The results will improve experiments with dark energy on large telescopes
DOE / LAWRENCE BERKELEY NATIONAL LABORATORY
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PICTURE: THE TOP LEFT PICTURE SHOWS THE SPECTRA – BRIGHTNESS VS WAVELENGTH – FOR TWO SUPERNOVA. One is near and one is very far away. To measure dark energy, scientists need to … read more. CREDIT: GRAPHIC CREDIT: ZOSIA ROSTOMIAN / BERKELEY LAB; PHOTO CREDIT: NASA / ESA)
Cosmologists have found a way to double the accuracy of measuring the distance to supernova explosions – one of their proven tools for studying the mysterious dark energy that makes the universe expand faster and faster. The results of the collaboration near the Supernova Factory (SNfactory), led by Greg Aldering of the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), will enable scientists to study dark energy with greatly improved precision and accuracy, and a powerful cross-check perform the technique over long distances and time. The results will also be central to important upcoming cosmological experiments that use new ground and space telescopes to test alternative explanations of dark energy.
Two articles published in the Astrophysical Journal report these results, with Kyle Boone as the lead author. Boone is currently a postdoctoral fellow at the University of Washington and a former PhD student to Nobel Prize winner Saul Perlmutter, Berkeley Lab senior scientist and professor at UC Berkeley who led one of the teams that originally discovered dark energy. Perlmutter was also a co-author of both studies.
Supernovae were used in 1998 to make the surprising discovery that the expansion of the universe is accelerating rather than slowing down as expected. This acceleration – attributed to dark energy which makes up two-thirds of all energy in the universe – has since been confirmed by a variety of independent techniques, as well as more detailed studies of supernovae.
The discovery of dark energy was based on the use of a certain class of supernovae, Type Ia. These supernovae always explode with almost the same intrinsic maximum brightness. Since the supernova’s observed maximum brightness is used to infer its distance, the small remaining variations in intrinsic maximum brightness limited the precision with which dark energy could be tested. Despite 20 years of improvements by many groups, supernovae studies of dark energy have so far been limited by these variations.Quadrupling the number of supernovae
The new results announced by the SNfactory come from a multi-year study that is exclusively dedicated to increasing the precision of cosmological measurements with supernovae. The measurement of dark energy requires comparisons of the maximum brightness of distant supernovae billions of light years away with those of neighboring supernovae “only” 300 million light years away. The team examined hundreds of such supernovae in close proximity in great detail. Each supernova was measured several times at intervals of a few days. Each measurement examined the spectrum of the supernova and recorded its intensity over the wavelength range of visible light. An instrument tailored for this study, the SuperNova Integral Field Spectrometer, installed on the 2.2-meter telescope at the University of Hawaii in Maunakea, was used to measure the spectra.
“We had the idea for a long time that if the physics of the explosion of two supernovae were the same, their maximum brightness would be the same. With the spectra of the supernova factory nearby as a kind of CAT scan through the supernova explosion, we were able to test this idea, ”said Perlmutter.
A few years ago, the physicist Hannah Fakhouri, then a doctoral student at Perlmutter, made a discovery key for today’s results. Looking at a large number of spectra recorded by the SNfactory, she found that in a number of cases the spectra of two different supernovae looked almost identical. Some of the 50 or so supernovae were practically identical twins. When the shaky spectra of a pair of twins were superimposed, there was only a single trace to the eye. The current analysis builds on this observation in order to model the behavior of supernovae in the time near the time of their maximum brightness.
The new work nearly quadruples the number of supernovae used in the analysis. This made the sample large enough to use machine learning techniques to identify these twins, leading to the discovery that Type Ia supernova spectra only vary in three ways. The intrinsic brightnesses of the supernovae also depend mainly on these three observed differences, which makes it possible to measure supernova distances with a remarkable accuracy of about 3%.
Equally important, this new method does not suffer from the distortions that occurred with previous methods when comparing supernovae found in different types of galaxies. Since nearby galaxies are slightly different from distant galaxies, there has been serious concern that such a dependency would lead to incorrect readings when measuring dark energy. Now, by measuring distant supernovae with this new technique, this problem can be greatly reduced.
In describing this work, Boone noted, “Conventional measurement of supernova distances uses light curves – images captured in multiple colors as a supernova brightens and fades. Instead, we used a spectrum from each supernova. These are so much more detailed, and machine learning techniques then made it possible to discover the complex behavior that was critical to measuring more accurate distances. “
The results of Boone’s work will benefit two upcoming large-scale experiments. The first experiment will be conducted at the 8.4-meter-long Rubin Observatory in Chile, which is under construction and whose Legacy for Space and Time, a joint venture between the Department of Energy and the National Science Foundation, was created. The second is NASA’s upcoming Nancy Grace Roman Space Telescope. These telescopes measure thousands of supernovae to further improve the measurement of dark energy. You can compare your results with measurements made with complementary techniques.
Aldering, also co-author of the article, stated that “this distance measurement technique is not only more accurate, but only requires a single spectrum that is recorded when a supernova is brightest and therefore easiest to observe – a game changer!” A multitude of techniques is particularly valuable in this area where prejudices have been shown to be false and there is a great need for independent verification.
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The SNfactory collaboration includes the Berkeley Lab, the Laboratory for Nuclear Physics and High Energy at the University of Sorbonne, the Center for Astronomical Research in Lyon, the Institute for Physics of the 2 Infinities at the University of Claude Bernard, the Yale University, the Humboldt University in Germany, the max Planck Institute for Astrophysics, China’s Tsinghua University, Center for Particle Physics of Marseille and Clermont Auvergne University.
This work was supported by the Office of Science of the Department of Energy, NASA’s Astrophysics Division, the Gordon and Betty Moore Foundation, the French National Institute of Nuclear and Particle Physics, and the National Institute of Earth Sciences and Astronomy of the French National Center for Scientific Research, the German Research Foundation and the German Aerospace Center, the European Research Council, Tsinghua University and the National Science Foundation of China.Additional background
In 1998, two competing groups studying supernovae, the Supernova Cosmology Project and the High-z Supernova Search Team, announced that they had found evidence that contrary to expectations, the expansion of the universe was not slowing down, but getting faster and faster. Dark energy is the term used to describe the cause of the acceleration. The 2011 Nobel Prize was awarded to heads of the two teams: Saul Perlmutter from Berkeley Lab and UC Berkeley, head of the Supernova Cosmology Project, as well as Brian Schmidt from the Australian National University and Adam Riess from Johns Hopkins University from the Highz Team.
Additional techniques for measuring dark energy include the DOE-assisted Dark Energy Spectroscopic Instrument, led by Berkeley Lab, which will use spectroscopy on 30 million galaxies in a technique called Baryon Acoustic Oscillation. The Ruby Observatory will also use another so-called weak gravitational lens.
The Lawrence Berkeley National Laboratory (https://www.lbl.gov/) was founded in 1931 with a belief that the greatest scientific challenges can best be tackled by teams, and its scientists have been awarded 14 Nobel Prizes. Today, researchers at the Berkeley Lab are developing sustainable energy and environmental solutions, creating useful new materials, pushing the limits of computing, and exploring the secrets of life, matter, and the universe. Scientists from around the world rely on the laboratory’s facilities for their own science of discovery. The Berkeley Lab is a national multi-program laboratory administered by the University of California for the US Department of Energy’s Office of Science.
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– From Bob Cahn
From EurekAlert!
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