If you take a step back and look at what we know about the Universe you’ll quickly realize that ALL our knowledge about distant space comes from measuring light that is emitted by far-away astronomical objects.Thus any experimental technique that promise to dramatically increase the accuracy and stability of measuring light is a big deal for observational astronomers. In a recent issue of Science such a big deal was published. It’s called a Laser Frequency Comb and works by overlaying laser pulses of extremely short duration and extremely stable recurring frequency on top of the optical spectrum of the astronomical object under investigation. In essence, this technique allows the measurement of tiny variations over long periods of time in the frequency of light emitted by astronomical objects.
Space is expanding, so that two objects that are separated by a certain distance drift apart with a speed that is proportional to the separation distance: The further an object is away from us the faster it moves away from us. So, if you could measure the red-shift of this object over some time, then you would be able to detect that said red-shift increases as the object moves away further and further. You would not need to reference the red-shift in the object’s spectral lines to some calibration source that was measured here on earth, as is done currently. Think of this as an improved laser gun for your local traffic cop who will not only be able to tell your exact speed, but also whether you were slowing down or accelerating when he pointed the laser at you.
This technique promises to revolutionize deep-space astronomy AND near space astronomy as it will become possible to detect planets moving around their primary star, at least if the orbit is roughly within the line-of-sight from earth. Exciting times.
The basics of a laser frequency comb. A mode-locked laser creates femtosecond pulses at gigahertz frequencies, ƒrep (top), that are synchronized with an atomic clock. A spectrum of the pulses (bottom) is composed of many modes that are uniformly spaced in wavelength (or frequency) and cover a spectral bandwidth given roughly by the inverse of the pulse duration. Each mode’s wavelength (or frequency) does not have to be measured, but instead is given by a mathematical relation that includes ƒrep, known a priori with very high accuracy. Laser frequency combs could therefore become the perfect wavelength calibration technique for astrophysical experiments that require high accuracy and long-term stability.
CREDIT: ADAPTED BY P. HUEY/SCIENCE