Breaking the Surface

Transcription

1 Breaking the Surface Like the seafarers of centuries ago, the scientists of HETDEX are sailing into uncharted waters Water covers about 70 percent of Earth s surface. And while the ocean depths are dark, mysterious, and largely unexplored, we at least understand what water is, and people have known its extent since sailors began mapping the oceans many centuries ago. But imagine that no one had ever seen an ocean or drawn the outlines of the continents. In such a world, a single stroll on a sandy beach, watching the waves roll ashore and feeling the cool salt spray in the air, would start a scientific revolution. Astronomers are at the dawn of such a revolution today. In the 1990s, they discovered a previously unknown dark energy that is causing the universe to expand faster as it ages. They calculated that this energy constitutes more than 70 percent of all the matter and energy in the universe. So far, though, scientists don t know what dark energy is. Learning about dark energy is far more difficult than sticking your toe in the ocean or toting a bucket of water back to the laboratory, though. Trying to find Dark energy is not only terribly important for astronomy, it s the central problem for physics. It s been the bone in our throat for a long time. Steven Weinberg, Nobel Laureate University of Texas at Austin TIM JONES/NASA/STScI something that they didn t even know existed until a few years ago will require scientists to devise clever ways to probe the universe and study the history of its birth and evolution, and engineers to design new tools to study them. One of those new tools is HETDEX, the Hobby-Eberly Telescope Dark Energy Experiment. An international team of astronomers and physicists will use the Hobby-Eberly Telescope at McDonald Observatory to study more than one million galaxies that are more than nine billion light-years from Earth, which means we are seeing them as they looked when the universe was less than one-third its present age. Measuring how fast these galaxies are moving away from us will reveal how the effects of dark energy have changed over time. And measuring the distribution of the galaxies will reveal details about the aftermath of the Big Bang, in which dark energy left its imprint. Their work will help us understand the vast cosmic ocean of dark energy.

2 Dark Universe While dark energy repels, dark matter attracts. And dark matter s influence shows up even in individual galaxies, while dark energy acts only on the scale of the entire universe Our universe may contain 100 billion galaxies, each with billions of stars, great clouds of gas and dust, and perhaps scads of planets and moons. The stars produce an abundance of energy, from radio waves to X-rays, which streaks across the universe at the speed of light. Yet everything that we can see is like the tip of the cosmic iceberg it accounts for only about four percent of the total mass and energy in the universe. About one-quarter of the universe consists of dark matter, which releases no detectable energy but exerts a gravitational pull on all the visible matter in the universe. Everything else more than two-thirds of everything in the entire universe consists of dark energy. Because of the names, it s easy to confuse dark matter and dark energy, but their effects are quite different. While dark matter pulls matter inward, dark energy pushes it outward. Also, while dark energy shows itself only on the largest cosmic scale, dark matter exerts its influence on individual galaxies as well as the universe at large. Astronomers discovered dark matter while studying the outer regions of our galaxy, the Milky Way. They found that stars at the edge of the galaxy s wide, flat disk were orbiting the center of the galaxy at much higher speed than expected. Calculations showed that the stars were pulled by the gravity of some unseen matter outside the galaxy s bright disk. Since then, observations of other galaxies and clusters of galaxies have confirmed dark matter s existence and allowed astronomers to calculate its abundance. Dark energy was discovered in the late 1990s when two teams of astronomers were trying to measure the rate at which the universe is expanding. Astronomers expected the expansion, which is a result of the Big Bang 13.7 billion years ago, to slow down as the gravity of all the visible galaxies and the invisible dark matter pulled on each other. Instead, they found that the expansion was accelerating. Later observations showed that the rate of expanion has been increasing for several billion years. DARK MATTER NORMAL MATTER DARK ENERY

3 What is Dark Energy? Like the dark side of the Moon or the 19th-century European concept of dark Africa, dark energy represents the unknown In the late 1990s, astronomers discovered that the universe is expanding faster today than they had expected. But they don t know what is causing the acceleration, so for now, they simply call it dark energy. Even so, theorists have developed several explanations for dark energy. HET- DEX will help them select the correct one. Vacuum Energy The early favorite is a concept known as vacuum energy. It suggests that space itself produces energy, which is pushing the universe outward. Vacuum energy could explain why the acceleration started fairly recently on the cosmic timescale. In the early universe, matter was packed much more densely than it is today, so there was less space between galaxies. ravity was the dominant force, slowing the acceleration of the universe that had been imparted in the Big Bang. In addition, since there was less space in the universe, and the vacuum energy comes from space itself, the energy played a smaller role. Today, 13.7 billion years after the Big Bang, the universe has grown much larger, so the galaxies are not so tightly packed. Their pull on each other is weakened, allowing the vacuum energy to play a more dominant role. Yet vacuum energy should be far too weak to account for the acceleration seen in the present-day universe, although it is the most complete scenario to date.??? in the early universe, but exert a powerful influence today. Flawed ravity For a century, Albert Einstein s Theory of eneral Relativity has reigned as the explanation for how gravity works. Yet there are problems with eneral Relativity and questions about its effects. All the experiments to date have confirmed eneral Relativity s effects on large scales, but scientists still aren t certain whether gravity has remained constant since the Big Bang, whether it acts the same in all regions of the universe, or whether it retains its grip on the very largest scales. It is possible that gravity weakens on the largest scales, thereby allowing the universe to expand faster as it ages. A Surprise Dark energy might prove to be something completely different from the possibilities that scientists are already considering. Part of the fun of science is that, like any journey into the unknown, you never know what you might find. New Physics Physicists who feel that vacuum energy has too many problems are looking for other solutions, and one contender is a new type of particle, created in the Big Bang, that would permeate the universe. Another contender is a field called quintessence. One key difference from the vacuum energy is that quintessence would vary with time, so it might not show up at all

4 Classroom Activities McDonald Observatory has developed classroom activities to help students understand astronomical observations and techniques, including several with application to dark energy. Journey Into Spectroscopy Subjects: Tools of the Astronomer rade Levels: 9-12 Students make their own spectroscope as they explore and observe spectra of familiar light sources. Extension activities expand their understanding of different kinds of spectra and sharpen their observing skills. mcdonaldobservatory.org/teachers/classroom/spectroscopy/ sp.html Color of Stars Subjects: Stars rade Levels: 9-12 Students observe colors in the flame of a burning candle to explore connections between matter, light, color, and temperature basic concepts of matter and energy. They elaborate on these basic concepts in a new context of astronomy and stars. The second half of the activity investigates star colors and relative sizes. stardate.org/teachers/plans/ color-stars A diffraction grating on one of the HET s instruments creates a rainbow effect around the reflected image of an astronomer. Exploring Light: The Optics of Refraction Subjects: Electromagnetic Radiation rade Levels: 9-12 Astronomers use diffraction of light to disperse (or spread out) colors of light from astronomical light sources into a spectrum. The spectrum is then used to measure the physical characteristics of that source. This activity provides an opportunity for hands-on understanding of the phenomenon of diffraction of light. mcdonaldobservatory.org/ teachers/classroom/diffraction/ Diffraction.html MARTIN HARRIS Telescope Technology Subjects: Tools of the Astronomer rade Levels: 6-8, 9-12 Large telescope designs have changed significantly over the last few decades, with a growing emphasis on using segmented mirrors. This activity series consists of four challenges that students complete to discover how and why astronomers design and use segmented-mirror telescopes. mcdonaldobservatory.org/teachers/classroom/ttt/ TelescopeTechnology.html HETDEX Collaboration HETDEX is a collaboration of The University of Texas at Austin, Pennsylvania State University, Texas A&M University, Universitats- Sternwärte Munich, Astrophysical Institute Potsdam, Institut fuer Astrophysik oettingen, and Max-Planck-Institut fuer Extraterrestrische Physik. Financial support is provided by the State of Texas, the United States Air Force, the National Science Foundation, and the generous contributions of many private foundations and individuals. Poster Credits Writer/Editor Damond Benningfield Design/Production Tim Jones Editorial Assistance Rebecca Johnson HETDEX.org Webmaster Doug Addison

5 HETDEX: Leading the Revolution HETDEX will use the Hobby-Eberly Telescope and innovative spectrographs to map the positions of more than one million galaxies HETDEX is leading the revolution in physics that will reveal the nature of dark energy. It combines the immense light-gathering power of the Hobby-Eberly Telescope, one of the world s largest, with an array of new instruments for analyzing the light from distant galaxies. During three years of observations, HETDEX will collect data on more than one million galaxies that are 9 billion to 11 billion light-years away, yielding the largest threedimensional map of the universe ever produced. The map will allow HETDEX astronomers to measure how fast the universe was expanding at different times in its history. Changes in the expansion rate will reveal the role of dark energy at different epochs. Various theories for dark energy predict different changes in the expansion rate, so by providing exact measurements of the expansion, HETDEX will eliminate some of the competing ideas. HETDEX also will study baryonic acoustic oscillations, which are sound waves produced during the first 400,000 years of the universe. Disturbances in the early universe created sound waves that rippled across the universe. At 400,000 years, the universe became cool and thin enough that the ripples became frozen at a unique size. The peaks of these waves formed a basic yardstick. As the universe expands, that yardstick is maintained, although it grows with the universe itself. HETDEX will find this yardstick by measuring the distances between galaxies at different times in the early universe. Careful analysis of the map of distant galaxies will reveal how the yardstick changed with time, telling us the size of the universe at different epochs. Comparing the size and expansion rate at different times in the history of the universe, which are influenced by the effect of dark energy, will reveal dark energy s true nature. HETDEX will produce its map by using a set of 150 spectrographs mounted on HET. They are known as VIRUS Visible Integral-Field Replicable Unit Spectrographs. These units will gather the light from distant galaxies and split the light into its individual wavelengths, known as a spectrum. A spectrum reveals an object s chemical composition, its temperature, and how fast it is moving toward or away from us. For distant galaxies, astronomers can convert its motion away from us into its distance, producing precise 3-D maps. Full data-gathering operations will begin in 2012, with the survey completed three years later. HETDEX will then provide the first major test of the evolution of dark energy with time, providing a key understanding of the nature of dark energy. TIM JONES/DAMOND BENNINFIELD HETDEX and other searches will map galaxies at different times in the history of the universe to try to find the imprint of sound waves from the very early universe. Left: A field of galaxies for study. Center: Astronomers will measure the distances between millions of galaxy pairs, and use statistics to find a common scale length. Right: This common length is the imprint of the early sound waves, which shows up as ripples in the distribution of galaxies. The scale length changes as the universe ages, so determining that length at different epochs will reveal how the expansion rate of the universe has changed over time.

6 Scanning alactic Barcodes HETDEX will measure the distances and velocities to its target galaxies through the cosmic barcodes known as spectra HETDEX will determine the distances to more than one million galaxies and the speeds at which the galaxies are moving away from us. These measurements will show how the expansion of the universe has changed over time, which in turn will reveal the influence of dark energy. Measuring distances to astronomical objects is tricky because even the closest stars are extremely far away. So astronomers have devised a distance ladder than provides several steps for measuring distances. The first step is the most direct. Astronomers plot a star s position throughout the year and see how it moves against the background of more-distant stars. It s like holding your finger at arm s length and looking at it through first one eye and then the other the finger appears to shift position with respect to background objects. The same thing applies to stars. Those that are close will appear to shift position back and forth by a tiny amount. The shift is called parallax, and the size of the back-andforth shift reveals the star s distance. Each step after that increases the scale to greater and greater distances: variable stars known as Cepheids, a particular class of exploding stars, and the redshift in the light of distant galaxies. (See graphic at right.) To obtain the distances to galaxies, HETDEX will measure each galaxy s redshift, which is the stretching of its lightwaves due to the expansion of the universe. The tool for measuring redshift is a spectrograph, which splits the light from an astronomical object into its individual wavelengths or colors. Each chemical element produces its own unique signature, which shows up as an increase or decrease in the intensity of light at specific wavelengths. That produces a series of bright or dark lines in an object s spectrum, like a celestial barcode. From these barcodes, astronomers can measure the composition of a galaxy, star, or other object. If an object is moving toward us, its light waves are compressed, so the wavelengths of its component elements are shifted toward the blue end of the spectrum. But if an object is moving away from us, its light waves are stretched out, producing a shift toward the red end of the spectrum. Redshift is a powerful tool for analyzing galaxies because all but a handful of them are moving away from us as a result of the expansion of the universe. The greater the redshift, the faster the galaxy is receding, and therefore the farther away it must be. A spectrograph breaks an astronomical object s light into its individual wavelengths, forming a rainbow of color. Each chemical element leaves a unique imprint in this rainbow a series of dark lines like a barcode. If the object is moving away from the observer, the lines are shifted toward the red end of the spectrum.

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8 Upgrading for the Future Hobby-Eberly Telescope, one of the world s largest, will provide wide, deep views of distant galaxies HETDEX has two critical technological requirements. One is a set of instruments that can quickly gather and analyze the light from many distant galaxies at once. The other is a giant telescope that can see far into the universe, providing good views of galaxies that existed when the universe was only a few billion years old. The first requirement is met by VIRUS, a set of 150 spectrographs. The second is met by the Hobby-Eberly Telescope (HET), one of the largest optical telescopes in the world. A set of upgrades will improve its capabilities and provide an even more powerful platform for HETDEX. The HET s mirror consists of 91 identical six-sided segments that fit together like the tiles on a floor. Each segment, made from a glass/ceramic material, is one meter across and two inches thick. Small computer-controlled motors adjust the positions of individual segments to maintain the proper overall mirror shape. The combined segments yield a light-gathering surface equivalent to a single 11-meter (36-foot) telescope. Not all of this surface can be used at one time, though, because unlike most other telescopes, HET s mirror is always tilted at the same angle, 55 degrees above the horizon. To compensate for this, a tracking system is installed at the top of the telescope above the primary mirror. As the tracker moves, it captures light from different portions of the mirror. With this system, up to 9.2 meters of the mirror can be used for any given observation. The telescope rotates between exposures to view different regions of the sky. The combination of this rotation and the tracker s motion allows the HET to cover 70 percent of the sky that is visible from McDonald Observatory. A series of upgrades is increasing HET s capabilities for HETDEX. A new tracker assembly at the top of the telescope incorporates new optics that greatly widen the field of view. The wider view is critical for HETDEX because the project must survey a large swatch of sky to amass data on enough galaxies to meet its goal. Without the larger field of view, it would take decades to complete the project. The new tracker is larger and heavier than the original unit to accommodate new electronics and other critical telescope systems. The upgrades will not only accommodate HETDEX, they will improve the telescope s overall science capabilities for many other research projects. HETDEX will search a large region of the sky overlapping the Big Dipper. While the Dipper s stars are only a few dozen light-years away, though, the target galaxies are up to 10 billion light-years away. The targets are young, vigorous lyman-alpha galaxies, which are giving birth to new stars. Energy from these young stars excites the hydrogen gas around them, producing a bright glow. MARTIN HARRIS TIM JONES