Title: Anthropic versus cosmological solutions to the coincidence problem Authors: A. Barreira, P.P. Avelino

In this paper we investigate possible solutions to the coincidence problem in flat phantom dark energy models with a constant dark energy equation of state and quintessence models with a linear scalar field potential. These models are representative of a broader class of cosmological scenarios in which the universe has a finite lifetime. We show that, in the absence of anthropic constraints, including a prior probability for the models inversely proportional to the total lifetime of the universe excludes models very close to the \Lambda {CDM} model. This relates a cosmological solution to the coincidence problem with a dynamical dark energy component having an equation of state parameter not too close to -1 at the present time. We further show, that anthropic constraints, if they are sufficiently stringent, may solve the coincidence problem without the need for dynamical dark energy.

NASA's Hubble Rules Out One Alternative to Dark Energy

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 neighbourhood. If we lived near the centre 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 Equation of State) team that works to refine the accuracy of the Hubble constant to a precision that allows for a better characterisation of dark energy's behaviour. 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 of 2009. Riess' results appear in the April 1 issue of The Astrophysical Journal.

Title: Modelling Time-varying Dark Energy with Constraints from Latest Observations Authors: Qing-Jun Zhang, Yue-Liang Wu

We introduce a set of two-parameter models for the dark energy equation of state (EOS) w(z) to investigate time-varying dark energy. The models are classified into two types according to their boundary behaviours at the redshift z=(0,\infty) and their local extremum properties. A joint analysis based on four observations (SNe + BAO + CMB + H_0) is carried out to constrain all the models. It is shown that all models get almost the same \chi²_{min}\simeq 469 and the cosmological parameters (\Omega_M, h, \Omega_bh²) with the best-fit results (0.28, 0.70, 2.24), although the constraint results on two parameters (w_0, w_1) and the allowed regions for the EOS w(z) are sensitive to different models and a given extra model parameter. For three of Type I models which have similar functional behaviours with the so-called CPL model, the constrained two parameters w_0 and w_1 have negative correlation and are compatible with the ones in CPL model, and the allowed regions of w(z) get a narrow node at z\sim 0.2. The best-fit results from the most stringent constraints in Model Ia give (w_0,w_1) = (-0.96^{+0.26}_{-0.21}, -0.12^{+0.61}_{-0.89}) which may compare with the best-fit results (w_0,w_1) = (-0.97^{+0.22}_{-0.18}, -0.15^{+0.85}_{-1.33}) in the CPL model. For four of Type II models which have logarithmic function forms and an extremum point, the allowed regions of w(z) are found to be sensitive to different models and a given extra parameter. It is interesting to obtain two models in which two parameters w_0 and w_1 are strongly correlative and appropriately reduced to one parameter by a linear relation w_1 \propto (1+w_0).

Title: Is Dark Energy a Cosmic Casimir Effect? Authors: Kevin Cahill

Unknown short-distance effects cancel the quartic divergence of the zero-point energies. If this renormalisation took effect in the early universe after the last phase transition and applied only to modes whose wavelengths (over 2 pi) were shorter than the Hubble length 1/H at that time, then the zero-point energies of the modes of longer wavelengths can approximately account for the present value of the dark-energy density. The model makes two predictions.

That's why a new observation by scientists at the National Radio Astronomy Observatory, in Virginia, could be so important. By linking a group of far-flung radio telescopes into a virtual telescope thousands of miles across, James Braatz and Cheng-Yu Kuo have measured the distance to galaxy NGC 6264 to an accuracy of 450 million light-years from Earth, give or take 9%. That's crucial, because while it's simple to measure how fast a galaxy is moving, you also need to know exactly where it is. Imagine that a car is accelerating toward you, and you want to know when it will zip by. To calculate that, you need to know not only how fast it's going at any given moment, but also how far away it is. They've homed in on the black hole at the core of NGC 6264 - or more precisely, the disk of gas that swirls around it before being sucked into oblivion. Water molecules in the disk act as natural masers - essentially, they're lasers that transmit in radio frequencies rather than visible light. With those masers acting as beacons, the astronomers used a single radio telescope to figure out the actual size of the disk. Then they used their virtual radio to measure its apparent size - how tiny it looks at such an enormous distance. Read more

Our limited view of the cosmos obscures the identity of the mysterious forces that are responsible for the accelerating expansion of the Universe. Physicists at the University of Cambridge, UK, now say in two papers that the 'cosmological constant' - which is used to represent the Universe's expansion in cosmological equations - depends on the time and location where it is measured. This could explain long-standing problems with the constant and help physicists to explain the Universe's expansion. Read more

Title: A New Solution of The Cosmological Constant Problems Authors: John D. Barrow, Douglas J. Shaw (Version v3)

We extend the usual gravitational action principle by promoting the bare cosmological constant (CC) from a parameter to a field which can take many possible values. Variation leads to a new integral constraint equation which determines the classical value of the effective CC that dominates the wave function of the universe. In a realistic cosmological model, the expected value of the effective CC, is calculated from measurable quantities to be O(t_U), as observed, where t_U is the present age of the universe in Planck units,. Any application of our model produces a falsifiable prediction for \Lambda in terms of other measurable quantities. This leads to a specific falsifiable prediction for the observed spatial curvature parameter of Omega_k0=-0.0055. Our testable proposal requires no fine tunings or extra dark-energy fields but does suggest a new view of time and cosmological evolution.

Title: Reconciling the local void with the CMB Authors: Seshadri Nadathur, Subir Sarkar

In the standard cosmological model, the dimming of distant Type Ia supernovae is explained by invoking the existence of repulsive 'dark energy' which is causing the Hubble expansion to accelerate. However this may be an artifact of interpreting the data in an (oversimplified) homogeneous model universe. In the simplest inhomogeneous model which fits the SNe Ia Hubble diagram without dark energy, we are located close to the centre of a void modelled by a Lemaitre-Tolman-Bondi metric. It has been claimed that such models cannot fit the CMB and other cosmological data. This is however based on the assumption of a scale-free spectrum for the primordial density perturbation. An alternative physically motivated form for the spectrum enables a good fit to both SNe Ia (Constitution/Union2) and CMB (WMAP 7-yr) data, and to the locally measured Hubble parameter. Constraints from baryon acoustic oscillations and primordial nucleosynthesis are also satisfied.

Distant Galaxies Confirm Dark Energy's Existence and Universe's Flatness

In the late 1990s, two teams of astronomers stunned the scientific community with the finding that the universe is accelerating in its expansion, somehow overpowering the constant pull of gravity that should be slowing it down. The culprit pressing the cosmic accelerator goes by the name "dark energy," which is an appropriately enigmatic moniker for something that remains so poorly understood. Read more

Geometric test supports the existence of a key thread in the fabric of the Universe.

The claim that mysterious dark energy is accelerating the Universe's expansion has been placed on firmer ground, with the successful application of a quirky geometric test proposed more than 30 years ago. The accelerating expansion was first detected in 1998. Astronomers studying Type 1a supernovae, stellar explosions called "standard candles" because of their predictable luminosity, made the incredible discovery that the most distant of these supernovae appear dimmer than would be expected if the Universe were expanding at a constant rate. This suggested that some unknown force - subsequently dubbed dark energy - must be working against gravity to blow the universe apart. Read more