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Using Dark Matter to Sense Dark Energy

By Claudia MignoneMar. 25, 2010 , 11:43 AM

It's a weird, weird, weird universe we live in. Cosmologists and astronomers know that only 5% of it consists of ordinary matter of the sort found in stars and planets. Another 23% consists of mysterious dark matter that (so far) manifests itself only through its gravity. And a whopping 72% of the universe consists of bizarre, space-stretching dark energy which is speeding up the expansion of the universe. Scientists don't know exactly what dark matter and dark energy are. But now they've pulled off a bit of black magic and used the subtle effects of one to study the other.
Dark matter gives structure to the cosmos. Space is filled with a vast "cosmic web" of strands and clumps of dark matter, which have grown from microscopic variations in the original, nearly smooth distribution dark matter after the big bang. Through their gravity, the clumps draw in ordinary matter, so the galaxies form and reside within these clumps. Responding to their own gravity, the clumps and strands also grow denser and more compact. At the same time, dark energy stretches the very fabric of space. So if scientists can study the evolution of the cosmic web, they ought to be able to see the effects of dark energy setting in and slightly slowing the growth and coalescence of the clumps.
And that's what astrophysicist Tim Schrabback of the Leiden Observatory in the Netherlands, and colleagues have done. Their study uses data from the Cosmic Evolution Survey, or COSMOS, the largest galaxy survey ever conducted with NASA's orbiting Hubble Space Telescope. Thanks to an improved algorithm to analyze the images, Schrabback's team could study in great detail the shapes of over 446,000 galaxies in a 1.64 square degree patch of sky. During its journey to Earth, the light from these faint galaxies must pass through the lumps and filaments of dark matter in the cosmic web. The gravity exerted by the clumps bends the paths of light rays and distorts the images of the galaxies, so rather than appearing as randomly oriented ellipses on the sky, neighboring galaxies align a bit like fish in a school. The distortions are tiny, but starting in 2000, astronomers managed to detect the effect, which is known as cosmological weak lensing, in surveys of thousands of galaxies. In recent years, they've even begun to trace, at least crudely, the three-dimensional structure of the cosmic web.
Now, Shrabback and colleagues have gone a key step further and have actually detected the effects of dark energy on the evolution of the cosmic web. This was possible thanks to additional ground-based data which allowed them to estimate distances to almost half of the observed galaxies. Together with the shape measurements, the distances helped the researchers produce a 3D "picture" of the distribution of the dark matter in the cosmic web. This tomographic approach is the cosmic analogous of the "reconstruction of the skeleton from a CT scan," adds team member F. William High from Harvard University.
This 3D view on the "cosmic web" shows how many clumps are to be found at different distances from us, and how massive they are. Comparing the results obtained on the different distance slices, the team recorded a slowing of the growth of cosmic structures, a sign that the dark energy is driving the universe to expand faster and faster. The excellent agreement between these and many other measurements indicates that "cosmologists seem to be on the right track on their quest to understand the properties and evolution of the Universe," notes Schrabback.
"This important result shows the power of weak lensing studies from space to track down dark energy," says Yun Wang, a cosmologist at the University of Oklahoma, Norman. She also points out that, in order to fully exploit this method and attempt to understand what dark energy actually is, "a much wider survey is necessary," such as those planned for the future space-telescope missions such as the United States proposed Joint Dark Energy Mission and Europe's proposed Euclid satellite. "This study demonstrates that the method can work, and anticipates the success of these future experiments," she says.

Dark energy, repulsive force that is the dominant component (73 percent) of the universe. The remaining portion of the universe consists of ordinary matter and dark matter. Dark energy, in contrast to both forms of matter, is relatively uniform in time and space and is gravitationally repulsive, not attractive, within the volume it occupies. The nature of dark energy is still not well understood.
A kind of cosmic repulsive force was first hypothesized by Albert Einstein in 1917 and was represented by a term, the “cosmological constant,” that Einstein reluctantly introduced into his theory of general relativity in order to counteract the attractive force of gravity and account for a universe that was assumed to be static (neither expanding nor contracting). After the discovery in the 1920s by American astronomer Edwin Hubble that the universe is not static but is in fact expanding, Einstein referred to the addition of this constant as his “greatest blunder.” However, the measured amount of matter in the mass-energy budget of the universe was improbably low, and thus some unknown “missing component,” much like the cosmological constant, was required to make up the deficit. Direct evidence for the existence of this component, which was dubbed dark energy, was first presented in 1998.
Dark energy is detected by its effect on the rate at which the universe expands and its effect on the rate at which large-scale structures such as galaxies and clusters of galaxies form through gravitational instabilities. The measurement of the expansion rate requires the use of telescopes to measure the distance (or light travel time) of objects seen at different size scales (or redshifts) in the history of the universe. These efforts are generally limited by the difficulty in accurately measuring astronomical distances. Since dark energy works against gravity, more dark energy accelerates the universe’s expansion and retards the formation of large-scale structure. One technique for measuring the expansion rate is to observe the apparent brightness of objects of known luminosity like Type Ia supernovas. Dark energy was discovered in 1998 with this method by two international teams that included American astronomers Adam Riess (the author of this article) and Saul Perlmutter and Australian astronomer Brian Schmidt. The two teams used eight telescopes including those of the Keck Observatory and the MMT Observatory. Type Ia supernovas that exploded when the universe was only two-thirds of its present size were fainter and thus farther away than they would be in a universe without dark energy. This implied the expansion rate of the universe is faster now than it was in the past, a result of the current dominance of dark energy. (Dark energy was negligible in the early universe.)
Studying the effect of dark energy on large-scale structure involves measuring subtle distortions in the shapes of galaxies arising from the bending of space by intervening matter, a phenomenon known as “weak lensing.” At some point in the last few billion years, dark energy became dominant in the universe and thus prevented more galaxies and clusters of galaxies from forming. This change in the structure of the universe is revealed by weak lensing. Another measure comes from counting the number of clusters of galaxies in the universe to measure the volume of space and the rate at which that volume is increasing. The goals of most observational studies of dark energy are to measure its equation of state (the ratio of its pressure to its energy density), variations in its properties, and the degree to which dark energy provides a complete description of gravitational physics.
In cosmological theory, dark energy is a general class of components in the stress-energy tensor of the field equations in Einstein’s theory of general relativity. In this theory, there is a direct correspondence between the matter-energy of the universe (expressed in the tensor) and the shape of space-time. Both the matter (or energy) density (a positive quantity) and the internal pressure contribute to a component’s gravitational field. While familiar components of the stress-energy tensor such as matter and radiation provide attractive gravity by bending space-time, dark energy causes repulsive gravity through negative internal pressure. If the ratio of the pressure to the energy density is less than −1/3, a possibility for a component with negative pressure, that component will be gravitationally self-repulsive. If such a component dominates the universe, it will accelerate the universe’s expansion.
The simplest and oldest explanation for dark energy is that it is an energy density inherent to empty space, or a “vacuum energy.” Mathematically, vacuum energy is equivalent to Einstein’s cosmological constant. Despite the rejection of the cosmological constant by Einstein and others, the modern understanding of the vacuum, based on quantum field theory, is that vacuum energy arises naturally from the totality of quantum fluctuations (i.e., virtual particle-antiparticle pairs that come into existence and then annihilate each other shortly thereafter) in empty space. However, the observed density of the cosmological vacuum energy density is ~10⁻¹⁰ ergs per cubic centimetre; the value predicted from quantum field theory is ~10¹¹⁰ ergs per cubic centimetre. This discrepancy of 10¹²⁰ was known even before the discovery of the far weaker dark energy. While a fundamental solution to this problem has not yet been found, probabilistic solutions have been posited, motivated by string theory and the possible existence of a large number of disconnected universes. In this paradigm the unexpectedly low value of the constant is understood as a result of an even greater number of opportunities (i.e., universes) for the occurrence of different values of the constant and the random selection of a value small enough to allow for the formation of galaxies (and thus stars and life).
Another popular theory for dark energy is that it is a transient vacuum energy resulting from the potential energy of a dynamical field. Known as “quintessence,” this form of dark energy would vary in space and time, thus providing a possible way to distinguish it from a cosmological constant. It is also similar in mechanism (though vastly different in scale) to the scalar field energy invoked in the inflationary theory of the big bang.
Another possible explanation for dark energy is topological defects in the fabric of the universe. In the case of intrinsic defects in space-time (e.g., cosmic strings or walls), the production of new defects as the universe expands is mathematically similar to a cosmological constant, although the value of the equation of state for the defects depends on whether the defects are strings (one-dimensional) or walls (two-dimensional).
There have also been attempts to modify gravity to explain both cosmological and local observations without the need for dark energy. These attempts invoke departures from general relativity on scales of the entire observable universe.
A major challenge to understanding accelerated expansion with or without dark energy is to explain the relatively recent occurrence (in the past few billion years) of near-equality between the density of dark energy and dark matter even though they must have evolved differently. (For cosmic structures to have formed in the early universe, dark energy must have been an insignificant component.) This problem is known as the “coincidence problem” or the “fine-tuning problem.” Understanding the nature of dark energy and its many related problems is one of the most formidable challenges in modern physics.

Dark energy, repulsive force that is the dominant component (73 percent) of the universe. The remaining portion of the universe consists of ordinary matter and dark matter. Dark energy, in contrast to both forms of matter, is relatively uniform in time and space and is gravitationally repulsive, not attractive, within the volume it occupies. The nature of dark energy is still not well understood.
A kind of cosmic repulsive force was first hypothesized by Albert Einstein in 1917 and was represented by a term, the “cosmological constant,” that Einstein reluctantly introduced into his theory of general relativity in order to counteract the attractive force of gravity and account for a universe that was assumed to be static (neither expanding nor contracting). After the discovery in the 1920s by American astronomer Edwin Hubble that the universe is not static but is in fact expanding, Einstein referred to the addition of this constant as his “greatest blunder.” However, the measured amount of matter in the mass-energy budget of the universe was improbably low, and thus some unknown “missing component,” much like the cosmological constant, was required to make up the deficit. Direct evidence for the existence of this component, which was dubbed dark energy, was first presented in 1998.
Dark energy is detected by its effect on the rate at which the universe expands and its effect on the rate at which large-scale structures such as galaxies and clusters of galaxies form through gravitational instabilities. The measurement of the expansion rate requires the use of telescopes to measure the distance (or light travel time) of objects seen at different size scales (or redshifts) in the history of the universe. These efforts are generally limited by the difficulty in accurately measuring astronomical distances. Since dark energy works against gravity, more dark energy accelerates the universe’s expansion and retards the formation of large-scale structure. One technique for measuring the expansion rate is to observe the apparent brightness of objects of known luminosity like Type Ia supernovas. Dark energy was discovered in 1998 with this method by two international teams that included American astronomers Adam Riess (the author of this article) and Saul Perlmutter and Australian astronomer Brian Schmidt. The two teams used eight telescopes including those of the Keck Observatory and the MMT Observatory. Type Ia supernovas that exploded when the universe was only two-thirds of its present size were fainter and thus farther away than they would be in a universe without dark energy. This implied the expansion rate of the universe is faster now than it was in the past, a result of the current dominance of dark energy. (Dark energy was negligible in the early universe.)
Studying the effect of dark energy on large-scale structure involves measuring subtle distortions in the shapes of galaxies arising from the bending of space by intervening matter, a phenomenon known as “weak lensing.” At some point in the last few billion years, dark energy became dominant in the universe and thus prevented more galaxies and clusters of galaxies from forming. This change in the structure of the universe is revealed by weak lensing. Another measure comes from counting the number of clusters of galaxies in the universe to measure the volume of space and the rate at which that volume is increasing. The goals of most observational studies of dark energy are to measure its equation of state (the ratio of its pressure to its energy density), variations in its properties, and the degree to which dark energy provides a complete description of gravitational physics.
In cosmological theory, dark energy is a general class of components in the stress-energy tensor of the field equations in Einstein’s theory of general relativity. In this theory, there is a direct correspondence between the matter-energy of the universe (expressed in the tensor) and the shape of space-time. Both the matter (or energy) density (a positive quantity) and the internal pressure contribute to a component’s gravitational field. While familiar components of the stress-energy tensor such as matter and radiation provide attractive gravity by bending space-time, dark energy causes repulsive gravity through negative internal pressure. If the ratio of the pressure to the energy density is less than −1/3, a possibility for a component with negative pressure, that component will be gravitationally self-repulsive. If such a component dominates the universe, it will accelerate the universe’s expansion.
The simplest and oldest explanation for dark energy is that it is an energy density inherent to empty space, or a “vacuum energy.” Mathematically, vacuum energy is equivalent to Einstein’s cosmological constant. Despite the rejection of the cosmological constant by Einstein and others, the modern understanding of the vacuum, based on quantum field theory, is that vacuum energy arises naturally from the totality of quantum fluctuations (i.e., virtual particle-antiparticle pairs that come into existence and then annihilate each other shortly thereafter) in empty space. However, the observed density of the cosmological vacuum energy density is ~10⁻¹⁰ ergs per cubic centimetre; the value predicted from quantum field theory is ~10¹¹⁰ ergs per cubic centimetre. This discrepancy of 10¹²⁰ was known even before the discovery of the far weaker dark energy. While a fundamental solution to this problem has not yet been found, probabilistic solutions have been posited, motivated by string theory and the possible existence of a large number of disconnected universes. In this paradigm the unexpectedly low value of the constant is understood as a result of an even greater number of opportunities (i.e., universes) for the occurrence of different values of the constant and the random selection of a value small enough to allow for the formation of galaxies (and thus stars and life).
Another popular theory for dark energy is that it is a transient vacuum energy resulting from the potential energy of a dynamical field. Known as “quintessence,” this form of dark energy would vary in space and time, thus providing a possible way to distinguish it from a cosmological constant. It is also similar in mechanism (though vastly different in scale) to the scalar field energy invoked in the inflationary theory of the big bang.
Another possible explanation for dark energy is topological defects in the fabric of the universe. In the case of intrinsic defects in space-time (e.g., cosmic strings or walls), the production of new defects as the universe expands is mathematically similar to a cosmological constant, although the value of the equation of state for the defects depends on whether the defects are strings (one-dimensional) or walls (two-dimensional).
There have also been attempts to modify gravity to explain both cosmological and local observations without the need for dark energy. These attempts invoke departures from general relativity on scales of the entire observable universe.
A major challenge to understanding accelerated expansion with or without dark energy is to explain the relatively recent occurrence (in the past few billion years) of near-equality between the density of dark energy and dark matter even though they must have evolved differently. (For cosmic structures to have formed in the early universe, dark energy must have been an insignificant component.) This problem is known as the “coincidence problem” or the “fine-tuning problem.” Understanding the nature of dark energy and its many related problems is one of the most formidable challenges in modern physics.

Named after the Greek word for unstable (astatos), Astatine is a naturally occurring semi-metal that results from the decay of uranium and thorium. In its most stable form - astatine-210 - it's got a half-life of just 8.1 hours, which means even if you did happen to stumble on some of it, half of it would be gone by the end of a work day. Depending on how it decays, it'll either turn into the isotopes bismuth-206 or polonium-210.
This instability, combined with its actual scarceness, means that at any one time, there's less than 30 grams of it in the Earth's crust. If scientists need to use it, they have to produce it from scratch, that said, only 0.05 micrograms (0.00000005 grams) of astatine have been produced to date. No one's ever seen it in its elemental state, because if you had enough of it to see it with the naked eye, it would have already been vaporised by the heat of its own radioactivity.That said, scientists assume it would take on a dark or metallic appearance if you could see it.

Measurements of an artificial radioactive element called lawrencium could revive an arcane controversy over the element’s position in the periodic table — and the structure of the table itself.
An international team of physicists and chemists reports in Nature¹ that it takes very little energy to strip an electron out of an atom of lawrencium, element 103. The measurement is a tour de force of chemistry, because the radioactive element does not exist in nature, can be synthesized only in vanishingly small amounts, and lasts for mere seconds.
Lawrencium, named after physicist Ernest Lawrence, the inventor of the cyclotron particle accelerator, is the heaviest element for which researchers have yet measured the fundamental property known as the first ionization energy — the energy required to turn the atom into an ion by ripping out its most easily accessible electron. That measurement underpins researchers’ understanding of an atom’s chemistry, but until now had been known only for the elements up to einsteinium (atomic number 99).

Welcome to UnicoreElements/We make the Periodic Table of the Elements come alive. We have been offering display-quality Element samples to the public since 2003. We cater to element collectors and any person or organization that needs small quantities of high quality element samples. We have specialized in Rare-Earth metals since the beginning. Samples of most of the elements from the Periodic Table of the Elements are in stock. Our most popular product line is the Elements Coin series - a series of standard-sized coins made from elements from the Periodic Table.

RARE EARTH ELEMENTS

The International Union of Pure and Applied Chemistry (IUPAC) defines rare earth elements (REE) or rare earth metals as a collection of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides (Note: Even though lanthanoid means 'like lanthanum' and as such should not include lanthanum it has become included through common usage.) plus scandium and yttrium (Figure 1). Scandium and yttrium are considered rare earth elements since they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties.

LREE and HREE

The rare earth elements are often described as being a 'light-group rare earth element' (LREE) or 'heavy-group rare earth element' (HREE). The definition of a LREE and HREE is based on the electron configuration of each rare-earth element. The LREE are defined as lanthanum, atomic number 57 through gadolinium, atomic number 64. The LREE have in common increasing unpaired electrons, from 0 to 7. The HREE are defined as terbium, atomic number 65 through lutetium, atomic number 71, and also yttrium, atomic number 39. All of the HREE are differ from the first eight lanthanides in that they have 'paired' electrons (a clockwise and counter-clockwise spinning election). The LREE have no paired electrons. Yttrium is included in the HREE group based on its similar ionic radius and similar chemical properties. Scandium is also trivalent; however, its other properties are not similar enough to classify it as either a LREE or HREE.

Abundance and use

Despite their name, rare earth elements (with the exception of the radioactive promethium) are relatively plentiful in the Earth's crust. The more abundant REE are each similar in crustal concentration to commonplace industrial metals such as chromium, nickel, copper, or lead (Figure 2). Even the least abundant REE, thulium, is nearly 200 times more common than gold. However, in contrast to ordinary base and precious metals, REE have very little tendency to become concentrated in exploitable ore deposits.
The largest rare earth metal deposits, in the form of bastnäsite and monazite, are found in China and the United States. From 1965 through the mid-1980s, Mountain Pass in California, United States, was the dominant source of REE. Since 1985, production of REE in China has increased dramatically and now China controls more than 90 % of the global supply of rare-earth minerals.
The separation of rare-earth elements had been very difficult due to their similar chemical properties. The chemists have succeeded, for the first time, in separating rare earth elements in very small amounts and of modest purity by fractional crystallization. The method, however, was extremely laborious and time consuming. The American chemist Charles James (1880-1928) performed 15 000 recrystallizations in order to obtain good quality thulium bromate. In the beginning of 1947 the American chemist Frank Harold Spedding (1902-1984) and his colleagues at Iowa State College published a series of papers in which they had described practical methods for preparative separation of the rare earths by displacement ion-exchange chromatography. As a result, for the first time, chemists could deal with pure rare earths in substantial quantities.
The rare earth elements are essential for a diverse and expanding array of high-technology applications, which constitute an important part of the industrial economy of the 21st century. As a matter of fact, rare earth has been listed in the category of strategic elements in many countries, such as the USA and Japan. The unsaturated 4f electronic structure of rare earth elements makes them have special properties of luminescence, magnetism and electronics, which could be used to develop many new materials. Europium, for example, provides red phosphor for TVs and computer monitors and it has no known substitute. Cerium similarly rules the glass-polishing industry.
The rare earth elements are often used without separation, for instance in steel to improve strength and workability, and in magnesium alloys in the production of lighter flints.

The Rare Earth Dilemma

Rare earth extraction is one of the most environmentally destructive and toxic producing of all mining practices. Excessive rare earth mining has resulted in landslides, clogged rivers, environmental pollution emergencies and even major accidents and disasters, causing great damage to people's safety and health and the ecological environment. According to statistics conducted within Baotou, where China's primary rare earth production occurs, all the rare earth enterprises in the Baotou region, Inner Mongolia, produce approximately ten million tons of wastewater every year and most of that waste water is discharged without being effectively treated. In addition, each ton of rare earths produces 2000 tons of mine tailings, which often contain radioactive thorium.
In an effort to reduce dependence on foreign imported oil and natural gas, many countries are turning more and more to green technologies, all of which require an abundance of REEs. Neodymium, one of the most common rare earths, is a key part of magnets used in hyper-efficient motors and generators. Around two tons of neodymium is needed for each wind turbine. Lanthanum is a major ingredient for hybrid car batteries (each Prius uses up to 15 kg), while terbium is vital for low-energy light bulbs and cerium is used in catalytic converters. The fact that REEs are needed for green technologies is shrouded in irony because of the great potential environmental implications of mining and processing them.
China, which possesses one-third of the world's rare-earth reserves and provides more than 90 percent of the world's supplies, has seen environmental damage from the mining and processing and depletion of the resource. Because of this, the government of China implemented a number of measures to improve the sustainable development of the industry. The implement of new standards cut exports of rare earths for the first half of 2011 by 35 percent, following a 72 percent reduction for the second half of year before. China says it's being stingy for environmental reasons, not economic leverage, but the cutbacks have nonetheless caused major price spikes.