In 1917, not long after publishing the theory of general relativity, Albert Einstein played some mathematical gymnastics with his field equations -- a set of equations that relate the curvature of spacetime to the amount of matter and energy moving through a region of spacetime. At the time, everybody thought that the universe is stationary. For the framework of his theory of general relativity to make any sense under these conditions, Einstein inserted a term called the cosmological constant (denoted by the Greek capital letter lambda).
But almost a decade later, Edwin Hubble proved beyond a doubt that the universe was not static -- it was, in fact, expanding. Upon hearing the news, Einstein abandoned the cosmological constant, calling it the "biggest blunder" of his life.
However, this wasn't the end of it. In 1998, scientists discovered that the universe wasn't only expanding, it was doing so at an accelerated rate. Some unknown force was overcoming gravity, making galaxies move away from each other increasingly faster. This force is known today as dark energy, and it's true nature still remains a mystery.
Ironically, physicists had to reintroduce the cosmological constant into Einstein's field equations in order to account for this new force, which constitutes about 70% of the energy content in the universe. This constant employ a different value than Einstein would have thought, but the idea is still exactly what Einstein came up with.
In the current standard model of cosmology, the cosmological constant estimates its value as 10-52 per square meter -- that's incredibly tiny but over the scale of the universe, this constant becomes significant enough to accelerate the expansion of space.
The cosmological constant also includes "vacuum energy" or "zero point energy" -- the energy density of empty space. When physicists try to calculate its contribution to the cosmological constant, they end up with an absurd value in the order of 10 120 (yes, 10 followed by 120 zeroes). The discrepancy between the two proposed values of the cosmological constant is unacceptable, to say the least.
This may mean that Einstein's original field equations for gravity are wrong -- but that is extremely unlikely. The theory of general relativity is one of the most tested frameworks in physics, having stood scrutiny time and time again. The fairly recent detection of gravitational waves by the LIGO experiment suggests that Einstein's theory is the best we've got so far that explains gravity.
Instead of doubting Einstein's theory of general relativity, Lucas Lombriser, an assistant professor of theoretical physics at the University of Geneva in Switzerland, simply added a new equation on top of the field equations. Essentially, what Lombriser did was to assume that the gravitational constant (the one first used by Isaac Newton in his laws of gravity) can change. Yes, constants have really lost their semantics in modern theoretical physics.
At any rate, the Lombriser version of general relativity assumes that the gravitational constant remains the same within the observable universe but can change beyond it. In other words, his theory assumes that there are multiple universes -- that we live in a multiverse -- some of which may function with different values for the fundamental constants.
After accounting for the estimated mass of all the galaxies, stars, and dark matter in the universe, Lombriser found that this framework returned a value for the cosmological constant that closely agrees with experimental observations. Specifically, he found the universe is made of 74% dark energy whereas observations estimate 68.5% -- a huge improvement, to say, the least over the previous discrepancy.
Unfortunately for Lombriser, who published his work in the journal Physics Letters B, there's no way to actually test his theory -- at least not yet.
What's shocking. though, if you think about it for a second is that even Einstein's 'bad' ideas were brilliant!