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EDGE: Explorer of Diffuse high redshift Galaxy Emission
The EDGE experiment is a balloon experiment that will study variations in the cosmic infrared background over the sky to further our understanding of large-scale structures in the Universe. A key goal of modern observational cosmology is to understand and trace the formation of large-scale structure throughout the history the Universe. We now know from studies of the Cosmic Microwave Background Radiation (CMBR) that the matter distribution in the Universe was extremely uniform when it was around 300,000 years old. As time progressed, gravitational collapse amplified the small deviations from uniformity into the rich structures that we see in the Universe today. In the nearby Universe, we have made great progress reaching back to a time when the Universe was perhaps half its current age, at approximately 6 billion years old. What happened in the time between 300,000 and 6 billion years? A key remaining observational milestone occurred when the Universe was only a few billion years old - when the first objects in the Universe were sufficiently compressed to ignite. This light was absorbed by dust in the local protogalaxy environment and then re-emitted in the infrared. The integrated light from all such dust in the line-of-sight is the Cosmic Infrared Background (CIB). Because the CIB is emitted by these first galaxies, it traces their distribution in space, thus providing a new observational handle on this crucial early era. EDGE will measure the 'clumpiness' of this light from the Universe over the sky, back to the time the first galaxies ignited. This will complement the measurements from the CMBR and from the modern era to form a more complete history of large-scale structure formation.
The EDGE scientific goals are an integral part of the NASA Office of Space Science strategic plan: "Understand the structure of the Universe, from its earliest beginnings to its ultimate fate," and the specific goal: "Determine the size, shape, age, and energy content of the Universe." EDGE also directly addresses two "key problems" in the McKee-Taylor Decadal Report of 2001 - "determine the large-scale properties of the Universe," and "study the dawn of the modern Universe."
The main scientific questions that EDGE aims to answer are:
The picture below shows how EDGE fits in with the other experimental and theoretical work that makes up our current understanding of the origin and evolution of our Universe. One can note that EDGE does indeed bridge a gap in our understanding of the universe between the time that structures formed when matter and radiation separated from each other generating the cosmic microwave background and the galactic structures we see in the Universe today. More details of EDGE's science capabilities are provided below.
EDGE also has the following secondary science goals:
The EDGE experiment will be a payload on a large balloon. It is expected to fly continuously for at least a week around the south pole during the month of January. If it is feasible for the balloon to stay aloft longer, it could collect data a week or two beyond the initial 7 days. The EDGE team hopes that the payload will be successfully recovered after the first run so that the experiment could fly a second time one year later. It is hoped that EDGE will fly as early as January of 2007.
The balloon will fly at a height of around 120,000 ft or 36 km. This is well suited to studying the universe in the infra-red as the telescope will be above the infra-red absorbing water vapor layers in the Earth's atmosphere. The reason for choosing the south pole for this experiment is that there is very little diurnal variation in the atmosphere to affect the height of the balloon significantly over the course of the flight. In addition, the south pole is devoted to scientific studies, making it an ideal location to carry out this experiment.
The region of sky that EDGE will observe is one of the darkest in the far-infrared with minimal Galactic dust foreground emission. This is shown in the figure below, which is centered on the south celestial pole. The total area of the sky that the observations will cover is around 400 sequare degrees (which is around 1% of the whole sky). It will be possible to obtain a signal to noise of around 4/pixel for the cosmic infra-red background by studying this region of sky. The area is also suitable for observation from a circumpolar balloon, since it is at a convenient declination and far from the Sun in January. The size of the region observed will allow for around 50 spatial scales to be sampled. Repeated observations of this region will enable EDGE to reduce instrumental drifts and noise as well as to reduce systematic effects caused by atmospheric emission.
EDGE's infra-red telescope is 1m in diameter and will be mounted on the balloon payload as seen in the schematic below:
The EDGE mission will make extensive use of hardware, software and expertise developed by the EDGE team for previous missions. For example, the telescope structure and primary mirror are existing hardware. The large degree of reuse at all levels minimizes development risk for major portions of the experiment.
The primary telescope of the EDGE experiment is 1 m in diameter. It will have a Frequency Selective Bolometer array as its detector. (An infrared bolometer measures changes in the heat input from the surroundings and converts this into a measurable quantity such as a voltage or current. A more general definition of a bolometer is at encyclopedia.com - opens in new window.) EDGE's bolometer array will provide a 7 pixel, 8 spectral band resolution, and allows for simultaneous observations of the source. This technology is essential for EDGE science because unambiguous detection of the CIB requires a spectral decomposition of the sky signal, using multiple spectral bands, in order to separate it from Galactic dust emission. The bolometer array is an innovative, compact, and extendable design that is applicable to upcoming space missions. Details of the EDGE spectral bands are shown in the table below. The angular resolution of EDGE is around 6'. It will spend around 150s on a patch of sky, for each pointing. And the total area of the sky that it will cover is around 400 sequare degrees (which is around 1% of the whole sky).
Additional Resources: (Links are correct as of Sept 2003, and all open in another window)