How did we get here? Where are we going? And how long will it take? These questions are as old as humanity itself, and potentially much older than that if they have already been asked by other species in the universe. These are also some of the fundamental questions we try to answer in the study of the universe, called cosmology. One of the mysteries of cosmology is the expansion rate of the universe, as measured by a number called the Hubble constant. And there’s some tension around that.

In two recent papers led by my colleague Patrick Kelly from the University of Minnesota, we’ve successfully used a new technique to measure the Hubble constant – capturing light from an exploding star that comes to Earth through several detours through the expanding universe. Published articles Science And Astrophysical Journal. If our results don’t fully resolve the tension, they give us another clue and more questions to ask.

We’ve known since the 1920s that the universe is expanding. Around 1908, American astronomer Henrietta Leavitt devised a way to measure a star’s own brightness, called the Cepheid variable. Cepheids brighten and darken in a regular cycle, and Leavitt showed that intrinsic brightness is related to the length of this cycle.

Levitt’s law, as they are now called, allows scientists to use Cepheids as “standard candles”: objects whose intrinsic brightness is known and therefore their distance can be calculated.

How does it work? Imagine that it is night and you are standing in a long, dark street with only a few light poles. Now imagine that each light post has the same type of bulb with the same wattage. You will notice that distant images appear dimmer than those closer.

We know that according to the inverse square law of light, light weakens in proportion to its distance. Now, if you can measure how bright each light looks to you, and if you already know how bright it should be, you can determine how far away each light pole is. In 1929, another American astronomer, Edwin Hubble, was able to locate a few of these Cepheid stars in other galaxies and measure their distances, and based on these distances and other measurements, he was able to determine that the universe is expanding.

Different methods give different results

This standard candle method is powerful and allows us to measure the vast universe. We are always looking for different candles that can be measured better and seen from a much greater distance.

Some recent efforts to measure the universe farther from Earth, such as the SH0ES project I’m involved in, led by Nobel laureate Adam Riess, have used Cepheids alongside an exploding star called a Type Ia supernova, which can also be used as a Standard. candle. There are other methods of measuring the Hubble constant, such as those using the cosmic microwave background, such as residual light or radiation that began circulating in the universe shortly after the Big Bang.

The problem is that the two measurements, one using supernovas and Cepheids nearby, and the other much farther away, using the microwave background, differ by almost 10%. Astronomers call this difference the Hubble voltage and are looking for new measurement techniques to solve it.

A new method: gravitational lensing

In our new study, we have successfully used a new technique to measure this expansion rate of the universe. The study is based on a supernova called Supernova Refsdal. In 2014, our team observed multiple images of a single supernova; This is the first time such a “lensed” supernova has been observed. Instead of the Hubble Space Telescope seeing one supernova, we saw five!

How does this happen? The light from the supernova shone in all directions, but bent part of the light’s path and traveled through space distorted by the vast gravitational fields of a large galaxy cluster that took multiple routes to reach Earth. . Each occurrence of a supernova has reached us from different paths in the universe.

Consider three trains leaving the same station at the same time. But one goes straight to the next station, the other makes a wide journey through the mountains, the other between the coasts. They all go to and arrive at the same stations but on different journeys, so they arrive at different times even though they leave at the same time.

Our lensed images show the same supernova exploding at a given point in time, but each image takes a different path. By looking at the arrival of each supernova occurrence to Earth (one of which occurred in 2015, after the exploding star was observed), we were able to measure their travel times and thus how much the universe had grown while the image was there. path

Have we arrived yet?

This has given us a different but unique dimension to the growth of the universe. In the papers, we found that this measurement is closer to measuring the cosmic microwave background than measuring Cepheids and nearby supernovae. However, it should be closer to the Cepheid and Supernova measurement due to its location.

While this doesn’t quite settle the debate, it does give us another clue to consider. It could be a problem with the meaning of supernova, or with our understanding of galaxy clusters and the patterns that apply to lensing, or something else entirely. We still don’t know, much like children in the back seat of a car ride asking “are we here”.