Why map the universe?

In the 1920s, Edwin Hubble famously observed that the Universe was expanding. Soon after, physicists attributed the expansion to the remnants of the Big Bang. Over a long enough period of time, they speculated that the expansion would be slowed by gravity. This was a sensible conclusion, as gravity was the only known force that operated over such long distances.

In the 1990s, however, we discovered that the Universe's expansion was accelerating, not slowing down. Our current Standard Model of physics does not possess any forces that could drive the accelerating expansion; this indicates that our understanding of physics is fundamentally incomplete. The mysterious force that is driving the accelerating expansion is called "dark energy". Uncovering the nature of dark energy is a critical step to a deeper understanding of the physical laws that govern our Universe.

The first step to solving dark energy is to precisely measure the expansion rate across the Universe's history. Recording dark energy's behavior establishes a foundation for future theories to decipher the mechanism behind it. The current timeline of the Universe's history is shown on the left.

Image Credits: NASA/WMAP Science Team; Dana Berry

Measuring Dark Energy

Now that we understand the motivation for measuring the Universe's expansion rate, how do we measure it? There are a few different ways, but fortunately the Universe provided a ruler for us to use. How this ruler comes about is a fair bit complicated, but the video above from CAASTRO provides a very useful visualization of this process.

The early Universe consisted of a fluid mixture of light and matter, which were coupled together because of the Universe's extremely high density or compactness. "Coupled" in this case indicates that the physical properties (in particular, the temperature and pressure) of light and matter were linked by their constant interactions. Any changes in one would also occur in the other. During an event called "inflation", quantum fluctuations perturb the light-matter fluid, forming ripples, almost as if someone dropped a bunch of pebbles into the fluid at the same time. These ripples all travel across the fluid at a constant speed, as is shown in the first 30 seconds of the video, forming rings of approximately the same size.

However, the Universe's expansion eventually pulled the light and matter apart, destroying the fluid mixture. We still see light from this event coming from every direction! This is called the Cosmic Microwave Background, which we won't go into here. The ripples freeze and create nearly identical rings in the matter distribution. Galaxies then form inside of these rings. By studying the spatial distribution of galaxies statistically, we can measure how the size of these rings change with time due to the Universe's expansion. However, studying the entire Universe one galaxy at a time can be quite expensive -- you can imagine trying to measure the depth of the ocean with a bucket, it's a lot of work and very time-intensive. Financially, it's expensive too, especially when we spend billions of dollars to create state-of-the-art buckets that we launch into space.

Tianlai

The Tianlai dish array, pictured here with the Milky Way overhead.

Cosmic Radio Static

An alternative method of mapping the Universe is presented by using its most abundant element: hydrogen. Hydrogen emits radio waves, which luckily for us is easily observed on Earth and doesn't require complicated methods of determining the position of a galaxy. This technique is called 21cm Intensity Mapping, named after the wavelength of light that hydrogen emits. We're still in the developmental stages of this technique, which is where Tianlai comes in. Tianlai's goal is to demonstrate the potential of this technique to map the Universe, and prepare analytical pipelines and methodology for future intensity mapping instruments.

I began my research career working with Peter Timbie, on the Tianlai array. To test this instrument, I built electromagnetic simulations of the array in order to predict and characterize its response to astronomical and test signals, which was used during this instrument's calibration phase (Zhang+21). I then used observational data to quantify variations in the response of the different dishes in the array (Wu+21).

Cross Correlations

section coming soon!