How fast is the Universe really expanding? Multiple views of an exploding star raise new questions

How did we get here? Where are we going? And how long will it take? These questions are as old as humanity itself, and, if they’ve already been asked by other species elsewhere in the Universe, potentially very much older than that.

They are also some of the fundamental questions we are trying to answer in the study of the Universe, called cosmology. One cosmological conundrum is how fast the Universe is expanding, which is measured by a number called the Hubble constant. And there is quite a bit of tension around it.

In two new papers led by my colleague Patrick Kelly at the University of Minnesota, we have successfully used a new technique – involving light from an exploding star that arrived at Earth via multiple winding routes through the expanding Universe – to measure the Hubble constant. The papers are published in Science^[1] and The Astrophysical Journal^[2].

And if our results don’t quite resolve the tension, they do give us another clue – and more questions to ask.

Standard candles and the expanding Universe

We have known since the 1920s that the Universe is expanding.

Around 1908, US astronomer Henrietta Leavitt found a way to measure the intrinsic brightness of a kind of star called a Cepheid variable – not how bright they appear from Earth, which depends on distance and other factors, but how bright they really are. Cepheids grow brighter and dimmer in a regular cycle, and Leavitt showed the intrinsic brightness was related to the length of this cycle.

Leavitt’s Law, as it is now called, lets scientists use Cepheids as “standard candles”: objects whose intrinsic brightness is known, and therefore, whose distance can be calculated.

How does this work? Imagine it is night, and you are standing on a long, dark street with only a few light poles going down the road. Now imagine every light pole has the same type of light bulb, with the same power. You’ll notice the distant ones appear fainter than the nearby ones.

We know that light fades proportionately to its distance, in something called the inverse-square law for light. Now, if you can measure how bright each light appears to you, and if you already know how bright it should be, you can then figure out how far away each light pole is.

In 1929, another US astronomer, Edwin Hubble, was able to find a number of these Cepheid stars in other galaxies and measure their distance – and from those distances and other measurements, he could determine that the Universe was expanding.

Different methods give different results

This standard candle method is a powerful one, allowing us to measure the vast Universe. We are always looking for different candles that can be better measured, and seen at much greater distances.

Some recent efforts to measure the Universe further from Earth, like the SH0ES project I was a part of, led by Nobel laureate Adam Riess, have used Cepheids alongside a type of exploding star called a Type Ia supernova, which can also be used as a standard candle.

There are also other methods to measure Hubble’s constant, such as one that uses the cosmic microwave background – relic light or radiation that began to travel through the Universe shortly after the Big Bang.

The problem is that these two measurements, one nearby using supernovae and Cepheids, and one much farther away using the microwave background, differ by nearly 10%. Astronomers call this difference the Hubble tension, and have been looking for new measurement techniques to resolve it.

A new method: gravitational lensing

References

1. ^{^} Science (www.science.org)
2. ^{^} The Astrophysical Journal (iopscience.iop.org)
3. ^{^} P.L. Kelly et al., Science 10.1126/science.abh1322 (2023) (www.science.org)