Does dark energy change over time? A puzzling model of this as yet undetected entity that is thought to be accelerating the universe's expansion could explain some puzzling observations of galaxy clusters. But it will have to jump many more hurdles to compete with the simplest and so far most successful model of the elusive entity.So lets see that how it will be helpful in crossing those hurdles !!
The model known as the cosmological constant, holds that there is a certain amount of repulsive energy in every cubic centimetre of space, and the amount stays the same over time. As the universe expands, more space exists, and hence the expansion also gets accelerated.According to Edoardo Carlesi of the Autonomous University in Madrid, Spain, the colleagues have simulated a universe where the amount of repulsive energy per unit of volume changes with time i.e the repulsive energy per unit of volume is directly proportional to the time.According to them the model can explain how several galaxy clusters grew to weigh as much as a quadrillion (1015) suns by the time the universe was just 6 billion years old. Here is a puzzle because in sight of some researchers 6 billion years would not have been enough time for gravity to amass such large structures.For easy understanding the puzzle has been compared with a standard "recipe" for the universe which is used. The ingredients for that recipe are a large amount of dark energy, in the form of a cosmological constant, and a dollop of matter. Their ratio has been calculated by studying the cosmic microwave background, radiation that reveals the distribution of matter and energy in the early universe.Looking at the cosmic microwave background data through the lens of a different dark energy model can produce different ratios of ingredients. The cosmological constant model allows for matter to make up 27 per cent of the universe's energy density, whereas the dark energy model studied by Carlesi's team provides a more generous help of 39 per cent.
According to Carlesi massive clusters can form up to 10 times as often using this recipe, they can explain current observations within a model that allows much more matter. As a result, galaxies attract other galaxies through their gravitational pull, so massive clusters form faster.Keeping with view the cluster problem may not even be a problem, though, as the jury is still out on whether the clusters challenge the leading cosmological model, because there is a lot of uncertainty about their mass, most of which is thought to be tied up in invisible dark matter.The cosmological constant has so far been able to explain a wide range of observations, so turning to a relatively unproven model to account for a few galaxy clusters that may be heavier than expected is like using a huge hammer to kill a tiny fly.
So according to the researchers it is just the first test of the model,and should undergo further tests that the cosmological constant has already passed. For example, its effects should be consistent with the integrated Sachs-Wolfe effect, in which photons from the cosmic microwave background experience slight changes in wavelength as they feel the gravity of superclusters of galaxies they pass through.So lets see if further information can be dug out about this old galaxy clusters !!!
[via:New Scientist]
[via:New Scientist]
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