* Astronomy

Blobrana · L

RE: Dark Energy

Blobrana · L

Title: What Drives Our Accelerating Universe?
Authors: Sidney Bludman (Universidad de Chile, Santiago)
(Version v3)

The homogeneous expansion history H(z) of our universe measures only kinematic variables, but cannot fix the underlying dynamics driving the recent acceleration: cosmographic measurements of the homogeneous universe, are consistent with either a static finely-tuned cosmological constant or a dynamic 'dark energy' mechanism, which may be material Dark Energy or modified gravity (Dark Gravity). Resolving the composition and foreground noise to reduce their large systematic errors.dynamics of either kind of 'dark energy', will require complementing the homogeneous expansion observations with observations of the growth of cosmological fluctuations. Because the 'dark energy' evolution is at least quasi-static, any dynamical effects on the fluctuation growth function g(z) will be minimal. They will be best studied in the weak lensing convergence of light from galaxies at 0<z<5, from neutral hydrogen at 6<z<20, and ultimately from the CMB last scattering surface at z=1089. Galaxy clustering also measures g(z), but requires large corrections for baryonic composition and foreground noise. Projected observations potentially distinguish static from dynamic 'dark energy', but distinguishing dynamic Dark Energy from Dark Gravity will require a weak lensing shear survey more ambitious than any now projected. Low-curvature modifications of Einstein gravity are also, in principle, observable in the solar system or in isolated galaxy clusters. The cosmological constant can only be fine-tuned, at present. The Cosmological Coincidence Problem, that we live when the ordinary matter density approximates the 'gravitational vacuum energy', on the other hand, is a material problem, calling for an understanding of the observers' role in cosmology.

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Title: Dark energy and the future fate of the Universe
Authors: Yungui Gong, Yuan-Zhong Zhang
(Version v3)

We consider the possibility of observing the onset of the late time inflation of our patch of the Universe. The Hubble size criterion and the event horizon criterion are applied to several dark energy models to discuss the problem of future inflation of the Universe. We find that the acceleration has not lasted long enough to confirm the onset of inflation by present observations for the dark energy model with constant equation of state, the holographic dark energy model and the generalised Chaplygin gas (GCG) model. For the flat \Lambda CDM model with \Omega_{m0}=0.3, we find that if we use the Hubble size criterion, we need to wait until the a_v which is the scale factor at the time when the onset of inflation is observed reaches 3.59 times of the scale factor a_T when the Universe started acceleration, and we need to wait until a_v=2.3 a_T to see the onset of inflation if we use the event horizon criterion. For the flat holographic dark energy model with d=1, we find that a_v=3.46 a_T with the Hubble horizon and a_v=2.34 a_T with the event horizon, respectively. For the flat GCG model with the best supernova fitting parameter \alpha=1.2, we find that a_v=5.50 a_T with the Hubble horizon and a_v=2.08 a_T with the event horizon, respectively.

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Blobrana · L

In the early Universe, small fluctuations in energy density and pressure caused oscillations. Although tiny in the beginning, these ripples have been magnified by the expansion of the Universe so that they stretch 500 million light-years across today.
The clouds of neutral hydrogen should follow the same ripple pattern, so astronomers will know they're looking at those first, primordial clouds, and not some closer ones.
And so, astronomers will be able to look back in time, and study the distance to the clouds at each epoch in our Universe's expansion. They should be able to trace how much dark energy was affecting space at each time, and get a sense if this energy has always remained constant, or if it's changing.

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Ed ~ ignore the `Big Bang doubtful` part, as that will only be of concern to beings 300 billion years from now.

Blobrana · L

The average brightness of stellar explosions that astronomers rely on to measure dark energy the mysterious force causing our universe to expand faster and faster has actually changed over time, a new study reports. The authors say uncertainties in gauging the brightness might throw off future measurements of dark energy in unpredictable ways.
To measure the expansion of the universe and discern the effects of dark energy, scientists rely on explosions called type Ia supernovae, which are thought to signal the deaths of white dwarf stars.

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Title: The Destiny of Universes After the Big Trip
Authors: A.V. Yurov

The big trip can be describe with the help of the Wheeler-DeWitt wave equation {\hat H}\psi(w,a)=0. The probability to find the universe after big trip in the state with w=w_0 will be maximal if \partial\psi(w,a)/\partial w|_{w=w_0}=0 for any values of the scale factor a. It is shown that this will be the case if and only if w_0=-1/3. This fact allows one to suggest that vast majority of universes in multiverse must be in this state after their big trips.

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Dark energy may not be needed to explain why the expansion of space appears to be speeding up. If our universe is like Swiss cheese on large scales with dense regions of matter and holes with little or no matter it could at least partly mimic the effects of dark energy, suggests a controversial new model of the universe.

In 1998, astronomers found that distant supernovae were dimmer, and thus farther away, than expected. This suggested the expansion of the universe was accelerating as a result of a mysterious entity dubbed dark energy, which appears to make up 73% of the universe.
But trying to pin down the nature of dark energy has proven extremely difficult. Theories of particle physics suggest that space does have an inherent energy, but this energy is about 10120 times greater than what is actually observed.
This has caused some cosmologists to look for alternative explanations.

"I don't have anything against dark energy, but we ought to make all possible efforts to see whether we can avoid this exotic component in the universe" says Sabino Matarrese of the University of Padova in Italy.

So he and colleagues, including Edward Kolb of the Fermi National Accelerator Laboratory in Batavia, Illinois, US, decided to model the universe as having large-scale variations in density.
That contradicts the standard model of cosmology, which assumes that the universe is homogeneous on large scales. In the homogeneous model, known as the Friedmann-Robertson-Walker (FRW) universe, the effect of dark energy is to stretch space, thus increasing the wavelength of photons from the supernovae.

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'Seesaw' explains light dark-energy particles
Eight fields would interact to produce a single particle

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High-z Supernova Search Team Wins Gruber Cosmology Prize
Brian Schmidt (Australian National University) and the High-z Supernova Search team have been awarded the 2007 Gruber Cosmology Prize for their discovery that the universes expansion is accelerating under the influence of dark energy. They will share the honour with Saul Perlmutter (University of California at Berkeley) and the Supernova Cosmology Project, a second team of astronomers who also observed the accelerating expansion.

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The High-z Supernova Search team announced their discovery of the accelerating universe in February 1998. A detailed paper was submitted to the Astronomical Journal in March 1998 and published in September (Riess et al. 1998, AJ, 116, 1009).

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Blobrana · L

Four astronomers at the Space Telescope Science Institute in Baltimore, Md. are on two teams sharing the $500,000 2007 Gruber Cosmology Prize for their discovery that the expanding universe is accelerating under a mysterious cosmic force called dark energy.

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