Dark energy. From Andrew Masterson at Cosmos:
The question of dark energy in one universe does not require others to provide an answer.
A hypothetical multiverse seems less likely after modelling by researchers in Australia and the UK threw one of its key assumptions into doubt.
Dark energy is supposed to be responsible for the accelerating expansion of our universe but current theory suggests that there should be much more than there is, so:
The multiverse idea to an extent accounts for and accommodates this oddly small – but life-permitting – dark energy quotient. Essentially it permits a curiously self-serving explanation: there are a vast number of universes all with differing amounts of dark energy. We exist in one that has an amount low enough to permit stars and so on to form, and thus life to exist. (And we find ourselves here, runs the logic, because we couldn’t find ourselves anywhere else.)
It’s really an argument against the evidence for fine-tuning of our universe. The cosmologists ran simualtions that showed that stars and planets could form with either very little dark energy or a great deal.
“The multiverse was previously thought to explain the observed value of dark energy as a lottery – we have a lucky ticket and live in the universe that forms beautiful galaxies which permit life as we know it,” says Barnes.
“Our work shows that our ticket seems a little too lucky, so to speak. It’s more special than it needs to be for life. This is a problem for the multiverse; a puzzle remains.”
It is a puzzle that goes right to the heart of the matter: if the dark energy assumptions are flawed, does a multiverse even exist? The researchers acknowledge that their results do not preclude it – but they do diminish the likelihood. More.
Despite these and any number of findings, the multiverse cannot be diminished by evidence against it because no evidence for it is felt to be truly needed. It is a proposed alternative to the massive evidence for fine-tuning of our universe. It can be embraced by faith alone despite lack of evidence or contrary evidence.
The battle is being fought elsewhere now. Multiverse theorists wage war against falsifiability as a concept, so as to make their faith the untestable bedrock of cosmology.
See also: Science at sunset: Dark energy, if it exists, might make a multiverse hospitable to life, if it exists
Stephen Hawking’s final theory scales back multiverse
Cosmic inflation theory loses hangups about the scientific method
What becomes of science when the evidence does not matter?
and
The multiverse is science’s assisted suicide