The Universe is expanding. Almost everything in the Universe is getting further away from Earth because of this expansion. The current rate of this expansion, as measured by the Hubble constant, and the expansion history of the Universe depend on the content of the Universe.

Two decades ago, astronomers using Type Ia supernovae determined that the Universe's expansion is accelerating, propelled by dark energy. This 2011 Nobel Prize in Physics was awarded for this discovery. Today, we continue to use Type Ia supernovae to precisely measure the Universe's expansion history to understand the nature of dark energy.

Additionally, Type Ia supernovae are a primarily way of measuring the Hubble constant. Recent measurements indicate that the "local" supernova measurement is higher than the inferred "reverse distance ladder" measurement. While this difference may be caused by subtle errors in one or both of the analyses, it is also possible that there is "new" physics that is unaccounted in our current cosmological model.

The UC Santa Cruz transient team is making the most precise local measurements with the Foundation Supernova Survey, the Swope Supernova Survey, and the Young Supernova Experiment. We track supernovae to large distances with the Dark Energy Survey. We are also preparing for the next generation of surveys, to be executed with the Vera C. Rubin Observatory and Nancy G. Roman Space Telescope.

Gravitational waves were first detected by LIGO in 2015 when two black holes merged. The 2017 Nobel Prize in Physics was awarded for this discovery. In the following years, LIGO has discovered dozens of gravitational wave events, providing a new window to the Universe.

In 2017, LIGO detected gravitational waves from the merger of neutron stars for the first time. Unlike black holes, merging neutron stars were expected to also produce a flash of light. Our 1M2H team searched for and discovered the first optical counterpart of a gravitational wave source, SSS17a, with the Swope Telescope at Las Campanas Observatory. This result was the start of a new scientific field, multi-messenger astronomy, and was considered the 2017 Science Breakthrough of the Year.

With our observations of SSS17a, our team and others were able to determine it was a kilonova, the theorized outcome of a neutron star merger. It produced a large amount of heavy, r-process material, suggesting that perhaps all of the heavy elements in the Milky Way were made in neutron star mergers. This watershed moment created huge advances in our knowledge of cosmology to nuclear physics.

Through the 1M2H collaboration, we will search for and analyze new gravitational wave counterparts.

Type Ia supernovae are basically thermonuclear bombs caused by a runaway nuclear reaction on a white dwarf. Although we use Type Ia supernovae as one of primary cosmological probes, we still do not know basic things about the star systems that create them or the details of the explosions. To be fully confident in our cosmological measurements, we must know these details.

While we are confident that Type Ia supernovae come from white dwarf stars in a binary system, we do not know what kind of star the companion is. It could be another white dwarf, a star similar to our Sun, or a red giant star. Some of our recent observations indicate that Type Ia supernovae could come from all of these possibilities. If confirmed, determining the fraction from each channel now – and when the Universe was younger – will be critical to making precise dark energy measurements.

Similarly, there could be a diversity of how the star explodes. Does the explosion start deep in the star or at the surface? Does it happen while the second star is being tidally disrupted? Is the explosion subsonic or supersonic?

With new, earlier discoveries and more intense, detailed observations, we are quickly making progress. Our imaging with the Swope Supernova Survey and the Young Supernova Experiment, as well as spectroscopy from the Lick and Keck Observatories is producing one of the largest samples of Type Ia supernovae ever.

For the past century, systematic SN searches have discovered thousands of SNe. Nearly all of these SNe fall into 3 classes: Type Ia, Type II, and Type Ib/c. However, over the last decade with the implementation of large SN searches, we have begun to discover many astrophysical transients that do not fall into the well-delineated classes mentioned above. It turns out that there were about a dozen new classes of "exotic" or "peculiar" transients lurking in the shadows. These classes include .Ia SNe, Ca-rich SNe, fallback SNe, kilonovae, luminous red novae, luminous SNe IIn, pair-instability SNe, SN impostors, SNe Iax, SN 2006bt-like SNe, and tidal disruption flares. These newly identified classes have very diverse observational and physical properties: some are 1/100 as bright as typical SNe with durations of a few days instead of weeks, while others are 100 times brighter than the average SN and last for years. Some of these classes only have a handful of events detected, but because of improving surveys, culminating with the Vera C. Rubin Observatory, we expect those classes to have hundreds or thousands of members in less than a decade.

Exotic transients are not just an idle curiosity. These events are the result of the most extreme physical conditions imaginable; they come from the most massive stars, or the weakest explosions, or trigger the creation of neutron stars with the largest magnetic fields, or are the result of truly unexplored phenomena, etc. Quite simply, exotic transients explore new endpoints of stellar evolution and the boundaries of possible physical conditions of stellar systems. Meanwhile, the explosions probe interesting corners of physics such as r-process element creation and the first stars.

Beyond their intrinsically interesting qualities, exotic transients are also incredibly useful for understanding more normal stellar explosions. From this, exotic transients can help us understand varied topics such as neutron stars, black holes, heavy element abundance, and dark energy.

We are continually discovering new classes with the Young Supernova Experiment and following these rare events with the Swope Supernova Survey, and with the Lick and Keck Observatories.

The Young Supernova Experiment is a novel transient survey using the Pan-STARRS telescopes to discover and follow more than 10,000 transients over the next few years. It is the only 4-band transient survey currently running, and our red bands are unique among transient surveys. We combine the Pan-STARRS data with other public data to have images of our transients every ~1.5 days. With these data, we will detect supernovae right after explosion, discover new classes of transients, and produce the largest sample of cosmologically useful Type Ia supernovae yet.

The Young Supernova Survey is also the best current analog to the Rubin Observatory's Legacy Survey of Space and Time (LSST). Through newly developed machine learning techniques, we will help with planning LSST and improve its utility. Learn more here.

The Roman Space Telescope is NASA's flagship mission for the 2020's. Its goal is to solve basic problems about dark energy and exoplanets through several dedicated surveys, including a supernova survey. Prof. Foley is in charge of a Science Investigation Team to prepare for this survey and make recommendations to NASA. The proposed supernova survey will run for two years taking up six months of telescope time. It will discover more than 10,000 Type Ia supernovae and performed the most detailed study of dark energy yet.

Our One Meter, Two Hemispheres (1M2H) team discovered the first optical counterpart to a gravitational wave source. This collaboration uses several smaller telescopes to search for counterparts and follows them with larger telescopes including Keck, Magellan, and Gemini. We are currently analyzing data from LIGO's third observing run and preparing for the fourth run, after LIGO has its instruments upgraded. Learn more here.

We are using the one-meter Swope telescope at Las Campanas Observatory to follow hundreds of transients a year. This survey builds on the legacy of the Carnegie Supernova Project, using the same telescope and camera. We are building a high-fidelity sample of >400 nearby Type Ia supernovae and monitoring dozens of other exotic transients and interesting supernovae.