The cosmos didn’t just leak in here drop by drop from another dimension, as the once-popular Steady State theory suggested. It began all at once and then briefly expanded much faster than the speed of light. That’s the only way the speed of light and the value of such constants as gravity and the mass of fundamental subatomic particles can be so precisely identical everywhere.
This being so, any physical truth that applies to our galaxy, such as the strength of the four fundamental forces, is identical everywhere else and throughout all of time. And until 1998, one additional universal reality that seemed inarguable was that the cosmos’ expansion is slowing down.
The Big Bang theory, strongly supported by the cosmic microwave background and the omnipresent expansion, says that starting 13.8 billion years ago, everything initially raced outward from everything else, and this cosmic expansion was explosively rapid at first. But the gravitational attraction of every galaxy group on every other kept tugging at this expansion like a rubber band, slowing it down. 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?”
Nobody entertained a different and illogical possibility, which took the world by storm in 1998. 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 for the first half of its life. But then around seven 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 that even exceeds light speed at a particular distance from us.
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 we’re observing.
Since nobody has a clue to what’s going on, we posit that space itself must have an anti-gravity repulsive property and we call this ‘dark energy.’ We assume that this dark energy was responsible for blasting out the cosmos in the first place, in the Big Bang, but it then lost its dominance to gravity. When galaxies grew far enough apart so that empty space got vast enough, this anti-gravity force gained the upper hand. Now it will push harder and harder and everything will explosively fly apart forever.
This leads us to the nature of the empty space that’s doing all that pushing-apart. To do the job, this “nothingness” must haves such vast power that it comprises three-quarters of the substance of the entire universe.
And though we don’t understand its underlying nature, this hasn’t stopped us from giving it a series of different names. You can call it vacuum energy or dark energy or the cosmological constant, or zero-point energy. The latter name owes its origin to the fact that it only starts to reveal itself when its temperature is zero degrees Kelvin, or -459.67 degrees below zero Fahrenheit. Any warmer than that, and the kinetic energy inherent in moving atoms and molecules dominates the picture and obscures this vacuum energy.
Though it’s blowing the universe apart, which may someday leave us looking into a starless sky, much is unknown about dark energy. Everything, actually. Although it must be nature’s dominant item, it might — for all we know — weaken or even reverse itself over time. Perhaps the cosmos could eventually come back together after all.
And if you find all this just a tad peculiar, rest assured you’re not alone.
Join the family! 
