We have come far in black hole research, but there is much still to discover. We know that there are “stellar-mass” black holes, formed when a star collapses at the end of its life, and “supermassive” ones found in the cores of galaxies. But, is there anything in between? Do intermediate-mass, or “medium-sized” black holes exist? Did you know that while black holes suck everything in, they also power some of the most luminous objects in the universe and may have helped make the universe transparent? Dr. Philip Kaaret, Professor of Physics and Astronomy at the University of Iowa, addresses fundamental questions about black holes focusing on the search for medium-sized black holes that exist and also studying what effect the first black holes formed from the first stars had on the early universe. Building instrumentation for the X-ray and gamma-ray bands and conducting observations of black holes at X-ray, gamma-ray, optical, and radio wavelengths, he is advancing astronomical research on all fronts and shedding light on important principles of the universe.

Collaborating with researchers at NASA’s Goddard Space Flight Center in Greenbelt, MD, and with researchers at the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, Dr. Kaaret’s robust team of undergraduate and graduate students is a world leader in the search for medium-sized black holes and one of only a few groups in the world studying the impact of stellar-mass black holes on the early universe. In order to examine some of the most exotic and mind-bending objects known, Dr. Kaaret is building a CubeSat-based X-ray telescope. This compact and low-cost observatory would have a major impact on the field and could energize a whole new generation of low-cost space-based observatories. Making frequent progress on the black hole research almost every year, Dr. Kaaret and his team hope to broaden our understanding of the universe and the world in which we are placed.

Current research projects include:

• Building CubeSat-based X-ray observatories: Dr. Kaaret is working towards building an X-ray telescope to measure the polarization of X-rays. X-rays do not penetrate Earth's atmosphere, so X-ray telescopes are necessarily flown on satellites. The high costs of building new satellites has prevented the launch of an X-ray polarimeter since the 1970s. New technologies for X-ray detection and the advent of CubeSats, tiny satellites the size of a loaf of bread enabled by microelectronics, have made possible a new generation of low-cost X-ray observatories. Due to their affordability and mass availability, the CubeSats also help engage and train students better. Dr. Kaaret is developing a CubeSat-based astronomical observatory to provide the first X-ray polarization measurements, which could energize a whole new generation of low-cost space-based observatories.
• Looking for medium-sized black holes: A prime focus of Dr. Kaaret's research has been the search for intermediate-mass or "medium-sized" black holes. Weighing more than the most massive stars, but less than the 100,000 solar masses of the smallest supermassive black holes, the medium-sized black holes have been suggested as being an important factor in forming larger, supermassive black holes. Discovery of a medium-sized black hole would also add a completely new class of objects to the cosmic zoo. Dr. Kaaret uses X-ray observations made at an international suite of observatories to seek out new medium-sized black hole candidates and understand their properties.
• Black holes in the early universe: It is difficult to study the galaxies formed in the early universe because they are so far away, but there are galaxies relatively close to us that have properties similar to those of the early galaxies; the key property is the fraction of atoms heavier than hydrogen and helium, what astronomers call “metallicity”. Galaxies with low metallicity, similar to early galaxies, contain an unusual abundance of black hole binaries -- Dr. Kaaret's group has recently shown that there are almost 10 times as much X-ray emission coming from binaries in low-metallicity galaxies as compared with “normal” galaxies," such as our own home, the Milky Way. This discovery impacts our understanding of the first galaxies and how the universe made the transition from being an opaque cloud of gas to being transparent.