Simons Observatory goes online to study the oldest light

The Simons Observatory is fully online and conducting science operations from Chile’s Atacama Desert, the University of Chicago said Nov. 4, 2026.

The facility carries 98,000 detectors collecting microwave light, a count more than 16 times that of its predecessor, the Atacama Cosmology Telescope. The greater number lets it map the sky several times faster than before, and early observations have already produced a cosmological dataset comparable to earlier state-of-the-art versions.

It operates four telescopes in the Atacama Desert: three Small Aperture Telescopes and one Large Aperture Telescope. The Small Aperture Telescopes came online first and already hold several years of data, while the newly commissioned Large Aperture Telescope scans half the celestial sky every couple of days at high resolution and makes daily maps to search for transient objects. Mark Devlin, observatory co-director and cosmologist at the University of Pennsylvania, said, “The scaling is phenomenal. It changes the whole game.”

Early observations captured activity around a supermassive black hole, giant flares from a star in the Milky Way, and asteroids crossing the solar system. The Large Aperture Telescope also produced the most comprehensive polarized microwave images of the Milky Way, revealing new information about galactic magnetic fields important for star formation and evolution. Suzanne Staggs, a Simons Observatory co-director at Princeton University, said beautiful crisp images came essentially right out of the box.

Beyond the background, the observatory aims to study galaxy evolution on large scales, transient objects such as supernovae and asteroids, and elementary particles known as neutrinos. Jeffrey McMahon, a University of Chicago professor and lead Simons Observatory scientist, said early maps place the team at the forefront of the field and the group plans to go much further. The team is shifting focus from construction to optimizing the observatory and scientific analysis, which McMahon called an exhilarating period with opportunities for students.

Erin Healy, a University of Chicago postdoctoral researcher who helped scale detector count while a graduate student at Princeton, said the key to this era of cosmology experiments is scale, with plans to build 10 times more detectors at 10 times the previous rate. University of Chicago led development of new metamaterial lenses for the observatory; research assistant professor Rahul Datta led coatings and fabricated 50 lenses, with metamaterials fabricated at Fermi National Accelerator Laboratory and tested at UChicago’s Eckhart Research Center. Metamaterials are synthetically engineered materials with extraordinary properties, and Datta said the impact of the lenses on the observatory and field is significant.

The cosmic microwave background is the oldest visible light, emitted roughly 380,000 years after the universe’s first second when nuclei formed mostly hydrogen atoms and the cosmos went from opaque to transparent, allowing heat and light to escape. As space expanded, that light stretched from visible through infrared and microwaves to radio waves, and today it is the dimmest background heat, something only radio telescopes can detect. Scientists first detected it in the 1960s and study it as the universe’s baby pictures. Zeeshan Ahmed of Stanford University and SLAC National Accelerator Laboratory said the photons have traveled for 13.8 billion years, and his work concerns the final microsecond of that journey.

The background holds steady at just below 3 kelvins across the whole universe, so an object floating between galaxies could never drop below that temperature because the radiation keeps it warm. The background holds most of the universe’s photons, and its energy density exceeds all light ever created by stars. Tiny ripples in the background carry information about early inflation, revealing the seeds of galaxies and the universe’s composition, age and expansion rate.

Physicists think cosmic inflation occurred in a tiny fraction of a second following the Big Bang some 13.8 billion years ago, rapidly expanding the universe and creating ripples in space-time that mark the oldest light we can see. In that instant, the cosmos expanded from sub-proton size to a marble-sized patch. Within the first second, it expanded from smaller than an electron to wider than 100 trillion miles, or 161 trillion kilometers.

About 10 millionths of a second after the start of the universe, quarks and gluons merged into protons and neutrons. During inflation, shifts in the energy field powering expansion turned into differences in the quark-gluon soup and then into temperature differences in the cosmic microwave background.

Scientists expect that the splitting of the field that creates gravity in the earliest moments, combined with expansion, created primordial gravitational waves. Inflation alone could boost that signal enough to detect, so detecting them would be solid evidence supporting inflation. Kimmy Wu, an assistant professor at the California Institute of Technology, said these waves would help scientists understand gravity at extreme scales.

Polarization describes the orientation of light as it spreads through space, and the cosmic microwave background’s polarization is almost perfectly even; flaws in that evenness might point to primordial gravitational waves. John Carlstrom, a University of Chicago professor and director of the South Pole Telescope, led the Degree Angular Scale Interferometer project at the South Pole that detected CMB polarization in 2002.

Recalling the 2002 detection, Carlstrom said, “We were sitting around with a small team, looking at our computer. There were peaks and wiggles, just as predicted.” The detection opened a new area of research into dark energy and dark matter.

Both E-modes and B-modes in CMB polarization are much weaker than temperature ripples; the E-mode is one-tenth as strong as temperature unevenness and B-modes are even weaker. Zeeshan Ahmed, an associate professor at Stanford University and researcher at SLAC National Accelerator Laboratory, said measurements require a precision of a few nanokelvin on top of the CMB’s 2.725 degrees, a precision like spotting a 10-kilometer peak with an error thinner than paper.

E-modes arise from changes in spacetime density at the universe’s start. Scientists distinguish E-modes from B-modes by pattern shape, with E-modes looking like straight lines while B-modes look swirly, but E-modes can disguise themselves as B-modes. Large structures bend the light and turn some CMB E-modes into B-modes, which creates extra noise for hunters of the primordial signal. Milky Way dust that polarizes light might also look like B-modes.

Detecting B-modes robustly would be bulletproof evidence for inflation. If scientists cannot find primordial gravitational wave B-modes, they can rule out some inflation models and possibly rework or discard them. Seeing B-modes or not seeing them will shape ideas about the early universe.

High altitude, dry air and thin atmosphere make the South Pole one of the world’s top sites for millimeter wavelength measurements, and an international radio-quiet designation protects the location for radio astronomy. Researchers reach the station by flying to New Zealand, boarding U.S. Antarctic Program military planes, and then taking ski-equipped turboprop planes to the pole once they have gathered bulky cold-weather gear. Argonne physicist Lindsey Bleem, who visited in 2008 and 2014, described flying over parts of Antarctica where no human has ever set foot.

The journey supports fieldwork at the bottom of the world during the Antarctic summer under persistent daylight. John Carlstrom said the sun being always up during the Antarctic summer throws off scientists’ body clocks. During the Antarctic winter, no planes can reach the station for months, so winter-overs keep experiments running. Kimmy Wu called it a team effort and thanked the winter-overs who keep experiments running through that time.

Keeping telescopes working is hard all year because the site is so remote.

Low humidity at the pole raises the risk of static shocks to personnel and electronics, drifting snow can interfere with equipment, and the remoteness makes it hard to get parts. Doing basic work while wearing gloves and mittens requires patience. Amy Bender of Argonne said researchers must plan as though launching hardware to space, since no hardware store is nearby, and all South Pole cosmic microwave background detectors are fully custom.

The South Pole Telescope has a 10-meter (33-foot) mirror, measures mainly at millimeter wavelengths, sees arcminute-scale detail about as sharp as human eyesight for light that is not visible, and has tens of thousands of supersensitive detectors. Initially installed in 2007, it is now on its third major upgrade and completed surveys in 2011 using the SPT-SZ camera and in 2016 using SPTpol. It currently uses the SPT-3G camera for surveying large areas of the sky, and its low-noise, high-resolution data work well for mapping CMB polarization.

Scientists have used the South Pole Telescope to identify more than 7,000 galaxy clusters, providing insights into dark matter and dark energy. Argonne National Laboratory built parts of the telescope. The SPT’s arcminute resolution supports detailed CMB mapping.

The BICEP Array, located not far from the South Pole Telescope, is the fifth generation of BICEP instruments at that location; the first BICEP began taking data in 2006, and BICEP3 and the BICEP Array currently take data. Its four telescopes use half-meter (1.6-foot) lenses and gather data at five frequencies, optimized for constrained, clean systematics, though limited resolution restricts correction of lensed B-modes.

The two instruments operate like a married couple: BICEP brings intense sensitivity to large scales and SPT produces gravitational lensing maps; lensing maps from SPT keep systematics from limiting BICEP, and together they achieve targeted goals. Herschel and Planck data add galactic foreground information and maps showing the matter behind E-mode lensing, and the telescopes allow researchers to check results against one another.

In 2013, Planck published its first full-sky CMB map, and SPT’s new SPTpol camera spotted the first B-modes ever detected. Argonne built parts of the SPT. Scientists used Herschel data to map large-scale structures that bent E-modes, then computed the lensing potential using a DOE Office of Science User Facility for the calculation.

In March 2014, the BICEP2 team said it had detected B-modes from primordial gravitational waves using three seasons of data. The announcement faced challenges. Early work showed researchers can separate lensing B-modes from primordial gravitational wave B-modes, and the paper described the technique as a promising avenue.

A separate line of work examines cosmic birefringence, a small twist in the orientation of the cosmic microwave background’s polarization, that scientists suspect could link to hypothetical particles called axions and might point to new physics that breaks left-right symmetry. This faint afterglow of the Big Bang contains information about the early universe. Previous work put the birefringence angle at roughly 0.3 degrees.

The study is the first to numerically examine uncertainty in the cosmic birefringence angle and suggests the rotation may be larger than the reported 0.3 degrees. Fumihiro Naokawa, a University of Tokyo Graduate School of Science PhD candidate, led the research reported in Physical Review Letters, working with Toshiya Namikawa, Project Associate Professor at Kavli IPMU. Nailing down the angle matters for testing possible new physics.

Cosmic birefringence carries a 180-degree phase ambiguity: rotation readings of 0.3, 180.3 or 360.3 degrees look the same from today’s observations alone, like clock hands without a reference date. Fumihiro Naokawa said of observing the CMB state: “Like a clock, the CMB we can observe is only in its current state.” Studying fine details in the EB correlation signal could reveal the true cosmic birefringence angle and remove the 180-degree phase ambiguity, providing a tool for future high-precision observations.

If scientists include phase uncertainty, birefringence affects the CMB EE correlation signal that scientists use to estimate optical depth when studying cosmic reionization, which might mean earlier optical depth estimates need another look. Researchers uncovered the link between birefringence and the EE correlation, and Physical Review Letters published the findings.

Another Naokawa study proposes checking cosmic birefringence using radio galaxies powered by supermassive black holes to verify the effect and learn about dark energy. Future experiments such as the Simons Observatory and LiteBIRD could apply this birefringence method to test new theories of fundamental physics.