In recent observations it is strongly believed that the universe is experiencing an accelerated expansion. The type Ia Supernovae and Cosmic Microwave Background (CMB) observations have shown the evidences to support cosmic acceleration. This acceleration is caused by some unknown matter which has the property that positive energy density and negative pressure satisfying ρ + 3p < 0 is dubbed as “dark energy” (DE). If ρ + p < 0, it is dubbed as “phantom energy”. The combined astrophysical observations suggests that universe is spatially flat and the dark energy occupies about 70% of the total energy of the universe, the contribution of dark matter is ∼ 26%, the baryon is 4% and negligible radiation. A cosmological property in which there is an infinite expansion in scale factor in a finite time termed as ‘Big Rip’. In the phantom cosmology, big rip is a kind of future singularity in which the energy density of phantom energy will become infinite in a finite time. To realize the Big Rip scenario the condition ρ + p < 0 alone is not sufficient. Distinct data on supernovas showed that the presence of phantom energy with – 1.2 < w < – 1 in the Universe is highly likely. In this case the cosmological phantom energy density grows at large times and disrupts finally all bounded objects up to subnuclear scale.
A wormhole is a feature of space that is essentially a “shortcut” from one point in the universe to another point in the universe, allowing travel between them that is faster than it would take light to make the journey through normal space. So the wormholes are tunnels in spacetime geometry that connect two or more regions of the same spacetime or two different spacetimes. Wormholes may be classified into two categories – Euclidean wormholes and Lorentzian wormholes. The Euclidean wormholes arise in Euclidean quantum gravity and the Lorentzian wormholes which are static spherically symmetric solutions of Einstein’s general relativistic field equations. In order to support such exotic wormhole geometries, the matter violating the energy conditions (null, weak and strong), but average null energy condition is satisfied in wormhole geometries. For small intervals of time, the weak energy condition (WEC) can be satisfied.
Ujjal Debnath and colleagues in their work studied effects of accretion of the dark energies onto Morris-Thorne wormhole using a dark-energy accretion model for wormholes which have been obtained by generalizing the Michel theory. They found that for quintessence like dark energy, the mass of the wormhole decreases and phantom like dark energy, the mass of wormhole increases.
They have also assumed recently proposed two types of dark energy like variable modified Chaplygin gas (VMCG) and generalized cosmic Chaplygin gas (GCCG). They obtained the expression of wormhole mass in both cases and found that the mass of the wormhole is finite at late universe. Their dark energy fluids violate the strong energy condition (ρ + 3p < 0 in late epoch), but do not violate the weak energy condition (ρ + p > 0). So the models drive only quintessence scenario in late epoch, but do not generate the phantom epoch (in their choice). So wormhole mass decreases during evolution of the universe for these two dark energy models.
Since their considered dark energy candidates do not violate weak energy condition, so the dynamical mass of the wormhole are decaying by the accretion of their considered dark energies, though the pressures of the dark energies are outside the wormhole. From figures 1 and 2, they observed that the wormhole mass decreases as z increases for both VMCG and GCCG, which accrete onto the wormhole in our expanding universe.
Next they have assumed 5 kinds of parametrizations (Models I-V) of well known dark energy models (some of them are Linear, CPL, JBP models). These models generate both quintessence and phantom scenarios for some restrictions of the parameters. So if these dark energies accrete onto the wormhole, then for quintessence stage, wormhole mass decreases upto a certain value (finite value) and then again increases to infinite value for phantom stage during whole evolution of the universe. They also showed these results graphically clearly. Figures 3-7 shows the mass of wormhole first decreases to finite value and then increases to infinite value.
In future work, it will be interesting to show the natures of mass for various types of wormhole models if different kinds of dark energies accrete upon wormhole in accelerating universe also.— said debnath, lead author of the study.
Reference: Debnath, U. Accretions of various types of dark energies onto Morris–Thorne wormhole. Eur. Phys. J. C 74, 2869 (2014). https://link.springer.com/article/10.1140/epjc/s10052-014-2869-4 https://doi.org/10.1140/epjc/s10052-014-2869-4
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