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Research

I'm broadly interested in early-universe astrophysics and cosmology. My recent work has been on understanding the epochs of cosmic dawn and reionization, especially on what the first galaxies can tell us about dark matter. If you want to see a fairly recent talk on my work here's a link to the Clay Lecture I gave at the Harvard-Smithsonian CfA.

You can find the codes that I have written in the github, including:

Zeus21: An analytic code for 21-cm at cosmic dawn

21cmvFAST: 21-cm simulations to include dark matter-baryon relative velocities.

RelicFast: a code to find scale-dependent bias from light relics and massive neutrinos.

I have also contributed to developing 21cmFASTv3 (see EOS21 page) and GALLUMI (link, see below).

Here is a list of some of the topics I'm excited about, and some of the results my group has gotten. If you're interested in any of this please get in touch!

21-cm Cosmology

A big part of my work is on extracting cosmology and astrophysics from 21-cm data. Here are some examples:

In two companion papers (1 and 2) I introduced 21cmvFAST, a version of 21cmFAST that included the effect of the dark matter-baryon relative velocities on the first galaxies. I showed that these produce velocity-induced acoustic oscillations (VAOs, or wiggles) in the 21-cm signal during cosmic dawn, which can be used as a standard ruler to cosmic dawn (z=10-20). In this paper I made the velocities part of the standard 21cmFAST software, and updated our reference 21-cm models of the first galaxies (see EOS21 for details and nice animations).

I have also developed the 21-cm line as a tool to learn about dark matter (DM). Here we argued that cosmic dawn is an ideal location to search for millicharged DM, and that only a sub-percent fraction of the DM is required to explain the EDGES anomaly at z=17. I have led the HERA theory analysis of DM, and found more stringent limits, though at z=8-10. Upcoming high-z HERA data will be able to test the origin of the EDGES depth, if real. The 21-cm line also has information about the timing of the first galaxies, which  in this paper I showed makes it is sensitive to the small-scale nature of dark matter, at much higher wavenumbers than currently accessible (k~50/Mpc). This can help identify self-interacting DM, like in the ETHOS models.

The future of 21-cm is to map the fluctuations across many redshifts and scales, which show a rich phenomenology of processes, ranging from heating of the IGM (likely due to high-mass X-ray binaries), to its reionization (due to galaxies of different masses). Key to understand all these processes are fast simulations across a range of scales. These are needed to explore the vast astrophysical---and cosmological---parameter space. Below I show a lightcone of the 21-cm signal across cosmic dawn and the EoR, along with the densities that seed the fluctuations.

21cmLC

HERA Data Analysis

All these simulations and predictions are not just beautiful pictures, but also signals to compare with the results of current and upcoming 21-cm experiments. I'm a member of HERA, where we have been able to set the deepest limits on the 21-cm power spectrum during reionization (z=8-10), and are now pushing to do the same at cosmic dawn (z=14-20). I lead the dark-matter theory group in HERA, and have worked on translating the 21-cm limits into inferences on the spin temperature of gas during unexplored eras. See for instance the plot I made with a summary of the first HERA limits, in our first interpretation paper (see also the most recent one, with a similar plot, here).

Tslims

Minihaloes and PopIII stars

If you're curious to see how we can use 21-cm to detect and understand the first PopIII stars that formed in the universe, you may be interested to see the EOS 2021 (the Evolution of Structure in 21-cm, 2021) simulations. Here is an entire page dedicated to this project that you can peruse:

EOS 2021 simulation project

Cosmology from HST/JWST

Many of us are familiar with the beautiful Hubble ultra-deep field images. Not only do these teach us a lot about the formation of the first galaxies during the epoch of reionization, but also about cosmology and the nature of dark matter. In these two papers (1 and 2, led by grad student Nash Sabti) we developed a pipeline to measure the clustering of matter at z=4-10 marginalizing over the astrophysical uncertainties of this era (chiefly the unknown halo-galaxy connection). We obtained a measurement of the matter power spectrum up to k=10/Mpc, reaching smaller scales---and earlier times---than other cosmic probes.

Our measurement (black crosses) is shown along with other cosmic data (compiled in this paper, all linearly extrapolated to z=0, where the standard LCDM prediction is the black line) in this figure. The UVLFs allow us to access the unknown era of z=4-10 at smaller scales than we currently probe (albeit only at ~30% precision!) We used a similar technique to constrain primordial non-Gaussianity at small scales here.

matterpower_UVLF

Zeus21

Most recently I have introduced an effective model for the 21-cm signal, based on log-normal fields. You can find it in arXiv and in the public software package Zeus21 (Git). This code can reproduce the results of expensive simulations in ~1 s, and gives us a new way to find the evolution of the 21-cm signal across cosmic dawn. To give you an idea, here is the prediction of Zeus21 against the popular 21cmFAST simulations for 3 different astrophysical parameter sets. Top panel shows the global signals, and bottom is the power spectrum at k=0.3/Mpc. Solid lines are Zeus21, which can reproduce the simulations (dashed) really well, and very fast!

Zeus21showcase

Light but Massive Relics (LiMRs)

If you follow beyond-standard-model cosmology, you'll be familiar with Neff constraints on new physics. In a recent series of papers we studied how to use cosmological data to search for relics that are light but not massless. We call these LiMRs. In our first two papers we showed their effect on the matter power spectrum (here and here). LiMRs tend to suppress power for scales smaller than their free-streaming, like neutrinos, so measurements of galaxies and weak lensing are ideal to look for them.

In our most recent work, led by Linda Xu, we found the strongest constraints on light relics to date. Using SDSS+Planck+CFHTLens data we ruled out gravitinos with masses above ~2 eV (an order of magnitude tighter constraints than the Lyman-alpha forest!) 

Here are our constraints in the 2D space of relic temperature (directly related to Neff) and mass. The Neff constraint corresponds to the low-mass limit of this plot, and by using the power spectrum we can test heavier relics with even lower temperatures.

LiMR_T_m

Does JWST break our cosmological models?

Following up on our HST work described above, we set out to test if JWST is breaking our cosmological models. In particular we seeked for enhancements of power that could explain the overabundance of ultramassive galaxies claimed to have been observed in JWST (see Labbe+23  and Boylan-Kolchin 23). We used the same HST data (taken over decades!) to show that extra structure formation in a way that would produce more dark-matter halos for JWST at z~5-10 would be in conflict with the HST galaxies. In the left panel below you can see quantitatively how. The 2D posteriors are the power-spectrum enhancement you'd need over LCDM to explain the JWST (blue and green, assuming either 10% or 30% of the gas converts to stars), whereas HST does not allow anything on top of the yellow region. The right panel shows the same information as a function of the density of galaxies above different stellar-mass cuts, where JWST needs to be at the red star, whereas HST only allows to be within (or below) the blue region). In other words, HST does not let you break the universe as much as the claimed JWST galaxies need! The solution may be astrophysical instead, some of the galaxies may host AGNs which make them appear redder (see for example Endsley+23)

HST_constraintsabundance_jwst

Paper is here, and it was summarized in the APS magazine Physics here (and other news outlets linked at the bottom).

Press Releases

Our recent work constraining cosmological solutions to the JWST "universe breaking" galaxies was highlighted by PRL and has been featured on KXAN, Scientific American, ScienceNews, Physics, Phys.org, astrobites, and UT News, amongst others.

I was recently featured as one of ScienceNews 10 scientists to watch in 2023 (SN10 link).

The VAOs that I developed as a standard ruler during cosmic dawn have were selected as PRL Editor's choice, and featured on the APS magazine Physics, Phys.org, and astrobites.

My work on millicharged dark matter published in Nature has been featured in the Spanish newspaper El País, in Physics World,  Cosmos Magazine, and Live Science, as well as in podcasts such as El Método and Señal y Ruido (in Spanish), amongst others.

Work done during my PhD on microlensing of Fast Radio Bursts, published in PRL, was also featured in Futurity and the JHU HUB.

And I was interviewed on Fox 7 Good Day Austin about the 2024 total eclipse (link), and recently was in a podcast with UT colleagues on whether cosmology is in crisis (link).