Lesson video

Hello, my name's Dr.

George.

This lesson is called The Changing Universe, and we'll be looking at how the universe began.

The lesson is part of the unit, gravity in space.

The outcome of the lesson is I can describe the Big Bang theory and evidence that supports it.

These are the keywords to the lesson, which I'll explain as they come up.

Come back to this slide anytime if you want to remind yourself of the meanings.

The lesson has three parts.

They're called moving galaxies, the Big Bang theory and understanding the universe.

You may note that when an object that emits light is moving away from us, that light gets redshifted.

We can see that when we split the light from stars or galaxies into a spectrum.

Within the spectrum of stars and galaxies, there are dark lines called absorption lines.

When a galaxy is moving away from us, its light is redshifted, and we can see that by the change in position of these absorption lines.

Here's the spectrum for a relatively slow moving galaxy compared with light from the Milky Way itself.

And a faster moving galaxy.

These are simplified spectra, but this is a sort of thing we observe.

The greater a galaxy speed away from us, the greater the shift in frequency as shown by the absorption lines.

The absorption spectrum of the Milky Way is shown at the top and three other galaxies' absorption spectra shown below it, which galaxy is moving away from the Milky Way at the lowest speed? I'll wait five seconds each time I ask a question, but if you need longer, press pause and press play when you have your answer ready.

The correct answer is the spectrum in which the lines have moved least compared with the Milky Way.

So it's A.

In 1924, Edwin Hubble observed several objects thought to be nebulae, clouds of gas and dust, inside the Milky Way.

And he used the changes in brightness of variable stars called Cepheid variables within them to measure their distance.

He found that they were much further away than previously thought.

They were far too distant to be within the Milky Way and must be completely separate clusters of stars.

Hubble had discovered that the universe contained many galaxies, not just the Milky Way.

Here's a photo of one of those galaxies, the Triangulum galaxy.

Hubble also measured the redshift of some of these newly discovered galaxies and used this information to calculate their velocities.

He plotted a graph of the distance of each galaxy from us against its velocity compared with us, and he found a correlation between the two.

He found that the further away a galaxy is from us, the greater its velocity away from us.

So which of the sets of plotted data shown here most closely matches Hubble's results for Galaxy movement? And the correct answer is C.

He found positive correlation, but it wasn't extremely strong unlike the graph shown in A.

Over the next few decades, astronomers observed many more galaxies and improved their measurements of the distances to these galaxies.

So the results became more like this, a stronger correlation was seen.

And this more clearly defined pattern led to the development of Hubble's Law.

Hubble's Law says that, "The distance of a galaxy from the Milky Way is directly proportional to the galaxy's velocity." Further analysis showed that the galaxies weren't just moving away from the Milky Way, that we're moving away from each other as well.

In fact, an observer in another galaxy would see the same effect as we do.

They would see that nearly every galaxy is moving away from theirs.

That observer would also say nearly all other galaxies are moving away from us at a velocity proportional to their distance from us.

So the Milky Way is not in a special location within the universe.

It's not some centre point of the universe.

Is this true or false? Over time, astronomers became more certain that galaxies move away from us as a velocity proportional to their distance away.

I'd also like you to choose a justification for your answer out of these three.

There are two justifications you could use here.

They collected more observations which fitted the predicted pattern.

So that made them more sure that the pattern really existed and they were able to take better measurements to reduce the uncertainties in the measurements of distances to observe galaxies.

And now here are a couple more questions for you to share what you've learned.

Press pause while you discuss or write down your answers and press play when you're ready to check them.

Here are example answers.

In question one you were asked to describe how the velocity of other galaxies can be measured from Earth.

You could say something like this.

The change in frequency and wavelength of the light from the distant galaxies can be measured.

This is the redshift.

The greater the redshift, the faster the galaxy is moving.

And then in question two, Sam said, "The Milky Way is a special galaxy because most of the other galaxies are moving away from us.

That means we are in the centre of the universe." Sam is correct in saying that most galaxies are moving away from us, but is not correct in saying that this makes us the centre.

Nearly every galaxy is moving away from every other galaxy, so there is no centre of the universe.

And Izzy also made a statement that was partly correct.

She said, "Edwin Hubble measured the exact distances and speeds of all the galaxies to show that their distance away is proportional to their speed." It is correct that Hubble measured the speeds of galaxies and their distance from us, but the distances were not exact, they had large errors, and he only examined a small sample of galaxies.

Not all of them.

Well done if your answers included the same points.

And now let's learn about the Big Bang theory.

The movement of the galaxies away from each other forms part of the evidence for the Big Bang theory.

Galaxies are spreading apart from each other over time.

And because they're spreading apart, they must have been closer together in the past.

Analysis of the velocities of the galaxies shows that about 13.

8 billion years ago, all galaxies occupied the same space.

So as we go back in time, they get closer and closer together and they meet about 14 billion years ago.

And which of the images below represents the positions of the galaxies when the universe was youngest? And it's the image in which the galaxies are closest together, C.

The Big Bang theory states that all the matter in the universe occupied the same tiny region of space in the distant past.

That would mean the region would be incredibly dense and also incredibly hot.

In the Big Bang Theory, a tiny region of space expanded rapidly, although it wasn't an explosion.

As the universe expanded, its temperature fell.

The initial material formed into the types of particles we see today, electrons, protons, and neutrons.

And these then formed atoms as they cooled further.

Gravitational forces pulled atoms together to form stars and galaxies.

Over the next 13.

8 billion years, the universe evolved into its current state.

How long ago did the universe start to expand according to the Big Bang theory? And the correct answer is about 14 billion years ago.

That's 14,000 million years ago.

You may remember that our sun is about 5 billion years old, so it's been around for a little over a third of the age of the universe.

Evidence for the universe's initial very high temperature state was first found in 1964.

The particles in the early universe were moving at extremely high speeds, and because of that, they would've emitted high energy, electromagnetic radiation with very short wavelengths which spread throughout the universe as it expanded.

Over the next 13.

8 billion years, the wavelengths of this radiation have been stretched significantly by the expansion of the universe, and it's now microwave radiation found throughout all of space known as cosmic microwave background radiation or CMBR.

Cosmic microwave background radiation is evidence that the universe was much hotter in the past.

We don't have any other explanation for why it should be there, and it certainly fits with the idea of a very hot early universe.

We detect microwave radiation coming from every direction throughout all of space.

So in the early universe, high energy, short wavelength radiation was produced.

And in the current universe, the wavelength of this radiation have expanded with the universe to become longer wavelength, microwave radiation.

So which of the following are features of the Big Bang theory of the universe? The correct answers are B and C.

The Big Bang Theory does predict that galaxies become further apart from each other and that the temperature of the universe has decreased over time.

Big Bang theory doesn't predict that the density of the universe is constant over time.

The universe expands and the density decreases.

And it doesn't predict that matter is constantly being created to keep a constant density as the universe expands.

Well done if you picked out B and C.

Before the discovery of this microwave background radiation, there was an alternative model to explain the movement of the galaxies that Hubble had discovered.

This model was called the steady state theory.

In this model, new matter is constantly being generated somehow as the galaxies move apart so that the density and temperature of the universe don't change over time.

The discovery of CMBR couldn't be explained by the steady state model, which was eventually then rejected by scientists.

Which of these are features of the steady state model of the universe? And they all are.

Matter is constantly being created to keep a constant density as the universe expands.

Galaxies are becoming further apart from each other.

The average temperature of the universe is not changing and the density of the universe is not changing.

Well done if you realise those were all correct.

And now here are some more written questions for you to try.

Press pause when you do this and press play when you're ready.

Here are example answers.

First describing the initial state of the universe.

Initially, the universe was very dense containing all of the matter in one place and as an extremely high temperature.

In two part A, explaining how the apparent movement of the galaxies supports the Big Bang theory.

The universe is expanding with most galaxies moving further apart from each other over time, which of course suggests that they were closer together in the past.

And then explaining how the existence of the microwave background radiation supports the Big Bang theory, you could say something like this, high energy electromagnetic radiation produced in the early stages of the universe at very high frequencies and short wavelengths.

As the universe expanded and cooled, the wavelength of the radiation also expanded.

It was stretched.

Eventually the wavelengths increased until they fell into the microwave region, which has been detected throughout all of space.

And finally in question three, you were asked why the steady state model was rejected in favour of the Big Bang theory.

The steady state model was eventually disregarded because it could not explain key observations including the cosmic microwave background radiation as simply as the Big Bang theory can.

Another observation by the way that the steady state model couldn't easily explain was the discovery of ultra bright objects called quasars.

We can see that quasars were more common in the distant past billions of years ago, but this suggests that the universe has been changing over time, which doesn't easily fit the steady state model.

Now let's move on to the third part of this lesson, understanding the universe.

Our earliest understanding of the universe began with observations of the sun, moon, and stars.

Many cultures gave special meaning to the movement of the sun and moon across the sky, and they learned how these changed as months and seasons passed.

Many ancient monuments like Stonehenge seem to have strong links to the summer or winter solstice, the time when the days are longest or the days are shortest.

Being able to predict these changes of the seasons was important to hunting and farming and so to people's survival.

Most civilizations imagine patterns in the unchanging positions of the stars, and they often link these shapes to special animals, legends, heroes or mythical beings.

What you can see here is the constellation, Orion, the hunter.

These groups called constellations.

Orion's belt is particularly easy to spot in the sky.

These three bright stars that are almost in a straight line.

Is it true or false? The stars in a constellation must all be close to each other in space.

And choose a justification for your answer.

And the correct answer is false because some of the stars may be much brighter and further away.

So the stars we see in a constellation aren't all at the same distance from us.

If we were looking from a different position in the galaxy, the constellations will look very different from the way they look here.

Most of our understanding of the universe is based on observing light and then later other parts of the electromagnetic spectrum.

At first, we could only use our eyes, which allowed us to discover thousands of stars and identify what was called wondering stars among them.

These are the planets.

And in fact, the word planet comes from a word meaning wonderer.

Optical telescopes led to the discovery of new stars, planets, moons and galaxies, each containing billions of stars.

We've developed telescopes to observe x-rays, ultraviolet radiation, infrared radiation, and radio waves.

Observations from these telescopes have greatly expanded our knowledge of the current structure and early state of the universe.

Which of the following regions of the electromagnetic spectrum are used by astronomers? The correct answer is all of these.

They're all produced by objects in space, and they're all observed by different types of telescope.

There are still, of course, many things we do not understand, and scientists aim to build new tools and develop new theories to fill the gaps in our knowledge.

Two particular areas of exploration involve the missing mass of the universe and the increasing rate of expansion of the universe.

I'll tell you a bit more about each of these.

Observations of the speed of rotation of spiral galaxies showed that they contained far more mass than could be directly observed from their stars.

So we can estimate the total mass of a galaxy based on how bright it is because the brightness lets us estimate the number of stars and from that, we can estimate the mass.

But what's been found is stars near the edges of galaxies are moving more quickly than expected.

The more massive a galaxy, the faster you would expect stars to go round.

But these stars were going too fast from our estimated masses based on the brightness.

Some of the missing mass could be dust or gas, which doesn't contribute to the brightness of the galaxy, but this wouldn't be enough to explain the high speed of rotation.

So astronomists proposed that galaxies contain large amounts of what's called dark matter, which can be directly detected.

Scientists don't yet know what dark matter is.

Which of the following pieces of evidence indicates that some of the mass of the universe cannot currently be detected? The correct answer is B.

The speed of stars orbiting spiral galaxies is faster than expected.

Statements A and B are correct statements, but they don't provide evidence that dark matter exists.

And what about dark energy? Well, astronomists thought that the attractive gravitational forces in the universe would gradually slow its rate of expansion over time.

But more recent observations using supernova explosions in distant galaxies has shown us that the rate of expansion of the universe seems to be increasing rather than slowing down.

So there appears to be some sort of unknown dark energy causing this faster rate of expansion.

But scientists don't yet understand what this is either.

Which of the following has been proposed to help explain the increasing rate of expansion of the universe? And as I said, it's dark energy.

And now the final written task of this lesson, press pause while you answer these questions and press play when you're ready to check them.

Here are some example answers.

So to explain how early cultures used observations of the sky to help them understand the universe, you could say something like this.

Early cultures used the movement of the sun, moon, and stars to predict seasonal changes crucial for survival.

They created constellations based on patterns in the stars to aid navigation and mark important times of the year.

Question two asked how the development of optical and advanced telescopes enhance our understanding of the universe.

The invention of optical telescopes allows us to observe new stars, planets, moons, and galaxies.

Advances in technology have enabled us to study x-rays, ultraviolet, infrared and radio waves, providing insights into the universe's structure and history beyond what optical light reveals.

Question three, asks what's the significance of dark matter and dark energy in our understanding of the universe? You could say dark matter and dark energy are crucial to our understanding of the universe because they seem to make up most of its mass and energy.

Dark matter affects the motion of galaxies and their clusters while dark energy is thought to be driving the accelerated expansion of the universe.

And finally, you are asked to research the challenges scientists face in investigating dark matter and dark energy.

Scientists face challenges in studying these phenomena because dark matter does not emit light or interact with electromagnetic radiation, making it difficult to detect directly.

Dark energy is even more mysterious as its nature is not yet understood and it does not have a clear method of detection.

Well done if you've got some or all of these answers right.

And now we've reached the end of the lesson, and I'll finish with a summary.

Analysis of redshift of light shows that the distance between galaxies is increasing and that the universe is expanding.

The Big Bang theory is the accepted model for the origin of the universe.

It describes how the universe started as a very, very hot and dense region about 13.

8 billion years ago.

The early universe expanded rapidly and cooled to its current state.

The movement of galaxies away from each other and the existence of cosmic microwave background radiation, provide evidence for this theory.

Concepts like dark matter and dark energy are being used to try to explain observations that we still cannot explain about the universe.

Well done for working through this lesson.

I hope you enjoyed it and found it interesting, and perhaps you'll want to go and find out more about our universe and how we're trying to understand it.

Bye for now.