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TOPIC: Dark Energy
Blobrana


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Posts: 131433
Date:
Vacuum Energy
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Title: Vacuum Energy: Myths and Reality
Authors: G.E. Volovik


We discuss the main myths related to the vacuum energy and cosmological constant, such as: ''unbearable lightness of space-time''; the dominating contribution of zero point energy of quantum fields to the vacuum energy; non-zero vacuum energy of the false vacuum; dependence of the vacuum energy on the overall shift of energy; the absolute value of energy only has significance for gravity; the vacuum energy depends on the vacuum content; cosmological constant changes after the phase transition; zero-point energy of the vacuum between the plates in Casimir effect must gravitate, that is why the zero-point energy in the vacuum outside the plates must also gravitate; etc. All these and some other conjectures appear to be wrong when one considers the thermodynamics of the ground state of the quantum many-body system, which mimics macroscopic thermodynamics of quantum vacuum. In particular, in spite of the ultraviolet divergence of the zero-point energy, the natural value of the vacuum energy is comparable with the observed dark energy. That is why the vacuum energy is the plausible candidate for the dark energy.

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Blobrana


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The Largest Map
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An international team of astronomers released today (15 May) new results on the Cosmos, based on the largest map of the heavens ever produced.

This massive atlas emphatically confirmed recent findings that the Universe is full of ’dark energy’, a mysterious substance that makes up three-quarters of our Universe, together with ’dark matter’ which accounts for most of the remaining quarter. Understanding this composition is now one of the most important problems facing the whole of science.


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"We now have a precise view of what makes up our Universe, but little idea as to why. It is intriguing that the ordinary matter our bodies are made of and that we experience in everyday life only accounts for a few percent of the total cosmic budget" - Prof. Ofer Lahav, a member of the international team and the Head of the Astrophysics Group at University College London.

Our Universe contains billions of galaxies of all shapes and sizes. In recent years astronomers have used increasingly large surveys to map out the positions of these galaxies, stepping their way out into the Cosmos.
The new cosmic map unveiled today is the largest to date -- a three-dimensional atlas of over a million galaxies spread over a distance of more than 5 billion light years. The findings confirm that we live in a Universe filled with mysterious dark matter and dark energy.

"We have analysed the patterns in this map and discovered waves of structure over a billion light years across. These waves were generated billions of years ago and have been vastly stretched in size by the expanding Universe" - Dr. Chris Blake of the University of British Columbia, principal author of the study.

Construction of the cosmic atlas was led by co-author Dr. Adrian Collister of the University of Cambridge, as part of his PhD work, using a novel Artificial Intelligence technique he developed with his supervisor Prof. Ofer Lahav.

"By using very accurate distances of just 10,000 galaxies to train the computer algorithm we have been able to estimate reasonably good distances for over a million galaxies. This novel technique is the way of the future" - Dr. Adrian Collister.

The original 2-dimensional positions of colours of the one million galaxies were from the Sloan Digital Sky Survey.
The precise observations of the 10,000 galaxy distances were made as part of an international collaboration between U.S., U.K. and Australian teams using data from the Sloan Digital Sky Survey and the Anglo-Australian Telescope.
By measuring the positions of galaxies, astronomers can unravel the balance of forces that govern our Universe: the force of gravity which pulls everything together, and the competing effect of the expanding Universe which smoothes things out. These cosmic forces have arranged the galaxy distribution into a complex network of clusters, filaments and voids.

"The galaxy map can tell us the amount of ordinary ’baryonic’ matter relative to the amount of mysterious ’dark matter’. We have confirmed that over 80% of the material in the Universe consists of an invisible dark matter whose nature is not yet understood” - co-author Dr. Sarah Bridle, University College London.

The cosmic atlas of a million galaxies will shortly be made freely available on the World Wide Web for the benefit of other researchers. This free exchange of data is an important feature of modern astronomy, since many discoveries are only possible when different observations are combined.
The key problem in mapping the cosmos is determining the distance to each galaxy. Researchers can measure these distances because as the Universe expands, the colour of each galaxy changes as their emitted light waves are stretched or ‘redshifted’.

Traditionally, astronomers have needed to take a "spectrum" of each galaxy to determine this distance, splitting its light into many components to reveal sharp features with which to measure the amount of redshifting. This requires a time-consuming, individual observation of each galaxy.
The new cosmic map has been constructed using a novel technique focusing on a special class of galaxy whose intrinsic colour is very well known. For these ‘Luminous Red Galaxies’ researchers can measure the amount of colour distortion, and hence the approximate distance of the galaxy, just by looking at digital images of the sky, without the need to obtain a full spectrum.

All that is needed to exploit the technique is accurate observations of a small sample of the galaxies. In this case, precise measurements of just 10,000 galaxies were used to produce the atlas of over a million galaxies. These techniques will be very important for future large astronomical projects such as the Dark Energy Survey, scheduled to start in 2009, in which University College London and the universities of Portsmouth, Cambridge and Edinburgh are key partners.

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Cosmological parameters from a million photometric redshifts of SDSS LRGs
Authors: Chris Blake (UBC), Adrian Collister (IoA), Sarah Bridle (UCL), Ofer Lahav (UCL)

We analyse MegaZ-LRG, a new photometric-redshift catalogue of Luminous Red Galaxies (LRGs) based on the imaging data of the Sloan Digital Sky Survey (SDSS) 4th Data Release. MegaZ-LRG, presented in a companion paper, contains > 10^6 photometric redshifts derived with ANNz, an Artificial Neural Network method, constrained by a spectroscopic sub-sample of ~13,000 galaxies obtained by the 2dF-SDSS LRG and Quasar (2SLAQ) survey. The catalogue spans the redshift range 0.4<z<0.7 with an r.m.s. redshift error ~ 0.03(1+z). We present the first cosmological parameter fits to galaxy angular power spectra from a photometric redshift survey. Combining the redshift slices with appropriate covariances, we determine the matter density Omega_m and baryon density Omega_b in the combinations Omega_m h = 0.20 ±0.03 and Omega_b/Omega_m = 0.14 ±0.04. These results are in agreement with and independent of the latest studies of the Cosmic Microwave Background radiation, and their precision is comparable to analyses of contemporary spectroscopic-redshift surveys. We find visual suggestions of baryon oscillations in the clustering pattern, with a "model-independent" statistical significance of less than 3 sigma. On the largest scales probed by the survey we measure an excess of power with respect to the model with a significance of approximately 2 sigma. We perform an extensive series of tests which conclude that our power spectrum measurements are robust against potential systematic photometric errors in the catalogue. We conclude that photometric-redshift surveys are competitive with spectroscopic surveys for measuring cosmological parameters in the simplest "vanilla" models. Future deep imaging surveys have great potential for further improvement, provided that systematic errors can be controlled.

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Blobrana


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Posts: 131433
Date:
type Ia Distance
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Until now, scientists had used only one class of supernova - type Ia - to determine the distance to distant galaxies. All type Ia supernovae reach the same peak intrinsic brightness, so calculating their distance is a simple matter of quantifying their apparent brightness.
New research has found an independent way of checking such huge distances.

Astronomers at Lawrence Berkeley National Laboratory, California, have for the first time used the light from type II-P supernovae, a more common type, to estimate distances in comparison to Ia supernovae. The work will appear in a forthcoming issue of Astrophysical Journal

Researcher Eddie Baron of the University of Oklahoma ran numerous models of Type IIP supernovae to determine the extragalatic distance scale and determining the nature of dark energy. The dark energy has been discovered using Type Ia supernovae by two teams - the Berkeley Lab-based Supernova Cosmology Project and the High-Z Supernova Search Team, the two international groups of astronomers and physicists who discovered the accelerating expansion of the universe. This unknown energy, which is thought to make up about two thirds of the universe and acts to overcome gravity, can be verified independently by the use of Type II supernovae which Baron used the new NERSC machine to calculate. Baron said the models ran up to four times faster on the new IBM than on previous supercomputers.


Towards a Cosmological Hubble Diagram for Type II-P Supernovae

Authors: Peter Nugent (1), Mark Sullivan (2), Richard Ellis (3), Avishay Gal-Yam (3 and 4), Douglas C. Leonard (3 and 5), D. Andrew Howell (2), Pierre Astier (6), Raymond G. Carlberg (2), Alex Conley (2), Sebastien Fabbro (7), Dominique Fouchez (8), James D. Neill (9), Reynald Pain (6), Kathy Perrett (2), Chris J. Pritchet (9), Nicolas Regnault (6) ((1) Lawrence Berkeley National Laboratory, (2) University of Toronto, (3) California Institute of Technology, (4) Hubble Postdoctoral Fellow, (5) NSF Astronomy and Astrophysics Postdoctoral Fellow, (6) LPNHE, CNRS-IN2P3 and University of Paris VI & VII, (7) CENTRA, (8) CPPM, CNRS-IN2P3 and University Aix Marseille II, (9) University of Victoria)

We present the first high-redshift Hubble diagram for Type II-P supernovae (SNe II-P) based upon five events at redshift up to z~0.3. This diagram was constructed using photometry from the Canada-France-Hawaii Telescope Supernova Legacy Survey and absorption line spectroscopy from the Keck observatory. The method used to measure distances to these supernovae is based on recent work by Hamuy & Pinto (2002) and exploits a correlation between the absolute brightness of SNe II-P and the expansion velocities derived from the minimum of the Fe II 516.9 nm P-Cygni feature observed during the plateau phases.
We present three refinements to this method which significantly improve the practicality of measuring the distances of SNe II-P at cosmologically interesting redshifts. These are an extinction correction measurement based on the V-I colors at day 50, a cross-correlation measurement for the expansion velocity and the ability to extrapolate such velocities accurately over almost the entire plateau phase. We apply this revised method to our dataset of high-redshift SNe II-P and find that the resulting Hubble diagram has a scatter of only 0.26 magnitudes, thus demonstrating the feasibility of measuring the expansion history, with present facilities, using a method independent of that based upon supernovae of Type Ia.

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-- Edited by Blobrana at 20:44, 2006-04-26

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Blobrana


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Posts: 131433
Date:
Gravastar Theory
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Dark energy and dark matter, two of the greatest mysteries confronting physicists, may be two apects of the same phenomena; as a yet undiscovered kind of star , that could also in turn, remove the need for black holes.


The Gravastar theory is a proposal by Pawel Mazur and Emil Mottola to replace the black hole. Instead of a star collapsing into a pinpoint of space with virtually infinite density, the gravastar theory proposes that as an object gravitationally collapses, space itself undergoes a phase transition preventing further collapse, being transformed into a spherical void surrounded by a form of super-dense matter.

http://en.wikipedia.org/wiki/Gravastar

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Blobrana


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RE: Dark Energy
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Dark energy and dark matter, two of the greatest mysteries confronting physicists, may be two apects of the same phenomena; as a yet undiscovered kind of star , that could also in turn, remove the need for black holes.

George Chapline, a physicist at Lawrence Livermore National Laboratory in California, and Nobel laureate Robert Laughlin of Stanford University and their colleagues. Last week at the 22nd Pacific Coast Gravity Meeting in Santa Barbara, California, Chapline suggested that the objects that till now have been thought of as black holes could in fact be dead stars that form as a result of an obscure quantum phenomenon.

They propose that dead stars pass through a "quantum critical phase transition" and time would slow down on the surface and it would behave just like a black hole's event horizon.
The collapse of massive stars would create a thin quantum critical shell. The size of which would depend on the star's mass and, crucially, does not contain a space-time singularity. Instead, the shell contains a vacuum, just like the energy-containing vacuum of free space. As the star's mass collapses through the shell, it is converted to energy that contributes to the energy of the vacuum.

The team calculated that the vacuum energy inside the shell would act like an anti-gravity effect, just like the dark energy that appears to be causing the expansion of the universe to accelerate. Chapline has dubbed the objects produced this way "dark energy stars".
Francisco Lobo of the University of Lisbon in Portugal, has calculated that the anti-gravity effect will not to blow the star's shell apart. He calculate that stable dark energy stars can exist for a number of different models of vacuum energy.

"Dark energy stars and black holes would have identical external geometries, so it will be very difficult to tell them apart. All observations used as evidence for black holes - their gravitational pull on objects and the formation of accretion discs of matter around them - could also work as evidence for dark energy stars" - Francisco Lobo.

He and his colleagues have also calculated the energy spectrum of the released gamma rays.

"It is very similar to the spectrum observed in gamma-ray bursts" - George Chapline.

As for Darkmatter:

"The big bang would have created zillions of tiny dark energy stars out of the vacuum. Our universe is pervaded by dark energy, with tiny dark energy stars peppered across it" - George Chapline.

These small dark energy stars would behave just like dark matter particles.



Dark Energy Stars
Authors: G. Chapline

Event horizons and closed time-like curves cannot exist in the real world for the simple reason that they are inconsistent with quantum mechanics. Following ideas originated by Robert Laughlin, Pawel Mazur, Emil Mottola, David Santiago, and the speaker it is now possible to describe in some detail what happens physically when one approaches and crosses a region of space-time where classical general relativity predicts there should be an infinite red shift surface. This quantum critical physics provides a new perspective on a variety of enigmatic astrophysical phenomena, including supernovae explosions, gamma ray bursts, positron emission, and dark matter.

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Blobrana


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Dark energy, the mysterious stuff invoked to explain why the expansion of the universe is accelerating, could have a simple explanation. The energy may be coming from neutrinos that were created in copious quantities just after the big bang.
The leading candidate for dark energy is Einstein's "cosmological constant", which proposes that the vacuum of space has an inherent energy that counters gravity.
But if you calculate the density of this energy using quantum theory, it works out at nearly 120 orders of magnitude greater than what would fit with cosmological observations. So physicists have proposed ever more exotic explanations for dark energy that require, for instance, the existence of extra dimensions.

Now a team of Italian physicists says the answer has been under our noses all along. Antonio Capolupo and Giuseppe Vitiello of the University of Salerno and Salvatore Capozziello at the University of Naples claim that dark energy is the result of Neutrino 'mixing' that leads to a non-zero contribution to the cosmological constant. This contribution is of a completely different nature with respect to the usual one by a massive spinor field.



Neutrino mixing as a source for cosmological constant
Authors: Massimo Blasone, Antonio Capolupo, Salvatore Capozziello, Sante Carloni, Giuseppe Vitiello

We report on recent results showing that neutrino mixing may lead to a non-zero contribution to the cosmological constant. This contribution is of a completely different nature with respect to the usual one by a massive spinor field. We also study the problem of field mixing in Quantum Field Theory in curved space-time, for the case of a scalar field in the Friedmann-Robertson-Walker metric.

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Blobrana


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Title: Dark Energy from Brane-world Gravity
Authors: Roy Maartens (Portsmouth)


Recent observations provide strong evidence that the universe is accelerating. This confronts theory with a severe challenge. Explanations of the acceleration within the framework of general relativity are plagued by difficulties. General relativistic models require a "dark energy" field with effectively negative pressure. 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 also faces severe difficulties, but does provide a new angle on the problem. This review considers an example of modified gravity, provided by brane-world models that self-accelerate at late times.



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Blobrana


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Constraining Inverse Curvature Gravity with Supernovae
Authors: Olga Mena, Jose Santiago, Jochen Weller

Researchers show that the current accelerated expansion of the Universe can be explained without resorting to dark energy.

Models of generalized modified gravity, with inverse powers of the curvature can have late time accelerating attractors without conflicting with solar system experiments. they have solved the Friedman equations for the full dynamical range of the evolution of the Universe. This allows them to perform a detailed analysis of Supernovae data in the context of such models that results in an excellent fit.
Hence, inverse curvature gravity models represent an example of phenomenologically viable models in which the current acceleration of the Universe is driven by curvature instead of dark energy.
When they further include constraints on the current expansion rate of the Universe from the Hubble Space Telescope and on the age of the Universe from globular clusters, they obtain that the matter content of the Universe is 0.07

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Blobrana


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Astronomer Bradley Schaefer of Louisiana State University in Baton Rouge, US, has used observations of 52 Gamma-Ray Burst (GRBs) to suggest that dark energy has changed over time.

In the largest GRB study of its kind, Schaefer found that 12 of the most distant GRBs – lying nearly 13 billion light years away – were all brighter than expected, suggesting the universe was expanding at a slower rate than it is today.
The fact that all 12 were brighter than would be predicted by a cosmological constant increases his confidence in the data.

"It's like if you flip a coin and get 12 heads in a row" - Bradley Schaefer .

Instead of pushing space apart, dark energy appears to have changed over time and was in fact drawing space together in the early universe. What that means for the fate of the universe is not clear.

"With quintessence, you can do anything you want" - Bradley Schaefer.

But other researchers are yet to be convinced. Type 1a supernovae all explode with the same intrinsic energy, making them ideal "standard candles" to measure distance. But GRBs explode with a variety of energies. So Schaefer used five observed properties of the bursts – such as how their brightness changes over time – to estimate their intrinsic brightness, and thus their distance.

So far, supernova studies have supported the cosmological constant – one recent study of 70 supernovae reported that the strength of repulsion given by dark energy could not have changed by more than about 20% over the past eight billion years.
But supernovae are too dim to be seen over the largest cosmic distances. So some astronomers argue that gamma-ray bursts (GRBs) – violent, fleeting explosions that accompany the deaths of some massive stars – are better signposts. At about 100 times brighter than supernovae, they can be seen at much greater distances.

GRB expert Dale Frail of the National Radio Astronomy Observatory in New Mexico, US, says GRBs vary too widely and are not understood well enough to make such inferences.

"I think it's premature to start using GRBs as standard candles" - Dale Frail .

"You've got kind of a blunt tool to measure a very delicate effect" - Robert Kirshner, astronomer at Harvard University in Cambridge, Massachusetts, US, and a pioneer of supernova dark energy studies.

Schaefer acknowledges the research is preliminary, but says the analysis will improve as more GRBs are discovered and studied.
The research was presented on Wednesday at a meeting of the American Astronomical Society in Washington DC, US.

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Blobrana


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The first set of spectra of high-redshift supernovae based on the Supernova Legacy Survey (SNLS) conducted with MegaPrime/MegaCam at the Canada-France-Hawaii Telescope have appeared in two scientific papers by North American and European groups led by D.A. Howell (University of Toronto) and another by P. Astier (LPNHU, Centre National de la Recherche Scientifique (CNRS) and Universités Paris VI & VII).


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SNLS uses Type Ia supernovae as standard candles to study the acceleration of the universe. Programs are underway at Gemini and other large telescopes to characterize the dark energy driving this expansion by measuring its average equation of state, w = p/ρ.
Dark energy is a new, unaccounted-for form of energy that opposes the self-attraction of matter (due to gravity) and accelerates the expansion of the universe. The equation of state defines the time dependence of the dark energy density.



The goal of the SNLS is to obtain 700 well-observed Type Ia supernovae in the redshift range between 0.2 and 0.9 to increase the statistical significance for values of w.

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