Every proper scientific theory must have evidence to support
it. In science-speak, "theory" actually means "an
established framework" and is not nearly as flimsy a thing as
the word implies. A theory is not just any crazy idea:
it must meet certain criteria. If the idea can be
tested, it is formed into an hypothesis. An hypothesis
graduates to Theory status only when 1) there is
substantial evidence to back it up, 2) when it demonstrates
predictive ability, and 3) after any possible evidence
against it has been searched for but not found. Finally,
every possible alternative has to be thoroughly explored
before the new theory will be widely accepted.
And then there's the unimaginative name. "The Big
Bang" was originally coined as a term of derision by people
who didn't think it would ever make it as a theory. But
as the evidence for it mounted, the name stuck. I am a
fan of cartoonist Bill Waterson's improved name for this
theory, "The Tremendous Space Kablooie," but I am
in the minority on this point.
For something as impossible-sounding as the Big Bang
Theory, there'd better be some darn good evidence. Are
you ready to find out what it is?
None of us were there,
so what makes us think there might have been a Big Bang?
The first clue is all around us: Matter. We now
know matter is composed of large numbers of just three
basic parts: protons, neutrons and electrons. (Please Visit
my blog for an explanation of the structure of
The next logical step is to figure out where those pieces
came from. Electrons, for example, are formed when a
particle of light in the high x-ray range or beyond
suddenly disappears, leaving an electron and its arch-nemesis,
the anti-electron, behind. Like being born with an evil
How do we know this? By experiment. Anyone can
do it - all you need is a source of x-rays and a way of
recording what happens next. This DIY cloud
chamber can help. But since x-rays can sometimes be
dangerous to your health, best leave it to the pros.
Electrons and anti-electrons usually destroy each other and
turn back into a flash of light. However, a tiny
fraction of anti-electrons are "defective" in some way that
we're still trying to discover (much like a certain brand of
import car). A miniscule percentage of them break down,
leaving single electrons free to exist unmolested by their
evil twins. Those lucky ones survive to this day and
comprise everything we know in and around us. For more
information about this enthralling mystery of modern Science I
recommend the book The
Mystery of the Missing Antimatter by Helen Quinn and Yossi Nir.
For so many electrons to exist in such numbers, the
universe must have been very hot, dense and full of
x-rays at some time. Apparently it didn't stay that way
because, well, here we are. The conditions favouring
electron production are obviously not prevalent in the
universe we know today. One might ask, "Why was it so
before, and why isn't it so now?" This provides us with
the first inkling that we inhabit an ever-changing universe.
The next clue was stumbled upon by investigators doing
spectrophotometric chemical analysis of distant galaxies.
(Yes, amazingly, there are people who do that sort of
thing. Why? Because they're scientists.)
It was noticed that the wavelengths of light coming from
distant galaxies were always longer than they ought to be, and
by a fairly predictable amount. The further away a
galaxy was, the more stretched out the wavelengths were.
Finally someone correctly deduced that this was an
intergalactic example of the Doppler Effect. A train's
horn seems to have a higher pitch when it is coming towards
you, and a lower pitch when receding from you, even though
nothing about the horn actually changes as it speeds
The inescapable conclusion was that all galaxies are
spreading out from each other in an expanding space. And
if that is happening now, they must have been closer
together in the past. In fact, there must have been
a "time zero" when the universe was infinitely dense and hot;
hot enough to cook up billions of galaxies-worth of matter.
From their observations, Astronomers have worked out when
that "time zero" was: 13.8 billion years ago.
But where's the smoking gun? If such an event really
took place, it must have left some trace or imprint, some
detectable feature on the universe, some afterglow
or reverberation. If the Big Bang consisted of a massive
flash of light, where is that light today? Shouldn't we
still be able to see it way off in the distance?
Actually, we can. The theory predicts that this
afterglow should show up in the microwave part of the
spectrum. When we tune in with microwave antennas,
there is a faint background hiss coming from all
directions that never goes away. To someone
with "microwave eyes" the Big Bang can still be seen in the
sky surrounding us, and it looks like this:
Want to know more about the Big Bang? I'd like to
personally recommend to you one of my favorite books, The
First Three Minutes: A Modern View Of The Origin Of The
Universe by Steven Weinberg. I first read it in
1989, and the book has only become better with the continued
expansion of the universe.