The multiverse – a key ingredient to many a sci-fi film, however the topic has now become a fairly serious area of thought in theoretical physics. The universe we know and live in is characterised by key fundamental constants which are thought to be the same thorough-out. For example G, the gravitational constant, which quantifies the gravitational force of attraction felt between objects in the universe. Λ, the cosmological constant, behind the accelerating nature of the universe’s expansion. h, Plank constant, a constant central in quantum mechanics which defines the size of the quantum realm. c, the speed of light, 3×10^8 m/s, never more, never less. The list goes on… All these fine tuned constants together make our universe as we know it, they dictated its evolution and allowed life, consciousness and ultimately us to come about. If any of these constants had been changed just a little bit the evolutionary path of the universe would not have been the same and we would not have existed.
Now this suggests we are very very special indeed and it seems like a coincidence almost too hard to swallow that this universe developed with just the right conditions to create us, just by chance. This, my friends, is the problem of fine-tuning. Now there are ways to get around this, one of these being the idea of the multiverse. This being that there are a great many number of universes, all with different values of the constants and it is our section bias, our observance of only this universe that makes it seem so rare. This universe has the tuning that is compatible with life and hence allows us to be here, witness it and reflect upon it in the first place. This is, in essence, the anthropic principle. It’s all to easy to be consumed into a view that emphasises our rarity, but if there are in fact many of these universes and ours has the right conditions to allow life, it seems not so puzzling at all that we are here reflecting upon it. It’s a question whose answer is in the asking of the question itself.
This post will focus on the Multiverse as seen by the theory of Eternal Inflation. To possibly understand the theory of eternal inflation we must first understand the workings of normal inflationary theory. Now, inflationary theory came about to resolve multiple problems with the standard Big Bang model – however never-mind the workings of those now. Inflation represents a phase of extreme expansion of spacetime, the mechanisms behind its workings are less understood though it is thought to be linked to the phase transitions which occurred during the symmetry-breaking of the four fundamental forces early on the in the universe. From when they acted similarly, to when they broke away and adopted unique roles with different strengths.
Eternal Inflation Theory believes that this state of inflationary expansion continues forever and it is only in regions that certain quantum fluctuations occur. These can cause a limited region to drop out of the inflationary state, forming an bubble which continues to expand at a steady universe-like state. [Forgive me reader, I know ‘drop out’ is not a satisfactory explanation but this is a hard topic in theoretical physics and I pretend not to properly understand the mechanics of it myself. This post is just to present the concepts behind what is now a cosmological theory which holds substantial ground.] So what we have is this material in the inflationary state with regions of quantum fluctuations that cause bubbles to drop out into normal-state like universes; expanding at a normal universe-like rate. Multiple 3D (working with only spatial dimensionals here) universes arising within a fabric of unknown dimensionality.
These bubbles are thought to continually form out of the expanding inflationary state and one of these many bubbles is our bubble, our universe. So from our point of view, if we were able to travel far enough out we would encounter a transition, we would ‘leave’ our 3D bubble and enter an area of inflationary-state like matter. And if we could have the power to zoom out to an even further vantage point we would see multiple bubbles like our own separated by distances increasing at an inflationary rate!
Now who is to say the workings of life, the laws of physics, the nature of matter is the same within each of these bubbles? Yes they arise from the same inflationary-state but the initial conditions of each universe may well be influenced by the locality surrounding the region and the nature of the quantum fluctuation. The physics is unclear whether the value of fundamental constants, masses and charges of particles is a product of initial conditions, i.e. locally dependent or arise from the physical laws dictating the arena in the inflationary-state. If it is this way, then yes we can expect that these different bubble universes have different physical constants, different evolution histories and as such the vast majority may not have developed in a way to allow life. So, the values for the fundamental constants may be a totally random product of the locality of the bubble. If there are millions of these bubbles, the statistical argument for there being one that has the constants that allow the state of evolution such that grows to welcomes life, makes the phenomena seem not so rare. And of course we are only in this bubble because it allowed life in the first place, it’s not like we had a choice of which bubble to grow up in and we magically chose the 1 in a million (figure made up) bubble that allowed life. We are here because we can be.
Now the eternal inflation multiverse theory gets a lot of stick because it is most likely untestable. A key pillar of the scientific method is that theories should be empirically testable/falsifiable in order to hold weight and as such critics say the multiverse is non-scientific. However to me this resounds loudly with the sound of human-arrogance. Why should a theory be ruled out because we as human beings – tiny 3D creatures who have only been around for a blip in the lifetime of the universe – be equipped with the tools to experimentally test a theory, for it to be considered credible?! It seems ludicrous to me. The physics is strong, the mathematics is strong, the predictive power of the theory is strong, it is based off solid reasoning – it may well hold weight. Moreover perhaps one day we will be able to probe far (and it would be very far) enough such that we can detect the inflationary-state past the edge of our bubble, though this remains as with all too many things, uncertain.
All there is left to say is the Multiverse Theory, based on eternal inflation, leaves us feeling all that much tinier than before. We may no longer be just one tiny planet in a galaxy with thousands of planets, in a universe with billions of other galaxies – we could now be merely a bubble of a universe in a sea of many other bubbles. I hope this leaves you with the feeling that mankind is not special as it believes itself to be – we just happened to exist in a smidgen that allowed our particular form of consciousness to exist and with the lack of competition reaching our tiny corner we proclaim ourselves remarkable.