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Science Forum Index » Astro - Amateur Forum » Unmasking Dark Energy...
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| Sam Wormley... |
 Posted: Sat Aug 02, 2008 6:13 pm |
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Unmasking Dark Energy
http://sciencenow.sciencemag.org/cgi/content/full/2008/801/3
By Phil Berardelli
ScienceNOW Daily News
1 August 2008
Astronomers have taken another significant step toward proving the existence of dark
energy, the mysterious force that seems to be stretching the cosmos at an accelerating
rate. A team from the University of Hawaii, Honolulu, has observed how dark energy
directly affects the largest structures in the universe. If confirmed, the findings could
bring astronomers closer to understanding a phenomenon that has bedeviled them for more
than a decade.
In the mid-1990s, an international team of researchers attempted to measure the rate by
which the expansion of the universe--begun 13.7 billion years ago in the big bang--was
decelerating. They planned to do so by carefully calibrating the brightness of a
well-studied type of supernova in very distant galaxies. Because the brightness of each of
these supernovae should be exactly the same, the researchers could calculate the distance
and velocity of their host galaxies. But by 1998, the team had reached a stunningly
different conclusion: The expansion of the universe was not slowing down--it was
accelerating, driven by some unknown effect that came to be called dark energy. Ever
since, researchers have struggled to detect the strange force, which also seems to
constitute 76% of the mass-energy balance of the universe.
To conduct the new study, the Hawaiian team, led by astronomer Istvan Szapudi, combined
two large-scale observations of the cosmos that already had been completed: the cosmic
microwave background (CMB), which represents the last, dying embers of the big bang, and
the Sloan Digital Sky Survey, which comprises images of millions of galaxies. Current
theory predicts that dark energy would slightly heat the CMB radiation passing through
dense regions of galaxies called superclusters, each about a half-billion light-years
across. In contrast, dark energy would slightly cool the microwaves passing through
equally vast areas of mostly empty space called supervoids. As they will report in an
upcoming issue of Astrophysical Journal Letters, based on data collected from observations
of 50 superclusters and 50 supervoids, the CMB was either heated or cooled almost
precisely as expected.
The study is "the best of its type undertaken to date," says astrophysicist Adam Riess of
the Space Telescope Science Institute in Baltimore, Maryland. Riess, whose team published
the first paper on dark energy, adds, however, that although the method used by the
Hawaiian team provides "fairly direct evidence for dark energy," the force itself still
hasn't been detected. Nor do the results help scientists determine which of their theories
best describes what dark energy is--in particular, whether it is "vacuum energy" inherent
in space itself or some sort of "quintessence" force that might change over time. "We've
still got a cosmic mystery on our hands," Riess says. |
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| gubbenimanen... |
| Posted: Sun Aug 03, 2008 5:33 am |
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A simple explanation would be that the CMB is cool and that where
everywhere there are stars and matter there is a simple microwave part
of the black-body radiation that makes the background look warmer.
Just like stones heated by the sun are radiating at night and making
warmer localities.
Roger Persson |
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| Shawn... |
| Posted: Sun Aug 03, 2008 9:20 am |
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Sam Wormley wrote:
snip
Quote: To conduct the new study, the Hawaiian team, led by astronomer Istvan
Szapudi, combined two large-scale observations of the cosmos that
already had been completed: the cosmic microwave background (CMB), which
represents the last, dying embers of the big bang, and the Sloan Digital
Sky Survey, which comprises images of millions of galaxies. Current
theory predicts that dark energy would slightly heat the CMB radiation
passing through dense regions of galaxies called superclusters, each
about a half-billion light-years across. In contrast, dark energy would
slightly cool the microwaves passing through equally vast areas of
mostly empty space called supervoids. As they will report in an upcoming
issue of Astrophysical Journal Letters, based on data collected from
observations of 50 superclusters and 50 supervoids, the CMB was either
heated or cooled almost precisely as expected.
snip
I don't understand what they mean by "heating" and "cooling" of the CMB.
Do they mean red-shifting (cooling) and blue-shifting (heating)? If
so, I'm familiar with gravitational red-shifting as light (e.g. the CMB)
travels away from a mass, but this sounds like the opposite.
Red-shifting (cooling?) across voids, or any span of space for that
matter, is what we've come to expect from dark energy. Thoughts?
Shawn |
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| Stuart Levy... |
| Posted: Sun Aug 03, 2008 12:54 pm |
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On 2008-08-03, gubbenimanen <gubbenimanen at (no spam) yahoo.com> wrote:
Quote: A simple explanation would be that the CMB is cool and that where
everywhere there are stars and matter there is a simple microwave part
of the black-body radiation that makes the background look warmer.
Just like stones heated by the sun are radiating at night and making
warmer localities.
Roger Persson
But no, a situation like that -- with some warm, mostly-transparent matter
whose radiation was superimposed on a cooler blackbody spectrum --
wouldn't give a spectrum that looked like that of a blackbody with a higher
temperature. It'd be a blackbody plus some shorter-wavelength lumps.
If you put a cold body in space in that situation, it would
come to equilibrium at a warmer temperature than the original
blackbody -- that's the analog of being warmed by the earth after dusk.
But a spectral measurement would show the difference. |
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| Sam Wormley... |
| Posted: Sun Aug 03, 2008 10:16 pm |
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Shawn wrote:
Quote: Sam Wormley wrote:
Unmasking Dark Energy
http://sciencenow.sciencemag.org/cgi/content/full/2008/801/3
snip
To conduct the new study, the Hawaiian team, led by astronomer Istvan
Szapudi, combined two large-scale observations of the cosmos that
already had been completed: the cosmic microwave background (CMB),
which represents the last, dying embers of the big bang, and the Sloan
Digital Sky Survey, which comprises images of millions of galaxies.
Current theory predicts that dark energy would slightly heat the CMB
radiation passing through dense regions of galaxies called
superclusters, each about a half-billion light-years across. In
contrast, dark energy would slightly cool the microwaves passing
through equally vast areas of mostly empty space called supervoids. As
they will report in an upcoming issue of Astrophysical Journal
Letters, based on data collected from observations of 50 superclusters
and 50 supervoids, the CMB was either heated or cooled almost
precisely as expected.
snip
I don't understand what they mean by "heating" and "cooling" of the CMB.
Do they mean red-shifting (cooling) and blue-shifting (heating)? If
so, I'm familiar with gravitational red-shifting as light (e.g. the CMB)
travels away from a mass, but this sounds like the opposite.
Red-shifting (cooling?) across voids, or any span of space for that
matter, is what we've come to expect from dark energy. Thoughts?
Shawn
A systematic temperature increase in the CMB spectra associated with
super clusters might not be explainable other than by effects of Dark
energy. Some think that idea is worth looking at. |
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