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How did we get here? where are we going? How long will it take? These questions are as old as humanity itself, and if they were actually asked by other species elsewhere in the universe, they are likely much older than that.
They are also some of the fundamental questions we try to answer in the study of the universe, which is called cosmology. One cosmological dilemma is how fast the universe is expanding, which is measured by a number called the Hubble constant. There is a great deal 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 reached Earth via multiple zigzag paths through the expanding universe — to measure the Hubble constant. The papers have been published in Sciences And Astrophysical Journal.
And if our results don’t completely resolve the tension, they give us another 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 Levitt found a way to measure the intrinsic brightness of a type of star called a Cepheid variable—not how bright they are from Earth, which depends on distance and other factors, but how bright they really are. Cephids grow brighter and dimmer in a regular cycle, and Levitt showed that intrinsic brightness is related to the length of this cycle.
Levitt’s law, as it is now called, allows scientists to use Cepheids as “normative candles”: objects whose internal brightness is known and, therefore, their distance can be calculated.
How does this work? Imagine it’s night, and you’re standing in a long, dark street with a few lampposts running down the road. Now imagine that each lamppost has the same type of lamp with the same power. You will notice that the far ones appear weaker than the near ones.
We know that light fades in proportion to its distance, in something called the inverse square law of light. Now, if you could measure how bright each light was to you, and if you actually knew how bright it was, then you could tell how far each light pole was.
In 1929, another American astronomer, Edwin Hubble, was able to find a number of these Cepheid stars in other galaxies and measure the distance between them – and from those distances and other measurements, he was able to determine that the universe is expanding.
Different methods give different results
This standard candle method is a powerful one, as it 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 efforts to measure the universe far from Earth, such as the SH0ES project I was part of, led by Nobel Prize winner Adam Riess, have used kyphids along with a type of exploding star called a Type Ia supernova, which can also be used as a standard candle supernova.
There are also other ways to measure the Hubble constant, such as those that use the cosmic microwave background — 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 kevidids, and one very far away using the microwave background, differ by about 10%. Astronomers call this divergence the Hubble tension, and they are looking for new measurement techniques to solve it.
New method: gravitational lensing
In our new work, we have successfully used a new technique to measure the expansion rate of the universe. The work is based on a supernova called Supernova Refsdal.
In 2014, our team observed multiple images of the same supernova — the first time such a “lenticular” supernova had been observed. Instead of seeing one supernova, the Hubble Space Telescope saw five!
How does this happen? The light from the supernova shot off in all directions, but it traveled through space warped by the massive gravitational fields of a huge cluster of galaxies, bending some of the light’s path in such a way that it ended up on Earth via multiple paths. . Every supernova we come across is a different path through the universe.
Imagine three trains leaving the same station at the same time. However, one goes straight to the next stop, one makes a wide trek through the mountains, and one across the coast. They all leave and arrive at the same stations, but they take different journeys and so while they leave at the same time, they will arrive at different times.
So our lens images show the same supernova, which exploded at a certain point in time, but each image travels a different path. By looking at the arrival of each supernova apparition to Earth — one of which happened 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 grown while the apparition was in transit.
have we arrived
This gave us a different but unique measure of the growth of the universe. In research papers, we found this measurement to be closer to that of the cosmic microwave background, not to that of Cepheid and the nearby supernova. However, based on its location, it should be closer to Cepheid and Supernova measurement.
While this doesn’t settle the debate at all, it does give us another clue to consider. There could be a problem with the supernova value, our understanding of galaxy clusters and models that can be applied to lensing, 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.
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