Dark Matter and Dark Energy… the Same Thing?

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I’ve said it many times, but it bears repeating: regular matter only accounts for 4% of the Universe. The other 96% – dark matter and dark energy – is a total mystery. Wouldn’t it be convenient if we could find a single explanation for both? Astronomers from the University of St. Andrews are ready to decrease the mysteries down to one.

Dr. HongSheng Zhao at the University of St. Andrews School of Physics and Astronomy has developed a model that shows how dark energy and dark matter are more closely linked than previously thought.

Dr Zhao points out, “Both dark matter and dark energy could be two faces of the same coin. “As astronomers gain understanding of the subtle effects of dark energy in galaxies in the future, we will solve the mystery of astronomical dark matter at the same time.”

Just a quick explainer. Dark energy was discovered in the late 1990s during a survey of distant supernova. Instead of finding evidence that the mutual gravity of all the objects in the Universe is slowing down its expansion, researchers discovered that its expansion is actually accellerating.

Dark matter was first theorized back in 1933 by Swiss astronomer Fritz Zwicky. He noted that galaxies shouldn’t be able to hold themselves together with just the regular matter we can see. There must be some additional, invisible matter surrounding the regular matter that provides the additional gravitational force to hold everything together.

And since their discoveries plenty of additional evidence for both dark energy and dark matter have been seen across the Universe.

In Dr. Zhao’s model, dark energy and dark matter the same thing that he calls a “dark fluid”. On the scale of galaxies, this fluid behaves like matter, providing a gravitational force. And in the large scales, the fluid helps drive the expansion of the Universe.

Dr. Zhao’s model is detailed enough to produce the same 3:1 ratio of dark energy to dark matter measured by cosmologists.

Of course, any theory like this only gains ground when it starts making predictions that can be tested through observation. Dr. Zhao expects the work at the Large Hadron Collider to be fruitless. If he’s right, dark matter particles will have such low energy that the collider won’t be able to generate them.

The paper was recently published in the Astrophysical Journal Letters in December 2007, and Physics Review D. 2007.

Original Source: University of St. Andrews News Release

Building a Map of Dark Energy

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Understanding dark energy is probably one of the largest mysteries in cosmology right now. Figure this out, and a Nobel Prize in physics is assured. But it won’t give up its secrets so easily. Since they can’t measure dark energy directly, astronomers can only measure its effects on the large-scale structures of the Universe, and then tease out some of its characteristics.

In a recent survey using the European Southern Observatory’s Very Large Telescope, and international team of 51 astronomers from 24 institutions gathered the light from more than 13,000 galaxies. This survey contains galaxies within a 25 million cubic-light years volume.

Using this survey, they were able to map out the large scale structures in the nearby Universe, including clusters of galaxies and filamentary superclusters. By measuring the velocity of the different clusters as they speed away from us, at different times in the past, the astronomers were able to measure the speed distortion contributed by dark energy.

Their conclusions lend evidence to the so-called cosmological constant theory originally introduced by Albert Einstein – there’s some constant repulsive force that exists throughout space. But the uncertainties in their calculations doesn’t rule out other possibilities.

One of the most popular of these is known as modified gravity. In this situation, gravity just acts differently at large scales than it does in the small scale.

But the researchers feel that a measurement that might cover 10 times the volume of this survey could be able to tell the difference between a dark energy force of exotic origin or an actual modification of theory of gravity.

Original Source: ESO News Release

Large Hadron Collider Could Detect “Unparticles”

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Understanding the mysterious dark matter in our universe is paramount to cosmologists. Dark matter and dark energy makes up the vast majority of mass in the observable universe. It influences galaxy rotation, galactic clusters and even holds the answer to our universe’s fate. So, it is unsurprising to hear about some outlandish physics behind the possible structure of this concealed mass. A Harvard scientist has now stepped up the plate, publishing his understanding about dark matter, believing the answer may lie in a type of material that has a mass, but doesn’t behave like a particle. “Unparticles” may also be detected by the high energy particle accelerator, the Large Hadron Detector (LHD) at CERN going online in a few weeks time. High energy physics is about to get stranger than it already is…

Dark matter is theorized to take on many forms, including: neutron stars, weakly interacting massive particles (WIMPs), neutrinos, black holes and massive compact halo objects (MACHOs). It is hard, however, to understand where the majority of mass comes from if you can’t observe it, so much of what we “know” about this dark source of matter and energy will remain theory until we can actually find a way of observing it. Now, we have a chance, not only to observe a form of dark matter, but also to generate it.
A simulation of a LHC collision (credit:CERN)
Professor Howard Georgi, a Harvard University physicist, wants to share his idea that the “missing mass” of the universe may be held in a type of matter that cannot be explained by the current understanding of physics. The revelation came to him when he was researching what can be expected from the future results of LHC experiments. Beginning with quantum mechanics (as one would expect), he focused on the interactions between sub-atomic particles. Using the “Standard Model”, which describes everything we know and understand about matter in our universe (interactions, symmetry, leptons, bosons etc.), Georgi soon came to a dead end. He then side stepped a basic premise of the standard model: the forces that govern particle interactions act differently at different length scales.

I did think I was crazy,” Prof. Georgi on the moment he stumbled on the “unparticle theory”.

This is one of the major failings of the standard model – the unification of the four universal forces: weak force, strong force, electromagnetic force and gravitational force. The standard model unites the first three, but neglects gravity. Gravity simply does not fit. So Georgi took the bold step and calculated what could be generated by the LHC without the help of standard sub-atomic thinking and scale length restrictions.

The unparticle would therefore be “scale invariant”, a property of fractals. Unparticles can interact over any scale lengths without restriction. When viewed, the unparticle will act as a fractal and will look similar over any scale (a characteristic known as self-similarity). Unparticles will also take on any mass, some or all the mass, depending on the scale you are viewing at. Now the implication of mass suddenly becomes interesting to the dark matter hunters out there. Unparticles could be a huge source of dark matter.

As they have mass, unparticles would also possess an “ungravity”. Ungravity should have a strong, short-distance effect on matter in the observable world, and so, may be observed by sufficiently sensitive gravity probes.

Whether unparticles exist or not, exploring the possibility that standard thinking may need to be slightly re-jigged for the extreme world of high energy particle collisions will surely lead to some healthy scientific debate. For now, we wait in anticipation for when the LHC goes online in May this year.

Source: Telegraph.co.uk

Finding Dark Energy in a Supercomputer

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Dark energy is probably the most influential force in the cosmos, overwhelming the pull of dark matter, and absolutely dominating the meager impact of regular matter. And scientists have absolutely no idea what it is. But a new supercomputer simulation by cosmologists at Durham University might give astronomers a few places to look; to know how to measure this mysterious force.

When dark energy was discovered in 1998, it came as a complete surprise. By measuring the distance to supernovae, astronomers were hoping to calculate the rate at which the Universe’s expansion is slowing down. Instead of slowing down, though, they found that the expansion of the Universe is actually accelerating. Instead of coming together in a big crunch, it looks like dark energy will spread the Universe out faster and faster.

Physicists now believe that dark energy makes up 70% of the Universe, with the remaining amount made of mostly dark matter, and a sprinkling of regular matter. Since that discovery, astronomers haven’t been able to find the source of this dark energy.

So a new simulation, run on Durham University’s Cosmology Machine supercomputer could help astronomers in their search. The simulation looked at the tiny ripples in the distribution of matter in the Universe made by sound waves a few hundred thousand years after the Big Bang. These ripples have long since been destroyed by the 13.7 billion years of the lifetime of the Universe, but the simulations show they might have survived in some conditions.

By changing the nature of dark energy, the researchers found that the ripples changed in length. In other words, if astronomers can find the ripples in the real Universe, this can help constrain the parameters for dark energy.

Durham University Professor Carlos Frenk said, “the ripples are a gold standard. By comparing the size of the measured ripples to the gold standard we can work out how the Universe has expanded and from this figure out the properties of the dark energy.”

An upcoming ESA mission called the SPectroscopic All-sky Cosmic Explorer (SPACE) should have the capabilities to detect these ripples, and so help put some constraints on the nature of dark energy.

If all goes well, SPACE will launch in 2017.

Original Source: Durham University News Release

Has Dark Energy Always Been Constant?

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Dark energy is that mysterious force that seems to be accelerating the expansion of the Universe. But the question is: has it always been pushing the Universe apart with the same force, or was it weaker or stronger in the past, and will it get stronger in the future? Researchers from the Harvard-Smithsonian Center for Astrophysics have a plan to study distant clumps of hydrogen, to get to the bottom of this question, once and for all.

Dark energy was first discovered nearly a decade ago, when astronomers noticed that distant supernovae were further away than their calculations were expecting. Some mysterious force appears to be accelerating the expansion of the Universe from every point in space. As space expands, more dark energy seems to appear. And although the amount of dark energy in any one point in space is tiny, across the vast reaches of space, it really adds up, accounting for more than 70% of the Universe.

If dark energy is increasing, however, you could imagine it eventually becoming so strong that it starts to tear galaxy clusters apart, and then galaxies themselves, and even star systems. Maybe it might even become so strong that it tears apart atoms and even the fabric of space itself. Astronomers call this theory the “Big Rip”. Or maybe just the opposite is true, and dark energy will eventually become negligible to the expansion of the Universe.

In order to see if the strength of dark energy is changing over time, astronomers are planning to carefully plot the position of clouds of neutral hydrogen, shortly after they formed from the Big Bang. Although it’s not possible now, future planned observatories should be able to trace this material all the way back to a time when the Universe was only 200 million years old.

In the early Universe, small fluctuations in energy density and pressure caused oscillations. Although tiny in the beginning, these ripples have been magnified by the expansion of the Universe so that they stretch 500 million light-years across today. The clouds of neutral hydrogen should follow the same ripple pattern, so astronomers will know they’re looking at those first, primordial clouds, and not some closer ones.

And so, astronomers will be able to look back in time, and study the distance to the clouds at each epoch in our Universe’s expansion. They should be able to trace how much dark energy was affecting space at each time, and get a sense if this energy has always remained constant, or if it’s changing.

Their answers will shape our understanding of the Universe’s evolution, and its future.

Original Source: CfA News Release

Chandra Furthers Understanding About Dark Energy

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Dark energy. Does it exist, and what are its properties? Using galaxy-cluster images from NASA’s Chandra X-ray Observatory, astronomers have applied a powerful, new method for detecting and probing dark energy. The results offer intriguing clues about the nature of dark energy and the fate of the Universe. The Marshall Center manages the Chandra program.
Photo: Composite image of the galaxy cluster Abell 2029 (Optical: NOAO/Kitt Peak/J.Uson, D.Dale; X-ray: NASA/CXC/IoA/S.Allen et al.)

Astronomers have detected and probed dark energy by applying a powerful, new method that uses images of galaxy clusters made by NASA’s Chandra X-ray Observatory. The results trace the transition of the expansion of the Universe from a decelerating to an accelerating phase several billion years ago, and give intriguing clues about the nature of dark energy and the fate of the Universe.

“Dark energy is perhaps the biggest mystery in physics,” said Steve Allen of the Institute of Astronomy (IoA) at the University of Cambridge in England, and leader of the study. “As such, it is extremely important to make an independent test of its existence and properties.”

Allen and his colleagues used Chandra to study 26 clusters of galaxies at distances corresponding to light travel times of between one and eight billion years. These data span the time when the Universe slowed from its original expansion, before speeding up again because of the repulsive effect of dark energy.

“We’re directly seeing that the expansion of the Universe is accelerating by measuring the distances to these galaxy clusters,” said Andy Fabian also of the IoA, a co-author on the study. The new Chandra results suggest that the dark energy density does not change quickly with time and may even be constant, consistent with the “cosmological constant” concept first introduced by Albert Einstein. If so, the Universe is expected to continue expanding forever, so that in many billions of years only a tiny fraction of the known galaxies will be observable.

If the dark energy density is constant, more dramatic fates for the Universe would be avoided. These include the “Big Rip,” where dark energy increases until galaxies, stars, planets and eventually atoms are eventually torn apart. The “Big Crunch,” where the Universe eventually collapses on itself, would also be ruled out.

Chandra’s probe of dark energy relies on the unique ability of X-ray observations to detect and study the hot gas in galaxy clusters. From these data, the ratio of the mass of the hot gas and the mass of the dark matter in a cluster can be determined. The observed values of the gas fraction depend on the assumed distance to the cluster, which in turn depends on the curvature of space and the amount of dark energy in the universe.

Because galaxy clusters are so large, they are thought to represent a fair sample of the matter content in the universe. If so, then relative amounts of hot gas and dark matter should be the same for every cluster. Using this assumption, Allen and colleagues adjusted the distance scale to determine which one fit the data best. These distances show that the expansion of the Universe was first decelerating and then began to accelerate about six billion years ago.

Chandra’s observations agree with supernova results including those from the Hubble Space Telescope (HST), which first showed dark energy’s effect on the acceleration of the Universe. Chandra’s results are completely independent of the supernova technique – both in wavelength and the objects observed. Such independent verification is a cornerstone of science. In this case it helps to dispel any remaining doubts that the supernova technique is flawed.

“Our Chandra method has nothing to do with other techniques, so they’re definitely not comparing notes, so to speak,” said Robert Schmidt of University of Potsdam in Germany, another coauthor on the study.

Better limits on the amount of dark energy and how it varies with time are obtained by combining the X-ray results with data from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), which used observations of the cosmic microwave background radiation to discover evidence for dark energy in the very early Universe. Using the combined data, Allen and his colleagues found that dark energy makes up about 75% of the Universe, dark matter about 21%, and visible matter about 4%.

Allen and his colleagues stress that the uncertainties in the measurements are such that the data are consistent with dark energy having a constant value. The present Chandra data do, however, allow for the possibility that the dark energy density is increasing with time. More detailed studies with Chandra, HST, WMAP and with the future mission Constellation-X should provide much more precise constraints on dark energy.

“Until we better understand cosmic acceleration and the nature of the dark energy we cannot hope to understand the destiny of the Universe,” said independent commentator Michael Turner, of the University of Chicago.

The team conducting the research also included Harald Ebeling of the University of Hawaii and the late Leon van Speybroeck of the Harvard-Smithsonian Center for Astrophysics. These results will appear in an upcoming issue of the Monthly Notices of the Royal Astronomy Society.

NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA’s Office of Space Science, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Additional information and images are available at:

http://chandra.harvard.edu/
and
http://chandra.nasa.gov/

Original Source: NASA News Release