David Lambert
Director's Message: HETDEX’s Unique Texan View on Dark Energy
As BoV members, you have heard many times about dark energy from me and from Karl Gebhardt and Gary Hill, the leaders of our Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) team. As HETDEX is overcoming some significant challenges, the facts are so amazing, and HETDEX is so significant, that they bear repeating: First detected in the late 1990s and later confirmed by other studies, dark energy is causing the Universe’s expansion to accelerate when it should be slowing. Dark Energy dominates the Universe: It comprises some 73 percent of the Universe’s energy/matter content (with the rest made up of dark matter at 23 percent, and “normal” matter, made of atoms, at a mere four percent).
Is dark energy a property of space itself? Does it arise from an undiscovered subatomic particle? Has dark energy evolved or stayed constant? Does the theory of gravity need rewriting? By international consensus, dark energy’s nature constitutes the greatest mystery in all the sciences. Leaders of the two teams whose observations first revealed dark energy were awarded the 2011 Nobel Prize in Physics. Unlocking its secrets will no doubt win similar accolades.
When we began HETDEX in 2005, we knew we wanted to accomplish what no one else in the world was planning, which was to look back 11 billion years to the era of the earliest galaxy formation. That way we could show whether dark energy had evolved or stayed constant, thereby providing the strongest possible discrimination among the different theories about dark energy. We developed a plan to expand the field of view of the Hobby-Eberly Telescope, one of the world’s largest, and to add a suite of revolutionary integral-field spectrographs called by the acronym VIRUS. Together, they would create an unmatched capability to survey the sky with spectroscopy, 10 times faster than any current or planned facility.
The new tracker for HETDEX undergoing testing at the Center for Electromechanics in Austin
In 2005, there were seven major competing projects planned. Today, only two are underway -- The Baryon Oscillation Spectroscopic Survey (BOSS) and the Dark Energy Survey (DES). Both projects will measure the properties of dark energy in the relatively recent past (at least from a cosmic perspective). BOSS, utilizing the 2.5-meter Sloan Telescope in New Mexico, got underway in 2010 and in 2012 released preliminary results, including a map of 250,000 luminous red galaxies at a redshift of 0.55 (the equivalent of nearly six billion light years). BOSS is slated for completion in 2014. The second competing experiment, DES, run by the Department of Energy’s Fermilab, uses a four-meter telescope in Chile with a new camera. DES announced first light in September 2012, and is projecting completion in 2017. Like BOSS, DES is designed to study the Universe at around six billion years ago, gathering data on galaxy clusters, weak lensing, Baryon Acoustic Oscillations, and Type 1a supernovae.
HETDEX is different. It is designed to find galaxies up to 11 billion light years away (at redshift 3) — far beyond those that DES and BOSS can reach.
Light from the Hobby-Eberly Telescope’s primary mirror (with its maximum pupil expanded from 9.2 to 10 meters) will be focused by the new wide-field corrector onto optical fibers feeding 75 pairs of VIRUS spectrographs, capturing the spectrum of every object that falls onto one of the 34,500 fibers in the array. Each night, these spectra will provide three-dimensional locations for up to 10,000 star-forming high-redshift galaxies — a million such galaxies over the course of the three-year survey. Generating the deepest 3-D map ever created, HETDEX scientists will show how fast the Universe was expanding at different times in its history. Scores of theories about dark energy predict different changes in the expansion rate. HETDEX will provide the first-ever high-precision measurements of that expansion far back in time, testing all dark energy theories in a way that no other project can. Only such detail and reach can really move the understanding of dark energy forward.
In addition, HETDEX will measure the (potential) curvature of the Universe to a precision of .1 percent — ten times more exact than the best measurement to date (which was provided by the WMAP study of the Cosmic Microwave Background). BOSS and DES will not be able, on their own, to separate the effects of dark energy from the effects of curvature. Thus, HETDEX’s observations will be crucial to our competitors’ success. In fact, the three projects are truly complementary, but HETDEX holds the crucial information.
From the start, the science we planned challenged the state of the art in mirror making, fiber optics, and other technologies. Generous support from BoV members and many others, along with federal agencies and the State of Texas (through the McDonald Observatory operating budget and University of Texas funds) and partner contributions made it possible for our scientists to undertake this most ambitious, most important project in McDonald Observatory’s history.
Significant progress has been made on the two major phases of HETDEX construction. The new Wide Field Corrector and its larger new tracker assembly have been built and are being tested in Austin. VIRUS production is well underway, with parts arriving or on order and within the original budgets, and with several pairs already assembled. The huge cryogenic system that delivers liquid nitrogen to each of the VIRUS units is under contract, and we are completing the design of the spectrograph enclosures.
A pair of VIRUS spectrographs, packed to ship to Germany, where it is being used to test fiber optics
Over the past year, however, some of our contractors have encountered challenges that affect both HETDEX’s timeline and its budget. The critical pacing items for HETDEX deployment are the four state-of-the-art mirrors for the new Wide Field Corrector, and delays in polishing the mirrors have added a year to our schedule. Polishing the mirrors is now complete and their figures meet specifications, which is an important milestone for the project. Contracts for the very high performance reflective coatings have been placed, and the coatings for the four mirrors will be finished by early 2013. Significant additional funds have been required by these changes.
At The University of Texas at Austin’s Center for Electromechanics (CEM), the tracker is assembled and functional, but final work on its software will not be completed until January 2013. The effort going forward has been divided between programmers at CEM and at McDonald Observatory, and has required additional funding.
For the HETDEX schedule, these changes to the Tracker and the Wide Field Corrector mean that the earliest we can take down the existing HET and begin HETDEX installation is July 2013, and it moves our projected completion date from 2014 to 2015. We will cover much of the additional funding needed from the McDonald Observatory operating budget, and we are working with the University of Texas at Austin administration and partners on other funds. In addition, we are in conversation with potential private donors about funding for the additional needs that have developed.
I am proud that The University of Texas at Austin is leading this world-class project, which is substantially complete. HETDEX will provide unique data for solving the most important mystery in our Universe, along with revolutionary science in a host of other areas. There is a great deal of excitement about the prospect for data flowing in about 18 months among the Faculty, researchers, and graduate students at UT, and preparations are under way to exploit the opportunities that HETDEX provides not just for unraveling dark energy, but for a host of other science applications. HETDEX is crucial to the future of McDonald Observatory and The University of Texas at Austin. I will report on additional milestones soon.
[David L. Lambert]
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