Jun 12

Dark Universe mission blueprint complete

Source: ESA

Artist’s impression of Euclid.
Credits: ESA - C. Carreau.

ESA’s Euclid mission to explore the hidden side of the Universe – dark energy and dark matter – reached an important milestone today that will see it head towards full construction. (read more)

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Jul 11

‘Zombie' Stars Key to Measuring Dark Energy

Source: UCSB Press Release

Chandra X-ray image of Tycho's supernova remnant.
Image credit: NASA/Chandra X-ray Observatory.

"Zombie" stars that explode like bombs as they die, only to revive by sucking matter out of other stars. According to an astrophysicist at UC Santa Barbara, this isn't the plot for the latest 3D blockbuster movie. Instead, it's something that happens every day in the universe –– something that can be used to measure dark energy.

This special category of stars, known as Type Ia supernovae, help to probe the mystery of dark energy, which scientists believe is related to the expansion of the universe. (read more)

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May 11

Galaxy Evolution Explorer finds Dark Energy repulsive

Source: NASA News

Artist's impression of GALEX.
Image credits: NASA/JPL-Caltech.

A five-year survey of 200,000 galaxies, stretching back seven billion years in cosmic time, has led to one of the best independent confirmations that dark energy is driving our universe apart at accelerating speeds.

The survey used data from NASA's space-based Galaxy Evolution Explorer and the Anglo-Australian Telescope on Siding Spring Mountain in Australia.

The findings offer new support for the favored theory of how dark energy works - as a constant force, uniformly affecting the universe and propelling its runaway expansion. They contradict an alternate theory, where gravity, not dark energy, is the force pushing space apart. According to this alternate theory, with which the new survey results are not consistent, Albert Einstein's concept of gravity is wrong, and gravity becomes repulsive instead of attractive when acting at great distances. (read more)

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Mar 11

Hubble Space Telescope rules out one alternative to dark energy

Source: NASA - Hubble Space Telescope

Astronomers using NASA's Hubble Space Telescope have ruled out an alternate theory on the nature of dark energy after recalculating the expansion rate of the universe to unprecedented accuracy.

The universe appears to be expanding at an increasing rate. Some believe that is because the universe is filled with a dark energy that works in the opposite way of gravity. One alternative to that hypothesis is that an enormous bubble of relatively empty space eight billion light-years across surrounds our galactic neighborhood. If we lived near the center of this void, observations of galaxies being pushed away from each other at accelerating speeds would be an illusion.

This hypothesis has been invalidated because astronomers have refined their understanding of the universe's present expansion rate. Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University in Baltimore, Md., led the research. The Hubble observations were conducted by the SHOES (Supernova Ho for the quation of State) team that works to refine the accuracy of the Hubble constant to a precision that allows for a better characterization of dark energy's behavior. The observations helped determine a figure for the universe's current expansion rate to an uncertainty of just 3.3 percent. The new measurement reduces the error margin by 30 percent over Hubble's previous best measurement in 2009. Riess's results appear in the April 1 issue of The Astrophysical Journal.(read more)

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Jan 11

The Best Way to Measure Dark Energy Just Got Better

Credit: Harvard-Smithsonian Center for Astrophysics (CfA)

Computer simulation of dark matter distribution in a
galaxy cluster formed in the universe with dark energy.
Image credit: Andrey Kravtsov.

Dark energy is a mysterious force that pervades all space, acting as a "push" to accelerate the Universe's expansion. Despite being 70 percent of the Universe, dark energy was only discovered in 1998 by two teams observing Type Ia supernovae. A Type 1a supernova is a cataclysmic explosion of a white dwarf star.

These supernovae are currently the best way to measure dark energy because they are visible across intergalactic space. Also, they can function as "standard candles" in distant galaxies since the intrinsic brightness is known.

Just as drivers estimate the distance to oncoming cars at night from the brightness of their headlights, measuring the apparent brightness of a supernova yields its distance (fainter is farther). Measuring distances tracks the effect of dark energy on the expansion of the Universe.

The best way of measuring dark energy just got better, thanks to a new study of Type Ia supernovae led by Ryan Foley of the Harvard-Smithsonian Center for Astrophysics.(read more)

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Dec 10

Comparison of dark energy models: A perspective from the latest observational data

Physicists at the Institute of Theoretical Physics, Chinese Academy of Sciences and the Department of Physics at Northeastern University have made a comparison of a number of competing dark energy models. They have tested and compared nine popular dark energy models using the latest observational data. The study is reported in Issue 9 (Volume 53) of SCIENCE CHINA Physics, Mechanics & Astronomy because of its significant research value.

Web-like structure of  'superclusters' of galaxies, leaving ever larger voids behind.
The greater rate of expansion in the voids may account for the observations usually attributed to dark energy.(Illustration credit: Center for Cosmological Physics/U Chicago)

Over the past decade, cosmologists around the world have accumulated conclusive evidence for the fact that the cosmic expansion is accelerating. Within the framework of the standard cosmological model, this implies that about two-thirds of the cosmos is composed of an exotic component, “dark energy”, which, unlike any known form of matter or energy, is gravitationally repulsive. To explain the gravitationally repulsive dark energy, physicists and cosmologists have proposed a variety of theoretical models. However, none of them are commonly accepted as the convincing theoretical explanation for dark energy. Understanding the nature of dark energy continues to be one of the major missions for fundamental physics.

In the absence of clear theoretical guidance, the physics community is reliant on comparison to observational data in selecting a correct dark energy model. In this work, nine popular dark energy models are tested and compared using the latest observational data, which includes type Ia supernovae, baryon acoustic oscillation, and cosmic microwave background. The models under consideration are the cosmological constant (CC) model, two equation of state parameterization models, the generalized Chaplygin gas model, two Dvali-Gabadadze-Porrati models, and three holographic dark energy models. All of these models are well-known dark energy candidates and have attracted considerable attention in the past.

The dark energy models have been fitted to the observational data. In this procedure, each model is assessed with a number called the IC value. Statistically, a model with fewer parameters and with a better fit to the data has a lower IC value. The models under consideration have been compared and ranked according to their IC values.

Consequently, it has been found that the CC model fits the observational data best. In addition to the CC model, five other models also provide a good fit to the current data, while the remaining three models, a Dvali-Gabadadze-Porrati model and two holographic models, clearly do not fit the observational data well. (For simplicity, we call them the five good models and the three bad models.) It is interesting to note that four of the five good models are closely related to the CC model, which may be the reason they fit the data so well, while none of the bad models can be reduced to the CC model.

Furthermore, it is interesting to note that there is one holographic dark energy model, which is not reducible to the CC model, yet it still provides a good fit to the current observational data. This model was proposed by Professor Miao Li, one of the authors of this work. As the name implies, this holographic dark energy model arises from the holographic principle of quantum gravity. The holographic principle determines the range of validity for a local effective quantum field theory to be an accurate description of the world involving dark energy by imposing a relationship between the ultraviolet and infrared cut-offs. In his well-known article, “A Model of Holographic Dark Energy”, Li has shown that a form of holographic dark energy emerges if the future event horizon size of the universe is chosen as the infrared cut-off scale in the effective quantum field theory.

In summary, the CC model is still the best candidate for dark energy, according to the results from fitting models to current observational data. The authors concluded that “Given the current quality of the observational data, and with the assumption of a flat universe, information criteria indicate that the cosmological constant model is still the best one and there is no reason to prefer any more complex model”. However, as new observations are made, it is possible that the preferred model could change. It is important to keep in mind that there are limitations associated with the current observational data. To precisely determine the nature of dark energy, an improvement in the accuracy of data is of essential importance. “We look forward to seeing whether this conclusion [that the CC model is the best candidate for dark energy] can be changed by future more accurate data,” said the authors in the final statement of the paper.

Reference: Li M, Li X.D. and Zhang X. SCIENCE CHINA Physics, Mechanics & Astronomy 2010; 53: 1631-1645

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Nov 10

Galaxies pin down dark energy

Source: Physics World
Hubble Space Telescope image of a pair of spiral galaxies with swirling arms.
The galactic system is located in the constellation of Draco, about 350 million light-years away.
Image credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration
and A Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

A new way of measuring the geometry of the universe confirms that dark energy dominates the cosmos and bolsters the idea that this unusual form of energy is described by Einstein's cosmological constant. The technique, developed by physicists in France, involves a relatively easy measurement of the orientation of distant pairs of galaxies.

Over the past decade or so, several kinds of observation, such as measurements of the distances of remote supernovae, have provided strong evidence that the expansion of the universe is accelerating. Cosmologists believe that this expansion is being driven by what is known as dark energy – a substance with negative pressure that opposes the pull of gravity. Unfortunately, however, they have little idea of what dark energy actually is, having been unable to measure its properties well enough to distinguish between rival hypotheses.(read more)

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