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Post Info TOPIC: Primordial deuterium abundance


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Ultra-dense deuterium
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A material that is a hundred thousand times heavier than water and more dense than the core of the Sun is being produced at the University of Gothenburg. The scientists working with this material are aiming for an energy process that is both more sustainable and less damaging to the environment than the nuclear power used today.
Imagine a material so heavy that a cube with sides of length 10 cm weights 130 tonnes, a material whose density is significantly greater than the material in the core of the Sun. Such a material is being produced and studied by scientists in Atmospheric Science at the Department of Chemistry, the University of Gothenburg.
So far, only microscopic amounts of the new material have been produced. New measurements that have been published in two scientific journals, however, have shown that the distance between atoms in the material is much smaller than in normal matter. Leif Holmlid, Professor in the Department of Chemistry, believes that this is an important step on the road to commercial use of the material.

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QSO SDSS1558-0031
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Title: The Deuterium to Hydrogen Abundance Ratio Towards the QSO SDSS1558-0031
Authors: John M. O'Meara, Scott Burles, Jason X. Prochaska, Gabe E. Prochter, Rebecca A. Bernstein, Kristin M. Burgess

Researchers present a measurement of the D/H abundance ratio in a metal-poor damped Lyman alpha (DLA) system along the sightline of QSO SDSS1558-0031. The DLA system is at redshift z = 2.70262, has a neutral column density of log(NHI)=20.67 0.05 cm^2, and a gas-phase metallicity (O/H)= -1.49 which indicates that deuterium astration is negligible. Deuterium absorption is observed in multiple Lyman series with a column density of log(NDI)=16.19 0.04 cm, best constrained by the deuterium Lyman-11 line. The researchers measure log(D/H) = -4.48 0.06, which when combined with previous measurements along QSO sightlines gives a best estimate of log(D/H) = -4.55 0.04, where the 1-sigma error estimate comes from a jackknife analysis of the weighted means.
Using the framework of standard big bang nucleosynthesis, this value of D/H translates into a baryon density of Omega_b h = 0.0213 0.0013 0.0004 where the error terms represent the 1-sigma errors from D/H and the uncertainties in the nuclear reaction rates respectively. Combining their new measurement with previous measurements of D/H, they no longer find compelling evidence for a trend of D/H with NHI.

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RE: Primordial deuterium abundance
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Milky Way's gas may be down to cannibal diet
The researchers identified dusty, relatively undisturbed regions by searching for low levels of gaseous silicon and iron these elements had presumably condensed into solid dust grains. The deuterium to hydrogen ratio in these dusty regions was as low as 5 parts per million (ppm).
According to Jeffrey Linsky of the JILA research institute in Boulder, Colorado, US, the most likely explanation for the higher ratio is that the galaxy has absorbed a lot more "pristine" gas which has not been altered much by stars than previously believed.

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The total ratio of deuterium to hydrogen in gas between stars (out to 3,000 light years from the sun) is 23 parts per million. That ratio is only slightly smaller than the best estimates of the ratio at the beginning of the universe, which was about 28 parts per million.
The ability to accurately measure it today will allow astronomers to have a better underststanding of supernovae and of the chemical evolution of the Milky Way and other galaxies.

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3 minutes after the big bang, the expanding universe had cooled to below about 10^9 K so that protons and neutrons could fuse to make stable deuterium nuclei (a hydrogen isotope with one proton and one neutron). Most of the helium in the universe was created from the primordial neutrons and protons (Although stars do produce some of the helium visible today), by the time the nucleosynthesis epoch ended. Stars to fuse hydrogen nuclei to make a helium nucleus use fusion. The fusion chain process in the early universe was slightly different than what occurs in stars because of the abundant free neutrons in the early universe. However, the general process is the same: protons react to produce deuterium (heavy hydrogen), deuterium nuclei react to make Helium-3 nuclei, and Helium-3 nuclei react to make the stable Helium-4 nucleus.
The amount of the final Helium-4 product is not as sensitive to the density of the early universe
The deuterium nucleus is the weak link of the chain process, so the fusion chain reactions could not take place until the universe had cooled enough. The exact temperature depends sensitively on the density at that time. Extremely small amounts of Lithium-7 were also produced during the early universe nucleosynthesis process. Lithium-7 and deuterium density depends sensitively upon the density of protons (2 up + 1 down quarks) and neutrons during this time. If the universe were too dense, then most of the deuterium would have fused into helium. The more neutrons that decay before combining with protons, the smaller the abundances of heavier elements. Only in a low-density universe can the deuterium survive. A denser universe would have had more deuterium fused to form helium, so the amount of the remaining deuterium seen today is used as a probe of the early density because of the sensitivity of its production to the density of the protons and neutrons and temperature in the early universe.
Comparing the observed densities of the primordial isotopes to those computed from models and translating the results into Omega, the density parameter, gives Omega = 0.015/h where h is the Hubble parameter divided by 100 km/sec/Mpc. The smaller Ho, the larger Omega; if Ho=50, Omega is approximately 0.06, whereas Ho=100 gives Omega of only 0.015. This range is still much less than Omega=1, but nucleosynthesis limits can indicate only the density of baryons, because only baryons participate in nuclear reactions. Hence we must conclude that the universe contains less than the critical density of baryons.

After about 15 minutes it was too cold for fusion. Free neutrons and protons were synthesised into the light elements: deuterium (D), helium-3, and helium-4. The universe consisted of 10% helium and 90% hydrogen, (25% helium and 75% hydrogen, by mass).
There were also extremely small amounts of the Lithium-7 produced.
The elements heavier than helium were produced in the cores of stars.
The number of deuterium nuclei that do not later undergo fusion reaction to make Helium-3 nuclei also depends sensitively on the temperature and density of the protons and neutrons. A less dense universe would have had more deuterium remaining. Therefore, measurement of the primordial deuterium can show if there is enough matter to make the universe positively-curved and eventually stop the expansion.

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A heavy form of hydrogen created just moments after the Big Bang has been found to exist in larger quantities than expected in the Milky Way, a finding that could radically alter theories about star and galaxy formation, says a new international study led by the University of Colorado at Boulder.

CU-Boulder astrophysicist Jeffrey Linsky said new data gathered by NASA's Far Ultraviolet Spectroscopic Explorer, or FUSE, satellite, shows why deuterium appears to be distributed unevenly in the Milky Way Galaxy. It apparently has been binding to interstellar dust grains, changing from an easily detectable gaseous form to an unobservable solid form, said Linsky, a fellow of JILA, a joint institute of CU-Boulder and the National Institute of Standards and Technology.
The FUSE deuterium study, six years in the making, solves a 35-year-old mystery concerning the distribution of deuterium in the Milky Way while posing new questions about how stars and galaxies are made, according to the research team. A paper on the subject by a team of international researchers led by Linsky is being published in the August 20 issue of The Astrophysical Journal.

"Since the 1970s, we have been unable to explain why deuterium levels vary all over the place. The answer we found is as unsettling as it is exciting" - Jeffrey Linsky.

Since deuterium -- a hydrogen isotope containing a proton and a neutron -- is believed burned and lost forever during star formation, scientists think the amount of deuterium present in the universe is "pure" and serves as a tracer for star creation and galaxy building over billions of years. While primordial deuterium in the distant, early universe has been measured at concentrations of about 27 parts per million parts hydrogen atoms, measurements by FUSE and NASA's Copernicus satellite have shown a "patchy" distribution of the element in the Milky Way galaxy, often at far lower levels.
In 2003, Princeton University's Bruce Draine, a co-author on the new study, developed a model showing that deuterium, when compared to hydrogen, might preferentially bind to interstellar dust grains. The observations by FUSE -- which can detect the telltale spectral fingerprints of deuterium in the ultraviolet energy range -- strongly support the theory, according to The Astrophysical Journal paper authors.

"Where there are high concentrations of interstellar dust in the galaxy, we see lower concentrations of deuterium gas with FUSE. And where there is less interstellar dust, we are measuring higher levels of deuterium gas" - Jeffrey Linsky.

In relatively undisturbed areas of the universe -- like regions around Earth's sun, for example -- deuterium atoms systematically "leave" the gas phase and replace normal hydrogen atoms in dust grains. When a pocket of the universe is disturbed by events like a supernova shock wave or violent activity triggered by nearby hot stars, the dust grains are vaporized, releasing deuterium atoms back into a gas, which has been measured by FUSE, the researchers said.
Scientists assumed from astrophysical theories that at least one-third of the primordial deuterium present in the Milky Way was destroyed over time as it cycled through the stars. But according to the new FUSE findings, the present-day deuterium abundance is less than 15 percent below the primordial values.

"This implies that either significantly less material has been converted to helium and heavier elements in stars or that much more primordial gas has rained down onto the galaxy over its lifetime than had been thought. In either case, our models of the chemical evolution of the Milky Way will have to be revised significantly to explain this important new result" - Jeffrey Linsky.

Launched in 1999, FUSE is a NASA Explorer mission developed in cooperation with the French and Canadian Space Agencies and by Johns Hopkins University, CU-Boulder and the University of California, Berkeley. CU-Boulder's Center for Astrophysics and Space Astronomy designed and built the mission's $9 million spectrograph, which collects and funnels UV light from the satellite's four telescopes.
The paper was co-authored by scientists from Princeton, Johns Hopkins and Northwestern universities, the Space Telescope Science Institute, CU-Boulder, the University of Wisconsin-Madison, the University of Texas-Austin, NASA-Goddard, the Laboratoire d'Astrophysique in Marseille, France, and the Observatoire de Paris-Meudon in Meudon, France. Other CU-Boulder co-authors include JILA's Brian Wood, CASA's Michael Shull and CASA doctoral graduate Seth Redfield.

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Alan E. E. Rogers and colleagues at Massachusetts Institute of Technology's (MIT) Haystack Observatory in Westford, Massachusetts, have detected deuterium's radio signature for the first time. They measured the deuterium-to-hydrogen ratio as 23 parts per million, which is close to the Wilkinson Microwave Anisotropy Probe prediction of 25 parts per million.

They detected the signal at a frequency of 327 megahertz, which corresponds to a wavelength of 92 centimetres. Deuterium's 92-centimeter signature emission line is similar to hydrogen's 21-centimeter line



The amount of deuterium detected puts a constraint on the ratio of radiation-to-ordinary matter. This ratio is related to the amount of matter in the early universe, therefore giving cosmologists clues about the nature of dark matter.

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If you want to hear a little bit of the Big Bang, just switch off your TV.

That's what neighbours of MIT's Haystack Observatory were asked to do.



And now the results are in: Scientists at Haystack have made the first radio detection of deuterium, an atom that is key to understanding the beginning of the universe. The findings are being reported in an article in the September. 1 issue of Astrophysical Journal Letters.
The team of scientists and engineers, led by Alan E.E. Rogers, made the detection using a radio telescope array designed and built at the MIT research facility in Westford, Mass. Rogers is currently a senior research scientist and associate director of the Haystack Observatory.

After gathering data for almost one year, a solid detection was obtained on May 30.
The detection of deuterium is of interest because the amount of deuterium can be related to the amount of Dark Matter in the universe, but accurate measurements have been elusive. Because of the way deuterium was created in the Big Bang, an accurate measurement of deuterium would allow scientists to set constraints on models of the Big Bang.

Also, accurate measurement of deuterium would be an indicator of the density of cosmic baryons, and that density of baryons would indicate whether ordinary matter is dark and found in regions such as black holes, gas clouds or brown dwarfs, or is luminous and can be found in stars. This information helps scientists who are trying to understand the very beginning of our universe.

Until now the deuterium atom has been extremely difficult to detect with instruments on Earth. Emission from the deuterium atom is weak since it is not very abundant in space-there is approximately one deuterium atom for every 100,000 hydrogen atoms, thus the distribution of the deuterium atom is diffuse. Also, at optical wavelengths the hydrogen line is very close to the deuterium line, which makes it subject to confusion with hydrogen; but at radio wavelengths, deuterium is well separated from hydrogen and measurements can provide more consistent results.

In addition, our modern lifestyle, filled with gadgets that use radio waves, presented quite a challenge to the team trying to detect the weak deuterium radio signal. Radio frequency interference bombarded the site from cell phones, power lines, pagers, fluorescent lights, TV, and in one case from a telephone equipment cabinet where the doors had been left off.
To locate the interference, a circle of yagi antennas was used to indicate the direction of spurious signals, and a systematic search for the RFI sources began.


The Deuterium Array at Haystack is a soccer-field size installation conceived and built at the Haystack facility with support from the National Science Foundation, MIT and TruePosition Inc.

At times, Rogers asked for help from Haystack's neighbours, and in several instances replaced a certain brand of answering machine that was sending out a radio signal with one that did not interfere with the experiment. The interference caused by one person's stereo system was solved by having a part on the sound card replaced by the factory.
The other members of the team working with Rogers are Kevin Dudevoir, Joe Carter, Brian Fanous and Eric Kratzenberg (all of Haystack Observatory) and Tom Bania of Boston University.

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Researchers suggest that by measuring fluctuations in the absorption of cosmic microwave background (CMB) photons by neutral gas during the cosmic dark ages, at red shifts z ~ 7--200, could reveal the primordial deuterium abundance of the Universe.
The strength of the cross-correlation of brightness-temperature fluctuations due to resonant absorption of CMB photons in the 21-cm line of neutral hydrogen with those due to resonant absorption of CMB photons in the 92-cm line of neutral deuterium is proportional to the fossil deuterium to hydrogen ratio fixed during big bang nucleosynthesis.
Although technically challenging, this measurement could provide the cleanest possible determination of the deuterium/hydrogen ratio, free from contamination by structure formation processes at lower red shifts, and has the potential to improve big bang nucleosynthesis constraints to the baryon density of the Universe \Omega_{b} h^2.
The researchers present their results for the thermal spin-change cross-section for deuterium-hydrogen scattering.

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