The twinkling starlight of
the night sky is not just pretty to look at — it also carries with it the
secrets of distant suns.
From this light, scientists
can glean information about what stars are made
of, as well as the speed and direction of their movements across the
heavens. This data, in turn, can help unravel riddles about cosmic expansion
and reveal if other Earth-like worlds exist.
Now, German researchers
have more accurately scanned starlight for these telltale details, according to
a Sept. 5 study in the journal Science. The paper's authors tested an
improved stellar speedometer, called a laser frequency comb, on a telescope for
the first time earlier this year. Scientists say the Nobel Prize-winning
technology, which debuted in 2005, still needs some work. But it may prove a
big step forward in astronomical research.
"I'm very excited
about the discoveries this laser frequency comb should make possible,"
says Thomas Udem, an author of the paper and an optical physicist at the Max
Planck Institute for Quantum Optics in Garching, Germany.
Red and blue shifts
For its first trial run,
researchers hooked the speedometer up to a telescope in Germany in March and took basic readings of the Sun's light. The comb device looks like a large
oven-pan-size motherboard of circuitry with cables running to other instruments
and monitors. Nevertheless, this contraption helps astronomers tease out
important information from the complexity of a star's light.
As stars travel through the
universe, certain signatures in their light move, too. Scientists are after
these so-called spectral lines that result from elements like helium and oxygen
heating up in stars. When starlight is splayed out by a prism, the lines look
like thin, dark gaps in the rainbow of colors that the human eye can see,
called the visible spectrum. Each element, from hydrogen on up, possesses its
own unique barcode of spectral lines.
When a star moves away from
Earth, these spectral lines shift toward the red,
low-energy part of the spectrum. This happens as light waves and their sets
of spectral lines stretch out while spanning the trillions upon trillions of
miles between their stellar sources and our telescopes. This wave-stretching
phenomenon, called the Doppler Effect, works for both sound and light: Think of
how an ambulance's siren fades as the vehicle zooms off, says Udem. For sound waves,
this expansion means lower pitch, while for light it means longer, redder
wavelengths.
Accordingly, spectral lines
shift into the bluer, higher-energy end of the spectrum when a star is getting
closer, just as an approaching ambulance siren gets louder. By spotting these
blue or red shifts, scientists can tell if stars are heading hither or thither.
To detect these usually small shifts, scientists need a standard for comparing
the shifted light to normal, unshifted light, says Udem.
How the 'comb' works
Laser frequency combs
answer this need for greater precision by using laser pulses to generate many
distinct energies of light. This light is filtered through a prism, which then
makes evenly spaced "spikes" on a spectrum. These spikes correspond
to "teeth" in the comb, says Udem.
By way of another analogy,
this "color ruler" has clear separations between units so scientists
can more easily read the pattern of spectral lines in a star's light, says
Michael Murphy, a co-author of the paper and an astrophysicist at Swinburne
University of Technology in Hawthorne, Australia. The light collected by the
telescope is routed through the comb, which overlays its own color ruler on top
of the incoming spectrum from the stellar object of interest.
Current telescope
calibration techniques use special lamps containing elements like thorium and
argon to set spectral lines for comparison. "But these lamps change over
time and can throw off your results," says Scott Diddams, a physicist at
the National Institute of Standards and Technology in Boulder, Colo., who is
also working on laser frequency combs.
Measures of stars'
velocities vary by 10 meters per second with present instrumentation — too
much to suss out some finer details, like Earth-sized planets. But with comb
calibration, the German team achieved state of the art variation at 9 meters
per second, says Udem, even though some equipment of theirs, like the atomic
clock that ensures accuracy, was modest in capacity. Theoretically, 1,000-fold
more precision should be possible, he adds, down to one centimeter per second.
Tracking quasars
Once armed with precision
of that caliber, scientists plan to investigate the universe's expansion to see
if it is really accelerating. Current evidence, like the extreme red shifts of
distant supernovae, indicates this is so, but finding a mechanism for this
galloping growth has baffled cosmologists. They have proposed the existence of
a mysterious, as-yet-undetected force called dark energy as a tentative
explanation.
Probing this acceleration
will be an ambitious, 20-year project, says Udem. The proposed experiment would
track far-flung young galaxies
called quasars on a massive telescope, appropriately called the European
Extremely Large Telescope, which is not slated to begin operating until nearly 2020.
The extent of the quasars' red shift over the planned two-decade period of
observation should indicate if they are receding at a quickening rate.
Laser frequency combs
should also enhance the detection of small, Earth-like exoplanets that orbit
other stars, says Udem. Planets cause gravitational disturbances in their suns'
velocities by tugging them back and forth, making the stars appear to wobble
very slightly. With today's astronomical equipment, though, scientists have a
hard time finding Earth-size exoplanets because they generate such small
wobbling effects.
"These experiments are
really screaming out for help from frequency combs," says Swinburne's
Murphy.
Comb to hunt exoplanets
For now, the technology is
still too cutting edge to produce immediate, significant results, says Udem.
The combs will need more teeth, for example: The device that his team used in Germany only provided calibration in the infrared part of the spectrum. (Elements produce
spectral lines in the infrared, too, not just in visible light.)
Even so, a U.S. team will try out a laser frequency comb this fall when hunting for exoplanets
around some low-mass stars — the first test for such a device on a star other
than the sun. "It's one step at a time with these instruments," says
Ronald Walsworth, a physicist at the Harvard-Smithsonian Center for
Astrophysics in Cambridge, Mass., who is involved with the effort.
The final design
specifications, based on which telescopes use them, will determine the combs' overall
costs, Udem adds.
Researchers hope that this
calibration tool will push the limits of cosmological discovery. Yet another
test made possible through enhanced instrumentation is exploring if the laws of
nature —as we understand them — hold true in distant, ancient quarters of the
cosmos, says Udem. The speed of light and power of gravity, for example, "could
just be sort-of 'by-laws' for our local time and place in the universe,"
says Murphy. If these physical constants turn out to be variable, then this
finding may break new ground on the fundamental theories of how the universe
works "that people have been chasing since Einstein," says Murphy.
And that's just what's on
the docket so far. "It's possible that new experiments that we have not
even thought of before will pop up," says Murphy, "and lead to
revolutionary results."
This story is provided by Scienceline, a project of New York University's Science, Health and Environmental Reporting Program.
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