Monday, August 30, 2021

Perhaps there is no problem in the different measures of the expansion rate of the universe!

 The Hubble constant, which determines the rate of expansion of the universe, has been one of the hottest debates among astronomers.  Is that, currently, there are three main ways to measure the speed at which the universe expands, but each one of them presents a different result.  But maybe that's not all that important — at least for some experts in these measurements.
 Astronomers find new challenges in their quest for the universe's expansion rate
 Astronomers use gravitational lenses to measure the universe's expansion rate
 Measurement suggests the universe is 13.7 billion years old, but there are still uncertainties
 Among them is researcher and professor Wendy Freedman, who made some of the first measurements of the universe's expansion rate.  She has been working for more than 20 years on this endeavor, but in a new article, she presented some conclusions that may frustrate those who advocate one method over another, or even those who are looking for a new physics to explain the discrepancy between the results.
 Before we get to Freedman's conclusion — which isn't quite the end point — it's important to point out that accurate measurement of the rate of expansion is important for astrophysicists because mathematical models and things like the age of the universe depend on it.  So different methods were used, all theoretically correct, but each gave slightly different results.  It's like using three rulers to measure a wall and get numbers a few inches apart.
 All the different methods have been tested several times, and the results remain inconsistent.  Are they:
 Cosmic background radiation: This dim light left over from the Big Bang is one of the most important tools in cosmology today.  The measurements that used it as a “ruler” resulted in a rate of 67.4 km/s/megaparsec.
 Stars and Galaxies in the Near Universe: Knowing how fast the universe expands, we can calculate how fast galaxies are moving away from the Milky Way.  To accurately calculate the distance of galaxies, Freedman, a great expert in this method, uses Cepheid stars, reaching a value of 72 km/s/megaparsec.
 Red Giants: These huge stars at the end of their lives always reach the same maximum brightness before disappearing.  If their distances are measured accurately, distances to their host galaxies can reveal the speed of expansion of the universe.  Freedman used this method in 2019 and the result was 69.8 km/s/Mpc
 The first version of the red giant study used a single, very close galaxy to calibrate the luminosity of red giant stars.  Over the next two years, which bring us to the present day, the professor and her colleagues did the calculation for several galaxies and different populations of stars.  "There are now four independent ways to calibrate the brightness of the red giants, and they agree to be within 1% of each other," said Freedman.
 What can this tell scientists?  Is there an unknown element in the physics of the universe?  If that is the case, a lot would have to be re-evaluated in cosmological models and in physics in general.  But Freedman doesn't think that will be necessary.  For her, the small difference that the technique with red giants “indicates that this is a very good way to measure distance”.
 expansion of the universe
 Furthermore, she argues that the differences in outcome between the methods — 67.4, 72, and 69.8 — may not be all that relevant.  In fact, maybe there is no conflict.  "No new physics is needed," said Freedman.  "The Cepheid stars have always been a little noisier and trickier to understand; they're young stars in the active star formation regions of galaxies, and that means there's the potential for things like dust or contamination from other stars to get in the way of their measurements," she explained. .
 This doesn't mean that Cepheids are a bad method of measuring distances, not for experienced astronomers like Freedman.  “I really wanted to look carefully at both the Cepheids and the red giants,” she said. “I know their strengths and weaknesses well.” But the results with the red giants “show they are consistent.” Finally, she argues that the value obtained with the red giants is virtually the same value with respect to measurements with the cosmic background radiation.
 But that doesn't convince astrophysicists who are looking for precise, unambiguous numbers for math to work in their models.  To resolve the impasse, Freedman suggests the next instruments, such as the James Webb telescope.  It "will give us greater sensitivity and resolution, and the data will get better very, very soon," she said, concluding that the results so far are actually consistent. "That's the way the science goes," said Freedman.  "You kick the tires to see if something deflates and, so far, no flat tires", she concludes

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