[tt] NS: 'Hot' photons cast dark energy into the light

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'Hot' photons cast dark energy into the light
http://space.newscientist.com/article/mg19826583.500-hot-photons-cast-dark-energy-into-the-light.html

30 May 2008
Zeeya Merali

SOME people were beginning to wonder if it was just an illusion, but
it looks as if dark energy is real and here to stay.

Dark energy was dreamed up to explain why the expansion of the
universe appears to be speeding up. Since then, astronomers have
struggled to explain what dark energy actually is - leading to
speculation that it may not exist at all. For instance, some claim
that an uneven distribution of matter within the universe could be
distorting our measurements of the distance to supernovae, fooling
us into thinking that they are moving away faster than they really
are (New Scientist, 8 March, p 32).

In a bid to solve the dark energy question once and for all, István
Szapudi at the University of Hawaii in Honolulu turned to the cosmic
microwave background - the radiation left behind by the big bang.
Detailed maps of the CMB show hot and cold spots that reflect
variations in the density of matter in the early universe.
"We have shown the imprint on the cosmic microwave background of
dark energy at work"

When dark energy was proposed, astronomers realised it could explain
some of the temperature bumps on the map. These variations occur
because the energy of photons zipping across the universe from the
CMB towards us changes depending on whether they have passed through
a region of dense matter or a sparser region, called a void. The
greater the energy a photon has upon reaching us, the greater the
temperature shown in the CMB map.

A photon gains energy when it enters a dense region with greater
gravity, such as a galaxy cluster, as it falls into the cluster's
gravity well. When it leaves the cluster, it must use up the energy
it had gained in order to climb back out of the gravity well. So in
a universe without dark energy, the energy a photon gains is
virtually cancelled out by the energy it loses. In the presence of
dark energy, however, the universe expands quickly enough to stretch
the gravity well while the photon is still inside it. This shallower
well is easier for the photon to climb out of, so a photon
travelling through a cluster gains more energy on its approach than
it loses upon its departure. This differential gives it a little
energy kick, resulting in a hotter than expected spot on images of
the CMB. A photon that has passed through a void would result in a
cold spot.

It's tough to discern the effects of dark energy on photons because
the phenomenon gives only a slight nudge to the temperature - a
change that is swamped by the normal temperature variations seen in
the CMB, says Szapudi. To tackle this, his team focused on the
regions surrounding over 50 superclusters and 50 supervoids, where
you would expect to see the biggest effect. They found the regions
did indeed tally with enhanced hot and cold spots in the CMB. The
team are submitting the work to Astrophysical Journal Letters
(www.arxiv.org/0805.3695).

While other studies have reported signs of this effect, they were
open to alternative explanations, says Szapudi. By contrast, his
calculations suggest there is only a 1-in-200,000 chance that his
result is down to anything other than dark energy. "We have shown
the imprint on the CMB of dark energy at work," he says. "In this
sense, we have imaged dark energy."

Ofer Lahav, an astrophysicist at University College London, is not
convinced that Szapudi's team has proved the existence of dark
energy, but agrees the work supports it. "If you take Occam's razor
approach, this fits with the picture that dark energy is a
significant component of the universe we live in."