HETDEX is the first major experiment to search for dark energy in the early universe.
It uses the giant Hobby-Eberly Telescope at McDonald Observatory and a set of spectrographs to map the three-dimensional positions of over one million galaxies.
The Experiment
The Hobby-Eberly Telescope Dark Energy Experiment — HETDEX — at The University of Texas at Austin McDonald Observatory is the first major experiment to search for dark energy in the early universe. It uses the giant Hobby-Eberly Telescope and an innovative instrument to map three-dimensional positions of over one million galaxies.
HETDEX is currently collecting data on at least one million galaxies that are 9 billion to 11 billion light-years away, yielding the largest map of the early universe ever produced. The map will tell the astronomers how fast the universe is expanding at different times in its history, which will help reveal the role that dark energy has played at different epochs. Various explanations for dark energy predict different changes in the expansion rate, so by providing precise measurements of the expansion, the HETDEX map will eliminate many competing ideas.
How the Project Works
HETDEX studies baryonic acoustic oscillations — “sound waves” from the first 400,000 years of the universe. In those early years, the universe was a dense “soup” of particles of matter and particles of energy. Disturbances created sound waves that rippled across the dense universe. At 400,000 years, the universe became cool and thin enough that the ripples created during the Big Bang became frozen at a unique size.
HETDEX measures the distances between galaxies at different times in the early universe. Careful mathematical analysis of its map of distant galaxies will tell us the size of the universe at different epochs. Comparing the size and expansion rate at different times in the history of the universe, which are influenced to different degrees by the “repulsive” force of dark energy, will reveal the true nature of dark energy.
Survey Area & Timing
HETDEX surveys a large area of the sky that encompasses most of the Big Dipper. This region is far above the plane of the Milky Way galaxy, which is filled with clouds of gas and dust that block the view of distant galaxies.
The rotation of the earth causes stars and galaxies close to the pole to move across the sky more slowly than objects near to the celestial equator. The northern objects therefore remain in the telescope’s
view longer, and require fewer position changes during a night of observing.
HETDEX observations are scheduled around the time of new Moon when there is no moonlight to overpower the feeble glow of distant galaxies. Observing takes place from late winter into early summer, when the target region wheels high overhead during the night, putting it in a prime viewing location. Each observation covers a segment of sky that is a little smaller than the Moon. In all, HETDEX will survey an area that’s the equivalent of about 2,000 full Moons.
Hobby-Eberly Telescope
- While other large optical telescopes perform a variety of roles, the Hobby-Eberly Telescope is designed to observe astronomical objects spectroscopically — in other words, to split light from an object into its component wavelengths.
- In most telescopes, the primary mirror is a single piece of glass. The HET’s mirror, though, consists of 91 identical six-sided segments that fit together like the tiles on a floor. Each segment, which is made from a glass/ceramic material, is one meter across and two inches thick. Small computer-controlled motors attached to the back of each segment periodically adjust the positions of individual segments to maintain the proper overall mirror shape.
- The combined segments yield a light-gathering surface equivalent to a single 11-meter telescope. Unlike most other telescopes, HET’s mirror is always tilted at the same angle, 55 degrees above the horizon. A tracking system at the top of the telescope above the primary mirror moves to capture light from different portions of the mirror.
- The telescope rotates between exposures to view different regions of the sky. The combination of this rotation and the tracker’s motion in six directions allows the HET to cover 70 percent of the sky that is visible from McDonald Observatory.
Instrument
HET’s wide field of view is critical for HETDEX to survey a large swatch of sky to amass data on at least a million galaxies to measure the effects of dark energy at earlier times in the history of the universe.
Visible Integral-Field Replicable Unit Spectrographs
To perform the HETDEX survey, University of Texas astronomers designed an innovative instrument that uses 78 bundles of optical fibers to gather light from distant galaxies, and 156 spectrographs to break the light into its individual wavelengths or colors. Each fiber bundle contains 448 fibers like those that carry telephone calls and computer data. The entire array consists of over 34,000 fibers, each of which looks at a tiny piece of the sky.
HETDEX uses a set of 156 spectrographs mounted on the structure of the HET itself. Each unit gathers the light from distant galaxies and splits the light into its individual wavelengths, i.e., it makes a spectrum of the light detected by each fiber. The spectrum of an object reveals its chemical composition, temperature, and how fast it is moving toward or away from us. For distant galaxies, astronomers can convert these motions away from us into distances, and produce a precise 3-D map of the distant universe.
Scientific Requirements
HETDEX science verification began in the early spring of 2017, and the HETDEX observations began in December 2017. The baseline parameters of the survey are:
TOTAL AREA OF SURVEY
540 square degrees with a filling factor of 1 in 4.5
SPRING FIELD
56×7 square degrees centered on 13.5h, +53d (in the Big Dipper)
FALL FIELD
30×5 square degrees centered on 1.5h,0d (between Eridanus and Orion)
FIELD OF VIEW
22 arcmin in diameter
WAVELENGTH COVERAGE
3500 to 5500 Angstroms
SPECTRAL RESOLUTION
4.7 to 5.6 AA (resolving power of 750-950)
EXPOSURE TIME PER FIELD
1200 seconds (using 3 separate dithered exposures)
END OF SURVEY: 2024
1200 hours of Observing Time (140 nights over 3 years)
EMISSION LINE SENSITIVITY LIMIT AT 5000 ANGSTROMS
3.5 x 10-17 ergs/cm2/sec.
SCIENCE GOALS
Direct detection of dark energy at z=2.4, measure of the Hubble Expansion to 0.9%, and measure of the angular diameter distance to 0.9%
The main survey will produce:
- a direct detection of Dark Energy at z = 2.5 (for a simple Λ model).
- a measurement of the curvature of the universe to about 10-3, i.e., 10 times better than the current measurement.
- a small improvement in the present day equation of state, without a significant improvement in the measurement of any change in w with redshift.
- a measurement of H(z=2.8) to 0.9%. a measurement of angular diameter distance DA(z=2.8) to 0.9%.
- a measurement of the galaxy power spectrum to 1.5% for structure growth.
These precisions can be improved with additional survey time.
Meet the Team
HETDEX is a large collaboration that involves astronomers, engineers, technicians, graduate students, and others from six academic institutions in the United States and Germany.