An article published in “The Astrophysical Journal” reports a new measurement of the expansion of the universe, which is approximately 9% faster than the estimates made by studying the early universe. A team of astronomers led by Nobel laureate Adam Riess combined observations made with the Hubble Space Telescope of 70 variable stars called Cepheid variables used for measurements with others conducted by the Araucaria project to obtain extremely precise measurements of their brightness. The discrepancy between the measurements of the expansion of the near universe and those of the early universe remains and it’s important to improve the measurements to obtain clues to the origin of the discrepancy.
Adam Riess was among the scientists who won the Nobel Prize for the discovery that the expansion of the universe is accelerating. His research on the subject has been going on for decades trying to obtain increasingly precise measurements of the phenomenon. The expansion speed of the universe is also called the Hubble constant after Edwin Hubble, the astronomer who provided the first evidence of the expansion of the universe.
What was defined as a tension in the field of astrophysics is given by the discrepancy between the measurements made 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, and the ones carried out by studying the early universe with the Planck Surveyor space probe.
The discrepancy is a problem of which currently we don’t understand the reason. Adam Riess explained that these are not just two experiments in disagreement but two fundamentally different measurements: one is the measure of how the universe is expanding today while the other is a prediction based on the early universe physics and on measures of how fast it should expand. For this reason he stated that there’s a strong probability that we lack something in the cosmological model that connects the two eras, repeating a concept he expressed in the past, for example on the occasion of the publication again in “The Astrophysical Journal” in July 2018 of a previous estimate of the speed of expansion of the universe.
For this new measurement, Adam Riess’s team focused on Cepheid variables using a new method to observe a dozen at a time using the Hubble Space Telescope. Previously in the same period of time only one of them was observed and this allowed to study 70 of these stars in the Large Magellanic Cloud. Other data were collected thanks to the Araucaria project, an international collaboration that aims to improve the measurement of distances of extragalactic objects, including the Cepheid variables object of this research.
The image (NASA, ESA, A. Riess (STScI/JHU) and the Palomar Digitized Sky Survey) shows an image of the Large Magellanic Cloud captured by a ground-based telescope and in the inset a star cluster containing Cepheid variables photographed by the Hubble Space Telescope.
The precision achieved in this new research thanks to the new data accumulated on the 70 Cepheid variables reduces the measurement uncertainty to 1.9% but it doesn’t solve the problem. Not only does the discrepancy between the measurements made with the two methods remain, but it indicates even more that our models are incomplete. In fact, before this last calculation there was a 1 in 3,000 probability that the discrepancy was due to problems related to measurements, the greater precision achieved brings that probability to 1 in 100,000.
There’s no explanation for this discrepancy. On the other hand, the acceleration of the expansion of the universe hasn’t a proven explanation either because dark energy is currently a hypothesis and there are various alternatives such as that of a universe at the edge of a bubble in a five-dimension space-time. The measurements continue with the confidence that increasingly accurate observations can lead to the identification of the cause of the discrepancy.