Karkare’s group works with laboratories across the country on macro telescopes in the South Pole to create a 3D map of the universe.
by Danny Giancioppo, Photos by Kelly Peña
“How did the universe begin? Why are we here?
To Assistant Professor Kirit Karkare (PHYS), this is the most fundamental scientific question one can ask. It’s also the basis behind his research as a faculty member in the newly founded Cosmology Group at Boston University, where he studies the Cosmic Microwave Background (CMB)––the radiation left by the Big Bang and its initial expansion. A group of three faculty members are working to cover the cosmological side of physics, stretching across theory, experiments, and analysis. Karkare’s team aims to map out the universe in three dimensions, from its inception to the modern day.
“Most people who are interested in space probably got into it at an early age,” Karkare says. “You either wanted to be an astronaut or a firefighter––and I definitely wanted to be an astronaut! I was certainly space-obsessed as a kid.”
Where Karkare may have diverged from other young astronauts was in his proficiency for physics and engineering––building computers and discovering the fundamentals of the universe in high school and undergrad. At Caltech, Karkare worked at a radio observatory in the high California desert, building a telescope receiver 100 feet above the desert floor. Clambering around a 40-meter dish in the elements, his education was anything but typical.
“It was just so much fun,” he explains. “At Caltech I got exposure to the Microwave Background research––not just with pencil and paper, but with turning screws and liquid nitrogen.”
This exposure to major CMB research groups led Karkare to Harvard University, where he was introduced to the development of telescopes at the South Pole. These telescopes seek to uncover the fundamental physics behind the Big Bang, and the stars and galaxies that emerged in its early years.
But how does research like this come to be? In measuring the CMB and the first light of the universe, it’s only natural to assume that optics and photonics research is a necessary component to mapping our reality. So Karkare and team turned to the Photonics Center for help.
MAPPING OUT THE UNIVERSE WITH PRISMS
“The recipe is: point at galaxies, see if there’s a line that pops out––that tells you how far away it is––do this for every galaxy out there, and then build up a three-dimensional map of the universe.”
“The light that we see with our eyes has wavelengths of hundreds of nanometers,” Karkare says. “But it turns out that if you can make cameras that detect light emitting at one or two or three millimeters in wavelength, you can observe the Microwave Background. You can observe some of the light from these first stars and galaxies.”
However, building millimeter-wave telescopes isn’t enough to spot some of the earliest products of the universe. While typical CMB image processing involves detectors measuring three colors––analgous to red, green, and blue––this only helps to distinguish the CMB from the light emitted by our galaxy…not so much the space in between. During his studies, Karkare and team showed that by expanding the use of color from these three filters to a prism, more “constituent colors” can be used to mark out hundreds upon thousands of emissions from the first galaxies formed after the Big Bang. On top of the Microwave Background signal, they could now find signals from the first objects formed in its wake.
Using narrow spectrometers to measure distant structures, known as Line Intensity Mapping (LIM), requires an additional layer of architecture that allows the separation of light to such an advanced degree. In Karkare’s own words, “we’re using all the infrastructure that we’ve built up from the CMB research, and all this knowledge on how to build the detector in the first place, but now we have to add an additional photonics bit in front to separate out the light.”
A natural question to the necessity of the prism might be: what is it about the celestial objects which came so soon after the Big Bang that cause them to emit different wavelengths? As Karkare explains, it’s as simple as this: movement creates fingerprints.
“It turns out that every atom and molecule has a chemical fingerprint,” he says. “Such that if you have a filter at just the right wavelength, [the molecule] will light up.” One of the most common molecules the team searches for is carbon monoxide––a common byproduct of the formation of stars. “We know that carbon monoxide has this fingerprint, this emission line, at about two millimeters in wavelength. So if I take a spectrometer and I look at the galaxies, they will light up roughly at two millimeters.” From there the team can measure how much carbon monoxide there is by how strong the light is. The dimmer and longer the light, the further away the galaxy is.
This stretching of light is known as the Doppler effect, or “redshifting.” When objects move away, from, or toward us, the light they emit is either stretched or compressed, respectively. In cosmology, this is the means of mapping how far the universe has expanded. All objects are moving away from each other in the universe. The further they are, the faster they are moving.
“If I can measure what [an object’s] line has redshifted to,” Karkare says, “I can tell how far away the object is.”
This is the key of Line Intensity Mapping. By tracking the earliest creations of the universe’s redshifting with their telescopes, Karkare and team can fill in the void between the Big Bang and the universe’s first billion years of existence.
“The recipe is: point at galaxies, see if there’s a line that pops out––that tells you how far away it is––do this for every galaxy out there, and then build up a three-dimensional map of the universe.”
SNOW WORK LIKE FIELD WORK
“We are uncovering the grand mystery. Practical applications aside, this is literally a question that every single person on Earth has wondered about.”
Understandably, it takes both an influx of resources and time to build, maintain, and utilize the instruments necessary to chart out the universe. One technological challenge is in building these cameras and telescopes from scratch. Thankfully, it isn’t a one-lab project.
Over the last few decades, researchers around the country have built improved millimeter-wave detectors that can measure light which was previously impossible to see. All of this work, in fact, is highly collaborative. Karkare and team work very closely with colleagues in universities such as Harvard, Stanford, and Chicago––totaling some 50 team members, ––to further the mission of identifying our universe. Through this collaboration, Karkare and team are able to create these millimeter-wave cameras with superconducting metals deposited onto silicon wafers.
For Karkare specifically, this is done in the Photonics Center’s shared facilities. By working directly with lab managers, Karkare and team find personalized attention to their needs, allowing for uninterrupted progress.
Their camera isn’t dissimilar to a phone camera, except for its need to be placed on cryogenic telescopes in the middle of the Antarctic plateau. And if that wasn’t enough? It takes years to make the measurement, and our own planet is actually standing in the way.
“Our telescopes are designed to measure the Microwave Background only, so that incorporates a lot of design features that are unique to our measurement,” Karkare describes. “Not only does the atmosphere emit in microwaves, the ground is warm––it’s 300 Kelvin. Even at the South Pole, the ground is very warm compared to the thing that we’re trying to observe.”
“The [CMB] light is so faint,” he adds, “that if you just have a camera at room temperature, it’s too warm.” The telescopes therefore require an intensely cold environment, just a fraction of a degree above absolute zero. Therefore, Karkare and team keep their instruments at 100 Millikelvin, or -459.49º Fahrenheit.
In order to detect the CMB and earliest galaxies amidst the temperate noise of Earth, one must stare at the sky for months upon years. Analyzing the data to extract the celestial signal can also take years––but above all else, telescope design is integral to success.
“This is all extremely hands-on,” Karkare says. “The fun part is, this is all macroscopic-scale. So a lot of this really does involve climbing around on the outside of your telescope, waving a calibration source in front of the telescope, and then yelling down into the receiver cabin to somebody who’s looking at the data coming out and saying, ‘hey, do you see that?’”
And that’s just the tip of the iceberg. Karkare’s students are also driving snowmobiles across the Antarctic tundra, or placing items on masts rising fifty feet into the air with hand cranks. Each year, a handful of the Karkare lab goes to the South Pole during its austral summer (November to February.) Then, during the austral winter (February to November), when the sun goes down for six months, the station closes for the season, and students are able to record data in a stiller atmosphere than anywhere else on Earth.
It’s this reason that Karkare is looking for graduate students with a “sense of adventure” to join his lab. “It’s going to be messy. We’re out in the elements, it’s dusty, it’s cold, sometimes you can’t feel your fingers. And that’s the fun of it!”
Beyond the hand-on work, this research is also highly translatable for graduate students. Their practical work hones skills in advanced engineering, readying them for all manner of work: nano-fabrication, cryogenics, microwave engineering, large-scale data analysis, and so on. “We’re developing an extremely talented STEM workforce.”
Whether it’s in the lab, the South Pole, or research institutions around the world, their work touches on an innate human curiosity. Perhaps the oldest curiosity.
“We are uncovering the grand mystery,” Karkare says. “Practical applications aside, this is literally a question that every single person on Earth has wondered about. Everybody has asked, ‘where did we come from?’ From the perspective of pure scientific curiosity, answering this question is totally universal.”