[DIGEST: CNN, Space.com, Universe Today, NASA]

It has been one hundred years since Albert Einstein came up with his crowning intellectual achievement–the theory of relativity. That theory, published in 1915, posited that space and time are part of a single, interwoven continuum called spacetime. More importantly, spacetime itself is malleable and “warps” under the influence of matter. What we know as gravity is actually the bending of spacetime by objects with mass.

Einstein’s theory has survived over a hundred years, having been put to rigorous testing and come out unscathed. But recent conclusions on the accelerating expansion of the universe corroborate a further idea Einstein had posited–and rejected–nearly a century ago.

#### Defying Gravity

Einstein, like most in his time, held to a “static” view of the universe, believing that without a “cosmological constant” in his equations (a repulsive force that countered the effect of gravity), the matter in the universe ultimately would draw itself into a single point. So Einstein added that cosmological constant to his initial equations in 1917 as a kind of “fudge factor,” seeking to zero out the effect of gravity so that the universe stayed in a constant state.

Then, in 1929, the cosmologist Edwin Hubble proved through his observations of infrared light that the galaxies were actually moving away from us. The universe was not static at all, it was expanding. This startling observation suggested that at one point the entire universe arose from a single point, giving rise to what is now the “Big Bang.” The proof of the expanding universe caused Einstein to regret having added his cosmological constant, so he removed it from his equation, reportedly calling it his “biggest blunder.”

But was it? Scientists now believe that Einstein’s dismissal of the cosmological constant may actually have been premature, and that he would have been right, in a sense, to have included it all along.

**The Ever-Expanding Universe**

Prior to 1998, common scientific thought was that gravity would eventually pull space inward, creating what they called a very hot “Big Crunch.” Essentially, if the universe began with the Big Bang, which inflated the universe with a huge expanding force, then the

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Big Crunch would be the natural response to this expansion. Once the universe’s expansion slowed, gravity eventually would then pull everything back to center, ending everything in a slow compacting.

But via the Hubble space telescope, astronomers observed in 1998 that the universe’s expansion was *accelerating*, not slowing down. This threw the Big Crunch into serious doubt. If these observations were confirmed–and most cosmologists now concur–then at some point in the very distant future, only local galaxies would be visible from Earth, as others would have long since drifted off.

This discovery also had an effect on the underlying theory of general relativity: Some astronomers now suggest that Einstein’s constant–or something like it–should be reinserted into the equation. Cosmologists now theorize that the observable effect of a more rapidly expanding universe is created by some unknown, repulsive force called “dark energy”–a theoretical energy field that, while never actually been observed, is sufficient to overpower the pull of gravity. It is dark energy that is causing the universe to expand ever and ever quicker.

Could Einstein’s cosmological constant be re-substituted in for the effect of dark, repulsive energy? Some argue that a new “fudge factor” to account for dark energy is needed, and that while a constant density and the expanding volume of the universe means

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an increasing rather than constant energy, a constant is still permitted within the rules of general relativity.

But others are dubious, questioning whether this dark energy would ever truly be a constant, yielding the same force throughout time. Indeed, observational evidence suggests that the repulsive effect has been varying across time, and not constant.

**Earth, Wind and Fire… and Quintessence?**

Enter the notion of quintessence, named after the ancient Greeks’ notion of an unseen “fifth element” beside earth, air, fire and water. Quintessence is posited as “a dynamic, time-evolving and spatially dependent form of energy with negative pressure sufficient to drive the accelerating expansion,” according to Paul Steinhardt of Princeton University, one of the originators of the inflation theory who predicted an accelerating universe back in 1995.

But whether it’s a constant or quintessence, dark energy most likely exists and has to be accounted for. To the amazement of scientists today, somehow Einstein anticipated this energy nearly a century ago. Yet he went to his grave thinking that including that factor was his greatest error. In a twist of irony, it now seems that something close to what Einstein described belongs in that equation, though not for the reason he thought. One hundred years after his great equations were published, Einstein continues to demonstrate his uncanny genius. Some now note that Einstein’s too-hasty removal of this repulsive energy “fudge factor” actually may have been his “greatest blunder.”