The Big Bang Theory
"The evolution of the world can be compared to a display of fireworks
that has just ended; some few red wisps, ashes and smoke. Standing on a
cooled cinder, we see the slow fading of the suns, and we try to recall
the vanishing brilliance of the origin of the worlds." Lemaitre
An overwhelming weight of evidence has convinced cosmologists that the
universe came into existence at a definite moment in time, some 13 billion
years ago, in the form of a superhot, superdense fireball of energetic radiation.
This is known as the Big Bang theory. Until the arrival of the Big Bang
theory the universe was believed to be essentially eternal and unchanging,
represented by the Steady State model. The first clear hint that the universe
might change as time passes came in 1917 when Albert Einstein developed
his General Theory of Relativity. Einstein realised that his equations said
that the universe must be either expanding or contracting, but it could
not be standing still, because if it were then gravity would attract all
the galaxies towards one another. This was, at the time, a revolutionary
concept, so revolutionary that Einstein refused to believe it and introduced
his infamous 'cosmological constant' into the equations so that the sums
agreed that the universe could be static. He later claimed it was the biggest
blunder of his career. It was in 1920 that Edwin Hubble discovered that
the universe was expanding by measuring the light from distant galaxies.
This discovery was followed in 1927 by Georges Lemaitre, a Belgian astronomer,
who was the first person to produce a version of what is now known as the
Big Bang model.
It is necessary to understand that the Big Bang did not begin as a huge
explosion within the universe, the Big Bang created the universe.
A popular misconception is that it happened within the universe and that
it is expanding through it. This causes people to wonder where in the universe
it started, as if by running the clock backwards we would reach the point
where all the galaxies come together in the centre of the universe. The
universe does not have a centre, any more than the surface of a sphere has
a centre, there is no preferred place that could be termed the centre. I
know this sounds odd, it must have a centre, mustn't it? The problem we
have here is we are trying to visualise the universe in the standard 3 dimensions
that we are familiar with and therefore expect to find a centre to an expanding
sphere. The universe, however, is not an expanding 3 dimensional sphere,
it contains also the dimension of time (see 'What is Time?') and many other
dimensions as well. By way of an illustration imagine a balloon with dots
painted on the surface to represent the galaxies. If the balloon is now
inflated we can see that all the dots are moving away from one another,
just as the galaxies are in the real universe, and we can also see that
on the surface of the balloon there is no centre point from which all the
galaxies are moving away from. I am not suggesting that we are existing
on the 'outside' of an expanding bubble, only that we cannot visualise the
entire expanding universe.
Let's begin with a brief look at how the Big Bang describes the creation
and evolution of the universe before moving on to some of the evidence to
support the theory and the problems associated with the theory.
The Big Bang theory
The standard model of the Big Bang theory proposes that the universe
emerged from a singularity, at time zero, and describes all that has happened
since 0.0001 (10-4) of a second after this moment of creation. The temperature
of the universe at that time was 1,000 billion degrees Kelvin (1012) and
had a density that of nuclear matter, 1014 grams per cubic centimetre (the
density of water is 1 gram per cubic centimetre). Under these extreme conditions,
the photons of the 'background' radiation carry so much energy that they
are interchangeable with particles. Photons create pairs of particles and
antiparticles which annihilate one another to make energetic photons in
a constant interchange of energy in line with Einstein's equation E = mc2.
Because of a small asymmetry in the way the fundamental interactions work,
slightly more particles were produced than antiparticles - about one in
a billion more particles than antiparticles.
When the universe had cooled to the point that photons no longer had
the energy required to make particles, all the paired particles and antiparticles
annihilated, and the one in a billion particles left over settled down to
become stable matter.
One-hundredth of a second after time zero the temperature had fallen
90% to 100 billion K. By one-tenth of a second after time zero the temperature
was down to 30 billion K. The temperature after 13.8 seconds was down to
3 billion K, and by three minutes and two seconds had cooled to 1 billion
K, only 70 times hotter than the centre of the Sun today. At this temperature
nuclei of deuterium and helium could be formed and stick together despite
collisions with other particles.
During the fourth minute after time zero reactions took place that locked
up the remaining neutrons in helium nuclei, as described by Gammow et
al in 1940 and Fred Hoyle and others in the 1960's. This epoch ended
with just under 25% of the nuclear material converted into helium, and the
rest left behind as lone protons - hydrogen nuclei.
By just over 30 minutes after time zero, all of the positrons had annihilated
with almost all of the electrons - with again one in a billion left over
- to produce the background radiation proper, and the temperature had dropped
to 300 million K, and the density was only 10% of that of water. At this
temperature stable atoms were still not able to form.
The interactions between electrons and photons continued for 300,000
years, until the universe had cooled to 6000 K, roughly the temperature
of the surface of the Sun, and the photons were becoming too weak to knock
electrons off atoms.
Over the next 500,000 years the background radiation decoupled, and had
no more significant interaction with matter. The Big Bang was in effect
over, and the universe left to expand and cool. About 1 million years after
time zero, stars and galaxies could begin to form. Nucleosynthesis inside
stars convert hydrogen and helium to make heavier elements, eventually giving
rise to our Sun, the Earth and ourselves.
This is only a very brief overview of the main points describing the
evolution of the universe, a number of books have been published that describe
just the first four minutes or less!
So how does it all stack up? How much evidence do we actually have to
support the Big Bang model of the universe?
Einstein's Theory of Relativity.
This is a theory of spacetime, offering a complete mathematical description
of the universe. Relativity, along with Quantum Mechanics, (see "What
is Quantum Mechanics?") is considered to be the most complete and accurate
theory ever devised, mathematically describing such diverse phenomenon as
the constant speed of light and the formation of black holes. Einstein's
equations tell us - apart from many other things - that the universe is
expanding, and that by going back in time there must have been a time when
all the galaxies were very close together. And further back when all the
stars must have been touching each another, merging to make one great fireball
as hot as the inside of a star at 15 million degrees Kelvin (Kelvin is absolute
zero temperature). Einstein's equations actually go further back than that,
to a time when all the matter and energy of the universe emerged from a
single point of zero size, a singularity. This is how the Big Bang theory
describes the birth of the universe.
Expansion of the universe.
One of the reasons the universe is believed to be expanding is because of
the phenomenon known as 'red shift'. Light, or other electromagnetic radiation
from an astronomical object may be stretched, (due to a number of reasons)
making its wavelength longer. Because red light has a longer wavelength
than blue light, the effect of this stretching on features in the optical
spectrum is to move them towards the red end of the spectrum. If then the
optical spectrum of a distant galaxy shows features that are shifted towards
the red end of the spectrum (red shifted), it can be due to one or more
of the following three reasons:
1) Motion. The galaxy is moving away from us, this is known as the Doppler
effect. The same effect can be detected in sound. When a police car is speeding
towards us the sound waves made by its siren are 'squashed' and the pitch
sounds higher. As it passes us and starts to move away the sound waves are
'stretched' and the pitch sounds lower. In the 1920's Edwin Hubble observed
that all galaxies (apart from a few local ones attracted towards our own
and showing blueshift) show red shift. This indicates that the galaxies
are all flying away from us, as in a Big Bang explosion.
2) Expansion of the universe. Einstein's famous equations show that the
universe should be expanding, not because the galaxies were moving through
space, but because the 'empty' space between them (spacetime) is expanding.
This cosmological redshift results because the light from the distant galaxies
is stretched by the amount that space expands while the light is en route
to us. This also reveals that the Earth is not at the centre of the universe
with all the galaxies moving away from us, but that due to the expansion
of the universe, all the galaxies are moving away from each other, like
painted dots on a balloon moving apart as it is inflated.
3) Gravity. This is also explained by Einstein's general theory. Light
moving outwards from a star is moving 'uphill' in the star's gravitational
field, and loses energy as a result. Because light cannot slow down - it
always travels at the same speed - when it loses energy its wavelength increases,
in other words, it is redshifted. It does however, require a very powerful
gravitational field for this effect to be measurable, such as created by
a white dwarf star.
All three kinds of redshift can be at work at the same time. If we had
telescopes sensitive enough to see light from a white dwarf star in a distant
galaxy, the overall redshift in that light would be due to a combination
of Doppler, cosmological and gravitational redshifts.
The fact that we can measure redshift in the light from distant galaxies
tells us that the galaxies are receding from us, and from each other. It
only takes a little logical deduction to conclude that as they are now all
receding from one another, then at some finite point in the past (believed
to be around 13 billion years or so) they must have all been at the same
Microwave Background Radiation. The universe is filled with a sea of radiation
at a temperature of just over 2.7 degrees Kelvin, detectable at microwave
radio frequencies both by Earth based radio telescopes and by instruments
onboard artificial satellites. This is interpreted as direct evidence of
the Big Bang fireball in which the universe was born, being the remnant
of the superhot radiation from the fireball that has cooled down as the
universe expanded. The discovery of the background radiation is therefore
the most important observation made in cosmology since the discovery by
Edwin Hubble that the universe was expanding. The existence of the background
radiation, and its temperature, was accurately predicted by the Big Bang
theory. When it was later discovered, by chance as it happens, this was
yet another confirmation of the theory.
Nucleosynthesis of the light elements.
As the universe expanded and cooled, so the process of matter building
began that led to the formation of stars, planets, galaxies etc. The process
began with the simplest element, hydrogen, then helium, and then eventually
onto more complex elements. The observed abundance of hydrogen, the simplest
element and the most common in the universe, followed by helium, is yet
further confirmation of the Big Bang theory.
The study of stars reveals how their internal nuclear interactions cause
simple atoms, such as hydrogen and helium, to create more complex elements.
Stars are in fact gigantic matter producing factories, converting hydrogen
and helium into carbon and heavier elements by the process of nucleosynthesis
deep within their interiors. If stars did not exist, we would not be here,
our own atoms that make us, were formed by the stars, and by the supernovas
at the death of certain size stars. All as described by the Big Bang theory.
Formation of galaxies and large-scale structure
the Big Bang model provides a framework in which to understand the collapse
of matter to form galaxies and other large-scale structures observed in
the universe today. At about 10,000 years after the Big Bang, the temperature
had fallen to such an extent that the average density of the universe began
to be dominated by massive particles, rather than by light and other radiation.
This change meant that the gravitational forces between the particles could
begin to take effect, so that any small perturbations in their density would
grow. These small perturbations led to the formation of galaxies.
These are the principle observed phenomenon that go to support the Big
Bang theory. Is it proof enough? Do we have a universe that was born out
of a singularity, that is expanding and cooling, and is therefore finite
in both age and size? See Can anything
'real' be infinite? for an explanation of a finite universe.
What do I think?
The Big Bang theory clearly still has a long way to go in order to be
able to explain the origin of the universe. That is not to say that the
theory as it stands is in any way wrong.
The staggering amount of evidence in support of the Big Bang theory is
simply overwhelming. So much so that the theory simply cannot now be overturned.
What is known to agree with the theory today cannot be changed tomorrow,
by any theory, to make it disagree.
The situation is such that any new theory, far from displacing the Big
Bang, would have to incorporate it. In other words, it can only be improved
upon in much the same way that Einstein incorporated Newton's theory of
gravity into his own theory of relativity. Relativity did not overthrow
Newton's theory, it incorporated and developed it.
I realise that some people do not support the Big Bang theory, and a
few have contacted me to inform me of this. Okay, agreed, it is 'only a
theory.' It is also 'only a theory' that the Earth orbits the Sun, and not
the other way around. But eventually, the mass of accumulated evidence for
'only a theory' becomes so powerful and persuasive it becomes impossible
to ignore, whether or not you happen to like it.
For more information on the Big Bang and related topics, the following
is an excellent site: In
POLL & MESSAGE BOARD
Is the Big Bang theory correct?
In a nut shell, here are the reasons why the Big Bang theory is believed
to be correct. Taken from Ask
the Space Scientist
1.... The universe is expanding.
2.... There exists a cosmic background radiation field detectable at
3.... The cosmic microwave background field is measurably isotropic to
better than a few parts in 100,000 after compensation is made for the relativistic
Doppler effect caused by Earth/Sun/Milky Way motion.
4.... The cosmic microwave background radiation field is precisely that
of a black body.
5.... The cosmic microwave background radiation field has a temperature
of 2.7 K.
6.... There does exist a universal abundance ratio of helium to hydrogen
consistent with the current expansion rate and cosmic background temperature.
7.... The cosmological abundance of deuterium relative to hydrogen and
helium is consistent with the levels expected given the current expansion
rate and density.
8.... There are only three families of neutrinos.
9.... The night sky is not as bright as the surface of the Sun.
10... The cosmic background radiation field is slightly lumpy at a level
of one part in 100,000 to 1,000,000.
11... There are no objects that have ages indisputably greater than the
expansion age of the universe.
12... There are about 10,000,000,000 photons in the cosmic background
radiation field for every proton and neutron of matter.
13... The degree of galaxy clustering observed is consistent with an
expanding universe with a finite age less than 20 billion years.
14... There are no elements heavier than lithium which have a universal
15... The universe was once opaque to its own radiation.
16... The universe is now dominated exclusively by matter and not a mixture
of matter and anti-matter.
I should point out that, although some rival theories have proposed alternate
explanations to a few of these observations, there are no rivals that provide
a simple explanation for ALL of these remarkable observations. What could
be simpler than a universe expanding from a hot, dense state to the present
cool, rarified one with all of the above features emerging! No new physics
I think that is pretty convincing, don't you?
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