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(麻豆淫院Org.com) -- "In many ways, the standard model of cosmology works very well," Jose Cembranos tells 麻豆淫院Org. "However, there are very basic features that we just do not know. We have dark energy and dark matter. They dictate the evolution of late time cosmology. They both together constitute more than 95 percent of the energy content of the present Universe." If this is the case, why do we trust the standard model? It can鈥檛 explain such a large portion of the universe.

Cembranos points this out about why we continue to place our faith in the standard model: 鈥淪ometimes it is hard to explain why you trust a model if you don鈥檛 understand the most part of it. The reason is that you do not need to know the fundamental nature of dark energy or dark matter to compute the evolution of the Universe. With the ingredients we have, the model fits the different observations we have done so far.鈥� But many scientists are not satisfied with a model that leaves out the natures of dark energy and dark matter. They, like Cembranos, are looking for ways to modify the standard model so that these mysterious parts of the universe may eventually be revealed.

Cembranos is a scientist at the William I. Fine Theoretical 麻豆淫院ics Institute at the University of Minnesota in Minneapolis. He has been working on a model of cosmology that includes a modification of Einstein gravity. 鈥淕ravity is the main issue in any model of cosmology; it is the force that drives the dynamic. However, we know that the Einstein theory of gravity cannot be the final word on gravity. It is not consistent at high energies.鈥� Cembranos has developed a model that uses an effective approach and illustrates the idea that new gravitational states could account for dark matter with R² gravity. His work is described in 麻豆淫院ical Review Letters: 鈥淒ark Matter from R² 骋谤补惫颈迟测.鈥�

鈥淚 used R² gravity because it is the simplest way to modify Einstein gravity,鈥� Cembranos explains. 鈥淲e know how to deal with gravity as an effective field theory, working at low energies. At low energies you have the opportunity for a perturbative expansion, everything working order by order. Unfortunately, you can鈥檛 go to very high energies. From a quantum standpoint this approach does not work.鈥�

鈥淚n addition to a constant that may explain the dark energy, and a linear term in the space-time curvature that defines Einstein gravity, the next term of the expansion is quadratic,鈥� Cembranos explains. 鈥淲hen you introduce this R² term, gravity is modified by the introduction of a new mediator of the gravitational interaction. However, it is completely different from its cousin: The standard graviton predicted by the linear term is a mass-less particle of spin 2. This new graviton has mass and spin 0.鈥�

The model that Cembranos developed also allows you to tune the parameters of the system in order to explain dark matter. 鈥淚 wanted to focus on the dark matter issue,鈥� he says, 鈥渂ecause dark matter seems a little more straightforward. All you need to do is introduce more stuff into the model. We can do this with R2 gravity.鈥�

Cembranos points out that while R2 gravity is an interesting approach to the problem, it doesn鈥檛 hold all the answers. 鈥淢any people have used different modifications of gravity in order to explain dark matter and even dark energy,鈥� he says. 鈥淗owever, usually these explanations end up being worse than Einstein gravity. Einstein gravity clearly has problems, but nearly all the other explanations are worse.鈥�

What makes the model studied by Cembranos more promising, he insists, is that using the R² term isn鈥檛 worse than Einstein鈥檚 gravity theory. 鈥淚t鈥檚 not any better, but it鈥檚 also not any worse. It鈥檚 more or less the same, but a little more complicated.鈥�

The additional gravitational states allowed in the R² gravity modification introduce new degrees of freedom to the standard model. 鈥淚鈥檓 hoping that by studying this model,鈥� Cembranos points out, 鈥渢hat we can get a general idea of what can be signals or observations that are apparent if dark matter is really related to the gravitational sector.鈥�

More information: Cembranos, Jose, 鈥�Dark Matter from R² 骋谤补惫颈迟测.鈥� 麻豆淫院ical Review Letters (2009). Available online: .

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