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miércoles, 3 de junio de 2009

Particles Larger Than Galaxies Fill the Universe?

Every day billions of subatomic particles called neutrinos are streaming at Earth from the sun. But these particles have no charge, very tiny masses, and move near the speed of light, making them especially hard to detect (above, a diver services a tank of water in Michigan used to detect neutrinos).

In May 2009 astronomers trying to measure neutrino masses found that the oldest known neutrinos might have been stretched by the expansion of the universe so that each particle encompasses a space larger than thousands of galaxies.

The oldest of the subatomic particles called neutrinos might each encompass a space larger than thousands of galaxies, new simulations suggest.

Neutrinos as we know them today are created by nuclear reactions or radioactive decay.

According to quantum mechanics, the "size" of a particle such as a neutrino is defined by a fuzzy range of possible locations. We can only detect these particles when they interact with something such as an atom, which collapses that range into a single point in space and time.

For neutrinos created recently, the ranges they can exist in are very, very small.

But over the roughly 13.7-billion-year lifetime of the cosmos, "relic" neutrinos have been stretched out by the expansion of the universe, enlarging the range in which each neutrino can exist.

"We're talking maybe up to roughly ten billion light-years" for each neutrino, said study co-author George Fuller of the University of California, San Diego.

"That's nearly on the order of the size of the observable universe."

"Small" Physics, Writ Large

Neutrinos have no charge, and their masses are so tiny they have yet to be accurately measured.

This means that neutrinos, which zip around at nearly the speed of light, can pass through normal matter largely undisturbed.

Most neutrinos that affect Earth come from the sun. Billions of solar neutrinos pass through the average human every second.

While trying to calculate masses for neutrinos, Fuller and his student Chad Kishimoto found that, as the universe has expanded, the fabric of space-time has been tugging at ancient neutrinos, stretching the particles' ranges over vast distances.

Such large ranges can remain intact, the scientists suggest in the May 22 issue of Physical Review Letters, since neutrinos pass right through most of the universe's matter.

An open question is whether gravity—say, the pull from an entire galaxy—can force a meganeutrino to collapse down to a single location.

"Quantum mechanics was intended to describe the universe on the smallest of scales, and now here we're talking about how it works on the largest scales in the universe," Kishimoto said.

"We're talking about physics that hasn't been explored before."

According to physicist Adrian Lee at the University of California, Berkeley, who was not part of the study team, "gravity is a real frontier these days that we don't really understand.

"These neutrinos could be a path to something deeper in our understanding with gravity."

Follow the Gravity?

But answers to such questions depend on eventually detecting these predicted meganeutrinos.

Although they should be extraordinarily common in the universe, the relic neutrinos now have only about one ten-billionth of the energy of neutrinos generated by the sun.

"This makes relic neutrinos near impossible to detect directly, at least with anything one could build on Earth," study co-author Fuller said.

Still, the fact that there are so many relic neutrinos means that together they likely exert a significant gravitational pull—"enough to be important for how the universe as a whole behaves," Fuller added.

Dark matter, for example, has never been directly observed. But astrophysicists have found proof that dark matter exists based on its effect on colliding galaxies.

"So by looking at the growth of structures in the universe," Fuller said, "you might be able to detect relic neutrinos indirectly by their gravity."

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