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Figure 16. The star formation history of the universe is illustrated, as well as important milestones in the evolution of the universe, from the Big Bang to the current epoch. Credit: A. Feild (STSci).

It is important to realize that at this point, no stars have formed. There are no self-luminous bodies, only expanding and cooling gas with intermittent eddies and ripples of local density enhancements. The term Dark Age is apt, because we are thoroughly "in the dark" about what happened next in the development of the universe. Clearly, stars must have formed at some point but the formation mechanism in these unique conditions is poorly understood at best. We also know that these first stars did not survive in great numbers, since they would have been metal free (metals were only formed through stars, not the Big Bang), yet we see metals in all of the stars observed to date. In addition, the universe must have been flooded with enough UV radiation to return back to an ionized state, which in turn suggests a radiation source consistent with massive stars at some point in the early universe. While we can look back across time to the CMB and study this event through ever more detailed satellite observations, somewhere in the foreground lie the first stars and one of the great quests in astronomy is to find them. At least a couple of things are sure about what will be required to detect these mysterious objects. It will (1) either take a large ground based or space based telescope and (2) observations will have to be made in the near-infrared, since light from these stars will be so highly red-shifted that the peak in their spectral energy distributions will fall in the 1-2 ^m range. CCDs will be of no use in this quest - like the Galactic center, this cutting-edge research will rely exclusively on infrared array technology.

There are applications for CCDs in similarly grandiose future explorations of the universe. These applications will be based upon observations of galaxies that are much closer in space and time than the so-called first light targets mentioned before. Surveys to date have employed 8-12 CCDs in a single mosaic and have been used to map large swaths of the sky to measure the three-dimensional distribution of galaxies. The surveys of the future will be far greater in scope and include observations of the entire sky on short timescales to detect synoptic phenomena that current relatively narrow field telescopes are effectively blind to. Science applications of such observations are manifold and include mapping the structure of galaxies across a ~1000 deg2 field and using weak lensing as a probe of dark matter across the voids between the galaxies spread across the universe. This application is particularly important, as it stands to "shed light" on one of the greatest mysteries of the universe. Observations of the motions of stars and gas in galaxies have long been known not follow the speeds expected, given the amount of luminous matter present in these galaxies and simple gravitational laws. A nearly endless string of theories has been postulated in recent decades about what this so-called missing mass is, ranging from failed stars (brown dwarves) to exotic massive particles that have the bizarre property of not interacting with radiation. Thus far, none have been proven to account for dark matter. Since astronomers rely almost exclusively on studying the radiation from distant objects to learn about the universe, they are at an enormous disadvantage in studying dark matter, which appears to be oblivious to all forms of radiation. It does interact gravitationally with normal or baryonic matter though. Thus far one of the best probes of the nature of dark matter has been gravitational lensing as light passes through dark matter clumps to map out its spatial distribution. In massive galaxy clusters this is relatively easy to do. However, to map the distribution of dark matter across the entire cosmos requires imaging field galaxies across great expanses of space to detect minute deformation in the shapes of galaxies that is characteristic of intervening dark matter. The objective of such measurements is to measure the temperature and clumpiness of dark matter with high fidelity over various spatial scales. These data can be used to constrain the multitude of theories in astronomy which attempt to explain dark matter. Ultimately, the importance of dark matter lies not just in the enigma which it poses, but in its overwhelming quantity, because dark matter is by far the dominant form of matter in the universe. What we know as normal or baryonic matter, and learned about in high school chemistry through the periodic table, has little to do with dark matter. Stars, planets, and even humans are the exception in a universe dominated by dark matter.

The enormous surveys planned to probe weak lensing across the sky will require Giga-pixel class focal planes that sample 2-3 degrees of the sky in a single observation. Figure 17 shows some examples of the new observatories planned that will rely exclusively on camera systems incorporating >109 pixels. To reduce the overhead of such enormous data streams to acceptable levels will require the use of advanced high speed and highly multiplexed array controllers. These are capable of reading out enormous focal planes in just seconds - something that many modern controllers cannot achieve with much smaller mosaics.

Figure 17. A couple examples of next-generation observatories employing Giga-pixel class optical focal planes: (left) the LSST and (right) Pan-STARRS.

Another great mystery of our time is dark energy - a more recent discovery that arguably has even deeper consequences for the universe than dark matter. Observations of supernovae during the past decade have been used to measure the expansion rate of the universe. In essence, a certain type of supernova is a fairly accurate "standard candle", hence measuring the brightness of supernovae as a function of red-shift is a gauge of their distances. Though simple in concept these measurements require considerable finesse to properly deal with systematic uncertainties, not to mention an unorthodox technique in which large telescopes (necessary to measure the spectra of distant supernovae) are called into service on very short notice to measure spectra during the early peak phase in their apparent brightness evolution. As shown in Figure 18, these measurements indicate a slow roll-off in the measured brightness of supernovae compared to what is expected, demonstrating that they are further than expected and that the universe is in fact expanding at an accelerating rate. This discovery has triggered an enormous amount of research in astronomy, as the search continues for the source of energy behind this expansion. Like dark matter, the underlying physics behind dark energy remains essentially unknown, though theoreticians have characteristically left us awash with possibilities. Like the early days after the discovery of the Big Bang, a broad range of theories were posed in astronomy to explain the earliest moments in the universe. However it was not until follow-up observations, particularly by COBE and WMAP, that the vast majority of these theories were ruled out, leaving us with the demonstrably sound theoretically basis we have today for the origin of the universe. New observations will be used to systematically reduce the theoretical possibilities behind dark energy. In the process, they will leave us with a much more complete picture of our universe.

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Figure 18. Adapted from Riess et al. [5], the downturn in the brightness of supernovae at large red-shifts indicates the acceleration of the universe.

Figure 18. Adapted from Riess et al. [5], the downturn in the brightness of supernovae at large red-shifts indicates the acceleration of the universe.

Dark energy has a number of important implications, beyond opening new avenues of physics. For example, prior to this discovery it was understood that the universe is slowly decelerating from its initial expansion, with progressively finer measurements used to pin-down this expansion rate.

With dark energy the ultimate fate of the universe now appears to involve ultimately shearing apart the large scale structures we commonly see today, like galaxies, stellar clusters, and even stars and planets. Beyond the obvious question of where this energy comes from is that of why did it begin to dominate the energy density of the universe ~5 Gyr ago. Prior to that time the universe was a matter dominated realm but now dark energy accounts for ~70% of the energy density content of the universe. This even has certain anthropic implications, since it appears impossible for life to exist early in the universe (insufficient time for stars to form, which generated the heavy elements necessary for life), and it appears impossible for life to exist late in the universe due to its accelerated expansion. Hence we appear to live in a unique phase over the lifetime of the universe.

Key to understanding dark energy will be a range of observations made over the next decade by various instruments. Among these will be next-generation wide field spectrometers offering the same sorts of enormous multiplex gains over current spectrometers as the imagers of tomorrow. Figure 19 shows a couple of examples of optical spectrometers that will provide nearly an order of magnitude multiplex gain over any astronomical spectrometers existing today.

Figure 19. Next-generation spectrometers with multiplex gains are nearly an order of magnitude greater than anything available today. (left) The Gemini/Subaru WFMOS, shown deployed on Subaru. (Right) LAMOST, which is being built in China.

One way such spectrometers may be used for dark energy measurements is to make large scale measurements of the 3D distribution of galaxies at different epochs to evaluate the time-evolution of the expansion of the universe using a "standard ruler" metric. Specifically, after the Big Bang there were low-level local density peaks that propagated across the then highly-compact universe at the speed of sound. The physics behind the propagation of sound waves in the fluid medium of the early universe is quite simple, since structure was nearly absent and the universe existed in a very well mixed and uniform phase. As mentioned before, when the universe went dark, due to the temperature and pressure dropping to the point that neutral atoms could form, is the time when these physical over-densities were frozen into the tapestry of matter, forever acting as the seeds of collapse for the largest structures in the universe. Evaluation of the power spectrum of spatial structure in the CMB reveals that most structure existed on the ~1° scale at this point in the evolution of the universe (see Figure 20).

Angular Scale

Angular Scale

Figure 20. Adapted from Bennett et al. [6], these oscillations on the power spectrum of structure in the universe, as derived from CMB observations, demonstrate a "preferred" spatial scale or standard ruler in the universe.

Thereafter in time, or at closer distances in our line-of-sight to the edge of the universe, emission that we detect is essentially due to stars associated with galaxies, all of which formed in clusters with some typical spatial scale. This is where the next generation of highly multiplexed spectrometers come into play. These machines will be used to map out the three-dimensional structure of the universe across vast regions of space. Evaluating the spatial power spectrum of these enormous distributions of matter (galaxies) will yield a set of baryonic power spectrum "wiggles" which are the tell-tale signatures of the initial seeds of structure, dating back to the so-called dark ages in the history of the universe. This characteristic spatial scale or "standard ruler" is essentially an invariant metric unless the fabric of spacetime is changing over time due, for example, to dark energy. By measuring the length of this standard ruler at different epochs or look-back times in the universe, it will be possible to measure the time-evolution of dark energy to much higher fidelity than any measurement achieved to date, thereby providing important clues about its origin and nature.

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