Universe Today

Image credit: NASA
Astronomers who want to study the early universe face a fundamental problem. How do you observe what existed during the “dark ages,” before the first stars formed to light it up? Theorists Abraham Loeb and Matias Zaldarriaga (Harvard-Smithsonian Center for Astrophysics) have found a solution. They calculated that astronomers can detect the first atoms in the early universe by looking for the shadows they cast.

To see the shadows, an observer must study the cosmic microwave background (CMB) – radiation left over from the era of recombination. When the universe was about 370,000 years old, it cooled enough for electrons and protons to unite, recombining into neutral hydrogen atoms and allowing the relic CMB radiation from the Big Bang to travel almost unimpeded across the cosmos for the past 13 billion years.

Over time, some of the CMB photons encountered clumps of hydrogen gas and were absorbed. By looking for regions with fewer photons – regions that are shadowed by hydrogen – astronomers can determine the distribution of matter in the very early universe.

“There is an enormous amount of information imprinted on the microwave sky that could teach us about the initial conditions of the universe with exquisite precision,” said Loeb.

Inflation and Dark Matter
To absorb CMB photons, the hydrogen temperature (specifically its excitation temperature) must be lower than the temperature of the CMB radiation – conditions that existed only when the universe was between 20 and 100 million years old (age of Universe: 13.7 billion years). Coincidentally, this is also well before the formation of any stars or galaxies, opening a unique window into the so-called “dark ages.”

Studying CMB shadows also allows astronomers to observe much smaller structures than was possible previously using instruments like the Wilkinson Microwave Anisotropy Probe (WMAP) satellite. The shadow technique can detect hydrogen clumps as small as 30,000 light-years across in the present-day universe, or the equivalent of only 300 light-years across in the primordial universe. (The scale has grown larger as the universe expanded.) Such resolution is a factor of 1000 times better than the resolution of WMAP.

“This method offers a window into the physics of the very early universe, namely the epoch of inflation during which fluctuations in the distribution of matter are believed to have been produced. Moreover, we could determine whether neutrinos or some unknown type of particle contribute substantially to the amount of ‘dark matter’ in the universe. These questions – what happened during the epoch of inflation and what is dark matter – are key problems in modern cosmology whose answers will yield fundamental insights into the nature of the universe,” said Loeb.

An Observational Challenge
Hydrogen atoms absorb CMB photons at a specific wavelength of 21 centimeters (8 inches). The expansion of the universe stretches the wavelength in a phenomenon called redshifting (because a longer wavelength is redder). Therefore, to observe 21-cm absorption from the early universe, astronomers must look at longer wavelengths of 6 to 21 meters (20 to 70 feet), in the radio portion of the electromagnetic spectrum.

Observing CMB shadows at radio wavelengths will be difficult due to interference by foreground sky sources. To gather accurate data, astronomers will have to use the next generation of radio telescopes, such as the Low Frequency Array (LOFAR) and the Square Kilometer Array (SKA). Although the observations will be a challenge, the potential payoff is great.

“There’s a gold mine of information out there waiting to be extracted. While its full detection may be experimentally challenging, it’s rewarding to know that it exists and that we can attempt to measure it in the near future,” said Loeb.

This research will be published in an upcoming issue of Physical Review Letters, and currently is available online at http://arxiv.org/abs/astro-ph/0312134.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: Harvard CfA News Release

Image credit: Fermilab
With the first data from their underground observatory in Northern Minnesota, scientists of the Cryogenic Dark Matter Search have peered with greater sensitivity than ever before into the suspected realm of the WIMPS. The sighting of Weakly Interacting Massive Particles could solve the double mystery of dark matter on the cosmic scale and of supersymmetry on the subatomic scale.

The CDMS II result, described in a paper submitted to Physical Review Letters, shows with 90 percent certainty that the interaction rate of a WIMP with mass 60 GeV must be less than 4 x 10-43 cm2 or about one interaction every 25 days per kilogram of germanium, the material in the experiment’s detector. This result tells researchers more than they have ever known before about WIMPS, if they exist. The measurements from the CDMS II detectors are at least four times more sensitive than the best previous measurement offered by the EDELWEISS experiment, an underground European experiment near Grenoble, France.

“Think of this improved sensitivity like a new telescope with twice the diameter and thus four times the light collection of any that came before it,” said CDMS II cospokesperson Blas Cabrera of Stanford University. “We are now able to look for a signal that is just one-fourth as bright as any we have seen before. Over the next few years, we expect to improve our sensitivity by a factor of 20 or more.”

The results are being presented at the April Meeting of the American Physical Society on May 3 and 4 in Denver by Harry Nelson and graduate student Joel Sanders, both of the University of California-Santa Barbara, and by Gensheng Wang and Sharmila Kamat of Case Western Reserve University.

“We know that neither our Standard Model of particle physics nor our model of the cosmos is complete,” said CDMS II spokesperson Bernard Sadoulet of the University of California at Berkeley. “This particular missing piece seems to fit both puzzles. We are seeing the same shape from two different directions.”

WIMPs, which carry no charge, are a study in contradictions. While physicists expect them to have about 100 times the mass of protons, their ghostly nature allows them to slip through ordinary matter leaving barely a trace. The term “weakly interacting” refers not to the amount of energy deposited when they interact with normal matter, but rather to the fact that they interact extremely infrequently. In fact, as many as a hundred billion WIMPs may have streamed through your body as you read these first few sentences.

With 48 scientists from 13 institutions, plus another 28 engineering, technical and administrative staffers, CDMS II operates with funding from the Office of Science of the U.S. Department of Energy, from the Astronomy and Physics Divisions of the National Science Foundation and from member institutions. The DOE’s Fermi National Accelerator Laboratory provides the project management for CDMS II.

“The nature of dark matter is fundamental to our understanding of the formation and evolution of the universe,” said Dr. Raymond L. Orbach, Director of DOE’s Office of Science. “This experiment could not have succeeded without the active collaboration of the DOE’s Office of Science and the National Science Foundation.”

Michael Turner, Assistant Director for Math and Physical Sciences at NSF, described identifying the constituent of the dark matter as one of the great challenges in both astrophysics and particle physics.

“Dark matter holds together all structures in the universe-including our own Milky Way-and we still do not know what the dark matter is made of,” Turner said. “The working hypothesis is that it is a new form of matter-which, if correct will shed light on the inner workings of the elementary forces and particles. In pursuing the solution to this important puzzle, CDMS is now at the head of the pack, with another factor of 20 in sensitivity still to come.”

Dark matter in the universe is detected through its gravitational effects on all cosmic scales, from the growth of structure in the early universe to the stability of galaxies today. Cosmological data from many sources confirm that this unseen dark matter totals more than seven times the amount of ordinary visible matter forming the stars, planets and other objects in the universe.

“Something out there formed the galaxies and holds them together today, and it neither emits nor absorbs light,” said Cabrera. “The mass of the stars in a galaxy is only 10 percent of the mass of the entire galaxy, so the stars are like Christmas tree lights decorating the living room of a large dark house.”

Physicists also believe WIMPs could be the as-yet unobserved subatomic particles called neutralinos. These would be evidence for the theory of supersymmetry, introducing intriguing new physics beyond today’s Standard Model of fundamental particles and forces.

Supersymmetry predicts that every known particle has a supersymmetric partner with complementary properties, although none of these partners has yet been observed. However, many models of supersymmetry predict that the lightest supersymmetric particle, called the neutralino, has a mass about 100 times that of the proton.

“Theorists came up with all of these so-called ‘supersymmetric partners’ of the known particles to explain problems on the tiniest distance scales,” said Dan Akerib of Case Western Reserve University. “In one of those fascinating connections of the very large and the very small, the lightest of these superpartners could be the missing piece of the puzzle for explaining what we observe on the very largest distance scales.”

The CDMS II team practices “underground astronomy,” with particle detectors located nearly a half-mile below the earth’s surface in a former iron mine in Soudan, Minnesota. The 2,341 feet of the earth’s crust shields out cosmic rays and the background particles they produce. The detectors are made of germanium and silicon, semiconductor crystals with similar properties. The detectors are chilled to within one-tenth of a degree of absolute zero, so cold that molecular motion becomes negligible. The detectors simultaneously measure the charge and vibration produced by particle interactions within the crystals. WIMPS will signal their presence by releasing less charge than other particles for the same amount of vibration.

“Our detectors act like a telescope equipped with filters that allow astronomers to distinguish one color of light from another,” said CDMS II project manager Dan Bauer of Fermilab. “Only, in our case, we are trying to filter out conventional particles in favor of dark matter WIMPS.”

Physicist Earl Peterson of the University Minnesota oversees the Soudan Underground Laboratory, also home to Fermilab’s long-baseline neutrino experiment, the Main Injector Neutrino Oscillation Search.

“I’m excited about the significant new result from CDMS II, and I congratulate the collaboration,” Peterson said. “I’m pleased that the facilities of the Soudan Laboratory contributed to the success of CDMS II. And I’m especially pleased that the work of Fermilab and the University of Minnesota in expanding the Soudan Laboratory has resulted in superb new physics.”

As CDSMII searches for WIMPs over the next few years, either the dark matter of our universe will be discovered, or a large range of supersymmetric models will be excluded from possibility. Either way, the CDMS II experiment will play a major role in advancing our understanding of particle physics and of the cosmos.

The CDMS II collaborating institutions include Brown University, Case Western Reserve University, Fermi National Accelerator Laboratory, Lawrence Berkeley National Laboratory, the National Institutes of Standards and Technology, Princeton University, Santa Clara University, Stanford University, the University of California-Berkeley, the University of California-Santa Barbara, the University of Colorado at Denver, the University of Florida, and the University of Minnesota.

Fermilab is a DOE Office of Science national laboratory operated under contract by Universities Research Association, Inc.

Original Source: Fermilab News Release

Image credit: Hubble
University of California scientists working at Los Alamos National Laboratory have proposed a new theory to explain the movement of vast energy fields in giant radio galaxies (GRGs). The theory could be the basis for a whole new understanding of the ways in which cosmic rays — and their signature radio waves — propagate and travel through intergalactic space.

In a paper published this month in The Astrophysical Journal Letters, the scientists explain how magnetic field reconnection may be responsible for the acceleration of relativistic electrons within large intergalactic volumes. That is, the movement of charged particles in space that are originally energized by massive black holes.

“If our understanding of this process is correct,” says Los Alamos astrophysicist Philipp Kronberg, “it could be a paradigm shift in current thinking about the nature of GRGs and cosmic rays.”

Researchers still do not fully understand why magnetic field reconnection occurs, but this much is known: a deeper understanding of the mechanism could have important applications here on Earth, such as the creation of a system of magnetic confinement for fusion energy reactors.

If the Los Alamos scientists’ theory is correct, the discovery also has wide-ranging astrophysical consequences. It implies that magnetic field reconnection or some other highly efficient field-to-particle energy conversion process could be a principal source of all extragalactic radio sources, and possibly also the mysterious “Ultra High Energy Cosmic Ray particles”.

Giant radio galaxies are vast celestial objects that emit a continuum of radio wavelengths detectable with radio telescopes like those at the Very Large Array in Socorro, N.M. Using comprehensive data on seven of the largest radio galaxies in the Universe gathered over the past two decades, the researchers were able to study cosmic ray energy fields that are expelled from the GRGs centers — which are almost certain to contain supermassive black holes — outward as much as a few millions of light years into intergalactic space (1 light year = 5,900,000,000,000 miles).

What the Los Alamos researchers concluded was that the high energy content of these giant radio galaxies, their large ordered magnetic field structures, the absence of strong large-scale shocks and very low internal gas densities point to a direct and efficient conversion of the magnetic field to particle energy in a process that astrophysicists call magnetic field reconnection. Magnetic field reconnection is a process where the lines of a magnetic field connect and vanish, converting the field’s energy into particle energy. Reconnection is considered a key process in the sun’s corona for the production of solar flares and in fusion experiment devices called tokamaks. It also occurs in the interaction between the solar wind and the Earth’s magnetic field and is considered a principal cause of magnetospheric storms.

The research determined that the measurement of the total energy content of at least one of these giant radio galaxies — which is believed to have at its center a black hole with a mass equal to 100 million times that of our sun — was 10 61 ergs. Ergs are a measure of energy where one erg is the amount of energy needed to lift one gram of weight a distance of one centimeter. This energy level of 10 61 ergs is several times more than the thermonuclear energy that could be released by all the stars in a galaxy, offering substantial proof to the researchers that the source of the measured energy could not be typical solar fusion or even supernovae.

In addition to the high energy content, the large, orderly structure of the magnetic field and the absence of strong large-scale shocks — like those that might be present from a supernova explosion — led the scientists to believe that the process of magnetic field reconnection is at work.

In addition to Kronberg, the theory is the result of work by Los Alamos scientists Stirling Colgate, Hui Li and Quentin Dufton. The research was funded by Los Alamos Laboratory-Directed Research and Development (LDRD) funding. LDRD funds basic and applied research and development focusing on creative concepts selected at the discretion of the Laboratory Director.

Los Alamos National Laboratory is operated by the University of California for the National Nuclear Security Administration (NNSA) of the U.S. Department of Energy and works in partnership with NNSA’s Sandia and Lawrence Livermore national laboratories to support NNSA in its mission.

Los Alamos enhances global security by ensuring safety and confidence in the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction and improving the environmental and nuclear materials legacy of the cold war. Los Alamos’ capabilities assist the nation in addressing energy, environment, infrastructure and biological security problems.

Original Source: Los Alamos National Laboratory News Release

Image credit: University of Chicago
University scientists are preparing to run the most advanced supercomputer simulation of an exploding star ever attempted.

Tomasz Plewa, Senior Research Associate in the Center for Astrophysical Thermonuclear Flashes and Astronomy & Astrophysics, expects the simulation to reveal the mechanics of exploding stars, called supernovae, in unprecedented detail.

The simulation is made possible by the U.S. Department of Energy?s special allocation of an extraordinary 2.7 million hours of supercomputing time to the Flash Center, which typically uses less than 500,000 hours of supercomputer time annually.

?This is beyond imagination,? said Plewa, who submitted the Flash Center proposal on behalf of a research team at the University and Argonne National Laboratory.

The Flash Center project was one of three selected to receive supercomputer time allocations under a new competitive program announced last July by Secretary of Energy Spencer Abraham.

The other two winning proposals came from the Georgia Institute of Technology, which received 1.2 million processor hours, and the DOE?s Lawrence Berkeley National Laboratory, which received one million processor hours.

The supercomputer time will help the Flash Center more accurately simulate the explosion of a white dwarf star, one that has burned most or all of its nuclear fuel. These supernovae shine so brightly that astronomers use them to measure distance in the universe. Nevertheless, many details about what happens during a supernova remain unknown.

Simulating a supernova is computationally intensive because it involves vast scales of time and space. White dwarf stars gravitationally accumulate material from a companion star for millions of years, but ignite in less than a second. Simulations must also account for physical processes that occur on a scale that ranges from a few hundredths of an inch to the entire surface of the star, which is comparable in size to Earth.

Similar computational problems vex the DOE?s nuclear weapons Stockpile Stewardship and Management Program. In the wake of the Comprehensive Test Ban Treaty, which President Clinton signed in 1996, the reliability of the nation?s nuclear arsenal must now be tested via computer simulations rather than in the field.

?The questions ultimately are how is the nuclear arsenal aging with time, and is your code predicting that aging process correctly?? Plewa said.

Flash Center scientists verify the accuracy of their supernovae code by comparing the results of their simulations both to laboratory experiments and to telescopic observations. Spectral observations of supernovae, for example, provide a sort of bar code that reveals which chemical elements are produced in the explosions. Those observations currently conflict with simulations.

?You want to reconcile current simulations with observations regarding chemical composition and the production of elements,? Plewa said.

Scientists also wish to see more clearly the sequence of events that occurs immediately before a star goes supernova. It appears that a supernova begins in the core of a white dwarf star and expands toward the surface like an inflating balloon.

According to one theory, the flame front initially expands at a relatively ?slow? subsonic speed of 60 miles per second. Then, at some unknown point, the flame front detonates and accelerates to supersonic speeds. In the ultra-dense material of a white dwarf, supersonic speeds exceed 3,100 miles per second.

Another possibility: the initial subsonic wave fizzles when it reaches the outer part of the star, leading to a collapse of the white dwarf, the mixing of unburned nuclear fuel and then detonation.

?It will be very nice if in the simulations we could observe this transition to detonation,? Plewa said.

Flash Center scientists already are on the verge of recreating this moment in their simulations. The extra computer time from the DOE should push them across the threshold.

The center will increase the resolution of its simulations to one kilometer (six-tenths of a mile) for a whole-star simulation. Previously, the center could achieve a resolution of five kilometers (3.1 miles) for a whole-star simulation, or 2.5 kilometers (1.5 miles) for a simulation encompassing only one-eighth of a star.

The latter simulations fail to capture perturbations that may take place in other sections of the star, Plewa said. But they may soon become scientific relics.

?I hope by summer we?ll have all the simulations done and we?ll move on to analyze the data,? he said.

Original Source: University of Chicago News Release