Dark Energy Found Stifling Growth in Universe

Dark Energy Found Stifling Growth in Universe

December 17, 2008

For the first time, astronomers have clearly seen the effects of "dark energy" on the most massive collapsed objects in the universe using NASA's Chandra X-ray Observatory. By tracking how dark energy has stifled the growth of galaxy clusters and combining this with previous studies, scientists have obtained the best clues yet about what dark energy is and what the destiny of the universe could be.

This work, which took years to complete, is separate from other methods of dark energy research such as supernovas. These new X-ray results provide a crucial independent test of dark energy, long sought by scientists, which depends on how gravity competes with accelerated expansion in the growth of cosmic structures. Techniques based on distance measurements, such as supernova work, do not have this special sensitivity.

Scientists think dark energy is a form of repulsive gravity that now dominates the universe, although they have no clear picture of what it actually is. Understanding the nature of dark energy is one of the biggest problems in science. Possibilities include the cosmological constant, which is equivalent to the energy of empty space. Other possibilities include a modification in general relativity on the largest scales, or a more general physical field.

To help decide between these options, a new way of looking at dark energy is required. It is accomplished by observing how cosmic acceleration affects the growth of galaxy clusters over time.

"This result could be described as 'arrested development of the universe'," said Alexey Vikhlinin of the Smithsonian Astrophysical Observatory in Cambridge, Mass., who led the research. "Whatever is forcing the expansion of the universe to speed up is also forcing its development to slow down."

Vikhlinin and his colleagues used Chandra to observe the hot gas in dozens of galaxy clusters, which are the largest collapsed objects in the universe. Some of these clusters are relatively close and others are more than halfway across the universe.

The results show the increase in mass of the galaxy clusters over time aligns with a universe dominated by dark energy. It is more difficult for objects like galaxy clusters to grow when space is stretched, as caused by dark energy. Vikhlinin and his team see this effect clearly in their data. The results are remarkably consistent with those from the distance measurements, revealing general relativity applies, as expected, on large scales.

"For years, scientists have wanted to start testing how gravity works on large scales and now, we finally have," said William Forman, a co-author of the study from the Smithsonian Astrophysical Observatory. "This is a test that general relativity could have failed."

When combined with other clues -- supernovas, the study of the cosmic microwave background, and the distribution of galaxies -- this new X-ray result gives scientists the best insight to date on the properties of dark energy.

The study strengthens the evidence that dark energy is the cosmological constant. Although it is the leading candidate to explain dark energy, theoretical work suggests it should be about 10 raised to the power of 120 times larger than observed. Therefore, alternatives to general relativity, such as theories involving hidden dimensions, are being explored.

"Putting all of this data together gives us the strongest evidence yet that dark energy is the cosmological constant, or in other words, that 'nothing weighs something'," said Vikhlinin. "A lot more testing is needed, but so far Einstein's theory is looking as good as ever."

These results have consequences for predicting the ultimate fate of the universe. If dark energy is explained by the cosmological constant, the expansion of the universe will continue to accelerate, and the Milky Way and its neighbor galaxy, Andromeda, never will merge with the Virgo cluster. In that case, about a hundred billion years from now, all other galaxies ultimately would disappear from the Milky Way's view and, eventually, the local superclusters of galaxies also would disintegrate.

Astrophysicists recreate stars in the lab

Astrophysicists recreate stars in the lab
by European Science Foundation

Astronomers are recruiting the physics laboratory to unravel the high energy processes involved in formation of stars and other critical processes within the universe. Experiments with high energy radiation and plasmas in the laboratory involving temperatures and magnetic fields over a million times greater than normally encountered on earth are also producing spin off benefits for important applications, notably in the drive towards nuclear fusion as a source of clean carbon-neutral energy.

Although a great deal has been learnt through a combination of theoretical models and observation of the universe right across the electromagnetic spectrum including visible light with conventional optical telescopes, many questions on energetic processes taking place billions of miles away still remain unanswered.This is why astrophysicists are turning to a third ingredient, the high energy laboratory, fusing results obtained there with theoretical models and direct observation through instruments. The state of this highly promising field was discussed at a recent workshop organised by the European Science Foundation (ESF), which also set out a roadmap for future collaborative research in Europe over the next five years.

The workshop is setting up a European framework for conducting coordinated experiments in Extreme Laboratory Astrophysics (ELA), aiming to simulate the high temperatures and magnetic fields experienced in a variety of formative processes occurring throughout the universe's history. Full blown ELA builds on earlier more tentative initiatives, such as the JETSET network, which is a four-year Marie Curie Research Training Network (RTN) funded by the European Commission, designed to build a vibrant interdisciplinary European Research and Training community centred on rigorous and novel approaches to plasma jet studies, with a focus on flows produced during star formation. Plasma jets comprise high energy atomic nuclei stripped of their electrons, expelled from stars during their formation and early in their lives.

ELA experiments however, as discussed at the ESF workshop, go much further than the study of plasma jets, and therefore expand on the foundations created by JETSET. "The JETSET network was truly innovative in that it combined not only theoretical and observational astrophysics, but also for the first time experiments," said Andrea Ciardi, convenor of the ESF workshop and plasma physicist at the Ecole Normale Superieure in Paris. "However JETSET was limited in terms of astrophysical phenomena studied (jets from young stars) and in terms of groups involved. The workshop aims at the creation of an XLA framework combining numerical modelling, experiments and theory, to complement observations in the study of a broader range of astrophysical phenomena."

The workshop fulfilled its objectives of stimulating the required interdisciplinary research effort, and providing a broad outlook of future objectives. Furthermore it generated great excitement about prospects for the field, according to Ciardi. "The workshop covered a large spectrum of research both in astrophysics and in laboratory plasma physics: from cosmic rays acceleration, to the properties of fast winds in stars, and from high-power lasers aimed at achieving fusion to experiments producing magnetic bubbles expanding at hundreds of kilometres per second," said Ciardi. "Indeed the excitement comes from being able to re-create in the laboratory astrophysical phenomena taking place in some of the most extreme and exotic objects in the universe."

The ELA experiments should also have practical benefits. "ELA research has an inherent duality: experiments developed initially for laboratory astrophysics, including new diagnostics, theoretical and numerical models, can be useful for example to fusion research, which is pursuing a clean source of energy, which in some cases uses similar theoretical and experimental techniques," said Ciardi.

ELA research could also help improve weather forecasts by leading to better understanding of cosmic rays that strike the earth's atmosphere and have a significant effect on cloud formation and thunderstorm activity.

Breakthrough experiment on high-temperature superconductors

Breakthrough experiment on high-temperature superconductors

New information about the metallic state from which high temperature superconductivity emerges, has been revealed in an innovative experiment performed at the University of Bristol.

The international team of physicists, led by Professor Nigel Hussey from the University’s Physics Department, publish their results today in Science Express, a rapid online access service for important new publications in the journal Science.

Superconductivity is a process by which a pair of electrons travelling in opposite directions and with opposite spin direction suddenly become attracted to one another. By pairing up, the two electrons manage to lose all their electrical resistance. This superconducting state means that current can flow without the aid of a battery.

Historically, this remarkable state had always been considered a very low temperature phenomenon, thus the origin of the superconductivity peculiar to very unusual metallic materials termed ‘high temperature superconductors’, still remains a mystery.

Hussey and his team used ultra-high (pulsed) magnetic fields – some of the most powerful in the world – to destroy the superconductivity and follow the form of the electrical resistance down to temperatures close to absolute zero.

They found that it was as the superconductivity becomes stronger, so does the scattering that causes the resistance in the metallic host from which superconductivity emerges. At some point however, the interaction that promotes high temperature superconductivity gets so strong, that ultimately it destroys the very electronic states from which the superconducting pairs form. The next step will be to identify just what that interaction is and how might it be possible to get around its self-destructive tendencies.

In doing this experiment, the team was able to reveal information that will help theorists to develop a more complete theory to explain the properties of high temperature superconductors.

“Indeed”, said Hussey, “if researchers are able to identify what make these superconductors tick, and the electrons to pair up, then material scientists might be able to create a room temperature superconductor. This holy grail of superconductivity research holds the promise of loss-free energy transmission, cheap, fast, levitated transport and a whole host of other revolutionary technological innovations.”

Related Journal : Anomalous Criticality in the Electrical Resistivity of La_2-xSr_xCuO₄

Researchers explain mystery of gravity fingers

Researchers explain mystery of gravity finger
by: MIT Writer

Researchers at MIT recently found an elegant solution to a sticky scientific problem in basic fluid mechanics: why water doesn't soak into soil at an even rate, but instead forms what look like fingers of fluid flowing downward.

Scientists call these rivulets "gravity fingers," and the explanation for their formation has to do with the surface tension where the water—or any liquid—meets the soil (or other medium). Knowing how to account for this phenomenon mathematically will have wide-ranging impact on science problems and engineering applications, including the recovery of oil from reservoirs and the sequestration of carbon underground.

The solution reported in the Dec. 12 issue of Physical Review Letters involves borrowing a mathematical phrase, if you will, from the mathematical description of a similar problem, a solution both simple and elegant that had escaped the notice of many researchers in earlier attempts to describe the phenomenon.

Co-authors Luis Cueto-Felgueroso and Ruben Juanes of the MIT Department of Civil and Environmental Engineering discovered the solution while studying the larger question of how water displaces oil in underground reservoirs. (Petroleum engineers commonly flush oil reservoirs with water to enhance oil recovery.)

"Our paper addresses a long-standing issue in soil physics," said Cueto-Felgueroso. "Lab experiments of water infiltration into homogeneous, dry soil, repeatedly show the presence of preferential flow in the form of fingers. Yet, after several decades, the scientific community has been unable to capture this phenomenon using mathematical models."

"This was the type of problem that required someone from a different research discipline to take a look at it and come up with the solution," said Juanes, the ARCO Assistant Professor in Energy Studies. "Luis applied his expertise to a fluid mechanics problem in another medium—porous media flows—and quickly figured out the solution."

Cueto-Felgueroso, a post-doctoral associate who has previously worked primarily on airflow fluid mechanics problems, had a Eureka! moment when he realized that gravity fingers in soil (or clay or sand) look very similar to water flowing down a window pane, a fairly well-understood phenomenon. He and Juanes then pulled the mathematical explanation (think of it as a phrase of words or music) from the equation describing water on a window, and included that mathematical phrase in the equation describing liquid moving downward through soil.

After rigorous comparison of data produced by the new mathematical model with observed phenomena, the two realized they'd found the solution, a solution described by one scientist reviewing the paper in Physical Review Letters as "simple and elegant" and a "major breakthrough" in the field.

The Cueto-Felgueroso and Juanes solution also describes one aspect of the water-flowing-down-a-windowpane phenomenon that previously was not understood by scientists, who actually refer to this as "the flow of thin films": why water builds up at the tips of the fingers. Again, the answer has to with the surface tension. Before the water can flow down the film, it must build up enough energy to overcome the tension holding it in place.

So what was missing from earlier models of water moving downward through soil that made it appear to move as a steady, horizontal front, rather than in finger-like paths—even when the soil was homogenous in particle size and shape?

The missing mathematical phrase describes the surface tension of the entire finger of water, which may be several centimeters in width, as opposed to the tension existing at the micron-scale of pores between soil particles.

And that phrase will sound like music to the ears of physicists and engineers.