Decade-long Dark Energy Survey offers new insights into the expansion of the universe

In August 2016, Dave Harding received an email. The U.S. Department of Energy’s Oak Ridge National Laboratory needed magnets for a particle accelerator upgrade. These weren’t your usual refrigerator magnets: the scientists at Oak Ridge were looking for large, specially-built, one-of-a kind magnets essential to steering the beam of their particle accelerator. To get these magnets made, Oak Ridge contacted DOE’s Fermi National Accelerator Laboratory. Harding, a senior scientist at Fermilab who led the Accelerator Support Group, took on the project.

“They came to us because of Fermilab’s expertise in designing and building magnets,” said Harding. “It was an engaging challenge because there were a few special things that Oak Ridge needed.”

Fermilab holds a special niche in the DOE system that often leads to collaborations with other national laboratories: Fermilab excels at designing, fabricating and testing magnets for steering, bending and otherwise manipulating particle beams.

Since the initial request, Dave Harding and Thomas Strauss, another Fermilab scientist, have worked with Fermilab’s Magnet Systems Department to lead the design and fabrication of three custom magnets. They are for Oak Ridge’s Proton Power Upgrade to its Spallation Neutron Source.

“These three magnets are right at the heart of the SNS, and a crucial part of the upgrade that we had to get right the first time,” said Nick Evans, accelerator physicist at Oak Ridge. “We knew Fermilab had all of the expertise to design and build these complex, one-off devices and the collaborative spirit we needed to make the most of a partnership like this. We wouldn’t be where we are without their contribution.”

Speedy particles

The SNS is a particle accelerator that creates an abundance of neutrons for research. “The point of it is to slam protons into a target to produce a lot of neutrons, which can then be used to study the properties of various materials and structures that are of interest for a wide range of scientific disciplines,” said Harding.

The SNS consists of four main parts, starting with a linear accelerator that propels a high-intensity beam of negatively charged hydrogen ions (a proton and two electrons). Then, a series of four magnets called a chicane steers the incoming beam onto the path of the circulating beam in the accumulator ring, while a thin foil strips the electrons from the protons. Third, the protons circulate in the accumulator ring until they are ready to be fired at the target, which, lastly, releases the desired neutrons when bombarded with the proton beam.

The Proton Power Upgrade project aims to double the power of the proton beam hitting the target to create more neutrons to drive research forward. To achieve this goal, Oak Ridge will increase the energy that the H-minus ions have when they exit the linear accelerator by 30 percent and enhance the linear accelerator to handle 50% more particles in each pulse.

The linear accelerator improvements mean that the H-minus beam will have more energy than the current chicane magnet system can handle.

“Injection into the ring is like merging onto an interstate,” said Harding. “You want to keep feeding cars in without disturbing the ones that are already there. With the power upgrade, the system will run with an increased speed. And if your original design had a fairly sharp turn on the entrance — splat!”

The upgraded chicane magnets need to be longer for smoother merging of the higher-energy particles. That requires precise and powerful magnets. The solution: the magnets the Fermilab team designed and built.

Special requirements

The Oak Ridge upgrade team had specific requirements for the magnets that made the designs technically challenging. The SNS accelerator system starts with H-minus ions but then turns them into protons to merge them into the storage ring. This means that the electrons must be removed, and the stripping must happen within the magnets that direct the particles from the linear accelerator into the accumulator ring. The solution: a thin foil filter placed inside the chicane magnet system to knock the electrons off the protons.

However, the electrons that are set free in this process pose a challenge.

“If you did not direct the electrons, they would spiral back on a very tight spiral and smash back into this foil, causing it to deteriorate more quickly,” Harding said. The Fermilab team had to design the magnets so that they direct the electrons away from the foil. “That was a serious three-dimensional challenge. It meant that the top and bottom of the magnet had to be different from each other.”

The strong, room temperature magnets for Oak Ridge are shaped like the letter C. The conducting copper wire wraps around the center of the structure, and the iron magnetic poles reach toward each other with a gap between them. This shape concentrates the magnetic field in that gap and allows for easy access to the magnet’s interior.

The magnets that Fermilab created are asymmetrical images of each other. The top half of the C is smaller than the bottom half in one magnet, and the second magnet is a flipped version of the first.

The team also designed and built a third magnet for the Oak Ridge system. Though the thin foil in the chicane strips off most of the electrons from the protons in the beam, some of the H-minus ions will make it through the filter intact. If any electrons remain on a proton, the particle will have the wrong electrical charge from the rest of the beam; it will not be directed into the accumulator ring with the rest of the beam. The third magnet functions as the off-ramp for those particles.

Putting it all together

In February 2023, Dave Harding turned leadership of the Oak Ridge magnets project over to Strauss, the group leader of Fermilab’s Accelerator Support Group. In addition, the project had recently gained a new lead engineer, Sherry Baketz, and was replacing retiring technicians.

Despite the challenges of bringing new personnel up to speed, the team built the two required chicane magnets by April 2023 and shipped all three magnets to Oak Ridge in August. They were installed in the SNS tunnel in the fall.

“I worked really closely with the technicians to find ways to streamline things,” said Baketz. “It turned out really well. Oak Ridge had their magnets before they needed them, and they work!”

The final step of this project is currently underway; a spare chicane magnet and coil system will be shipped to Oak Ridge this month.

“It’s most exciting when we are actually building the magnet and the coils, just the actual production work,” said Baketz. “You start with nothing, and you end up with this whole magnet. That’s what I like about being a mechanical engineer — turning concepts into reality.”

A mystery

Before sending the magnets to Oak Ridge, a Fermilab team from the Test and Instrumentation Department did a detailed measurement of the magnets and made sure they met all required specifications.

“An Oak Ridge review of this project just gave us some very high praise for the work that we have done,” said Mike Tartaglia, the former head of the T&I Department and now head of the Magnet Test Support Group.

At the same time, the T&I team noticed something that is of scientific interest.

“We measured all three of the magnets,” said Tartaglia. “With the first magnet, we took some preliminary measurements to check whether the field distribution looked correct. We found some regions where the field is not identical to what the model simulated.”

Though the deviations they found won’t affect the Oak Ridge beam, they’re still an intriguing mystery for Tartaglia.

“I have a few ideas about what these deviations could be. I’m excited to analyze the testing data and understand this slight anomaly compared to the model.”

Future collaborations

With this project coming to a close, the Magnets Systems Department has begun to look toward its next collaborations with Oak Ridge and other DOE national laboratories.

“We are currently in the early stages of working with the Second Target Station project at Oak Ridge,” said Harding. The project will add a second neutron source to the SNS that will generate high-brightness, cold neutrons. “There, the challenges are different, but they’re interesting magnets; they could be twice the size, about two meters high.”

Fermilab’s magnets team is also partnering with other labs, including DOE’s SLAC National Accelerator Laboratory and the European laboratory CERN.

“Oak Ridge has been a great partner,” said Strauss, who will manage future magnet collaborations. “It’s always nice to work with people in other labs because there’s a very similar mindset, but you have different capabilities. We have some upcoming work for other national labs because Fermilab has the expertise in the DOE system to make these unconventional magnets.”

The Proton Power Upgrade project of the Spallation Neutron Source is funded by the Basic Energy Science program of the DOE Office of Science. 

Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

The Deep Underground Neutrino Experiment, or DUNE, has brought together researchers from around the world to explore elusive neutrinos in the largest high-energy physics experiment in the U.S. The Spanish Centre for Energy, Environmental and Technological Research, also known as CIEMAT, is one of the key contributors to the international experiment.

A delegation led by Yolanda Benito Moreno, CIEMAT’s director general, recently visited the U.S. Department of Energy’s Fermi National Accelerator Laboratory, which hosts DUNE.

Since 2015, Spanish research groups have played a critical role in DUNE’s research and development phase. They are contributing to the ultra-sensitive liquid-argon detectors of DUNE. When neutrinos interact with the cryogenic liquid argon inside DUNE’s neutrino detectors, photons—particles of light—will be produced. CIEMAT researchers are developing a photon detection system that will be triggered by the interactions between neutrinos and liquid argon. The system will provide a complementary signal to the traditional ionization signal in liquid argon. In addition, the groups are creating a temperature-monitoring system to track the temperature of the liquid argon within the detectors more precisely.

On the first day of the visit, the CIEMAT delegation met with Fermilab’s leadership to sign a commemorative certificate to acknowledge the official partnership that went into effect in December 2022.

“Studying neutrinos is a monumental challenge,” said Lia Merminga, Fermilab’s director. “The DUNE collaboration brings together some of the brightest minds from around the world to create solutions that will provide humanity with knowledge about the nature of mysterious neutrinos. The work done by CIEMAT and Spanish partners is critical to making this effort possible.”

After signing the certificate, the CIEMAT delegation toured the Short-Baseline Near Detector at Fermilab, a neutrino experiment at Fermilab to which CIEMAT is also contributing. The neutrino detector uses similar technology to the detectors that will make up DUNE.

Critically important to the DUNE experiment is PIP-II, a state-of-the-art particle accelerator. The machine, which relies on superconducting technologies, will generate the powerful particle beam to produce an intense beam of neutrinos for DUNE’s detectors. Steve Gourlay, the director of Fermilab’s magnet technology division, and Rich Stanek, the interim PIP-II project director, provided an overview of Fermilab’s research in superconducting accelerator technology that will enable PIP-II.

Silvia Zorzetti, engineer at the Fermilab-hosted Superconducting Quantum Materials and Systems Center, provided a tour of the center. She explained how SQMS Center researchers aim to advance quantum computing and sensing.

The CIEMAT delegation rounded out their visit with a tour of Fermilab’s Lederman Science Center. The visitors saw firsthand how the lab approaches educational outreach. They received a summary of the lab’s efforts in emerging technologies from Panagiotis Spentzouris, the associate lab director for Emerging Technologies, to learn more about Fermilab’s broader impact on society. The delegation also learned about the Illinois-Express Quantum Network, a fiber-optic cable network with different nodes at participating research institutions across Illinois.

“Visiting the facilities and discussing with the scientists firsthand has provided me with a clear picture of the importance of this research line for CIEMAT,” said CIEMAT Director General Yolanda Benito Moreno. “A fundamental aim in our scientific strategy is collaborating with the best teams worldwide. I am now sure that our neutrino program is in the best hands.”

Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Engineers and scientists at the U.S. Department of Energy’s Fermi National Accelerator Laboratory have a new, leading-edge building to advance high-energy physics projects and technology innovation.

The DOE has awarded Fermilab the Project Completion and Start of Operations approval for its award-winning Integrated Engineering Research Center. Known as CD-4, the last of the Critical Decisions in the DOE Project Management of Capital Assets process, this approval affirms Fermilab’s completion of construction and readiness to operate this new state-of-the-art facility.

Completed on time and under budget despite the pandemic, the IERC is an 80,000-square-foot, multi-story laboratory and office building adjacent to the iconic Wilson Hall on the Fermilab campus. The new research center, funded by DOE’s Science Laboratory Infrastructure program, is intended to meet current and future needs for research performed at Fermilab for the DOE Office of Science.

“Successful completion of the IERC demonstrates a job really well done by the project team,” said Randy Ortgiesen, IERC project director. “Its completion will consolidate engineering and technical staff from remote areas across the site to yield many long-lasting benefits.”

Providing professional workspace for about 100 engineers and technicians, the research center features high-bay laboratory spaces and a high-quality cleanroom to limit dust particles and other contaminants during the production of ultra-precise electronic equipment. It also includes offices, meeting rooms and collaboration areas, and it provides much-needed space for Deep Underground Neutrino Experiment-related work, as well as other experiments that require research and development laboratories.

Its infrastructure will enable technological development for particle detectors, including electronics and application-specific integrated circuits, and much more. The building’s flexible design will allow for adaptation to meet future science needs with minimal down-time. Importantly, the IERC with its ample meeting space will enhance collaboration among the researchers and foster innovation.

The architect’s design for the IERC reflects the iconic shape of Fermilab’s 16-story Wilson Hall, which is located next to the IERC. The completed IERC building garnered an award: The project won the Engineering News Record Midwest 2023 “Best Project” award in the higher education category.

The IERC was built with sustainability in mind. The building boasts energy-efficient lighting, ventilation, heating and air-conditioning, as well as large windows for natural lighting and low-flow water fixtures. Its exterior lays claim to sustainable features as well, with native and drought-tolerant plantings, and a 20,000-square-foot green roof.

“I am pleased and excited the IERC has achieved CD-4 status,” said Fermilab Director Lia Merminga. “With it, Fermilab has the go-ahead from DOE to pursue our mission in this beautiful space. We can now bring together engineers and technicians into this state-of-the-art facility where they can better collaborate and innovate to enable our laboratory’s important scientific mission synergistically.”

The construction of the Integrated Engineering Research Center was funded by the Science Laboratory Infrastructure program within the Department of Energy Office of Science.

Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

By Madeleine O’Keefe

In 1998, astrophysicists discovered that the universe is expanding at an accelerating rate, attributed to a mysterious entity called dark energy that makes up about 70% of our universe. While foreshadowed by earlier measurements, the discovery was somewhat of a surprise; at the time, astrophysicists agreed that the universe’s expansion should be slowing down because of gravity.

This revolutionary discovery, which astrophysicists achieved with observations of specific kinds of exploding stars, called type Ia (read “type one-A”) supernovae, was recognized with the Nobel Prize in Physics in 2011.

Now, 25 years after the initial discovery, the scientists working on the Dark Energy Survey have released the results of an unprecedented analysis using the same technique to further probe the mysteries of dark energy and the expansion of the universe. They placed the strongest constraints on the expansion of the universe ever obtained with the DES supernova survey. In a presentation at the 243^rd meeting of the American Astronomical Society on Jan. 8 and in a paper submitted to the Astrophysical Journal in January titled, “The Dark Energy Survey: Cosmology results with ~1500 new high-redshift type Ia supernovae using the full 5-year dataset,” DES astrophysicists report results that are consistent with the now-standard cosmological model of a universe with an accelerated expansion. Yet, the findings are not definitive enough to rule out a possibly more complex model.

Taking a unique approach to analysis

The Dark Energy Survey is an international collaboration comprising more than 400 astrophysicists, astronomers and cosmologists from over 25 institutions led by members from the U.S. Department of Energy’s Fermi National Accelerator Laboratory. DES mapped an area almost one-eighth the entire sky using the Dark Energy Camera, a 570-megapixel digital camera built by Fermilab and funded by the DOE Office of Science. It was mounted on the Víctor M. Blanco Telescope at the National Science Foundation’s Cerro Tololo Inter-American Observatory, a Program of NSF’s NOIRLab, in 2012. DES scientists took data for 758 nights across six years.

To understand the nature of dark energy and measure the expansion rate of the universe, DES scientists perform analyses with four different techniques, including the supernova technique used in 1998.

This technique requires data from type Ia supernovae, which occur when an extremely dense dead star, known as a white dwarf, reaches a critical mass and explodes. Since the critical mass is nearly the same for all white dwarfs, all type Ia supernovae have approximately the same actual brightness and any remaining variations can be calibrated out. So, when astrophysicists compare the apparent brightnesses of two type Ia supernovae as seen from Earth, they can determine their relative distances from us.

Astrophysicists trace out the history of cosmic expansion with large samples of supernovae spanning a wide range of distances. For each supernova, they combine its distance with a measurement of its redshift — how quickly it is moving away from Earth due to the expansion of the universe. They can use that history to determine whether the dark energy density has remained constant or changed over time.

“As the universe expands, the matter density goes down,” said DES director and spokesperson Rich Kron, who is a Fermilab and University of Chicago scientist. “But if the dark energy density is a constant, that means the total proportion of dark energy must be increasing as the volume increases.”

The culmination of a decade of effort

The standard cosmological model is ΛCDM, or Lambda Cold Dark Matter, a model based on the dark energy density being constant over cosmic time. It tells us how the universe evolves, using just a few features, such as the density of matter, type of matter and behavior of dark energy. The supernova method constrains two of these features very well: matter density and a quantity called w, which indicates whether the dark energy density is constant or not.

According to the standard cosmological model, the density of dark energy in the universe is constant, which means it doesn’t dilute as the universe expands. If this is true, the parameter represented by the letter w should equal –1.

When the DES collaboration internally unveiled their supernova results, it was a culmination of a decade’s worth of effort and an emotional time for many of the astrophysicists involved.

“I was shaking,” said Tamara Davis, a professor at the University of Queensland in Australia and co-convener of DES’s supernova working group. “It was definitely an exciting moment.”

The results found w = –0.80 +/- 0.18 using supernovae alone. Combined with complementary data from the European Space Agency’s Planck telescope, w reaches –1 within the error bars.

“w is tantalizingly not exactly on –1, but close enough that it’s consistent with –1,” said Davis. “A more complex model might be needed. Dark energy may indeed vary with time.”

To come to a definitive conclusion, scientists will need more data. But DES won’t be able to provide that; the survey stopped taking data in January 2019. The supernova team, led by many Ph.D. students and postdoctoral fellows, will soon have extracted all they can from the DES observations.

“More than 30 people have been involved in this analysis, and it is the culmination of almost 10 years of work,” said Maria Vincenzi, a research fellow at Duke University who co-led the cosmological analysis of the DES supernova sample. “Some of us started working on this project when we were barely at the beginning of our Ph.D., and we are now starting faculty positions. So, the DES Collaboration contributed to the growth and professional development of an entire generation of cosmologists.”

Pioneering a new approach

This final DES supernova analysis made many improvements upon DES’s first supernova result released in 2018 that used just 207 supernovae and three years of data.

For the 2018 analysis, DES scientists combined data about the spectrum of each supernova to determine their redshifts and to classify them as type Ia or not. They then used images taken with different filters to identify the flux at the peak of the light curve — a method called photometry. But spectra are hard to acquire, requiring lots of observing time on the largest telescopes, which will be impractical for future dark energy surveys like the Legacy Survey of Space and Time, LSST, to be conducted at the Vera C. Rubin Observatory, operated jointly by NSF’s NOIRLab and DOE’s SLAC National Accelerator Laboratory.

The new study pioneers a new approach to use photometry — with an unprecedented four filters — to find the supernovae, classify them and measure their light curves. Follow-up spectroscopy of the host galaxy with the Anglo-Australian Telescope provided precise redshifts for every supernova. The use of the additional filters also enabled data that is more precise than previous surveys and is a major advancement compared to the Nobel-winning supernovae samples, which only used one or two filters.

DES researchers used advanced machine-learning techniques to aid in supernova classification. Among the data from about two million distant observed galaxies, DES found several thousand supernovae. Scientists ultimately used 1,499 type Ia supernovae with high-quality data, making it the largest, deepest supernova sample from a single telescope ever compiled. In 1998, the Nobel-winning astronomers used just 52 supernovae to determine that the universe is expanding at an accelerating rate. “It’s a really massive scale-up from 25 years ago,” said Davis.

There are minor drawbacks of the new photometric approach compared to spectroscopy: Since the supernovae do not have spectra, there is greater uncertainty in classification. However, the much larger sample size enabled by the photometric approach more than makes up for this.

The innovative techniques DES pioneered will shape and further drive future astrophysical analyses. Projects like Rubin’s LSST and NASA’s Nancy Grace Roman Space Telescope will pick up where DES left off. “We’re pioneering these techniques that will be directly beneficial for the next generation of supernova surveys,” said Kron.

“This new supernova result is exciting because this means we can really tie a bow on it and hand it out to the community and say, ‘This is our best attempt at explaining how the universe is working,’” said Dillon Brout, an assistant professor at Boston University who co-led the cosmological analysis of the DES Supernova sample with Vincenzi. “These constraints will now be the gold standard in supernova cosmology for quite some time.”

Even with more advanced dark energy experiments forthcoming, DES scientists emphasized the importance of having theoretical models to explain dark energy in addition to their experimental observations. “All of this is really unknown territory,” said Kron. “We do not have a theory that puts dark energy into a framework that relates to other physics that we do understand. For the time being, we in DES are working to constrain how dark energy works in practice with the expectation that, later on, some theories can be falsified.”

DES scientists continue to use the supernova results in more analyses by integrating them with results obtained with the other DES techniques. “Combining the DES supernova information with these other probes will even better inform our cosmological model,” said Davis.

“Even if we measure dark energy infinitely precisely, it doesn’t mean we know what it is,” she said. “Dark energy is still out there to be discovered.”

Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Funding Authority for Funding and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, Brazilian National Council for Scientific and Technological Development and the Ministry of Science and Technology, the German Research Foundation and the collaborating institutions in the Dark Energy Survey.

The U.S. National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab) operates the Cerro Tololo Inter-American Observatory (CTIO) and Vera C. Rubin Observatory (operated in cooperation with the U.S. Department of Energy’s SLAC National Accelerator Laboratory). The research community is honored to have the opportunity to conduct research on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the local communities in Chile.

Based in part on data acquired at the Anglo-Australian Telescope for the Dark Energy Survey by OzDES. We acknowledge the traditional custodians of the land on which the AAT stands, the Gamilaraay people, and pay our respects to elders past and present.

Fermilab is America’s premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance LLC. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @Fermilab.

The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.