With my hosts in the CMB Lab. Brianna Jane Cantrall, me and Zeeshan Ahmed
Some experiences rearrange the furniture in your head. My recent visit to KIPAC, the Kavli Institute for Particle Astrophysics and Cosmology at Stanford, was one of them.
I was invited by Zeeshan Ahmed (he goes by Zeesh), an Associate Professor of Particle Physics and Astrophysics at KIPAC. My co-host was his research colleague, Brianna Jane Cantrall, a young and passionate experimental physicist.
They invited me to see the latest equipment for the BICEP telescopes, a family of instruments at the South Pole that stare at the oldest light in the universe. They are looking for an imprint left by gravity itself, from a moment so early that the word “moment” barely applies. In the photo above we are in their lab looking at a cryostat, a supercool (and supercooled) instrument built by Zeesh’s team and currently making astonishing observations at the South Pole.
I am not a physicist. I walked in curious and walked out a little stunned.
The oldest light we can see
Start with the cosmic microwave background, or CMB. For roughly the first 380,000 years after the Big Bang, the universe was a hot plasma. Then it cooled enough for neutral atoms to form, and light was set free. Stretched by billions of years of cosmic expansion, it now arrives as faint microwaves with a temperature of about 2.7 degrees above absolute zero, almost exactly the same in every direction.
Almost is the important word. The CMB varies by about a ten-thousandth of a degree from one patch of sky to the next. And about ten percent of the light is polarized, meaning its waves have a preferred orientation. That polarization is where the BICEP team does its hunting.
Swirls in the sky
Physicists sort the polarization pattern into two kinds: orderly E-modes and swirling B-modes. B-modes are far fainter, and they are special because gravity is one of the few things that can make them.
According to the theory of cosmic inflation, in the first tiny fraction of a second after its creation the universe expanded exponentially. Our current conjecture about quantum gravity tells us that the expansion should have produced primordial gravitational waves, ripples in spacetime itself, and those ripples would leave a faint B-mode pattern on the CMB at scales of about a degree on the sky.
Finding that pattern would be direct evidence for inflation and would also validate some aspects of quantum gravity.
A cautionary tale, and a better one
Some readers may remember the headlines from March 2014. The BICEP2 team reported inflationary gravitational waves. Then science did what science does: our own galaxy is full of dust grains that glow with polarized light, and that glow can mimic the gravitational signal. In 2015, a joint analysis of BICEP2, Keck Array, and Planck satellite data found strong evidence for dust and no statistically significant evidence for primordial gravitational waves – a huge disappointment after the flush of exhilaration from the supposed groundbreaking discovery!
I find that story more inspiring than a clean victory would have been. The collaboration built instruments that observe at more frequencies, so dust could be measured and subtracted, and kept going. Their 2021 analysis, tightened the limit to r , a parameter that depicts the ratio of swirls to the orderly gradient, to < 0.036 at 95 percent confidence. (Any confirmed value above zero would be evidence of B modes and potential gravity imprint.) This was the strongest constraint on primordial gravitational waves when it was published. Zeesh told me that they continue to tighten the error bands and the end result may well be the required accuracy of 99.99..% (or 5 sigma) that would definitely prove primordial gravity’s imprint. As far as I could find, the collaboration’s latest published limit on r has ruled out several once-popular models of inflation.
Why the South Pole
The BICEP telescopes sit at the geographic South Pole because the site is high and dry, with a stable atmosphere. The program has gone through generations: BICEP1, BICEP2, the Keck Array, BICEP3, and now BICEP Array, which replaced the Keck Array. When complete, BICEP Array’s four receivers will train more than 30,000 detectors on the southern sky in bands from 30 to 270 GHz.
That spread of frequencies is the lesson of 2014 built into hardware. Galactic dust and synchrotron radiation change brightness with frequency in a different way than the CMB does, so seeing many “colors” lets you pull them apart. The fourth and final receiver, observing at 90 and 150 GHz, is planned to go to the Pole this coming austral summer, 2026-27.
Detectors on the edge
The signal is faint and the telescopes use amazing cryogenic temperatures and ingenious sensitive measurements to tease out minute changes in the CMB. The instrument in the telescope is the Cryostat which is built and tested at KIPAC’s lab.
Examining a Cryostat during my visit
Here I am standing right next to a cryostat, the kind of deep-cold refrigerator that superconducting detectors need in order to reach a fraction of a degree above absolute zero. With its gold-plated stages, copper parts, and nests of fine wiring, it was strangely beautiful up close.
The detectors in these telescopes are transition-edge sensors, or TESs. Each one is a tiny superconducting film held right at the edge of its transition between superconducting and normal states, at roughly half a degree above absolute zero. Balanced on that edge, the slightest warmth from incoming microwaves produces a measurable change in its electrical resistance. Thousands of them are patterned onto silicon wafers. For BICEP Array, the detector wafers are fabricated at NASA’s Jet Propulsion Laboratory, and Stanford and SLAC teams work on much of the rest. SLAC scientists developed modular detector components, lenses, and filters for BICEP3, and Stanford produces refractive optics for the BICEP Array receivers.
That raises a practical problem: you cannot run a separate wire out of the deep cold for every detector without letting heat in. The answer, I learned, is the SQUID, or superconducting quantum interference device: a tiny superconducting loop interrupted by weak links called Josephson junctions, and one of the most sensitive magnetic-field sensors known.
A TES signals with a minuscule change in current, and a SQUID turns that current’s faint magnetic field into something electronics can read. Thanks to my hosts I got to observe the tiny circuitry of a SQUID through a microscope.
I was blown away by its tiny size, how cold it runs, and how much precision goes into a single part.
What stays with me
Somewhere in that faint, swirling polarization there may be a record of the first instant of time, and a team right here in Silicon Valley is building instruments sensitive enough to read it.
The wonder I keep coming back to is this: we may soon detect the imprint of primordial gravity on the oldest light, and if we do, it would revolutionize physics, opening a direct window on the birth of the universe and on energies no laboratory can reach. If the signal stays hidden a while longer, ruling out more possibilities still tells us something real about where everything came from. Either way, this is measurement, done with extraordinary patience, and that is what blew my mind.
My warm thanks to Zeeshan and Brianna for inviting me in, for their generosity with their time, and for letting me, a curious visitor, glimpse work this beautiful. I now know that behind every star I see in the night sky is a faint glow from the beginning moment of “Creation”, and that people I have met are trying to read gravity’s fingerprint on it.
In essence Modern Monetary Theory (MMT) says government deficits are not necessarily bad. Since our government has the authority to “print” money it cannot go bankrupt. In this way the government is not like a household which must balance its income with its expenditures. We shouldn’t even label the gap between taxes and public spending as a “deficit”. To the economy at large, such as the American households, the small businesses, the NGO’s, the corporations etc. the extra spending by government means a surplus. So the deficit is really a surplus when viewed from the point of view of the actual economy.
