New Data Strengthens A Huge Problem That Currently Exists In Cosmology

Artist's conception illustrating a disk of maser emission from a water-bearing gas orbiting the supermassive black hole at the core of a distant galaxy. By observing maser emission from such disks, astronomers can use geometry to measure the distance to the galaxies, a key requirement for calculating the expansion rate of the universe. Sophia Dagnello, NRAO/AUI/NSF

Our best understanding of the universe as a whole is gathered together in the Standard Model of Cosmology. Over the last few years, researchers have found a major tension within one of the central tenets. Estimates of the expansion rate of the universe obtained with different methods end up producing different numbers, suggesting the universe might be more complex than we have come to believe.

A new study has strengthened this tension by adding new data to the problem. The measure of the expansion rate of the universe obtained with the cosmic microwave background is about 67.66 (km/s)/Mpc where Mpc stands for Megaparsec (a million parsec). However, measurements from galaxies over the last few years suggest that the value is something around 74 (km/s)/Mpc. 

The (km/s)/Mpc unit might seem peculiar but it becomes clearer once its history is known. It was devised by astronomer Edwin Hubble who first discovered that almost every galaxy in the universe is moving away from us. He noticed the relation, now known as Hubble’s law, between the speed at which the galaxy is receding (the km/s part) and the distance from us (expressed in megaparsec or Mpc). Measuring the recession speed of a galaxy is surprisingly easy but its distance is a whole other ballpark.

The new research, published in The Astrophysical Journal Letters, used improved measurements of the distance of four special galaxies. These galaxies host masers – radio waves analogous to lasers – produced by an active supermassive black hole. This rare configuration allows astronomers to refine distance measurements. They found that their expansion rate value is around 73.9 (km/s)/Mpc. 

“Testing the standard model of cosmology is a really challenging problem that requires the best-ever measurements of the Hubble Constant. The discrepancy between the predicted and measured values of the Hubble Constant points to one of the most fundamental problems in all of physics, so we would like to have multiple, independent measurements that corroborate the problem and test the model. Our method is geometric, and completely independent of all others, and it reinforces the discrepancy,” lead author Dom Pesce, a researcher at the Center for Astrophysics – Harvard and Smithsonian, said in a statement.

It is unclear exactly why this discrepancy exists, so it's difficult to know how to improve the Standard Model of Cosmology. Hoepfully, more observations will provide fresh insight into the cause of the deviation.

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