Seeing with a mirror darkly

Astronomy has always struck me as a science where the more we know, the more there is to know. Although the "Big Bang" theory of an expanding universe is generally accepted by astronomers, we don't know what is driving matter outward at ever-faster speeds.

Jun 1st, 2002

By Jeff Bairstow, ATD Online Editorial Director
jbairstow@pennwell.com

Astronomy has always struck me as a science where the more we know, the more there is to know. Although the "Big Bang" theory of an expanding universe is generally accepted by astronomers, we don't know what is driving matter outward at ever-faster speeds. Some scientists have suggested that there is a mysterious force called "dark energy" that is accelerating matter. However, no one has yet succeeded in measuring dark energy. But a group of Princeton University and Bell Labs scientists have come up with a detailed proposal for an optical telescope that might help us "see" dark matter.

If you'd like to know more about this search for the invisible, I recommend the web site for the Large-aperture Synoptic Survey Telescope (LSST) at www.dmtelescope.org. To give you some idea of the problems the LSST researchers face, I have excerpted sections of the web site in this column.

For decades, say the LSST designers, we have been trying to understand the physical structure of our universe on the basis of severely limited observations. All the galaxies, stars, and other cosmic matter that we see—even with the full spectrum of ground- and space-based telescopes in operation today—amount to luminous foam on an ocean of dark matter. This mysterious form of matter is transparent and emits no radiation, and so is invisible. Yet dark matter dominates the mass of the universe and drives the development of structure through its gravitational influence. Over the course of billions of years, mountains of dark matter have shepherded the assembly of ordinary matter into stars, planets, and life.

Synergy between fundamental science and advanced technology has given us the opportunity, say the LSST designers, to view the panorama of dark matter directly and to chart the development of structure on a cosmic time scale. The proposed Dark Matter Telescope can provide this unique view, which would test theories on the nature of dark matter, as well as theories on the origin and fate of our universe.

Nonluminous matter takes multiple forms, from dense clusters of unknown composition to nearby asteroids that could threaten Earth. The Dark Matter Telescope would image huge dark-matter structures directly; it would provide unprecedented insight into our evolving and transient universe; and it could chart all significant near-Earth objects. The project engages novel developing technology—optics, software, and high-throughput data analysis—to image and map nonluminous matter.

This astronomical project calls for deep images recorded over a wide field of view. The researchers have proposed an 8.4-m aperture, three-mirror telescope that would take advantage of advances in optical fabrication and detectors to obtain remarkable imaging capability. Its etendue, the product of collecting area and field of view, is 260 (m · degrees)2, or ten times greater than any other current or planned telescope. The three-degree field would be imaged at f/1.25 by circular mosaics of imaging detectors 55 cm in diameter, using charge-coupled devices for wavelengths between 0.3 and 1 μm.

The telescope design is very compact, little longer than the primary mirror diameter. In three or four clear nights, the fast-slewing telescope could survey the entire visible sky (20,000 square degrees). Such a telescope need not be operated in space, which reduces the anticipated cost of the project from the billion-dollar range of a satellite-borne telescope to less than $120 million, say the researchers. The LSST has yet to be funded and a suitable site would have to be found. Clearly, it would tell us more than we now know about dark matter, but could leave us with much more to discover.

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