Lesson Video: The Big Bang and the Fate of the Universe Physics

In this video, we will learn how to describe the theories for how the universe began and the different eventualities of the universe.


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.

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