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

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how did we get here Where are we going? How long does it take?

These questions are as old as mankind itself, and if they have been asked by other species elsewhere in the universe, perhaps even older.

These are also some of the fundamental questions that we try to answer as part of the study of the universe, known as cosmology.

A cosmological mystery is the rate at which the universe is expanding. This is measured using a number called the Hubble constant. And there’s quite a bit of tension.

In two recent papers led by my University of Minnesota colleague Patrick Kelly, we have successfully used a new technique — light from an exploding star that made its way to Earth via several tortuous routes through the expanding universe — to calculate the Hubble constant measure.

The articles will be published in Science and The Astrophysical Journal.

And if our findings don’t quite resolve the suspense, they give us another clue—and more questions to ask.

Standard candles and the expanding universe

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

Around 1908, US astronomer Henrietta Leavitt found a way to measure the intrinsic brightness of a type of star called the Cepheid variable – not how bright they appear from Earth, which depends on distance and other factors, but how bright they actually are .

Cepheids brighten and dimmer on a regular cycle, and Leavitt showed that intrinsic brightness is related to the length of this cycle.

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

How does this work? Imagine it’s night and you’re standing on a long, dark street with only a few light poles along it.

Now imagine that each light tower has the same type of bulb with the same wattage. You will notice that the distant objects appear dimmer than the nearby ones.

We know that light weakens proportionally with its distance, which is called the inverse square law of 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 find out how far away each light pole is.

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

Different methods lead to different results

This standard candle method is powerful and allows us to survey the vast universe. We’re always looking for other candles that can be better measured and seen from much greater distances.

Some recent attempts to map the universe further from Earth, like the SH0ES project I was involved in and led by Nobel laureate Adam Riess, have used Cepheids alongside a type of exploding star called a Type Ia supernova, the can also be used as a supernova standard candle.

There are other methods of measuring the Hubble constant, such as one that uses the cosmic microwave background – relic light, or radiation, that began traveling through the universe just after the Big Bang.

The problem is that these two measurements – one close by using supernovae and Cepheids and one much further away using the microwave background – differ by almost 10%.

Astronomers call this difference the Hubble voltage and have been looking for new measurement techniques to resolve it.

A new method: gravitational lenses

In our new work, we have successfully used a new technique to measure this rate of expansion of the universe. The work is based on a supernova called Supernova Refsdal.

In 2014, our team discovered multiple images of the same supernova – the first time such a “lensing” supernova had been observed. Instead of the Hubble Space Telescope seeing one supernova, we saw five!

How does this happen? The light from the supernova emanated in all directions, but it traveled through space, distorted by the enormous gravitational fields of a giant galaxy cluster, which bent part of the light’s path so that it eventually reached Earth via multiple paths.

Each supernova phenomenon had reached us on a different path through the universe.

Imagine three trains leaving the same station at the same time. However, one goes straight to the next station, the other makes a long journey through the mountains, and another over the coast.

They all depart from and arrive at the same stations but take different trips, so they depart at the same time but arrive at different times.

So our lens images show the same supernova exploding at a given point in time, but each image traveled a different path.

By observing the arrival of each supernova phenomenon on Earth – one of which occurred in 2015, after the exploding star had already been sighted – we were able to measure their travel time and hence how much the universe had grown while the recording was en route.

Are we already there?

This gave us a different but unique measure of the growth of the universe.

In the papers we note that this measurement is closer to the cosmic microwave background measurement than to the nearby Cepheid and Supernova measurement.

However, due to its location, it should be closer to the Cepheid and Supernova measurement.

While this is by no means the end of the debate, it does give us another clue to look at. There could be a problem with the supernova value, or our understanding of galaxy clusters and the models to apply to lensing, or something else entirely.

Just like the kids in the back of the car on a car ride ask, “Are we there yet?” we still don’t know.

Written by Brad E Tucker. The conversation.

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