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 measuring recent expansion in the Universe. 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.

Tianlai

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

Cosmic Radio Static

Galaxy surveys are expensive cheap alternative still in developmental stages Tianlai's goal is to demonstrate the potential of this technique to constrain cosmological parameters. The image on the left shows the array of radio telescopes I used to begin my research career with Peter Timbie, with the Milky Way overhead. These telescopes are a proof-of-concept for a technique called Intensity Mapping, which intends to provide a cheap and efficient alternative to galaxy surveys, where observing each individual galaxy becomes an increasingly complicated challenge the further out we observe. 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!