Does the Universe Need God?


- by Sean Carroll

In the beginning, God created the heavens and the earth. 

In many religious traditions, one of the standard roles of the deity has been to create the universe.  The first line of the Bible, Genesis 1:1, is a plain statement of this role. Much has happened, both in our scientific understanding  of the universe and in the development of  theology, since that line was first written.  It’s worth examining what those developments imply for the relationship between God and cosmology.

In some ways of thinking about God, there’s no relationship  at all; a conception of divinity that is sufficiently ineffable and transcendent may be completely separate from the workings  of  the  physical  world.  For  the  purposes  of  this  essay,  however,  we  will  limit ourselves to versions of God that play some role in explaining the world we see.  In addition to the role of creator, God may also be invoked as that which sustains the world and allows it  to  exist,  or  more  practically  as  an  explanation  for  some  of  the  specific  contingent properties of the universe we observe.

Each of these possibilities necessarily leads to an engagement with science. Modern cosmology  attempts to come up with the most powerful  and economical  possible understanding of the universe that is consistent with observational data. It’s certainly conceivable  that  the  methods  of science  could  lead  us  to a self-contained  picture  of  the universe that doesn’t involve God in any way.  If so, would we be correct to conclude that cosmology  has undermined  the reasons for believing  in God, or at least a certain kind of reason?

This is not an open-and-shut question.  We are not faced with a matter of judging the merits  of a mature and compelling scientific theory, since we don’t yet have such a theory. Rather, we are trying to predict the future: will there ever be a time when a conventional scientific model provides a complete understanding of the origin of the universe?  Or, alternatively,  do we already know enough to conclude that God definitely helps us explain the universe we see, in ways that a non-­‐theistic approach can never hope to match?

Most  modern  cosmologists  are  convinced  that  conventional  scientific progress  will ultimately  result  in  a  self-­‐contained  understanding  of  the  origin  and  evolution  of the universe,  without  the need  to invoke  God  or any  other  supernatural  involvement.(1)   This conviction  necessarily  falls  short  of  a  proof,  but  it  is  backed  up  by  good reasons.    While  we don’t  have  the  final  answers,  I  will  attempt  to  explain  the  rationale  behind  the belief  that science will ultimately understand the universe without involving God in any way.

The Universe We Know


A century ago, we knew essentially none of what are now considered the basic facts of  cosmology.   This situation changed rapidly, first on the theoretical  front in the 1910’s, then on the observational front in the 1920’s.

Cosmology studies the universe on the largest scales, and over large scales the most important force of nature is gravity.   Our modern understanding of gravity is the theory of general relativity, proposed by Einstein in 1915.   The key insight in this theory is the idea that  space  and  time  can  be  curved  and  have  a dynamical  life  of  their  own,  changing  in response  to  matter  and  energy. As  early  as  1917,  Einstein  applied  his  new  theory  to cosmology, taking as an assumption something we still believe is true:   that on the largest scales,  matter  in  the  universe  (or  at  least  our  observable  part  of  it)  is  uniform through space.   He also assumed,  consistent  with the apparent  implication  of observations  at the time, that the universe  was static.   To his surprise,  Einstein  found  that general  relativity implied that any uniform universe would necessarily be non-­‐static – either expanding or contracting.    In response  he suggested  modifying  his theory  by  adding  a new parameter called the “cosmological constant,” which acted to push against the tendency of matter to contract together.   With that modification, Einstein was able to find a static (but unstable) solution if the cosmological constant were chosen precisely to balance against the attraction of matter on large scales.

This  discussion   became   somewhat   academic   when   Edwin   Hubble   and   Milton Humason announced in 1929 that the universe is expanding:   distant galaxies are receding from us at speeds  that are proportional  to their distance.   It had only been in 1924  that Hubble had established that the spiral nebulae, which many thought were clouds within our own galaxy, were separate  galaxies in their own right, demonstrating  the true vastness of the universe.  The collection of stars we live in, the Milky Way galaxy, contains something over 100 billion stars, and there are over 100 billion such galaxies within the observable universe.

If the universe is expanding now, it was smaller in the past.  (More properly, galaxies were closer together and the universe was more dense; it’s possible that space is actually infinite in extent.)  Using the rules provided  by general relativity,  and some assumptions about the types of matter and energy that pervade the universe, we can play the movie backwards in time to reconstruct the past history of our universe.   Eventually – about 13.7 billion years ago, according to our best current estimates – we reach a moment of infinite density and spacetime curvature.  This singularity is known as the “Big Bang.”  Confusingly, the  phrase  “Big  Bang  model”  refers  to the entire  history  of the expanding  universe  that began in a hot, dense state, the broad outlines of which are established beyond reasonable doubt.   In contrast, the “Big Bang event” is not really an event at all, but a placeholder for our lack of complete understanding.

While we don’t claim to understand the absolute beginning of the universe, by the time one second has elapsed we enter the realm of empirical testability.  That’s the era of primordial nucleosynthesis,  when protons and neutrons were being converted into helium and other light elements.  The theory of nucleosynthesis makes precise predictions for the relative abundance of these elements, which have passed observational muster with flying colors, providing impressive  evidence in favor of the Big Bang model.

Another important test comes from the cosmic microwave background (CMB), the relic radiation left over from the moment the primordial plasma cooled off and became transparent, about 380,000 years after the Big Bang.  Together, observations of primordial element abundances and the CMB provide   not   only   evidence  in  favor of  the  basic cosmological   picture,  but stringent constraints on the parameters describing the composition of our universe.

One implication of these data is that only about 4% of the total energy of the current universe  is in  the  form  of  “ordinary  matter”  –  the  atoms  and  molecules  consisting  of protons, neutrons, and electrons, as well as photons and neutrinos and all the other known elementary  particles.   Another  23%  of the universe  is “dark  matter”  – a completely  new kind of particle, as yet undiscovered here on Earth.   In addition to constraints from nucleosynthesis and the CMB, strong evidence for dark matter comes from the dynamics of galaxies, clusters of galaxies, and large-­‐scale  structure in the universe (see Komatsu et al. 2001).

This leaves us with 73% of the universe in an even more mysterious form – “dark energy.”  Once the expansion of the universe was discovered, Einstein’s original motivation for introducing the cosmological constant evaporated.   But the idea didn’t go away, and physicists later realized that this parameter had a very natural interpretation – the energy density of empty space, or “vacuum energy” for short.  In 1998 two groups of astronomers made a surprising discovery:   the universe is not only expanding, but accelerating – distant galaxies are moving away from us faster and faster over time (Riess et al. 1998, Perlmutter et al. 1999).  This is contrary to our expectation that the gravitational pull between galaxies should slow   the   expansion   down.  The   most   straightforward   explanation   for   this acceleration is to posit dark energy – a smooth, persistent form of energy that isn’t localized into particles, but is spread throughout space.  Vacuum energy, or Einstein’s cosmological constant,  is the  simplest  candidate  for  dark  energy;  it  features  a  density  that  is  strictly constant,  unchanging  through  space or time.   But more complicated  models  are possible, and cosmologists  are currently  working hard to test the hypothesis  that the dark energy density is truly a constant.  If it is, we can predict the future of the universe – it will expand forever, gradually cooling and diluting away until nothing is left but empty space.

While the Big Bang model – the picture of a universe expanding from a hot, dense state over the course of billions of years – is firmly established, the Big Bang itself – the hypothetical  singular   moment   of   infinite   density   at   the   very   beginning   –   remains mysterious. Cosmologists sometimes talk about the Big Bang, especially in popular-­‐level presentations,  in ways that convey more certainty  than is really warranted,  so it is worth our time to separate what we know from what we may guess.

The success of primordial nucleosynthesis  gives us confidence that we understand what the universe was doing about one second after the Big Bang, but anything before that is necessarily  speculative.    Even  the formulation  “one  second  after  the Big Bang”  should really be interpreted as “one second after what would be the moment of infinite curvature in the most straightforward extrapolation to earlier times.”  But there are different degrees of speculation.

From one second back to about 10-­‐43 seconds, we expect the kinds of physics we understand  –  general  relativity  and  quantum  field  theory  – to be  applicable,  even  if the details are unclear.  That is, we think we can successfully model the world in terms of fields that obey the rules of quantum mechanics, evolving within a curved spacetime obeying the laws of general  relativity.   The value 10-­‐43   seconds  is the “Planck  time,” before  which  we expect  spacetime  itself  to  be  subject  to  quantum  behavior.    Currently  we don’t  have  a reliable theory that describes gravity in quantum-­‐mechanical terms; the search for a theory of “quantum gravity” is one of the foremost goals of modern physics.  The leading candidate for such a synthesis, string theory, has been the subject of an enormous amount of attention in recent decades. Unfortunately,  despite  a number  of intriguing  theoretical  discoveries, string theory  has neither  made direct contact  with experiments,  nor  provided  an unambiguous answer to what happened at the Big Bang.

One sometimes  hears the claim that the Big Bang was the beginning  of both time and space; that to ask about spacetime “before the Big Bang” is like asking about land “north of the North Pole.”  This may turn out to be true, but it is not an established understanding. The singularity at the Big Bang doesn’t indicate a beginning to the universe, only an end to our theoretical comprehension.   It may be that this moment does indeed correspond to a beginning,  and  a  complete  theory  of  quantum  gravity  will  eventually  explain  how  the universe started at approximately this time.  But it is equally plausible that what we think of as the Big Bang is merely a phase in the history of the universe, which stretches long before that  time  –  perhaps infinitely  far  in  the  past.    The  present  state  of  the  art  is  simply insufficient  to decide between these alternatives;  to do so, we will need to formulate  and test a working theory of quantum gravity.

Theories of Creation



The inability of established physics to describe the Big Bang event makes it tempting to consider the possibility that God has a crucial role to play at this unique moment in the history of the universe. If we were able to construct a complete and compelling naturalistic account,  the  necessity  of  appealing  to  God  would  be  diminished.    A  number  of  avenues toward this goal are being explored. They can be divided into two types: “beginning” cosmologies,  in which  there  is a first  moment  of  time,  and  “eternal” cosmologies,  where time stretches to the past without limit.

There are a number of avenues currently being explored by physicists that hope to provide  a  complete  and  self-­‐contained  account  of  the  universe,  including  the  Big  Bang. Roughly  speaking  they can be divided  into  two  types:  “beginning”  cosmologies,  in which there is a first moment of time, and “eternal” cosmologies, where time stretches to the past without limit.

“Beginning” cosmologies typically attempt to replace the Big Bang singularity of classical general relativity with some sort of quantum-­‐mechanical event, and often go by the name  “quantum  cosmology”  (Hartle  and  Hawking  1983,  Vilenkin  1984).    These  models imagine that spacetime is a classical approximation to some sort of quantum-­‐mechanical structure.  (Even if we don’t have a complete theory of quantum gravity, the hope is that the basic features of quantum mechanics and general relativity are sufficiently robust that the details aren’t important for this particular question.)   In particular, time may be just an approximate notion, useful in some regimes but not others.  Near the Big Bang is an obvious candidate   for  an  era  in  which   time  loses  its  conventional   meaning.  The  important ingredient  is then  a “boundary condition”  that  describes  the  state  of the universe  at the moment when time is first an intelligible concept.   The most famous example is the “no-­‐ boundary proposal” of Hartle and Hawking, which constructs the state of the universe by integrating  over  all possible  Euclidean  geometries  with  no other  boundaries. By “Euclidean” we mean geometries in which all four dimensions are spatial, in contrast to the “Lorentzian” geometry of spacetime with its distinction between timelike and spacelike directions.   One occasionally speaks of “imaginary time,” a phrase that has probably not increased the total amount of understanding in the universe.

A provocative way of characterizing these beginning cosmologies is to say that “the universe was created from nothing.”  Much debate has gone into deciding what this claim is supposed to mean.  Unfortunately, it is a fairly misleading natural-­‐language translation of a concept that is not completely well-­‐defined even at the technical level. Terms that are imprecisely  defined  include  “universe,”  “created,”  “from,”  and  “nothing.”    (We  can  argue about “was.”) The  problem  with  “creation  from  nothing”  is that  it  conjures  an  image  of  a  pre-­existing “nothingness” out of which the universe spontaneously appeared – not at all what is actually  involved  in  this  idea.  Partly  this  is  because,  as  human  beings  embedded  in  a universe with an arrow of time, we can’t help but try to explain events in terms of earlier events, even when the event we are trying to explain is explicitly stated to be the earliest one.   It would be more accurate to characterize  these models by saying “there was a time such that there was no earlier time.”

To make sense of this, it is helpful to think of the present state of the universe and work backwards,  rather  than  succumbing  to  the  temptation  to  place  our  imaginations “before” the universe came into being.  The beginning cosmologies posit that our mental journey backwards in time will ultimately reach a point past which the concept of “time” is no longer applicable. Alternatively, imagine a universe that collapsed into a Big Crunch, so that  there  would  be  a future  end  point  to  time.     We aren’t  tempted  to  say  that  such  a universe  “transformed  into nothing”;  it simply  has a final moment  of its existence.  What actually  happens  at  such  a  boundary  point  depends,  of  course,  on  the  correct  quantum theory of gravity.

The important point is that we can easily imagine self-­‐contained descriptions of the universe that have an earliest moment of time.  There is no logical or metaphysical obstacle to completing the conventional temporal history of the universe by including an atemporal boundary  condition  at the beginning. Together  with  the successful  post-Big-Bang cosmological  model already in our possession, that would constitute a consistent and self-­‐ contained description of the history of the universe.

Nothing in the fact that there is a first moment of time, in other words, necessitates that  an external  something  is required  to bring  the universe  about  at that  moment. As Hawking (1988, 156) put it in a celebrated passage:
So long as the universe had a beginning, we could suppose it had a creator. But if the universe is really self-­‐contained, having no boundary or edge, it would have neither beginning nor end, it would simply be. What place, then, for a creator?
The issue of whether or not there actually is a beginning to time remains open.  Even though  classical  general  relativity  predicts  a  singularity  at  the  Big  Bang,  it’s  completely possible that a fully operational theory of quantum gravity will replace the singularity by a transitional stage in an eternal universe.  A variety of approaches along these lines are being pursued by physicists:   bouncing cosmologies in which a single Big Crunch evolves directly into our observed Big Bang (Gasperini and Veneziano 1993; Bojowald 2001; Khoury et al. 2001), cyclic cosmologies in which there are an infinite number of epochs separated by Big Bangs (Steinhardt  and Turok 2002; Penrose 2001), and baby-universe  scenarios in which our Big  Bang arises  spontaneously  out  of  quantum  fluctuations  in  an  otherwise  quiet spacetime (Farhi et al. 1990; Fischler et al. 1990; Carroll and Chen 2004).  There is no way to decide between beginning and eternal cosmologies on the basis of pure thought; both possibilities are being actively pursued by working cosmologists, and a definitive judgment will have to wait until one or the other approach  develops into a mature scientific theory that makes contact with observations.

Interestingly, many (although certainly not all) natural theologians have managed to resist  the temptation  to point  to the Big Bang  as  evidence  of God’s  existence.  Since  the Fourth Lateran Council declared that the universe had a beginning in time and was created by God ex nihilo, the Big Bang would seem to fit relatively naturally into Christian theology. One figure who gave into temptation was Pope Pius XII, who in 1951 argued:
In  fact,  it  would  seem  that  present-­‐day  science,  with  one  sweeping  step  back across  millions    of    centuries,    has    succeeded    in    bearing    witness    to    that
primordial Fiat Lux uttered  at the moment  when,  along with  matter,  there burst forth from nothing a sea of light and radiation, while the particles of chemical elements split and formed into million of galaxies… Therefore, there is a Creator. Therefore, God exists!
(quoted in Singh 2005, 360)
However,  one figure who famously  did not take that route was Georges Lemaître, the Belgian priest and physicist who in the 1920’s developed the original Big Bang model (which he called the “primeval atom”). Lemaître resolutely declined to draw any theological conclusions from his theory, preferring  to keep his religious beliefs strictly separate from his  scientific  work  (Lemaître  1958,  1). He  later  served  as  a  member  of  the  Pontifical Academy of Sciences, and advised Pius against using scientific discoveries as evidence in theological arguments.

Why This Universe?

In recent years, a different aspect of our universe has been seized upon by natural theologians as evidence for God’s handiwork – the purported fine-­‐tuning of the physical and cosmological  parameters  that  specify our  particular  universe  among  all  possible  ones. These parameters are to be found in the laws of physics (the mass of the electron, the value of the vacuum energy) as well as in the history of the universe (the amount of dark matter, the smoothness of the initial state).  There’s no question that the universe around us would look very different  if some  of   these   parameters were changed   (Rees   1999). The controversial claims are two: that intelligent life can only exist for a very small range of parameters, in which our universe just happens to find itself; and that the best explanation for this happy circumstance is that God arranged it that way.

The clearest example of apparent fine-­‐tuning  is the vacuum energy (Carroll 2001). As discussed above, vacuum  energy  is the leading  candidate  for the dark energy  causing distant galaxies to accelerate;  but even if the vacuum energy is exactly  zero and the dark energy  is something  else,  we  can  safely  say  that  the  value  of  the  vacuum  energy  is not greater than that of the dark energy, about 10-­‐8  ergs per cubic centimeter.  Using techniques from  quantum  field  theory,  we can do a rough  calculation  of what  we  would  expect  the vacuum  energy to be, if we hadn’t already  measured  it.   The answer is quite a bit larger: about 10112 ergs per cubic centimeter.  The fact that the actual value of the vacuum energy is at least 120 orders of magnitude smaller than its natural value is a fine-­‐tuning by anyone’s estimation.

Cosmologists don’t have a compelling model for why the vacuum energy is so much smaller than it should be.  But if it were anywhere near its “natural” value, we would not be here talking about it.  Vacuum energy pulls objects away from each other, and a value much larger than what is observed would prohibit galaxies and stars from forming, presumably making it harder for life to exist.

Other  constants  of nature,  such as those  that  govern  atomic  and  nuclear  physics, seem natural by themselves, but would give rise to very different macroscopic phenomena if they were changed even slightly.  For example, if the mass of the neutron were a bit larger (in comparison  to the mass of the proton) than its actual value, hydrogen would not fuse into deuterium and conventional stars would be impossible; if the neutron mass were a bit smaller, all the hydrogen in the early universe would fuse into helium, and helium stars in the late universe would have much shorter lifetimes (Hogan 2000; Collins 2003).   (On the other  hand,  Adams  has  argued  that  a  wide  range  of  physical  parameters  leads  to  stars sustained by nuclear fusion (Adams 2008).)
  1. In the face of these apparent fine-­‐tunings, we have several possible options:Life is extremely  robust,  and  would  be likely  to arise even if the parameters were very different, whether or not we understand what form it would take.
  2. There is only one universe, with randomly-­‐chosen parameters, and we just got lucky that they are among the rare values that allow for the existence of life.
  3. In different regions of the universe the parameters take on different values, and we  are fooled  by  a  selection   effect:   life  will  only   arise  in  those  regions compatible with the existence of life.
  4. The  parameters  are not chosen  randomly,  but designed  that  way  by a deity.
Generally, not nearly enough credence is given to option #1 in this list.   We know very little about the conditions under which complexity, and intelligent life in particular, can possibly form. If, for example, we were handed the Standard Model of particle physics but had no actual knowledge of the real world, it would be very difficult to derive the periodic table  of  the  elements,  much  less  the  atoms  and  molecules  on which  Earth-­‐based   life depends.   Life may be very fragile, but for all we know it may be ubiquitous (in parameter space); we have a great deal of trouble even defining “life” or for that matter “complexity,” not to mention “intelligence.”  At the least, the tentative nature of our current understanding of these issues  should  make us reluctant  to draw  grand  conclusions  about  the nature  of reality from the fact that our universe allows for the existence of life.

Nevertheless,  for  the sake  of  playing  along,  let’s imagine  that  intelligent  life  only arises under a very restrictive set of circumstances.   Following Swinburne (1990), we can cast the remaining choices in terms of Bayesian probability.   The basic idea is simple:   we assign some prior probability – before we take into account what we actually know about the  universe  –  to  each  of  the  three  remaining  scenarios.    Then  we  multiply  that  prior probability by the probability that intelligent life would arise in that particular model.  The result is proportional to the probability that the model is correct, given that intelligent life exists.(2)  Thus, for option #2 (a single universe, no supernatural intervention), we might put the prior probability at a elatively high value by virtue of its simplicity, but the probability of life arising (we are imagining) is extremely small, so much so that this model could be considered unlikely in comparison with the other two.

We  are  left  with  option  #3,  a  “multiverse”  with  different  conditions  in  different regions  (traditionally  called  “universes”  even  if  they  are  spatially  connected),  and  #4,  a single universe with parameters chosen by God to allow for the eventual appearance of life. In either case we can make a plausible argument that the probability of life arising is considerable.  All of the heavy lifting, therefore, comes down to our prior probabilities – our judgments  about  how a priori   likely   such  a  cosmological   scenario   is. Sadly,   prior probabilities are notoriously contentious objects.

I will consider  more  carefully  the status  of the “God  hypothesis,”  and its corresponding  prior  probability,  in  the  final  section.  For  now,  let’s  take  a  look  at  the multiverse.

The Multiverse and Fine-­Tuning


There  are (at least)  two  popular  mechanisms  to obtain  a  multiverse. One  is the many-worlds  or  Everett  interpretation  of  quantum  mechanics;  I  won’t  discuss  this  idea here, because the various “branches  of the wave function”  describing  different worlds  all share the same basic laws of physics. The other kind of multiverse is in some sense more prosaic,  in  that  it  simply  posits  regions  of  spacetime  outside  our  observable  horizon,  in which conditions are very different – including, in principle and often in practice, the parameters specifying the laws of physics, such as the mass of the neutron or the vacuum energy.

This latter scenario has garnered a great deal of attention in recent years, in part because  it  seems  to  be  a  natural  outcome  of  two  powerful  ideas  that  were  originally pursued for other reasons:  inflationary cosmology, and superstring theory.  Inflation uses the fact that dark energy makes the universe accelerate, but posits an initially small region of space filled with a temporary form of super-­‐dark-­‐energy at an enormously high density. This causes this small region  to grow to fantastic  size, before  the dark energy  ultimately decays.  In many versions of the theory, the decay isn’t complete, and at least some region is always  undergoing  ultra-­‐fast  inflationary  expansion  (Guth  1998).   From string theory  we get the idea of a “landscape” of possible vacuum states.   A “vacuum state” is simply a configuration of empty space with an associated set of physical laws.  That is, what we think of as spacetime  comes in a variety  of phases,  much like  water  can be in  solid,  liquid,  or gaseous  forms. In  string  theory  there  seems  to  be a mind-b­oggling number  of  possible phases (over 10500), each characterized by different physical constants, including the set of  elementary particles and the number of macroscopic dimensions of space (Vilenkin 2007; Susskind 2006; Greene 2011).

The  multiverse  comes  to  life  by  combining  inflation  with  string  theory. Once inflation starts, it produces a limitless supply of different “pocket universes,” each in one of the possible phases in the landscape of vacuum states of string theory.  Given the number of potential   universes,  it  wouldn’t  be  surprising   that  one  (or  an  infinite  number)  were compatible  with  the  existence  of  intelligent  life.  Once  this  background  is  in  place,  the “anthropic principle” is simply the statement that our observable universe has no reason to be representative  of the  larger  whole:  we  will  inevitably  find  ourselves  in a  region  that allows for us to exist.

What prior likelihood should we assign to such a scenario?  One popular objection to the multiverse is that it is highly non-­‐parsimonious; is it really worth invoking an enormous number of universes just to account for a few physical parameters?  As Swinburne (1996, 68) says:
To postulate  a  trillion  trillion  other  universes,  rather  than  one  God  in order to explain the orderliness of our universe, seems the height of irrationality.
That  might  be  true,  even  with  the  hyperbole,  if  what  one  was  postulating  were simply “a  trillion  trillion  other  universes.”    But  that  is  a  mischaracterization  of  what  is involved.   What one postulates are not universes, but laws of physics.   Given inflation and the  string  theory  landscape  (or  other  equivalent  dynamical  mechanisms),  a  multiverse happens, whether you like it or not.

This is an important point that bears emphasizing. All else being equal, a simpler scientific theory is preferred over a more complicated one.  But how do we judge simplicity? It certainly doesn’t mean “the sets involved in the mathematical description of the theory contain the smallest possible number  of elements.”  In the Newtonian clockwork  universe, every cubic centimeter contains an infinite number of points, and space contains an infinite number  of  cubic  centimeters,  all  of  which  persist  for  an  infinite  number  of separate moments each second,  over an infinite number  of seconds.   Nobody  ever claimed that all these infinities were a strike against the theory.   Indeed, in an open universe described by general relativity, space extends infinitely far, and lasts infinitely long into the future; again, these features are not typically seen as fatal flaws. It is only when space extends without limit  and  conditions  change  from  place  to  place,  representing separate  “universes,”  that people grow uncomfortable.   In quantum mechanics, any particular system is potentially described by an infinite number of distinct wave functions; again, it is only when different branches  of  such  a  wave  function  are  labeled  as  “universes”   that  one  starts  to  hear objections, even if the mathematical description of the wave function itself hasn’t grown any more complicated.

A scientific theory consists of some formal structure, as well as an “interpretation” that matches that structure onto the world we observe.  The structure is a statement about patterns  that  are exhibited  among  the various  objects  in the theory.   The  simplicity  of a theory is a statement about how compactly we can describe the formal structure (the Kolmogorov complexity), not how many elements it contains. The set of real numbers consisting of “eleven, and thirteen times the square root of two, and pi to the twenty-­‐eighth power, and all prime numbers between 4,982 and 34,950” is a more complicated  set than “the integers,” even though the latter set contains an infinitely larger number of elements. The physics of a universe containing 1088  particles that all belong to just a handful of types, each particle behaving precisely according to the characteristics of its type, is much simpler than that of a universe containing only a thousand particles, each behaving completely differently.

Likewise, a multiverse  that arises due to the natural dynamical consequences  of a relatively simple set of physical laws should not be discounted because there are a lot of universes  out  there.    Multiverse  theories  certainly  pose  formidable  problems,  especially when it comes to making predictions and comparing them with data; for that reason, most scientists  would  doubtless  prefer  a  theory  that  directly  predicted  the parameters  we observe   in  nature   over   a  multiverse   ensemble   in  which  our  local  environment was explained  anthropically. But  most  scientists  (for  similar  reasons)  would  prefer  a theory that was completely free of appeals to supernatural agents.

The multiverse is not a theory; it is a prediction of a theory, namely the combination of inflationary cosmology and a landscape of vacuum states.  Both of these ideas came about for other reasons, having nothing to do with the multiverse.   If they are right, they predict the existence of a multiverse in a wide variety of circumstances.   It’s our job to take the predictions of our theories seriously, not to discount them because we end up with an uncomfortably large number of universes.

The multiverse, by itself, doesn’t offer an explanation for every cosmological fine-­tuning problem.   If a parameter  needs to be smaller  than a certain value for life to exist, there’s no anthropic reason for it to be much smaller than that value.   We therefore have a prediction: anthropically-­‐selected parameters should be of the same order of magnitude as the  largest  value  compatible   with   the  existence   of  life. Indeed,   this  prediction   was successfully made by Steven Weinberg for the vacuum energy, over a decade before it was actually discovered (Weinberg 1987).

An example  of fine-­‐tuning  well beyond anthropic  constraints  is the initial state of the  universe,  often  characterized  in  terms  of  its extremely  low  entropy  (Penrose  1989). Roughly speaking, the large number of particles in the universe were arranged in an extraordinarily   smooth  configuration,   which  is  highly  unstable  and  unlikely  given  the enormous gravitational forces acting on such densely-­‐packed matter.  While vacuum energy is tuned to one part in 10120, the entropy of the early universe is tuned to one part in ten to the power of 10120, a preposterous number.  The entropy didn’t need to be nearly that low in order  for life  to come into  existence.  One way  of thinking  about  this is to note  that  we certainly don’t need a hundred billion other galaxies in the universe in order for life to arise here  on Earth;  our single  galaxy  would  have  been  fine,  or for  that  matter  a  single  solar system.

That  doesn’t  mean  that  we  can’t  possibly  explain  the  low  entropy  of  our  early universe  by  invoking  the  multiverse;  it  just  means  that  the  explanation  must  rely  on detailed  dynamical  properties  of the multiverse,  rather than simply  the requirement  that life  can  exist.    What  we  would  need  to  show  is  that,  in  the  context  of  the  particular multiverse scenario under consideration, when life arises at all it typically does so in the aftermath of an extremely low-­‐entropy event like our Big Bang.  This is a challenge, but not obviously  an insuperable  one, and researchers  are actively  tackling  this question (Carroll 2010).

If anything, the much-­‐more-­‐than-­‐anthropic tuning that characterizes the entropy of the universe is a bigger problem for the God hypothesis than for the multiverse.  If the point of arranging the universe was to set the stage for the eventual evolution of intelligent life, why all the grandiose excess represented by the needlessly low entropy at early times and the universe’s hundred billion galaxies? We might wonder whether those other galaxies are spandrels – not necessary for life here on Earth, but nevertheless a side effect of the general Big  Bang  picture,  which  is  the  most  straightforward   way  to  make  the  Earth  and  its biosphere. This turns out not to be true; quantitatively,  it’s easy to show that almost all possible  histories  of the universe that involve  Earth as we know  it don’t  have any other galaxies at all.3   It’s unclear why God would do so much more fine-tuning of the state of the universe than seems to have been necessary.

Accounting for the World


So  far  we’ve  been  discussing  roles  for  God  that  match  those  of  a  conventional scientific  theory  –  providing  a  clear  and  compelling  account  of  the  observational  facts. There is another angle often taken by natural theologians in explaining God’s usefulness to cosmology: that, whatever the facts of the world might be and whatever patterns they might follow, only a divine being can offer a “reason why” things are that way, over and above the facts and patterns themselves.

This  approach  takes  a  number  of  different  forms.  One  is  to  give  God  credit  for simply allowing the universe to exist:
For Judeo-­‐Christianity,  God is not a person in the sense that Al Gore arguably is… He  is,  rather,  the  condition  of  possibility  of  any  entity  whatsoever,  including ourselves.  He  is  the  answer  to  why  there  is  something  rather  than  nothing. (Eagleton 2006, 32)
Another  is to  sustain  the  existence  of  the  universe.    In  response  to  Hawking’s  question “What place, then, for a creator?”, John Polkinghorne (1994, 73) answers:
[I]t would be theologically  naïve to give any answer other than: “Every place – as the sustainer of the self-­‐sustained spacetime egg and as the creator of its quantum laws.”
Along similar lines, God is sometimes credited with maintaining the regularities observed in nature, which would otherwise simply be a coincidence.
The same laws of nature govern the most distant galaxies we can observe through our telescopes as operate on earth, and the same laws govern the earliest events in time to which we can infer as operate today… If there is no cause of this, it would be a most extraordinary coincidence – too extraordinary for any rational person to believe.  (Swinburne 1996, 49)
A final example comes from the traditional “cosmological” arguments for God’s existence.  In the “Kalam” formulation championed by William Lane Craig (1979), the first premise of the argument  states  “everything  that  has  a  beginning  in  time  has  a  cause.”  Things  cannot simply begin; something must begin them.

For convenience I am brutally lumping together quite different arguments, but hopefully the underlying point of similarity is clear.  These ideas all arise from a conviction that, in various contexts, it is insufficient to fully understand what happens; we must also provide an explanation for why it happens – what might be called a “meta-­‐explanatory” account.

It can be difficult to respond to this kind of argument.  Not because the arguments are especially persuasive, but because the ultimate answer to “We need to understand why the universe  exists/continues  to exist/exhibits  regularities/came  to be” is essentially  “No we don’t.”  That is unlikely to be considered a worthwhile comeback to anyone who was persuaded by the need for a meta-­explanatory understanding in the first place.

Granted, it is always nice to be able to provide reasons why something is the case. Most scientists, however, suspect that the search for ultimate explanations eventually terminates in some final theory of the world, along with the phrase “and that’s just how it is.”  It is certainly conceivable that the ultimate explanation is to be found in God; but a compelling  argument  to that effect would consist  of a demonstration  that God provides a better explanation (for whatever reason) than a purely materialist picture, not an a priori insistence that a purely materialist picture is unsatisfying.

Why  are  some  people  so  convinced  of  the  need  for  a  meta-­‐explanatory  account, while others are perfectly happy without one?  I would suggest that the impetus to provide such  an  account  comes  from  our  experiences  within  the world,  while  the suspicion  that there is no need comes from treating the entire universe as something unique, something for which a different set of standards is appropriate.

For example, we could imagine arguing that there is no puzzle associated with the value of the vacuum energy.   It had to be some number, and we have (perhaps) measured what  that  value  is,  and  there’s  nothing  more  to  be  said.    (Some  physicists,  although  a minority, do hold this view, and similarly for other fine-­‐tuning problems.)   The counter-­‐ argument is that the vacuum energy is really a parameter that we measure in the “effective field theory” that governs physics at low energies, regardless of the virtual high-energy processes we have not yet explored in experiments.  Even though there is only one universe, there are many  effective  field theories,  and  many  parameters  in the theories  relevant  to low-­‐energy physics.  So the vacuum energy is not a unique object; we have expectations for it  based  on  our  experience  with other  parameters  in  effective  field  theories,  and  can sensibly   compare   its  measured   value to those expectations.  It  is  in  terms  of   that comparison that we can legitimately call the vacuum energy finely-tuned.

States of affairs only require an explanation  if we have some contrary expectation, some reason to be surprised  that they hold.   Is there any reason to be surprised  that the universe exists, continues to exist, or exhibits regularities?   When it comes to the universe, we don’t have any broader context in which to develop expectations.   As far as we know, it may simply exist and evolve according to the laws of physics.   If we knew that it was one element of a large ensemble of universes, we might have reason to think otherwise, but we don’t.  (I’m using “universe” here to mean the totality of existence, so what would be called the multiverse” if that’s what we lived in.)

In Aristotle’s Metaphysics, he suggested the need for an “unmoved mover” to explain the  motion  of  ordinary  objects.    That  makes  sense  in  the  context  of  Aristotle’s  physics, which was fundamentally teleological:  objects tended toward their natural place, which is where they wanted to stay.   How, then, to account for all the motion we find everywhere around  us?     But  subsequent   developments   in  physics  –  conservation   of  momentum, Newton’s laws of motion – changed the context in which such a question  might be asked. Now  we  know  that  objects  that  are  moving  freely  continue  to  move  along  a  uniform trajectory, without anything moving them.  Why?  Because that’s what objects do.  It’s often convenient,  in the context  of everyday  life, for us to refer to this or that event as having some particular cause.  But this is just shorthand for what’s really going on, namely: things are obeying the laws of
physics.

Likewise for the universe.   There is no reason, within anything we currently understand about the ultimate structure of reality, to think of the existence and persistence and regularity of the universe as things that require external explanation.  Indeed, for most scientists,  adding  on  another  layer  of  metaphysical   structure  in  order  to  purportedly explain these nomological facts is an unnecessary complication.  This brings us to the status of God as a scientific hypothesis.

God as a Theory


Religion serves many purposes other than explaining the natural world.   Someone who  grew  up  as  an  altar  server,  volunteers  for  their  church  charity,  and  has  witnessed dozens of weddings and funerals of friends and family might not be overly interested in whether God is the best explanation for the value of the mass of the electron.   The idea of God has functions other than those of a scientific hypothesis.

However, accounting for the natural world is certainly a traditional role for God, and arguably a foundational  one.   How we think about other religious practices depends upon whether our understanding of the world around us gives us a reason to believe in God.  And insofar   as  it  attempts   to  provide   an  explanation   for  empirical   phenomena,   the   God hypothesis should be judged by the standards of any other scientific theory.

Consider a hypothetical world in which science had developed to something like its current state of progress, but nobody had yet thought of God.  It seems unlikely that an imaginative thinker in this world, upon proposing God as a solution to various cosmological puzzles, would be met with enthusiasm.   All else being equal, science prefers its theories to be precise, predictive, and minimal – requiring the smallest possible amount of theoretical overhead.    The  God  hypothesis  is  none  of  these.    Indeed,  in  our  actual  world,  God  is essentially  never invoked in scientific discussions.   You can scour the tables of contents in major physics journals, or titles of seminars and colloquia in physics departments and conferences, looking in vain for any mention of possible supernatural intervention into the workings of the world.

At  first  glance,  the  God  hypothesis  seems  simple  and  precise  –  an  omnipotent, omniscient, and omnibenevolent being.  (There are other definitions, but they are usually comparably   terse.)      The   apparent   simplicity   is   somewhat   misleading,   however. In comparison  to a purely  naturalistic  model,  we’re not simply  adding  a new element  to an existing ontology (like a new field or particle), or even replacing one ontology with a more effective one at a similar level of complexity (like general relativity replacing Newtonian spacetime, or quantum mechanics replacing classical mechanics).   We’re adding an entirely new metaphysical category, whose relation to the observable world is unclear.  This doesn’t automatically disqualify God from consideration as a scientific theory, but it implies that, all else being equal, a purely naturalistic model will be preferred on the grounds of simplicity.

There is an inevitable tension between any attempt to invoke God as a scientifically effective explanation  of the workings of the universe,  and the religious  presumption  that God is a kind of person, not just an abstract principle.  God’s personhood is characterized by an essential unpredictability  and the freedom to make choices.  These are not qualities that one  looks  for  in  a  good  scientific   theory.  On   the  contrary,   successful   theories   are characterized  by  clear  foundations  and  unambiguous  consequences. We  could  imagine boiling  God’s  role  in  setting  up  the  world  down  to  a  few  simple  principles  (e.g., “God constructs   the   universe   in   the   simplest   possible   way   consistent   with   the eventual appearance of human beings”).  But is what remains recognizable as God?

Similarly, the apparent precision of the God hypothesis evaporates when it comes to connecting to the messy workings of reality.   To put it crudely, God is not described in equations, as are other theories of fundamental physics.   Consequently, it is difficult or impossible  to make predictions.   Instead, one looks at what has already  been discovered, and agrees that that’s the way God would have done it.   Theistic evolutionists  argue that God uses natural  selection  to develop  life on Earth;  but religious  thinkers  before  Darwin were unable to predict that such a mechanism would be God’s preferred choice.

Ambitious  approaches  to contemporary  cosmological  questions,  such  as quantum cosmology, the multiverse, and the anthropic principle, have not yet been developed into mature scientific theories.   But the advocates of these schemes are working hard to derive testable   predictions   on   the   basis   of   their   ideas:   for   the   amplitude   of   cosmological perturbations (Hartle, Hawking, and Hertog 2008), signals of colliding pocket universes in the cosmic microwave background (Aguirre and Johnson 2009), and the mass of the Higgs boson  and  other  particles  (Feldstein  et  al.  2006).    For  the  God  hypothesis,  it  is unclear where one would start.  Why does God favor three generations of elementary particles, with a wide pectrum of masses?  Would God use supersymmetry or strong dynamics to stabilize the hierarchy  between  the weak  scale and the Planck  scale,  or simply  set it that way by hand? What would God’s favorite dark matter particle be?

This is a venerable  problem,  reaching  far beyond  natural  theology.   In numerous ways, the world around us is more like what we would expect from a dysteleological set of uncaring  laws  of  nature  than  from  a  higher  power  with  an  interest  in  our  welfare.  As another  thought  experiment,  imagine  a  hypothetical  world  in  which  there  was  no  evil, people  were  invariably  kind,  fewer  natural  disasters  occurred,  and  virtue  was  always rewarded.   Would inhabitants of that world consider these features to be evidence against the existence  of  God?   If not,  why  don’t  we consider  the contrary  conditions  to be such evidence?

Over the past five hundred years, the progress of science has worked to strip away God’s  roles  in  the world. He  isn’t  needed  to  keep  things  moving,  or  to  develop  the complexity of living creatures, or to account for the existence of the universe. Perhaps the greatest triumph of the scientific revolution has been in the realm of methodology. Control groups,  double-blind  experiments,  an  insistence  on  precise  and testable  predictions  –  a suite of techniques constructed to guard against the very human tendency to see things that aren’t there.  There is no control group for the universe, but in our attempts to explain it we should aim  for  a  similar  level  of  rigor. If  and  when  cosmologists  develop  a  successful scientific understanding of the origin of the universe, we will be left with a picture in which there is no place for God to act – if he does (e.g., through subtle influences on quantum-­mechanical transitions or the progress of evolution), it is only in ways that are unnecessary and imperceptible. We can’t be sure that a fully naturalist understanding  of cosmology is forthcoming, but at the same time there is no reason to doubt it. Two thousand years ago, it was perfectly reasonable to invoke God as an explanation for natural phenomena; now, we can do much better.

None of this amounts to a “proof” that God doesn’t exist, of course. Such a proof is not forthcoming; science isn’t in the business of proving things. Rather, science judges the merits of competing models in terms of their simplicity, clarity, comprehensiveness,  and fit to the data. Unsuccessful theories are never disproven, as we can always concoct elaborate schemes to save the phenomena; they just fade away as better theories gain acceptance. Attempting to explain the natural world by appealing to God is, by scientific standards, not a very successful  theory. The fact that we humans  have been able to understand  so much about  how  the  natural  world  works,  in  our  incredibly  limited  region  of  space  over  a remarkably short period of time, is a triumph of the human spirit, one in which we can all be justifiably proud.
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Footnotes

1.  See also Carroll 2005, 62.   For a different view, see chapters in this volume by Don Page, 
Robin Collins, and Steve Barr.

2.  It’s not obvious  that this line reasoning  is valid. One could certainly  imagine  taking the 
position that our existence offers exactly zero information about the probability of any 
cosmological   scenario,   because   if  we  didn’t  exist  we  wouldn’t   be  here  debating   the 
alternatives.  But for the moment we are playing along.

3.  Given laws of motion, the space of histories of the universe is isomorphic to the space of 
states at some fixed time.   The entropy is the logarithm of the number of macroscopically similar 
states.   The fact that we can imagine much higher-­‐entropy configurations of the universe  today  
without disturbing  the Earth (e.g., by putting the rest of the universe  into black holes) 
demonstrates that histories like ours are an incredibly tiny fraction of histories that give rise 
to something like our current Earth.

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