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The Cosmic Jerk
« on: 2003-10-16 09:14:39 » |
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[rhinoceros]
It seems gravity was deccelerating the expansion of the Universe until 5 billion years ago. That decceleration was important for the formation of galaxies.
At that point, "dark energy", the unknown repulsive force which is held responsible for the accelerating expansion of the Universe we observe today, broke even and then took the upper hand.
Here is how it was reported in New Scientist:
Astronomers date Universe's 'cosmic jerk'
Source: New Scientist
Author: Sharmila Kamat
Dated: 2003-10-13
The point when the repulsive force of dark energy overwhelmed gravity and started the accelerating expansion of the Universe that continues today has been revealed.
"It happened five billion years ago," says Adam Riess, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland. "That was when the Universe stopped slowing down and began to accelerate, experiencing a cosmic jerk."
Astronomers discovered that the Universe is expanding at an accelerating rate in 1998 by studying exploding stars known as Type 1a supernovae. These supernova always emit the same amount of energy, so their brightness indicates how far away they are.
Because the Universe is expanding, the light from the supernovae shifts towards the red end of the spectrum. The 1998 observations revealed that light from such supernovae appeared dimmer than their red shifts predicted - showing that the expansion of the Universe was accelerating.
But there was always the worry that other factors, like intergalactic dust, could make the objects seem less bright. Now, Reiss's findings have set aside these doubts.
Stop and start
Reiss and his colleagues analysed more distant supernovae using the Hubble Space Telescope. They spent two years observing the light from six supernovae which was emitted between about 9 and 11 billion years ago.
They found the light was brighter than expected from their red shifts, indicating that expansion was decelerating at that time. "Such a deceleration is vital to allow for galaxy formation," notes Michael Turner of the University of Chicago.
So after the big bang, matter was still relatively dense in the Universe and therefore gravity braked expansion. But as galaxies moved farther apart, dark energy began to exert a more significant influence. For a brief period, two forces balanced and "the expansion of the Universe coasted along at a steady rate, like a car in cruise control," says Riess.
But then, five billion years ago, dark energy got the upper hand. No-one yet knows what dark energy is, but supernova observations provides some constraints. "It gives a handle on how much there is," says Saul Perlmutter of Lawrence Berkeley National Laboratory in California.
The new results were announced at the CERCA-Kavli Future of Cosmology conference at Case Western Reserve University in Cleveland, Ohio.
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