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RE: Dark Energy
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The first results from the Supernova Legacy Survey (SNLS) project, an international effort to probe the nature of dark energy, suggests that Dark energy has remained constant over the life of the universe.
It will not fade away as some hypotheses have suggested.
The data shows that the strength of repulsion cannot have changed by more than about 20% over the past eight billion years, when the universe was just half of its current size.
Many alternative theories predicted that dark energy fades in strength.
One idea that was ruled out was that the repulsion comes from topological defects – fractures in space that might have been left behind as the universe cooled after the big bang. But the density of topological defects would fade too fast.

The results, to be published in Astronomy and Astrophysics, fit the most conservative theory of dark energy – that space has some inherent, and constant, energy density.

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Supernova Legacy Survey
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The enigmatic dark energy that drives the accelerating expansion of the universe behaves just like Einstein's famed cosmological constant, according to the Supernova Legacy Survey (SNLS), an international team of researchers in France and Canada that collaborated with large telescope observers at Oxford, Caltech and Berkeley.

Their observations reveal that the dark energy behaves like Einstein's cosmological constant to a precision of 10 per cent.

"The significance is huge. Our observation is at odds with a number of theoretical ideas about the nature of dark energy that predict that it should change as the universe expands, and as far as we can see, it doesn't" - Professor Ray Carlberg, Department of Astronomy and Astrophysics at U of T.

"The Supernova Legacy Survey is arguably the world leader in our quest to understand the nature of dark energy" - Chris Pritchet, a professor of physics and astronomy at the University of Victoria in British Columbia, Canada, study co-author.

The researchers made their discovery using an innovative, 340-million pixel camera called MegaCam, built by the Canada-France-Hawaii Telescope and the French atomic energy agency, Commissariat a l'Energie Atomique.

"Because of its wide field of view — you can fit four moons in an image — it allows us to measure simultaneously, and very precisely, several supernovae, which are rare events" - Pierre Astier, Centre National de la Recherche Scientifique (CNRS) in France.

"Improved observations of distant supernovae are the most immediate way in which we can learn more about the mysterious dark energy. This study is a very big step forward in quantity and quality" - Richard Ellis, a professor of astronomy at the California Institute of Technology.

"The data is more beautiful than we could have imagined 10 years ago — a real tribute to the instrument builders, the analysis teams and the large scientific vision of the Canadian and French science communities" - Saul Perlmutter, a physics professor at the University of California, Berkeley, Study co-author.

The findings will start a dramatic new generation of cosmology work using supernovae.

The results will be published in an upcoming issue of the journal Astronomy & Astrophysics.

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SuperNova legacy survey first results describing the nature of dark energy

The SuperNova Legacy Survey is an international collaboration involving about 40 researchers, which aims to discover several hundred far supernovae and measure their distance. The team's first results will be published in a coming issue of Astronomy & Astrophysics.
The SuperNova Legacy Survey is the largest observational project of its kind. It started in 2003 and will last for five years.
So far, the team has measured the distance to 71 supernovae that exploded between 2 and 8 billion years ago. Many of the largest telescopes worldwide are involved in this project; the imaging part of the programme is carried out at the Canada-France-Hawaii Telescope (CFHT), in the framework of the CFHT Legacy Survey. Spectroscopic observations are obtained at the ESO/Very Large Telescope, the Gemini and Keck observatories.

Measuring the distance to faraway supernovae is a key tool for cosmologists. Supernovae are exploding stars, known to have similar brightness whatever their location in other galaxies. Observing these exploding stars can thus make it possible to measure their distances: they are known as "standard candles" for measuring long distances in the Universe.



This supernova is as bright as 100 billion Sun-like stars. It exploded 3 billion years ago.
Credit CFHTLS/SNLS/Terapix

Measurement of these distances revealed a startling phenomenon; in the late 1990s, astronomers found that the expansion of the Universe is accelerating. American astronomer Edwin Hubble first discovered this expansion in 1929. The expansion of the Universe was thought to be slowing down because of the gravitational attraction of matter. Astronomers were thus very surprised to discover this was not the case at all.
Theorists then attempted to explain the acceleration of expansion through various cosmological models. These models all involved the so-called "dark energy" concept, which is a kind of repulsive force against gravitational attraction. Nobody knows what dark energy is, but we can make an attempt to understand how it behaves.

In recent years, cosmological observations have supported that the Universe is made of about 25 % of matter and 75 % of dark energy. Unlike matter, which dilutes with expansion, dark energy appears to stay roughly constant.
The new results, to be published by the SuperNova Legacy Survey team, put strong constraints on the absence of dilution of dark energy. Einstein himself already foresaw such a kind of dark energy when he introduced the famous "cosmological constant" into his General Relativity equations. Such a constant was needed for the equations to be consistent with a static universe, as it was believed to be at that time.
When the Universe was discovered to be expanding, it seemed that the cosmological constant was no longer needed in the equations. Later, Einstein referred to it as his "greatest blunder".
The discovery of the accelerating Universe expansion suggested the need for a cosmological constant that might, among other models, explain the acceleration of the expansion. The first results of the Legacy Survey indeed show that the existence of a cosmological constant is the best way to fit their observations.
Once completed, by the end of 2008, their Survey will bring even more restrictive constraints to these cosmological models. It will help us better understand the physical nature of this cosmological constant: 80 years later, "Einstein's greatest blunder" is perhaps less of a blunder after all.

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The 'Big Trip'
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The universe may end with everything being swallowed by a giant wormhole – a scenario dubbed the 'Big Trip'.
According to cosmologist Pedro Gonzalez-Diaz at the Institute of Mathematics and Fundamental Physics, CSIC, Madrid, the gradual inflow of so-called phantom energy into a wormhole could cause it to swell up so much so that it would eventually engulf the entire universe. Phantom energy is a hypothetical explanation for dark energy; the puzzling stuff thought to be responsible for the accelerated expansion of the universe. The defining property of phantom energy is that its energy density – the amount of energy per unit volume – increases over time.
The energy density of all known matter, by contrast, decreases or remains constant over time. This has led some theorists to suggest that phantom energy will cause the universe to grow ever faster until it eventually blows up in a scenario dubbed the 'Big Rip'

Phantom inflation and the "Big Trip"
Authors: Pedro F. Gonzalez-Diaz (IMAFF, CSIC), Jose A. Jimenez-Madrid (IAA, CSIC)

Primordial inflation is regarded to be driven by a phantom field which is here implemented as a scalar field satisfying an equation of state p = wp, with w < -1. Being even aggravated by the weird properties of phantom energy, this will pose a serious problem with the exit from the inflationary phase. We argue however in favour of the speculation that a smooth exit from the phantom inflationary phase can still be tentatively recovered by considering a multiverse scenario where the primordial phantom universe would travel in time toward a future universe filled with usual radiation, before reaching the big rip. We call this transition the "big trip" and assume it to take place with the help of some form of anthropic principle which chooses our current universe as being the final destination of the time transition.

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On the accretion of phantom energy onto wormholes
Authors: Pedro F. Gonzalez-Diaz (IMAFF, CSIC)

By using a properly generalized accretion formalism it is argued that the accretion of phantom energy onto a wormhole does not make the size of the wormhole throat to comovingly scale with the scale factor of the universe, but instead induces an increase of that size so big that the wormhole can engulf the universe itself before it reaches the big rip singularity, at least relative to an asymptotic observer.

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Expansion
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Cosmologists have long accepted that the universe is expanding.
The space between galaxies is stretching.

But why is it that normal thing like tables and chairs don’t expand?

Now, Richard Price, a physicist at the University of Texas at Brownsville, has worked out why some objects are stretched by cosmological expansion, and others are not.
Atoms are made up mostly of empty space, with electrons "orbiting" the nucleus at distances typically many hundreds of times its diameter, it seemed reasonable to ask whether the electrons would be dragged away from the nucleus by the stretching of space.
Price examined the simplest system, that of a hydrogen atom, with one negative electron orbiting a positive proton. He found if the force involved - electromagnetic in the case of atoms - binding the system together is stronger than a certain critical value, the system will be entirely unaffected by the cosmological expansion

"This means that the solar system - which is quite tightly bound by gravity - doesn't expand. Your desk doesn't expand. Your dog doesn't expand. But clusters of galaxies, which are only loosely bound by gravity, will feel this effect" - Richard Price.

In an expanding universe, what doesn't expand?
Authors: Richard H. Price

The expansion of the universe is often viewed as a uniform stretching of space that would affect compact objects, atoms and stars, as well as the separation of galaxies. One usually hears that bound systems do not take part in the general expansion, but a much more subtle question is whether bound systems expand partially. In this paper, a very definitive answer is given for a very simple system: a classical ``atom'' bound by electrical attraction. With a mathemical description appropriate for undergraduate physics majors, we show that this bound system either completely follows the cosmological expansion, or -- after initial transients -- completely ignores it. This ``all or nothing'' behaviour can be understood with techniques of junior-level mechanics. Lastly, the simple description is shown to be a justifiable approximation of the relativistically correct formulation of the problem.

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Source for previous article and image.

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Seven years ago, two research groups turned the field of cosmology on its head with their announcements that the expansion of the universe was accelerating. Their theories were based on the red-shift of light from distant supernovae, and their findings suggested that the expansion of the universe was actually accelerating, not decelerating.

The mysterious force behind this accelerated expansion was given the name "dark energy". Prior to this, conventional scientific wisdom held that the Big Bang had resulted in an expansion of the universe that would gradually be slowed down and eventually reverse, ending in a Big Crunch.
Now, cosmologists are scrambling to determine what exactly dark energy is and understand how it affects the expansion of the universe. Back in 1917, Einstein amended his General Theory of Relativity with a cosmological constant, which, if the value was right, would allow the universe to exist in a perfectly balanced, static state. Einstein later called the addition of this constant his "greatest blunder," but the notion of dark energy has revived the idea.

"The cosmological constant was a vacuum energy (the energy of empty space) that kept gravity from pulling the universe in on itself. A problem with the cosmological constant is that it is constant, with the same energy density, pressure, and equation of state over time. Dark energy, however, had to be negligible in the universe's earliest stages; otherwise the galaxies and all their stars would never have formed" - Eric Linder, Berkeley Lab.

To answer the questions posed by dark energy, Linder and Robert Caldwell of Dartmouth, have proposed several scenarios for how dark energy might interact with the universe. Their paper, to be published in Physical Review Letters, identifies models of dark energy which could be used to rule out Einstein's cosmological constant and explain the nature of dark energy. And, scientists should be able to determine which of these scenarios is correct with the experiments being planned for the Joint Dark Energy Mission (JDEM) that has been proposed by NASA.

Scientists have been arguing the question 'how precisely do we need to measure dark energy in order to know what it is?'.

JDEM could be part of the SuperNova/Acceleration Probe (SNAP), a three-mirror, 2-meter reflecting telescope in deep-space orbit that would be used to find and measure thousands of Type Ia supernovae each year. These measurements should provide enough information to clearly point towards either the thawing or freezing scenario - or to something else entirely new and unknown.

In their paper, Linder and Caldwell describe two scenarios, one they call "thawing" and one they call "freezing," which point toward distinctly different fates for our permanently expanding universe. Under the thawing scenario, the acceleration of the expansion will gradually decrease and eventually come to a stop. Expansion may continue more slowly, or the universe may even collapse.
Under the freezing scenario, acceleration continues indefinitely, like a car with the gas pedal pushed to the floor. Both of these scenarios rule out Einstein's cosmological constant. Under any scenario, however, dark energy is a force that must be reckoned with.

"Because dark energy makes up about 70 percent of the content of the universe, it dominates over the matter content. That means dark energy will govern expansion and, ultimately, determine the fate of the universe" - Eric Linder.

So where does dark energy come from? Enter the concept of quintessence. "Quintessence is a dynamic, time-evolving, and spatially dependent form of energy with negative pressure sufficient to drive the accelerating expansion. Whereas the cosmological constant is a very specific form of energy - vacuum energy - quintessence encompasses a wide class of possibilities" - Robert Caldwell.
Linder and Caldwell have proposed tests to see if quintessence - named after the fifth element of the ancient Greeks - is the source of dark energy.
As the basis for those tests, Linder and Caldwell used a scalar field as their model. A scalar field possesses a measure of value, but not direction, for all points in space. With this approach, they were able to show quintessence as a scalar field relaxing its potential energy down to a minimum value. An analogy might be a set of springs under tension and exerting a negative pressure that counteracts the positive pressure of gravity.

"A quintessence scalar field is like a field of springs covering every point in space, with each spring stretched to a different length. For Einstein's cosmological constant, each spring would be the same length and motionless" - Eric Linder.

Under the thawing scenario, the potential energy of the quintessence field was "frozen" in place until the decreasing material density of an expanding universe gradually released it. In the freezing scenario, the quintessence field has been rolling towards its minimum potential since the universe underwent inflation, but as it comes to dominate the universe it gradually becomes a constant value.

"If the results from measurements such as those that could be made with SNAP lie outside the thawing or freezing scenarios, then we may have to look beyond quintessence, perhaps to even more exotic physics, such as a modification of Einstein's General Theory of Relativity to explain dark energy" - Eric Linder.

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Astronomers are developing the Wide-Field Multi-Object Spectrograph (WFMOS), a powerful instrument to hunt for rings of galaxies that formed on the edges of the sound waves that formed shortly after the big bang.
The rings would offer an alternative to using supernovae to illuminate the nature of dark energy and the expansion rate of the universe.

Statistically galaxies are currently slightly more likely to be 500 million light years apart than any other distance.

Predictions made 30 years ago map out how the nearly featureless infant universe should have matured into the structured place seen today.
The early quantum fluctuations bubbling through space created pockets of different density in hot ionised plasma and photons that eventually clumped into structures like stars and galaxies.

But when the clumps of gas began to collapse and grow, photons trapped in the hot dense ionised plasma exerted an outward pressure that counteracted the growth. These opposing forces set off pressure, or sound waves that oscillated within the gas.

About 400,000 years after the big bang, the universe had expanded and cooled enough for electrons and protons in the plasma to start combining into neutral hydrogen, and freeze the shape of the sound waves. The process lasted about 600,000 years, forming sound waves about 500,000 light years long that then "froze" and expanded with the rest of space.

Galaxies seen today outline those early frozen shapes.
The instrument called the Wide-Field Multi-Object Spectrograph (WFMOS), which may be installed on either the 8.2-metre Subaru telescope or the 8.0-metre Gemini telescope in Hawaii.
WFMOS will determine the locations of 4000 galaxies at once, using optical fibres to channel their light to a spectrograph that separates the light into its component wavelengths. Over 150 nights, it could map 2 million galaxies, probing the structure of the universe when it was between one-fifth and one-half its present age. This represents 10 times the number of galaxies studied with the 2-degree Field Galaxy Redshift Survey, and explores about five times as far back in time as the mammoth Sloan Digital Sky Survey.

"These galaxies are so much fainter and so much farther away, which is why this is 10 times harder to do than those surveys" - Karl Glazebrook, a WFMOS team member at Johns Hopkins University in Baltimore, Maryland, US.

The WFMOS survey would study the expansion of the universe as a function of time. But instead of measuring distance by how much the light from each galaxy had been red-shifted by the expansion of space, as is usually done, it would look for the statistical signal of "rings" of galaxies of a characteristic size.
Studies of the cosmic microwave background - the afterglow of the big bang - and more recent observations of galaxies, show that more galaxies formed on the leading edges of sound waves generated in the first million years after the big bang.
These ripples provide a "cosmic yardstick", allowing astronomers to calculate distance by comparing the apparent width, or angular size, of the ripples to their actual physical size.

"It is important to use all good observational tests to measure dark energy" - Alan Dressler, astronomer at the Carnegie Observatories in Pasadena, California, US.

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Cosmologists from Princeton University have proposed a new technique that will be able to determine if the cosmic acceleration (expansion of the universe) is due to a yet unknown form of Dark Energy in the universe or if it is a signature of a breakdown of Einstein's theory of General Relativity at very large scales of the universe.
"The accelerating expansion of the universe constitutes one of the most intriguing and challenging problems in astrophysics. Moreover, it is related to problems in many other fields of physics. Our research work is focused on constraining different possible causes of this acceleration." - Dr. Mustapha Ishak-Boushaki, research associate at Princeton University.
Several independent astronomical observations, during the last 8 years, have demonstrated that the expansion of the universe has entered a phase of acceleration.
"It could be a whole new form of energy or the observational signature of the failure of Einstein's theory of General Relativity. Either way, its existence will have profound impact on our understanding of space and time. Our goal is to be able to distinguish the two cases." - Professor David Spergel, from Princeton.
Some of the recently proposed modified gravity models are inspired by Superstring theory and extra dimensional physics.
The new proposed procedure shows that we distinguish between these two possibilities.
The basic idea is; if the acceleration is due to Dark Energy then the expansion history of the universe should be consistent with the rate at which clusters of galaxies grow. Deviations from this consistency would be a signature of the breakdown of General Relativity at very large scales of the universe.
The procedure proposed implements this idea by comparing the constraints obtained on Dark Energy from different cosmological probes and allows one to clearly identify any inconsistencies.
As an example, a universe described by a 5-dimensional modified gravity theory was considered in this study and it was shown that the procedure can identify the signature of this theory. Importantly, it was shown that future astronomical experiments can distinguish between modified gravity theories and Dark Energy models.
The research work on the results presented was led by Dr. Mustapha Ishak-Boushaki in collaboration with Professor David Spergel, both from the Department of Astrophysical Sciences at Princeton University, and Amol Upadhye, a graduate student at the Physics Department at Princeton University.


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