Page 390 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch14_351-376.indd Page 367 9/3/10 6:16 PM user-f465
tiL12214_ch14_351-376.indd Page 367 9/3/10 6:16 PM user-f465 /Users/user-f465/Desktop

A Closer Look

Dark Energy
he first evidence came from astron- By comparing these two light sources, as a function of space. This means there
Tomers trying to measure how fast astronomers would be able to calculate was not much—shall we say, dark energy—
the expanding universe is slowing. At the the expansion rate for now and in the dis- when the universe was smaller, and grav-
time, the generally accepted model of the tant past. ity slowed the expansion. As the distance
universe was that it was still expanding To their surprise, the astronomers did between galaxies increased, there was an
but had been slowing from gravitational not find a rate of slowing expansion. Rather, increase of dark energy, and the expan-
attraction since the big bang. The astrono- they found that the expansion is speed- sion accelerated. Einstein removed this
mers intended to measure the slowing rate ing up. There was no explanation for this constant when Edwin Hubble reported
to determine the average density of matter speeding up other than to assume that some evidence that the universe is expanding.
in the universe, providing a clue about the unknown antigravity force was at work. Removing the constant may have been a
extent of dark matter. Their idea was to This unknown force must be pushing the mistake. One of the problems in under-
compare light traveling from a supernova galaxies farther apart at the same time that standing what was happening to the early
in the distant universe with light traveling gravity is trying to pull them together. The universe compared to now was a lack of
from a much closer supernova. Light from unknown repulsive force became known as information about the past as represented
the distant universe is just now reaching “dark energy.” by the distant, faraway universe. Today,
us because it was emitted when the uni- The idea of an unknown repulsive this is changing as more new technology
verse was very young. The brightness of force is not new. It was a new idea when is helping astronomers study the faraway
the older and younger supernova would Einstein added a “cosmological constant” universe. Will they find the meaning
provide information about distance, and to the equations in the theory of general of dark energy? What is dark energy?
expansive redshift data would provide in- relativity. This constant represents a force The answer is that no one knows. Stay
formation about their speed of expansion. that opposes gravity, and the force grows tuned . . . this story will be continued.

to their distance away. It would not matter which raisin you were tinuing. All the evidence tells us it began by expanding with a
on since all the raisins would appear to be moving away to all the big bang. Will it continue to expand, becoming increasingly
other raisins. In other words, the galaxies are not expanding into cold and diffuse? Will it slow to a halt, then start to contract to a
space that is already there. It is space itself that is expanding, and fiery finale of a big crunch (Figure 14.19)? Researchers continue
the galaxies move with the expanding space (Figure 14.18). searching for answers with experimental data and theoreti-
The ultimate fate of the universe will strongly depend on cal models, looking for clues in matches between the data and
the mass of the universe and if the expansion is slowing or con- models.

FIGURE 14.18 Will the universe continue expanding as the dust and gas in galaxies become locked up in white dwarf stars, neutron stars,
and black holes?

14-17 CHAPTER 14 The Universe 367