A Brief History of the Universe: Crash Course Astronomy #44

Summary

This script by Phil Plait offers a fascinating journey through the history of the Universe, from the Big Bang to the present day. It explains how physics and astronomy help us understand the past, detailing the Universe's expansion, cooling, and evolution through various stages of matter formation. The concept of inflation is introduced as a rapid expansion smoothing out early lumpiness. The script also touches on the cosmic microwave background and the formation of galaxies, ending with a nod to the ongoing quest for answers in cosmology.

Takeaways

• 🌌 The Universe began with the Big Bang, starting from an unfathomably hot and dense state.
• 🔬 Understanding the Universe's past is based on current observations and the application of physics in reverse.
• 🌐 The Universe's expansion implies it was denser and hotter in the past, reaching temperatures beyond those at the core of a supernova.
• 🔬 Particle colliders replicate the high-energy conditions of the early Universe, allowing us to study its phases.
• 🌡 As we go back in time, the Universe transitions through phases, from a plasma of subatomic particles to a unified super force before splitting into distinct forces.
• 🕒 Physics cannot fully describe the first 10^-43 seconds after the Big Bang, marking a limit to our current understanding.
• 🚀 The first second of the Universe saw the formation of basic subatomic particles as it expanded and cooled.
• 🌟 Three minutes after the Big Bang, the Universe cooled enough for nuclear fusion to create the first atoms, predominantly hydrogen and helium.
• 🌞 At around 380,000 years, electrons combined with atomic nuclei in a process called recombination, making the Universe transparent for the first time.
• 🌌 The cosmic microwave background radiation we observe today is the light emitted at the time of recombination, now redshifted to microwave frequencies.
• 🔍 The uniformity of the cosmic microwave background suggests the Universe was extremely smooth, leading to the theory of cosmic inflation.
• 🌀 Cosmic inflation, proposed by Alan Guth, explains the uniformity and smoothness of the early Universe by an exponential expansion of space-time.
• 🌠 The tiny fluctuations in the cosmic microwave background are the seeds of the large-scale structures like galaxies and galaxy clusters we see today.
• 🤔 Despite significant progress, many questions remain in cosmology, such as the nature of dark energy, the role of dark matter, and the possibility of multiple universes.

Q & A

What is the main theme of the video script by Phil Plait?

The main theme of the video script is the history and evolution of the Universe, explaining how physics, math, and astronomy can be used to understand what the Universe was like in the past, all the way back to the moment of the Big Bang.

How does the script describe the early Universe?

The script describes the early Universe as being unfathomably hot and dense, with a fundamental physical nature that was very different from what it is today due to the high energy levels.

What role do particle colliders play in understanding the early Universe?

Particle colliders play a crucial role in understanding the early Universe by recreating the high-energy conditions that existed when the cosmos was very young. They allow scientists to study the behavior of particles and the creation of subatomic particles, similar to what occurred in the early Universe.

What is the significance of the 'inflation' theory in cosmology?

The 'inflation' theory is significant because it explains why the Universe appears so smooth at the time of recombination. It suggests that the Universe underwent a rapid expansion, smoothing out any lumps in matter, and is now almost universally agreed upon by astronomers.

What is the 'recombination' event mentioned in the script?

The 'recombination' event refers to the moment approximately 380,000 years after the Big Bang when the Universe had cooled enough for electrons to combine with protons and helium nuclei, forming stable neutral atoms for the first time. This made the Universe transparent to light, allowing photons to travel freely.

Why is the cosmic microwave background radiation important?

The cosmic microwave background radiation is important because it provides a snapshot of the Universe at a very early stage, shortly after recombination. It tells us about the distribution of matter and temperature uniformity in the early Universe and is a key piece of evidence supporting the Big Bang model.

What does the script suggest about the distribution of matter in the early Universe?

The script suggests that the distribution of matter in the early Universe was very even, with only tiny variations in temperature across different regions. These small variations, or lumps, were the seeds for the formation of galaxies and larger structures.

How does the script explain the formation of the first stars?

The script explains that the first stars formed about 400 million years after the Big Bang from the denser spots in the Universe, which were the result of small perturbations during inflation. These spots grew as gravity attracted dark matter, and normal matter collected there, eventually condensing to form stars.

What unanswered questions about the Universe does the script mention?

The script mentions several unanswered questions, including the nature of dark energy, the role of dark matter in the early Universe, the origin of the Universe, and the possibility of the existence of other universes beyond our own.

What is the significance of the timeline of the Universe's history in the script?

The timeline of the Universe's history is significant as it outlines the stages of the Universe's evolution, from the Big Bang to the formation of atoms, the inflation period, and the eventual formation of galaxies and larger structures. It provides a framework for understanding the development of the Universe.

Outlines

00:00

🌌 The Birth and Evolution of the Universe

In this introductory paragraph, Phil Plait sets the stage for a journey through the history of the Universe. He emphasizes that our understanding of physics, math, and astronomy allows us to trace the Universe's evolution from its birth to the present day. Starting from a state of nothingness, the Universe expanded and cooled, undergoing significant changes in density, temperature, and the behavior of fundamental particles. Plait highlights the importance of particle colliders in simulating the early Universe and understanding its properties. The paragraph also touches on the concept of running the 'clock backwards' to study the past states of the Universe, leading to insights about its initial conditions.

05:00

🔥 The Early Universe: From Big Bang to Atom Formation

This paragraph delves into the details of the Universe's early moments, starting from the Big Bang. It describes the initial state of the Universe as being incredibly hot and dense, with particles and forces behaving differently than they do today. As the Universe expanded and cooled, atoms and nuclei began to form, leading to the creation of the first elements like hydrogen, helium, and lithium. The paragraph also discusses the concept of 'recombination', when electrons combined with atomic nuclei to form neutral atoms, making the Universe transparent for the first time. This allowed photons to travel freely, which we now observe as the cosmic microwave background. The uniformity of this radiation suggests that the Universe was remarkably smooth at the time of recombination, leading to the proposal of 'inflation' as a mechanism to explain this uniformity.

10:03

🌟 The Formation of Galaxies and the Role of Inflation

In this concluding paragraph, Phil Plait discusses the formation of galaxies and the role of inflation in the early Universe. He explains that the tiny fluctuations observed in the cosmic microwave background are the result of quantum fluctuations that were stretched to macroscopic sizes during inflation. These denser regions attracted matter, leading to the formation of the first stars and eventually galaxies. Plait also reflects on the progress made in understanding the Universe's history and the many questions that remain, such as the nature of dark energy and dark matter, and the possibility of multiple universes. The paragraph ends with a celebration of the scientific endeavor to answer these profound questions.

Keywords

💡Big Bang

The Big Bang refers to the prevailing cosmological model for the beginning of the observable universe, where all space and time began in an infinitely dense and hot state. In the video, it is described as the starting point of the universe's history, where everything began from an unfathomably hot and dense state, setting the stage for the evolution of the cosmos.

💡Universe Expansion

Universe Expansion is the idea that the universe is getting larger over time. The script explains that because the universe is expanding, it was denser, more crowded, and hotter in the past. This concept is crucial for understanding the evolution of the universe from a hot, dense state to its current, more diffuse state.

💡Particle Colliders

Particle colliders are experimental facilities that accelerate particles to high speeds and smash them into each other to study the resulting subatomic particles. The script mentions that these colliders replicate conditions of the early universe, allowing scientists to investigate the high-energy environment that existed shortly after the Big Bang.

💡Phase Change

A phase change is a transformation in the state of matter, such as from solid to liquid or liquid to gas, due to changes in temperature or pressure. The video uses the melting and vaporization of a snowball as an analogy to describe how the universe underwent similar changes in its fundamental physical nature as it cooled and expanded.

💡Quarks

Quarks are elementary particles and a fundamental constituent of matter. The script explains that at extremely high temperatures, even protons and neutrons can break apart into their constituent quarks, indicating a state of the universe where matter as we know it did not exist.

💡Unified Force

Unified Force refers to a hypothetical state where the four fundamental forces of nature—gravity, electromagnetism, and the strong and weak nuclear forces—were indistinguishable from one another. The video describes how, in the early universe, these forces were all part of one unified super force before they separated as the universe cooled.

💡Inflation

Inflation is a theory in cosmology that describes a period of extremely rapid (exponential) expansion of the universe during the first fraction of a second after the Big Bang. The script discusses inflation as a critical event that smoothed out the universe, explaining the uniformity observed in the cosmic microwave background.

💡Recombination

Recombination is the point in the universe's history when electrons combined with atomic nuclei to form stable neutral atoms for the first time. The script highlights this as a pivotal moment when the universe transitioned from being opaque to transparent, allowing photons to travel freely.

💡Cosmic Microwave Background

The Cosmic Microwave Background (CMB) is a remnant thermal radiation from the early universe, observed today as a faint glow permeating the entire sky. The video explains that the CMB is the light emitted at the time of recombination, which has been traveling through the universe ever since, providing crucial information about the early conditions of the universe.

💡Nuclear Fusion

Nuclear fusion is the process by which atomic nuclei combine to form heavier nuclei, releasing energy in the process. The script describes how, within the first 17 minutes after the Big Bang, the universe was cool enough for nuclear fusion to occur, leading to the formation of light elements like hydrogen, helium, and lithium.

💡Quantum Fluctuations

Quantum fluctuations refer to temporary changes in energy in a point in space, which can have an impact on the fabric of space-time itself. The video mentions that the small variations observed in the CMB are the result of quantum fluctuations that were stretched to macroscopic scales during inflation, seeding the formation of galaxies.

Highlights

The Universe's history can be traced back to its very first moment using physics, math, and astronomy.

The Universe began as an incredibly hot and dense state, evolving from a singularity to its current expansion.

The expansion of the Universe implies a denser, hotter past, reaching temperatures hotter than the core of a star.

Particle colliders simulate the early Universe's conditions, allowing us to study its phases by recreating high-energy collisions.

As the Universe cooled, it underwent phase changes, from a plasma of subatomic particles to the formation of atoms.

The fundamental forces of nature were once a unified super force in the extremely high-energy early Universe.

Our understanding of physics is limited in describing the first 10^-43 seconds after the Big Bang.

The Universe's expansion and cooling led to the separation of the four fundamental forces and the formation of subatomic particles within the first second.

Nuclear fusion occurred between 3 to 20 minutes after the Big Bang, creating light elements like hydrogen, helium, and lithium.

The primordial ratio of hydrogen to helium in the Universe is consistent with measurements of the Sun's composition.

The Universe's structures, such as galaxies, began to form as gravity overcame the heat and allowed matter to clump together.

Recombination at 380,000 years post-Big Bang allowed electrons to combine with atomic nuclei, making the Universe transparent to light.

The cosmic microwave background radiation is the afterglow of the Big Bang, providing a snapshot of the early Universe.

Tiny fluctuations in the cosmic microwave background radiation indicate the initial seeds of cosmic structures.

Alan Guth's theory of cosmic inflation explains the uniformity of the Universe and the origin of its large-scale structure.

Inflation smoothed out the Universe, allowing for the formation of galaxies and clusters of galaxies from quantum fluctuations.

Our galaxy and others originated from these primordial fluctuations, growing over billions of years into the structures we observe today.

Despite significant progress, many questions about the Universe's origin, dark energy, and the possibility of multiverses remain unanswered.

The journey from the Big Bang to the present demonstrates the power of scientific inquiry in unraveling the mysteries of the cosmos.

Transcripts

Hey folks, Phil Plait here, and for the past few episodes, I’ve been going over what

we know about the structure, history, and evolution of the Universe, and how we know it.

Now it’s time to put that into action. We can use this knowledge of physics, math, and

astronomy to figure out what the Universe was like in the past, going all the way back

to literally the first moment after it was born.

So, here you go: a brief history of the Universe:

In the beginning there was nothing. Then there was everything.

Oh, you want more?

It may seem a little weird to suppose that we can understand how the Universe got its

start. But it’s much like any other field of science: We have clues, observations, based

on what we see going on now. Knowing the rules of physics we can then run the clock backwards

and see what things were like farther and farther into the past.

For example, as I’ve talked about in the past couple of episodes, the Universe is expanding.

That means in the past it was denser, more crowded, and hotter. At some point it was

hotter than the surface of a star, hotter than the core of a star, hotter than the heart

of a supernova. And as we push the timer back even farther we find temperatures and densities

that make a supernova look chilly and positively rarefied.

A lot of what we know about the early Universe comes from experiments done in giant particle

colliders. When the cosmos was very young and very hot, particles were whizzing around

at high speeds and slamming into each other, creating other subatomic particles in the

process. That’s exactly what colliders do! The higher energy we can give our colliders,

the faster we can whack particles together, and the earlier phase of the Universe we can

investigate. That’s one of the main reasons why we keep making ‘em bigger and more powerful,

to test our ideas of what the young cosmos was like.

So let’s wind the clock back. Looking around us, peering into the Universe both near and

far, what can we say about the beginning of everything?

When the Universe got its start, it was unfathomably hot and dense. It was totally different then

than it is today, because when you pump more energy into something, the way it behaves,

even its fundamental physical nature, changes.

If you take a snowball and heat it up, it’ll melt. We call that a phase change, or a change

of state. Heat it more and it vaporizes, changing into a gas. It’s still water, still composed

of water molecules, but it looks and acts pretty differently, right?

When you heat something up what you’re doing is giving it more energy. In a solid this

means the atoms wiggle around more and more until they break free of their restrictive

bonds with each other, and the solid melts. The atoms are still bound by other forces,

but if you heat them more they break free of those, too, and the liquid becomes a gas.

Heat them more, give them more energy, and the atoms whiz around faster and faster. Heat

them to millions or billions of degrees, and the atoms themselves fall apart. They collide

so violently they can overcome the hugely strong forces holding their nuclei together,

and you get a soup of subatomic particles; electrons, neutrons, and protons.

Heat them more and even protons and neutrons will collide hard enough to shatter into their

constituent subatomic particles, which are called quarks. And as far as we know, quarks

and electrons are basic particles, so they can’t be subdivided any more.

Maybe you can see where I’m going here. As we wind the clock backwards, the Universe

gets denser and hotter. At some point in the past it was so hot that atoms wouldn’t have

been able to hold on to their electrons. A little farther back and it was so hot that

nuclei couldn’t stay together, and the Universe was a small, ultra dense ball of energy mixed

with neutrons, protons, and electrons.

Go a wee bit farther back and even that changes. Neutrons and protons couldn’t form, because

the instant they did they’d whack into each other hard enough to fall apart. The Universe

was a sea of electrons and quarks.

The cosmos was a bizarre, unfamiliar place back then. Even the basic forces we see today

— gravity, electromagnetism, and the two nuclear forces responsible for holding atomic

nuclei together as well as letting them disintegrate in radioactive decay — were all squeezed

together into one unified super force.

Like the snowball melting and vaporizing, each of these moments in the history of the

Universe was like a phase change. The very nature of reality was changing, its laws and

behavior different. At some point, we go so far back, so close

to that first moment in time, that our laws of physics… well, they don’t break down

so much as say, “Here Be Dragons.” We just don’t understand the rules well enough

to be able to say anything about that first razor thin slice of time.

How far back are we talking here? If we call the instant of the Big Bang “time zero,”

then our physics cannot describe what happens in the first 10-43 seconds.

Now let me just say, only semi-sarcastically, that that’s not so bad. The Universe is

13.82 billion years old, so being able to go back to that very first one-ten-millionth

of a trillionth of a trillionth of a trillionth of a second is a massive triumph of physics!

What happened after that fraction of a second is better understood. The Universe expanded

and cooled, the four forces went their separate ways, and the first basic subatomic particles were

able to hold themselves together. This all happened in the very first second of the Universe’s existence.

Three minutes later — yes, three minutes — the Universe cooled enough that these

subatomic particles could start to stick together. For the next 17 minutes, the Universe did

something remarkable: it made atoms.

It was still ridiculously hot, like the core of a star, but it’s at those temperatures

that nuclear fusion can occur. For a few minutes the particles smashed together, forming deuterium,

an isotope of hydrogen, helium, and just a smattering of lithium. A little bit of beryllium

was made as well, but it was radioactive, and rapidly decayed into lithium.

Then, at T+20 minutes, the Universe cooled enough that fusion stopped. When it did, there

was three times as much hydrogen as helium in the Universe. This primordial ratio is

still pretty much true today. When we measure the Sun’s elemental abundance, we see it’s

roughly 75% hydrogen, 25% helium.

At this point the Universe is still hotter than a star’s surface, but it’s also still

expanding and cooling. As it does, structures start to form as the gravity of matter can

overcome the tremendous heat. These will become the galaxies we see today. This is important

and a bit weird, so I’ll get back to it in a minute.

The next big event happened when the Universe was at the ripe old age of about 380,000 years.

Up to this point, electrons couldn’t bond with the atomic nuclei zipping around; every

time they did it was so hot that random photons would blow them off again. The Universe was ionized.

But then, after 380 millennia, it had cooled enough that electrons could combine with protons

and helium nuclei, becoming stable neutral atoms for the very first time. We call this

moment “recombination.”

This was an important event! Free electrons are really really good at absorbing photons,

absorbing light. When the Universe was still ionized, prior to recombination, it was opaque.

A photon couldn’t get very far before an electron sucked it up.

But after recombination, the photons were free to fly. The Universe became transparent!

Why is this important?

Because the light emitted at this time is what we see as the cosmic microwave background

today! Those neutral atoms emitted light; they were as hot as a red dwarf star. Those

photons have been traveling ever since, fighting the expansion of the Universe, redshifting

into the microwave part of the spectrum, and seen today all over the sky. That background

glow predicted by the Big Bang model has been on its journey to Earth for almost 13.8 billion years!

This light is incredibly important, because it tells us what the Universe was like

not long after it formed.

For example, that light looks almost exactly the same everywhere you look in the sky. It

looks “smooth.” That tells us that matter was very evenly distributed everywhere in

the Universe at that time, and also that all the matter had the same temperature. If there

had been one spot that was denser, lumpier, then it would have been hotter, and we’d

see that in the background radiation as a patch of bluer light.

That’s pretty weird. When you look at the background radiation from opposite sides of

the sky, you’re seeing it coming from opposite ends of the universe! Even back then, those

regions of the Universe were separated by vast distances, and had plenty of time to

go their separate ways, change in different ways. They should look pretty different. But they don’t.

As telescopes got better, very tiny variations in the light were found. But they were really

teeny, only a factor of 1 in 100,000. In other words, one part of the sky may look like it

has a temperature of 2.72500 Kelvins, but another spot is at 2.752501.

The Universe had lumps, but they were far, far smaller than expected. Something must

have happened in the Universe to force it to be this smooth even hundreds of thousands

of years after the Big Bang.

This led theoretical physicist Alan Guth to propose a dramatic addition to the Big Bang

model: At some point in the very early Universe, the expansion suddenly accelerated vastly.

For the tiniest fraction of a second, space inflated hugely, far faster than the normal

expansion, increasing in size by something like a hundred trillion trillion times!

We call this super-expansion “inflation.” It sounds a little arbitrary, but it actually

has quite a bit of physical foundation now; in a sense it’s like one of the phase changes

of the Universe that happened in that first fraction of second dumped huge amounts of

energy into the fabric of space-time, causing it to swell enormously.

Inflation explains why the Universe was so smooth at the time of recombination: Space

expanded so rapidly that any lumps in it were smoothed out, like pulling on a bedsheet to

flatten out the wrinkles.

Inflation explains several other problems in cosmology as well, and although the details

are still being hammered out, the basic idea is almost – pardon the expression – universally

agreed upon by astronomers.

The fluctuations we see in the background glow now were actually incredibly small perturbations

in the fabric of space at the time of inflation, which got stretched by inflation to macroscopic

size. These denser spots were seeds, eventually growing even more, their gravity attracting

flows of dark matter.

Normal matter collected there too, condensing, eventually forming the first stars about 400

million years after the Big Bang. Eventually, those teeny little bumps from the beginning

of the Universe became galaxies and clusters of galaxies, now tens of billions of light

years away. Our own galaxy, our own piece of the Universe, started the same way, as

a quantum fluctuation in space 13.8 billion years ago.

Now look at us. How’s that for an origin story?

There are still many unanswered questions in our understanding of cosmology. What’s

dark energy? What was the role of dark matter in the early Universe? Where did the Universe

come from in the first place? Are there more universes out there? Hidden away where we

can’t see them? If time and space started in the Big Bang, does it even make sense to

ask what came before it, or is that like asking what’s north of the north pole?

We don’t know the answers to these questions, and trust me, there are thousands more just

like them. But here’s the fun part: we might yet be able to answer them! After all, even

asking if the Universe had a beginning, let alone what happened between then and now,

was nuts just a century or two ago. Now we have a decent handle on it, and our grip is

getting better all the time.

Science! Asking – and answering – the biggest questions of them all. I love this stuff.

Today you learned that the timeline of the Universe’s history can be mapped using modern

day physics and astronomical observations. It started with a Big Bang, when the Universe

was incredibly dense and hot. It expanded and cooled, going through multiple stages

where different kinds of matter could form. It underwent a phenomenally rapid moment of

expansion called inflation which smoothed out much of the lumpiness in the matter. Normal

matter formed atoms between 3 and 20 minutes after the bang, and the lumps left over from

inflation formed the galaxies and larger structures we see today.

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.