Theos-Talk Archives (March 2004 Message tt00008)

Dark Energy Changes the Universe	02.27.04 
Dark energy has the cosmoslogists scratching their heads. 
Observations taken by NASA's Hubble Space Telescope and future space 
telescopes will be needed in order to determine the properties of 
dark energy, which makes up about 70 percent of the universe.Probing 
dark energy, the energy in empty space causing the expanding universe 
to accelerate, calls for accurately measuring how that expansion rate 
is increasing with time. Dark energy is thought to drive space 
apart.The diagram at right shows the changes in the rate of expansion 
since the universe's birth 15 billion years ago. The more shallow the 
curve, the faster the rate of expansion. The curve changes noticeably 
about 7.5 billion years ago, when objects in the universe began 
flying apart at a faster rate. Astronomers theorize that the faster 
expansion rate is due to a mysterious, dark force that is pulling 
galaxies apart. Image courtesy of NASA/STScI/Ann Feild. Click on 
image for larger view.Astronomers used NASA's Hubble Space Telescope 
to hunt for supernovae (an energetic explosive event that occurs at 
the end of a star's lifetime), using their brightness, astronomers 
could measure if the universe was expanding faster or slower in the 
distant past. In its search, Hubble discovered 42 new supernovae, 
including six that are among the most distant ever found. The 
farthest supernovae show that the universe was decelerating long ago, 
but then "changed gears" and began to accelerate. Albert Einstein 
coined the term "cosmological constant" to represent the possibility 
that even empty space has energy and couples to gravity. Like other 
astronomers of the time, he thought that the universe was static and 
so proposed there was a repulsive force from space that kept the 
universe in balance.Einstein discarded his own findings in 1929, when 
Edwin Hubble found through his research that the universe was 
expanding and not static. Today, new data from Hubble may well prove 
Einstein was on the right track. The pull of gravity and the push of 
dark energy have been trying to outmuscle each other since the 
beginning of time. About seven billion years ago, dark energy got the 
upper hand because the universe had grown so large and matter (the 
source of gravity) had expanded and scattered.At left is an 
illustration of the possible fates of the universe. Unstable dark 
energy could cause a "big rip" (the universe expands violently, then 
the stars, planets and atoms come unglued) or a "big crunch" (the 
universe implodes or compresses). Image courtesy of NASA/STScI/Ann 
Feild. Click on image for larger view.Cosmologists believe about 70 
percent of the universe consists of dark energy, 25 percent is dark 
matter, and only four percent normal matter (the stuff that stars, 
planets and people are made of). Hubble observations suggest the dark 
energy may be Einstein's cosmological constant, an energy percolating 
out of the vacuum of the space between galaxies.The energy of the 
universe is dominated by empty space emitting a repulsive form of 
gravity that is pushing the universe apart. But what does all this 
mean to Earth? Even if Einstein's theory was correct, we won't have 
to be concerned about the "dark side" for about 30 billion years, 
according to Hubble researchers.For further information, visit: 
http://universe.gsfc.nasa.gov/progr am/darkenergy.htmlNASA's John F. 
Kennedy Space Center and the Space Telescope Science Institute