Looking into the past
Down at the South Pole, thousands of miles away from any possible source of interference is a set of powerful telescopes that give us the incredible ability to peer deep into space and far back through time. These telescopes don’t function like traditional, optical telescopes though. Instead, they read the echoes of radiation from the Big Bang and the rapid expansion that happened a fraction of a fraction of a fraction of a second after that first bang.
At UC Berkeley and Stanford, Kimmy Wu is studying a very different time in our universe’s history using data from these telescopes. Billions of years ago, the universe was thousands of times smaller. It wasn’t even full “cooked” yet, so to speak. In these early days of the universe, particles had yet to come together and form the planets, stars, and clouds of dust that now populate outer space. Instead, it was more of an opaque soup of subatomic particles like photons and electrons.
Before this small sliver of time that Wu is looking at, the universe was so hot that neutral hydrogen couldn’t form. Electrons and protons were unable to bind due to the constant high temperature. This resulted in a soup full of ionized electrons. At the same time, photons were shooting around as well. But these photons would be knocked off their path whenever they came into contact with a charged particle, and because this universe soup was very dense, it happened quite often.
But once the universe expanded and cooled enough to allow neutral hydrogen and helium to form, photons no longer were sent off their paths by charged particles and settled down, therefore making the universe transparent, like it is to us now.
It is this time that is crucial in our ability to look back into the universe’s history, as it is this point (called recombination, as it is the point where electrons and protons finally combined to form hydrogen) that cosmic microwave background (CMB) radiation comes from, and CMB is the oldest light in the universe – which is what the SPT and the BICEP/Keck telescopes observe.
Cosmic microwave background radiation forms a kind of canvas upon which other waves and fluctuations can leave their mark. The gravitational waves that Wu is looking for, act as evidence for inflation.
The first, rapid expansionary period of the universe is known as the inflationary epoch and lasted for a millionth of a millionth of a second after the Big Bang. Inflation explains why so many parts of the universe look the same; that is, why no one section of the universe is wildly different in terms of number of galaxies or stars than another, which suggests that everything expanded out from one smaller point.
What Wu is looking for is evidence of the existence of early universe gravitational waves, waves that came from this first inflationary epoch. Since it was so many billions of years ago, the waves themselves would now be too faint for any of our current technology to detect. However, the waves did leave an imprint on the cosmic microwave background, and by looking at the marks they have left behind we can infer their existence.
The CMB, when discovered in the 60’s, was essentially just a uniform temperature map of the universe (a temperature of around 2.7 Kelvin, which is further proof of the universe’s uniformity in all directions). Using our increasingly sophisticated technology, we are able to detect very slight deviations from this measurement of 2.7 Kelvin, deviations in the order of .0003 Kelvin hotter or cooler than what is expected.
The primordial gravitational waves stretched the fabric of spacetime, red- and blue-shifting the photons shooting around, causing fluctuations in their temperature, and leaving behind a kind of signature that proves the existence of these primordial waves in the form of a swirly polarisation pattern on the CMB. These polarisation patterns are called B-modes.
While we know that inflation predicts the existence of these waves, what isn’t known is how big the evidence of these waves will be, or if they will be visible at all with our current level of technology.
The tricky part comes in separating the red herrings found in these microwave telescope readings and the real gravitational wave signatures that physicists are looking for.
One of these red herrings is the phenomenon of gravitational lensing. Gravitational lensing describes the process by which the paths of photons are warped, creating patterns that could be mistaken for primordial gravitational wave B-modes
Since that first inflationary epoch, the universe has become filled with many things that have an impact on the curve of space-time. Large planets, super-massive stars, black holes, these all cause warps in the fabric of space-time which affect the path that photons take through the universe to our telescopes. In particular, dark matter act as lenses and deflect the paths of the CMB photons -- this we call gravitational lensing.
The effect of gravitational lensing on the CMB is the generation of B-mode polarisation, similar to the swirly, primordial gravitational wave generated polarisation patterns that Wu is looking for, but on a smaller scale. However, it’s not all bad, as by characterising the kind of waves resulting from gravitational lensing that come to us we can better understand the dark matter and galaxy clusters that lie between us.
Because we have a good idea of how much dark matter is in the universe and how it’s distributed, physicists are able to better identify the B-modes caused by gravitational lensing and therefore remove them from any readings taken by the telescopes. When gravitational lensing-caused B-modes are removed, it is just the primordial gravitational wave B-modes that are left behind, painting a picture of what this inflationary epoch looked like.
Deep into space, far back into time
And this is where the South Pole telescope and the BICEP/Keck Array come in. In a small patch of the South Pole station in Antarctica known as the dark sector, so named because signals - for example radio signals - that may interfere with the telescope’s readings are not permitted, this suite of powerful telescopes looks at a slice of the sky. While this slice represents only about 1% of the sky as a whole, the power of this incredibly sophisticated technology means that 1% is enough.
While satellites in the air may be able to look at the entire sky, or considerably larger patches, the smaller scope of the South Pole telescope and BICEP/Keck Array mean that scientists can make much deeper and more detailed maps.
Because the BICEP/Keck Array telescopes’ aperture are quite small, about 30 cm, they have a lower resolution. While the BICEP/Keck Array is able to look at larger swathes of the sky, smaller, finer features are reserved for the South Pole Telescope. The SPT has considerably larger, almost ten-meter dish, and is able to view with much higher resolution. Combined, these two telescopes provide both low-resolution and high-resolution glimpses of the sky.
The things these telescopes reveal to us answer contain answers to many of our questions about the origins of the universe. It’s up to scientists like Wu to sift through the data and pull out the clues to bring to light these mysteries about the beginning of time.
Dr Kimmy Wu, Croucher fellowship 2015, completed her undergraduate studies double majoring in physics and math at the University of Michigan at Ann Arbor and performed research with galaxy clusters using the Sloan Digital Sky Survey and radio source catalogs like FIRST and NVSS. She also spent a semester at the University of St. Andrews in Scotland and a summer at CERN for research. In graduate school at Stanford University, she focused her research in cosmology. She got her PhD in physics in September 2015. Her thesis is titled: “BICEP3 and CMB-S4: Current and Future CMB Polarization Experiments to Probe Fundamental Physics”.
To view Dr Wu’s personal Croucher profile, please click here.