Ph.D., 1983 - Massachusetts Institute of Technology
Louisiana State University
Department of Physics & Astronomy
243-A Nicholson Hall, Tower Dr.
Baton Rouge, LA 70803-4001
I have a wide range of interests throughout astrophysics, including Gamma-Ray Bursts, supernovae, supernova remnants, supernova progenitors, historical records of supernovae, novae, recurrent novae, superflare stars, low mass X-ray binaries, eclipsing binaries, Nereid, Pluto, Kuiper Belt Objects, sunspot counts, the visibility of objects in the sky (especially lunar crescents and heliacal rises), astronomical effects on history, the accuracy of the press in reporting astronomy, the origin of the Greek and Chinese constellations, archaeoastronomy, astronomical events in history (e.g., the Crucifixion and the Star of Bethlehem), and astronomical events in literature (especially in The Hobbit and in the Sherlock Holmes canon).
A primary thrust of research is to use photometry of exploding objects to get results of interest for cosmology:
For supernovae, as part of the Supernova Cosmology Project, a Hubble Diagram was created with high accuracy out to a red shift of ~1.0. This demonstrated that the Cosmological Constant is non-zero and causes our Universe's expansion to accelerate. This is the discovery of what is now called 'Dark Energy'. For this work, I was awarded a share of the $500,000 Gruber Prize for Cosmology in 2007, and awarded a share of the $3,000,000 Breakthrough Prize in Fundamental Physics in 2015. The 2011 Nobel Prize in Physics was awarded to Saul Perlmutter for his original work, inspiration, and leadership for the Supernova Cosmology Project.
We have work that has been getting answers to the notorious and highly-important Type Ia supernova progenitor problem. For example, in a recent Nature paper with graduate student Ashley Pagnotta, the lack of any possible ex-companion star in the center of SNR 0509-67.5 in the Large Magellanic Cloud proves that this Type Ia supernova must have come from a double-degenerate progenitor. For this one 400±50 year old Type Ia explosion, it is very confident that all published single-degenerate models are greatly rejected, because any remaining ex-companion star must be less luminous than MV=+8.4. With graduate students Zachary Edwards and Ashley Pagnotta, less-strong limits have been applied to three other supernova remnants in the Large Magellanic Cloud. In a further innovative study, with graduate student Zachary Edwards, we have measured the 'vertical' distribution of Type Ia and Core Collapse supernovae as viewed in exactly edge-on spiral galaxies. The Core Collapse supernovae are all closely concentrated to the galactic plane (although avoiding the exact plane due to the usual dust lanes), proving that their progenitors are a very young population. The Type Ia supernovae have a very large scale height, proving that their progenitors are a very old population, mostly older than ~2 billion years. This simple observation can be compared to the predictions for single-degenerate models giving an age distribution younger than a fraction of a billion years, and the predictions for a double-degenerate population giving an age distribution older than a billion years, with the strong conclusion that most Type Ia supernovae in spirals are from double-degenerate progenitors. With these two lines of evidence, we make a strong case that Type Ia supernovae are effectively all from double-degenerate progenitors (i.e., from the in-spiral of a close-in white-dwarf-plus-white-dwarf binary).
For Recurrent Novae, the question is whether they are the progenitors of Type Ia supernovae, as the knowledge of the progenitor type is required for any evolution calculation such as is needed for the future precision cosmology with supernovae. To answer the progenitor question, extensive work is being pursued to get Recurrent Novae orbital periods, accretion rates, outburst dates, eruption light curves, and the average magnitudes between outbursts. A centerpiece of this work is the timing of eclipses from four systems (U Sco, T Pyx, T CrB, and CI Aql) since 1855 to measure the orbital period changes across the latest eruptions (in 2010, 2011, 1946, and 2000 respectively) so as to get the first measures of the ejected mass, with the result in all four cases that the white dwarfs are ejecting much more mass than they had accreted in the previous cycle, so the white dwarfs are losing mass and recurrent novae are not supernova progenitors. The eruption of U Sco was predicted and occurred in 2010, with a large international collaboration (collecting data from 8 satellites and 30 telescopes) making this eruption the all-time best observed nova event. In 2011, the eruption of T Pyx was observed with ~100,000 magnitudes in the light curve. Further killer arguments against recurrent novae as supernova progenitors include the fact that most of the nova eruptions are very neon rich, and that the many close searches by many other researchers have always severely excluded the possibility of any red giant or sub-giant companion star. From these three convincing paths, we can be sure that recurrent novae (i.e., the most popular single-degenerate path to Type Ia supernova) certainly can contribute less than ~1% of all observed events. That is, recurrent novae are not progenitors.
For Gamma-Ray Bursts, the big advance has been the realization that the bursts are standard candles (like Type Ia Supernovae and Cepheids). With this, GRBs become tools for cosmology that can be seen out to red shifts from ~0.2 to around >8. This allows GRBs to be used to create a Hubble Diagram from 0.2<z<7, and this can independently prove that ΩM=0.39 (+0.12, -0.08) from the GRBs alone. Well, by the time I could make this independent derivation, the Supernova Cosmology Project, the cosmic microwave background work, and the galaxy's baryonic acoustic oscillations work had already demonstrated the existence of something like what we call the Cosmological Constant, so this GRB result serves merely as independent confirmation. But GRBs go out to greatly higher redshifts than do supernovae, so GRBs uniquely have the ability to record the changing equation of state of the Universe. As an example of this, my GRB Hubble Diagram conclusively rules out the models for 'Weyl gravity' and for a 'Chaplygin gas' equation of state. What I am finding is that the GRB Hubble Diagram is consistent with the concordance model and also with an unchanging Cosmological Constant.
For my work in the history of astronomy, I have often served as a technical calculator of celestial visibility and its myriad applications to old history:
One set of analyses relates to the visibility of the stars/planets/Moon low in the sky, often during evening twilight or the dawn sky. This is actually a complex problem needing fundamental physics input for the scattering and refraction of light in the atmosphere and the probability of detection by human eyes for a source viewed against a smooth background. In all cases, my style is always to collect large amounts of ground-truth data and compare it against my new best theory. The simplest case is just for knowing the altitude above the horizon (called the extinction angle) at which a rising star first becomes visible. With the results from this study, I could easily refute an influential set of claims for archaeoastronomical alignments (the megalithic lunar observatory claims by Thom) and refute several claims for how the Great Pyramid was aligned. The next harder case is that of 'heliacal rising', which is a star's first visibility on one day as it comes out from behind the Sun. Many ancient calendars are based on the heliacal rising of stars. My observations reported in the paper are the first observations published since c. 300 BC, and my theory is the first advance since 150 AD. My great leaps forward are simply because I am the first astrophysicist who has addressed these antique questions with the full tools of modern science. Applications of my heliacal rise model include for the Star of Bethlehem, the Egyptian Sothic cycle, and the peak magnitudes of ancient supernovae. The next step up is to determine the visibility of the thin crescent Moon, low in the western sky soon after sunset. This visibility defines the first day of the lunar month in most lunar and luni-solar calendars, and this is applicable to most cultures worldwide. I have spent about one man-year on this problem. Part of this is collecting large numbers of positive and negative sightings of the young crescent Moon. From this effort, I have a complex algorithm that predicts the date of first visibility for each lunar month for any site in the world for any year from 600 BC to far into the future. This explicitly accounts for the variations over time of the haziness of the atmosphere (the extinction coefficient). A comparison of my model versus the full data collection shows that my model is 3X better than any prior model or criterion. Again, this is not surprising, simply because I am the first astrophysicist to treat the entire problem as a physics and astronomy task with modern methods. My modern visibility algorithm has a strong application in dating the Crucifixion.
A frequently-used tool is 'precessional dating', where ancient reports on the positions of stars can be translated into both a year and a latitude for whoever made those observations. What is going on is that precession very slowly moves the stars around the sky (relative to the equinoxes), so that the stars form something like a giant clock, where each star operates as an hour-hand extending from the north pole reading out the time. The ancient observers are merely telling us the position of the stars, so we can work out the reported time. With the many sizeable data sets, the year can be determined to within a long-lifetime and the latitude can be determined to within ~50 miles. Here are my results: (1) The origin of the Chinese lunar lodge system (the hsiu) was in the year 3200±500 BC. (2) The origin of the Indian lunar lodge system (the nakshatra) was in the year 1700±500 BC. (3) The origin of the Arabic lunar lodge system (the manazil) was in 200±400 BC. (4) The origin of the 'Imperial' Chinese constellations, those described in the 'anciently' list of the Kai Yuan Zhan Jing, is 300±200 BC. (5) The star lore in the much-copied Babylonian tablet series called MUL.APIN dates back to 1370±100 BC with a latitude of 35.1°±1.2°, which is to say that the Mesopotamian constellation lore goes back to the time and place of the Assyrian cultural flowering, just when the archaeologists are returning the first evidence of their constellations. (6) The southern Greek constellations were invented in the year 690±360 BC from a latitude of 33°±2°, which is to say that they originally came from Mesopotamia. (7) The astronomical observations reported in Eudoxus' best-selling book The Phaenomena date back to 1130±80 BC, with a latitude of 36.0°±0.9°, which is to say that Eudoxus was merely repeating work passed down to him over 800 years from some observer in Mesopotamia. (8) Hipparchus' only surviving book is a lesser work called The Commentary, which describes many star positions, presumably from his famous star catalog. From detailed analysis of all his reported constraints, I derive an epoch of 136±8 BC, a latitude of 36.31°±0.23°, an adopted obliquity of 23.8°±0.2°, and a total 1-sigma error bar of 1/3° for individual reported star positions. (9) The marble statue called the "Farnese Atlas" in Naples depicts the full set of Greek constellations, with the positions for the stars in 125±55 BC, with the only realistic candidate for the source of the positions is the great astronomer Hipparchus, who famously has a star catalog from that time. (10) In Ptolemy's Almagest, the southern stars inside the first three quadrants date to an epoch of 500±200 AD and a latitude of 32.0°±0.8°, and were certainly from Ptolemy (c. 150 BC) himself in Alexandria (31.2° north). The southern stars inside the fourth quadrant have an epoch of 600±500 BC and a latitude of 35.7°±1.1° and apparently were selected and positioned by Hipparchus (c. 130 BC) in Rhodes (36.4° north).
I have a wide variety of overview articles on many topics (not related to celestial visibility). I am most proud of a long series of 19 feature articles in the magazine Sky & Telescope with the common theme of showing how astronomical events in the sky relate to real people on the ground, both famous figures and common folk. For the big international quadrennial Oxford Conference series (Oxford VII) in June 2004, I presented the Keynote address titled "Case Studies of Three of the Most Famous Claimed Asrchaeoastronomical Alignments in North America", where I was chastising many in the archaeoastronomy community for poor methodology, and emphasizing the need to present evidence that proves the intention by the original builders of any claimed alignment.
I have a distinctive style of combining history and astrophysics, where I use old or very-old data to critically answer modern front-line science questions. For example, I have used Tycho Brahe's original astrometric measures of the position of the 1572 supernova to solve the long-running big-time controversy as to whether the so-called 'Star G' is the ex-companion star of the exploding system, with my answer being 'no', thus breaking one of the few good arguments for the single-degenerate progenitor model. I have used detailed and exhaustive textual analysis of the ancient Chinese records of a transient event from 186 AD to prove that this was not a supernova, but rather was the known appearance of a periodic comet. I have used the photometric reports plus the heliacal rise/set dates (with my modern algorithm) to derive the peak magnitudes of the Type Ia supernovae of the years 1006, 1572, and 1604 to get peak magnitudes and then a value for the Hubble Constant. In an now-nearly-unique methodology, I have recovered vast amounts of data from dusty archives to collect light curves of recurrent novae, going far back in time. The most extreme example, is for the canonical recurrent nova T CrB, I have recovered >100,000 magnitudes from 1829 to present in both B and V colors, measuring the unique and weird pre-eruptions and post-eruption events that are identical across the 1866 and 1946 eruptions. Part of this means that I am predicting a third eruption in the year 2022. Most critically, I have measured the orbital period change across the eruption, with this requiring a tremendous amount of mass ejection, such that T CrB cannot become a Type Ia supernova. A frequently-used data source is the Harvard collection of archival sky photographs going back to 1889, where for many stars I can recover 1000-3000 magnitudes spread over 1889-1953 plus 1969-1989, so as to provide a near-unique record of the star's history. For a recent example, I have looked at the very-mysterious star KIC 8462852 (also called Boyajian's star after the LSU professor Tabetha Boyajian) to find that it is fading by ~19% from the early 1890s to the late 1980s. This provides a proof that the weird behavior of the star is not due to comet swarms, alien-mega-structures, or any of various other proposed ideas.
Nobel Prize in Physics 2011 (Prize went to team leader of the Supernova Cosmology Project, Saul Perlmutter, for this paper reporting discovery of Dark Energy) AND
Gruber Prize for Cosmology 2007 ($500,0000 shared by all Supernova Cosmology Project for discovery of Dark Energy).
“Measurements of Omega and Lambda from 42 High-Redshift Supernovae,” S. Perlmutter et al., Astrophysical Journal 517, 565 (1999) -Electronic Version