HETDEX - Dark Energy Experiment

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HETDEX is a project designed to understand the evolutionary history of dark energy. Dark energy is the mysterious property that consists of over 70% of the total energy in the Universe. We know very little about it, beyond its existence, but we do understand how to study it. The goal is to use results from HETDEX to precisely describe how the Universe expands, thereby illuminating the nature of dark energy.

The Problem

The Universe has been expanding since the Big Bang, 13.7 billion years ago. The matter in the Universe has been slowing down that expansion due to its gravitational attraction. However, recent measurements of exploding stars called supernovae have shown that there is an additional component in the Universe that has won over gravity and is causing the Universe to expand at an accelerating rate. This component is dark energy.

In 1920, Einstein invented the idea of dark energy to explain why the Universe feels a force that counteracts gravity. His explanation was quickly abandoned as unnecessary, and subsequently ignored for 70 years. Today we know that not only does dark energy exist, but it is expanding the Universe into darkness.

Although dark energy makes up 70% of the total energy in the Universe, we have no idea what it is or from where it comes. Without an accurate understanding of dark energy, we cannot make much progress toward understanding how the Universe evolves. We are truly walking into new territory. Discovering the nature of dark energy is certainly one of astronomy's most exciting prospects for this century.

Unique tracking system of the HET

The Solution

The first step is to measure the effect of dark energy on the Universe with very high precision--specifically, to measure exactly how the Universe has expanded over time. When we look at distant objects, due to the finite speed of light, we are able to see back in time. We call this look-back time. Hence, we can measure the properties of the Universe back in time, by observing more and more distant galaxies and supernovae. Using the supernovae that first demonstrated the existence of dark energy, we can probe the size of the Universe to 9 billion years. At greater look-back times, detection becomes too difficult.

The ideal tracer of the Universe's expansion history, all the way back to 12 billion years (nearly 90% of the age of the Universe), is the large-scale distribution of galaxies. As the Universe expands, the distance between galaxies increases. There are characteristic patterns in the distribution of galaxies which can be measured. These patterns increase in scale as the Universe expands. Therefore, by comparing the size of the patterns in the distribution of galaxies at different look-back times, one can measure the expansion of the Universe over cosmic time.

HETDEX

Uncovering the patterns in the distribution of galaxies requires a survey to map out the positions of a million galaxies in a volume ten times larger than any survey to date. This cannot be done with any existing instrument on any large telescope, but it can be achieved on the Hobby-Eberly Telescope with the new VIRUS instrument (HET).

The HET is a premier telescope for surveys, and it is an ideal telescope to solve the problem of dark energy. Run by a small consortium, it ranks among the world's largest telescopes, with an effective aperture of 9.2 meters (360 inches). Due to its revolutionary design, it was constructed at just 20% of the cost of comparable telescopes.

HETDEX promises the largest ever galaxy survey. Among several studies now planned targeting dark energy, none will obtain the early times HETDEX is designed to probe. Additionally, with funding, HETDEX can be complete within 8 years, sooner than other surveys. These factors promise to make HET a dominant player in the endeavor to understand dark energy.

University of Texas at Austin Astronomy
McDonald Observatory
McDonald Observatory Vistor's Center
Hobby-Eberly Telescope

Visible Integral-field Replicable Unit Spectrograph

Two things are needed to achieve HETDEX: a very large telescope and a revolutionary instrument
The VIRUS instrument uses the new concept of industrial replication to save 75% of the cost, versus a conventional astronomy instrument
HET with VIRUS can do the HETDEX survey in just 100 nights of observing

Mapping the galaxies

Collecting the light from distant galaxies with a large telescope is only half the story - the light must be spread out into a spectrum (like a rainbow) and detected by an instrument called a spectrograph.

The spectrum allows us to measure how far away a galaxy is, so we can map its position for the survey. In an expanding universe, every galaxy is moving away from every other galaxy, so, from our perspective living in one of them, galaxies that are further away are moving faster away from us. This allows us to determine how far away a galaxy is by measuring its speed, via the Doppler effect. A common example of the Doppler effect is the change in pitch one hears from a siren as an emergency vehicle passes - higher pitch as it approaches and lower pitch as it goes away. The Doppler effect in light means that objects moving away have their spectra shifted to the red, equivalent to a lower pitch. This effect is called redshift, and it allows us to determine the speed of a galaxy and hence its distance and position in space.

HET and VIRUS

Most spectrographs can observe only a few objects at one time. An integral field spectrograph improves on this by obtaining a spectrum of every bit of sky that it is observing simultaneously. Small integral field spectrographs exist but would take centuries to complete the HETDEX survey.

Instead, we will build the VIRUS spectrograph that is powerful enough to obtain the data in only 100 nights. We do this by creating more than 100 copies of a simple integral field spectrograph by industrial replication. This is a new concept in instrumentation for astronomy. It allows us to build an instrument that is much less expensive than a conventional one, where only one large instrument is built and more than half the cost is in the engineering expense. Each conventional astronomical instrument is usually a prototype and copies are rarely made. By contrast, we will replicate the simple VIRUS spectrograph module over a hundred times and spread the engineering cost so it will account for less than 10% of the total. The additional savings of using industrial manufacturing techniques will allow VIRUS to be built for only 25% of the cost of an equivalent traditional design.

VIRUS and HETDEX

VIRUS on HET will measure the positions in space of 10,000 galaxies every night, so can map out a million galaxies in 100 nights. This survey is sufficient to constrain the scale of the Universe to better than 1%, and will tell us whether dark energy is a constant effect over time, or whether it evolves. Evolution of dark energy will tell us a lot about its properties and will provide the first significant constraints on theories about its nature.

HETDEX has been initiated by Gary Hill, Karl Gebhardt, and Phillip MacQueen at the University of Texas and McDonald Observatory. HETDEX is led by McDonald Observatory and the University of Texas at Austin, with participation from the Universitäts-Sternwarte of the Ludwig-Maximilians-Universität München, the Max-Planck-Institut für Extraterrestriche-Physik, the Astrophysikalisches Institut Postdam, and the HET consortium. We are particularly grateful to Congressman Henry Bonilla for providing the seed that started this project, and to the George and Cynthia Mitchell Foundation for their contribution that has enabled development of the VIRUS unit spectrograph prototype.

The Hobby-Eberly Telescope (HET) is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximillians-Universität München, and Georg-August-Universität Goettingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly.

15 May 2008
Astronomy Program · The University of Texas at Austin · Austin, Texas 78712
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