As with most physicists, my research is primarily motivated by questions of what our universe is made of and how it works, and I have been approaching these questions using astrophysics measurements. Our work with supernovae, which was intended to measure the deceleration of the universe’s expansion due to gravity (in other words, we were “weighing the universe”), turned out to see an acceleration. This unexpected result suggests that most of the universe may be primarily (~75%) made of a previously unknown energy -- now called “dark energy” -- that is accelerating the expansion.
This dark energy is a new mystery, raising many new questions: What is the physics behind it? Is it vacuum energy? Does it behave as Einstein’s cosmological constant or has it been evolving with redshift?
I think we have an excellent chance to make progress on answering these fundamental questions in the next few years. We are aggressively extending our research with SNe in several programs that range from the nearby universe to the most distant observable SNe. In addition, we are also exploring complementary astrophysical techniques. For example, weak lensing, a technique that is rapidly maturing, allows us to measure the growth of structure in the universe, and infer properties of dark energy (as well as dark matter). Another direction we are pursuing is baryon acoustic oscillations, which lets us tie the perturbations of the CMB to the low redshift universe.
It is too soon to know what these methods will reveal about dark energy. It might turn out to be something completely unexpected – and this would be even more exciting.
The Supernova Cosmology Project
The Supernova Cosmology Project (SCP) studies type Ia supernovae at high redshift to map out the expansion history of the universe and measure cosmological parameters and the dark energy’s equation of state. For example, our most recent survey is a very large observing program using the Hubble Space Telescope (HST) to discover and follow SNe in high redshift (z > 1) galaxy clusters. The SNe are discovered in the HST data, and we study their spectra using the largest ground-based observatories (Keck, VLT, Subaru) to determine the supernova type and redshift. Study of SNe in different galactic environments (e.g. in elliptical galaxies in clusters compared to spiral galaxies) can give us a handle on different systematic effects, such as extinction by dust. Another very active program that we are working on is the French-Canadian SuperNova Legacy Survey (SNLS), a large statistics study of intermediate redshift SNe discovered with the CFHT observatory on Mauna Kea. For further study of these supernovae, we have a large spectroscopic program in progress using the Keck and Gemini Telescopes.
The Nearby Supernova Factory
The Nearby Supernova Factory (SNfactory) is an ambitious project that aims to discover and obtain lightcurve spectrophotometry for a large sample of Type Ia supernovae in relatively nearby supernovae (but just far enough into the Hubble flow that their redshifts directly tells us their relative distances). This is a key distance range for the cosmological measurements, which has up to now been very sparsely studied. This project has two scientific goals. The first is for cosmology: we will use this large, well-calibrated sample of Type Ia supernovae to significantly improve the low-redshift portion of the redshift-luminosity diagram. This will provide a crucial improvement in the statistical accuracy attainable by current high-redshift supernova cosmology programs such as SNLS (see above) and the future programs such as SNAP (see below). The second goal is use this rich dataset to explore the features of type Ia supernovae in detail to search for parameters that can further improve the capabilities of SNe Ia as cosmological measurement tools. The SNFactory is now in operation, continually adding SNe discoveries and their detailed spectra to our dataset.
SNAP (SuperNova/Acceleration Probe) Satellite Project
Our most ambitious project is SNAP, a proposed new space telescope designed specifically to obtain the exquisitely precise measurements needed to study the properties of dark energy. We are designing SNAP as a ~2-meter telescope with a field of view hundreds of times larger than that of the current Hubble Space Telescope. This project involved extensive instrumentation development. For example, it will have a unique large-format camera with CCD detectors that we have developed in our labs to have excellent response over a very wide wavelength range, and resistance to radiation damage due to cosmic rays in space. It will provide measurements with unprecedented accuracy of thousands of SNe out to very high redshift. In addition, the wide field of view and multiple filters will make it the instrument of choice for large weak-lensing surveys. It is expected to yield the extremely sensitive measurements required to look for a possible evolution of dark energy with redshift.
BOSS (Baryon Oscillation Spectroscopic Survey) at the SDSS Telescope
Working with my colleagues at LBNL, our group members are also involved in the newest approach to dark energy measurements, Baryon Acoustic Oscillations (BAO). By studying the clustering of nearby galaxies we can detect the imprint of the sound waves that were “frozen in” as the cosmic plasma of the early Universe cooled – the same acoustic phenomena that produced the anisotropies we see in the cosmic microwave background (CMB). Moreover, by comparing the structure in the CMB with that seen in the distribution of galaxies in the nearer universe, we obtain a measurement of the properties of dark energy that is independent of the one made using supernovae. Our LBNL groups are developing the Baryon Oscillation Spectroscopic Survey (BOSS), a dramatic step forward in BAO dark energy measurements. The survey is designed to obtain spectra of 1.5 million galaxies at z<0.7 and 160,000 quasars for the BAO signal at much higher redshifts. On the instrumentation side, we are upgrading the spectrographs with our red-sensitive CCDs and, on the theory side, we are working on large scale numerical simulations of the distribution of matter (and galaxies).
The Supernova Legacy Survey: ΩM, ΩΛ, and w from the First Year Data Set, P. Astier, et al., Astronomy and Astrophysics, 447,31 (2006)
New Constraints on ΩM, ΩΛ, and w from an Independent Set of Eleven High-Redshift Supernovae Observed with HST, R. A. Knop et al, (The Supernova Cosmology Project), Astrophysical Journal, 598,102, (2003)
Measuring Cosmology with Supernovae, Saul Perlmutter and Brian P. Schmidt, Supernovae & Gamma Ray Bursts, K. Weiler, Ed., Springer, Lecture Notes in Physics, astro-ph/0303428.
Supernovae, Dark Energy, and the Accelerating Universe, S. Perlmutter, Physics Today, April 2003
The Hubble Diagram of Type Ia Supernovae as a Function of Host Galaxy Morphology, M. Sullivan, et al., (The Supernova Cosmology Project), Monthly Notices of the Royal Astronomical Society, 340, 1057 (2003)
Overview of the SuperNova/Acceleration Probe (SNAP), G. Aldering, et al., (The SNAP Collaboration), in Future Research Direction and Visions for Astronomy. A. M. Dressler, ed., Proceedings of the SPIE, Vol. 4835, 146 (2002)
The distant Type Ia supernova rate, R. Pain, et al., (The Supernova Cosmology Project), Astrophysical Journal, 577, 120 (2002).
Measurements of the cosmological parameters ƒ¶ and Λ from 42 high-redshift supernovae. S. Perlmutter et al. (The Supernova Cosmology Project), Astrophysical Journal, 517, 565 (1999).
Constraining dark energy with SN Ia and large-scale structure. S. Perlmutter, M.S. Turner, and M. White, Physical Review Letters, 83, 670 (1999)
The Cosmic Triangle: Revealing the state of the universe. N.A. Bahcall, J.P. Ostriker, S. Perlmutter, and P.J. Steinhardt, Science, 284, 1481, (1999)
Discovery of a supernova explosion at half the age of the universe and its cosmological implications. S. Perlmutter et al. (The Supernova Cosmology Project), Nature, 391, 51 (1998).
A Type Ia supernova at z = 0.457. S. Perlmutter et al. (The Supernova Cosmology Project), Astrophysical Journal Letters, 440, L41 (1995).