杏吧原创

CAN science explain how the Universe began? Even suggestions to that effect
have provoked an angry and passionate response from many quarters. Religious
people tend to see the claim as a move to finally abolish God the Creator.
Atheists are equally alarmed, because the notion of the Universe coming into
being from nothing looks suspiciously like the creation, ex nihilo, of
Christianity. The general sense of indignation was well expressed by writer Fay
Weldon. 鈥淲ho cares about half a second after the big bang,鈥� she railed in 1991
in a scathing newspaper attack on scientific cosmology. 鈥淲hat about the half a
second before?鈥� What indeed. The simple answer is that, in the standard picture
of the cosmic origin, there was no such moment as 鈥渉alf a second before鈥�.

To see why, we need to examine this standard picture in more detail. The
first point to address is why anyone believes the Universe began at a finite
moment in time. How do we know that it hasn鈥檛 simply been around for ever? Most
cosmologists reject this alternative because of the severe problem of the second
law of thermodynamics. Applied to the Universe as a whole, this law states that
the cosmos is on a one-way slide towards a state of maximum disorder, or
entropy. Irreversible changes, such as the gradual consumption of fuel by the
Sun and stars, ensure that the Universe must eventually 鈥渞un down鈥� and exhaust
its supplies of useful energy. It follows that the Universe cannot have been
drawing on this finite stock of useful energy for all eternity.

Body of evidence

Direct evidence for a cosmic origin in a big bang comes from three
observations. The first, and most direct, is that the Universe is still
expanding today. The second is the existence of a pervasive heat radiation that
is neatly explained as the fading afterglow of the primeval fire that
accompanied the big bang. The third strand of evidence is the relative
abundances of the chemical elements, which can be correctly accounted for in
terms of nuclear processes in the hot dense phase that followed the big
bang.

But what caused the big bang to happen? Where is the centre of the explosion?
Where is the edge of the Universe? Why didn鈥檛 the big bang turn into a black
hole? These are some of the questions that bemused members of the audience
always ask whenever I lecture on this topic. Though they seem pertinent, they
are in fact based on an entirely false picture of the big bang. To understand
the correct picture, it is first necessary to have a clear idea of what the
expansion of the Universe entails. Contrary to popular belief, it is not the
explosive dispersal of galaxies from a common centre into the depths of a
limitless void. The best way of viewing it is to imagine the space between the
galaxies expanding or swelling.

The idea that space can stretch, or be warped, is a central prediction of
Einstein鈥檚 general theory of relativity, and has been well enough tested by
observation for all professional cosmologists to accept it. According to general
relativity, space-time is not a static arena, but an aspect of the gravitational
field. This field manifests itself as a warping, or curvature, of space-time
geometry, and when it comes to the large scale structure of the Universe, such a
warping occurs in the form of space being stretched with time.

A helpful, albeit two-dimensional, analogy for the expanding Universe is a
balloon with paper spots stuck to the surface. As the balloon is inflated, so
the spots, which play the role of galaxies, move apart from each other. Note
that it is the surface of the balloon, not the volume within, that represents
the three-dimensional Universe.

Now, imagine playing the cosmic movie backwards, so that the balloon shrinks
rather than expands. If the balloon were perfectly spherical (and the rubber
sheet infinitely thin), at a certain time in the past the entire balloon would
shrivel to a speck. This is the beginning.

Translated into statements about the real Universe, I am describing an origin
in which space itself comes into existence at the big bang and expands from
nothing to form a larger and larger volume. The matter and energy content of the
Universe likewise originates at or near the beginning, and populates the
Universe everywhere at all times. Again, I must stress that the speck from which
space emerges is not located in anything. It is not an object surrounded by
emptiness. It is the origin of space itself, infinitely compressed. Note that
the speck does not sit there for an infinite duration. It appears
instantaneously from nothing and immediately expands. This is why the question
of why it does not collapse to a black hole is irrelevant. Indeed, according to
the theory of relativity, there is no possibility of the speck existing through
time because time itself begins at this point.

This is perhaps the most crucial and most difficult aspect of the big bang
theory. The notion that the physical Universe came into existence with time and
not in time has a long history, dating back to St Augustine in the fifth
century. But it took Einstein鈥檚 theory of relativity to give the idea scientific
respectability. The key feature of the theory of relativity is that space and
time are part of the physical Universe, and not merely an unexplained background
arena in which the Universe happens. Hence the origin of the physical Universe
must involve the origin of space and time too.

But where could we look for such an origin? Well, the theory of relativity
permits space and time to possess a variety of boundaries or edges, technically
known as singularities. One type of singularity exists in the centre of a black
hole. Another corresponds to a past boundary of space and time at the big bang.
The idea is that, as you move backwards in time, the Universe becomes more and
more compressed and the curvature or warping of space-time escalates without
limit, until it becomes infinite at a singularity. Very roughly, it resembles
the apex of a cone, where the fabric of the cone tapers to an infinitely sharp
point and ceases. It is here that space and time begin.

Once this idea is accepted, it is immediately obvious that the question 鈥淲hat
happened before the big bang?鈥� is meaningless. There was no such epoch as
鈥渂efore the big bang鈥�, because time began with the big bang. Unfortunately, the
question is often answered with the bald statement 鈥淭here was nothing before the
big bang鈥�, and this has caused yet more misunderstandings. Many people interpret
鈥渘othing鈥� in this context to mean empty space, but as I have been at pains to
point out, space did not exist either prior to the big bang.

Absolutely nothing

Perhaps 鈥渘othing鈥� here means something more subtle, like pre-space, or some
abstract state from which space emerges? But again, this is not what is intended
by the word. As Stephen Hawking has remarked, the question 鈥淲hat lies north of
the North Pole?鈥� can also be answered by 鈥渘othing鈥�, not because there is some
mysterious Land of Nothing there, but because the region referred to simply does
not exist. It is not merely physically, but also logically, non-existent. So too
with the epoch before the big bang.

In my experience, people get very upset when told this. They think they have
been tricked, verbally or logically. They suspect that scientists can鈥檛 explain
the ultimate origin of the Universe and are resorting to obscure and dubious
concepts like the origin of time merely to befuddle their detractors. The
mind-set behind such outraged objection is understandable: our brains are
hard-wired for us to think in terms of cause and effect. Because normal physical
causation takes place within time, with effect following cause, there is a
natural tendency to envisage a chain of causation stretching back in time,
either without any beginning, or else terminating in a metaphysical First Cause,
or Uncaused Caused, or Prime Mover. But cosmologists now invite us to
contemplate the origin of the Universe as having no prior cause in the normal
sense, not because it has an abnormal or supernatural prior cause, but because
there is simply no prior epoch in which a preceding causative
agency鈥攏atural or supernatural鈥攃an operate.

Nevertheless cosmologists have not explained the origin of the Universe by
the simple expedient of abolishing any preceding epoch. After all, why should
time and space have suddenly 鈥渟witched on鈥�? The latest thinking is that this
spontaneous origination of time and space is a natural consequence of quantum
mechanics. Quantum mechanics is the branch of physics that applies to atoms and
subatomic particles, and it is characterised by Heisenberg鈥檚 uncertainty
principle, according to which sudden and unpredictable fluctuations occur in all
observable quantities. Quantum fluctuations are not caused by
anything鈥攖hey are genuinely spontaneous and intrinsic to nature at its
deepest level.

Impossible predictions

For example, take a collection of uranium atoms suffering radioactive decay
due to quantum processes in their nuclei. There will be a definite time period,
the half-life, after which half of the nuclei present should have decayed. But
according to Heisenberg it is not possible, even in principle, to predict when a
given nucleus will decay. If you ask, having seen a particular nucleus decay,
why the decay event happened at that moment rather than some other, there is no
deeper reason, no underlying set of causes, that explains it. It just
happens.

The key step for cosmogenesis is to apply this same idea not just to matter,
but to space and time as well. Because space-time is an aspect of gravitation,
this entails applying quantum theory to the gravitational field of the Universe.
The application of quantum mechanics to a field is fairly routine for
physicists, though it has to be said that there are special technical problems
associated with the gravitational case that have yet to be satisfactorily
resolved (鈥淐an gravity take a quantum leap?鈥�, 10 September 1994, p 28). The
quantum theory of the origin of the Universe therefore rests on shaky
ground.

In spite of these technical obstacles, one may say quite generally that once
space and time are made subject to quantum principles, the possibility
immediately arises of space and time 鈥渟witching on鈥�, or popping into existence,
without the need for prior causation, entirely in accordance with the laws of
quantum physics.

The details of this process remain both subtle and contentious, and depend to
some extent on the interrelationship between space and time. Einstein showed
that space and time are closely interwoven, but in the theory of relativity they
are still distinct. Quantum physics introduces the new feature that the
separate identities of space and time can be 鈥渟meared鈥� or 鈥渂lurred鈥� on an
ultra-microscopic scale. In a theory proposed in 1982 by Hawking and American
physicist Jim Hartle, this smearing implies that, closer and closer to the
origin, time is more and more likely to adopt the properties of a space
dimension, and less and less likely to have the properties of time. This
transition is not sudden, but blurred by the uncertainty of quantum physics.
Thus time does not switch on abruptly in Hartle and Hawking鈥檚 theory, but it
emerges continuously from space. There is no specific first moment at which time
starts, but neither does time extend backwards for all eternity (see
Diagram p
34).FIG-20274201.jpg

Unfortunately, the topic of the quantum origin of the Universe is fraught
with confusion because of the publicity given to a preliminary, and in my view
wholly unsatisfactory theory of the big bang based on an instability of the
quantum vacuum. According to this alternative theory, first mooted by Edward
Tryon in 1973, space and time are eternal, but matter is not. It suddenly
appears in a pre-existing and unexplained void due to quantum vacuum
fluctuations. In such a theory, it would indeed involve a serious misnomer to
claim that the Universe originated from nothing: a quantum vacuum in a
background space-time is certainly not nothing.

Law unto itself

However, if there is a finite probability of an explosive appearance of
matter, it should have occurred an infinite time ago. In effect, Tryon鈥檚 theory
and others like it run into the same problem of the second law of thermodynamics
as most models of an infinitely old Universe.

It will be obvious from what I have said that the attempt to explain the
origin of the Universe is based on an application of the laws of physics. This
is normal in science: one takes the underlying laws of the Universe as given.
But when tangling with ultimate questions, it is only natural that we should
also ask about the status of these laws. One must resist the temptation to
imagine that the laws of physics, and the quantum state that represents the
Universe, somehow exist before the Universe. They don鈥檛鈥攁ny more than they
exist north of the North Pole. In fact, the laws of physics don鈥檛 exist in space
and time at all. They describe the world, they are not 鈥渋n鈥� it. However, this
does not mean that the laws of physics came into existence with the Universe. If
they did鈥攊f the entire package of physical Universe plus laws just popped
into being from nothing鈥攖hen we cannot appeal to the laws to explain the
origin of the Universe. So to have any chance of understanding scientifically
how the Universe came into existence, we have to assume that the laws have an
abstract, eternal character. The alternative is to shroud the origin in mystery
and give up.

It might be objected that we haven鈥檛 finished the job by baldly taking the
laws of physics as given. Where did those laws come from? And why those laws
rather than some other set? This is a valid objection. I have argued that we
must eschew the traditional causal chain and focus instead on an explanatory
chain, but inevitably we now confront the logical equivalent of the First
Cause鈥攖he beginning of the chain of explanation.

In my view it is the job of physics to explain the world based on lawlike
principles. 杏吧原创s adopt differing attitudes to the metaphysical problem of
how to explain the principles themselves. Some simply shrug and say we must just
accept the laws as a brute fact. Others suggest that the laws must be what they
are from logical necessity. Yet others propose that there exist many worlds,
each with differing laws, and that only a small subset of these universes
possess the rather special laws needed if life and reflective beings like
ourselves are to emerge. Some sceptics rubbish the entire discussion by claiming
that the laws of physics have no real existence anyway鈥攖hey are merely
human inventions designed to help us make sense of the physical world. It is
hard to see how the origin of the Universe could ever be explained with a view
like this.

In my experience, almost all physicists who work on fundamental problems
accept that the laws of physics have some kind of independent reality. With that
view, it is possible to argue that the laws of physics are logically prior to
the Universe they describe. That is, the laws of physics stand at the base of a
rational explanatory chain, in the same way that the axioms of Euclid stand at
the base of the logical scheme we call geometry. Of course, one cannot prove
that the laws of physics have to be the starting point of an explanatory scheme,
but any attempt to explain the world rationally has to have some starting point,
and for most scientists the laws of physics seem a very satisfactory one. In the
same way, one need not accept Euclid鈥檚 axioms as the starting point of geometry;
a set of theorems like Pythagoras鈥檚 would do equally well. But the purpose of
science (and mathematics) is to explain the world in as simple and economic a
fashion as possible, and Euclid鈥檚 axioms and the laws of physics are attempts to
do just that.

In fact, it is possible to quantify the degree of compactness and utility of
these explanatory schemes using a branch of mathematics called algorithmic
information theory. Obviously, a law of physics is a more compact description of
the world than the phenomena that it describes. For example, compare the
succinctness of Newton鈥檚 laws with the complexity of a set of astronomical
tables for the positions of the planets. Although as a consequence of Godel鈥檚
famous incompleteness theorem of logic, one cannot prove a given set of laws, or
mathematical axioms, to be the most compact set possible, one can investigate
mathematically whether other logically self-consistent sets of laws exist. One
can also determine whether there is anything unusual or special about the set
that characterises the observed Universe as opposed to other possible universes.
Perhaps the observed laws are in some sense an optimal set, producing maximal
richness and diversity of physical forms. It may even be that the existence of
life or mind relates in some way to this specialness. These are open questions,
but I believe they form a more fruitful meeting ground for science and theology
than dwelling on the discredited notion of what happened before the big
bang.

* * *

World without end

How sure can we be that the Universe began with the big bang? Was there
only one big bang, or have there been many? Could the Universe really have begun
as a quantum blip? Some of the world鈥檚 leading cosmologists give their
views

TRY asking a bunch of cosmologists about the origin of the Universe, and it鈥檚
hard to get a clear answer. 鈥淭he Universe didn鈥檛 start. It鈥檚 infinite,鈥� says
British cosmologist Fred Hoyle. 鈥淚t鈥檚 an open question,鈥� says Steven Weinberg,
Nobel prizewinning particle physicist from the University of Texas. 鈥淚t鈥檚 up in
the air,鈥� says Paul Steinhardt from Pennsylvania State University, co-developer
in the 1980s of a key theory about the early Universe. 鈥淚t must have had a
beginning,鈥� says cosmologist Alexander Vilenkin of Crufts University in
Massachusetts. The standard big bang model is agreed, says Oxford mathematician
Roger Penrose, and everything else is 鈥渆mbellishments and flights of fancy鈥�.

So what gives? Well, Hoyle is convinced that the big bang is a myth, and that
the Universe is eternal, with matter continuously created at the centres of
galaxies. But virtually everyone else is happy with the big bang model, at least
as far back as the early stages of the Universe. Says Weinberg, 鈥淲e are in an
expanding Universe which at one time鈥攂efore any of the stars or galaxies
formed鈥攚as very hot and dense. I don鈥檛 think there鈥檚 any serious argument
that in that sense there was a big bang, and the part of the Universe that we
live in had a start. But beyond that we really don鈥檛 know.鈥�

To try to trace the history of the Universe back to its origin, cosmologists
picture the expansion running backwards to a point where the Universe was almost
unimaginably small and dense. The first problem they meet, when they do this, is
that the concept of time comes apart in their hands. The reason is that at the
so-called Planck scale (a mere 10^-35 metres), two theories begin to clash.
Einstein鈥檚 smooth, large-scale, classical theory of gravity makes no provision
for the fuzzy, indeterminate quantum theory of tiny particles, and all bets are
off. 鈥淨uestions about what happened before what begin to lose meaning,鈥� says
Steinhardt. 鈥淏efore only makes sense if there鈥檚 a sensible time ordering to
things, and that notion breaks down at the Planck scale.鈥� Weinberg agrees: 鈥淎ny
description that tries to go to earlier times has to give up the idea of time.
It鈥檚 no longer a meaningful concept.鈥�

Glimmers of hope for reconciling relativity and quantum theory come from an
idea called superstrings鈥攊n which all matter is made up of tiny
10-dimensional strings. Although we appear to live in a Universe with just four
dimensions, three for space and one for time, the theory goes that the other six
dimensions present are curled up so tightly that we can鈥檛 detect them directly.
But this causes even greater problems, because at the Planck scale the tightly
curled extra dimensions become significant. 鈥淵ou go back in time and it looks
like you鈥檙e heading towards a singularity and all of a
sudden鈥攚ham鈥攑hysics changes because all those extra dimensions that
you weren鈥檛 aware of suddenly come into play,鈥� says Steinhardt.

It is usually easy to tell time and space apart. But, says Steinhardt, 鈥淲hen
you unwrap the extra dimensions, you don鈥檛 know what they鈥檒l be like. It may be
that you even have two time-like coordinates, or more.鈥� The idea of before and
after would then be even shakier.

How the Universe could appear from nothing in the first place? In 1982,
Vilenkin came up with the idea that the Universe literally tunnelled its way
into existence, something allowed by quantum theory but impossible on an
everyday large scale. In the classical world, if you have a heavy object lying
in a dip it will need a push to climb over the edge and roll down the other
side. But in the quantum world, there is a small, but non-zero probability that
the object can simply tunnel to the other side of the dip without any outside
help. The only condition is that it does not gain any energy in the process.

So how does this relate to the Universe? Well, say you start with nothing at
all鈥攏ot even space or time. Presumably the total energy of this system
would be zero. Is it possible to make a Universe of space, time and matter whose
total energy is still zero? The answer is yes. 鈥淵ou can鈥檛 create something out
of nothing,鈥� says Vilenkin. 鈥淏ut the Universe is an exception. Gravitational
energy is negative and matter energy is positive. In a closed Universe鈥攐ne
where if you keep going in one direction you come back to the same
point鈥攖he negative energy of gravity exactly cancels the positive energy
of matter, so the total energy is zero.鈥�

In the classical picture, the Universe cannot appear out of nothing because
it is forbidden to adopt a certain range of sizes. But in quantum theory, the
Universe can tunnel through this size barrier, and appear spontaneously with a
size greater than the critical value.

Can we ever know if the Universe began at a single point, or has simply been
going on for ever? There is yet another complication, which may make the whole
question academic. It stems from an idea called inflation, first developed in
the early 1980s to solve some vexing problems with the standard big bang model.
In its earliest versions, inflation theory stipulated that, immediately after
the big bang, the Universe suddenly ballooned, increasing its diameter by more
than a trillion trillion times in just a tiny fraction of a second. After this,
the Universe switched to a non-inflationary phase, and expanded at a more sedate
rate. But in the mid-1980s, cosmologist Andrei Linde at Stanford University
realised that such a system would be self-replicating. Once you kicked it off
with a big bang, it would go on forever.

Even when most of the Universe had moved out of the inflationary phase, Linde
reasoned, tiny fluctuating regions would still be capable of undergoing
inflation. These would then go from being infinitesimal regions to sizeable
chunks of Universe in a split second, and would themselves go on to spawn new
patches of Universe and so on. In each case, once inflation was over, the patch
would evolve according to standard big bang theory.

If this is true, the whole Universe could be made up of a huge number of
expanding patches, which could be quite different from our own. The problem is
that we can never know. 鈥淲e are removed by a tremendous distance from regions
that underwent a different history,鈥� says Steinhardt. 鈥淚nflation casts a pall on
things because it makes the part of the Universe we see so infinitesimal
compared to the entire Universe, and perhaps not even representative. We will
never be able to see the edge of the patch we live in, and this puts us beyond
the ability to be able to probe things through observations.鈥�

What鈥檚 more, an eternal, self-replicating Universe may not even need a big
bang. Vilenkin says he has proved in a theorem that the inflationary Universe
must have had an origin, but Linde is skeptical. He thinks it likely but not
proved that there was an initial big bang from which all of the 鈥減retty big
bangs鈥� came. However, he adds that the question is so far removed from our
experience that it is irrelevant: 鈥淪ay you have an infinite number of bubbles,
all producing new ones. You live in one of these bubbles and you look at the
point the bubble was formed. For all practical purposes that鈥檚 the beginning of
your Universe.鈥� Because there are infinitely many such bubbles, we have no
reason to believe that ours is the first, or even the hundredth. It鈥檚 more
likely, says Linde, that our own personal big bang is actually a pretty
insignificant one, way down the line from the one that set the whole Universe
going.

Gabrielle Walker