Research

I am an astrophysicist -- now here come the caveats... I work mostly in the field of cosmology, that is, the study of the origin, evolution and large-scale structure of the Universe. Cosmology, almost by definition, trespasses on many other fields. In some of my work, I've put one foot in the realm of particle physics; in other work, the physics of the interstellar medium. At the same time, an important component of my research involves the building of new telescopes and, especially, the sensitive detectors that have (in my slightly biased view) transformed cosmology into a precision science.

Berkeley, November 2017

My dissertation

I wrote my PhD dissertation in a Kerouacian fit of inspiration in early 2018. I tried to produce a readable document, but I would guess that even the most highly motivated graduate student will struggle to make it past the acknowledgements. That is just an unfortunate truth of most dissertations. If you do manage to make it into the real content, please don't hesitate to send me any questions or criticisms.

Link to my dissertation.

Cosmic microwave background (CMB)

For several hundreds of thousands of years after the Big Bang, matter in the universe was mainly in the form of a plasma, a bit like the surface of the Sun. The afterglow of this primordial plasma is called the cosmic microwave background (CMB). It appears to us today as a faint radio signal emanating from all directions in the sky. Much of my work has been concerned with the design and construction of specialized telescopes to make measurements of this background radiation. Historically, measurements of the CMB were crucial in establishing the Big Bang model, and future measurements will contribute to a deeper, more precise understanding of the evolution of the universe from a fraction of a second after the Big Bang through the time of galaxy formation in the relatively recent past.

Very, very roughly, my work in CMB cosmology can be divided into three types. One is to use existing technologies and techniques to make measurements of the CMB, and this is typically accomplished in the context of a relatively large collaboration involving something like a hundred people. The second is to develop new technologies and techniques to enable better measurements in the future, and this typically involves something more like ten people from only a couple of institutions. The third is to use existing measurements in ways no one intended; for that, you're usually on your own...

Map of the cosmic microwave background from BICEP observations. The bright spots are showing you where the early universe was hotter and denser, and the dark spots are showing you where it was colder and more dilute. But these are subtle differences: to about one part in ten thousand, the early universe was uniform.

Cosmological cartography

We are map makers. We build sensitive radio telescopes to make maps of the CMB. Easy, you may say; just point the telescope at the sky region of interest and record the radio waves. The problem is that the CMB is quite faint. In fact, the CMB is about 100 times fainter than the radio waves emitted by your body -- yes, you are emitting radio waves (whether you like it or not). But, wait, it gets worse! The structure in the CMB is 10,000 times fainter still, and the yet subtler features, which are targeted by modern experiments, are fainter by at least another factor of 1,000. How do we overcome these difficulties? By trying everything we can think of cleverly removing spurious signals and patiently integrating observations over the course of years.

Cosmic inflation

A conspicuous absence in the standard Big Bang model is the actual bang. We don't know how to tell the story from the beginning (or if there even was a beginning). We are here in the present, and we try our best to figure out what happened in the past. By and large, it is easier to learn about the recent past than about the distant past. Amongst cosmologists, the history of the Universe is often told backwards, because that is approximately the direction in which our understanding progresses. We are fairly confident in the story back to about a millionth of a second after the mysterious "Big Bang singularity". That's pretty good, but you need to realize that the Universe was evolving very quickly. Part of what limits our understanding is that, before this time, particles were more energetic than any that have been produced in our laboratories on Earth, so we are necessarily in the realm of speculation and hypothesis. Our most popular paradigm is that of cosmic inflation, which describes a period of rapid expansion in the first trillionth of a trillionth of a trillionth of a second and accounts for several basic features of the large-scale structure of the Universe. We can test models of cosmic inflation by making very sensitive measurements of the cosmic microwave background.

Berkeley, July 2014

Detectors

A major effort in our field ever since its inception has been to develop ever more sensitive and efficient detectors. The particular type of detector that has become standard in our field is called a bolometer. There is a cute limerick, whose origin is unknown as far as I can tell, which will give you some sense for what a bolometer can do:

Oh, Langley devised the bolometer.

It's really a kind of thermometer

That can measure the heat

From a polar bear's feet

At a distance of half a kilometer.

To make these detectors reliably and in large numbers, we typically use nanofabrication techniques, which is how a graduate student like myself might end up spending several years doing "cosmology" in a clean-room bunny suit.

The particular type of bolometer that is now ubiquitous in CMB cosmology is called a transition-edge sensor (TES), which exploits the very steep temperature dependence of a metal near its superconducting transition.

Berkeley, February 2017

Our devices are small but not outrageously so. (Don't be too impressed!) The light, yellowish lines in the neighboring microscope image are narrower than a human hair. The height of this detail is about 1 mm. The hypnotic zigzag on the right is called a sinuous antenna; this is where the radio waves from the sky are coupled to the device. A TES is shown in the upper left.

For greatest sensitivity, we operate our detectors at extremely low temperatures, just a fraction of a degree above absolute zero (-273 C). So our telescopes are actually vacuum chambers equipped with powerful refrigerators, and a huge amount of effort in our field is devoted to cryogenics.

South Pole, December 2018. Each one of those metal tubes is a cryogenic telescope. (I am second from right.)

Telescopes

I'm a member of several CMB-related scientific collaborations. They are all in the business of building telescopes, usually in remote locations like the Atacama Desert, the South Pole or even outer space. The cosmic microwave background is strongest in -- well, the microwaves, that is, high-frequency radio waves with wavelengths between about a millimeter and a centimeter. Unfortunately, oxygen and water both tend to emit quite strongly at these wavelengths, so we choose our telescope locations to avoid the Earth's atmosphere as much as possible. Of course, a space telescope is best, but it is quite expensive and inaccessible post-launch. Some groups launch telescopes on balloons that float tens of kilometers above the Earth's surface. It's much cheaper than a satellite, but the observation time tends to be limited to just a few weeks. Most of the effort in our field is directed toward ground-based telescopes. To minimize contamination from oxygen and water, we install these telescopes in high-altitude deserts like the Atacama and the South Pole, locations which are awful for human beings but great for millimeter-wave astronomy.

BICEP

You can learn more about the BICEP series of experiments on Wikipedia or, if you dare, from the collaboration's publications.

South Pole Telescope

You can learn more about the South Pole Telescope on Wikipedia or on the collaboration website.

Simons Observatory

You can learn more about the Simons Observatory on Wikipedia or on the collaboration website

CMB-S4

You can learn more about CMB-S4 on the collaboration website.

LiteBIRD

You can learn more about LiteBIRD on Wikipedia or on the collaboration website.

POLARBEAR

You can learn more about POLARBEAR and the Simons Array on Wikipedia or on the collaboration website.

A prototype multiscale antenna array that I fabricated in the Marvell Nanolab at UC Berkeley in early 2017. The overlaid triangles show the effective antenna sizes for three different wavelength bands.

Multiscale antennas

We build antennas to catch the photons of the cosmic microwave background and other astrophysical radio emission. Depending on the radio wavelengths you are interested in measuring, you may want to build your antenna to be larger or smaller. In general, longer wavelengths demand larger antennas, and smaller wavelengths demand -- well, smaller antennas. (You are correct in assuming that almost all of the useful details have been swept under the rug.) If you want to build an antenna that is sensitive to a broad range of wavelengths, you will necessarily have to make some compromises...or do you? Well, that is roughly the motivation for building so-called multiscale antenna arrays. Suppose you have a grid of antennas (indicated by, e.g., the green circles in the neighboring photograph). Maybe each of those antennas has an appropriate size for the shortest wavelength of interest, so you let them operate independently. For a longer wavelength, however, you can combine neighboring antennas to form an effectively larger super-antenna (indicated by, e.g., the blue triangle). For even longer wavelengths, you can combine even more (red triangle). In this way, the same grid of antennas can operate at many wavelengths and with sizes that are appropriate for each wavelength.

A large part of my doctoral work was devoted to building antenna arrays that cover broader wavelength ranges (Westbrook et al., 2016) and with multiscale properties (Cukierman et al., 2018).

Superconducting resonators and SQUIDs that I spent many hours days weeks preparing for deployment to the South Pole (Stanford, September 2018).

Microwave multiplexing

FM radio -- now with superconductors!

So you have a fancy detector -- now how do you extract useful information from it? The readout electronics must preserve the highly sensitive information from the detectors, and this turns out to be very easy to screw up. Several schemes have been developed for use in CMB cosmology and related fields. The readout technology that I helped to develop at Stanford is called microwave multiplexing, which relies on some of the same principles and enjoys some of the same benefits as FM radio. We ensure adequate sensitivity (and also make our lives more difficult) by building some of the crucial circuitry from superconducting microwave resonators and superconducting quantum interference devices (SQUIDs). Much of the room-temperature control electronics was developed by the talented engineers at SLAC, where I spent about 30% of my time.

In the fall of 2018, we deployed a microwave-multiplexing system to the South Pole for operation on one of the telescopes of the BICEP program. The system observed for part of the following season, and the CMB map that we produced from it is (as of 2022 and to the best of my knowledge) the only one ever produced with microwave multiplexing (Cukierman et al., 2020). (But watch out for the Simons Observatory, which is deploying several new telescopes with microwave-multiplexing readout in the coming years.)

I gave a talk on this "microwave-multiplexing adventure" at the Low Temperature Detectors (LTD) conference in Milan in 2019, and you can watch it in the neighboring embedded YouTube video. But beware -- the target audience consisted of experts in cryogenic detectors.

Oscillating CMB polarization induced by the presence of hypothetical axion-like dark-matter particles.

Axion-like dark matter

The oscillating sky

It's fun to use an experiment in a way it was not at all intended to be used. I had an office next to a theoretical physicist who proposed that a CMB telescope could be repurposed as a detector for dark matter. The idea is that certain types of dark matter called axions can cause the CMB to change in time, whereas we normally assume that the CMB is static. In particular, the CMB polarization oscillates back and forth. So I used data from the BICEP telescopes to look for these axion-like dark-matter particles (BICEP/Keck Collaboration, 2021a and BICEP/Keck Collaboration, 2022a). I didn't find them; otherwise, you would have heard about this from the Nobel Committee. But the game's not over! Our CMB telescopes continue to improve and take data, which enhances our sensitivity on all fronts.

Measurements of hydrogen gas that have been processed (left to right) to accentuate the filamentary structure and infer the magnetic field lines (Cukierman, Clark, Halal, 2022).

Interstellar dust

Now for something a little different: In our measurements of the CMB, we have to wrestle with a number of foregrounds, i.e., all the other stuff in the Universe that emits microwaves. A primary foreground is due to the clouds of dust that occupy the space between the stars. For CMB cosmologists, dust is a nuisance, but it can also be studied for its own sake. In particular, the little dust grains tend to align with the Galactic magnetic field, and that produces polarized emission, which is picked up by CMB telescopes (and others). At the same time, the dust and gas follow the magnetic field lines and form slender, filamentary structures. By triangulating from multiple types of measurements, we can map the magnetic field of the Milky Way Galaxy.

Galactic dust varies dramatically across the sky, and it is illuminating to perform studies in regions with different properties. Some of my investigations have been focused on large fractions of the full sky (Cukierman, Clark, Halal, 2022), and some have homed in on the relatively small but well-studied BICEP observation region (BICEP/Keck Collaboration, 2022c).

For Parity Violation from Home, an asynchronous conference held in October 2023, I recorded a presentation on some of my work on interstellar dust filaments. You can watch it in the neighboring embedded YouTube video.

SPHEREx

In the summer of 2022, I moved to Caltech to work on a space telescope called SPHEREx that is scheduled for launch in early 2025. Unlike my previous work, SPHEREx is sensitive to infrared radiation, much closer to the wavelengths that the human eye can detect (but still invisible to you). SPHEREx targets the three-dimensional distribution of galaxies, the history of galaxy formation and also the distribution of water ice in the Milky Way Galaxy.

Watch a SPHEREx presentation that I gave jointly with my colleague Howard Hui at the Line Intensity Mapping 2024 conference at the University of Illinois, Urbana-Champaign.

Publications

If you dare, you can wade through this list of my publications on NASA's Astrophysics Data System (ADS), which I think may be the online database that confuses me the least with my brother lists my articles most completely.

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