I am interested in answering some of the most fundamental questions about the universe we live in through precise observations and laboratory measurements. I have recently been involved in projects that explore the nature of dark matter and dark energy as well as the relationship between gravity and quantum mechanics. In order to understand what our measurements mean, we must understand the shortcomings of our measurement procedures and analysis techniques. Consequently, my work focuses not only on observation and experiment, but also on how the experimental or observational procedure itself affects interpretation of the measurements.
Fiber-fed, multi-object echelle spectrographs are powerful instruments that are notoriously difficult to calibrate. Useful for studying clusters of stars and characterizing the properties of hundreds of target objects at a time, multi-object echelle instruments are becoming workhorses at large telescopes around the world. The Hectochelle spectrograph at the MMT is a good example. Two-hundred and forty fibers attached with magnets to the telescope's focal plane capture light collected by the telescope's 6.5-meter primary mirror. The fibers pipe starlight onto a giant echelle grating, which disperses it with a resolving power of ~40,000. Order-separating filters select which portion of the spectrum is imaged on the CCD. The difficulty in wavelength calibration results from the need to calibrate all 240 fibers simultaneously in a reasonable period of time with calibration light following the same optical path as starlight. Light reflected from a diffusive dome screen follows the approximate path of starlight incident on the telescope, but the dome screen attenuates calibration light by thirteen orders of magnitude. The ThAr hollow-cathode lamps typically used to calibrate high-resolution spectra are too faint for use with a dome screen. Instead, ThAr lamps shine directly onto the focal plane.
With the support of CfA instrument scientist Andy Szentgyorgyi and the Optical Technology Group at NIST, I am developing a wavelength calibrator for Hectochelle based on narrowband tunable lasers. The basic idea is to direct monochromatic laser light onto the dome screen, and tune the laser wavelength in regular steps across the spectral region spanned by an order-separating filter while leaving the camera shutter open. In esssence, we paint a regular comb of calibration lines onto the CCD, one line at a time. During each calibration scan, we record the laser wavelengths using a commercial wavemeter with a precision of <0.2 ppm. We have established tunable laser wavelength calibrations in six of Hectochelle's eleven filters. In each filter, the laser calibration yields more precise wavelength solutions than the ThAr lamp spectra. The laser calibrations are also more consistent when fibers are moved to different positions on the focal plane, and when spectra recorded in different fibers are compared to one another.
Tunable laser calibration is expected to improve the precision of Hectochelle to match its estimated photon-limited Doppler precision of 50 m/s in favorable observing conditions. This will enable new scientific goals to be achieved. I am currently collaborating with CfA astronomers to make improved measurements of young star velocities (J. Foster), compare stellar age indicators (A. Dupree), and characterize the distribution of dark matter in globular clusters (A. Dupree, A. Szentgyorgyi).
A recent conference paper:"A Tunable Laser System for the Wavelength Calibration of Astronomical Spectrographs"
C.E. Cramer, et al., OSA Technical Digest, JThE85 (2009)
The smallest extrasolar planets found to date have been discovered using the radial velocity method. Precise stellar spectra recorded over many planetary orbits reveal periodic Doppler shifts associated with center-of-mass motion in the extrasolar planetary system. Planets only a few times more massive than Earth have been discovered, but orbiting stars much smaller than our Sun with periods measured in days rather than years. To detect a truly Earth-like planet using the radial velocity technique, the stellar spectrum must be wavelengh-calibrated with a light source far more precise and more stable than the ThAr hollow cathode lamps currently in use.
Femtosecond combs may provide the answer. At first glance, a laser frequency comb appears to be an ideal wavelength calibrator. Thousands of equally spaced lines, known with exquisite precision and referenced to a stable clock should bring wavelength calibration uncertainty well below the level required to find Earth-like planets around Sun-like stars. The challenge to using a laser comb with an astronomical spectrograph lies in matching the comb line spacing to the spectrograph's resolving power. To a typical high-resolution spectrograph, a typical comb laser looks like white light. An interdisciplinary group of scientists from MIT's Research Lab for Electronics, the Harvard-Smithsonian Center for Astrophysics, and the Smithsonian Astronomical Observatory has built an "astro-comb" suitable for use with the TRES spectrograph on Mt. Hopkins, AZ by filtering the comb laser output with a Fabry- Perot cavity. After observing runs spanning one year, we have demonstrated that the astro-comb is a viable calibration light source for radial velocity measurements below the m/s level.
Selected references:"A laser frequency comb that enables radial velocity measurements with a precision of 1 cm/s"
C.E. Cramer, et al., in Transiting Planets, Proc. of the IAU, 253, p.499-501 (2009)
"A laser frequency comb that enables radial velocity measurements with a precision of 1 cm/s"
C.-h. Li, et al., Nature, 452, p.610-612 (2008)
Some of the most compelling evidence for Dark Energy comes from observations of distant supernovae. The lifecycle and brightness of type Ia supernovae are remarkably consistent regardless of when and where the progenitor stars explode. Teams based at Harvard and at U.C. Berkeley have exploited this to learn about the expansion history of the universe. Measuring the a supernova's brightness gives its distance, and measuring the Doppler shift of its spectrum gives its velocity. The cosmological model that best matches the results of the supernova surveys suggests that the universe is 70% dark energy -- a mysterious and disturbing phenomenon that is causing the universe to fly apart increasingly faster.
To learn more about dark energy, the supernova teams need to make more precise measurements of type Ia supernovae, as the surveys are currently limited by systematics. Relative photometry at the 1% level is required to bring the uncertainty on cosmological parameters down to 1%. One necessary precondition for 1% photometry is a thorough calibration of the telescope and camera system used for precise sky surveys. At Harvard's Laboratory for Particle Physics and Cosmology, Chris Stubbs is prototyping novel flat-field screen designs that will meet the needs of next-generation surveys. Instead of using a traditional incandescent lamp light source, the new designs use a broadly tunable laser, allowing the spectral response of the telescope and detector to be measured as a function of wavelength. Further, the calibrations are tied to NIST laboratory standards through the NIST-calibrated photodiode used to monitor the tunable laser. The new calibration screens must also have a low profile, as the Pan-STARRS and LSST domes are a tight fit, and restrict the angular distribution of light from the screen to reduce errors due to stray and scattered light. During my time at the LPPC, I participated in testing a screen design that used side-emitting optical fibers to distribute tunable laser light, and set up a prototype that used diverging optics to create a large spot on a rear-projection screen and a DLP micromirror array to control the intensity profile of the spot.
An account of the supernova discovery of dark energy:"The Extravagant Universe: Exploding Stars, Dark Energy, and the Accelerating Cosmos"
Robert Kirshner, Princeton Science Press, 2004
For more detail on the calibration scheme, see:
"Toward 1% Photometry: End-to-End Calibration of Telescopes and Detectors"
C.W. Stubbs and J.L. Tonry, Ap. J., 646, p.1436-1444 (2006)
If Lorentz symmetry is broken (see below), then the universe is full of a dynamical fluid that may have significant cosmological consequences, according to Harvard theorists. The fluid is composed of the Nambu-Goldstone bosons resulting from the broken symmetry, which couple only derivatively and are therefore a condensate of ghost particles. This ghost ether is capable of driving the accelerated expansion rate of the universe, and can, under the right circumstances, mimic dark matter. Conveniently, the ghost ether is also expected to interact with ordinary matter, opening the possiblilty of detecting it in the laboratory. In particular, it couples particles with non-zero intrinsic spin together, so they feel an unique force that depends on the relative orientation of their spins and their velocity with respect to the ether.
At the Center for Experimental Nuclear Physics and Astrophysics at the University of Washington, we searched for the interaction between the ghosts and electrons using a sensitive torsion balance containing a large net spin dipole and spin sources placed outside the torsion balance apparatus. The spin sources were made from SmCo magnets and magnet iron, making use of the same premise as the spin pendulum pucks described below. We found no evidence for the ghost condensate, and our experiment places a stringent new upper limit on anomalous forces between electron spins. Our experiment also places limits on axion-like particles, other exotic particles and unparticles, and the role of spin in gravity. Atomic experiments should soon place comparable bounds on similar forces between proton and neutron spins.
Theory references:"Ghost Condensation and a Consistent Infrared Modification of Gravity"
N. Arkani-Hamed, et al., JHEP 0405 (2004) 074
"Universal Dynamics of Spontaneous Lorentz Violation and a New Spin-Dependent Inverse-Square Law Force"
N. Arkani-Hamed, et al., JHEP 0507 (2005) 029
Experimental references: coming soon . . .
Precision frequency standards, in addition to being useful, have traditionally led to wide-ranging scientific advancements. A good example is the surprising discovery that the earth's rotation period can vary by up to 4 microseconds on a daily basis. This led to a veritable revolution in geophysical theory when the the first atomic clocks were turned on close to fifty years ago. In intervening years, atomic clocks have been put on chips, become an integral part of GPS, been flown around in commercial airplanes to make the first direct tests of special relativity, and compared to one another to test the time-dependence of fundamental constants. It is this last application which provides the primary scientific motivation to strive for higher precision in frequency standards.
Narrow optical transitions are appealing candidates for precision frequency standards because they have an intrinsically high quality factor. They have an additional advantage for measurements of the time-variation of the fine structure constant because ratios of optical transition frequencies typically depend only on powers of the fine structure constant and not other parameters that might also be varying in time. Consequently, optical clock comparison tests would be a valuable contribution to the vast but difficult to interpret body of existing measurements. The Fortson group at the University of Washington has chosen to explore neutral Yb as a candidate for a new precision frequency standard.
The Yb atom is a convenient and rich system to work with. It has two cooling and trapping transitions within the reach of modern lasers, and a doubly forbidden transition suitable for a clock with mHz precision (see figure). The ultimate goal is to trap up to a million Yb atoms in an optical lattice, combining the Doppler-eliminating Lamb-Dicke confinement of ion traps with the large ensemble of atoms available from neutral atom trapping techniques. The optical lattice would operate at a "magic" wagelength at which the light shift of the ground and excited states of the clock transition are the same, leaving the transition frequency itself unaffected. This particular clock transition is weakly allowed in the five odd Yb isotopes, allowing us to make one of the first measurements of its frequency by direct laser excitation in the more abundant 171 and 173 isotopes. Although the same transition is inaccessible by an electric dipole transition in the even isotopes, a four-level EIT scheme would also allow an extremely precise frequency determination. During my time in the Fortson lab, I participated in the direct measurement of the clock transition and a calculation to explore the possibility of an Yb EIT clock.
An accessible reference about atomic clocks:"Splitting the second: the story of atomic time"
Tony Jones, IOP Pub., 2000
Measurements of the Yb clock transition:
"Observation of the 1S0-3P0 transition in atomic ytterbium for optical clocks and qubit arrays"
T. Hong, C. Cramer, E. Cook, W. Nagourney, E.N. Fortson, Optics Letters, 30, 19, p.2644-6
"Observation and absolute frequency measurements of the 1S0-3P0 optical clock transition in neutral ytterbium"
C.W. Hoyt et al., Physical Review Letters, 95, 083003
EIT in Yb:
"Optical clocks based on ultranarrow three-photon resonances in alkaline Earth atoms"
T. Hong, C. Cramer, W. Nagourney, E.N. Fortson, Physical Review Letters, 94, 050801
Cavitation, the process by which bubbles form in a liquid, is both of great interest in a number of practical situations and a poorly understood physical process. Bubbles form easily when a liquid is agitated, as occurs when water rushes through hydroelectric power plants and boat propellers. These bubbles are capable of releasing a tremendous amount of energy as they collapse, damaging nearby surfaces and are thus a serious design concern. On the other hand, this cavitation erosion is convenient and useful as a mechanism for ultrasonic cleaning. Despite its ubiquity and simplicity, it is difficult to study cavitation in a controlled manner because bubbles form readily on any impurities present in a liquid and surfaces of the vessel containing it. To truly understand the physical process at work, one must carefully control impurities in the liquid and on container walls, which is difficult to accomplish under ordinary conditions.
At Brown University, Humphrey Maris has made a systematic study of cavitation in liquid helium. Helium is an ideal liquid in which to study cavitation because it remains in its liquid state down to absolute zero, allowing unwanted impurities to be filtered out by cooling the liquid to mK temperatures. Bubbles spontaneously appear in a cell filled with cold liquid helium when the pressure is sufficiently low. By focusing acoustic waves with a hemispherical piezoelectric transducer, we were able to create a well-defined region of low pressure in which we monitored bubble formation by observing laser light scattered by nucleating bubbles. We introduced impurities systematically by placing a radioactive beta-source in the helium cell which emitted electrons into the liquid. My undergraduate thesis work was the first measurement of bubble nucleation on electron impurities in liquid helium-3, laying the groundwork for a comparison of a viscous fermi liquid to superfluid helium-4.
An accessible reference:"Negative Pressures and Cavitation in Liquid Helium"
Humphrey Maris and Sebastien Balibar, Physics Today, Feb 2000, p.29-34
For more on comparing helium-3 and helium-4:
"Quantum Cavitation: A Comparison Between Superfluid Helium-4 and Normal Liquid Helium-3"
S. Balibar et al., Journal of Low Temperature Physics, 114, 3-4, Nov. 1998, p.459-71