Laser Frequency Combs for Astronomical Observations

TLDR

A direct measurement of the universe’s expansion history could be made by observing the real‑time evolution of cosmological redshift, but this requires Doppler velocity drifts of ~1 cm s⁻¹ yr⁻¹, a precision current spectrographs cannot yet achieve. The study aims to use laser frequency combs to model and remove systematic effects in spectrographs to the precision needed for future experiments to detect the universe’s acceleration. The authors employ a laser frequency comb to calibrate an astronomical telescope’s spectrograph, enabling precise wavelength calibration. They demonstrated the first use of a laser frequency comb for telescope calibration, achieving ~9 m s⁻¹ Doppler precision at 1.5 µm and showing its advantage in tracking complex, time‑varying systematic effects.

Abstract

A direct measurement of the universe's expansion history could be made by observing in real time the evolution of the cosmological redshift of distant objects. However, this would require measurements of Doppler velocity drifts of ∼1 centimeter per second per year, and astronomical spectrographs have not yet been calibrated to this tolerance. We demonstrated the first use of a laser frequency comb for wavelength calibration of an astronomical telescope. Even with a simple analysis, absolute calibration is achieved with an equivalent Doppler precision of ∼9 meters per second at ∼1.5 micrometers—beyond state-of-the-art accuracy. We show that tracking complex, time-varying systematic effects in the spectrograph and detector system is a particular advantage of laser frequency comb calibration. This technique promises an effective means for modeling and removal of such systematic effects to the accuracy required by future experiments to see direct evidence of the universe's putative acceleration.