Cosmic Inflation and the Multiverse Hypothesis

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A recent discovery in cosmology and the associated excitement in the scientific community has stirred up the debate related to the Multiverse Hypothesis.  Let's take a look at the new discovery, how it relates to the multiverse, and some of the shortcomings of the multiverse hypothesis.  But first, a little history.

From General Relativity to BICEP2

Of course any discussion of the history of modern cosmology has to include Einstein.  His theory of general relativity published in 1916 revolutionized the world of physics.  The theory explains that space and time are not completely distinct, but interwoven to form a spacetime.  Not only that, but spacetime is curved- it bends in the presence of large astronomical bodies.  Euclidean geometry is bunk; the shortest distance between two points in this universe isn't a straight line, strictly speaking.1  There are many other implications that arise out of the theory.  One is the existence of black holes; another is the existence of gravitational waves.  In relativity, gravity does not form an instantaneous interaction, as Newtonian physics predicts.  Rather, it spreads out like a wave.

Despite its many revolutionary aspects, Einstein's theory at first did not stray from one position firmly held by physicists of the day, namely, that the universe is static, eternal.  To make the universe static under general relativity, Einstein needed to include a cosmological constant.  Otherwise the series of equations would point towards an expanding universe, which would indicate it had a beginning in the finite past.

However, Edwin Hubble discovered that the light coming from distant stars and galaxies was red-shifted.  This Doppler shift meant that those stars and galaxies were moving away from earth.  It's clear that if everything is moving away from everything else, then in the past everything was closer together, and there must have been a certain point in the past when everything was infinitely close together.  The theory of the Big Bang was soon born.

Based on Big Bang cosmology, scientists predicted the existence of a cosmic background radiation.  Basically, during the very early stages of the universe, the tremendous heat and density of the universe made it impossible for electromagnetic waves to form.  Once the universe expanded and cooled sufficiently, the very first electromagnetic waves formed.  These primordial waves make up what is known as the cosmic microwave background (CMB).  The CMB is comprised of the earliest light in the universe, which formed about 380,000 years after the Big Bang (that may not sound very early, but next to 13.8 billion years it's a blink of an eye).

In 1964, two radio astronomers quite by mistake became the first in history to discover the cosmic microwave background.  They were later rewarded with Nobel prizes for their mistake.  This discovery strongly bolstered Big Bang cosmology.

However, as the CMB was more closely measured, something very puzzling became clear: it's incredibly smooth.  The temperature of the CMB looking at one direction of the universe is within two hundredths of a percent compared to any other direction.  Some unevenness was expected, given perturbations at the very beginning of the universe.  And while there is some unevenness, it is extremely slight.  The cause of this eluded physicists.

To solve this problem (and others), in 1980 Alan Guth (and later Endrei Linde) developed a theory known as cosmic inflation.  Extremely early in the universe's life, a period of rapid inflation occurred.  How early?  Inflation occurred between 10-36 seconds and 10-32 seconds after the Big Bang, during which the size of the universe increased by a factor of 1078.

The question is how on earth someone can test something like this.  Well, one of the predictions of cosmic inflation is that the cosmic microwave background will be polarized in a very specific way.  This polarization was caused by gravitational waves during inflation.  As mentioned earlier, gravitational waves are predicted by Einstein's theory of general relativity; if they exist at all, the waves propagating during inflation caused a polarization of the CMB.

Two experiments were devised to look for the polarization in the CMB: the BICEP2 telescope ("Background Imaging of Cosmic Extragalactic Polarization") in Antarctica (because that's about as close as you're going to get on earth to the conditions in space) and the Planck satellite.  On March 17 of this year, the results of the BICEP2 measurements were made public.  As it turns out, they found precisely the polarization that was predicted by cosmic inflation, to a very high degree of confidence.

Up to this point, much of the early universe was a complete mystery, and all we had were speculative theories.  Now we have all but confirmed that cosmic inflation is the right theory.  Further, it is the first indirect experimental evidence of gravitational waves.  However, the results won't officially be accepted by the scientific community until results from the Planck satellite's measurements are published.  Should the Planck satellite confirm the measurements of BICEP2, it will be the greatest discovery in cosmology since the discovery of the CMB itself.  Nobel prizes will almost certainly be awarded to those involved.

The Multiverse

What does this all have to do with the multiverse hypothesis?  Well, most inflationary models predict the existence of the multiverse through what is called eternal inflation.  The idea of the multiverse2 is fairly self-explanatory.  Our universe is but one of a plethora of universes, each forming its own "bubble".  New bubbles keep forming wherever inflation has stopped in another universe.  This is all caused by an eternal inflation.

Since inflation is all but confirmed and most inflationary models include eternal inflation, it's pretty obvious why the multiverse should become a hot-button issue once again.  The idea of a multiverse seems to be gaining in popularity in the scientific community.  That being said, the multiverse hypothesis has its detractors, both within science and without.

One of the purposes of positing a multiverse is to answer the age-old conundrum: why does the universe appear to be so finely tuned for life?   Of the many fundamental constants governing the universe, adjust one by a smidgen and life would not exist.  This apparent fine-tuning is a huge issue that has filled up volumes; it would take too long to summarize all the discussion.  For our present purposes, suffice it to say that the multiverse's answer has been criticized as being too convenient.  It doesn't really delve into the implications of fine-tuning, and offers a pat answer.

Another criticism is that the multiverse is not a testable hypothesis, and so is considered by some to be barely science or not science at all.  Even in our own universe, some of our probings into its fundamental workings must be indirect.  And any possible measurement of a multiverse would be even more indirect.  It's possible that any indirect evidence for a multiverse could simply be some part of our own universe that we haven't fully understood.  Because the other supposed universes are fundamentally unobservable, the hypothesis is essentially untestable.  Keep in mind that there are inflationary models that do not require a multiverse, and some argue that those, for the present, should be preferred for the sake of simplicity.

There are also philosophical issues that the multiverse hypothesis raises.  For example, even if we grant that the multiverse hypothesis in conjunction with the Anthropic Principle answers the fine-tuning problem, there is still the issue of figuring out why there is anything rather than nothing.  Even proponents of eternal inflation have shown that the inflation in fact started in the finite past.  So it does not answer the ultimate question.

Further, the multiverse hypothesis posits the existence of preposterous number of universes, all with different fundamental constants.  If that is the case, then it is actually far more probable that we would be in a universe that frankly doesn't make as much sense as this one- rather than an orderly universe, a universe in which objects pop into and out of existence seemingly at random.  It would be impossible to be able to come up with any sort of rationale to explain events in such a universe, even if that universe is governed by fundamental laws.  But our universe is orderly enough to the point that we have discovered a tremendous amount about it already, and will likely discover much more.  It seems likely, though, that the orderly and discoverable universes are greatly outnumbered by the ones that are strange and seemingly unpredictable.

The Christian's Response

How should a Christian view the multiverse hypothesis?  From my own layman's perspective, I really don't know if the multiverse even makes sense scientifically; I'm a bit skeptical, as it seems too convenient.  At the same time, I don't think Christians should be against it solely because it posits the existence of other universes.  C.S. Lewis was open to the idea of other life existing in this universe; he did not find that it upset his understanding of God or the Scriptures.  In a similar way, I don't see why God couldn't create a multitude of universes.

That being said, it is clear that the multiverse cannot be used to argue away the existence of God.  It is no substitute.  The cause of inflation and the existence of all the universes remains to be answered, even if eternal inflation is correct.  When it comes to answering the ultimate question (why does anything exist rather than nothing), it only kicks the can down the road.  And when it comes to Okham's razor, philosopher Keith Ward (among others) is of the opinion that the multiverse as an explanation for the existence of the universe is even worse than God.3

Ultimately, any mathematical, scientific model of the universe will fall short of explaining the universe's (or universes') existence.  As Stephen Hawking puts it,
Even if there is only one possible unified theory, it is just a set of rules and equations.  What is it that breathes fire into the equations and makes a universe for them to describe?  The usual approach of science of constructing a mathematical model cannot answer the questions of why there should be a universe for the model to describe.  Why does the universe go to all the bother of existing?4
That really is the question, isn't it?  From a Christian's perspective, that ultimate question is answered by the existence of God.  And it's not that he exists contingently and therefore his own existence requires an explanation.  If the great Christian thinkers of the ages are right, then God exists necessarily.  Nor is this some sort of God-of-the-gaps.  For one, it can be demonstrated that no set of mathematical equations can completely explain the universe, let alone how it got started (more on that in a future post, I hope); this isn't some problem that will be solved in the future, leaving God out of the picture.  For another, belief in the existence of God does not somehow limit the scientific endeavor.  Knowing that God exists does not keep one from trying to discover the mechanisms he put in place.  The book of natural revelation is laid open for us to read, and science is one of our best tools to discover its secrets.

1.  That being said, for almost all but the most extreme cases - those in which there are tremendous gravitational forces, such as near a black hole or a giant star or some other enormous celestial body - we can assume Euclidean geometry. 
2. The multiverse should not be confused with the many-worlds interpretation of quantum mechanics.  They are two very different ideas.  The multiverse deals with other "bubble" universes that are predicted by inflationary models.  In contrast, the many-worlds interpretation of quantum mechanics maintains that the probability wave function of a particle does not actually collapse, but causes the universe to split.  From the perspective of one of these universes, the wave function collapsed.
3.  I can't remember which lecture he mentioned this, but it was one of his Gresham College lectures, found here.
4.  Hawking, Stephen, A Brief History of Time. p. 174

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