Video transcript

The universe is expanding at an accelerating rate, but just how fast is that expansion?

We thought we knew the answer to that question. But recent findings have cast doubt on our understanding.

This story begins in the US in the 1920s. Back then, most astronomers believed that our galaxy was all that existed – it was the entire universe.

But observations by the astronomer Edwin Hubble would transform our understanding of our place the cosmos.

During the previous decade, the American astronomer Henrietta Swan Leavitt had identified the relation between the luminosity and the period for a class of pulsing star known as Cepheid variables.

This key discovery enabled astronomers to accurately predict the distance to these stars, providing a sort of cosmic tape measure for measuring far larger distances than had been possible before.

While studying cepheid variables in the spiral nebulae, Hubble realised that these stars must be located far beyond our galaxy.

That led to the realisation that the Milky Way was just one galaxy among many.

Soon, Hubble discovered that almost all of these other galaxies are moving away from us. This led the Belgian cosmologist Georges Lemaître to conclude that the universe is expanding.

Hubble and Lemaître independently derived a mathematical relationship to describe this expansion. The recession velocity (v) of a galaxy is equal to its distance (D) multiplied by the Hubble constant – a value which describes the rate of expansion at the current time.

Traditionally, the Hubble Constant is determined by measuring the distance and recession velocity of galaxies, by using astronomical objects of known brightness. These so-called “standard candles” include type 1a supernovae – which are basically exploded white-dwarf stars that have a certain critical mass.

Now, let’s fast-forward to 2013.

By analysing maps of the Cosmic Microwave Background from the Planck mission, astronomers were able to calculate our most precise value yet of the Hubble constant: 67.4 kilometers per second per megaparsec.

In itself, it can be hard to get your head around what the Hubble Constant means. But it matters because it dictates the age of the universe, and our understanding of how it evolved.

So it was a big shock in 2016 when a project led by Nobel Prize winner Adam Riess arrived at a significantly higher value for the Hubble Constant: 73.2 kilometers per second per megaparsec. That value would suggest the universe is younger than we thought.

Riess led the SH0ES Project, which involved measuring our old friends the Cepheid variables, the same type of star that had enabled Swann Leavitt’s and Hubble’s breakthroughs.

The SH0ES result appears to be backed up by another project with a quirky name, H0LiCOW.

That project, led by Sherry Suyu, calculates the Hubble constant using another ingenuous method: the gravitational lensing of light emitted from quasars. These are luminous active galactic nuclei that exhibit brightness variations.

Looking ahead, if the discrepancy between the Planck measurement and the more local measurements of the Hubble constant becomes stronger, then we might be looking at new physics.

Another different measurement of the Hubble constant may come from the emerging field of gravitational-wave astronomy.

In theory, astronomers could get an accurate value of the Hubble Constant by observing the gravitational waves and light emitted by the merger of two neutron stars. But unfortunately, these incredibly energetic events, known as kilonovae, are proving to be few and far between.

Find out more about the Hubble constant mystery in the July 2020 issue of Physics World.