* Astronomy

Blobrana · L

RE: Dark Energy

Blobrana · L

This time ten years ago, two independent teams of researchers in the US were deliberating over whether to go public with a discovery that would change our view of the universe forever. It concerned observations of distant supernovae that appeared to be moving away from each other faster than they should have been. A few weeks later the world found out that the expansion of the universe is accelerating, probably driven by some kind of gravitationally repulsive dark energy that makes up 75% of the universe.

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Galaxies today are struggling to clump together against the incredible repulsive power of dark energy, hints a new survey of thousands of galaxies. Measuring this anti-clumping effect puts a new arrow in the quiver of cosmologists seeking to uncover the nature of the mysterious force.
Scientists proposed the existence of a mysterious repulsive force called dark energy in 1998 to explain supernova observations showing the universe is expanding at ever faster rates.
Since then, researchers have been trying to measure the properties of dark energy more precisely, in the hope of discovering what it is. Possible explanations include fluctuating energy fields from quantum physics and the effects of unseen extra spatial dimensions. In some scenarios, the strength of dark energy changes with time in characteristic ways.
Now, a study led by Luigi Guzzo of Brera Astronomical Observatory in Merate, Italy, may pave the way for researchers to decide between the different theories. They wanted to see if dark energy had any effect on the motion of galaxies at different times since the big bang.

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Cosmologists have run a series of huge computer simulations of the Universe that could ultimately help solve the mystery of dark energy.
Results of the simulations, carried out by Durham Universitys world-leading Institute for Computational Cosmology (ICC), tell researchers how to measure dark energy a repulsive force that counteracts gravity.
The findings, published today (Friday, January 11) in the Monthly Notices of the Royal Astronomical Society, will also provide vital input into the design of a proposed satellite mission called SPACE the SPectroscopic All-sky Cosmic Explorer - that could unveil the nature of dark energy.
The discovery of dark energy in 1998 was completely unexpected and understanding its nature is one of the biggest problems in physics.
Scientists believe dark energy, which makes up 70 per cent of the Universe, is driving its accelerating expansion. If this expansion continues to accelerate experts say it could eventually lead to a Big Freeze as the Universe is pulled apart and becomes a vast cold expanse of dying stars and black holes.
The Durham research was funded by the Science and Technology Facilities Council and the European Commission
The simulations, which took 11 days to run on Durhams unique Cosmology Machine (COSMA) computer, looked at tiny ripples in the distribution of matter in the Universe made by sound waves a few hundred thousand years after the Big Bang.
The ripples are delicate and some have been destroyed over the subsequent 13 billion years of the Universe, but the simulations showed they survived in certain conditions.
By changing the nature of dark energy in the simulations, the researchers discovered that the ripples appeared to change in length and could act as a standard ruler in the measurement of dark energy.

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. Astronomers are stuck with the one universe we live in. However, the simulations allow us to experiment with what might have happened if there had been more or less dark energy in the universe - Carlos Frenk, ICC Director Professor.

In the next five to 10 years a number of experiments are planned to explore dark energy. The Durham simulation has demonstrated the feasibility of the SPACE satellite mission proposed to the European Space Agencys (ESA) Cosmic Vision programme.
The project has been put forward by an international consortium of researchers including the Durham team.
SPACE, which is led by Bologna University, in Italy, is through to the next round of assessment by the ESA and if successful is planned to launch in 2017.

Thanks to the ICC simulations it is possible to predict what SPACE would observe and to plan how to develop the mission parameters in order to obtain a three-dimensional map of the Universe and to compare it with the predictions of the simulations. Thanks to this comparison it will be possible to unveil the nature of dark energy and to understand how the structures in the Universe built up and evolved with cosmic time - Co-principal investigator Professor Andrea Cimatti, of Bologna University.

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-- Edited by Blobrana at 10:07, 2008-01-11

Blobrana · L

Title: Local Void vs Dark Energy: Confrontation with WMAP and Type Ia Supernovae
Authors: Stephon Alexander, Tirthabir Biswas, Alessio Notari, Deepak Vaid

It is now a known fact that if we happen to be living in the middle of a large underdense region, then we will observe an "apparent acceleration", even when any form of dark energy is absent. In this paper, we present a "Minimal Void" scenario, i.e. a "void" with minimal underdensity contrast (of about -0.4) and radius (~ 200-250 Mpc/h) that can, not only be consistent with the supernovae data, but also with the 3-yr WMAP data. We also discuss consistency of our model with various other measurements such as Big Bang Nucleosynthesis, Baryon Acoustic Oscillations and local measurements of the Hubble parameter. We also point out possible other observable signatures.

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It might seem as if astronomers and astrophysicists have had enormous success at unlocking the mysteries of space.
Impressive evidence has been gathered to support the theory that our universe was created about 13.7 billion years ago with an explosion of energy that eventually formed the innumerable galaxies still spinning away from one another to uncharted expanses of space.
We've discovered distant planets that might be friendly to life as we know it and have estimated distances to remote pulsing stars to help map the universe. We've assessed the power of black holes and remain awestruck by the extraordinary beauty of images of distant galaxies, young and old, captured by the Hubble Space Telescope.
But central mysteries of the universe remain unsolved, and from a scientific viewpoint, there are still more questions than answers. Most scientists would agree that we know very little about what really makes up our universe - and little about its origin and possible fate.

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Mankind 'shortening the universe's life'
Forget about the threat that mankind poses to the Earth: our very ability to study the heavens may have shortened the inferred lifetime of the cosmos.
But there is an odd feature of the theory that philosophers and scientists still argue about. In a nutshell, the theory suggests that quantum systems can exist in many different physical configurations at the same time. By observing the system, however, we may pick out one single 'quantum state', and therefore force the system to change its configuration.

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Have we hastened the demise of the universe by looking at it? Thats the startling question posed by a pair of physicists, who suggest that we may have accidentally nudged the universe closer to its death by observing dark energy, which is thought to be speeding up cosmic expansion.
Lawrence Krauss of Case Western Reserve University in Cleveland, Ohio, and colleague James Dent suggest that by making this observation in 1998 we may have caused the universe to revert to a state similar to early in its history, when it was more likely to end.

Incredible as it seems, our detection of the dark energy may have reduced the life-expectancy of the universe - Lawrence Krauss.

The researchers came to their conclusion by calculating how the energy state of our universe might have evolved. Until recently, cosmologists thought that the big bang 13.7 billion years ago occurred after a bubble of weird high-energy false vacuum with repulsive gravity decayed into a zero-energy ordinary vacuum. The energy released during this transition could have made matter and heated it to a ferocious temperature, which essentially created the massive explosion of the big bang. The discovery of dark energy - and the realisation that the universes expansion is accelerating - reveals that the vacuum may not have decayed to zero energy, but to another false vacuum state. In other words, some energy was retained in this vacuum, and this is accelerating the universes expansion.

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Title: Dark Energy and Dark Gravity
Authors: Ruth Durrer, Roy Maartens

Observations provide increasingly strong evidence that the universe is accelerating. This revolutionary advance in cosmological observations confronts theoretical cosmology with a tremendous challenge, which it has so far failed to meet. Explanations of cosmic acceleration within the framework of general relativity are plagued by difficulties. General relativistic models are nearly all based on a dark energy field with fine-tuned, unnatural properties. There is a great variety of models, but all share one feature in common -- an inability to account for the gravitational properties of the vacuum energy. Speculative ideas from string theory may hold some promise, but it is fair to say that no convincing model has yet been proposed. An alternative to dark energy is that gravity itself may behave differently from general relativity on the largest scales, in such a way as to produce acceleration. The alternative approach of modified gravity (or dark gravity) provides a new angle on the problem, but also faces serious difficulties, including in all known cases severe fine-tuning and the problem of explaining why the vacuum energy does not gravitate. The lack of an adequate theoretical framework for the late-time acceleration of the universe represents a deep crisis for theory -- but also an exciting challenge for theorists. It seems likely that an entirely new paradigm is required to resolve this crisis.

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Title: Is the evidence for dark energy secure?
Authors: Subir Sarkar (Oxford U.)

Several kinds of astronomical observations, interpreted in the framework of the standard Friedmann-Robertson-Walker cosmology, have indicated that our universe is dominated by a Cosmological Constant. The dimming of distant Type Ia supernovae suggests that the expansion rate is accelerating, as if driven by vacuum energy, and this has been indirectly substantiated through studies of angular anisotropies in the cosmic microwave background (CMB) and of spatial correlations in the large-scale structure (LSS) of galaxies. However there is no compelling direct evidence yet for (the dynamical effects of) dark energy. The precision CMB data can be equally well fitted without dark energy if the spectrum of primordial density fluctuations is not quite scale-free and if the Hubble constant is lower globally than its locally measured value. The LSS data can also be satisfactorily fitted if there is a small component of hot dark matter, as would be provided by neutrinos of mass 0.5 eV. Although such an Einstein-de Sitter model cannot explain the SNe Ia Hubble diagram or the position of the `baryon acoustic oscillation' peak in the autocorrelation function of galaxies, it may be possible to do so e.g. in an inhomogeneous Lemaitre-Tolman-Bondi cosmology where we are located in a void which is expanding faster than the average. Such alternatives may seem contrived but this must be weighed against our lack of any fundamental understanding of the inferred tiny energy scale of the dark energy. It may well be an artefact of an oversimplified cosmological model, rather than having physical reality.

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