Almost 90 years ago Edwin Hubble demonstrated that the universe was expanding. A decade ago two teams of astronomers proved that not only is the universe expanding, but it’s speeding up due to a mysterious force called dark energy. It was this discovery which gave us the most commonly used model of the universe – the Lambda-CDM. This tells us that that universe is made up of around 70% dark energy, around 25% dark matter, and around 5% of what we consider to be normal matter (the stuff we can see).

For years this Lambda-CDM fit in nicely with decades of data gathered about the universe. But then in 2016 a research team used the Hubble Space Telescope to do some digging around and discovered that the universe was expanding at a much faster rate than once thought.





The key to understanding the rate of expansion is lying in strongly lensed Type la supernovae, according to astrophysics at Berkeley Lab. In a recent paper published in the Astrophysical Journal, they explain how to control microlensing and also demonstrate how to identify these events in real time.

“Ever since the CMB result came out and confirmed the accelerating universe and the existence of dark matter, cosmologists have been trying to make better and better measurements of the cosmological parameters, shrink the error bars,” explains Peter Nugent one of Berkeley Lab’s astrophysicists and co-author on the paper. “Our paper presents a path forward for determining whether the current disagreement is real or whether it’s a mistake.”

In space, the farther away an object is, the longer it takes its light to reach Earth. Therefore, the farther out you look, the further back in time you’ll see. Type la supernovae have always been useful in measuring distance as they’re extremely bright and can be seen from almost anywhere in the cosmos. It was looking at these that helped scientists discover that dark energy is causing the universe to expand.

But last year an even more accurate marker was discovered in the form of strongly lensed Type la supernova. This occurs when the gravitational field of a large object bends and distorts light coming from a Type la behind it. The gravitational lensing causes the supernova to appear even brighter than normal and sometimes in multiple places. By tracking the time in which it takes for light to arrive from different images of the Type la event, astrophysicists can determine an accurate measurement of the rate of expansion of the cosmos.





After the completion of several simulations of supernova light at the National Energy Research Scientific Computing Center (NERSC), the researchers are confident they’ll find around 1,000 strongly lensed Type la supernovae in the data. “With three lensed quasars – cosmic beacons emanating from massive black holes in the centers of galaxies – collaborators and I measured the expansion rate to 3.8% precision,” explains Thomas Collett, an astrophysicist at the University of Portsmouth and co-author on the paper. “We got a value higher than the CMB measurement, but we need more systems to be really sure that something is amiss with the standard model of cosmology.”

The simulations also helped the researchers prove how strongly lensed Type la supernovae are in fact very accurate cosmological probes. “When cosmologists try to measure time delays, the problem they often encounter is that individual stars in the lensing galaxy can distort the light curves of the different images of the event, making it harder to match them up,” explains Danny Goldstein, a UC Berkeley graduate student and lead author on the paper. “This effect, known as ‘microlensing’ makes it harder to measure accurate time delays, which are essential for cosmology.”

However, after running their first set of simulation, the researchers found that microlensing had no effect on the color changing of the strongly lensed Type la supernovae during their early phases. This allowed the researchers to remove the unwanted effects of microlensing, making it easier to take more accurate cosmological measurements.

The supernovae were then modeled using the SEDONA code. “The simulations give us a dazzling picture of the inner workings of a supernova, with a level of detail that we could never know otherwise,” says Daniel Kasen, an astrophysicist in Berkeley Lab’s Nuclear Science Division and a co-author on the paper. “Advances in high-performance computing are finally allowing us to understand the explosive death of stars, and this study shows that such models are needed to figure out new ways to measure dark energy.”

Just a few years from now, in 2023, the Large Synoptic Survey Telescope (LSST) will begin its full survey operations. What this means is that it will then begin scanning the entire sky over a 10-year period. Researchers in charge of the mission is expecting LSST to deliver around 200 petabytes of data.





Nugent and Goldstein are also hoping to contribute to the mission by running some of this data through Nugent’s Real-Time Transient Detection pipeline located at NERSC in hopes of finding transient objects that change in brightness. Nugent confirms that once researchers have identified the first light emanating from a strongly lensed supernova event, they can use computational modeling to predict precisely when the next light is going to appear.

“I came to Berkeley Lab 21 years ago to work on supernova radiative-transfer modeling and now for the first time we’ve used these theoretical models to prove that we can do cosmology better,” says Nugent. “It’s exciting to see DOE reap the benefits of investments in computational cosmology that they started making decades ago.”

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