Astronomers are coming closer to a more accurate inventory of the universe and everything in it thanks to efforts to gauge the true shapes and orientations of galaxies.
Clare Lamman is part of a team of astrophysicists using data from the Dark Energy Spectroscopic Instrument to map as many as 50 million galaxies. In this talk, delivered in April 2024 at the annual Harvard Horizons Symposium, Lamman describes her distinctive contribution to this effort—gauging the “intrinsic alignment” of galaxies to better understand the universe and how it evolves.
This transcript has been edited for clarity and correctness.
In dark skies, you can see an uncountable number of stars. But deep in the darkness behind them lie an even greater number of galaxies. These galaxies are unimaginably distant. But let's take a slice, sour far away from Earth, and rotate our perspective.
One of the first things you might notice is that galaxies aren't randomly scattered about. They trace strands and clumps, which make up the cosmic web. This large-scale structure is the result of tiny fluctuations in the early universe, which have grown to massive scales as gravity draws stuff together. The universe is like old milk. The longer it sits out, the clumpier it gets. We want to measure the structure of the universe and explore the forces which shape it on the largest scales.
But how do you make a map of galaxies? Well, that's a story I wish I had time to tell. It's the result of decades and hundreds of people working together from all over the world. I am one of them. And as part of a collaboration called DESI, we are building the most complete map of the nearby universe.
This wedge contains the positions of some of the galaxies we've measured. It's 3 billion light years across, which is a distance you can only really comprehend through orders of magnitude. Me compared to the size of the solar system is about the same as the size of the solar system compared to the expanse of this wedge. It's really big. And yet this little slice represents less than 0.01% of the total volume we're surveying. Altogether, DESI is going to measure the positions of 50 million galaxies. That's more than I can even show you on a computer screen, which typically only has about 2 million pixels.
And when you're measuring things on these scales, we need to consider that light takes time to travel. Light from distant galaxies traveled for hundreds of thousands of years, carrying information of an older universe. Mapping out the universe at different distances reveals not just this structure but how the structure changed in time.
So we want to measure this cosmic web. But one thing I haven't told you up until now is that most of it is actually made of dark matter, which unfortunately is invisible. However, it's illuminated by glowing galaxies, which fall along the web. I want to maximize the information we can get from our map of galaxies. And to do so, we need to acknowledge that the connection between them and the web is tangled. They don't perfectly trace it out. They tend to form more in dense regions.
Also, galaxies are more complicated than just points. They have shapes and orientations, which are affected by where they fall in the web. Galaxies are also connected to each other, forming clusters and groups. Not to mention, all of these galaxies are moving, swirling around each other, and falling in along the cosmic strands.
My research explores this tangle of correlations from two main perspectives, how they skew traditional measurements of the web, and then what extra information we can get out of it. So first of all, these correlations can create systematic errors in important measurements because galaxy characteristics are also tangled with the criteria we use to select galaxies for the survey, resulting in shifted measurements of the underlying matter. I have found correlations between galaxy shapes and the web which create clustering patterns in our data that affect measurements of how fast structure grows. My corrections are necessary to distinguish between gravity and dark energy.
Second, all of these pesky correlations are actually quite useful because it means that every galaxy has a lot of extra information about its environment, and some of these connections span remarkable scales. For instance, I have found groups of galaxies and measured their average orientation. Although invisible, this is connected to the blob of dark matter surrounding it. These orientations may seem random, but when you measure them over many groups of galaxies, on average, they tend to point towards regions of higher density.
This is the result of tidal forces created by the cosmic web. It's the same principle that creates tides on Earth. It's sort of incredible when you think about the fact that something as far away as the moon can have such a strong effect on our oceans. And yet I found a connection that spans much further, over remarkable scales. I've found that a clump of matter can have an effect on galaxies, even if they're millions of light years away. The effect is subtle, but it can be detected if you have a map of millions of galaxies, revealing a unique way to quantify the largest structures in the universe.
Now, exactly how you go from my measurements to a quantitative statement about the underlying matter is really difficult to verbalize. It can be elegantly summarized through math. But instead of going into the details, let's take a step back and think about what it really means to measure these correlations. Internalizing just how massive these structures of matter are and why they even matter at all is difficult. So for another perspective, let's fly through the map of galaxies.
Understanding connections between them and the web is complicated because there are so many factors at play, bright young stars, colliding galaxies, black holes spewing material. But on very large scales, these little effects average out, and the remaining correlations are a clean result of the delicate balance between gravity and dark energy. Mapping out this structure, exploring its evolution, and searching for far-reaching connections within it reveal the nature of the largest forces in the universe.
Although these are scales well beyond our daily experiences, we're addressing questions that are very fundamental, very human. What does the universe look like? How does it change, and why?