The cosmic expansion was explosively rapid at first. But the gravitational attraction of every galaxy on every other kept tugging at this expansion like a rubber band, slowing it down. Astronomers spoke of a “deceleration parameter” that quantified the amount of this reduction in speed. The big question of the 20th century’s last half was: Will everything come to a stop in the far future? Will the cosmos then go the other way, and collapse into a “Big Crunch”?
Until 1998, nobody entertained a different and illogical possibility. But then, two teams of astronomers, examining the brilliant lights of past supernovae to obtain better-than-ever determinations of galactic distances, independently came to an astonishing conclusion: The universe indeed slowed its expansion during the first half of its life. But then it stopped slowing. Some 6 or 7 billion years ago, galaxies everywhere started speeding up their expansion from their neighbors. As the eons passed since then, this expansion has accelerated until, now, all galaxies fly away from each other in an ever-increasing frenzy.
We know this is impossible. Galaxies don’t have rocket engines attached to them. What could possibly make them zoom faster and faster? And yet, this is exactly what astronomers seem to be observing everywhere they look.
Since nobody has a clue to what’s going on, scientists posit that space itself must have an anti-gravity, repulsive property, which they call “dark energy.” They assume this dark energy was responsible for blasting out the cosmos in the Big Bang, but that it then lost its dominance to gravity. When galaxies grew far enough apart, so that empty space started to control the picture, this anti-gravity force again gained the upper hand. Now, it pushes harder and harder, and everything will explosively fly apart forever.
Lest the concept of a darker and ever-lonelier cosmos drive you to antidepressants, be aware that much remains unknown about dark energy — well, everything, actually. Because it takes quite a bit of oomph to blow a universe apart, we know that this entity must constitute 74 percent of the mass-energy of the entire cosmos. Dark energy, whatever it is, must be nature’s dominant item. But for all we know, it might weaken or even reverse itself over time. Perhaps the cosmos could eventually cease racing and come back together, after all.
Do you find this just a tad peculiar? You’re not alone. That’s why the accelerating universe can’t be cheated of a place at the podium on our 50 Weirdest countdown, in our theater of the bizarre.