* Astronomy

Blobrana · L

RE: Dark Energy

Blobrana · L

Some had hoped it might be just an illusion. But it looks like dark energy is real and here to stay, as astronomers "image" the mysterious entity in action.
In 1998, astronomers found that distant supernovae were dimmer, and thus farther away, than expected. This suggested that the expansion of the universe is accelerating and "dark energy" was named as the culprit.
Since then, astronomers have struggled to explain what dark energy actually is leading some to speculate that it may not exist at all.

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Blobrana · L

Title: Holographic Dark Energy in Braneworld Models with Moving Branes and the w=-1 Crossing
Authors: E. N. Saridakis
(Version v2)

We apply the bulk holographic dark energy in general 5D two-brane models. We extract the Friedmann equation on the physical brane and we show that in the general moving-brane case the effective 4D holographic dark energy behaves as a quintom for a large parameter-space area of a simple solution subclass. We find that w_\Lambda was larger than -1 in the past while its present value is w_{\Lambda_0}~-1.05, and the phantom bound w_\Lambda=-1 was crossed at z_{p}~0.41, a result in agreement with observations. Such a behaviour arises naturally, without the inclusion of special fields or potential terms, but a fine-tuning between the 4D Planck mass and the brane tension has to be imposed.

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Title: Is dark energy from cosmic Hawking radiation?
Authors: Jae-Weon Lee, Hyeong-Chan Kim, Jungjai Lee

We suggest that dark energy is the Hawking radiation from a cosmic horizon. Despite of the extremely low Hawking temperature this dark energy could have the appropriate magnitude and the equation of state to explain the observed cosmological data, thank to its huge entropy proportional to the horizon area. If the horizon is an event horizon and the entropy of the radiation satisfies the holographic principle, then the radiation gives the holographic dark energy with the parameter d\simeq 1, as observed. Albeit simple, this model could explain many mysteries of dark energy in a consistent way.

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Data transmissions by the Wilkinson Microwave Anisotropy Probe (WMAP) provide multiple insights into the formation of the universe and its infancy, said several University researchers who were involved in designing and launching the satellite. The probes mission is led by a partnership between NASA and the University, in collaboration with scientists at several other institutions.

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A Nasa space probe measuring the oldest light in the Universe has found that cosmic neutrinos made up 10% of matter shortly after the Big Bang.
Five years of study data also shows that the first stars took over half a billion years to light up the Universe.
WMAP launched in 2001 on a mission to measure remnants of light left over from the Big Bang.
Scientists say it is collecting a "treasure trove" of information about the Universe's age, make-up and fate.

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Title: Dark Energy and the Accelerating Universe
Authors: Joshua Frieman (Chicago/Fermilab), Michael Turner (Chicago), Dragan Huterer (Michigan)

The discovery ten years ago that the expansion of the Universe is accelerating put in place the last major building block of the present cosmological model, in which the Universe is composed of 4% baryons, 20% dark matter, and 76% dark energy. At the same time, it posed one of the most profound mysteries in all of science, with deep connections to both astrophysics and particle physics. Cosmic acceleration could arise from the repulsive gravity of dark energy -- for example, the quantum energy of the vacuum -- or it may signal that General Relativity breaks down on cosmological scales and must be replaced. We review the present observational evidence for cosmic acceleration and what it has revealed about dark energy, discuss the various theoretical ideas that have been proposed to explain acceleration, and describe the key observational probes that will shed light on this enigma in the coming years.

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Blobrana · L

The WMAP (Wilkinson Microwave Anisotropy Probe) mission is designed to determine the geometry, content, and evolution of the universe via a 13 arcminute FWHM resolution full sky map of the temperature anisotropy of the cosmic microwave background radiation.

NASA's Wilkinson Microwave Anisotropy Probe (WMAP) has revealed new data about the early universe.

The top image shows a pie chart of the relative constituents observed in todays Universe.
A similar chart below shows the composition when the Universe was 380,000 years old (13.7 billion years ago).
The composition varies as the universe expands: the dark matter and atoms become less dense as the universe expands, but the photon and neutrino particles also lose energy as the universe expands, so their energy density decreases faster, compared to the matter.
The photon and neutrinos formed a larger fraction of the early universe.
Interestingly, it appears that the dark energy density does not decrease. It now dominates the universe even though it was perhaps a tiny contributor 13.7 billion years ago.

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Blobrana · L

Interstellar space may be strewn with tiny whiskers of carbon, dimming the light of far-away objects. This discovery by scientists at the Carnegie Institution may have implications for the dark energy hypothesis, proposed a decade ago in part to explain the unexpected dimness of certain stellar explosions called Type1a supernovae.
Type1a supernovae are among the brightest objects in the universe. Astronomers use them as standard candles to gauge cosmological distances: brighter-appearing supernovae are closer, dimmer ones are farther away. In the late 1990s some astronomers noticed that some seemed too dimtoo far awayto be explained by conventional theories of the universes expansion. This led to the hypothesis that the expansion was accelerating, pushed along by an unknown form of energy dark energy.

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Dr HongSheng Zhao, of the University's School of Physics and Astronomy, has shown that the puzzling dark matter and its counterpart dark energy may be more closely linked than was previously thought.
Only 4% of the universe is made of known material - the other 96% is traditionally labelled into two sectors, dark matter and dark energy.

"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" - Dr HongSheng Zhao, Advanced Fellow of the UK's Science and Technology Facilities Council.

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