Dark Energy

Cosmology is the study of the big questions: where did the universe come from? How does it evolve? What will happen to it? We know that the universe is expanding, and its expansion is increasing. We’re not sure why, but we attribute it to an unknown force called “dark energy”.

We can actually “see” how the universe has evolved since the formation of the earliest galaxies, thanks to the fact that light takes time to travel. The further away in space we look, the further we look back in time. By mapping out the location of galaxies all around us, we can study both the large-scale structure of matter in the universe, and how it has changed over time. The components of our universe affect its evolution, so the best way to observe the nature of dark energy on large scales is to map out this evolution. We use simulations like the one pictured above to understand how different theories of dark energy affect the structure we observe.

More about AbacusSummit simulations

Dark Energy Spectroscopic Instrument

I am part of the Dark Energy Spectroscopic Instrument (DESI) collaboration, a group of hundreds of astronomers from all over the globe who are mapping out the nearby universe. DESI is located on the 4m Mayall Telescope at Kitt Peak, AZ. We are working on measuring the distances to over 50 MILLION galaxies, creating the most complete map of the universe to date. DESI is built to observe as many galaxies as possible, as quickly as possible. It does this thanks to 5,000 individually-controlled robotic positioners which can each gather the light of an individual galaxy. This light then gets passed down to a spectrograph room, where it is broken up and measured. These measurement can tell us a lot about each galaxy: from its distance to what it's made of and even some of its dynamics.

DESI's website | 23-minute video about DESI

See this page for a series of comics featuring DESI's ambassador, BaoBan.

Intrinsic Alignments

The large-scale structure of the Universe leaves its mark on visible matter through subtle correlations involving galaxy shapes, galaxy spins, and the underlying cosmic web. These “Intrinsic Alignments” can only be observed with tens of thousands of galaxies. But how do we quantify them? For the long answer, The IA Guide ! For the short answer, keep reading.

Individual galaxies are beautifully unique and complex objects. But as an observational cosmologist, I see them as millions of flat ovals on the sky. Each galaxy can be described by a number, ellipticity, which contains information about how round the galaxy is and its orientation.

We explore how these shapes correlate with other properties by measuring them relative to some direction, such as another galaxy or dense regions of the cosmic web. My cat kindly agreed to demonstrate various ellipticities relative to the most important direction, her food.

Simulations tell us that, statistically, galaxy shapes have “intrinsic alignment” relative to clusters and filaments in the cosmic web. But the large-scale structure of the Universe also systematically distorts the light of galaxies, known as “weak lensing”. This is like the distortion you see in galaxy shapes in the JWST deep field, except very subtle.

There are many correlations involving galaxy shapes, positions, and lensing. It's important to untangle them in order to get accurate cosmological information out of weak lensing observations and other surveys. It also can be used to directly explore cosmology by studying the imprint that cosmological effects have on Intrinsic Alignments.

It's tricky to keep track of the different ways to quantify these correlations! There are many estimators, depending on what information you`re interested in and what data you`re working with. Therefore, a group of other early career scientists and myself put together The IA Guide. The IA Guide contains a collection of IA formalisms, estimators, modeling approaches, alternative notations, and useful references. We hope others find it as useful as we do!

Galaxy Alignment and Cosmology

^{Studying the 3D shapes of galaxies involves modeling lots of ellipsoids and understanding how they project to a surface, like the sky.}

Click here for an accessible summary of my latest RSD paper!

I am an observational cosmologist. Matter in the universe, and therefore galaxies, forms what we call a large-scale structure. It basically looks like a giant sponge – made up of strands and sheets of matter, with large voids that contain almost no matter. There are several statistics that we use to quantify this structure. They basically measure how “clumpy” the distribution of galaxies are.

DESI needs to measure these very precisely in order to distinguish between different models of dark energy, which means that we have to consider MANY difference sources of systematic errors. My work is exploring one of these sources.

The types of galaxies I look at are basically 3D ovals, and DESI is more likely to observe a galaxy if its long axis is pointed at us. This is because its light is more concentrated on the sky and it appears to have a higher surface brightness. This is a problem because the orientations of galaxies are ALSO aligned with the matter density that we are trying to measure.

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Here is how I would summarize my work with IA and RSD to different audiences:

Kindergartener

I use a telescope to look very far away at big clumps of stars. We wonder about where everything came from, and how it all started.

High Schooler

The universe is expanding, and its expansion is increasing. We’re not sure what force is driving this, but we call it dark energy. We’re using a telescope to observe 30 MILLION galaxies and making the most detailed map of the universe yet! I help measure the statistics which quantify how galaxies are distributed and how that distribution has changed over time. This will help us better understand what’s up with dark energy.

Astronomy Undergraduate Student

I’m helping with a spectroscopic galaxy survey, DESI. My project is measuring the correlation between galaxy shapes and the underlying density field. This is important because DESI’s target selection is dependent on galaxy orientation, which is tied to said density field.

Cosmologist

I’m measuring the projected shape-density correlation of Luminous Red Galaxies in DESI’s Legacy Imaging Survey. I’m also modeling the net polarization of LRGs due to DESI’s fiber magnitude-based selection. These effects combine to create a systematic bias in DESI’s measurement of the RSD quadrupole. We’re interpreting the IA signal via a suite of cosmological n-body simulations, AbacusSummit, and a linear tidal model.

A New Perspective on Intrinsic Alignments

Intrinsic Alignmnets can bias measurements within cosmology, but they can also be useful! We often trace large-scale structure (LSS) using only the positions of galaxies. However, intrinsic alignmnets mean that each galaxy also contains information about the direction of gravitational forces created by LSS. This could be used to detect many features of LSS, some of which can't be detected with traditional methods. However, using IA in practice is limited by two key issues: (1) there are many subtle problems assicoated with imaing the shapes of indiual galaxies (2) not all galaxies display alignment

To solve these issues, we present a new way of detecting IA: using the orientations of small sets of galaxies (or "multiplets") instead of individual galaxies. Using DESI's Year-one data, we detect alignment using all galaxy types and further back in the history of the universe than has been done before. This could be a useful new way to measure the largest structure in the universe and my current research is developing multiplet alignment as a cosmological tool.

Accessible Summary of the full paper | Link to original paper | 10-minute scientific talk about this work