Submm Interferometry | THz Astronomy
Dark Matter Detection
I collaborate with the Dark Matter Time Projection Chamber group at MIT to build detectors that are sensitive to the direction of arrival of dark matter. Dark matter searches are challenging because the expected event rates are tiny and are overwhelmed by background events that can masquerade as dark matter. Directional detectors provide a new handle on dark matter by looking for the signature of the motion of the Earth through the dark matter halo of our Galaxy. This produces a distinct signature in the arrival direction of dark matter.
The DMTPC detectors use diffuse-gas targets to search for this signature. If a dark matter particle strikes a target nucleus, that nucleus would recoil through the detector, typically traveling 1-3 millimeters. As it recoils, the nucleus ionizes the surrounding gas and produces a charge signal and scintillation light. This track length is long enough to be imaged by CCD cameras and therefore the direction of arrival of the dark matter can be inferred.
For more on directional dark matter detection and DMTPC, see:
Ahlen, S. et al. The Case for a Dark Matter Detector and the Status of Current Experimental Efforts, Int. J. Mod. Phys. A. 25, 1, 2010. arXiv:0911.0323
Lunar Laser Ranging
I have been on the APOLLO (Apache Point Observatory Lunar Laser-ranging Operation, PI: Prof. Tom Murphy, UCSD) team since 2004. We measure the Earth-Moon separation with millimeter precision by timing the round trip travel of pulsed laser light between a telescope in New Mexico and reflectors on the lunar surface. With these observations we test fundamental physical laws and principles such as gravity, Lorentz invariance and the existence of new fundamental forces.
For more on APOLLO, see:
Murphy, T.W., Adelberger,E.G., Battat, J.B.R., Carey, L.N., Hoyle, C.D., LeBlanc, P., Michelsen, E.L., Nordtvedt, K., Orin, A.E., Strasburg, J.D., Stubbs, C.W., Swanson, H.E. & Williams, E., APOLLO: the Apache Point Observatory Lunar Laser-ranging Operation: Instrument Description and First Detections, Publications of the Astronomy Society of the Pacific, accepted, 2008. [arXiv:0710.0890]
Lunar Laser Ranging (LLR) is a comprehensive probe of gravity. With it we are sensitive to violations of the equivalence principle, deviations from 1/r potentials, relativistic effects such as de Sitter and Lens-Thirring precession, and even time variability of the gravitational constant, G. With millimeter-precision measurements of the Earth-Moon range, APOLLO will improve these constraints by an order of magnitude.
Battat, J.B.R., Stubbs, C.W., Chandler, J.F. Solar System Constraints on the Dvali-Gabadadze-Porrati Braneworld Theory of Gravity, Phys. Rev. D, 78, 022003, 2008. [arXiv:0805.4466]
In 2006, Bailey & Kostelecky worked out the expected perturbations to the Earth-Moon range under a violation of Lorentz symmetry in the gravitational sector of the Standard Model Extension (SME). Using archival LLR observations with a typical range precision of a few centimeters, I (with John Chandler and Christopher Stubbs) placed the first observational constraints on a set of six SME parameters that govern Lorentz invariance.
Battat, J.B.R., Chandler, J.F., Stubbs, C.W., Testing for Lorentz Violation: Constraints on Standard-Model Extension Parameters via Lunar Laser Ranging, Phys. Rev. Lett., 99, 241103, 2007. [arXiv:0710.0702]
Bailey, Q. & Kostelecky, A., Signals for Lorentz Violation in Post-Newtonian Gravity, Phys. Rev. D 74 045001, 2006. [gr-qc/0603030]
The orbit of a test body around a central mass will be elliptical and closed (i.e. repeating the same path every orbit) if the interaction potential is inversely proportional to the separation between the bodies (as in Newtonian gravity). In other words, the 1/r potential of Newtonian gravity gives rise to Kepler's Laws. A deviation from a 1/r potential can lead to orbital precession. In fact, one of the great confirmations of Einstein's theory of General Relativity was that it could explain the excess precession of 43 arcsecond per century seen in Mercury's orbit (in the slow-motion, weak-field limit of GR, there is a 1/r3 term in the GR potential). There are many potential sources of non-1/r potentials: Yukawa interactions, braneworld gravity, etc. Through precision measurements of the precession of planets and the Moon, we can constrain these physical models.
Cosmology with Galaxy Clusters
PISCO: a photo-z machine
Galaxy clusters are the largest gravitationally bound objects. Tracking their number density and evolution with cosmic time teaches us about the expansion history of the Universe and can help us understand the nature of the dark energy. The South Pole Telescope will discover thousands of new clusters through the interaction of hot intracluster gas with the cosmic microwave background radiation (the Sunyaev-Zel'dovich effect). In order to extract cosmological constraints from the SPT data, the cluster redshifts must be independently determined. With Professor Christopher Stubbs, Antony Stark and others at Harvard, I am helping to develop a simultaneous multiband imager to determine the photometric redshifts of the SPT clusters. This instrument, called PISCO, the Parallel Imager for Southern Cosmological Observations, will be installed on one of the Magellan telescopes in Chile.
Atmospheric phase correction
Interferometry enables high resolution astronomical imaging by synthesizing a narrow beam with small telescopes. The Submillimeter Array (SMA) on Mauna Kea in Hawaii is capable of interferometric observations between 230 GHz and 690 GHz (wavelengths between 1.3 and 0.4 millimeters). Observations on long baselines and high frequencies can be limited by atmospheric turbulence. I designed and built a system to measure the atmospheric water vapor content along the line of sight of each telescope to correct the phase fluctuations introduced by the atmosphere. In essence, this is an adaptive optics system for submillimeter-wave interferometry. Through this work, our group developed techniques to stabilize heterodyne receivers to a few parts in 10,000.
Battat, J. B., Blundell, R, Hunter, T, Kimberk, R. Leiker, P. Tong, C-Y. E., Gain Stabilization of a Submillimeter SIS Heterodyne Reciever, IEEE Trans. on Microwave Theory and Techniques, 53, 2005. [arXiv:0710.0694]
Battat, J.B., Blundell, R., Moran, J.M. & Paine, S., Atmospheric Phase Correction Using Total Power Radiometry at the Submillimeter Array, Astrophysical Journal, 616, 1, 2004. [astro-ph/0407142]
From the ground?
Yes, there are a few places on Earth where the atmospheric transmission at 1 Terahertz is good enough for astronomy. One such region is the high, dry and mountainous Atacama Desert in Chile. I spent quite a bit of time at 18,000 feet (5,525 m) on Cerro Sairecabur (near the border of Chile, Argentina and Bolivia) observing star forming regions in the Orion Molecular Clouds using the RLT Telescope developed by the SMA receiver lab. Because of the small optical depth to high-level molecular rotational transitions (e.g. CO 9 -> 8 at 1.037 THz), our observations penetrated deep into the dense gas regions around new stars. Much deeper than millimeter astronomy could do. For example, these observations constrained the temperatures and densities of the OMC-1 region.
Marrone, D.P., Battat, J., Bensch, F., Blundell, R., Diaz, M., Gibson, H., Hunter, T., Meledin, D., Paine, S., Papa, D.C., Radford, S.J.E., Smith, M and Tong, E., A map of OMC-1 in CO 9-8, Astrophysical Journal 612, 2004. [astro-ph/0405530]