SPT-3G deployment: Going to the ends of the Earth to capture pictures of the infant universe

Apr 6, 2017

by Kyle Story

Photo courtesy K. Story

Last December, I travelled to the southernmost tip of the Earth to install a new camera on the South Pole Telescope (following a rich tradition of other KIPAC researchers who have travelled to Antarctica and returned to write about it, e.g. Val Monticue and Albert Wandui). This blogpost brings you along for a bit of that journey!

The author working on the primary mirror of the South Pole Telescope. (Photo courtesy K. Story.)
The author working on the primary mirror of the South Pole Telescope. (Photo courtesy Kyle Story.)

The South Pole Telescope (SPT) is a radio telescope at the South Pole in Antarctica. My collaborators and I use the SPT to make maps of the oldest light in the universe, the Cosmic Microwave Background (CMB).

Some quick background on this cosmic Background: The universe started out very hot shortly after the Big Bang and has been expanding and cooling ever since.  The CMB is the radiation left over from that time—in a sense, it is the “afterglow” of the Big Bang.

The CMB that we see today was formed about 380,000 years after the Big Bang when the average temperature of the Universe cooled enough for protons and electrons to combine to form hydrogen. This transition occurred relatively rapidly, and the Universe suddenly became transparent to light.  Before this time, the Universe was filled with a cloud of hot subatomic particles and a photon could not travel very far before running into a free electron, similar to being in a fog you can't see through.  But after the transition, the “fog” cleared and there were very few free electrons left, allowing photons to stream freely through the Universe.  The CMB that we see today is composed of the photons released at this time. So the maps we make of that radiation are effectively “baby pictures,” showing us what the Universe looked like at ~380,000 years after the Big Bang. Scientists have learned a huge amount about the history of the Universe and what it is made of from these baby pictures; for example, they are one of the most reliable sources telling us the age of the Universe—about 13.75 billion years old.

The daily commute at the South Pole. (Photo courtesty K. Story.)
The daily commute: walking home to the Amundsen-Scott South Pole Station after a hard day’s work, with a sundog in the background. (Photo courtesy of Kyle Story.)

We want to take even better ‘baby pictures’ of the Universe, so this year we installed the 3rd-generation (as in third major upgrade) camera on the SPT, called the SPT-3G camera. This camera has 10 times more detectors that the previous camera, a major upgrade! With this improved camera, we will be able to make much more precise (lower noise) maps of the CMB, especially of the polarization (essentially, the spin the CMB photons have). From these polarization maps, we hope to learn about one of the lightest types of particles—neutrinos—and about the earliest moments after the Big Bang, a hypothesized period called cosmic inflation, during which the universe expanded incredibly quickly, much faster than the speed of light.

The SPT-3G camera's first assembly. (Credit: Robert Guyser.)
The SPT-3G camera’s first assembly.  We tested the camera on the ground before lifting it into the telescope. (Credit: Robert Guyser.)

These maps of CMB radiation can teach us about neutrinos in several ways; I will describe two here.

First, the neutrino is one of the lightest particles we know of, but there are enough of them in existence to make up a noticeable portion of the total mass and energy in the universe. They even have an effect on the universe’s rate of expansion, which is dictated by how much of the total energy in the universe comes from radiation (photons), how much comes from matter—which includes neutrinos—considering this matter as energy via Einstein’s famous equation, E=mc2, and how much comes from dark energy. Because we measure the rate of expansion of the Universe over time with the CMB, this measurement is sensitive to the amount of energy in neutrinos. [Technically: the more radiative species in the Universe before decoupling, the more the CMB power spectrum is suppressed in the ℓ =2000-3000 range.]

Second, neutrinos are very, very light, but from experiments that study “neutrino oscillation” (neutrinos shifting between three different “flavors,” or types) we know they do have some mass. However, we have not yet measured the exact mass of the neutrino particles. Using cosmological measurements—and the CMB in particular—scientists hope to measure the mass of neutrinos with upcoming experiments. We can do this because neutrinos with mass affect the (very large scale) cosmic web of dark matter structure; by measuring that structure, we can learn something about the mass of the super-light neutrinos.  Information about this dark matter structure is encoded in the CMB by “gravitational lensing,” the bending of the travel paths of CMB photons by the gravitational pull of this dark matter.

The author with the South Pole Telescope. (Photo courtesy K. Story.)
The author and the South Pole Telescope. (Photo courtesy of Kyle Story.)

Thus, by measuring the gravitational lensing of the CMB—particularly using the polarization to which SPT-3G will be sensitive—we can use the entire universe to “weigh” the neutrino, one of the lightest known fundamental particles!

If you'd like to see more of my adventures at the South Pole, check out these photos on Flickr:

Kyle Story's South Pole Telescope Album