I am a Hubble fellow at Caltech. My research focuses on searching for strongly lensed supernovae and kilonovae with the Zwicky Transient Facility, the Large Synoptic Survey Telescope, and the Dark Energy Camera. I am the co-convener of the LSST Dark Energy Science Collaboration's strong lensing science working group.
For almost 100 years, we have known that the Universe is expanding. But exactly how fast it is expanding is a matter of debate. Recent measurements of the expansion rate using the cosmic distance ladder are in tension with the expansion rate inferred from the cosmic microwave background assuming our best cosmological model (ΛCDM) and the standard model of particle physics.
This tension could be due to physics beyond the standard model: if there were four neutrino species instead of the standard model's three, the measurements would be consistent. But it may also be a sign of systematic errors in the measurements. To determine the true origin of the discrepancy, independent probes of the expansion rate are needed.
Strongly lensed Type Ia supernovae, recently observed for the first time, provide a promising way to measure the expansion rate of the Universe (the Hubble constant) independently of both the cosmic microwave background and the distance ladder. Lately, I have been working on developing these objects as cosmological probes for the era of next-generation imaging surveys such as the Large Synoptic Survey Telescope and the Zwicky Transient Facility. My collaborators on this project are Peter Nugent, Eric Linder, Dan Kasen, Alex Kim, and Saul Perlmutter. My recent work has focused on how to discover these objects and how to measure their time delays in a way that mitigates the effects of microlensing, a systematic uncertainty.
To learn more about cosmology with strongly lensed Type Ia supernovae, check out the Facebook live video above.Papers on this Topic
The shock breakout light curves of core-collapse supernovae contain valuable information about their progenitor stars. The width of the shock breakout light curve reveals the light-crossing time of the star, and thus its size. Sampling the shock breakout light curves of many core-collapse supernovae could put stringent constraints on supernova explosion models.
Unfortunately, shock breakout is extremely difficult to detect, as it produces the very first visible emission from core-collapse supernovae, which only lasts for ~30 minutes. Only recently has a shock breakout light curve been detected (using Kepler), and these detections sampled the light curve only sparsely.
Strongly lensed core-collapse supernovae present a unique opportunity to detect shock breakout. By modeling strong lensing time delays, we can predict where and when a supernova image will appear, anticipating shock breakout before it occurs. I am involved in efforts to detect shock breakout light curves from the strongly lensed core-collapse supernovae that will be discovered by the Zwicky Transient Facility and the Large Synoptic Survey Telescope.
Dark energy makes up ~70% of the mass-energy of the Universe, but we know very little about it. Our simplest model of dark energy, the cosmological constant, fits the data well, but it does not elucidate dark energy's origin, physics, or connections to the other components of the standard model.
Time delays from strongly gravitationally lensed Type Ia supernovae probe the nature of dark energy through its equation of state in a manner that is highly complementary to other probes. I am involved in a number of efforts to constrain dark energy through lensed supernova time delays and the Type Ia supernova distance-redshift relation. Through these measurements, we seek to better understand the nature of dark energy and to determine if our best dark energy model, ΛCDM, breaks down.Papers on this Topic
Despite the major scientific successes that have resulted from their use as tools for cosmology, Type Ia supernovae remain poorly understood. Data, such as early-time observations of the very nearby SN Ia 2011fe, have indicated that the events result from the runaway thermonuclear explosion of at least one carbon-oxygen stellar core, most likely a white dwarf. However, a consensus on the nature of the progenitor system(s) and the explosion mechanism(s) of Type Ia supernovae has not emerged.
Many systems have been proposed as potential SN Ia progenitors. These can roughly be divided into two categories: single-degenerate and double-degenerate systems. With Dan Kasen, I have carried out detailed spectrum synthesis calculations of both thermonuclear and core-collapse supernovae. We are using the results of these calculations to measure the masses of Type Ia supernovae, in an attempt to test different progenitor scenarios.Papers on this Topic
Recently, the idea that a massive planet in the outer solar system can explain the unusual orbital clustering of distant trans-Neptunian objects has electrified the astronomical community. This bold prediction has launched a massive hunt for "Planet Nine," inspiring people around the world to look for the (potential) new planet in our cosmic backyard. I am involved in multiple efforts to find Planet 9.
I am the co-convener of the strong lensing working group in the LSST dark energy science collaboration. LSST is a transformational experiment that will survey a large fraction of the sky every few nights to unprecedented depth. It will discover thousands of strongly lensed galaxies and supernovae, as well as more exotic phenomena such as double source-plane lenses.
I am a builder of DES and a member of its supernova working group. I developed the machine learning pipeline used to discover transient candidates on difference images from the survey. As a graduate student, I have spent nearly 30 nights at CTIO in Cerro Tololo, Chile taking DES observations with the Dark Energy Camera (DECam). Above, a photo from the mountain.
email: danny [at] caltech [dot] edu