The Big Bang, Cosmology part 1: Crash Course Astronomy #42

Summary

This script delves into the nature of the Universe, exploring the shift from myth to scientific understanding. It outlines the transition from a static Universe model to the groundbreaking realization of an expanding cosmos, supported by observations like Vesto Slipher's redshifts and Edwin Hubble's law. The script explains the Big Bang theory, the cosmic microwave background, and the Universe's lack of a center, concluding with the current age of the Universe at 13.82 billion years. It highlights the remarkable journey of human comprehension from ancient beliefs to the empirical evidence that shapes our modern understanding of the cosmos.

Takeaways

• 🌌 The Universe's nature has been a subject of human curiosity and storytelling for a long time before science provided explanations.
• 🔬 The field of cosmology emerged to study the cosmos, leading to a better understanding of the Universe's nature through scientific methods.
• 🕰️ Early 20th-century scientists believed the Earth and Universe were ancient, but the exact age was unknown, with a popular model suggesting a static Universe.
• 🌌 Vesto Slipher's observations of 'spiral nebulae' revealed redshifts, indicating that these objects were moving away from us at high speeds.
• 🧐 Georges Lemaître's work on Einstein's equations suggested an expanding Universe, which aligned with Slipher's redshift observations.
• 🔭 Edwin Hubble and Milton Humason's work on galaxy distances and redshifts established the relationship between distance and speed of galaxies moving away from us.
• 🌌 The concept of an expanding Universe led to the idea that all matter was once compressed into an infinitely dense state, known as the 'Big Bang'.
• 💥 The Big Bang theory was initially met with skepticism but has since been confirmed by various observations, including the cosmic microwave background.
• 🚀 The 'lookback time' concept allows astronomers to observe distant objects and see the Universe as it was in the past, providing evidence for the Big Bang.
• 🌡️ The Universe's expansion and cooling have been confirmed by the cosmic microwave background radiation, which is the redshifted light from the early Universe.
• 📉 The Big Bang model also predicts the abundance of elements in the Universe, which matches what we observe, further supporting the theory.
• 🌐 The Universe's expansion is not galaxies moving through space but space itself expanding, carrying galaxies along with it, with no center or edge.

Q & A

What is the main subject discussed in the script?

The main subject discussed in the script is the nature of the Universe and the development of the Big Bang theory as the prevailing cosmological model.

How did humans initially respond to their lack of understanding about the Universe?

Humans initially responded by making up stories to help explain what they saw or to make themselves feel better about what they didn't understand.

What is the field of study that deals with the cosmos?

The field of study that deals with the cosmos is called cosmology.

What was the popular model for the Universe before the Big Bang theory?

The popular model for the Universe before the Big Bang theory was that it was static, unchanging, and had always existed in a balanced state.

What was Vesto Slipher's significant contribution to the field of cosmology?

Vesto Slipher's significant contribution was observing the spectra of 'spiral nebulae' and discovering that most of them exhibited a redshift, indicating they were moving away from us at high speeds.

What was Georges Lemaître's contribution to the understanding of the Universe's expansion?

Georges Lemaître contributed by studying Einstein's equations and proposing that an expanding or contracting Universe fit the equations better, leading to the idea that the Universe itself was getting bigger.

Which astronomers were pivotal in determining the distances to galaxies and their redshifts?

Edwin Hubble and his assistant Milton Humason were pivotal in determining the distances to galaxies and their redshifts, which supported the idea of an expanding Universe.

What is the term 'lookback time' in the context of observing the Universe?

The term 'lookback time' refers to the concept that the farther away something is, the farther in the past we see it, allowing us to observe the Universe when it was younger.

What evidence supports the Big Bang model of the Universe's origin?

Evidence supporting the Big Bang model includes the redshift of distant galaxies, the cosmic microwave background radiation, and the observed abundances of elements in the Universe.

What is the current best measurement of the Universe's age?

The current best measurement of the Universe's age is 13.82 billion years.

How does the script explain the misconception about the Big Bang being an explosion in space?

The script clarifies that the Big Bang was not an explosion in space but rather the initial exploding expansion of space itself, with galaxies not moving but being carried along by the expansion of space.

What does the script suggest about the physical nature of the Universe's expansion?

The script suggests that the Universe's expansion means that space itself is expanding, carrying galaxies along with it, and that there is no center or point of origin to the Universe.

Outlines

00:00

🌌 The Birth of Cosmology and the Expanding Universe

This paragraph delves into humanity's quest to understand the nature of the Universe. Initially, we relied on stories to explain the cosmos, but with the advent of science, we began to test our ideas and developed the field of cosmology. By the 20th century, it was understood that Earth was ancient, and the Universe was presumed to be static and unchanging. However, observations by Vesto Slipher of 'spiral nebulae' (galaxies) showed redshifts, indicating they were moving away from us at high speeds. This discovery, along with the theoretical work by Georges Lemaître and Alexander Friedmann, suggested an expanding Universe. Edwin Hubble and Milton Humason's work on galaxy distances and redshifts confirmed this expansion, leading to the understanding that the Universe is not static but growing.

05:00

💥 The Big Bang Theory and the Universe's Expansion

The second paragraph explores the implications of an expanding Universe, leading to the Big Bang theory. It explains how the Universe, being extremely hot and dense at its inception, began to cool as it expanded. This theory was initially met with skepticism but gained support through testable predictions. The 'lookback time' concept allows us to observe the Universe in its younger state by looking at distant objects. The cosmic microwave background (CMB), discovered in 1965, is the redshifted light from the early Universe, confirming the Big Bang theory. Additionally, the abundance of elements and the structure of the cosmos align with the predictions of the Big Bang model. This theory has become widely accepted by astronomers as the explanation for the Universe's beginning.

10:04

🔍 The Nature of the Universe's Expansion and Its Implications

This final paragraph clarifies misconceptions about the Big Bang, explaining that it was not an explosion in space but the initial expansion of space itself. It emphasizes that galaxies are not moving through space but are carried along by the expansion of space. The paragraph also discusses the concept that the Universe has no center, as every point within it appears to be the center due to the uniform expansion. The age of the Universe, calculated through observations and mathematical models, is estimated to be 13.82 billion years. The paragraph concludes by highlighting the significance of our understanding of the Universe's beginning and expansion, showcasing the intelligence and curiosity of humanity.

Keywords

💡Universe

The Universe refers to the entirety of space and all the matter and energy it contains. It is the central subject of the video, which discusses its nature, origin, and evolution. The script mentions that for a long time, humans were unsure about the nature of the Universe, relying on stories and later, scientific inquiry to understand it.

💡Cosmology

Cosmology is the scientific study of the origin, evolution, and eventual fate of the Universe. The script explains that the field of cosmology was born out of the desire to understand the cosmos itself, leading to the development of theories and models like the Big Bang.

💡Redshift

Redshift is a phenomenon where the wavelength of light or other electromagnetic radiation from an object is increased, or 'shifted' to the longer-wavelength (red) end of the spectrum. In the script, the redshift observed in the spectra of galaxies indicates that these galaxies are moving away from us, supporting the idea of an expanding Universe.

💡Big Bang

The Big Bang is the prevailing cosmological model for the observable Universe's origin from an extremely hot and dense state. The script describes the Big Bang as a moment when the Universe was unimaginably hot and dense, and then it expanded violently, cooling down over time. This concept is central to understanding the Universe's beginning.

💡Cosmic Microwave Background (CMB)

The Cosmic Microwave Background is a key piece of evidence for the Big Bang model. It is a faint glow, observed in every direction in the sky, which is the afterglow of the Big Bang. The script mentions the discovery of the CMB in the 1960s as a significant confirmation of the Big Bang theory.

💡Galaxy

A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, gas, dust, and dark matter. The script discusses the observation of galaxies, initially referred to as 'spiral nebulae', and how their redshift indicates the expansion of the Universe.

💡Expansion of the Universe

The expansion of the Universe is a key concept in cosmology, referring to the increase in the scale and size of the Universe over time. The script explains that the Universe is not expanding into something external but is expanding in space itself, carrying galaxies along with it.

💡Lookback Time

Lookback time is the time it takes light to reach us from distant objects in the Universe, effectively allowing us to see the Universe as it was in the past. The script uses the concept of lookback time to explain how we can observe the Universe at different stages of its evolution.

💡Element Abundance

Element abundance refers to the proportion of each chemical element in the Universe. The script mentions that the Big Bang model predicts specific ratios of elements compared to hydrogen, which have been observed and support the validity of the model.

💡Space

Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. The script discusses how space itself is expanding, which is a fundamental aspect of the Big Bang model and the observed redshift of galaxies.

💡Astronomer

An astronomer is a scientist who studies celestial objects, space, and the physical universe as a whole. The script mentions several astronomers, such as Vesto Slipher, Georges Lemaître, Edwin Hubble, and Milton Humason, who contributed significantly to our understanding of the Universe.

Highlights

The field of cosmology was born, the study of the cosmos itself.

Darwin’s Theory of Evolution and Lord Kelvin’s findings suggested Earth was millions of years old.

The Universe was initially thought to be static and unchanging.

Vesto Slipher discovered that most 'spiral nebulae' had highly redshifted spectra, indicating they were moving away from us at high speeds.

Georges Lemaître proposed an expanding Universe based on Einstein's equations and Slipher's observations.

Edwin Hubble and Milton Humason's work confirmed that galaxies' distances correlated with their redshifts, indicating an expanding Universe.

The concept of 'lookback time' allows us to observe the Universe in its younger state by looking at distant objects.

The Big Bang theory suggests that the Universe was once an infinitely dense and hot 'primeval atom' or 'cosmic egg'.

The term 'Big Bang' was popularized by Fred Hoyle, despite his initial skepticism of the model.

The cosmic microwave background radiation was discovered, confirming the Big Bang model's prediction of the early Universe's glow.

The Big Bang model predicts the abundance of elements in the Universe, which aligns with current observations.

The Universe's expansion means it has no center and no point of origin, making every location equally significant.

The Universe's expansion is not an explosion in space but the expansion of space itself, carrying galaxies along.

The age of the Universe has been calculated to be 13.82 billion years based on its expansion rate.

Crash Course Astronomy provides an educational series on the cosmos, with this episode focusing on the Big Bang and the Universe's nature.

Transcripts

What is the nature of the Universe?

How’s that for a question? For a long time we humans had no idea what was going on in

the Universe. To help, we made up stories to either help us explain what we saw, or

to make us feel better about what we didn’t understand.

But then science came along, and we started to understand more. We could test our ideas,

and as we got more confident in the process, our ideas grew. The field of cosmology was

born, the study of the cosmos itself. And now, after centuries of speculation and just-so-stories,

we’re starting to get a grasp on the biggest ideas there are.

What is the nature of the Universe? Let’s find out.

By the turn of the 20th century, scientists knew the Earth was old. Darwin’s Theory

of Evolution strongly implied the Earth was at least millions of years old, and Lord Kelvin,

a hugely respected physicist and engineer, confirmed the Earth was ancient, given that

it must have cooled from an initially molten state. That takes a while, at least a million years.

How old exactly, no one knew. As for the Universe itself, it logically must be as old or older

than Earth. A popular model for the Universe was that it was static: It is and always has

been as we see it now, and in general hasn’t changed. Stars may be born and they may die,

but overall things pretty much stayed in balance.

The Universe always existed, always will, always had galaxies in it, and so on. There

are variations on this idea, but that’s it in a nutshell, and it’s what many astronomers believed.

This is important. When we try to understand observations in astronomy, we fit them into

a framework of understanding, things we think we already know. When something doesn’t

fit, it’s a problem. Maybe the observation is wrong, or maybe we’re misinterpreting it.

Or maybe the framework is wrong! That’s a big step to undertake, and needs proper

contemplation and skepticism. Science is a tapestry, and when you yank at one thread,

the whole thing may need reweaving.

Sometimes – rarely, but sometimes – you have to yank that thread.

The thread that got pulled in this picture was first uncovered in 1912. That was when

astronomer Vesto Slipher — who has the uncontested coolest name for an astronomer ever — started

taking spectra of the so-called “spiral nebulae”, hoping to get some insight on

their characteristics (remember, this was before we understood what galaxies actually were).

It took him several years of observations, but by 1917 he had observed 25 of them…

and he found something astonishing. When he examined their spectra, he saw that almost

all of them were highly redshifted. In other words, it looked like most of these objects

were rushing away from us at high speed, millions of kilometers per hour!

What could that mean?

At this point, two different lines of work began to converge. One was by a Belgian theoretical

physicist named Georges Lemaître. In the 1920s he had been studying Albert Einstein’s

work, the equations dealing with the behavior of the Universe as a whole. Einstein had concluded

that the Universe was static, unchanging, but Lemaîtres disagreed. He found that an

expanding or contracting Universe fit the equations better — and, given the redshifts

observed by Slipher, he proposed the the Universe itself was getting bigger, which is why the

galaxies appeared to be moving away from us. Another brilliant physicist, Alexander Friedmann,

had also reached the same conclusion.

At the same time, astronomers were trying to determine the distances to the nebulae,

now understood to be galaxies in their own right. As I mentioned in our first episode

about galaxies, Edwin Hubble and his assistant Milton Humason were at the forefront of this.

They observed variable stars in the Andromeda Galaxy that allowed them to get the distance to the galaxy.

They then observed some of the same galaxies Slipher did, and measured their distances.

When they compared distances to the redshifts Slipher observed, they found that the farther

away the galaxy was, the faster it was moving away from us.

Let me repeat that, because it’s kind of important: The farther away a galaxy was,

the FASTER it appeared to recede from us.

Some other astronomers had also found similar results, but the work Hubble and Humason clinched

it. We now know it to be true for every distant galaxy we observe: They are all redshifted,

all heading away from us.

And this ties into what Lemaîtres had concluded: the Universe is expanding.

Wait, what? The Universe is getting bigger. How can that be? What does that even mean?

There are lots of different ways of looking at this. Lemaîtres himself suggested imagining

the cosmic clock running backwards. Right now, as time inexorably marches on, all the

galaxies in the sky are getting farther and farther away from us. But that means that

in the past they were closer together. Run the clock back far enough, and they get closer

and closer together until at some point in the past everything in the entire universe

was crammed together into an über-dense…thing.

That is a really, really weird thought. It’s hard to imagine everything in the whole cosmos

– every star, nebula, galaxy; every atom, electron, and proton – all squeezed together

into one infinitely dense blob. But that’s what the observations are telling us.

Lemaîtres called this a “primeval atom,” or, more colorfully, the “cosmic egg.” Fair enough.

But this has implications. If you squeeze all the energy everywhere into one place,

that place is going to be HOT. When the Universe was a tiny dot it would’ve been unimaginably,

hellishly hot. Then, for some reason, it suddenly expanded violently and started cooling.

This sounds an awful lot like an explosion – BANG! - involving the entire Universe,

which is big. What else would you call this but “the Big Bang”? In fact, the term

became popular when astronomer Fred Hoyle used it on a radio show, and later in a widely-read

magazine article. Ironically, he meant it somewhat disparagingly, since he didn’t

think the Big Bang model was correct. To his, and many other astronomers’, chagrin, the name stuck.

I like it. It’s not perfectly accurate, but it’s succinct.

Again, this is all pretty strange, and astronomers had a hard time accepting it. After all, it

went against everything they thought was true at the time!

In science, though, a hypothesis needs to make testable predictions before it can be

taken seriously. What predictions could the Big Bang model of the Universe make that we

can observe today?

The speed of light is fast: 300,000 kilometers per second, or about a billion kilometers per hour.

Like I said, fast.

But not infinitely fast.

The Sun is 150 million kilometers away. It takes light about 8 minutes to reach the Earth,

so in a sense you’re seeing the Sun as it was 8 minutes ago. The nearest star system

to us is Alpha Centauri, 4.3 light years away, so we see it as it was 4.3 years ago.

The Andromeda galaxy is about 2.5 million light years away. The light we see from it

now left that galaxy when Australopithecus walked the Earth.

The farther away something is, the farther in the past we see it. This is called the

“lookback time”, and it’s a crucial tool for cosmology: By observing very distant

objects, we can see the Universe when it was young!

You might think that we could see all the way back to the moment of the Big Bang, but

there’s a problem. At some point back in time, the Universe was so hot and dense that

it was the same temperature as the surface of a star. It would’ve been very luminous,

but also opaque!

As it expanded, it cooled, and became transparent. If we look back far enough, that moment in

time when it cleared up is as far back as we can see. What does that moment look like?

By looking at the physics of the Big Bang — the math that describes how matter, energy,

space, and time behave — astronomers could predict when this moment happened in the lifetime

of the Universe: a few hundred thousand years after the bang itself.

Using the idea of lookback time, they could predict how far away it would be from us,

and therefore calculate its redshift. Remember, redshift stretches the wavelength of light.

The light the Universe emitted at the time would’ve been like a star, in the visible

part of the electromagnetic spectrum, but the light that reaches us billions of years

later — now —should be redshifted into microwave wavelengths.

In 1965, a pair of radio astronomers announced they had found a signal in their radio telescope

that was like a background noise, coming from everywhere in the sky. They tried everything

they could to explain it — including scraping out the bird poop inside their radio telescope,

in case that might be causing it — but the only thing that made sense was that this was

indeed the redshifted light from the early Universe. They had discovered the glow of

the fireball leftover from the birth of the cosmos.

Later, in the 1990s, satellite observations further refined the measurements of this cosmic

microwave background, and now it’s essentially confirmed. This glow was successfully predicted

by the Big Bang model, and now we see it in exquisite detail. Its discovery was a

huge step in cosmology.

The redshift of distant galaxies and the cosmic background are not the only confirmations

we have that the Big Bang model is correct.

For example, the model also makes predictions about the elements we see in the Universe.

At first, when the Universe was dense and hot, only subatomic particles could exist.

But as the Universe cooled, for a brief time, they could fuse and form heavier elements.

The Big Bang model predicts certain abundances of elements — ratios of them, compared to

hydrogen — and that’s just what we see in the Universe at large.

Also, the size and shapes of large structures in the cosmos are in line with what a Big

Bang model predicts. There’s lots of other observational evidence as well. Pretty much

every modern astronomer on Earth understands that the Big Bang model of how the Universe

got its start is the correct one.

But what does it mean? I mean, physically?

It’s a very common misconception that the Big Bang was an explosion in space, with everything

rushing away from some point. But that’s not what’s really happening.

Remember, I’ve talked about space being a THING, in which matter and energy exist.

Space can be warped, bent, by mass, creating what we think of as gravity.

When we talk about the Universe expanding, we mean space itself is expanding, and when

it does it carries galaxies along with it. In a sense, it’s like having a rubber ruler.

When you pull on it, it gets longer, and the distance between the tick marks gets wider.

When the ruler doubles in length, the tick marks that started out a millimeter apart

are now TWO mm apart. But tick marks that were ten centimeters apart are now 20 cm apart!

In other words, the farther away a tick mark is, the faster it appears to move away.

Sound familiar? That’s just what galaxy redshifts are telling us. It also means that,

really, the galaxies aren’t actually doing any moving, it’s that space between them

is expanding. This may seem like a nitpicky semantic point, but it’s physically true.

The galaxies are, for all intents and purposes, standing still. The space in between them

is where all the action is.

And it gets even weirder: This is true no matter where you are in the Universe. From

any galaxy, it looks like all the others are rushing away from you. Look back at that ruler:

No matter what tick mark you start with, when the ruler stretches, from that spot it looks

like the tick marks are all moving away from you.

This is what Einstein’s equations showed, and what Lemaîtres saw in them. Space is

expanding! But that means the Big Bang wasn’t an explosion in some pre-existing space, it

was the initial exploding expansion of space itself. The Universe isn’t expanding into

anything, because it’s all there is. There’s nothing outside the Universe for it to expand into.

This also means the Universe has no center, no point of origin. Imagine the ruler is now

a circle, and the diameter is expanding. No tick mark is the actual center, yet no matter

where you are, on the ruler, every tick mark appears to move away from you. In a similar

way, every spot in the Universe appears like the center, which means…none is. No place

in the Universe is more special than any place else. We’re all in this together.

It can be hard to grasp, and I’ll admit we all have some difficulty with these concepts.

But the math bears them out, and so do essentially all the observations we make of the distant Universe.

And in all this weirdness, don’t lose sight of the big picture: The Universe had a beginning.

And we can see evidence of it!

Not only that, but by measuring how quickly it’s expanding, we can use math to run the

clock backwards and determine the age of the Universe. Currently, the best measurement

we have of the age of the Universe is 13.82 billion years.

Or perhaps I should say: 13.82 billion years!

That’s an amazing number. It’s a long, long time — three times older than the Earth

— but what gets me is that we can figure it out at all.

Pretty smart, us apes.

Today you learned that distant galaxies show a redshift in their spectra, which means they’re

moving away from us. The Universe is expanding! This means it used to be hot and dense, then

it started expanding and cooling. This model of the Universe’s early behavior is called

the Big Bang, and it was confirmed when the background radiation — the glow of the fireball

— was detected in the 1960s. Other lines of evidence support it as well. Using this

information, we have measured that the Universe is nearly 14 billion years old.

Crash Course Astronomy is produced in association with PBS Digital Studios. Head over to their

YouTube channel to catch even more awesome videos. This episode was written by me, Phil

Plait. The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller.

It was directed by Nicholas Jenkins, edited by Nicole Sweeney, the sound designer is Michael

Aranda, and the graphics team is Thought Café.