Measuring elusive solar neutrinos flowing through the Earth, physicists learn more about the sun

ScienceDaily (Oct. 11, 2011) — Using one of the most sensitive neutrino detectors on the planet, an international team including physicists Laura Cadonati and Andrea Pocar at the University of Massachusetts Amherst are now measuring the flow of solar neutrinos reaching earth more precisely than ever before. The detector probes matter at the most fundamental level and provides a powerful tool for directly observing the sun's composition.

Pocar, Cadonati and colleagues report in the current issue of Physical Review Letters that the Borexino instrument has now measured with high precision the flux of the beryllium seven (7Be) solar neutrino, abundant, low-energy particles once below the observable threshold. With this advance, they can now precisely study the behavior of solar neutrinos with kinetic energy below 1 megaelectron volt (MeV). Borexino scientists also recently reported the first observation of neutrinos produced in a little-studied solar nuclear process known as proton-electron-proton, or pep, and set of stringent limit on reactions involving carbon, nitrogen and oxygen (the CNO cycle) in the sun.

Cadonati says, "Borexino is the only detector capable of observing the entire spectrum of solar neutrinos at once. Our results, the culmination of 20 years of research, greatly narrow the observation precision. The data confirm the neutrino oscillations, flavor changes and flow predicted by models of the sun and particle physics."

Of particular interest, Pocar and Cadonati note, is the Borexino instrument's ability to more thoroughly test neutrino oscillation parameters, allowing an exploration of their characteristic non-zero mass, which does not fit the Standard Model of particle physics. "Our data can tell us about fundamental micro physics at the particle level," says Cadonati. "Borexino is using neutrinos to explore the interior of the sun, looking for new, exciting clues to the mysteries of the universe we cannot see." Pocar adds, "Our detector provides stringent tests of the three-neutrino oscillations model."

Solar neutrinos are produced in nuclear processes and radioactive decays of several elements during fusion reactions at the sun's core. As many as 65 billion of them stream out of the sun and hit every square centimeter of Earth's surface [or 420 billion every square inch] every second. But because they only interact through the nuclear weak force they pass through matter unaffected, making them very difficult to detect and to distinguish from the trace nuclear decays of ordinary materials. The weak force is one of the four fundamental forces of nature, with gravity, electromagnetism and the strong force. It is responsible for the radioactive decay of unstable subatomic particles, with a short range of influence, about 1 percent of the diameter of a typical atomic nucleus.

The Borexino instrument, housed far beneath Italy's Apennine Mountains, detects neutrinos as they interact with an ultra-pure organic liquid scintillator at the center of a large sphere surrounded by 2,000 tons of water. Its great depth and many onion-like protective layers maintain the core as the most radiation-free medium on the planet.

There are three neutrino types, or "flavors": electron, muon and tau. Those produced in the sun are the electron type. As they travel away from their birthplace, they oscillate, or change from one flavor to another. A detector like Borexino can observe all three types in real time and measures each one's energy, but it cannot distinguish between them. It's more sensitive to the electron type so they are more likely to be seen.

The 7Be neutrino flux now being detected by Borexino is predicted by the standard solar model to make up about 10 percent of solar flow, Cadonati says. Earlier instruments in Canada and Japan designed to detect higher-energy neutrinos had already observed evidence of their flavor oscillations, probing 1/10,000 of the solar neutrino flux and their oscillations as they travel through solar matter. However, without data in the low-energy range as scanned by Borexino, physicists were not able to confirm the specific energy-dependent effect of solar neutrino oscillations. Borexino has now filled this gap and for the first time observed evidence of neutrino oscillation in vacuum, as they travel between the sun and Earth.

Pocar says that from the astrophysics angle, Borexino's ability to conduct "precision physics" experiments and collect a large number of observations, with concomitant higher statistical power, is yielding data that show how our sun works. As for the possibility of discovering a new kind of neutrino coming from the sun, which is allowed by some theoretical extensions to the Standard Model of particle physics, he adds, "You always have the hope of seeing surprises, some small deviation from the expectations. And this you can only have if your accuracy and precision are good enough to see very small variations."

In a companion paper, the Borexino team says their 7Be solar neutrino flux measurements show no flow differences between day and night. Some had hypothesized that one might exist because neutrinos pass through Earth's bulk at night. But Pocar says, "The traverse through the Earth seems not to change neutrinos' flavor."

In the future, the researchers hope to identify the origin of every neutrino type coming from the sun, particularly to assess the relative levels of carbon, nitrogen and oxygen there, to deepen understanding of how the sun evolved and how its workings are related to that of larger stars.

Borexino is an international collaboration funded by the U.S. National Science Foundation, the Italian National Institute for Nuclear Physics (INFN) which manages the Gran Sasso labs and similar organizations in Germany, Russia and Poland.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Massachusetts at Amherst.

Journal Reference:

G. Bellini, J. Benziger, D. Bick, S. Bonetti, G. Bonfini, M. Buizza Avanzini, B. Caccianiga, L. Cadonati, F. Calaprice, C. Carraro, P. Cavalcante, A. Chavarria, D. D’Angelo, S. Davini, A. Derbin, A. Etenko, K. Fomenko, D. Franco, C. Galbiati, S. Gazzana, C. Ghiano, M. Giammarchi, M. Goeger-Neff, A. Goretti, L. Grandi, E. Guardincerri, S. Hardy, Aldo Ianni, Andrea Ianni, V. Kobychev, D. Korablev, G. Korga, Y. Koshio, D. Kryn, M. Laubenstein, T. Lewke, E. Litvinovich, B. Loer, F. Lombardi, P. Lombardi, L. Ludhova, I. Machulin, S. Manecki, W. Maneschg, G. Manuzio, Q. Meindl, E. Meroni, L. Miramonti, M. Misiaszek, D. Montanari, P. Mosteiro, V. Muratova, L. Oberauer, M. Obolensky, F. Ortica, M. Pallavicini, L. Papp, C. Peña-Garay, L. Perasso, S. Perasso, A. Pocar, R. Raghavan, G. Ranucci, A. Razeto, A. Re, A. Romani, A. Sabelnikov, R. Saldanha, C. Salvo, S. Schönert, H. Simgen, M. Skorokhvatov, O. Smirnov, A. Sotnikov, S. Sukhotin, Y. Suvorov, R. Tartaglia, G. Testera, D. Vignaud, R. Vogelaar, F. von Feilitzsch, J. Winter, M. Wojcik, A. Wright, M. Wurm, J. Xu, O. Zaimidoroga, S. Zavatarelli, G. Zuzel. Precision Measurement of the 7Be Solar Neutrino Interaction Rate in Borexino. Physical Review Letters, 2011; 107 (14) DOI: 10.1103/PhysRevLett.107.141302

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

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Measuring elusive solar neutrinos flowing through the Earth, physicists learn more about the sun

ScienceDaily (Oct. 11, 2011) — Using one of the most sensitive neutrino detectors on the planet, an international team including physicists Laura Cadonati and Andrea Pocar at the University of Massachusetts Amherst are now measuring the flow of solar neutrinos reaching earth more precisely than ever before. The detector probes matter at the most fundamental level and provides a powerful tool for directly observing the sun's composition.

Pocar, Cadonati and colleagues report in the current issue of Physical Review Letters that the Borexino instrument has now measured with high precision the flux of the beryllium seven (7Be) solar neutrino, abundant, low-energy particles once below the observable threshold. With this advance, they can now precisely study the behavior of solar neutrinos with kinetic energy below 1 megaelectron volt (MeV). Borexino scientists also recently reported the first observation of neutrinos produced in a little-studied solar nuclear process known as proton-electron-proton, or pep, and set of stringent limit on reactions involving carbon, nitrogen and oxygen (the CNO cycle) in the sun.

Cadonati says, "Borexino is the only detector capable of observing the entire spectrum of solar neutrinos at once. Our results, the culmination of 20 years of research, greatly narrow the observation precision. The data confirm the neutrino oscillations, flavor changes and flow predicted by models of the sun and particle physics."

Of particular interest, Pocar and Cadonati note, is the Borexino instrument's ability to more thoroughly test neutrino oscillation parameters, allowing an exploration of their characteristic non-zero mass, which does not fit the Standard Model of particle physics. "Our data can tell us about fundamental micro physics at the particle level," says Cadonati. "Borexino is using neutrinos to explore the interior of the sun, looking for new, exciting clues to the mysteries of the universe we cannot see." Pocar adds, "Our detector provides stringent tests of the three-neutrino oscillations model."

Solar neutrinos are produced in nuclear processes and radioactive decays of several elements during fusion reactions at the sun's core. As many as 65 billion of them stream out of the sun and hit every square centimeter of Earth's surface [or 420 billion every square inch] every second. But because they only interact through the nuclear weak force they pass through matter unaffected, making them very difficult to detect and to distinguish from the trace nuclear decays of ordinary materials. The weak force is one of the four fundamental forces of nature, with gravity, electromagnetism and the strong force. It is responsible for the radioactive decay of unstable subatomic particles, with a short range of influence, about 1 percent of the diameter of a typical atomic nucleus.

The Borexino instrument, housed far beneath Italy's Apennine Mountains, detects neutrinos as they interact with an ultra-pure organic liquid scintillator at the center of a large sphere surrounded by 2,000 tons of water. Its great depth and many onion-like protective layers maintain the core as the most radiation-free medium on the planet.

There are three neutrino types, or "flavors": electron, muon and tau. Those produced in the sun are the electron type. As they travel away from their birthplace, they oscillate, or change from one flavor to another. A detector like Borexino can observe all three types in real time and measures each one's energy, but it cannot distinguish between them. It's more sensitive to the electron type so they are more likely to be seen.

The 7Be neutrino flux now being detected by Borexino is predicted by the standard solar model to make up about 10 percent of solar flow, Cadonati says. Earlier instruments in Canada and Japan designed to detect higher-energy neutrinos had already observed evidence of their flavor oscillations, probing 1/10,000 of the solar neutrino flux and their oscillations as they travel through solar matter. However, without data in the low-energy range as scanned by Borexino, physicists were not able to confirm the specific energy-dependent effect of solar neutrino oscillations. Borexino has now filled this gap and for the first time observed evidence of neutrino oscillation in vacuum, as they travel between the sun and Earth.

Pocar says that from the astrophysics angle, Borexino's ability to conduct "precision physics" experiments and collect a large number of observations, with concomitant higher statistical power, is yielding data that show how our sun works. As for the possibility of discovering a new kind of neutrino coming from the sun, which is allowed by some theoretical extensions to the Standard Model of particle physics, he adds, "You always have the hope of seeing surprises, some small deviation from the expectations. And this you can only have if your accuracy and precision are good enough to see very small variations."

In a companion paper, the Borexino team says their 7Be solar neutrino flux measurements show no flow differences between day and night. Some had hypothesized that one might exist because neutrinos pass through Earth's bulk at night. But Pocar says, "The traverse through the Earth seems not to change neutrinos' flavor."

In the future, the researchers hope to identify the origin of every neutrino type coming from the sun, particularly to assess the relative levels of carbon, nitrogen and oxygen there, to deepen understanding of how the sun evolved and how its workings are related to that of larger stars.

Borexino is an international collaboration funded by the U.S. National Science Foundation, the Italian National Institute for Nuclear Physics (INFN) which manages the Gran Sasso labs and similar organizations in Germany, Russia and Poland.

Recommend this story on Facebook, Twitter,
and Google +1:

Other bookmarking and sharing tools:

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Massachusetts at Amherst.

Journal Reference:

G. Bellini, J. Benziger, D. Bick, S. Bonetti, G. Bonfini, M. Buizza Avanzini, B. Caccianiga, L. Cadonati, F. Calaprice, C. Carraro, P. Cavalcante, A. Chavarria, D. D’Angelo, S. Davini, A. Derbin, A. Etenko, K. Fomenko, D. Franco, C. Galbiati, S. Gazzana, C. Ghiano, M. Giammarchi, M. Goeger-Neff, A. Goretti, L. Grandi, E. Guardincerri, S. Hardy, Aldo Ianni, Andrea Ianni, V. Kobychev, D. Korablev, G. Korga, Y. Koshio, D. Kryn, M. Laubenstein, T. Lewke, E. Litvinovich, B. Loer, F. Lombardi, P. Lombardi, L. Ludhova, I. Machulin, S. Manecki, W. Maneschg, G. Manuzio, Q. Meindl, E. Meroni, L. Miramonti, M. Misiaszek, D. Montanari, P. Mosteiro, V. Muratova, L. Oberauer, M. Obolensky, F. Ortica, M. Pallavicini, L. Papp, C. Peña-Garay, L. Perasso, S. Perasso, A. Pocar, R. Raghavan, G. Ranucci, A. Razeto, A. Re, A. Romani, A. Sabelnikov, R. Saldanha, C. Salvo, S. Schönert, H. Simgen, M. Skorokhvatov, O. Smirnov, A. Sotnikov, S. Sukhotin, Y. Suvorov, R. Tartaglia, G. Testera, D. Vignaud, R. Vogelaar, F. von Feilitzsch, J. Winter, M. Wojcik, A. Wright, M. Wurm, J. Xu, O. Zaimidoroga, S. Zavatarelli, G. Zuzel. Precision Measurement of the 7Be Solar Neutrino Interaction Rate in Borexino. Physical Review Letters, 2011; 107 (14) DOI: 10.1103/PhysRevLett.107.141302

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

'Biological circuit' components developed; New microscope technique for measuring them

ScienceDaily (June 9, 2011) — Electrical engineers have long been toying with the idea of designing biological molecules that can be directly integrated into electronic circuits. University of Pennsylvania researchers have developed a way to form these structures so they can operate in open-air environments, and, more important, have developed a new microscope technique that can measure the electrical properties of these and similar devices.

The research was conducted by Dawn Bonnell, Trustee Chair Professor and director of the Nano/Bio Interface Center, graduate students Kendra Kathan-Galipeau and Maxim Nikiforov and postdoctoral fellow Sanjini Nanayakkara, all of the Department of Materials Science and Engineering in Penn's School of Engineering and Applied Science. They collaborated with assistant professor Bohdana Discher of the Department of Biophysics and Biochemistry at Penn's Perelman School of Medicine and Paul A. O'Brien, a graduate student in Penn's Biotechnology Masters Program.

Their work was published in the journal ACS Nano.

The development involves artificial proteins, bundles of peptide helices with a photoactive molecule inside. These proteins are arranged on electrodes, which are common feature of circuits that transmit electrical charges between metallic and non-metallic elements. When light is shined on the proteins, they convert photons into electrons and pass them to the electrode.

"It's a similar mechanism to what happens when plants absorb light, except in that case the electron is used for some chemistry that creates energy for the plant," Bonnell said. "In this case, we want to use the electron in electrical circuits."

Similar peptide assemblies had been studied in solution before by several groups and had been tested to show that they indeed react to light. But there was no way to quantify their ambient electrical properties, particularly capacitance, the amount of electrical charge the assembly holds.

"It's necessary to understand these kinds of properties in the molecules in order to make devices out of them. We've been studying silicon for 40 years, so we know what happens to electrons there," Bonnell said. "We didn't know what happens to electrons on dry electrodes with these proteins; we didn't even know if they would remain photoactive when attached to an electrode."

Designing circuits and devices with silicon is inherently easier than with proteins. The electrical properties of a large chunk of a single element can be measured and then scaled down, but complex molecules like these proteins cannot be scaled up. Diagnostic systems that could measure their properties with nanometer sensitivity simply did not exist.

The researchers therefore needed to invent both a new way of a measuring these properties and a controlled way of making the photovoltaic proteins that would resemble how they might eventually be incorporated into devices in open-air, everyday environments, rather than swimming in a chemical solution.

To solve the first problem, the team developed a new kind of atomic force microscope technique, known as torsional resonance nanoimpedance microscopy. Atomic force microscopes operate by bringing an extremely narrow silicon tip very close to a surface and measuring how the tip reacts, providing a spatial sensitivity of a few nanometers down to individual atoms.

"What we've done in our version is to use a metallic tip and put an oscillating electric field on it. By seeing how electrons react to the field, we're able to measure more complex interactions and more complex properties, such as capacitance," Bonnell said.

Bohdana Discher's group designed the self-assembling proteins much as they had done before but took the additional step of stamping them onto sheets of graphite electrodes. This manufacturing principle and the ability to measure the resulting devices could have a variety of applications.

"Photovoltaics -- solar cells -- are perhaps the easiest to imagine, but where this work is going in the shorter term is biochemical sensors," Bonnell said.

Instead of reacting to photons, proteins could be designed to produce a charge when in the presence of a certain toxins, either changing color or acting as a circuit element in a human-scale gadget.

This research was supported by the Nano/Bio Interface Center and the National Science Foundation.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Pennsylvania.

Journal Reference:

Kendra Kathan-Galipeau, Sanjini Nanayakkara, Paul A. O’Brian, Maxim Nikiforov, Bohdana M. Discher, Dawn A. Bonnell. Direct Probe of Molecular Polarization inDe NovoProtein–Electrode Interfaces. ACS Nano, 2011; 110603081000090 DOI: 10.1021/nn200887n

Note: If no author is given, the source is cited instead.

Disclaimer: This article is not intended to provide medical advice, diagnosis or treatment. Views expressed here do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

'Biological circuit' components developed; New microscope technique for measuring them

ScienceDaily (June 9, 2011) — Electrical engineers have long been toying with the idea of designing biological molecules that can be directly integrated into electronic circuits. University of Pennsylvania researchers have developed a way to form these structures so they can operate in open-air environments, and, more important, have developed a new microscope technique that can measure the electrical properties of these and similar devices.

The research was conducted by Dawn Bonnell, Trustee Chair Professor and director of the Nano/Bio Interface Center, graduate students Kendra Kathan-Galipeau and Maxim Nikiforov and postdoctoral fellow Sanjini Nanayakkara, all of the Department of Materials Science and Engineering in Penn's School of Engineering and Applied Science. They collaborated with assistant professor Bohdana Discher of the Department of Biophysics and Biochemistry at Penn's Perelman School of Medicine and Paul A. O'Brien, a graduate student in Penn's Biotechnology Masters Program.

Their work was published in the journal ACS Nano.

The development involves artificial proteins, bundles of peptide helices with a photoactive molecule inside. These proteins are arranged on electrodes, which are common feature of circuits that transmit electrical charges between metallic and non-metallic elements. When light is shined on the proteins, they convert photons into electrons and pass them to the electrode.

"It's a similar mechanism to what happens when plants absorb light, except in that case the electron is used for some chemistry that creates energy for the plant," Bonnell said. "In this case, we want to use the electron in electrical circuits."

Similar peptide assemblies had been studied in solution before by several groups and had been tested to show that they indeed react to light. But there was no way to quantify their ambient electrical properties, particularly capacitance, the amount of electrical charge the assembly holds.

"It's necessary to understand these kinds of properties in the molecules in order to make devices out of them. We've been studying silicon for 40 years, so we know what happens to electrons there," Bonnell said. "We didn't know what happens to electrons on dry electrodes with these proteins; we didn't even know if they would remain photoactive when attached to an electrode."

Designing circuits and devices with silicon is inherently easier than with proteins. The electrical properties of a large chunk of a single element can be measured and then scaled down, but complex molecules like these proteins cannot be scaled up. Diagnostic systems that could measure their properties with nanometer sensitivity simply did not exist.

The researchers therefore needed to invent both a new way of a measuring these properties and a controlled way of making the photovoltaic proteins that would resemble how they might eventually be incorporated into devices in open-air, everyday environments, rather than swimming in a chemical solution.

To solve the first problem, the team developed a new kind of atomic force microscope technique, known as torsional resonance nanoimpedance microscopy. Atomic force microscopes operate by bringing an extremely narrow silicon tip very close to a surface and measuring how the tip reacts, providing a spatial sensitivity of a few nanometers down to individual atoms.

"What we've done in our version is to use a metallic tip and put an oscillating electric field on it. By seeing how electrons react to the field, we're able to measure more complex interactions and more complex properties, such as capacitance," Bonnell said.

Bohdana Discher's group designed the self-assembling proteins much as they had done before but took the additional step of stamping them onto sheets of graphite electrodes. This manufacturing principle and the ability to measure the resulting devices could have a variety of applications.

"Photovoltaics -- solar cells -- are perhaps the easiest to imagine, but where this work is going in the shorter term is biochemical sensors," Bonnell said.

Instead of reacting to photons, proteins could be designed to produce a charge when in the presence of a certain toxins, either changing color or acting as a circuit element in a human-scale gadget.

This research was supported by the Nano/Bio Interface Center and the National Science Foundation.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Pennsylvania.

Journal Reference:

Kendra Kathan-Galipeau, Sanjini Nanayakkara, Paul A. O’Brian, Maxim Nikiforov, Bohdana M. Discher, Dawn A. Bonnell. Direct Probe of Molecular Polarization inDe NovoProtein–Electrode Interfaces. ACS Nano, 2011; 110603081000090 DOI: 10.1021/nn200887n

Note: If no author is given, the source is cited instead.

Disclaimer: This article is not intended to provide medical advice, diagnosis or treatment. Views expressed here do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Measuring the distant universe in 3-D using light from 14,000 quasars

ScienceDaily (May 2, 2011) — The biggest 3-D map of the distant universe ever made, using light from 14,000 quasars -- supermassive black holes at the centers of galaxies billions of light years away -- has been constructed by scientists with the third Sloan Digital Sky Survey (SDSS-III).

The map is the first major result from the Baryon Oscillation Spectroscopic Survey (BOSS), SDSS-III's largest survey, whose principal investigator is David Schlegel of the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab). The huge new map was presented at the April meeting of the American Physical Society in Anaheim, CA, by Anže Slosar of Brookhaven National Laboratory.

BOSS is the first attempt to use baryon acoustic oscillation (BAO) as a precision tool to measure dark energy. Baryon oscillation refers to how matter clumps in a regular way throughout the universe, a physical manifestation of the expansion of the universe. Until now, 3-D maps showing this oscillation have been based on the distribution of visible galaxies. BOSS is the first survey to map intergalactic hydrogen gas as well, using distant quasars whose light is produced by supermassive black holes at the centers of active galaxies.

"Quasars are the brightest objects in the universe, which we use as convenient backlights to illuminate the intervening hydrogen gas that fills the universe between us and them," Slosar says. "We can see their shadows, and the details in their shadows" -- specifically, the absorption features in their spectra known as the Lyman-alpha forest -- "allowing us to see how the gas is clumped along our line of sight. The amazing thing is that this allows us to see the universe so very far away, where measuring positions of individual galaxies in large numbers is impractical."

"BOSS is the first attempt to use the Lyman-alpha forest to measure dark energy," says principal investigator Schlegel. "Because the Sloan Telescope has such a wide field of view, and because these quasars are so faint, there was no one who wasn't nervous about whether we could really bring it off."

By using 14,000 of the quasars collected by the Sloan Telescope at Apache Point Observatory in New Mexico during the first year of BOSS's planned five-year run, the new map demonstrates that indeed it is possible to determine variations in the density of intergalactic hydrogen gas at cosmological distances and thus to measure the effects of dark energy at those distances.

Slosar, who leads BOSS's Lyman-alpha cosmology working group, says that while similar measurements have been made with individual quasars or small groups of quasars in the past, "These have given only one-dimensional information about fluctuations in density along the line of sight. Before now there has never been enough density of quasars for a 3-D view."

The distance scale of the new map corresponds to an early time in the history of the universe, when the distribution of matter was nearly uniform. Any effects of dark energy detected so early would settle basic questions about its nature.

Measuring the expansion history of the universe

Baryon acoustic oscillation is cosmologists' shorthand for the periodic clustering (oscillation) of matter (baryons), which originated as pressure (acoustic) waves moving through the hot, opaque, liquid-like early universe. The pressure differences resulted in differences in density and left their signature as small variations in the temperature of the cosmic microwave background. Later -- because the denser regions formed by the pressure waves seeded galaxy formation and the accumulation of other matter -- the original acoustic waves were echoed in the net-like filaments and voids of the clustering of galaxies and in variations in the density of intergalactic hydrogen gas.

The oscillations repeat at about 500-million-light-year intervals, and because this scale is firmly anchored in the cosmic microwave background it provides a ruler -- a very big one -- to measure the history of the expanding universe. With this cosmic yardstick it will be possible to determine just how fast the universe was expanding at the redshift of the objects in the BOSS survey -- in other words, how the expansion rate has changed over time. (Redshift is the degree to which the light from an object speeding away from the viewer is shifted toward the red end of the spectrum.) Knowing whether expansion has accelerated at a constant rate or has varied over time will help decide among the major theories of dark energy.

Over its five-year extent, BOSS is using two distinct methods to calibrate the markings on the cosmic yardstick. The first method, well tested, will precisely measure 1.5 million luminous red galaxies at "low" redshifts around z = 0.7 (z stands for redshift). The second method will eventually measure the Lyman-alpha forest of 160,000 quasars with high redshifts around z = 2.5. These redshifts correspond to galaxies at distances of 2 to 6 billion light years and quasars at 10 to 11 billion light years.

Lyman-alpha is the name given to a line in the spectrum of hydrogen, marking the wavelength of light emitted when an excited hydrogen electron falls back to its ground state; it's a strong signal in the light from quasars. As the quasar's light passes through intervening clouds of hydrogen gas, additional lines accumulate where the gas clouds absorb the signal, echoing it but shifting by different degrees according to factors including the redshift of the gas cloud and its density. The spectrum of a distant quasar may have hundreds of lines, clumped and blended into a messy, wiggly structure in the spectrum: this is what astronomers call the Lyman-alpha forest.

"In theory, you can turn any of these absorption lines directly into redshifts and locate the gas cloud precisely," says Bill Carithers of Berkeley Lab's Physics Division, who concentrates on extracting relevant information from the noisy data that comes straight from the telescope. "But in practice only the spectra of the very brightest quasars are clean enough to make things that simple."

Carithers says that "while a very long exposure could improve the signal-to-noise ratio, that comes at a price. We need lots and lots of quasars to make a map. We can only afford to spend so much telescope time on each."

Since the heart of BAO is the correlation distance among density oscillations, the trick turns out to be not overconcentrating on individual spectra but instead measuring the correlations among them. "For any correlation distance, many quasars will contribute," says Carithers, "so the noise will average and the signal will get stronger. We can say, 'I'll use my data, noise and all.'"

If the attempt to measure density variations in the intergalactic gas is indeed successful, what will the BAO correlation signal from the Lyman-alpha forest look like? Shirley Ho of Berkeley Lab's Physics Division, working with Slosar and Berkeley Lab's Martin White, developed simulations to find out.

""We modeled what you would see when you have a BOSS-like data set, and through the simulations we understand the possible sources of systematics when we try with real data to detect the acoustic peak from the Lyman-alpha forest, the signature of baryon acoustic oscillations," Ho says. Comparing the real data to the simulation confirms whether the search is working as hoped.

With Peter Nugent, who heads the Computational Cosmology Center at Berkeley Lab's National Energy Research Scientific Computing Center (NERSC), Ho established a 30-terabyte BOSS Project Directory to store the Lyman-alpha simulations, plus the entire Lyman-alpha raw data set as it arrives. The directory also contains a subset of galaxy data and is available to all BOSS collaborators and to the public. The total BOSS data set is stored in a dedicated cluster of computers nicknamed Riemann.

Targeting the search

The wide-field Sloan Telescope covers a wide expanse of sky at moderate magnification. To measure both galaxies and quasars, a thousand targets for each BOSS exposure are selected in advance from existing surveys. At the telescope's focal plane, "plug plates" are precision-machine-drilled with tiny holes at positions of known galaxies and quasars. These holes are plugged with optical fibers that channel the light from each chosen galaxy or quasar to a spectrograph, which isolates the spectrum of each individual object. Schlegel credits Berkeley Lab's Nicholas Ross for doing much of the "incredibly hard work" involved in this targeting.

Slosar says, "Our exploratory paper includes less than a tenth of the 160,000 quasars that BOSS will study, but already that's enough to establish a proof of the concept. This is a potentially revolutionary technique for mapping the very distant universe. We're paving the way for future BAO experiments like BigBOSS to follow suit." BigBOSS is a proposed survey that will find precise locations for 20 million galaxies and quasars and go beyond BOSS to encompass 10 times the volume of the finished BOSS map.

"By the time BOSS ends, we will be able to measure how fast the universe was expanding 11 billion years ago with an accuracy of a couple of percent," says Patrick McDonald of Berkeley Lab and Brookhaven, who pioneered techniques for measuring the universe with the Lyman-alpha forest. "Considering that no one has ever measured the cosmic expansion rate so far back in time, that's a pretty astonishing prospect."

Says Slosar, "We now know we can use the Lyman-alpha forest to look at the dark energy. There is all this structure at the distant universe that has never been seen before. Sometimes I feel like an adventuring cartographer from the Middle Ages!"

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by DOE/Lawrence Berkeley National Laboratory, via EurekAlert!, a service of AAAS.

Journal Reference:

Anže Slosar, Andreu Font-Ribera, Matthew M. Pieri, James Rich, Jean-Marc Le Goff, Éric Aubourg, Jon Brinkmann, Nicolas Busca, Bill Carithers, Romain Charlassier, Marina Cortês, Rupert Croft, Kyle S. Dawson, Daniel Eisenstein, Jean-Christophe Hamilton, Shirley Ho, Khee-Gan Lee, Robert Lupton, Patrick McDonald, Bumbarija Medolin, Jordi Miralda-Escudé, Adam D. Myers, Robert C. Nichol, Nathalie Palanque-Delabrouille, Isabelle Pâris, Patrick Petitjean, Yodovina Piškur, Emmanuel Rollinde, Nicholas P. Ross, David J. Schlegel, Donald P. Schneider, Erin Sheldon, Benjamin A. Weaver, David H. Weinberg, Christophe Yeche, Donald G. York. The Lyman-a forest in three dimensions: measurements of large scale flux correlations from BOSS 1st-year data. Journal of Cosmology and Astroparticle Physics, 2011; (submitted) [link]

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