Why Shouldn't the Universe Exist?

In 1967, cosmologists were grappling with the 'cosmological constant problem', a profound puzzle about why empty space doesn't weigh as much as one would expect. This issue lies at the intersection of quantum field theory and general relativity - two pillars of modern physics that describe how the universe works on vast scales and minute particles, respectively.

The 'cosmological constant', denoted by Lambda (Λ), was first introduced by Albert Einstein in his 1915 general theory of relativity as a way to maintain a static universe, but he later regarded it as his "biggest blunder" when the discovery showed that the universe is expanding. The cosmological constant appears in Einstein's field equations and represents an energy density filling space homogeneously.

According to quantum mechanics, empty space isn't truly empty but filled with virtual particles popping into existence and then annihilating each other almost instantaneously. This 'vacuum fluctuation' should result in a large value for Λ; however, observations suggest it is astonishingly small - about one part in 10^120.

This problem arises from the need to reconcile quantum mechanics and general relativity into a single theory that explains all physical phenomena. This unification attempt led to string theory - a candidate for 'Theory of Everything' that predicts an enormous number of possible vacuum states or 'vacua', each with different values for the cosmological constant. These are part of what is known as the 'String Theory Landscape'.

However, not everyone agrees on this explanation. Some physicists argue it isn't scientific due to its reliance on anthropic reasoning (i.e., only our universe could allow life because of specific conditions). Yet, others suggest that such reasoning can be falsified and continue exploring alternative ideas like supersymmetric large extra dimensions or vacuum energy sequestering among others.

The search for a solution to the cosmological constant problem continues as scientists weigh the universe - from stars, planets, interstellar gas clouds to dark matter halos enveloping galaxies and the vacuum of empty space itself. The paradox remains one of fundamental physics' most perplexing mysteries.