Variable dark energy could explain old galaxy clusters

Does dark energy change over time? An alternative model of the 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.

That model, called the cosmological constant, holds that there is a certain amount of repulsive energy in every cubic centimetre of space, and that amount stays the same over time. As the universe expands, more space exists, and so the expansion accelerates.

Now Edoardo Carlesi of the Autonomous University in Madrid, Spain, and his colleagues have simulated a universe where the amount of repulsive energy per unit of volume changes with time.

They say 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. That's a puzzle because some researchers say 6?billion years would not have been enough time for gravity to amass such large structures.

Standard recipe

The puzzle arises if the standard "recipe" for the universe 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 helping: 39 per cent.

Massive clusters can form up to 10 times as often using this recipe, the researchers say. "You can explain current observations within a model that allows much more matter," says Carlesi. As a result, galaxies attract other galaxies through their gravitational pull, so massive clusters form faster.

First hurdle

The cluster problem may not even be a problem, though, says Dragan Huterer at the University of Michigan in Ann Arbor. He says 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", he says.

Carlesi says this is just the first test of the model, and Cristian Armend?riz-Pic?n at Syracuse University in New York agrees. He says the model Carlesi is using 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.

Journal reference: Monthly Notices of the Royal Astronomical Society, DOI: 10.1111/j.1365-2966.2011.19660.x

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