Gulp! “Our Observed Universe is a Tiny Corner of an Enormous Cosmos”


“This new concept is, potentially, as drastic an enlargement of our cosmic perspective as the shift from pre-Copernican ideas to the realization that the Earth is orbiting a typical star on the edge of the Milky Way.” Sir Martin Rees, physicist, Cambridge University, Astronomer Royal of Great Britain.

Is our universe merely a part of an enormous universe containing diverse regions each with the right amount of the “dark energy” and each larger than the observed universe, according to Raphael Bousso, Professor of Theoretical Physics, U of California/Berkeley and Leonard Susskind, Felix Bloch Professor of Physics, Stanford University. The two theorize that information can leak from our causal patch into others, allowing our part of the universe to “decohere” into one state or another, resulting in the universe that we observe.

The many worlds interpretation of quantum mechanics is the idea that all possible alternate histories of the universe actually exist. At every point in time, the universe splits into a multitude of existences in which every possible outcome of each quantum process actually happens.The reason many physicists love the many worlds idea is that it explains away all the strange paradoxes of quantum mechanics.

Putting the many world interpretation aside for a moment, another strange idea in modern physics is the idea that our universe was born along with a large, possibly infinite, number of other universes. So our cosmos is just one tiny corner of a much larger multiverse.

Susskind and Bousso have put forward the idea that the multiverse and the many worlds interpretation of quantum mechanics are formally equivalent, but if both quantum mechanics and the multiverse take special forms.

Let’s take quantum mechanics first. Susskind and Bousso propose that it is possible to verify the predictions of quantum mechanics. In theory, it could be done if an observer could perform an infinite number of experiments and observe the outcome of them all, which is known as the supersymmetric multiverse with vanishing cosmological constant.

If the universe takes this form, then it is possible to carry out an infinite number of experiments within the causal horizon of each other. At each instant in time, an infinite (or very large) number of experiments take place within the causal horizon of each other. As observers, we are capable of seeing the outcome of any of these experiments but we actually follow only one.

Bousso and Susskind argue that since the many worlds interpretation is possible only in their supersymmetric multiverse, they must be equivalent. “We argue that the global multiverse is a representation of the many-worlds in a single geometry,” they say, calling this new idea the multiverse interpretation of quantum mechanics.

But we have now entered the realm of what mathematical physicist Peter Woit of Columbia calls “Not Even Wrong, because the theory lacks is a testable prediction that would help physicists distinguish it experimentally from other theories of the universe. And without this crucial element, the multiverse interpretation of quantum mechanics is little more than philosophy, according to Woit.

What this new supersymmetric multiverse interpretation does have is a simplicity– it’s neat and elegant that the many worlds and the multiverse are equivalent. Ockham’s Razor is fulfilled and no doubt, many quantum physicists delight in what appears to be an exciting. plausible interpretation of ultimate if currently untestable, reality.

Strictly speaking, our observable Universe coincides with something called the particle horizon. The particle horizon marks the distance to the farthest light that we can possibly see at this moment in time – photons that have had enough time to either remain within, or catch up to, our gently expanding Hubble sphere.

And just what is this distance? A little more than 46 billion light years in every direction – giving our observable Universe a diameter of approximately 93 billion light years, or more than 500 billion trillion miles.

(A quick note: the particle horizon is not the same thing as the cosmological event horizon. The particle horizon encompasses all the events in the past that we can currently see. The cosmological event horizon, on the other hand, defines a distance within which a future observer will be able to see the then-ancient light our little corner of spacetime is emitting today.

In other words, the particle horizon deals with the distance to past objects whose ancient light that we can see today; the cosmological event horizon deals with the distance that our present-day light that will be able to travel as faraway regions of the Universe accelerate away from us.)

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