Research Highlights


Novel Methods in Machine Learning and Statistics for Challenges in Cosmology:


  1. First attempt to infer the presence of dark matter substructure in strong lens images with a binary classifier, without having to do any intermediate lens modeling, using a Convolutional Neural Network (CNN): together with my Ph.D. student Ana Diaz Rivero we trained a CNN to classify images based on whether they have detectable substructure or not. Tens of thousands of new lenses are expected to become available in the near future. The new and fast approach to analyze strong lens images proposed in this work is more suited to this new era of large data sets.


Dark Matter, Light Relics, and Neutrinos:


  Some of my work to shed light in the dark sector includes:


  1. New Formalism for dark matter (DM) substructure statistics proposed to discern among different DM scenarios: in recent work with my group (published in PRD), I developed a general formalism to compute from first principles the projected mass density (convergence) power spectrum of the substructure in galactic halos under different populations of dark matter subhalos. We constructed a halo model-based formalism, computing the 1-subhalo and the 2-subhalo terms from first principles for the first time. We found that the asymptotic slope of the substructure power spectrum at large wavenumber reflects the internal density profile of the subhalos, and proposed this as a key observable to discern between different dark matter scenarios.


  In subsequent work (published in PRD), we applied our formalism to N-body simulations and found

  agreement with our predictions. Furthermore, we found that even at lower wavenumbers we can

  gain important information about dark matter. Comparing the amplitude and slope of the power

  spectrum on scales in the reach of current observations from lenses at different redshifts can help

  us distinguish between cold dark matter and other dark matter scenarios.


  1. New channel for DM Freeze-in: together with Katelin Schutz and Tongyan Lin, I identified an additional  production channel for DM produced through the freeze-in mechanism: the decay of photons that acquire an in-medium plasma mass (published in PRD). These plasmon decays are a dominant channel for DM production for sub-MeV DM masses, and including this channel leads to a significant reduction in the predicted signal strength for DM searches. The DM acquires a highly non-thermal phase space distribution, which impacts the cosmology at later times. This work was an Editors’ Suggestion in PRD.

  2. Strongest constraints to date on dark matter-baryon scattering implying that a baryon in the halo of a galaxy like the Milky Way cannot scatter from DM particles in the history of the universe: the shrinking of the canonical-WIMP parameter space from null LHC and direct searches, as well as possible difficulties for collisionless N-body simulations to reproduce observational data, provide motivation to consider stronger baryon-DM interactions.

  In work in collaboration with Kfir Blum and Marc Kamionkowski (published in PRD), I derived the

  strongest constraints to date on elastic scattering between baryons and DM for a wide range of

  velocity-dependent cross sections, using measurements of the CMB fluctuations by the Planck

  satellite and Lyman-alpha flux power spectrum measurements from the Sloan Digital Sky Survey

  (SDSS). These constraints imply, model-independently, that a baryon in the halo of a galaxy like

  our Milky Way cannot scatter from DM particles in the history of the universe.


  Recently, the EDGES collaboration has claimed a detection of neutral hydrogen in the early

  universe. Their measurement, taken at face value, disagrees with the standard prediction of the

  temperature of hydrogen gas, being colder than expected. Our work inspired a paper published in

  Nature as a potential explanation to the claimed observation, and a number of other papers.


  1. Probing sub-GeV DM with cosmology (complementary to direct detection searches): in recent work with my group (published in PRD), I showed that cosmology is complementary to dark matter direct detection experiments for DM masses below a GeV. We analyzed CMB data from Planck and Lyman-alpha forest data from the SDSS in the context of the sub-GeV DM scenario. Our analysis is particularly interesting given that lighter DM masses have remained unexplored by current direct detection experiments. The sub-GeV DM scenario scenario is now being considered as one of the main drivers of the Dark Matter science case for the proposed CMB-S4 experiment.

  Our paper captured attention from the community since our constraints rule out the possibility of

  DM-baryon scattering explaining the EDGES claimed detection of neutral hydrogen.


  1. Scale-dependent galaxy bias induced by light relics: with my postdoc Julian Muñoz, I  computed the scale-dependent galaxy bias induced by light relics (not limited to neutrinos) of

  different masses, spins, and temperatures. We also made publicly available a code (“RelicFast") 

  that efficiently computes the galaxy bias in under a second, allowing for this effect to be properly

  included in likelihood analyses with different cosmologies with light relics, at little computational

  cost.


  1. I led the Neutrino Mass from Cosmology paper submitted to the US Decadal Survey, where I

  argued that our understanding of the clustering of matter in the presence of massive neutrinos has

  significantly improved over the past decade, yielding cosmological constraints that are tighter than

  any laboratory experiment, and which will improve significantly over the next decade, resulting in a

  guaranteed detection of the absolute neutrino mass scale.



Inflation:


  1. Imprints of massive spinning particles in the large-scale structure of the universe: In work with my group (published in JCAP), I presented a theoretical template for the bispectrum generated by massive spinning particles in the early universe, valid for a general triangle configuration of momenta, when the approximate conformal symmetry of the inflationary background is broken.

  We investigated the prospects of measuring these signals with upcoming galaxy surveys, and our

  results suggest that two next-generation spectroscopic galaxy surveys, DESI and EUCLID, could   

  be sensitive to probing the effect of massive particles with non-zero spin.


  1. Gravitational waves (likelihood analysis of BICEP/Planck data): In 2015, I joined the joint analysis between BICEP2, the Keck array, and Planck collaborations. I worked on the likelihood analysis of a multi-component model that included galactic foregrounds and a possible contribution from inflationary gravity waves. The code that I wrote was made publicly, and it has been extensively used by the community. We reported no statistically significant evidence for primordial gravitational waves and a strong evidence for galactic dust (published in PRL). My code was subsequently used in the subsequent BICEP/Keck collaboration papers.


  1. Formalism for model-independent tests of slow-roll inflation: in a series of papers in collaboration with Wayne Hu (“Generalized slow roll approximation for large power spectrum features” (PRD), “CMB Constraints on Principal Components of the Inflation Potential” (PRD) and “Complete WMAP Constraints on Bandlimited Inflationary Features” (PRD)),  I developed a formalism, known as “Generalized Slow Roll", to test the hypotheses of slow-roll and single-field inflation in a general and model-independent way.

  This framework was used to map constraints from the CMB onto constraints on the shape of the

  inflationary potential beyond any specific model of inflation. It has been used and extended by

  many groups.

  The CMB-S4 experiment will use this formalism as its main way of probing features in the

  inflationary potential.


  1. Formalism for the bispectrum of inflationary models with features: I extended, together with collaborators, the “Generalized Slow Roll” formalism to the bispectrum in a series of papers (Fast Computation (PRD) and Non-Gaussianity from step features (PRD)).

  The Planck collaboration used our formalism to look for inflationary features and, more generally, it

  has been widely implemented to study different inflationary scenarios in the literature.


  1. Fundamental physics from large-scale CMB E-modes: I proposed for the first time that CMB

  polarization data from the Planck satellite has the statistical power to either confirm or rule out

  models that attempt to explain large-scale temperature anomalies.

  I also showed that the large-scale CMB polarization signal from reionization can be a source of

  confusion with inflationary features.

  These consistency checks were carried out by the Planck collaboration and other groups.



Epoch of Reionization:


  Since the beginning of my career, I keep an interest in the period of reionization.


  1. High-redshift ionization preferred by Planck data: the usual imposition of a steplike ionization

  history requires the optical depth to reionization to mainly come from low redshifts. Together with

  my graduate student Georges Obied and our collaborators, we relaxed this assumption and found

  that in the Planck 2015 data, there is a preference for a component of high redshift (z>10)

  ionization (early stars), in contradiction with claims made by the Planck collaboration. We found

  that marginalizing inflationary freedom does not weaken the preference for z>10 ionization.

  These findings prompted the Planck collaboration to revise their standard way of analyzing the

   reionization history and opened up an ongoing debate in the community.


  1. New CMB B-mode contribution from patchy reionization: I showed that existing calculations of

  the B-mode polarization power spectrum from reionization were incomplete by finding an additional

  source of B-modes. These B-modes have been sought for in simulations by many groups.


  1. Statistical technique for extracting the patchy reionization signal from CMB measurements: I developed a new statistical technique for extracting the inhomogeneous reionization signal from measurements of the CMB polarization. In this method, a quadratic combination of the E-mode and B-mode polarization fields is used to reconstruct a map of fluctuations in the CMB optical depth. This statistical technique has been widely used by the community, and it is one of the main ways in which CMB-S4 is planning to extract the inhomogeneous reionization signal from measurements of the CMB E-mode and B-mode polarization fields.

  I showed that the cross-correlation of this optical depth estimator with the 21-cm field is sensitive to

  the detailed physics of reionization, and can be measured with upcoming radio interferometers and

  CMB experiments (published in ApJ).