##### 1 Mar, 2021 | Prabin Pyakurel |

## What was Einstein’s Biggest Blunder?

Dark energy makes up about 70% of our universe, but we had no clue of its existence till 1998. Today, we know it does exist but very little about its nature or origin is known. This gives way to various theories attempting to model this energy and find out the consequences it has for our universe.

Serve yourself an introduction to dark energy so that you understand the blunder better.

### Einstein’s Space-time Equation

Dark energy is sometimes modeled as energy embedded in some material that remain undetected. At other times it is modeled as a constant that slightly changes the behavior of space-time.

This space-time is described by Einstein’s field equation which originally was

[latex] G_{\mu \nu}=\kappa T_{\mu \nu} [/latex]

The equation described a universe that would have an initial period of expansion and then begin to contract due to gravity as the initial force wears out.

### Einstein’s equation doesn’t make the cut

However, there was a problem with this behavior. At the time the prevalent idea was of a static universe. A Static universe would neither expand nor contract as time passes, contradicting Einstein’s equation.

To solve this contradiction, Einstein added a positive constant, known as the cosmological constant. This made his equation agree with observations of a static universe. The modified equation was:

[latex] G_{\mu \nu}+ \lambda g {\mu \nu}=\kappa T_{\mu \nu} [/latex]

The additional term lambda is the cosmological constant.

This equation, which was created rather artificially, ran into problems of its own as it didn’t produce any steady state solutions required to describe a static universe. However, astronomers couldn’t observe any expansion or contraction, so the constant remained for some time.

### Was the original correct?

Einstein would later call this addition his greatest blunder when Edwin Hubble discovered that the universe was expanding. Thus confirming Einstein’s original equation. Had Einstein accepted what the math him, he could have predicted an expanding universe years before any such observation was made.

Finally, the equation was back to its original form without the constant.

[latex] G_{\mu \nu}=\kappa T_{\mu \nu} [/latex]

The equation in its original form was in complete agreement with observations of an expanding universe. So by the end of the 20th century, astronomers thought they had now figured out the entire universe and the mathematical laws governing it.

### An Unexpected Discovery

Now they wanted to find out the amount of mass that the universe held. Their model as described earlier was that of a universe that would expand first and then start contracting. During the observations they carried out for this purpose. They found data suggesting that the universe was in fact expanding at an accelerating rate.

Thus came the idea of dark energy which can be modeled as a cosmological constant.

This cosmological constant is used at the same place in Einstein’s equation. But this time the new constant described an accelerated expansion rather than a static universe.

### Fate of the Universe

The cosmological constant or Dark energy makes up about 98% energy of the known universe as it pushes all matter away from each other.

Though we know very little about it. We do know that if dark energy is embedded in particles then it’s likely that this will be exhausted and we will be back to the model of gravitational contraction.

If its behavior can be described well with the cosmological constant, then the universe will keep on expanding forever. But at this point we really don’t know much about dark energy. So nothing can be said for certain.

So was it really Einstein’s blunder? The jury is still out on this one.

It’s a common misconception that dark matter is the same as dark energy, or that they are two aspects of the same thing. They are not. Find out more about dark matter here so that you stay on the know.