Almost all astronomers agree on the theory
of the Big Bang, that the entire Universe
is spreading apart, with distant galaxies
speeding away from us in all directions.
Run the clock backwards to 13.8 billion years
ago, and everything in the Cosmos started
out as a single point in space.
In an instant, everything expanded outward
from that location, forming the energy, atoms
and eventually the stars and galaxies we see
today.
But to call this concept merely a theory is
to misjudge the overwhelming amount of evidence.
There are separate lines of evidence, each
of which independently points towards this
as the origin story for our Universe.
The first came with the amazing discovery
that almost all galaxies are moving away from
us.
In 1912, Vesto Slipher calculated the speed
and direction of "spiral nebulae" by measuring
the change in the wavelengths of light coming
from them.
He realized that most of them were moving
away from us.
We now know these objects are galaxies, but
a century ago astronomers thought these vast
collections of stars might actually be within
the Milky Way.
In 1924, Edwin Hubble figured out that these
galaxies are actually outside the Milky Way.
He observed a special type of variable star
that has a direct relationship between its
energy output and the time it takes to pulse
in brightness.
By finding these variable stars in other galaxies,
he was able to calculate how far away they
were.
Hubble discovered that all these galaxies
are outside our own Milky Way, millions of
light-years away.
So, if these galaxies are far, far away, and
moving quickly away from us, this suggests
that the entire Universe must have been located
in a single point billions of years ago.
The second line of evidence came from the
abundance of elements we see around us.
In the earliest moments after the Big Bang,
there was nothing more than hydrogen compressed
into a tiny volume, with crazy high heat and
pressure.
The entire Universe was acting like the core
of a star, fusing hydrogen into helium and
other elements.
This is known as Big Bang Nucleosynthesis.
As astronomers look out into the Universe
and measure the ratios of hydrogen, helium
and other trace elements, they exactly match
what you would expect to find if the entire
Universe was once a really big star.
Line of evidence number 3: cosmic microwave
background radiation.
In the 1960s, Arno Penzias and Robert Wilson
were experimenting with a 6-meter radio telescope,
and discovered a background radio emission
that was coming from every direction in the
sky - day or night.
From what they could tell, the entire sky
measured a few degrees above absolute zero.
Theories predicted that after a Big Bang,
there would have been a tremendous release
of radiation.
And now, billions of years later, this radiation
would be moving so fast away from us that
the wavelength of this radiation would have
been shifted from visible light to the microwave
background radiation we see today.
The final line of evidence is the formation
of galaxies and the large scale structure
of the cosmos.
About 10,000 years after the Big Bang, the
Universe cooled to the point that the gravitational
attraction of matter was the dominant form
of energy density in the Universe.
This mass was able to collect together into
the first stars, galaxies and eventually the
large scale structures we see across the Universe
today.
These are known as the 4 pillars of the Big
Bang Theory.
Four independent lines of evidence that build
up one of the most influential and well-supported
theories in all of cosmology.
But there are more lines of evidence.
There are fluctuations in the cosmic microwave
background radiation, we don't see any stars
older than 13.8 billion years, the discoveries
of dark matter and dark energy, along with
how the light curves from distant supernovae.
So, even though it's a theory, we should regard
it the same way that we regard gravity, evolution
and general relativity.
We have a pretty good idea of what's going
on, and we've come up with a good way to understand
and explain it.
As time progresses we'll come up with more
inventive experiments to throw at.
We'll refine our understanding and the theory
that goes along with it.
Most importantly, we can have confidence when
talking about what we know about the early
stages of our magnificent Universe and why
we understand it to 
be true.
