When
Einstein formulated in 1915 his theory of general relativity, he soon applied it
to the entire universe, deriving the following cosmological equation:

It is
curious that this equation is identical to the equation that would result from Newton’s
theory of gravitation. There is only one difference: constant k represents, in Newton’s
case, the total energy of the universe; in Einstein’s case, its curvature.

Each term
of this equation contains a universal constant. Besides k, G is the
gravitational constant; L is called the

**, whose interpretation is not clear. Einstein initially thought he could eliminate this term by making L = 0, which simplifies the equation and makes it analytically solvable. Then he discovered that the solution, in that case, was a universe in constant expansion. Since he believed that the universe had to be stationary, he decided to assign the constant a critical value L = L***cosmological constant*_{c}, to make it be so.
Unfortunately
for Einstein, a few years later two things were discovered:

- The universe is actually expanding
(Lemaître and Hubble).
- Einstein’s universe with L=L
_{c}is stationary, but unstable. Any small disturbance would make it go into an endless expansion or a contraction to zero volume.

Albert Einstein |

When he
realized these two things, Einstein said that

**. However, in 1998, when the accelerating expansion of the universe was discovered , cosmologists recovered the cosmological constant to explain this phenomenon, and equated the third term of Einstein’s equation with the mysterious***introducing the cosmological constant was the biggest mistake he had made***.***dark energy*
The problem
is we do not know what dark energy may be. If it exists (this is not yet
proven) it would probably be a new fundamental interaction of the universe,
apart from the four we know. This would be as big news as the discovery of
radioactivity in the late nineteenth century: a whole new field would be open
to physics. Also its value seems to be quite small, very close to zero, which
does not square with quantum field theories that have tried to explain it as the
vacuum energy, according to which its value should be 10

^{120}higher. M.P. Hobson and his colleagues have called this the worst theoretical prediction of the history of physics.
In conclusion, we
have two possibilities:

- If dark energy does not exist, if the
accelerating universe were an artifact or could be explained otherwise
(some physicists are analyzing these two possibilities), perhaps Einstein
was right when he said that
.*introducing the cosmological constant was the biggest mistake he had made* - But if dark energy really exists and the
cosmological constant has the value the standard model assigns to it (10
^{-52}m^{-2}), then Einstein’s mistake would have been saying the words quoted in the previous paragraph.

**The same post in Spanish**

**Manuel Alfonseca**

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