The Hubble Constant

 

One property that astronomers have tried to use to help them do this, however, is a number known as the Hubble Constant.

“It’s a measure of how fast the universe is expanding at the current time,” says Wendy Freedman, an astrophysicist at the University of Chicago who has spent her career measuring it. “The Hubble Constant sets the scale of the Universe, both its size and its age.”

It helps to think about the Universe like a balloon being blown up. As the stars and galaxies, like dots on a balloon’s surface, move apart from each other more quickly, the greater the distance is between them. From our perspective, what this means is the further away a galaxy is from us, the faster it is receding.

Unfortunately, the more astronomers measure this number, the more it seems to defy predictions built on our understanding of the Universe. One method of measuring it directly gives us a certain value while another measurement, which relies on our understanding of other parameters about the Universe, says something different. Either the measurements are wrong, or there is something flawed about the way we think our Universe works.

But scientists now believe they are close to an answer, largely thanks to new experiments and observations aimed at finding out exactly what the Hubble Constant really is.

“What faces us as cosmologists is an engineering challenge: how do we measure this quantity as precisely and accurately as possible?” says Rachael Beaton, an astronomer working at Princeton University. To meet this challenge, she says, requires not only acquiring the data to measure it, but cross-checking the measurements in as many ways as possible. “From my perspective as a scientist, this feels more like putting together a puzzle than being inside of an Agatha Christie style mystery.”

The first ever measurement of the Hubble Constant in 1929 by the astronomer whose name it carries – Edwin Hubble – put it at 500km per second per megaparsec (km/s/Mpc), or 310 miles/s/Mpc. This value means that for every megaparsec (a unit of distance equivalent to 3.26 million light years) further away from Earth you look, the galaxies you see are hurtling away from us 500km/s (310 miles/s) faster than those a megaparsec closer.

Two competing forces

• the pull of gravity and the outwards push of radiation
• played a cosmic tug of war with the universe in its infancy

Over a century since Hubble’s first estimate for the rate of cosmic expansion, that number has been revised downwards time and time again. Today’s estimates put it at somewhere between 67 and 74km/s/Mpc (42-46 miles/s/Mpc).

Part of the problem is that the Hubble Constant can be different depending on how you measure it.

Most descriptions of the Hubble Constant discrepancy say there are two ways of measuring its value – one looks at how fast nearby galaxies are moving away from us while the second uses the cosmic microwave background (CMB), the first light that escaped after the Big Bang.

We can still see this light today, but because of the distant parts of the universe zooming away from us the light has been stretched into radio waves. These radio signals, first discovered by accident in the 1960s, give us the earliest possible insight into what the Universe looked like.

Two competing forces – the pull of gravity and the outwards push of radiation – played a cosmic tug of war with the universe in its infancy, which created disturbances that can still be seen within the cosmic microwave background as tiny differences in temperature.

Using these disturbances, it is then possible to measure how fast the Universe was expanding shortly after the Big Bang and this can then be applied to the Standard Model of Cosmology to infer the expansion rate today. This Standard Model is one of the best explanations we have for how the Universe began, what it is made of and what we see around us today.

But there is a problem. When astronomers try to measure the Hubble Constant by looking at how nearby galaxies are moving away from us, they get a different figure.

“If the [standard] model is correct, then you would imagine that the two values – what you measure today locally and the value that you infer from the early observations would agree,” says Freedman. “And they don’t.”

When the European Space Agency (ESA)’s Planck satellite measured discrepancies in the CMB, first in 2014 then again in 2018, the value that comes out for the Hubble constant is 67.4km (41.9 miles)/s/Mpc. But this is around 9% less than the value astronomers like Freedman have measured when looking at nearby galaxies.

Further measurements of the CMB in 2020 using the Atacama Cosmology Telescope correlated with the data from Planck. “This helps to rule out that there was a systematic problem with Planck from a couple of sources” says Beaton. If the CMB measurements were correct – it left one of two possibilities: either the techniques using light from nearby galaxies were off, or the Standard Model of Cosmology needs to be changed.

The technique used by Freedman and her colleagues takes advantage of a specific type of star called a Cepheid variable. Discovered around 100 years ago by an astronomer called Henrietta Leavitt, these stars change their brightness, pulsing fainter and brighter over days or weeks. Leavitt discovered the brighter the star is, the longer it takes to brighten, then dim and then brighten again. Now, astronomers can tell exactly how bright a star really is by studying these pulses in brightness. By measuring how bright it appears to us on Earth, and knowing light dims as a function of distance, it provides a precise way of measuring the distance to stars.