For the first time, astronomers have clearly seen the
effects of "dark energy" on the most massive collapsed objects in the
universe using NASA's Chandra X-ray Observatory. By tracking how dark
energy has stifled the growth of galaxy clusters and combining this
with previous studies, scientists have obtained the best clues yet
about what dark energy is and what the destiny of the universe could
be.
Above: The composite image on the left is of the galaxy cluster Abell 85, located about 740 million light years from Earth. The purple emission is multi-million degree gas detected in X-rays by NASA's Chandra X-ray Observatory and the other colors show galaxies in an optical image from the Sloan Digital Sky Survey. This galaxy cluster is one of 86 observed by Chandra to trace how dark energy has stifled the growth of these massive structures over the last 7 billion years. Galaxy clusters are the largest collapsed objects in the Universe and are ideal for studying the properties of dark energy, the mysterious form of repulsive gravity that is driving the accelerated expansion of the Universe.
The illustration on the right shows snapshots from a simulation by Volker Springel, representing the growth of cosmic structure when the Universe was 0.9 billion, 3.2 billion and 13.7 billion years old (now). This shows how the Universe has evolved from a smooth state to one containing a vast amount of structure. Gas is shown in these snapshots, where the yellow regions are stars and the brightest structures are galaxies and galaxy clusters. The growth of these structures was initially driven only by the attractive force of gravity, but then later there was competition with the repulsive force of dark energy.
|
This work, which took years to complete, is separate from other
methods of dark energy research such as supernovas. These new X-ray
results provide a crucial independent test of dark energy, long
sought by scientists, which depends on how gravity competes with
accelerated expansion in the growth of cosmic structures. Techniques
based on distance measurements, such as supernova work, do not have
this special sensitivity.
Scientists think dark energy is a form of repulsive gravity that now
dominates the universe, although they have no clear picture of what
it actually is. Understanding the nature of dark energy is one of the
biggest problems in science. Possibilities include the cosmological
constant, which is equivalent to the energy of empty space. Other
possibilities include a modification in general relativity on the
largest scales, or a more general physical field.
To help decide between these options, a new way of looking at dark
energy is required. It is accomplished by observing how cosmic
acceleration affects the growth of galaxy clusters over time.
"This result could be described as 'arrested development of the
universe', " said Alexey Vikhlinin of the Smithsonian Astrophysical
Observatory in Cambridge, Mass., who led the research. "Whatever is
forcing the expansion of the universe to speed up is also forcing its
development to slow down."
Vikhlinin and his colleagues used Chandra to observe the hot gas in
dozens of galaxy clusters, which are the largest collapsed objects in
the universe. Some of these clusters are relatively close and others
are more than halfway across the universe.
The results show the increase in mass of the galaxy clusters over time
aligns with a universe dominated by dark energy. It is more difficult
for objects like galaxy clusters to grow when space is stretched, as
caused by dark energy. Vikhlinin and his team see this effect clearly
in their data. The results are remarkably consistent with those from
the distance measurements, revealing general relativity applies, as
expected, on large scales.
"For years, scientists have wanted to start testing how gravity works
on large scales and now, we finally have, " said William Forman, a
co-author of the study from the Smithsonian Astrophysical
Observatory. "This is a test that general relativity could have
failed."
When combined with other clues -- supernovas, the study of the cosmic
microwave background, and the distribution of galaxies -- this new
X-ray result gives scientists the best insight to date on the
properties of dark energy.
The study strengthens the evidence that dark energy is the
cosmological constant. Although it is the leading candidate to
explain dark energy, theoretical work suggests it should be about 10
raised to the power of 120 times larger than observed. Therefore,
alternatives to general relativity, such as theories involving hidden
dimensions, are being explored.
"Putting all of this data together gives us the strongest evidence yet
that dark energy is the cosmological constant, or in other words,
that 'nothing weighs something', " said Vikhlinin. "A lot more testing
is needed, but so far Einstein's theory is looking as good as ever."
These results have consequences for predicting the ultimate fate of
the universe. If dark energy is explained by the cosmological
constant, the expansion of the universe will continue to accelerate,
and the Milky Way and its neighbor galaxy, Andromeda, never will
merge with the Virgo cluster. In that case, about a hundred billion
years from now, all other galaxies ultimately would disappear from
the Milky Way's view and, eventually, the local superclusters of
galaxies also would disintegrate.
The work by Vikhlinin and his colleagues will be published in two
separate papers in the Feb. 10 issue of The Astrophysical Journal.
NASA's Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra program for NASA's Science Mission Directorate in Washington.
The Smithsonian Astrophysical Observatory controls Chandra's science
and flight operations from Cambridge, Mass.
Additional information and images are available at: http://chandra.nasa.gov
US $85.00
