How did we get here? Where are we going? And how long does it take? These questions are as old as humanity itself, and if they have already been asked by other species elsewhere in the universe, they may be much older.
They are also some of the fundamental questions we try to answer in the study of the universe called cosmology. A cosmological conundrum is how fast the universe is expanding, which is measured by a number called the Hubble constant. And there’s a lot of tension around that.
In two new papers led by my colleague Patrick Kelly at the University of Minnesota, we successfully used a new technique – using light from an exploding star that arrived at Earth via multiple twisting routes through the expanding universe – to calculate the Hubble constant. to measure. The articles are published in Science and The Astrophysical Journal.
And if our results don’t quite resolve the tension, they give us one more clue — and more questions to ask.
Standard candles and the expanding universe
We’ve known since the 1920s that the universe is expanding.
Around 1908, American 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. are. Cepheids become brighter and fainter in a regular cycle, and Leavitt showed that the intrinsic brightness was related to the length of this cycle.
Leavitt’s law, as it is now known, allows scientists to use Cepheid variables as “standard candles”: objects whose intrinsic brightness is known and whose distance can be calculated.
How does this work? Imagine it’s night time and you’re standing in a long, dark street with only a few light poles crossing the road. Now imagine that every light pole has the same type of bulb, with the same wattage. You will notice that the distant ones seem fainter than the near ones.
We know that light fades proportionally to its distance, in something called the inverse square law for light. Now if you can measure how bright each light appears in front of you, and if you already know how bright it should be, you can figure out how far away each light pole is.
In 1929, another American astronomer, Edwin Hubble, was able to find some of these Cepheids in other galaxies and measure their distance. Based on those distances and other measurements, he was able to determine that the universe was expanding.
Different methods give different results
This standard candlestick method is a powerful one that allows 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 attempts to measure the universe farther from Earth, such as the SH0ES project I was part of, led by Nobel laureate Adam Riess, have used Cepheid variables in addition to a type of exploding star called a Type Ia supernova, which can also be used as a standard candle.
There are also other methods of measuring Hubble’s constant, such as one that uses the cosmic microwave background — relic light or radiation that began traveling through the Universe shortly 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 are looking for new measurement techniques to solve it.
A new method: gravitational lensing
In our new work, we successfully used a new technique to measure this expansion rate of the universe. The work is based on a supernova called Supernova Refsdal.
In 2014, our team saw multiple images of the same supernova – the first time such a “lens” supernova had been observed. Instead of the Hubble Space Telescope seeing one supernova, we saw five!
How did this happen? The supernova’s light went in all directions, but it traveled through space that was warped by the massive gravitational fields of a massive cluster of galaxies, bending part of the light’s path so that it eventually took multiple routes to Earth. came . Each supernova appearance had reached us by a different path through the universe.
Imagine three trains departing from the same station at the same time. One, however, goes straight to the next station, the other makes a wide trip through the mountains, and yet another along the coast. They all depart and arrive at the same station, but make different journeys and so, while departing at the same time, they will arrive at different times.
So our lensed images show the same supernova exploding at one point in time, but each image took a different path. By looking at the arrival at Earth of each supernova appearance – one of which occurred in 2015, after the exploding star had already been spotted – we were able to measure their travel time, and thus how much the Universe had expanded while the image was transiting .
Are we there yet?
This gave us a different, but unique, measurement of the growth of the universe. In the papers we find this measurement closer to the cosmic microwave background measurement, rather than the nearby Cepheid and supernova measurement. However, based on its location, it should be closer to the Cepheid and supernova measurement.
While this doesn’t settle the debate at all, it gives 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 lenses, or something else entirely.
Like the kids in the back of the car on a road trip asking “are we there yet”, we still don’t know.