the question. It revolves around the fact that different measurement methods have been determining two different values ​​for years, which are now quite clearly outside the respective error rate. It is unclear exactly what this is all about, even if there are repeated attempts to resolve the discrepancy.

Only a year ago, the most accurate measurements of the local propagation speed of the universe with the Hubble Space Telescope confirmed the mysterious discrepancy again. The researchers responsible for this had spoken of the Magnum Opus of the space telescope and pointed out that determining the value as precisely as possible also tells us how old the universe is as a whole. When Hubble was sent into space, information on the age of the cosmos still fluctuated between 8 and 20 billion years; the determined value is now around 13.8 billion years.


(mho)

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