The pictures above show the early Universe, in which luminous matter can be seen coalescing into huge
filaments and sheets, each tens of millions of light years across! These are the early galactic clusters
containing proto-galaxies, in the process of forming stars. The Universe at this stage was about one billion
years old. The lower picture is a closer view - note the
filaments and sheets of galaxy clusters and the voids.

When one looks out across space at the distant stars and galaxies,
one is looking back in time. This is because
of the finite (though enormous) speed of light (about 300 000 kilometres per second). The nearest star to the
Sun is a
red dwarf (of the flare star type) called Proxima Centauri, which is 4.22 light years from the Earth,
meaning that it takes light 4.22 years to travel the enormous distance from Proxima Centauri to an observer on
Earth and, therefore, the observer is seeing the star as it was 4.22 years ago. Galaxies have been seen as far
as 13.23 billion light years away from the Earth, so an observer on Earth (with a very powerful telescope!) sees
this galaxy as it was 13.23 billion years ago! This proves that the Universe is very old!

The Universe is made up of galaxies. Galaxies are enormous clusters of stars, typically each galaxy contains
100 billion (10^11) stars. The Sun is part of the Milky Way Galaxy, which is a spiral disc about 100 000 light
years in diameter and containing 100 billion stars, including the Sun. There are billions of known galaxies and
there are possibly an infinite number of galaxies in the Universe! (We can never observe them all, because we
can never observe further in light years than the Universe is old in years). The age of the Universe is estimated
(from observations) to be just under 14 billion years old.

Galaxies occur in
clusters containing tens to hundreds of galaxies. Your Milk Way Galaxy is one of a cluster,
called the Local Group, of about 30 galaxies. Clusters are typically about 10 million light years in diameter (but
vary tremendously) and the average distance between adjacent galaxy clusters is about 100 million light years.
Thus, on a gigantic scale, the Universe can be thought of as a 'gas' of particles, where each particle is a cluster
of galaxies! Clusters tend to associate loosely, forming filaments and walls of galaxy clusters, with vast voids
in-between, forming a kind of 'honeycomb' structure (as seen in the picture above).

However, the luminous matter(such as stars and nebulae) that make up the clusters only accounts for about
10% of the mass of the Universe, the rest is cold and non-luminous and so is called dark matter. We know it
exists because observations of the motions of stars in galaxies, as they revolve around the centre of their own
galaxy, shows the presence of this matter as its gravity effects the motions of the stars. (Dark matter should
really be called dark energy, since not all of it is mass, though most of it appears to be mass - it is energy that
generates a gravitational field, including energy locked up as mass. In the Universe today, most of the energy
appears to be in the form of mass, hence the term 'dark matter', while the term 'dark energy' has come to mean
something entirely different!).

When we look at these galaxy clusters, we notice something strange - they are
moving away from us. No matter
where you are in the Universe, the galaxy clusters around you are moving away from you. Extrapolating back in
time, one reaches the point where the galaxies merge together, and going back further still one ends up with all
the energy and mass in the Universe, compressed to a very high density. Observations also suggest that the
Universe is
open (or very nearly so) meaning that is has no maximum size and so will expand forever. A closed
, is one in which space is finite, but expanding, and in which space expands until a maximum size is
reached and then the galaxy clusters start to move closer together as space contracts, until the galaxies all
implode together into a
Big Crunch. Such a Big Crunch could lead to a Big Bounce in which the energy and
matter rebound, causing the Universe to expand again. Appealing to our natural instincts as this cyclical closed
Universe may be, it does appear that our Universe is open (though this has not been definitely settled). An
open universe has a definite beginning and end, or does it?

Note that a closed Universe is one in which space is globally curved, such that if you set out in one direction, in
a straight line, across the Universe, near to the speed of light, then you would eventually arrive back where you
It is the presence of energy that curves space and time - this is the essence of gravity. The Earth orbits
the Sun because it is bound by the Sun's strong gravitational field (the Sun has a lot of energy!). In fact, the
Earth would be moving along a straight line, were it not for the fact that the Sun's energy has curved space
around itself!! This curvature of space (and time) is the effect we call gravity! All energy, not just that in the
form of mass, generates a gravitational field (including gravity itself!). The curvature of spacetime caused by
the Sun is local, it has very little effect far from the Sun. However, if the density of energy in the Universe is
sufficiently high, then spacetime will be curved globally and may then curve around back on itself, rather like a
bubble, whereas an open Universe may be flat, as ours appears to be. Note that space is not curved in any
sense that we can ordinarily see. Rather, the three dimensions of space curve in the 4th (pseudo)spatial
dimension, rather like a two dimensional surface may curve around in three spatial dimensions to form the
surface of a sphere.

So, if our universe is expanding, and is open (and more or less flat), then how did it begin? The answer is not
known for certain, however, the movement of the galaxies away from one another does suggest that they are
moving away from some past time when they were closer together, indeed observations verify that the galaxies
in the early Universe were much closer together than they are today. Continuing back we arrive at a time when
all the energy of the Universe existed as a very hot and very dense mass of energy. This energy clearly got
propelled outwards, expanding and cooling as it did so. Once it cooled sufficiently, atoms could form, and then
eventually stars and galaxies. This outward 'explosion' of this primordial energy is called the
Big Bang. The
exact nature of the Big Bang is still uncertain, but we can see the 'glow' left over by it. This afterglow heats
space to the predicted 3 degrees Kelvin (it is cold microwave radiation and is called the
cosmic microwave
background radiation
). Furthermore, the Big Bang model accurately predicts the abundance of elements (such
as hydrogen, helium and lithium) that we see in the Universe today. Thus, all in all, there is very good empirical
evidence for the Big Bang - evidence for which there appears to be no alternative scientific explanation.

When did the Big Bang occur?

When we say that the Universe is about 13.8 billion years old, we really mean that the Big Bang occurred 13.8
billion years ago. Some argue that this moment was when time began, in which case nothing existed before the
Big Bang and nothing caused the Big Bang (as there was no time for the cause to act within). However, there
appears to be a finite division of time, which is the smallest meaningful unit of time, and is called the
. The Planck time is only about 5.39 x 10^-44 seconds 9or 0.000 000 000 000 000 000 000 000 000 000
000 000 000 000 0539 seconds!). It makes no real sense to talk about a smaller (or earlier) time as time begins
to fluctuate chaotically on this small scale, so time may not have had a definite beginning, but may simply 'melt
away' if we go back far enough! Indeed, it makes no sense to us to even talk of how long the Universe was in
this chaotic state before the Big Bang. What really happened is still beyond our science to ascertain at present,
but progress is being made all the time. Some models have a recurrent Big Bang, repeatedly sending out a
tremendous burst of matter forming energy into space. These models invoke
higher spatial dimensions (there is
reason to believe that there are 10 or 11 dimensions of space and one of time, although we can only see three
of the spatial dimensions and one of time).

Where did the Big Bang take place?

If the Big Bang took place at a certain location in space, then we are talking about matter and energy exploding
into a prior existing and (presumably) empty space. This would give the Universe a
geometric centre,
somewhere close to the point where the Big Bang occurred. There are models that have the Big Bang
occurring inside a black hole (with the black hole existing in the far future at the end of time, which does not
violate causality since no signal can cross a black hole and emerge, as far as we know!) or rather a white hole
(in some theories, black holes suck matter in and spew it out into another time and space via a white hole).
However, it is hard to reconcile the idea of the Universe having a centre with the fact that the galaxies move
away from any point in space, no matter where you are in the Universe, and also that the radiation emitted by
the Big Bang can be seen in all directions. This leads most cosmologists to regard the Big Bang as not
occurring in a pre-existing space, but rather it was the beginning of space and time, such that
space itself is
, in which case the Universe has no geometric centre, since the Big Bang occurred everywhere when
the Universe was much smaller than it is today. Within galaxies, the local curvature of spacetime by the high
concentrations of energy prevent space expanding on this local scale, but space between the galactic clusters,
in the near-empty voids, is expanding! Thus, you are not getting bigger, but galactic clusters are getting further
away from one another!

How big was the Universe in the very beginning?

Since space came into existence at the moment of the big Bang (or like time was chaotically fluctuating!) the
early Universe must have been as small as the smallest possible length, which is the
Planck length (a mere
1.616 x 10^-35 metres, or 0.000 000 000 000 000 000 000 000 000 000 000 01616 metres!). It makes no
sense to talk of anything smaller than the Planck length - space breaks down at this scale, much as time breaks
down in the Planck time. Thus, the Universe may have been one Planck length in diameter! The reason why all
the immense energy of the Universe confined in this small volume, reaching an infinite density, did not collapse
into a black hole, was because space was rapidly expanding! (This really does sound like a white hole!).
Personally, I cannot reconcile the fact that the Universe could expand from such a small finite size into an
infinite extent in a finite time at a finite speed (less than the speed of light?) - something seems wrong with this
theory somewhere! Perhaps the Universe is finite, or perhaps it was still infinite in extent (though much smaller
than it is today) at the time of the Big Bang! (If anyone knows the answer to this then please email Bot at Ok, I think I have thought of the answer - when we speak of the minute size of
the early Universe we really mean the 'observable Universe' - that is the part we see today. This region was
very tiny indeed, but the whole may still have had an infinite extent.

A Summary of Evidence for the Big Bang

1. Recession of the galaxies (galaxy clusters): we see galaxies speeding away from us in all directions and the
further away these galaxies are the faster they are moving. Since there is nothing unique about our own galaxy,
this implies that any observer anywhere in space would see the same phenomenon, as if the galaxies were
speeding away from his own galaxy. The implication is that space is expanding and all the galaxy clusters are
moving further apart.

The cosmic microwave background radiation: this radiation is very evenly dispersed throughout the whole of
space and is cold but nevertheless warmer than the predicted background radiation in the absence of a Big
Bang. The Big Bang predicts the formation of this radiation at the temperature we detect it at (3K). Only the
rapid expansion of spacetime shortly after the Big Bang can explain the uniformness (
homogeneity) of this
background radiation - it is essentially the same wherever one looks (apart from very slight but important
fluctuations or
inhomogeneities). Only the continued expansion of space apparently explains why this radiation
is so cold.

The Big Bang model accurately predicts the abundance of (light) elements (nuclear abundances) that we
see in the Universe today. Heavier elements were since manufactured by stars as stars reprocess the lighter
elements formed by the Big Bang (e.g. hydrogen, helium and lithium) into heavier ones. The contribution of
stars to the fine adjustments of present-day elemental abundances is, admittedly, difficult to predict.

Currently no other scientific theory can explain all of these phenomena. Theories only differ in the fine details
and such questions as the frequency of big bangs - was there only one or do they occur repeatedly, or are
there many different universes each formed by a big bang?

How it all Happened

According to current models...

The beginning

To begin with the Universe was incredibly dense and incredibly hot. Putting aside the problems of defining time
in the very first few moments when the Universe had the Planck density (and was at a very high temperature of
10^31 K, called the Planck temperature) we shall set the point of time at which time became a reasonably
ordered phenomenon as the beginning of time (t = 0). This is the point at which the Universe fell below the
Planck temperature and space as we understand it also came into being. Current theories break down at the
Planck temperature and Planck density, but once our defined origin of time is reached, then our theories begin
to kick-in. If this sounds confusing, then that is because nobody understands these very few moments when
time and space may have emerged from some undefinable 'pre-existing' chaotic state. (The problem is how can
anything pre-exist before time began, but as mentioned above, time seems to gradually melt away rather than
having a precise start).

The first microsecond (millionth of a second)

In the first 10^-36 of a second (0.000 000 000 000 000 000 000 000 000 000 000 001 s) the material of the
Universe was so dense that particles frequently collided with one another, ensuring an
even mixing of heat and
other energy
, so that the observable Universe (the bit we can see today, which is most likely not all of it!), at
this stage less than 10^-31 metres across, was slowly expanding and had an even temperature and density.
This stage is called
thermalisation. This continued until the Universe cooled, as it expanded (distributing the
heat energy throughout a larger volume lowers the temperature which is a measure of heat energy density) to
10^27 K.

Symmetry Breaking and Inflation

At this point the Universe was cool enough for a process called spontaneous symmetry breaking to occur. Prior
to this point, everything was so hot and energetic that all the particles behaved the same. Once symmetry was
broken, however, the
strong, weak and electromagnetic forces became distinguishable from one another (the
strong and weak forces govern certain aspects of sub-atomic particle behaviour, whilst the electromagnetic
force accounts for electricity and magnetism). Initially the strong became distinguishable from the
(the weak and electromagnetic forces still indistinguishable and so called the electroweak force) but later
(at 10^-12 seconds) the electromagnetic and weak forces became distinguishable after further symmetry
breaking. The onset of symmetry breaking is thought to have initiated a period of very rapid expansion, called
inflation. During inflation the observable Universe expanded from 10^-31 metres to 0.1 metre (10 centimetres!)
at a time from 10^-36 seconds to 10^-30 seconds.

Why this rapid burst of inflation?

If we point our telescopes far into the sky we can see galaxies existing very far away near to the beginning of
time, when the Universe was only half its present size. However, pointing in the opposite direction our
telescopes see the same numbers of galaxies just as far away on the other side. Additionally the cosmic
background microwave radiation is also similar in opposite directions. However, the light from these very distant
galaxies has only just reached us as we see them as they were billions of years ago. The Universe has not
existed for long enough to allow light from these distant galaxies we see in one direction, to reach those distant
galaxies we see in the opposite direction. If this has always been the case, then how did these two separate
regions of space become so similar in density of galaxies, if they had never been in contact with one another.
(Remember no signals can travel faster than light without violating
causality. By causality I mean that if event A
caused event B, then event A must always occur in the past of event B, unless we start reversing time!).
Inflation is necessary to explain the homogeneity of the Universe. Additionally, for the cosmic microwave
background to be so uniform over such a large region of space, it seems reasonable to suppose that these
regions of space were once connected such that they could exchange heat energy and light with one another.
Indeed energy must have been able to travel more or less across the entire region to even out the temperature.

Inflation explains the sameness or homogeneity of space in every direction. Since inflation involves a very rapid
expansion of space, it means that if inflation occurred for long enough, then a very tiny region of space will
have been expanded into a very large region. Prior to inflation, the observable Universe was small enough for
light to cross it from one end to the other, but after inflation the Universe became too big for light to have
crossed it, as it is today. Note that for inflation to work, spacetime must expand much faster than the speed of
light at this time.

The formation of matter as we know it

When the Universe was one microsecond (one millionth of a second) old, it had cooled to 10^13 K, which was
cool enough to allow particles called
quarks to combine to form protons and neutrons. A detailed analysis
predicts the final ratio to be 87% protons to 13% neutrons. This ratio is important when predicting the
abundance of the first elements that formed, since protons and neutrons are constituents of the atomic
nucleus. However, only after about the first minute when the Universe cooled to about 10^10 K was it cool
enough for the protons and neutrons to come together (they were fused together by
nuclear fusion) to form
atomic nuclei (with the protons and neutrons bonded together by the strong force) without collisions smashing
them apart. (One must understand that hotter particles move faster and so collide harder. Today we use huge
particle accelerators to accelerate particles to the tremendous speeds needed to smash nuclei into protons and
neutrons - nuclei are very strong indeed, but protons and neutrons are even harder and no particle accelerator
has yet managed to smash them into quarks). When the Universe was ten minutes old, it had finished
synthesising the lighter elements, such as hydrogen, helium, deuterium (heavy hydrogen) and lithium and
nuclear fusion fizzled out. The observable Universe continued to expand but was still only a tiny fraction of its
current size.

Radiation dominates matter

From the first hour until the Universe was about 10 000 years old, although it contained matter, most of its
energy was still in the form of energy (electromagnetic radiation). This is the opposite of what we see today,
where most of the Universe's energy is locked up as mass in stars and planets. However, as the Universe
continued to expand, the radiation diluted out faster than the matter and when the Universe was older than 10
000 years, matter came to dominate radiation - in that most of the energy was locked up as matter. The
Universe became a plasma (a hot gas of electrically charged particles - electrons and nuclei) and remained like
this for the next 300 000 years or so. A plasma as dense as this is
opaque - so light could not travel very far at

The formation of atoms

At an age of 300 000 years, the Universe had cooled enough to allow atoms to form without them being easily
smashed apart. Atoms formed as electrons and nuclei came together. This removed the free electrons as the
plasma became a gas of atoms. The free electrons were responsible for making the plasma opaque - free
electrons obstruct light and scatter it easily. Thus, when atoms formed, the Universe became
transparent, as it
is today. This allowed radiation to travel unimpeded and gave rise to the cosmic background radiation that we
see today. This is as far back as we can see in time, as our telescopes look across space, because before this
the Universe was opaque. As soon as it became transparent, the cosmic background radiation became visible.
This radiation continued to cool, reaching the cold 3K that it is at today. (We talk about the moment when the
Universe became transparent as the
decoupling of radiation and matter, meaning that most of the radiation
could travel far before being impeded by matter). At the time of decoupling, the Universe was 1200 times
smaller than it is today. This is already too large to allow enough time for light (and heat) to spread from one
end to the other, and yet the cosmic microwave background radiation gives a remarkable uniform temperature
(suggesting that heat was transported from one end of the Universe to the other at some point). This can only
be explained by inflation - the Universe was at one time much smaller, allowing heat and light to cross from one
end to the other and so even out the temperature, later on spacetime expanded faster than the speed of light
for a time, before slowing down again to give the Universe we see today, which is now too large and too young
for light to have crossed its diameter. (The light got left behind when spacetime expanded during inflation).

The formation of galaxies, stars and planets

After 10 million years, the clouds of atoms were cool enough to contract under their own gravity (they collapses
under their own weight!). As they collapsed into denser objects, they converted gravitational energy into heat
and they became hot dense objects - the first stars were born! Smaller fragments became proto-planets and
slowly cooled into planets proper, whilst the stars were hot enough to maintain their temperature by nuclear
fusion reactions in their cores - the stars shined as the planets cooled. Stars do not live forever, and stars died
as new ones were born. Indeed the remains of dead stars that get blasted across space form clouds of gas that
can condense into new stars, so generations of stars lived out their lives. Stars manufacture heavier elements,
such as carbon and oxygen, from the lighter elements present from the Big Bang (hydrogen and helium).
These heavier elements are essential to life as we know it, so only after generations of stars had enriched the
material of the Universe could life evolve! It is still not entirely clear how the stars came to be grouped into
galaxies, though it is assumed that the galaxies formed first or at least simultaneously with the first stars.

Star birth and death continue in galaxies, such as the Milky Way, to this day.
The Universe