Using red giants to estimate the speed of the universe expansion

An article being published in “The Astrophysical Journal” reports a new attempt to calculate the speed of the universe expansion, this time using red giant stars as a reference. A team of researchers coordinated by Carnegie Institution for Science and University of Chicago and led by astronomer Wendy Freedman used observations made with the Hubble space telescope to perform that calculation. The result has a probability peak at 69.8 km/s per megaparsec, between the values ​​calculated using the two methods that provided discrepant values.

What was defined as a tension in the field of astrophysics is given by the discrepancy between the measurements made using the so called standard candles, variable stars called Cepheid variables that have a very close correlation between their period of variability and their absolute brightness, and the ones carried out by studying the early universe with the Planck Surveyor space probe. An alternative to cepheids are supernovae but their use hasn’t changed the terms of the problem.

For years, the problem has been involving important people in the world of astronomy such as the astrophysicist Nobel prize winner Adam Riess. Wendy Freedman, former director of the Carnegie Observatories in Pasadena and now a professor at the University of Chicago, is an important researcher specializing in that type of study since in 2001 she directed a team that measured the speed of the universe expansion using cepheids as reference. Almost a century ago another Carnegie astronomer, Edwin Hubble, discovered the expansion of the universe and that’s why the speed at which it expands was called the Hubble constant.

A different method from the two used to calculate the Hubble constant could be useful to try to understand which of the ones calculated is closer to the right one, allowing to investigate with some certainty the farthest result to understand whether there’s any methodological error or something we don’t yet know of the universe that leads to systematic calculation errors. For example, there may be something in the cepheids used as standard candles that astronomers still don’t know and generate calculation errors. For this reason, Wendy Freedman’s team looked for another type of standard candle and ended up choosing red giant stars.

When a small or medium-size star, like the Sun, approaches the end of its life, it starts expanding becoming a red giant. A moment of that phase is what in jargon is called helium flash, when their temperature rises to a peak around 100 million degrees after which their brightness drops considerably. That peak can be used as a standard candle because its brightness is the same for all those stars, consequently by measuring the brightness seen from the Earth it’s possible to calculate its distance precisely.

The Hubble Space Telescope – also named after Edwin Hubble – is often at the center of this research because it’s used to observe various celestial objects. The image (NASA, ESA, W. Freedman (University of Chicago), ESO, and the Digitized Sky Survey) shows in the top row the galaxies in which the standard candles were searched for the new calculation and precisely in their halos, with the field of view indicated in the insets magnified in the central row where there are in turn other insets magnified again in the lower row, with the red giants in the yellow circles.

The result of the study didn’t give the outcome they hoped given that the calculated value for the Hubble constant has a probability peak at 69.8 km/s per megaparsec, between 74.03 km/s per megaparsec obtained using cepheids and 67.4 km/s per megaparsec obtained using the Planck Surveyor’s data even if it’s closer to the latter.

Recently an article published in the journal “Nature Astronomy” reported the use of observations of the neutron stars merger recorded on August 17, 2017 to try to calculate the value of the Hubble constant but it’s a long-term task because they need to find more events of that type to get a precise measurement.

Meanwhile, new possibilities could come from the Wide Field Infrared Survey Telescope (WFIRST) mission, a space telescope that could provide new observations of cepheids, supernovae and red giants to make even more precise calculations of the Hubble constant. There are some problems in the mission financing so we must hope that it will proceed and can offer new information that will help solve this crucial cosmological problem.

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