How quickly is the universe expanding? The Hubble constant estimates

Image from the Hubble Space Telescope of a doubly imaged quasar. How quickly is the universe expanding? The Hubble constant estimates
Image from the Hubble Space Telescope of a doubly imaged quasar. © NASA

One of the key questions for astronomers is how quickly the universe is expanding. Hubble constant estimates are central to answering this question.

A team of UCLA astronomers has created a novel way to measure how quickly the cosmos is expanding, which could lead to more precise measurements of the Hubble constant estimates.

What is the Hubble constant?

The Hubble constant is an essential aspect of the debate about the speed of the universe’s expansion. It is a number relating distances to the redshifts of galaxies (the amount of light which is stretched as it travels to Earth through the universe). The Hubble constant estimates range from around 67 to 73 kilometres per second per megasec. In other words, this means that two points in space 1 megaparsec apart, the equivalent of 3.26 million lightyears away, are moving away from each other at the speed of between 67 to 73 kilometres.

Due to the lack of a precise value for the Hubble constant, astronomers cannot accurately determine the sizes of remote galaxies. They are also unable to precisely determine the history of the universe’s expansion or the age of the universe.

Simon Birrer, who is the lead author of the story, and a UCLA postdoctoral scholar commented: “The Hubble constant anchors the physical scale of the universe.”

Are all of the estimates incorrect?

The team estimated about 72.5 kilometres per second per megaparsec. This figure is in line with what other Hubble constant estimates determined in research that used distances to supernovas as a method of measurement. Despite this, both estimates are approximately eight percent higher than one that relies on the cosmic microwave background (the faint glow from all over the sky, which is a relic of the Big Bang).

Tommaso Treu, a UCLA professor of physics and astronomy and the senior author of the paper, explained: “If there is an actual difference between those values, it means the universe is a little more complicated.” However, Treu also explained that it could mean that one of the measurements – or all three Hubble constant estimates – are incorrect.

The team are continuing their research to try and improve the precision of their Hubble constant measurement.

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