Connecting the dots in the sky could shed new light on dark matter

Galaxies. Amalgamations of stars, interstellar gas, dust, stellar debris and dark matter. They waltz through the cold universe, gravity nurturing their embrace. Occasionally, galaxies snowball into enormous galaxy clusters with masses averaging 100 trillion times that of our sun.

But this wasn’t always the case.

In the infant universe, temperatures were so high that electrons and protons were too hot to form atoms. Everything was a hot, ionized gas, not unlike the surface of the sun.

Over the next 400,000 years, the universe expanded and cooled to around 3,000 degrees Celsius, about the temperature of an industrial furnace. At these temperatures, electrons and protons combined into hydrogen atoms and released photons in the process. This light, called the cosmic microwave background radiation, has been traveling through space ever since, a watermark of space and time.

Now, scientists have found new ways to tease information out of this inexhaustible time machine.

Constraining cosmology with CMB polarization

In a study published in Physical Review Letters, Fermilab and University of Chicago scientist Brad Benson and colleagues use the polarization, or orientation, of the cosmic microwave background to calculate the masses of enormous galaxy clusters using a new mathematical estimator. This is the first time that scientists have measured these masses using the polarization of the CMB and the novel estimation method.

“Making this estimate is important because most of the mass of galaxy clusters isn’t even visible – it’s dark matter, which does not emit light but interacts through gravity and makes up about 85% of the matter in our universe,” Benson said.

The scientists’ work may eventually shed light on dark matter, dark energy and cosmological parameters that reveal more about structure formation in the universe.

Destination: Antarctica

At Amundsen-Scott South Pole Station, support staff and scientists, nicknamed “beakers,” work around the clock to manage the South Pole Telescope. It’s not easy work. Amundsen-Scott South Pole Station is located at the southernmost place on Earth, where the average temperature is minus 47 degrees Celsius and the sun rises and sets only once a year. But the South Pole Telescope, a 10-meter telescope charged with observing the cosmic microwave background, known as the CMB, is more than capable of achieving its scientific goals in this harsh environment.

The camera on the South Pole Telescope measures minuscule fluctuations in the polarization of CMB light across the southern sky on the order of 1 part in 100 million on average, more sensitive than any other experiment to date.

“These minuscule variations can be affected by large objects such as galaxy clusters, which act as lenses that create distinctive distortions in our signal,” Benson said.

The signal Benson and other scientists were looking for was a small-scale ripple around galaxy clusters — an effect called gravitational lensing. You can see a similar effect yourself by looking through the base of a clear wine glass behind which a candle is lit.

“If you look through the bottom of a wine glass base at a flame, you can see a ring of light. That’s like the effect we would see from a strong gravitational lens,” Benson said. “We are seeing a similar effect here, except the distortion is much weaker and the CMB light is spread out over a much larger area on the sky.”

There was a problem, however. Scientists estimated they would need to look at around 17,000 galaxy clusters to measure the gravitational lensing effect from the CMB and estimate galaxy cluster masses with any certainty, even using their new mathematical estimator. While the South Pole Telescope provided deeper and more sensitive measurements of the CMB’s polarization than ever before, its library of galaxy locations contained only about 1,000 galaxy clusters.

Destination: Chile

To identify more galaxy cluster locations from which to examine the gravitational lensing of CMB light around galaxy clusters, the scientists needed to travel roughly 6,000 kilometers north of the South Pole to the Atacama region of Chile, home to the Cerro Tololo Inter-American Observatory.

The Dark Energy Camera, mounted 2,200 meters above sea level on the 4-meter Blanco telescope at Cerro Tololo, is one of the largest digital cameras in the world. Its 520 megapixels see light from objects originating billions of light-years away and capture them in unprecedented quality. Most importantly, the camera captures the light and locations of the 17,000 galaxy clusters scientists needed to observe gravitational lensing of CMB light by galaxy clusters.

The scientists identified the locations of these clusters using three years’ worth of data from the Fermilab-led Dark Energy Survey and then put these locations into a computer program that searched for evidence of gravitational lensing by the clusters in the polarization of the CMB. Once evidence was found, they could calculate the masses of the galaxy clusters themselves using their new mathematical estimator.

Destination: Unspoiled places

In the current study, the scientists found the average galaxy cluster mass to be around 100 trillion times the mass of our sun, an estimate that agrees with other methods. A substantial fraction of this mass is in the form of dark matter.

To probe deeper, the scientists plan to perform similar experiments using an upgraded South Pole Telescope camera, SPT-3G, installed in 2017, and a next-generation CMB experiment, CMB-S4, that will offer further improvements in sensitivity and more galaxy clusters to examine.

CMB-S4 will consist of dedicated telescopes equipped with highly sensitive superconducting cameras operating at the South Pole, the Chilean Atacama plateau and possibly northern-hemisphere sites, allowing researchers to constrain the parameters of inflation, dark energy and the number and masses of neutrinos, and even test general relativity on large scales.

Anthony Bourdain, a gifted storyteller and food writer, once called Antarctica “the last unspoiled place on Earth … where people come together to explore the art of pure science, looking for something called facts.”

Scientists go far beyond Antarctica to another unspoiled place, the farthest reaches of our universe, to grapple with fundamental cosmological parameters and the behavior of structure in our universe.

Fermilab is a Department of Energy national laboratory funded by the Office of Science.

After riding in a cage with nickel miners, walking down drifts and stopping at the dry, SuperCDMS scientists enter their shotcrete igloo of discovery deep underground.

Translating this out of mining lingo: After taking an elevator down a two-kilometer mineshaft with nickel miners, Fermilab scientists walk through nearly two more kilometers of tunnels and then shower and change before entering the white-walled cavern that will house the Super Cryogenic Dark Matter Search, an experiment that will look for dark matter particles with masses ranging from half to 10 times the mass of a proton.

Dark matter, thought to make up approximately 85% of matter in the universe, emits no light and interacts rarely with normal matter. These characteristics make it difficult to detect, and only through very sensitive experiments such as SuperCDMS might it be possible to directly observe.

This summer, Fermilab scientists Matt Hollister and Dan Bauer will make this journey themselves to install the new SuperCDMS dilution refrigerator at SNOLAB near Sudbury, Ontario, Canada.

The coldest place at Fermilab

This development is made possible by a milestone reached in late October: SuperCDMS scientists cooled their dilution refrigerator, often just called a fridge, down to 5.3 millikelvin, only a few thousandths of a kelvin above absolute zero.

“The temperature we reached is the lowest ever achieved at Fermilab and is one of the lowest ever achieved with a modern, commercially built fridge,” said Matt Hollister, the main designer for the SuperCDMS cryogenic system.

To put this temperature into perspective, the coldest air ever recorded on Earth was 184 kelvins (around minus 89.2 degrees Celsius), logged at the Soviet Vostok Station in Antarctica in 1983. Outer space has a temperature of 2.7 kelvins (minus 270 degrees Celsius).

Extreme cold isn’t new for SuperCDMS scientists, who have run their experiments at successively lower temperatures over the past few decades to achieve new levels of sensitivity and precision in their data. SuperCDMS SNOLAB will be approximately 50 times more sensitive to low-mass dark matter particles than its previous iteration, which took data 700 meters underground at Soudan Underground Laboratory in northern Minnesota.

“SuperCDMS SNOLAB should be able to see the vibrations, or phonons, caused by a single dark matter particle bouncing off a nucleus or the electrons in the detector,” said Bauer, lead SuperCDMS scientist at Fermilab.

Temperatures that hover just above absolute zero are critical if SuperCDMS is to detect dark matter. The colder the crystals in the dark matter detectors, the easier it will be to spot vibrations caused by dark matter particles. It’s kind of like stepping outside during a Chicago winter with only a lightweight jacket. Just as people are more likely to notice you shivering if it’s 20 below rather than above freezing, so too are dark matter detectors more likely to spot a dark matter particle at lower temperatures.

Before they could begin searching for dark matter particles, though, the scientists needed a fridge.

Dilution fridges 101

The SuperCDMS SNOLAB dilution fridge arrived at Fermilab earlier this year from Leiden Cryogenics, a company based in the Netherlands. With twisting gold and copper tubes of various shapes and sizes adorning successively smaller platforms, the fridge looks like a tiered chandelier that delicately balances two fluids – helium-3 and helium-4.

Helium-3 and helium-4 are isotopes of helium that behave not unlike oil and water. At high temperatures, they stay mixed, but as the temperature is lowered, they separate. In SuperCDMS, helium-3 pumped into the fridge encounters helium-4 sitting at the bottom of the fridge. In a spontaneous quantum mechanical phase change, helium-3 “evaporates” into the helium-4 by pulling heat from a nearby energy source: the fridge itself. This phase change cools the fridge and its detectors when the helium-3 gets pumped out of the fridge.

The SuperCDMS fridge does not need expensive liquid cryogens and does not require nearly as much maintenance as older dilution refrigerators. As an added bonus, the adjacent gas exchange system that helps dissipate heat stored in the helium-3 also removes impurities, making the cooling process more efficient and sustainable long-term.

A fridge unlike any other

The fridge for SuperCDMS in its previous iteration at Soudan in Minnesota never dipped below 12 millikelvin. Getting colder for SuperCDMS SNOLAB required overcoming significant challenges. In fact, designing a system that would even work at such low temperatures for long periods of time was difficult.

To lower the backgrounds seen by its detectors, SuperCDMS scientists decided to house them several meters away from the fridge itself and link them thermally.

This decision led to some thought-provoking questions. Namely, how do you transfer heat away from the detectors in subkelvin temperatures when the fridge and the detectors are so far apart?

“Because there are so few people doing this kind of work, we couldn’t turn to the literature to see how a particular material will perform,” Hollister said.

Fermilab scientists spent several years designing and testing components such as a copper cryostat and thermal joints that would thermally connect the fridge and the detectors, making advances in materials science along the way.

“Hopefully others will find our work useful as well,” Hollister said.

There are also practical concerns for day-to-day operations, like the fact that SNOLAB is located in an active nickel mine. Because science is folded into the mining schedule, the scientists typically go down to SNOLAB – and return to the surface – at prearranged times. One consequence of this regimented schedule is that scientists and SNOLAB must coordinate well in advance what work needs to be done and when.

“You don’t have the access that you would if you were doing an experiment at Fermilab,” Bauer said. “You have to do a lot more planning to make sure your experiment is robust. You have to make sure that nothing will happen if the power goes off, for example, because you just can’t get down there all the time.”

From Fermilab to SNOLAB

With the fridge up and running at Fermilab, scientists must now plan the trip north to its new home near Sudbury, Ontario, in Canada.

After running checks to make sure that heat flows out of the fridge efficiently, scientists will disassemble the fridge at Fermilab and reassemble it at SNOLAB. Then, they’ll run it underground for several months next summer to check that the move didn’t affect its performance. Finally, they will integrate it with the rest of the experiment, which receives contributions from three DOE national laboratories as well as universities and partner institutions in the United States, Canada, France, the United Kingdom and India.

That’s when the scientists will start collecting data.

“Everybody really wants to get SuperCDMS SNOLAB operating,” Bauer said. “It’s going to be fun. Now that all the changes we’ve made to improve upon SuperCDMS Soudan are being realized, we’re all eager to get back in the game to find dark matter.”

Fermilab is a DOE national laboratory supported by the Office of Science. Construction of SuperCDMS SNOLAB is managed by DOE’s SLAC National Accelerator Laboratory for the international SuperCDMS collaboration. SuperCDMS also receives funding from the National Science Foundation, Canada Foundation for Innovation and SNOLAB.