By resolving light into its component frequencies, spectrographs can reveal, for example, the Doppler shift in light that travels to Earth from other solar systems with orbiting exoplanets or from the far reaches of the universe. But before it can be put to work, a spectrograph must be calibrated, and that process inevitably introduces measurement uncertainty. To reduce those uncertainties, Tilo Steinmetz and colleagues developed a calibration procedure that can dramatically improve spectroscopic precision, and they demonstrated their technique by obtaining the most accurate spectrum of the Sun’s photosphere to date. The key ingredient is a laser frequency comb, a series of equally spaced, precisely known frequency spikes that, as the white stripes in the figure show, can serve as a template with which astrophysical spectra can be compared (see PHYSICS TODAY, June 2000, page 19, and December 2005, page 19). Because the comb is reproducible, the calibration can be replicated from run to run. Moreover, frequency combs may help isolate systematic spectrograph uncertainties that, one hopes, can be understood and reduced. A plausible goal, according to Steinmetz and company, is to measure redshifts of objects whose speed along the line of sight is changing by 1 cm/s over a year’s time. That would allow for a direct measurement of cosmic acceleration. Closer to home, higher-precision spectroscopy would enable astrophysicists to identify Earth-like exoplanets by measuring the characteristic Doppler shifts experienced by a star as it is gravitationally tugged by an orbiting planet. (T. Steinmetz et al., Science 321, 1335, 2008.) — Steven K. Blau
Frequency combs help untangle astrophysical spectra
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Speaking of "direct measurement of cosmic acceleration", a really interesting test would entail the searching for asymmetric or symmetric changes in the expansion rate of our observable universe such as those which might be due to gravitational influence of other nearby universes located a relatively short distance away from our universe through higher dimensional or extra-dimensional space. It happens to be the case that part of the experimental program for the LHC will be to search for the indirect influences of the behavior of gravity due to the existence of higher dimensional space or extra hidden spatial dimensions such as those proposed in string theory.
Another interesting research agenda would entail looking for the collision of universes with our universe such as might occur if there where other big bangs a previously relatively short distance away from our observable universe in higher dimensional space. Such collisions, in theory, depending on the topology of the interacting universes might appear as a ring like disturbance or perturbation within the cosmic microwave background radiation, or perhaps as such in terms of patterns in space-time expansion rate changes due to stresses within the space times of the colliding big bangs or universes.