Physics  

Summary Dark energy is cosmology's biggest mystery—an anti-gravitational force that confounds the conventional laws of physics. It makes up more than two-thirds of the cosmos, but science is still grappling to explain what dark energy actually is. In this program, top physicists search for clues to this mystery in both the earliest moments of the universe and far into the future of the cosmos.

SOURCE  World Science Festival

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Cosmology  

Most of us understand the Big Bang as the concept that our entire universe came from a single point, what astrophysicists call a 'Singularity.' A new model however suggests that we might not need a singularity to have a Big Bang.

T
he question of cosmology of how do you get something from nothing has confounded philosophers for millennia. The question's accepted answer for the better part of recent history (outside of religious conceptions) has been the Big Bang.

The University of Lethbridge's Saurya Das and Ahmed Farag Ali – who was a PhD student at the U of L before taking a faculty position at Benha University in Egypt, recently put forth a mathematical model that assumes the Big Bang never happened and the universe has simply been eternal.

The paper explaining the model was recently published in the journal Physics Letters B, has received a lot of attention in the cosmology community for convincingly contradicting the conventional wisdom that the universe originated from a Singularity about 13.8 billion years ago.

While it explains a lot of what we have so far observed about the universe, the Big Bang theory and the Singularity in particular, leads to some conclusions which are very difficult to model and explain.

"The Singularity [in cosmology] is basically where all theories and all physics breaks down, so nobody really likes that,” Das said.

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The best current theories can only explain what happens immediately after the Big Bang, not during or before it. Even the idea of “before” the Big Bang is itself a conundrum, as the Singularity is thought to be the origin of space and time.

"We need more confirmations and more careful study of our model, for sure. We have to understand the mathematics better and whatever predictions it makes has to be tested with observations."

The new model developed by Das and Ali gets around that problem, as it simply states there was no singularity and the universe’s existence has been in a more or less steady state of existence.

Physicists have long believed that a quantum version of gravity would include a hypothetical particle, called the graviton, which generates the force of gravity. In their new model, Ali and Das propose that such gravitons could form this quantum fluid.

The new model moves away from the “expanding universe” theory. It also gets away from the matter of the universe’s previous infinite size and density, which the paper calls the “smallness problem,” by relying on a “cosmological constant” term that puts the universe at a finite size.

Speaking to Nature Middle East last month, Ali said the theory helped unify quantum mechanics and general relativity: "Our theory serves to complement Einstein’s general relativity, which is very successful at describing physics over large distances…But physicists know that to describe short distances, quantum mechanics must be accommodated."

Das said he’s been encouraged to see such interest in the model but cautioned there’s still plenty more work do to.

“We need more confirmations and more careful study of our model, for sure,” he said. “We have to understand the mathematics better and whatever predictions it makes has to be tested with observations.”

The new model also takes dark matter into account. So far, dark matter has only been perceptible through its gravitational effect on visible matter such as stars. When Das and a colleague set the mass of the graviton in the model to a small level, they could make the density of their fluid match the universe’s observed density of dark matter, while also providing the right value for dark energy’s push.

“This is the first time that anyone has shown that these two major problems in cosmology can be solved simultaneously by the quantum Raychaudhuri equation,” says Ali.

“We feel a deep sense of satisfaction that this model may resolve some of the most important cosmological issues in one stroke,” adds Das.

If his work turns out to fundamentally alter our understanding of where everything comes from, Das also suggests it would be far from the final answer.

“Of course, it brings new problems, new questions, like: What was the universe before what we think is the Big Bang?” he said. “That’s great, because a lot of us would be interested in investigating those things further and hopefully getting answers to them, in due course.”


SOURCE  Metro News

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 Dark Matter

For the first time, a team of astronomers has located dark matter connecting two neighboring galaxy clusters. Dark matter is a type of matter that interacts only very weakly with light and itself. The discovery provides the first direct evidence that the universe is filled by a lacework of dark matter filaments, upon which the visible matter in the universe is distributed.

R esearchers have, for the first time, directly detected part of the invisible dark matter skeleton of the universe, where more than half of all matter is believed to reside.

The discovery, led by a University of Michigan physics researcher, Jörg Dietrich confirms a key prediction in the prevailing theory of how the universe's current web-like structure evolved.

The map of the known universe shows that most galaxies are organized into clusters, but some galaxies are situated along filaments that connect the clusters. Cosmologists have theorized that dark matter undergirds those filaments, which serve as highways of sorts, guiding galaxies toward the gravitational pull of the massive clusters. Dark matter's contribution had been predicted with computer simulations, and its shape had been roughed out based on the distribution of the galaxies. But no one had directly detected it until now.

Dark matter was postulated by Fritz Zwicky in 1934, to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters. According to new supernova observations and Big Bang cosmology, dark matter accounts for 23 percent of the total mass-energy of the observable universe.

Dark matter, whose composition is still a mystery, doesn't emit or absorb light, so astronomers can't see it directly with telescopes. They deduce that it exists based on how its gravity affects visible matter. Scientists estimate that dark matter makes up more than 80 percent of the universe. To "see" the dark matter component of the filament that connects the clusters Abell 222 and 223, Dietrich and his colleagues took advantage of a phenomenon called gravitational lensing.

"We found the dark matter filaments. For the first time, we can see them," said Dietrich, a physics research fellow in the University of Michigan College of Literature, Science and the Arts. Dietrich is first author of a paper on the findings published online in Nature and to appear in the July 12 print edition.

The gravity of massive objects such as galaxy clusters acts as a lens to bend and distort the light from more distant objects as it passes. Dietrich's team observed tens of thousands of galaxies beyond the supercluster. They were able to determine the extent to which the supercluster distorted galaxies, and with that information, they could plot the gravitational field and the mass of the Abell 222 and 223 clusters. Seeing this for the first time was "exhilarating," Dietrich said.

"It looks like there's a bridge that shows that there is additional mass beyond what the clusters contain," he said. "The clusters alone cannot explain this additional mass," he said.

Scientists before Dietrich assumed that the gravitational lensing signal would not be strong enough to give away dark matter's configuration. But Dietrich and his colleagues focused on a peculiar cluster system whose axis is oriented toward Earth, so that the lensing effects could be magnified.

"This result is a verification that for many years was thought to be impossible," Dietrich said.

The team also found a spike in X-ray emissions along the filament, due to an excess of hot, ionized ordinary matter being pulled by gravity toward the massive filament, but they estimate that 90 percent or more of the filament's mass is dark matter.

In the video below, Robbert Dijkgraaf, Dutch mathematical physicist and string theorist, discusses the greatest "known unknown" of the universe, dark energy. Dark matter and energy comprise approximately 96% of the universe, but know physicists know little about the physical phenomena.   (The complete video available for free at http://fora.tv/2012/03/20/Robbert_Dijkgraaf_The_End_of_Space_and_Time)






SOURCE  University of Michigan

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