One or More?

Two years ago, I opened myself an account on Quora. It was one of the best decisions of my life. As an inquirer by nature – my name Anwesha means inquisitiveness too – this was paradise for me. I not only got a platform to ask my numerous questions, but also got a chance to share knowledge by answering questions. By now, I have answered several questions on astronomy with my basic understanding however there was one question that specifically caught my attention. And along with it, vanished my writer's block looking for a topic for my next article!

Can we prove the multiverse theory?

The multiverse theory was born out of the anthropic principle. Our universe is perfectly tuned to support life. If the universe had 10^-10% more dark energy than the value predicted to be in the universe, it would have expanded way too fast to allow nuclei to coalesce and form the magnificent structures that led to today. If there was a little less dark matter, matter would never have coalesced the way we know it and the universe would have collapsed in a Big Crunch even before the conditions were just right for life to exist. In a sense, we live in a Goldilocks universe.

If our universe has the perfect amount of dark energy, probabilistically, there is a chance of having another universe with a different quantity of dark energy. Ours just randomly happens to be one with the perfect quantities of ingredients. This is the foundation for the multiverse. If there is a world with the amount of dark energy that we have in our universe, there must be other worlds out there with different amounts of dark energy. It’s simple probability! And that is precisely why I don’t get along very well with the multiverse theory.

In July 2016, I visited the Perimeter Institute for Theoretical Physics in Waterloo, Canada as I attended a summer program run by the Institute. Perimeter Associate Faculty member Matthew Johnson was one of our keynote speakers. Johnson is a researcher primarily focusing on the multiverse theory. With my mind set on the multiverse theory being false, I mustered up the courage to ask him about potential proofs for the multiverse theory – can we prove it? To this, he answered there is no mechanism by which we can prove this today. As a 15 year-old passionately against the Multiverse theory, I felt like I had hit my jackpot – even a multiverse theorist was unable to give me a convincing answer, so this theory had to be false.

Fast forward to July 2017 and I came across an article on Perimeter’s website about the multiverse theory and guess who featured in it – Matthew Johnson. Though delightedly heartbroken, this was the answer I sought a year ago: a testable hypothesis. Johnson, along with Luis Lehner, another faculty member at PI, has been working on creating simulations whereby two bubble universes collide with one another. Such a collision should leave “a circular bruise in the cosmic microwave background” that will be detectable. Although the search for this bruise has yielded null, it is one giant leap for all the multiverse supporters out there. Johnson’s team has produced quantitative predictions for the observable signs of this cosmic collision and this is a first in cosmology (https://www.perimeterinstitute.ca/node/94107). This endeavour may as well have established that the Multiverse theory is indeed testable. But is it?

E=mc^2

Whether or not you remember the equation for kinetic energy from high school, you certainly haven’t forgotten this famed equation. Einstein’s theory of special relativity established the speed of light as the universal speed limit – nothing in space can go faster than the speed of light. Even gravity, which many think to be an instantaneous phenomenon, is transmitted at the speed of light. If the sun were to suddenly disappear, we wouldn’t feel the effects of its absence for 8 minutes since the calamity occurred.

So, logically, the best way to study the presence of a multiverse would be to send electromagnetic signals out of our universe or study the incoming electromagnetic signals from the external universes. This raises the question, why can’t we do this? The universal speed limit.

The universe is in a stage of accelerated expansion. In fact, the universe is expanding at a rate faster than the speed of light and this speed is increasing at the rate of 67km per second. That being said, we can’t send signals out of our universe neither can we intercept signals from external universes – if they have more dark energy than our universe, they would be expanding even faster and if they have less dark energy, they’d probably be in the contraction stage.

We just concluded that nothing in space can travel faster than the speed of light. Then how is it that the universe is expanding faster than the speed of light? This technically sounds bizarre as it violates Einstein’s theories! Note that nothing in space can travel faster than light. But space itself can travel faster than light! (What a loophole in a cosmological contracts! )Einstein’s theory is not violated in this process. While preserving this universal quantity, our access to the outside world is cut.

Crisis. Hypothesis. Theory. Prediction. This cycle is the centrepiece of science. Without a proof, science is stuck as theory. While a million observations may support a theory, a single odd observation is enough to dismiss it. Is the multiverse theory stuck in this third stage? As an avid opponent, I want to say I hope so. Do I have a darling theory to back me up? Absolutely! But perhaps that is another tale for another day.