EVERYTHING STARTS WITH THE BIG BANG
13.8 BILLION YEARS AGO
All ideas concerning the very early Universe are
speculative. No accelerator experiments have yet
probed energies of sufﬁcient magnitude to pro-
vide any experimental insight.
From zero to approximately 10
seconds... This is
the closest that current physics can get to the ab-
solute beginning of time. General relativity propos-
es a gravitational singularity before this time, and it
is hypothesized that the four fundamental forces
(electromagnetism, weak nuclear force, strong nu-
clear force and gravity) all have the same strength,
and are possibly even uniﬁed into one fundamen-
tal force, held together by a perfect symmetry.
At this point, the Universe spans a region of only
metres, and has a temperature of over 10
seconds to 10
seconds, the force
of gravity separates from the other fundamental
forces, and the earliest elementary particles and
antiparticles begin to be created.
The period of RAPID expansion
seconds to 10
by the separation of the strong nuclear force,
the universe undergoes an extremely rapid
exponential expansion, known as cosmic in-
ﬂation. The linear dimensions of the early Uni-
verse, during this period of a tiny fraction of a
second, increase by a factor of at least 10
around 10 centimetres.
seconds to 10
seconds, as the
strong nuclear force separates from the other
two, particle interactions create large numbers
of exotic particles, including W and Z bosons
and Higgs bosons. The Higgs ﬁeld slows par-
ticles down and confers mass on them, allow-
ing a universe made entirely out of radiation to
support things that have mass.
the universe cools
matter and antimatter
atomic nuclei can begin to form
seconds to 10
electrons and neutrinos form in large numbers
as the Universe cools off to below 10
the four fundamental forces assume their pre-
sent forms. Quarks and antiquarks annihilate
each other upon contact, but, in a process
known as ‘baryogenesis,’ a surplus of quarks
(about one for every billion pairs) survives,
which will ultimately combine to form matter.
seconds to 1 second, the tempera-
ture of the Universe cools to about 10
enough to allow quarks to combine to form
protons and neutrons. Electrons colliding with
protons fuse to form neutrons and give off
massless neutrinos, which continue to travel
freely through space today.
From 1 second to 3 minutes, after the majority
of hadrons and antihadrons annihilate each
other, leptons (electrons) and antileptons
(positrons) dominate the mass of the Universe,
and as they collide and annihilate each other,
energy in the form of photons is freed up, and
colliding photons in turn create more electron-
From 3 minutes to 20 minutes, the temperature
of the Universe falls to 10
°C, the point where
atomic nuclei can begin to form as protons and
neutrons combine through nuclear fusion to
form the nuclei of the simple elements of hy-
drogen, helium and lithium.
After about 20 minutes, the temperature and
density of the Universe has fallen to the point
where nuclear fusion cannot continue.
From 3 minutes to 240,000 years...
During this long period of gradual cooling, the
universe is ﬁlled with plasma, a hot, opaque
soup of atomic nuclei and electrons. After most
of the leptons and antileptons had annihilated
each other, the energy of the Universe is domi-
nated by photons, which continue to interact
frequently with the charged protons, electrons
RECOMBINATION // DECOUPLING
today’s cosmic microwave background (CMB) radiation
From this point onwards the physics
of the early Universe is better under-
stood, and less speculative.
From 240,000 to 300,000 years...
As the temperature of the Universe
falls to around 3,000 degrees and its
density also continues to fall, ionized
hydrogen and helium atoms capture
electrons (known as ‘recombination’),
thus neutralizing their electric charge.
With the electrons now bound to at-
oms, the Universe ﬁnally becomes
transparent to light, making this the
earliest epoch observable today. It
also releases the photons in the Uni-
verse which have up till this time been
interacting with electrons and pro-
tons in an opaque photon-baryon ﬂu-
id (known as ‘decoupling’), and these
photons (the same ones we see in to-
day’s cosmic background radiation)
can now travel freely. By the end of
this period, the universe consists of a
fog of about 75% hydrogen and 25%
helium, with just traces of lithium.
light makes its way
the early universe
The Universe is dominated by the mysterious “dark matter”
Although photons exist, the Universe at this time is literally dark, with no
stars having formed to give off light.
From 300,000 to 150 million years...
Although photons exist, the Universe at this
time is literally dark, with no stars having
formed to give off light. With only very little
diffuse matter, low energy levels and large
time scales, the Universe has tailed off dra-
matically. Little of note happens during this
period, and the Universe is dominated by
the mysterious ‘dark matter’.
From 150 million to 1 billion years...
The ﬁrst quasars form from gravita-
tional collapse, and the intense radia-
tion they emit reionizes the surround-
ing Universe, the second of two major
phase changes of hydrogen gas in
the Universe (the ﬁrst being the Re-
combination period). From this point
on, most of the Universe goes from
being neutral back to being com-
posed of ionised plasma.
From 300 - 500 million years on-
Gravity ampliﬁes slight irregularities
in the density of the primordial gas
and pockets of gas become more
and more dense, even as the Uni-
verse continues to expand rapidly.
These small, dense clouds of cosmic
gas start to collapse under their own
gravity, becoming hot enough to trig-
ger nuclear fusion reactions between
hydrogenatoms, creating the very
The ﬁrst stars are short-lived super-
massive stars, a hundred or so times
the mass of our Sun, known as Popu-
lation ‘III’ (or ‘metal-free’) stars.
From neutral black to ionized plasma
First stars and galaxies form
Population ‘II’ and ‘I’ stars also begin
to form from the material from previ-
ous rounds of star-making. Larger
stars burn out quickly and explode
in massive supernova events, their
ashes going to form subsequent gen-
erations of stars. Large volumes of
matter collapse to form galaxies and
gravitational attraction pulls galaxies
towards each other to form groups,
clusters and superclusters.
For the past 5 billion years, the expan-
sion of the Universe has been speed-
ing up, powered by the mysterious re-
pulsive force known as ‘dark energy.’
galaxy clusters form
Gravitational attraction pulls galaxies towards each other to form
groups, clusters and superclusters
VISIBLE MATTER DARK MATTER
is 13.8 billion years of age
...entire Universe vs. observable Universe...
The real size of the Universe is unknown, the region visible from Earth, the observable
Universe, is a sphere with a radius of about 46 billion light years.
The observable Universe contains 10
stars, and the approximate number of at-
oms is close to 10
The expansion of the Universe and recycling of star materials into new stars con-