life, the cosmos and everything

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Life, the cosmos and

everythingOct 3, 2001

Cosmologists who study the link between life

in the universe and the values of the physicalconstants were once viewed with suspicion byother scientists. But a recent high-profileconference at Cambridge showed that thesubject is fast becoming academically

respectable.

Stars in the making

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The notion that certain features of theuniverse, such as the values of the physicalconstants, may be constrained by the

requirement that intelligent observers can arisewas first mooted nearly 40 years ago. This"anthropic principle" has been a focus ofcontroversy (even intense antipathy in somequarters) ever since. However, judging by aconference that took place in Cambridge at theend of August, the notion seems to beattracting the interest of an increasing numberof eminent physicists. The meeting - the first

in a series supported in part by the TempletonFoundation - took place at the Cambridgehome of Martin Rees, one of the foremostadvocates of the anthropic principle. Futuremeetings will address the biological and

philosophical aspects of the subject.



What is the anthropic principle?

There are various versions of the anthropic

principle. The "weak" version accepts the lawsof nature and the values of the physicalconstants as given and claims that theexistence of life then imposes a selection effect

on where and when we observe the universe.

For example, the current age of the universecannot be less than the nuclear-burning time ofa massive star - otherwise there would nothave been enough time for the chemicalelements that are essential for life to have been

generated by stellar nucleosynthesis. On theother hand, the universe cannot be much olderthan this because the stars would have allburned out. This means that life can only exist

when the universe has roughly its observedage. This is a logical consequence of ourexistence and is relatively uncontroversial.

The "strong" version of the anthropic principlesuggests that the presence of observers



imposes constraints on the physical constantsthemselves. In other words, life could onlyarise if the constants were close to their

observed values. Some people might inferfrom this the existence of a creator who tailor-made the universe for our benefit. However,cosmologists have recently realized thatprocesses in the early universe may naturallyhave generated an ensemble of universes, eachhaving different values of the constants. Welive in one of the universes that is conducive tolife. Even though invoking multiple universes

is highly speculative, this makes the stronganthropic principle much more palatable froma physical point of view since it just becomesan aspect of the weak version.

In order to argue that the universe is fine-tunedfor the emergence of observers, one mustspecify who qualifies for this description, andnot everybody agrees on this. Brandon Carter,who first coined the term "anthropic principle"

in 1974, introduced the meeting by



emphasizing that the concept can be refined invarious ways according to whether oneincludes every conceivable observer (including

ants and extraterrestrials) or just Homosapiens. He proposed a "refined" anthropicprinciple, in which the observer is "weighted"according to the amount of informationprocessed. It is not clear, however, thatconsciousness is the key feature of theanthropic constraints. Other speakers stressedthat many of the fine-tunings are justassociated with the development of

complexity.

Evidence for the anthropic principle

As Virginia Trimble emphasized, the

prerequisites for getting out of bed in themorning are many and varied! In particular,the existence of life (or at least our particularform of it) requires the formation of ahierarchy of structures - planets, stars and



galaxies - and, as successive speakers pointedout, each of these seems to require ratherspecial conditions.

Carl Murray focused on planet formation. Thediscovery of several dozen extra-solarplanetary systems in recent years suggests thatour solar system is far from unique, although

he did emphasize that merely having planets isnot enough for life to occur since the Earthseems to have been fortunate in various otherways. For example, it is known that the Moonhas played an important role as a climate

regulator. If the Moon were much smaller, thespin axis of the Earth would changechaotically - leading to catastrophic weathervariations that could exclude the emergence of

life. Another fortunate aspect of our solarsystem is that the outer planets seem to haveplayed an important role in the formation ofthe inner ones.

Our presence on Earth might be regarded as an



example of the weak anthropic principle.Rather more controversial are the anthropicconditions that seem to be associated with

stars. I discussed in my talk how these involveconstraints between the dimensionless"coupling constants" that describe thestrengths of the fundamental interactions - inparticular the electric fine-structure constant a

= e2/h-bar c ~ 1/137, the gravitational fine-

structure constant aG

= Gmp2/h-bar c = 5 x 10-

39, and also the weak fine-structure constant

aW = gmec2

/h-bar3

x 10-10

, where G is thegravitational constant, g is the Fermi constant,m

pis the mass of a proton, h-bar is the Planck

constant divided by 2 pi, c is the speed of light

and me is the mass of an electron.

It seems that aG

must be roughly a20 for both

"convective" and "radiative" stars to exist(prerequisites for planets and supernovae,



respectively) and roughly aW

4 for neutrinos to

eject the envelope of a star in a supernovaexplosion (necessary for the dissemination ofheavy elements). These "coincidences" mightbe regarded as examples of the stronganthropic principle.

Several contributors highlighted an even more

striking example associated with stars. Thisinvolves the strong interaction and concernsthe generation of carbon (another prerequisite

for our form of life) in the helium-burning

phase of red giant stars. This occurs via areaction in which two alpha particles unite toform a beryllium nucleus that then combineswith another alpha particle to form carbon.However, as the late Fred Hoyle (see Sir Fred

Hoyle 1915 - 2001 and page 11 of this issue,print version) first pointed out, the berylliumwould decay before interacting with anotheralpha particle were it not for the existence of aremarkably finely tuned resonance in this

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interaction. This fact is sometimes presented asan anthropic prediction but, as Trimbleintriguingly pointed out, there may have been

evidence for this resonance in the data evenbefore Hoyle suggested that it be sought in thelaboratory.

Heinz Oberhummer, who has studied this

resonance in more detail, reported somebeautiful work showing how the amount ofoxygen and carbon produced in red giant starsvaries with the strength and range of thenucleon interactions. His work indicates that

the nuclear interaction must be tuned to at least0.5% if one is to produce both these elementsto the extent required for life.

Cosmological anthropic constraintsThe anthropic constraints associated with theformation of galaxies involve variouscosmological parameters, such as the density

of the matter in the universe, the amplitude of



the initial density fluctuations, the photon-to-baryon ratio and the cosmological constant (anextra term Einstein introduced into his field

equations for cosmological reasons and whichmay cause the universe to accelerate). Some ofthese parameters might be determined byprocesses in the early universe rather thanbeing prescribed freely as part of the initialconditions. However, as Martin Reesdiscussed, even small deviations from theobserved values of such parameters wouldexclude the formation of structures like

galaxies and their subsequent fragmentationinto stars.



Host with the most

An interesting twist on these arguments wasprovided by Anthony Aguirre, who described

anthropic constraints on so-called coldcosmological models, in which the initial ratioof photons to baryons (i.e. ordinary matter like

protons and neutrons) is much smaller thancurrently observed. He pointed out that such

models could provide life-supportingconditions with very different values of thecosmological parameters and couplingconstants to those found in our universe. Both

Rees and Aguirre stressed the importance of






calculating the probability distribution for suchparameters across the different universesbecause this is the only way of testing the

multiple universe or "multiverse" proposal.For example, if the distribution for theamplitude of the density fluctuations fell offtoo slowly, we would be surprised to be in auniverse with a value as small as is observed.

Fundamental constants

In assessing the anthropic principle, a key

issue is whether some fundamental theory willeventually determine all the constants uniquelyor whether some of them are contingent oninitial conditions or accidental features ofsymmetry breaking. In the first case there is no

room for the anthropic principle and theanthropic fine-tunings must just be regarded ascoincidental. In the second case, there may beroom for anthropic arguments.

One first has to decide which physical



constants should be regarded as fundamental.Unification theories predict relationshipsbetween some of the constants, so one is

certainly not free to vary all of them. CraigHogan identified the coupling constantsassociated with the four interactions and somebasic mass-scales (e.g. the masses of theelectron and the up and down quarks) asfundamental. Although features of biology arenot sensitive to the values of these constants,the existence of stable atoms and an interestingrange of chemical elements certainly are. For

example, even small changes in the quark andelectron masses would make the proton,deuteron or hydrogen atom unstable.

The particle physicists at the meeting

expressed various views on how likely suchtunings are to result from some fundamentaltheory. As John Donoghue emphasized, "fine-tuning" arises in various different contexts inparticle physics - why, for example, are the

cosmological constant and strong charge-



parity (CP) violation so small? - even thoughmost of these may have no anthropicsignificance. However, some of them do and

he particularly stressed anthropic constraintson the "vacuum expectation value" of theHiggs field, which determines the masses ofall the ordinary particles.

At least some physical parameters wouldappear to be contingent. For example, FrankWilczek, who first posited the existence of alight particle called the "axion" in order toexplain the lack of CP violation in strong

interactions, pointed out that the density ofthese particles would now be much larger thanthe baryonic density unless an angle associatedwith the initial conditions of the axion field

were tiny. Such a large axion density would beincompatible with the formation of galaxies -and so is anthropically disallowed. The onlyreasonable explanation for axions and baryonshaving comparable densities is to invoke an

early "inflationary" phase for the universe, in



which it expands exponentially fast due to theeffect of a cosmological constant. The axionangle would then have different values in

different places and we would necessarily livein a region where this angle was very small.

Most physicists would probably prefer theconstants to be determined by more

conventional physics. So how likely could thatbe? The current favourite candidate for afundamental theory is the string model. Thisposits that space-time is either 10-dimensional(superstring theory) or 11-dimensional (M-

theory), with four-dimensional physicsemerging from the compactification of theextra dimensions. Unlike the Standard Modelof particle physics, which does not incorporate

gravity and contains several dozen freeparameters, M-theory may predict all thefundamental constants uniquely. This pointwas emphasized by Malcolm Perry. The onlyinput would then be the string scale (related to

the size of the 11th dimension). However, the



situation is probably not as clear-cut as thissince M-theory only predicts that the numberof vacuum states should be discrete; the

constants may be uniquely determined withineach one but could be different across thestates themselves. The crucial issue is whetherthe number of vacuum states is sufficientlylarge and their spacing sufficiently small toallow some room for anthropic constraints.This issue remains unresolved.

A new twist arises if the (so-called) constantsvary in time even in our universe. This is

expected in many unification theories since theconstants should be related to the size of thecompact internal dimensions, which would beexpected to change during at least part of the

universe's history. This theme was takenfurther by John Barrow. He is part of a teamthat recently claimed to have found positiveevidence for a variation in a of about sevenparts in a million by studying absorption lines

in several hundred galaxies (see When

http://physicsworld.com/article/world/14/10/4





constants are not constant by Chris Carilli).His attempts to model this effect suggest that ashould remain constant during both the early

"radiation-dominated" phase of the universeand the late "curvature-dominated" or"cosmological-constant-dominated" phases.However, a can vary over the intermediatematter-dominated phase, which would make itdifficult to satisfy the anthropic constraints ona for an extended period if the curvature orcosmological constant were too close to zero.

Quantum cosmology

One reason why many cosmologists now takethe anthropic principle seriously is that the"many worlds" interpretation of quantum

mechanics seems to be the only sensiblecontext in which to discuss "quantumcosmology" - the branch of physics that triesto describe what happened near the big bang.As emphasized by Jim Hartle, quantum theory





allows many mutually incompatible histories.However, it only makes sense to consider theinitial conditions that led to the classical

behaviour that we observe today. (Withcomplete ignorance of the initial conditions,the quantum fluctuations could be arbitrarilylarge and the emergence of a classical worldwould not be possible.) Within this restriction,quantum cosmology allows many differentworlds or "branches", all with different valuesof the constants, and this validates the stronganthropic principle.

Nevertheless, the cosmologists present hadwidely different views on how the differentworlds might arise. Andrei Linde and AlexVilenkin invoked "eternal" inflation, in which

the universe is eternally self-reproducing. Thisversion of inflation predicts that there may bean infinite number of exponentially largedomains - all with different laws of low-energyphysics and different coupling constants.

Indeed, Linde regarded inflation as the only



plausible basis for anthropic arguments.Vilenkin argued that there is a well motivatedprescription within the eternal-inflation

scenario for calculating probabilitydistributions for the various constants,showing that the distributions should beweighted by the volume of the universe inwhich each set of values pertains.

Brief history of time

On the other hand, Stephen Hawking objectedto the eternal-inflation model on the grounds

that it extends to the infinite past and thusviolates his "no boundary" proposal for theorigin of the universe. This proposal requires






that the universe start at a finite time and itavoids the initial singularity by requiring timeto become imaginary there (i.e. time is

multiplied by (-1)1/2 so that the metric startsoff Euclidean rather than Lorentzian).Hawking uses the path-integral approach tocalculate the probability of a particular historybut only sums over those histories that lead to

observers.

Neil Turok elaborated on this theme, showingthat there are so-called instantons thatrepresent classical solutions of the Euclidean

equations that possess a continuation to realLorentzian space-time. Although the pathintegral favours inflationary periods shorterthan required, anthropic selection can salvage

this since one only considers historiescontaining observers. This permits either openor closed universes but he argued thatHawking's favoured (closed) solution is

unstable.



More radical physics?

The final day of the conference focused on

more radical deviations from standard physicsas well as some philosophical issues. RichardGott presented another version of the many-worlds principle, speculating how the

existence of closed timelike curves in general

relativity could allow the universe to createitself. Max Tegmark discussed anthropicconstraints on the dimensionality of space andtime: three spatial dimensions are required forthe stability of planetary orbits and more than

one time dimension would destroy causality.He also raised the issue of whether it issufficient to consider universes with differentvalues of the coupling constants, or whether

one should also contemplate universes withdifferent physical laws or even differentmathematical foundations. This might be theonly way to explain anthropic coincidences if

the physical constants within a given set of



laws turn out to be uniquely specified.

Bill Stoeger discussed the legitimacy of

anthropic arguments. He argued that the weakanthropic principle is a logical necessity - butthat the strong version only makes sense ifvariations in the initial conditions of theuniverse or the values of the constants or the

laws of nature allow some scope for anthropicselection. The multiverse proposal mayaccommodate this possibility, but howlegitimate is it, he argued, to invoke theexistence of other universes for which there

may never be any direct evidence?

Lee Smolin stressed that it is only justifiable if

one has a theory that independently predictsthe existence of these universes, and that such

a theory, to be scientific, must be falsifiable.He argued that most of the universes shouldhave properties like our own and that this neednot be equivalent to requiring the existence ofobservers.



Smolin's own approach invoked a form ofnatural selection. He argued that the formationof black holes might generate new universes in

which the constants are slightly mutated. Inthis way, after many generations, theparameter distribution will peak around thosevalues for which black-hole formation ismaximized. This proposal involves veryspeculative physics, since we have nounderstanding of how the baby universes areborn. However, it has the virtue of beingtestable since one can calculate how many

black holes would form if the parameters weredifferent.

A few speakers touched on the issue ofconsciousness. This is a topic usually

eschewed by physicists, but Don Pageemphasized that physics is primarilyconcerned with observations and these are, atroot, conscious perceptions. He argued that aparticular observer's experience should be a

random sample of all conscious experiences



and discussed how one might derive theprobability measure for this sample.Ultimately this must depend on unknown laws

connecting consciousness with physics. Lindealso proposed that consciousness might play acrucial role in the world, speculating that itmight exist (like space-time) even withoutmatter.

Such considerations may go beyond thedomain of legitimate science. But perhaps themain message of the meeting was thatdevelopments in modern physics may require

one to extend one's view of what constituteslegitimate science anyway. The anthropicprinciple may not yet have attained completescientific respectability, but it can no longer be

dismissed as nothing more than meremetaphysics.

"Anthropic arguments in fundamental physicsand cosmology" was held in Cambridge from30 August -1 September.



About the author

Bernard Carr is in the Astronomy Unit, School

of Mathematical Sciences, Queen Mary,University of London, UK.

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