Video Transcript
In this video, we’re going to be
discussing everything that has ever happened and everything that will ever
happen. In other words, we will discuss how
our universe began and the ways in which it might end. No big deal, let’s get into it. So first things first, we can ask
the question, has the universe always existed or did it have a beginning? This is a question that scientists
have asked for a very long time. And they think they have an answer
now.
The currently accepted theory,
based on all the experimental evidence that we have, is that the universe did have a
beginning. The universe started with what we
call the big bang. The whole universe as we know it
now, with all of its billions of galaxies and trillions of stars, was all
concentrated at an extremely tiny point, as small as can be right at the
beginning. The entire universe was extremely
hot and extremely tiny and extremely dense. Not very long afterwards, a few
millions of a second afterwards, a sort of explosion occurred. The universe expanded really
fast. And as it did, the initially very
hot, very small, very dense universe became a slightly cooler, slightly larger,
slightly less dense universe.
But there was still a huge amount
of energy present within the universe at this time. And this energy was flying around
in the form of electromagnetic radiation, EM waves, lots and lots of them. And because at this point in time
the universe was still relatively small, all of the electromagnetic radiation, the
energy that had come about because of the big bang, could spread out over the entire
universe. But in addition to this, as space
itself stretched out, the electromagnetic waves that were present in the universe
also stretched out. In other words, the wavelength of
the EM waves increased where before we had short wavelength electromagnetic
radiation. And as the universe expanded, the
wavelength of this radiation got longer and longer.
This increase in wavelength of all
the electromagnetic radiation in the universe will become important to us very
shortly. But for now, let’s continue
considering what happened to the universe as it expanded. Now, at some point in the rapid
expansion of the universe, the conditions became just right for the matter to
form. Alongside all of the
electromagnetic radiation that existed already, little quarks started to form at
first, making up what was known as the quark soup. Yes, that’s genuinely what it was
called. And then as the universe expanded
and cooled even further, some of these quarks came together to form protons and
neutrons, which meant that the very first element in the universe began to exist,
Hydrogen, because hydrogen contains one proton in its nucleus.
So an individual proton simply
flying around the universe is a hydrogen nucleus. And the universe was still hot
enough that nuclear fusion could also occur. So in some cases, a couple of
protons and maybe a neutron or two even came together to form helium nuclei. But the most common element in the
universe was still hydrogen, with small amounts of helium being formed. And in very rare instances, nuclei
with three protons in them were also formed, which meant that small amounts of
lithium were also present in the early universe. And the fact that hydrogen was the
most common element present in the universe at that time is still reflected in the
composition of the universe today.
Even now, hydrogen is the most
abundant element in the universe, followed by helium. The difference is that nowadays we
see a lot more elements. And these elements were allowed to
form because as the universe expanded even further, gravity started to bring lots of
hydrogen atoms together, resulting in clouds of gas forming. And these clouds of gas eventually
collapsed even further due to gravity forming the first stars. And if these stars were large
enough, then once they’ve completed their entire life cycle over a period of
billions of years, they would explode in what was a very high energy event that’s
forming lots of heavier elements.
But anyways, so coming back to the
formation of these stars from the hydrogen gas, eventually, as the universe got even
larger, many of these stars would come together due to gravity forming galaxies. And this whole process of stars
forming and clumping together forming galaxies and some stars dying continued for a
very long period of time. Eventually, a rather special star
formed at the edge of a galaxy known as the Milky Way. That star was our Sun. And in reality, there’s nothing
special about the Sun, of course. It’s just one of many billions of
stars in the Milky Way. And there are many billions of
galaxies in the universe by this point. But we say that it’s a special star
because it’s our star. It’s special to us.
And as the Sun formed, it began to
attract bits of matter surrounding it due to its gravitational pull, until
eventually all of the planets in our solar system were formed, including Earth. Now of course, lots of planets will
have formed already and orbited different stars in different galaxies and even stars
in our own galaxy. But what we’re most interested in
is the formation of the Sun and the formation of the Earth, as well as of course the
formation of the universe itself. And so on a very basic timeline, we
can say that the universe formed first. And then lots of stars and lots of
galaxies formed. And then the Sun came into
existence, followed by the Earth forming.
But after this entire explanation,
we might ask the question, how do we know this is what happened to the universe in
the past? What evidence do we have? Well, one really important piece of
evidence that we have that suggests that the universe was smaller in the past and
has since expanded to the size that we see that it is today is the radiation that
we’ve been constantly drawing as we’ve drawn the expansion of the universe. This electromagnetic radiation, as
we’ve already mentioned, is very long wavelength at the present time. But interestingly, scientists here
on Earth can detect this radiation coming from all directions in space.
Now, at this point in time, the
radiation is microwave radiation because it has a wavelength that falls into the
microwave category. And so if you take a microwave
detector and pointed at the sky, you can detect microwave radiation coming from all
directions. It doesn’t matter which direction
in the sky you point it. And all of this radiation seems to
be very consistent in terms of its wavelength, which means that there’s very little
variation in the wavelength of this radiation. And that suggests to us that it
must have all come from the same source because radiation coming from different
sources would have a larger variation in the wavelength. But as we’ve said already, all of
the radiation that we see seems to have a very narrow wavelength range. And it seems to be coming from all
directions.
But the problem is that if this
radiation came from one source at some point in time not too far in the past, then
the radiation would need time to spread out all over the universe because that’s the
only way we can detect it to be coming from all directions. However, we can recall that
electromagnetic radiation travels at a certain speed. It travels at the speed of
light. And so it would need a very large
amount of time to spread out over the entire universe because the universe as we
know it today is absolutely huge. And it would take much longer than
what we suspect to be the current lifetime of the universe in order for the
radiation to have spread out over the entire universe.
This gives us a clue that the
universe at some point in the past may have been much smaller because that way the
radiation would have to cover much smaller distances. And still it would permeate the
entire universe. Additionally, it makes sense for a
high energy event to have given off high energy radiation, which would then stretch
as time progressed and the universe got larger because, remember, high energy
radiation is low wavelength radiation. And as the wavelength increases,
the energy of the radiation decreases which means that the microwave radiation that
we see coming from all directions today is actually quite low energy.
And specifically, this radiation is
known as the cosmic microwave background radiation, or CMBR for short. Cosmic because it’s coming from the
cosmos from outer space. Microwave because it’s currently
microwave radiation. Background because it consistently
comes with same wavelength from all directions and with a relatively consistent
intensity from all directions. And radiation because, well, it’s
electromagnetic radiation. When this microwave background
radiation was emitted, it wasn’t microwave radiation. As we said already, it was much
higher energy radiation, specifically gamma rays. And those gamma rays have since
stretched out. Their wavelengths have
increased. And it’s now microwave
radiation.
Another piece of evidence we have
to suggest that the universe in the past would have been much smaller is that the
astronomer Edwin Hubble noticed all galaxies to be moving away from each other. In other words, all of the matter
in the universe was moving away from any other form of matter in the universe. The universe was expanding. But then if the universe was
constantly expanding, if it was getting bigger and bigger, then in the past, it
makes sense for it to have bean smaller, which lines up with our interpretation of
the cosmic microwave background radiation. Which suggest that the universe
would have had to be very small in order for it to be spread out over the entire
universe whilst limited by the fact that it could only travel at the speed of
light.
And so the cosmic microwave
background radiation, combined with Hubble’s observation that the universe was
expanding, are just two of the pieces of evidence that we have that suggest that the
universe was much smaller in the past and that it began with a big bang. And so the currently accepted model
of the beginning of the universe is that it started with a big bang and then the
universe rapidly expanded. A few 100 million years after the
big bang, the conditions in the universe would have been just right for hydrogen
atoms to clump together due to the force of gravity and begin the formation of the
stars. Eventually, our Sun have
formed. And as it did, its gravitational
pull attracted bits of matter around it. And these went on to form the
planets surrounding the Sun, including our Earth.
And that rather simplified
description of the universe brings us to today. Here we are, a civilisation on a
small planet, orbiting a relatively small star on an outer arm of a galaxy that’s
one of many billions in an amazingly large universe. But what’s going to happen to the
universe in the future? We’re pretty certain that it had a
beginning. But will it have an end? Well, there are lots of ideas
flying around as to what may happen many billions of years into the future. And we shall discuss two of them
here.
The first of these hypotheses is
that the universe might end in what is known as the big rip. As we know, currently, the universe
is expanding. All the galaxies are moving away
from each other. But not only is the universe
expanding, it’s in fact expanding at a faster and faster rate. The further and further away
galaxies get from each other, the faster and faster they move apart. And this accelerating expansion is
believed to be driven by something known as dark energy. We don’t know much about dark
energy, apart from the fact that it’s suspected to be strong enough to overcome the
forces of gravity that are trying to pull galaxies together and instead is causing
galaxies to fly apart from each other.
And if the density of stuff in the
universe, if the amount of matter in the universe, is smaller than a certain
critical value, then the force exerted by the dark energy trying to move matter
apart from each other will be large enough that not only do the galaxies move away
from each other, but eventually the stars forming the galaxies also start moving
away from each other. In other words, galaxies fall
apart. And then eventually the stars
forming the galaxies are torn apart as well. And then the atoms forming the
stars are torn apart. And this is the big rip hypothesis,
which is driven by repulsive forces caused by dark energy.
So that’s one potential way for the
universe to end. Another potential hypothesis
suggests the opposite will happened. This hypothesis is known as the big
crunch. And this one will occur if there is
enough matter in the universe that the force of gravity eventually overcomes the
force of the dark energy trying to push these galaxies apart. And so what the big crunch says is
that there is enough gravitational attraction between these galaxies to eventually
overcome the expansion of the universe and come back together. And the idea is that eventually
everything will collapse back in on itself into a very small region kind of similar
to how big the universe was at the big bang.
In other words, the big crunch
hypothesis suggests that the universe started out very small and then expanded. And then eventually, it will shrink
back down to the same size again. And that seems kind of satisfying,
right? Well, the problem is that at the
moment it doesn’t look like the expansion of the universe is set to stop. In fact, as we’ve already seen, the
expansion is accelerating. The universe is expanding faster
and faster. And so it doesn’t seem like gravity
will be able to overcome this and cause a collapse of the universe. Nonetheless, the big rip and the
big crunch are still both possibilities as to how the universe may end billions of
years into the future. So this means that we’ve now
discussed both the beginning of the universe and the end of the universe. Let’s get some familiarity with
these ideas by looking at an example question.
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.
So now that we’ve looked at an
example question, let’s summarise what we’ve talked about in this lesson. Firstly, we saw that the universe
began with a big bang. It was initially very small, hot,
and dense. And then it expanded rapidly. Secondly, we saw that the universe
may end with a big rip. Dark energy driving the expansion
of the universe currently will overcome gravity and pull all matter apart. Or we saw, the universe may end
with a big crunch. Gravity will overcome the expansion
and all matter will collapse to a single point.
