Video Transcript
List the following events in
chronological order: a) the formation of the Sun, b) the formation of Earth, c)
the big bang, d) the emission of cosmic microwave background radiation, e) the
discovery of the cosmic microwave background.
Okay, so in this question,
we’re trying to list these events in chronological order, which basically means
in time order or the order in which they happened, starting with a very first
event to have happened and then the next one and then the next one and so
on. So to answer this question, we
can start by recalling that the universe began with the event known as the big
bang. This was the very beginning of
the universe. And so nothing can have come
before it, which means that option c will be the very first event in our
list. Now, when the big bang
occurred, the universe was extremely small and extremely hot and extremely
dense. And very quickly the universe
expanded. Essentially, the tiny hot,
dense universe exploded, and it just got larger and larger very quickly. And very soon after the big
bang, this high energy expansion resulted in the emission of electromagnetic
radiation.
Now, at this time, the
electromagnetic radiation was in the form of gamma rays and it spread out over
the entire universe, which at that time was very small still. Now, this is where the question
is a little bit sneaky. If we look at option d, the
event that this option is discussing is the emission of cosmic microwave
background radiation. Now, as we’ve really seen, very
early on in the universe, some radiation was emitted. But this radiation was not
microwave radiation. It was gamma rays. However, if we fast forward
into the future slightly all the way to the present day, here on Earth, we see
microwave radiation coming towards us from all directions in outer space. And that’s because this
microwave radiation seems to be present over the entire universe.
And in fact, this microwave
radiation is what these gamma rays became because, as the universe expanded, the
gamma rays with their very short wavelength stretched out with the universe. They became much longer
wavelength microwave radiation. And so even though the cosmic
microwave background radiation that we see today was emitted as gamma rays, that
is still what the question is referring to when it says the emission of the
cosmic microwave background radiation. In other words, shortly after
the big bang, the cosmic microwave background radiation was emitted as gamma
rays. And so option d is the second
in our list.
Anyway, so we’re now going to
have to skip back slightly to a different point in time because, at this point
in time, a gas of hydrogen is coming together, shrinking in on itself due to the
force of gravity. And eventually, nuclear fusion
is starting to occur inside the core of this ball of hydrogen gas. This ball of hydrogen gas is
our Sun. And in fact, the formation of
the Sun is the third thing in our list. Now, not very long after the
formation of the Sun, large chunks of matter that surrounded the space around
the Sun were dragged into orbit around the Sun and clumped together. This is what formed the planets
in our solar system, including Earth. And so we can say that after
the formation of the Sun, the formation of Earth is the next thing on our
list.
And finally, the last thing on
our list is the discovery of the cosmic microwave background, which, as we’ve
already said, seems to be coming from all directions in space and was emitted
not very long after the big bang as gamma radiation. But it’s been stretched out to
be microwave radiation. And the important point here is
that, in order for us to discover the cosmic microwave background, humans would
have had to exist. And they couldn’t have done so
if Earth hadn’t formed first. And that wouldn’t have happened
if the Sun hadn’t formed.
And then going back in time, we
see that the cosmic microwave background was emitted. And just before that, the big
bang occurred. And so option e is the last
item on our list. Therefore, the five options
ordered in chronological order are the big bang, followed by the emission of
cosmic radiation, followed by the formation of the Sun and then the formation of
the Earth and then, finally, the discovery of the cosmic microwave background
radiation.
