Scientists have found more intriguing evidence for dark
energy — one of nature's most befuddling phenomena.
Dark energy is thought to make up about 74 percent of the
universe, while dark matter — a mysterious form of matter that scientists can
only detect by noting its gravitational pull on things — makes up about 22 percent.
That leaves only 4 percent of the universe composed of things we can see and
touch: the normal protons, electrons and neutrons called baryonic matter.
Scientists don't know what dark
energy is, but they observe its tugging effect, which causes the expansion
of the universe to accelerate. Now they have seen this mysterious force in some
of the largest known features of the cosmos, called superclusters and
supervoids.
The former are particularly crowded areas of space, each with
a lot of galaxies huddled in a region half a billion light-years across, while
the latter are the opposite, rather barren expanses notably lacking galaxies.
Astronomers led by István Szapudi of the University of
Hawaii Institute for Astronomy observed dark energy stretching out these areas
by detecting changes in rays of microwave light before and after they passed
through the regions.
"When a microwave enters a supercluster,
it gains some gravitational energy, and therefore vibrates slightly
faster," Szapudi said. "Later, as it leaves the supercluster, it
should lose exactly the same amount of energy. But if dark energy causes the
universe to stretch
out at a faster rate, the supercluster flattens out in the half-billion
years it takes the microwave to cross it. Thus, the wave gets to keep some of
the energy it gained as it entered the supercluster."
Szapudi, with University of Hawaii postdoctoral researcher
Mark Neyrinck and graduate student Benjamin Granett, analyzed a map of the
varying strength of the microwave radiation left over from the Big Bang, called
the cosmic microwave background radiation (CMB), across the universe. They
matched this data to a map of the universe with the 50 largest supervoids and
the 50 largest superclusters plotted, based on information from the Sloan
Digital Sky Survey, a project that mapped the distribution of galaxies over a
quarter of the sky.
As the researchers predicted, the microwaves were a bit
stronger if they had passed through a supercluster, and a bit weaker if they had
passed through a supervoid.
"With this method, for the first time we can actually see
what supervoids and superclusters do to microwaves passing through them,"
Granett said.
The team will detail their findings in the Astrophysical
Journal Letters in August or September.
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