If We Live in a Multiverse, How Many Are There?

by Nancy Atkinson on October 15, 2009

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Artist concept of the cyclic universe.

Artist concept of the cyclic universe.


Theoretical physics has brought us the notion that our single universe is not necessarily the only game in town. Satellite data from WMAP, along with string theory and its 11- dimensional hyperspace idea has produced the concept of the multiverse, where the Big Bang could have produced many different universes instead of a single uniform universe. The idea has gained popularity recently, so it was only a matter of time until someone asked the question of how many multiverses could possibly exist. The number, according to two physicists, could be “humongous.”

Andrei Linde and Vitaly Vanchurin at Stanford University in California, did a few back-of- the- envelope calculations, starting with the idea that the Big Bang was essentially a quantum process which generated quantum fluctuations in the state of the early universe. The universe then underwent a period of rapid growth called inflation during which these perturbations were “frozen,” creating different initial classical conditions in different parts of the cosmos. Since each of these regions would have a different set of laws of low energy physics, they can be thought of as different universes.

Linde and Vanchurin then estimated how many different universes could have appeared as a result of this effect. Their answer is that this number must be proportional to the effect that caused the perturbations in the first place, a process called slow roll inflation, — the solution Linde came up with previously to answer the problem of the bubbles of universes colliding in the early inflation period. In this model, inflation occurred from a scalar field rolling down a potential energy hill. When the field rolls very slowly compared to the expansion of the universe, inflation occurs and collisions end up being rare.

Using all of this (and more – see their paper here) Linde and Vanchurin calculate that the number of universes in the multiverse and could be at least 10^10^10^7, a number which is definitely “humungous,” as they described it.

The next question, then, is how many universes could we actually see? Linde and Vanchurin say they had to invoke the Bekenstein limit, where the properties of the observer become an important factor because of a limit to the amount of information that can be contained within any given volume of space, and by the limits of the human brain.

The total amount of information that can be absorbed by one individual during a lifetime is about 10^16 bits. So a typical human brain can have 10^10^16 configurations and so could never distinguish more than that number of different universes.

The number of multiverses the human brain could distinguish. Credit: Linde and Vanchurin

The number of multiverses the human brain could distinguish. Credit: Linde and Vanchurin

“So, the total number of possibilities accessible to any given observer is limited not only by the entropy of perturbations of metric produced by inflation and by the size of the cosmological horizon, but also by the number of degrees of freedom of an observer,” the physicists write.

“We have found that the strongest limit on the number of different locally distinguishable geometries is determined mostly by our abilities to distinguish between different universes and to remember our results,” wrote Linde and Vanchurin. “Potentially it may become very important that when we analyze the probability of existencse of a universe of a given type, we should be talking about a consistent pair: the universe and an observer who makes the rest of the universe “alive” and the wave function of the rest of the universe time-dependant.”

So their conclusion is that the limit does not depend on the properties of the multiverse itself, but on the properties of the observer.

They hope to further study this concept to see if this probability if proportional to the observable entropy of inflation.

Sources: ArXiv, Technology Review Blog

About 

Nancy Atkinson is Universe Today's Senior Editor. She also is the host of the NASA Lunar Science Institute podcast and works with Astronomy Cast. Nancy is also a NASA/JPL Solar System Ambassador.

Lawrence B. Crowell October 17, 2009 at 4:54 AM

We are limited, but not blind and incapable. Astronomy illustrates aspects of the large scale structure of the universe, such as the Hubble relationship v = Hd, the big bang and so forth. The mechanism for the big bang, or at least one phase of it, is inflation. Predictions from inflation include small anisotropes in the microwave cosmic background in a certain distribution, which have been found. There are also predictions of B-modes from gravitational decoupling. The Planck probe is currently suveying the cosmic background for signatures of these.

So the quest continues, and the next big issue is to understand how the cosmological constant, which drives the accelerated expansion of the universe, has the comparatively small value it has.

LC

DrFlimmer October 17, 2009 at 6:47 AM

It is always good for everyone to know his/her limits. I think I have an idea about my limits.

Still: Not imaginable is NOT not knowing. I can’t imagine the four dimensions of space-time, but I know that they exist. And there are many more examples….

Manu October 17, 2009 at 8:19 AM

@ Anacond:

“There are some things Man will never know”.

I couldn’t agree more. But please, lets not decide right away which ones these things are going to be.
We’ll write down the score the day the last human dies.

HeadAroundU October 17, 2009 at 8:28 AM

I’d like a bigger picture so I can read/see everything what’s in it. :(

Also, what’s white hole???

Jon Hanford October 17, 2009 at 10:31 AM

@HeadAroundU

Some info on white holes can be found here: http://en.wikipedia.org/wiki/White_holes .

RUF October 17, 2009 at 5:13 PM

“There are some things Man will never know”.

Will women know them?

BeckyWS October 19, 2009 at 10:39 AM

I’d just like to say that this could be the thread with the most impenetrable posts ever! But I like that, it tells me there are people out there specialising in amazing fields, with detailed knowledge and exploratory minds.
“Gobbledygook” it is not.

Jon Hanford October 19, 2009 at 3:44 PM

BeckyWS:

I most heartedly concur with your post. I find it fascinating that theorists are vigorously pursing these questions, wherever it leads. Definitely, “gobbledygook” it is not.

Paul Eaton-Jones October 23, 2009 at 1:46 AM

I had always thought that the multiverse referred to many bubble universes within The Universe whereas the Multi World Interpretation resulted in parallel universes. This would where you existed having made a decision ‘now’ to do this rather than that, the universe split, and one version of you went on to do ‘this’ and another version did ‘that’. Gregory Benford wrote a novel based on this idea – ‘Timeslip’- I think.

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