Postdoctoral Researcher studying dark matter

I am am a postdoctoral researcher at the University of California, Merced working with Anna Nierenberg.

I also have an associate member status at the Center for the Gravitational-Wave Universe

I was a postdoctoral researcher studying cosmology at the Korea Astronomy Space Science Institute with Arman Shafieloo.

Previously, I earned my PhD at UC Irvine working with Kevork Abazajian and also collaborating with Manoj Kaplinghat.

When able, I also play board/roleplaying games and train for marathons.

Research Interests

Dark Matter Phenomenology

I am interested in testing dark matter properties in astrophysica contexts. For instance, should dark matter be a WIMP an annihilation signal could be seen in dark matter haloes. The fact a gamma ray signal is not seen in the Milky Way's dwarf galaxies and the fact the gamma ray excess in the Galactic Center is more consistent with a nuclear and stellar bulge can possibly rule out WIMPs as the cosmological dark matter.

Dark sector models of dark matter generically predict dark matter would have a self-interaction cross section. Such an SIDM would have a number of interesting consequences for dark matter halos. For instance, in dwarf and low surface brightness galaxies, SIDM would predict cored density profiles. In baryon dominated systems, the baryons will contract the DM halo shrinking the core size and making a cuspy profile. This diversity in the predictions of SIDM matches the observed diversity in the rotation curves of actual galaxies, which CDM cannot explain. Further, satellite SIDM haloes that have passed close to their host can be tidally stripped, the mass loss of which can trigger a collapse of the isothermal core (arising from a negative heat gradient in gravitationally bound systems), ultimately leading to haloes much denser than CDM predictions. This has interesting implications for the Too Big To Fail problem.

Dark Energy Phenomenology

ΛCDM successfully explains individual cosmological datasets. However, a tension has emerged between the CMB's prediction of H0 and the local measurement of H0 from Cepheids and supernova. This tension may point to new physics beyond ΛCDM such as an evolving dark energy equation of state. Already, this tension may point to a transitional dark energy if the H0 measurement gets more precise. Regardless of the H0 tension, the dark energy equation of state will be determined from future BAO recontructions and growth rate measurements with DESI and from gravitational wave distances with LIGO/LISA/Einstein Telescope. The space of models of evolving dark energy is vast, however, and rather than iterative over a potentially infinite number of models and compare their evidences, I have been working on developing statistical tools can make inferences from cosmological datasets without assuming a model.

Inflationary Phenomenology

The simplest models of single-field slow-roll inflation generically predict a power-law, featureless primordial power spectra, which has successfully predicted the observed CMB anisotropies. As with my work on dark energy, rather than iterate over a large space of inflationary models, I have been using model-independent methods to try and search for deviations away from a power-law primordial power spectra.


Dark Matter Phenomenology

Tying Dark Matter to Baryons with Self-interactions

Self-interacting dark matter (SIDM) models have been proposed to solve the small-scale issues with the collisionless cold dark matter (CDM) paradigm. We derive equilibrium solutions in these SIDM models for the dark matter halo density profile including the gravitational potential of both baryons and dark matter. Self-interactions drive dark matter to be isothermal and this ties the core sizes and shapes of dark matter halos to the spatial distribution of the stars, a radical departure from previous expectations and from CDM predictions. Compared to predictions of SIDM-only simulations, the core sizes are smaller and the core densities are higher, with the largest effects in baryon-dominated galaxies. As an example, we find a core size around 0.5 kpc for dark matter in the Milky Way, more than an order of magnitude smaller than the core size from SIDM-only simulations, which has important implications for indirect searches of SIDM candidates.

Bright gamma-ray Galactic Center excess and dark dwarfs: Strong tension for dark matter annihilation despite Milky Way halo profile

We incorporate Milky Way dark matter halo profile uncertainties, as well as an accounting of diffuse gamma-ray emission uncertainties in dark matter annihilation models for the Galactic Center Extended gamma-ray excess (GCE) detected by the Fermi Gamma Ray Space Telescope. The range of particle annihilation rate and masses expand when including these unknowns. However, two of the most precise empirical determinations of the Milky Way halo's local density and density profile leave the signal region to be in considerable tension with dark matter annihilation searches from combined dwarf galaxy analyses for single-channel dark matter annihilation models. The GCE and dwarf tension can be alleviated if: one, the halo is very highly concentrated or strongly contracted; two, the dark matter annihilation signal differentiates between dwarfs and the GC; or, three, local stellar density measures are found to be significantly lower, like that from recent stellar counts, increasing the local dark matter density.

What the Milky Way's Dwarfs tell us about the Galactic Center extended excess

The Milky Way's Galactic Center harbors a gamma-ray excess that is a candidate signal of annihilating dark matter. Dwarf galaxies remain predominantly dark in their expected commensurate emission. In this work we quantify the degree of consistency between these two observations through a joint likelihood analysis. In doing so we incorporate Milky Way dark matter halo profile uncertainties, as well as an accounting of diffuse gamma-ray emission uncertainties in dark matter annihilation models for the Galactic Center Extended gamma-ray excess (GCE) detected by the Fermi Gamma-Ray Space Telescope. The preferred range of annihilation rates and masses expands when including these unknowns. Even so, using two recent determinations of the Milky Way halo's local density leave the GCE preferred region of single-channel dark matter annihilation models to be in strong tension with annihilation searches in combined dwarf galaxy analyses. A third, higher Milky Way density determination, alleviates this tension. Our joint likelihood analysis allows us to quantify this inconsistency. We provide a set of tools for testing dark matter annihilation models' consistency within this combined dataset. As an example, we test a representative inverse Compton sourced self-interacting dark matter model, which is consistent with both the GCE and dwarfs.

Strong constraints on thermal relic dark matter from Fermi-LAT observations of the Galactic Center

The extended excess towards the Galactic Center (GC) in gamma rays inferred from Fermi-LAT observations has been interpreted as being due to dark matter (DM) annihilation. Here, we perform new likelihood analyses of the GC and show that when including templates for the stellar galactic and nuclear bulges, the GC shows no significant detection of a DM annihilation template, even after generous variations in the Galactic diffuse emission models and a wide range of DM halo profiles. We include Galactic diffuse emission models with combinations of 3D inverse Compton maps, variations of interstellar gas maps, and a central source of electrons. For the DM profile, we include both spherical and ellipsoidal DM morphologies and a range of radial profiles from steep cusps to kiloparsec-sized cores, motivated in part by hydrodynamical simulations. Our derived upper limits on the dark matter annihilation flux place strong constraints on DM properties. In the case of the pure b-quark annihilation channel, our limits on the annihilation cross section are more stringent than those from the Milky Way dwarfs up to DM masses of ∼TeV, and rule out the thermal relic cross section up to ∼300 GeV. Better understanding of the DM profile, as well as the Fermi-LAT data at its highest energies, would further improve the sensitivity to DM properties.

Dark Energy Phenomenology

Model independent inference of the expansion history and implications for the growth of structure

We model the expansion history of the Universe as a Gaussian Process and find constraints on the dark energy density and its low-redshift evolution using distances inferred from the Luminous Red Galaxy (LRG) and Lyman-alpha (Lyα) datasets of the Baryon Oscillation Spectroscopic Survey, supernova data from the Joint Light-curve Analysis (JLA) sample, Cosmic Microwave Background (CMB) data from the Planck satellite, and local measurement of the Hubble parameter from the Hubble Space Telescope (H0). Our analysis shows that the CMB, LRG, Lyα, and JLA data are consistent with each other and with a ΛCDM cosmology, but the H0 data is inconsistent at moderate significance. Including the presence of dark radiation does not alleviate the H0 tension in our analysis. While some of these results have been noted previously, the strength here lies in that we do not assume a particular cosmological model. We calculate the growth of the gravitational potential in General Relativity corresponding to these general expansion histories and show that they are well-approximated by Ωm0.55 given the current precision. We assess the prospects for upcoming surveys to measure deviations from ΛCDM using this model-independent approach.

Implications of a transition in the dark energy equation of state for the H0 and σ8 tensions

We explore the implications of a rapid appearance of dark energy between the redshifts (z) of one and two on the expansion rate and growth of perturbations. Using both Gaussian process regression and a parameteric model, we show that this is the preferred solution to the current set of low-redshift (z<3) distance measurements if H0=73 km/s/Mpc to within 1% and the high-redshift expansion history is unchanged from the ΛCDM inference by the Planck satellite. Dark energy was effectively non-existent around z=2, but its density is close to the ΛCDM model value today, with an equation of state greater than −1 at z<0.5. If sources of clustering other than matter are negligible, we show that this expansion history leads to slower growth of perturbations at z<1, compared to ΛCDM, that is measurable by upcoming surveys and can alleviate the σ8 tension between the Planck CMB temperature and low-redshift probes of the large-scale structure.

Will Gravitational Wave Sirens Determine the Hubble Constant?

Lack of knowledge about the background expansion history of the Universe from independent observations makes it problematic to obtain a precise and accurate estimation of the Hubble constant H0 from gravitational wave standard sirens, even with electromagnetic counterpart redshifts. Simply fitting simultaneously for the matter density in a flat ΛCDM model can reduce the precision on H0 from 1% to 5%, while not knowing the actual background expansion model of the universe (e.g. form of dark energy) can introduce substantial bias in estimation of the Hubble constant. When the statistical precision is at the level of 1% uncertainty on H0, biases in non-ΛCDM cosmologies that are consistent with current data could reach the 3σ level. To avoid model-dependent biases, statistical techniques that are appropriately agnostic about model assumptions need to be employed.

Debiasing Cosmic Gravitational Wave Sirens

Accurate estimation of the Hubble constant, and other cosmological parameters, from distances measured by cosmic gravitational wave sirens requires sufficient allowance for the dark energy evolution. We demonstrate how model independent statistical methods, specifically Gaussian process regression, can remove bias in the reconstruction of H(z), and can be combined model independently with supernova distances. This allows stringent tests of both H0 and ΛCDM, and can detect unrecognized systematics. We also quantify the redshift systematic control necessary for the use of dark sirens, showing that it must approach spectroscopic precision to avoid significant bias.

A model-independent determination of the Hubble constant from lensed quasars and supernovae using Gaussian process regression

Strongly lensed quasar systems with time delay measurements provide "time delay distances", which are a combination of three angular diameter distances and serve as powerful tools to determine the Hubble constant H0. However, current results often rely on the assumption of the ΛCDM model. Here we use a model-independent method based on Gaussian process to directly constrain the value of H0. By using Gaussian process regression, we can generate posterior samples of unanchored supernova distances independent of any cosmological model and anchor them with strong lens systems. The combination of a supernova sample with large statistics but no sensitivity to H0 with a strong lens sample with small statistics but H0 sensitivity gives a precise H0 measurement without the assumption of any cosmological model. We use four well-analyzed lensing systems from the state-of-art lensing program H0LiCOW and the Pantheon supernova compilation in our analysis. Assuming the Universe is flat, we derive the constraint H0=72.2±2.1km/s/Mpc, a precision of 2.9%. Allowing for cosmic curvature with a prior of Ωk=[−0.2,0.2], the constraint becomes H0=73.0+2.8-3.0km/s/Mpc.

Determining Model-independent H0 and Consistency Tests

We determine the Hubble constant H0 precisely (2.3% uncertainty) in a manner independent of cosmological model through Gaussian process regression, using strong lensing and supernova data. Strong gravitational lensing of a variable source can provide a time-delay distance DΔt and angular diameter distance to the lens Dd. These absolute distances can anchor Type Ia supernovae, which give an excellent constraint on the shape of the distance-redshift relation. Updating our previous results to use the H0LiCOW program's milestone dataset consisting of six lenses, four of which have both DΔt and Dd measurements, we obtain H0=72.8+1.6−1.7 km/s/Mpc for a flat universe and H0=77.3+2.2−3.0 km/s/Mpc for a non-flat universe. We carry out several consistency checks on the data and find no statistically significant tensions, though a noticeable redshift dependence persists in a particular systematic manner that we investigate. Speculating on the possibility that this trend of derived Hubble constant with lens distance is physical, we show how this can arise through modified gravity light propagation, which would also impact the weak lensing σ8 tension.

Inflationary Phenomenology

Inflation Wars: A New Hope

We explore a class of primordial power spectra that can fit the observed anisotropies in the cosmic microwave background well and that predicts a value for the Hubble parameter consistent with the local measurement of H0=74 km/s/Mpc. This class of primordial power spectrum consists of a continuous deformation between the best-fit power law primordial power spectrum and the primordial power spectrum derived from the modified Richardson-Lucy deconvolution algorithm applied to the Cℓs of best-fit power law primordial power spectrum. We find that linear interpolation half-way between the power law and modified Richardson-Lucy power spectra fits the Planck data better than the best-fit ΛCDM by ΔlogLike=2.5. In effect, this class of deformations of the primordial power spectra offer a new dimension which is correlated with the Hubble parameter. This correlation causes the best-fit value for H0 to shift and the uncertainty to expand to H0=70.2±1.2 km/s/Mpc. When considering the Planck dataset combined with the Cepheid H0 measurement, the best-fit H0 becomes H0=71.8±0.9 km/s/Mpc. We also compute a Bayes factor of logK=5.7 in favor of the deformation model.


rkeeley [at] ucmerced [dot] edu