31.03.2017

Explaining the accelerating expansion of the universe without dark energy

Enigmatic 'dark energy', thought to make up 68% of the universe, may not exist at all, according to a Hungarian-American team. The researchers believe that standard models of the universe fail to take account of its changing structure, but that once this is done the need for dark energy disappears. The team publish their results in a paper in Monthly Notices of the Royal Astronomical Society.

A still from an animation that shows the expansion of the universe in the standard 'Lambda Cold Dark Matter' cosmology, which includes dark energy (top left panel, red), the new Avera model, that considers the structure of the universe and eliminates the need for dark energy (top middle panel, blue), and the Einstein-de Sitter cosmology, the original model without dark energy (top right panel, green). The panel at the bottom shows the increase of the 'scale factor' (an indication of the size) as a function of time, where 1Gya is 1 billion years. The growth of structure can also be seen in the top panels. One dot roughly represents an entire galaxy cluster. Units of scale are in Megaparsecs (Mpc), where 1 Mpc is around 3 million million million km. 

Our universe was formed in the Big Bang, 13.8 billion years ago, and has been expanding ever since. The key piece of evidence for this expansion is Hubble’s law, based on observations of galaxies, which states that on average, the speed with which a galaxy moves away from us is proportional to its distance.

Astronomers measure this velocity of recession by looking at lines in the spectrum of a galaxy, which shift more towards red the faster the galaxy is moving away. From the 1920s, mapping the velocities of galaxies led scientists to conclude that the whole universe is expanding, and that it began life as a vanishingly small point.

In the second half of the twentieth century, astronomers found evidence for unseen 'dark matter' by observing that something extra was needed to explain the motion of stars within galaxies. Dark matter is now thought to make up 27% of the content of universe (in contrast 'ordinary' matter amounts to only 5%).

Observations of the explosions of white dwarf stars in binary systems, so-called Type Ia supernovae, in the 1990s then led scientists to the conclusion that a third component, dark energy, made up 68% of the cosmos, and is responsible for driving an acceleration in the expansion of the universe.

In the new work, the researchers, led by Phd student Gábor Rácz of Eötvös Loránd University in Hungary, question the existence of dark energy and suggest an alternative explanation. They argue that conventional models of cosmology (the study of the origin and evolution of the universe), rely on approximations that ignore its structure, and where matter is assumed to have a uniform density.

"Einstein’s equations of general relativity that describe the expansion of the universe are so complex mathematically, that for a hundred years no solutions accounting for the effect of cosmic structures have been found. We know from very precise supernova observations that the universe is accelerating, but at the same time we rely on coarse approximations to Einstein’s equations which may introduce serious side-effects, such as the need for dark energy, in the models designed to fit the observational data." explains Dr László Dobos, co-author of the paper, also at Eötvös Loránd University.

In practice, normal and dark matter appear to fill the universe with a foam-like structure, where galaxies are located on the thin walls between bubbles, and are grouped into superclusters. The insides of the bubbles are in contrast almost empty of both kinds of matter.

Using a computer simulation to model the effect of gravity on the distribution of millions of particles of dark matter, the scientists reconstructed the evolution of the universe, including the early clumping of matter, and the formation of large scale structure.

Unlike conventional simulations with a smoothly expanding universe, taking the structure into account led to a model where different regions of the cosmos expand at different rate. The average expansion rate though is consistent with present observations, which suggest an overall acceleration.

Dr Dobos adds: "The theory of general relativity is fundamental in understanding the way the universe evolves. We do not question its validity; we question the validity of the approximate solutions. Our findings rely on a mathematical conjecture which permits the differential expansion of space, consistent with general relativity, and they show how the formation of complex structures of matter affects the expansion. These issues were previously swept under the rug but taking them into account can explain the acceleration without the need for dark energy."

If this finding is upheld, it could have a significant impact on models of the universe and the direction of research in physics. For the past 20 years, astronomers and theoretical physicists have speculated on the nature of dark energy, but it remains an unsolved mystery. With the new model, Csabai and his collaborators expect at the very least to start a lively debate.

Quelle: The Royal Astronomical Society

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Update: 5.04.2017

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For the past 20 years, physicists have known that the expansion of the universe is accelerating, as if some bizarre “dark energy” is blowing up space like a balloon. In fact, cosmologists’ well-tested standard model assumes that 69% of the content of the universe is dark energy. However, there may be no need for the mysterious stuff, a team of theorists claims. Instead, the researchers argue, the universe’s acceleration could be driven by variations, or inhomogeneities, in its density. If so, then one of the biggest mysteries in physics could be explained away with nothing other than Albert Einstein’s familiar general theory of relativity. Other researchers are skeptical, however.

“If it’s right, somebody is going to have to take back Nobel prizes” awarded in 2011 for the discovery of the accelerating expansion of the universe, says Nick Kaiser, a cosmologist at the University of Hawaii in Honolulu. Tom Giblin, a computational cosmologist at Kenyon College in Gambier, Ohio, who has worked on a similar analysis, says, “I would love if inhomogeneities explained dark energy.” However, he says, “I don’t see any evidence from our simulations to expect it to be as big an effect as they see here.”

At issue is the way cosmologists calculate how the universe evolved over the past 13.8 billion years. Roughly speaking, they rely on two equations. One describes how matter coalesces into galaxies and clusters of galaxies. The other, known as the Friedmann–Lemaître–Robertson–Walker (FLRW) metric, comes out of Einstein’s theory of gravity, or general relativity, and scientists use it to calculate how much the universe has expanded at any time. At each step in time in a simulation, the cosmologists’ program uses the FLRW metric to calculate the “scale factor,” which specifies how much the universe has grown. The program then uses the scale factor as an input to calculate how the formation of galaxies and clusters advances in that step.

Strictly speaking, however, the FLRW equation applies to a smooth and homogeneous universe. So to calculate the scale factor at each step, cosmologists typically assume the universe is smooth and use its average density—determined from the simulation—as the FLRW metric’s input. That’s a bit dicey, because general relativity says that mass and energy warp spacetime. As a result, space should expand faster in emptier regions and slower in crowded ones, where the galaxies’ gravity pulls against the expansion. Thus, in principle, inhomogeneities in the universe can feed back through the dynamics and affect the universe’s expansion.

Gábor Rácz and László Dobos, astrophysicists at Eötvös Loránd University in Budapest, and their colleagues set out to capture that “backreaction.” They simulated a cube of space measuring 480 million light-years along each side. Instead of using the FLRW metric to calculate at each time step a single scale factor for the entire cube, they broke the cube into 1 million miniuniverses and then used the equation to calculate the scale factor in each of them. “We assume that every region of the universe determines its expansion rate itself,” Dobos says. The researchers then calculated the average of the many scale factors, which can differ from the scale factor calculated from the average density.

The team’s virtual universe evolved much as the real one has, with its expansion accelerating over the past few billion years. That happened even without adding space-stretching dark energy to the simulation, the researchers report in a paper in press at the Monthly Notices of the Royal Astronomical Society. The results suggest that it may be possible to explain away dark energy as an illusion, Dobos says.

Others are cautious. Giblin notes that the simulation he and his colleagues performed differs from the new one. The new work tracks the evolution of the universe to finer spatial scales, but involves certain assumptions and approximations, he says. In spite of the differences, Giblin says, his work suggests that backreaction would change the expansion rate of the universe by less than a percent, whereas the new simulation suggests an effect in excess of 20%.

Kaiser also expects the effects of inhomogeneity to be small. He notes that the best evidence for the accelerated expansion of the universe comes from measuring the distances and ages of stellar explosions known as type 1a supernovae in the relatively nearby universe. However, in the local universe, plain Newtonian gravity should work well enough. That suggests the difference in how the scale factor is determined in a relativistic theory shouldn’t exert a big effect. “If they’re right, there’s something very funny going on,” he says.

Still, experts say it’s reasonable to investigate backreaction. “I would say that this is now part of the mainstream in that people want to calculate the size of this effect,” says Thomas Buchert, a cosmologist at the University of Lyon in France, who pioneered the topic in the 1990s. Giblin notes “mainstream cosmology has done such a bad job of solving the dark energy problem that it will likely be some nonmainstream idea like this that does.” But, he adds, “I don’t know if this is the one.”

Quelle: AAAS