The Accelerating Universe
If the expansion of the universe is in fact slowing with time,
as these simple models using gravity predict, then when the
universe was young it must have been expanding more rapidly
than it is today. Objects that are very far away should
have larger velocities than Hubble’s law derived from local
galaxies would suggest. This is the same as saying that the
plots in Figure 22.1 are all curving away from a straight
line. (If gravity were not present, the Hubble expansion
[see Chapter 19] would not be changing, and the plots would
instead be straight lines.)
During the 1990s, some groups of astronomers began
using the Hubble Space Telescope, telescopes in Chile,
and the giant Keck Observatory telescopes in Hawaii
to test this prediction. They measured the brightness of
Type Ia supernovae in very distant galaxies and compared
the brightness of those supernovae with their expected
brightness based on the redshift distances of those galaxies.
(Recall from Chapters 19 and 20 that Type Ia supernovae
have a very high peak luminosity that can be calibrated,
and therefore they are used as “standardizable”
candles for measuring
distances to galaxies.) The
findings of these studies
sent a wave of excitement
through the astronomical
community. The observational data of the Type Ia supernovae
at different distances are plotted in Figure 22.2.
Rather than showing that the expansion of the universe
has slowed down over time, the data indicated that it is
speeding up—accelerating. For this to be true, a force must
be pushing the entire universe outward in opposition to
The Accelerating UniverseIf the expansion of the universe is in fact slowing with time,as these simple models using gravity predict, then when theuniverse was young it must have been expanding more rapidlythan it is today. Objects that are very far away shouldhave larger velocities than Hubble’s law derived from localgalaxies would suggest. This is the same as saying that theplots in Figure 22.1 are all curving away from a straightline. (If gravity were not present, the Hubble expansion[see Chapter 19] would not be changing, and the plots wouldinstead be straight lines.)During the 1990s, some groups of astronomers beganusing the Hubble Space Telescope, telescopes in Chile,and the giant Keck Observatory telescopes in Hawaiito test this prediction. They measured the brightness ofType Ia supernovae in very distant galaxies and comparedthe brightness of those supernovae with their expectedbrightness based on the redshift distances of those galaxies.(Recall from Chapters 19 and 20 that Type Ia supernovaehave a very high peak luminosity that can be calibrated,and therefore they are used as “standardizable”candles for measuringdistances to galaxies.) Thefindings of these studiessent a wave of excitementthrough the astronomicalcommunity. The observational data of the Type Ia supernovaeat different distances are plotted in Figure 22.2.Rather than showing that the expansion of the universehas slowed down over time, the data indicated that it isspeeding up—accelerating. For this to be true, a force mustbe pushing the entire universe outward in opposition to
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