Hobby-Eberly Dark Energy Experiment

In 1998, studies of brightnesses of Type Ia supernovae showed us that the universe is dominated by something called Dark Energy, which is causing the expansion of the universe to accelerate. Since then, numerous experiments have been proposed (and initiated), all aimed at measuring the precise amount of Dark Energy now contained in the universe.

But exactly how does this Dark Energy work? Einstein wrote his field equations with a Cosmological Constant which defines a precise rule for the cosmic acceleration: the greater the size of the universe, the greater the universal acceleration. In other words, the pressure pushing the universe apart is inversely proportional to density, P ∝ ρ^w, with w = -1.

But is this true? Alternatives do exist to the Cosmological Constant model: for instance, if the cosmic acceleration is due to something called "quintessence", then w may vary through space and time. The Hobby-Eberly Telescope Dark Energy Experiment is aimed not only at measuring the amount of Dark Energy contained in the universe, but also determining its evolution. This makes HETDEX unique, as it is one of the only Dark Energy experiments currently planned to test the Cosmological Constant paradigm. Put mathematically, the question is simple: is w = -1 at all times?

HETDEX is a collaboration between Penn State, the University of Texas at Austin, Texas A&M, the Universitats-Sterwarte Munich, the the Astrophysical Potsdam, and the Max-Planck-Institut fuer Extraterrestrische Physik. The project's Principal Scientist is Karl Gebhardt (University of Texas); the Principal Investigator is Gary Hill (University of Texas). At Penn State, the lead scientists are Robin Ciardullo, Caryl Gronwall, Don Schneider, and Larry Ramsey.

How It Will Work

 The Big Bang did not create a perfectly smooth universe, and the slightly over- and under-densities of the explosion are imprinted in the microwave background. We can see these fluctuations, and measure the characteristic separation between the high and low-density regions using experiments like the Wilkinson Microwave Anisotropy Probe.  After recombination, however, these fluctuations were frozen into the matter distribution of the universe, so the characteristic scale imprinted on the microwave background is still with us today.  In fact, it can be used as a standard ruler.  If we could measure the scale of these fluctuations over cosmic time --- observe them at different redshifts --- then we could trace the expansion history of the universe itself, via the angular diameter-based distances, D_A(z), and the Hubble parameter, H(z). Both these parameters, of course, depend critically on the amount of Dark Energy in existence at any epoch.

Well, we can trace history of the microwave fluctuations. By observing the large scale distribution of galaxies which form from the fluctuations, and measuring how the characteristic spatial separation between clusters and voids changes with redshift, we can determine the expansion history of the universe to great precision. All we need is to obtain redshifts for large number of galaxies at each epoch.

The Instrument

To measure the expansion history of the universe, HETDEX will need to measure the redshifts of close to 1 million galaxies between redshifts z = 1.9 and z = 3.5. To do this, the experiment will use the Hobby-Eberly Telescope and what will be the world's largest integral field spectrograph, an instrument known as the Visible Integral-Field Replicable Unit Spectrograph, or VIRUS.

VIRUS is actually a very simple instrument. With the plate scale of the HET, it is relatively simply to make a cluster of about 200 optical fibers, each with a diameter of 1.5 arcsec, and bring these fibers down to a spectrograph which is optimized for the wavelength range between 3500 and 5500 A. This will generate a set of 200 spectra over about 350 square arcseconds of sky. Most of these spectra will be of blank sky, but if an emission-line galaxy happens to fall on one of the fibers, we will be able to obtain its redshift. More specifically, if a galaxy in the redshift range 1.9 < z < 3.5 has Lyα in emission, will we be able to see this emission.

Of course, 350 square arcseconds is not a very large area of sky, and finding 1,000,000 galaxies in this way would be very time-consuming. But we can accelerate the process by using industrial replication techniques and cloning the instrument about 150 times. This is a new concept in astronomy, and it allows us to build a very large spectrograph that is much less expensive than a conventional multi-object instrument. Normally, more than half the cost of a major astronomical project is in the engineering design, and copies of instruments are rarely made. By contrast, the HETDEX project will replicate the simple VIRUS spectrograph module over a hundred times, making the engineering cost less than 10% of the total. In addition, because the design is modular, the full experiment, from design, through data acquisition, to data reduction, can be debugged and optimized using a single prototype unit. In fact, a survey with the prototype VIRUS spectrograph, VIRUS-P, has been ongoing since 2006.

VIRUS on HET will measure the positions and redshifts of 10,000 galaxies every night, and a million galaxies in 100 nights. This will be sufficient to fix the expansion history of the universe, H(z) to better than 1% over the redshift range 1.9 < z < 3.5, and thereby tell us the evolution of Dark Energy during this epoch. It will define angular diameter distances, D_A(z) to better than 1%, and also allow us to measure the curvature of the universe to one part in a thousand.

Timescale and Details of the Survey

The baseline parameters of the HETDEX survey are:

• Area of Survey: 300 square degrees
• Filling Factor: 1 in 4.5
• Wavelength Range: 3500 to 5500 Angstroms
• Spectral Resolution: 6.4 Angstroms (R = 800)
• Exposure Time per field: 1200 seconds (using 3 separate dithered exposures)
• Total Time of Survey: 1200 hours of Observing Time (140 nights)
• Emission Line Sensitivity Limit: 3.5 x 10^-17 ergs/cm²/sec.
• Continuum Sensitivity Limit: S/N of 10 at R = 22 mag

• a direct detection of Dark Energy at z = 2.5 (for a Λ model).
• a measurement of curvature to about 10^-3, i.e., 10 times better than the current measurement.
• a small improvement in the present day equation of state, w₀
• 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 D_A(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.

Expected Detections

HETDEX will have the following continuum and line sensitivities:

With these sensitivities, the baseline survey will obtain spectra for

• 0.8 million Lyα emitting galaxies between 1.9 < z < 3.5
• 1.0 million [O II] emitting galaxies with z < 0.5
• 0.4 million other galaxies
• 0.25 million Milky Way stars
• 2000 galaxy clusters with Abell richnesses > 1
• Between 10,000 and 50,000 AGN at z < 3.5

Additional Science

Because the HETDEX survey is untargeted, it will obtain spectra for everything in the field of view. Some of the science projects that HETDEX will address are

• The first detection of Dark Energy at z > 2
• A large improvement in the measurement of curvature in the universe.
• The first detection of the cosmic web in emission
• The best measurement of the total neutrino mass
• The measurement of any non-Gaussian features in primeval density perturbations
• The best measurement of the luminosity function and equivalent width distribution of Lyα emitting galaxies
• The detection of 25000 active galactic nuclei with no selection biases, and the measurement of correlations between AGN and galaxies.
• The measurement of the local (z < 0.4) star-formation rate density through the detection of 1 million [O II] emission-line galaxies
• The measurement of dark matter in galaxies, via measurements of stellar velocity dispersions at large radii.
• Metallicity and age estimates for stellar populations at large galactic radii
• The detection of intracluster stars using planetary nebulae as tracers
• The detection of low-metallicity stars in the Milky Way
• The measurement of Milky Way structure and stellar kinematics

Some of the projects led by Penn State astronomers include the analysis of emission-line galaxy luminosity functions, the detection of galaxy cluster evolution via the kinematics of intracluster stars, and the measurement of the relationship between emission-line galaxies and Active Galactic Nuclei. In addition, Penn State astronomers are also leading the effort to conduct a deep, broadband imaging survey over the entire HETDEX survey field. This parallel survey is crucial for obtaining the maximum amount of science out of HETDEX.

Additional Information

Additional information can be found at

• The main HETDEX web page
• The University of Texas' HETDEX page