0.1-Hz Detector Could Measure Cosmic Acceleration
Daniel S. Burgess
We are on the cusp of the gravity-wave revolution in astronomy. Numerous detectors -- powered by bars of metal or by beams of light, on the ground or in orbit -- are scheduled to come online over the next 10 years or are awaiting approval to do so, but their designers have skipped over the 0.01- to 10-Hz band. A team of scientists in Japan is arguing that a search for space-time distortions at these frequencies would enable measurement of the acceleration of the universe, opening the door to a more precise cosmology.
Researchers argue for a search for gravitational waves at 0.1 Hz. A 10-year study of the distortions generated by pairs of neutron stars would enable cosmologists to determine the curvature of the universe without appealing to theory.
The researchers suggest that a scaled-down version of the Laser Interferometer Space Antenna spacecraft that is under development by NASA and the European Space Agency -- which has a tentative launch date later in this decade -- would have 10 times the sensitivity at 0.1 Hz. Such a device, which they call the Decihertz Interferometer Gravitational Wave Observatory, could use a 1-W laser and 30-cm optics, but it would feature 500,000-km arms instead of the 5 million planned for the other detector. Presuming advances in laser and optics technology, they claim an ultimate sensitivity a million times better than that, or 3 x 10-27 Hz1/2.
A 10-year study at 0.1 Hz of the gravitational waves generated by coalescing pairs of neutron stars at a red shift of 1, they argue, would yield a value for the time lag on the waves resulting from cosmic acceleration. This would enable cosmologists to determine the curvature of the universe without appealing to theories of the prevalence of matter.
"By direct measurement of the cosmic acceleration, we can observationally determine the pressure and density at each epoch of our past universe," explained Naoki Seto, a postdoctoral researcher at Osaka University in Toyonaka who proposed the system. "Then we can study what kind of matter -- or energy -- dominates the universe, and we might also predict its fate."
Social forces rule
The team notes that there are many challenges to overcome before the Decihertz Interferometer Gravitational Wave Observatory can fly, not the least of which is suppressing the sources of noise to enable quantum-limit sensitivity. But it counters that no one predicted that gravity-wave astronomy would emerge so rapidly. In the end, social forces rather than technological ability will determine the fate of such an instrument.
"It totally depends on how we will be allowed to deal with space programs from now on, which depends heavily on our economic situation in the future," said Seiji Kawamura of the National Astronomical Observatory in Mitaka, Japan, a researcher on the project with Seto and Takashi Nakamura of Kyoto University. "I would say that I might be able to see it, and my kids will probably see it, and my grandchildren will definitely see it."
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