Dark energy

However, in recent years a new factor has entered the equation. Cosmologists using a new method to cross-check the rate of universal expansion (see panel, supernova cosmology

The Supernova Cosmology Project searches for Type Ia supernovae detonating in remote galaxies. These supernovae, caused by white dwarfs collapsing to form neutron stars (see p.67), all have the same intrinsic brightness, so their distance can be calculated by their apparent brightness. Distances calculated in this way are consistently larger than the theoretical distances calculated from the red shifts of the supernovae host galaxies, suggesting that the expansion of the universe is accelerating.



above) discovered that the most distant galaxies are farther away than they should be if expansion has been slowing down since the Big Bang—in other words, something is giving expansion a "boost." This "dark energy" is seen as a force that causes space itself to stretch apart at an increased rate. Improved measurements suggest that this force may be increasing over time, and has only overcome the general deceleration in the past 6 billion years. Its presence seems to indicate that the universe will expand forever, but if its strength is increasing, it could have more drastic effects.

cold death

In the "Big Chill" scenario, all galaxies will merge to form giant ellipticals like NGC 1316 (left). Star formation will end, and eventually the last stars will go out. Over trillions of years, even the matter within them will disintegrate.

Possible destinies

Debates about the future of the universe used to focus on two alternatives— "Big Crunch" and "Big Chill." The discovery of dark energy seems to have ruled out a Big Crunch, but has opened up two new possibilities. The four possible scenarios are shown here.

Big Chili _

Modified Big Chill _

Big Rip Big Crunch /

_ Big Bang time

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