The measurement of the universe expansion based on the Hubble and Gaia space telescopes’ data shows the limits of current models

Scheme of Hubble and Gaia at work (Image NASA, ESA, and A. Feild (STScI))
Scheme of Hubble and Gaia at work (Image NASA, ESA, and A. Feild (STScI))

An article published in the journal “The Astrophysical Journal” describes a new measurement of the expansion of the universe. A team of astronomers led by Nobel Prize winner Adam Riess combined observations made with the Hubble Space Telescope and those made with ESA’s Gaia space probe, an observatory that specifically aims to map billions of objects in the sky including the variable stars called Cepheid variable used for those measurements. The new results increase the accuracy but also the discrepancy between the measures of the expansion of the near universe and those of the early universe.

Adam Riess has been busy researching the expansion of the universe for decades and was among the scientists who received the Nobel Prize for the discovery that the expansion of the universe is accelerating. To explain this phenomenon the hypothesis of the existence of dark energy was made but we know only this effect and very little else about it.

To try to understand the causes of the acceleration of the expansion of the universe, we need among other things to obtain precise measurements of this phenomenon. In honor of Edwin Hubble, the astronomer who provided the first evidence of the expansion of the universe, its estimated velocity was named the Hubble constant. A further problem emerging from the research is that the measurements are leading to what’s called a tension, an incompatibility between the present and the early universe.

The measurements of the expansion of the present universe are carried out using what are called standard candles, variable stars called Cepheid variables that have a very close correlation between their period of variability and their absolute brightness. Some type Ia supernovae can also be used for this purpose. In an article published in June 2016 again in the journal “The Astrophysical Journal” a team led again by Adam Riess used observations of the Hubble Space Telescope, which was named after Edwin Hubble as well.

Meanwhile, ESA launched its Gaia space probe to create a precise map of the sky and on April 25, 2018 published the second map, providing more very precise data on the distance and brightness of the Cepheid variables used to measure the expansion of the universe. The combination of Hubble and Gaia data indicate that the expansion rate is 73.52 ± 1.62 kilometers per second per megaparsec.

This new result is excellent but contradicts the one of about 67 kilometers per second per megaparsec obtained by studying the early universe with the Planck Surveyor space probe. This is another ESA special space telescope designed in this case to study the Cosmic Microwave Background Radiation, used as a basis for the other Hubble constant measurement method.

Even taking into account the uncertainty of the measurements, reduced to 2.2% in this new research thanks to the combination of Hubble and Gaia’s data, there’s a discrepancy between the measurements made with the two methods. Adam Riess’s team intends to keep on refining the measurement based on Cepheid variables using the data that Gaia keeps on collecting but the discrepancy requires an explanation.

The possible explanations are based on the characteristics of dark matter or dark energy but these are hypotheses. More and more sophisticated instruments allow to collect new data to better understand this mystery. It’s precisely where inconsistencies with the existing models are found that scientists investigate to obtain progress that will give us a higher level of understanding of the cosmos.

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