Dark Energy, Cosmology part 2: Crash Course Astronomy #43

Summary

This episode of Crash Course Astronomy explores the surprising discovery that the Universe's expansion is accelerating, contrary to expectations. It delves into the concept of dark energy, which is thought to be responsible for this acceleration, and discusses the implications for the Universe's future, including the shrinking of the observable Universe.

Takeaways

• 🌌 The Universe is expanding, with galaxies moving away from each other due to the expansion of space itself.
• 🔴 The Big Bang is the current working model for the start of the Universe, occurring about 13.82 billion years ago when all matter and energy were compressed into a singularity.
• 🌐 On large scales, the expansion of the Universe is faster, causing distant galaxies to appear as if they are rushing away from us.
• 🌠 The expansion of the Universe is influenced by gravity, but on small scales, gravity can overcome the expansion, as seen with the Andromeda galaxy moving towards us.
• 💥 The discovery of dark matter was expected to slow down the expansion of the Universe, but instead, observations showed an acceleration.
• 🌌 Astronomers used Type Ia supernovae as standard candles to measure the expansion rate of the Universe, finding that it is accelerating rather than slowing down.
• 🚀 The acceleration of the Universe's expansion is attributed to dark energy, an invisible form of energy that permeates space and pushes on the expansion.
• 🌟 Dark energy is estimated to make up about 68% of the total energy content of the Universe, significantly more than normal matter.
• 🌍 The geometry of the Universe, which describes its overall curvature, was once thought to determine its fate, but the presence of dark energy has complicated this.
• 🌠 The observable Universe has a radius of about 45 billion light years, but due to the accelerating expansion, this cosmic horizon is effectively moving closer, making the Universe appear to shrink in our view.

Q & A

What is the singularity in the context of the Big Bang theory?

The singularity refers to the point in time, 13.82 billion years ago, when all of space, time, matter, and energy were compressed into an infinitely dense point, initiating the Big Bang and the expansion of the Universe.

Why do distant galaxies appear to be moving away from us faster?

Distant galaxies appear to be moving away faster due to the expansion of space itself; the larger the scale, the faster the expansion, which is why their recession velocities are higher.

How does the Andromeda galaxy's motion counteract the expansion of space?

The Andromeda galaxy is moving toward us due to the mutual gravitational pull between our galaxies, which is strong enough to overcome the expansion of space on a local scale.

What is the significance of Type Ia supernovae in measuring cosmic distances?

Type Ia supernovae are significant because they have a consistent peak luminosity, making them 'standard candles'. By comparing their known intrinsic brightness with their observed brightness, astronomers can calculate their distances.

Why was the discovery that the Universe's expansion is accelerating so surprising?

It was surprising because astronomers expected gravity from all the matter in the Universe to slow down the expansion. Instead, they found evidence of an unknown force, later termed 'dark energy', that is causing the expansion to accelerate.

What is dark energy and how does it affect the Universe's expansion?

Dark energy is a mysterious form of energy that permeates all of space, exerting a negative pressure that causes the expansion of the Universe to accelerate, contrary to the deceleration expected due to gravity.

How does the concept of 'escape velocity' relate to the expansion of the Universe?

Escape velocity is analogous to the Universe's expansion in that if an object (or galaxy) is moving fast enough, it can escape the gravitational pull (or expansion) and move away indefinitely. If not, it slows down and falls back.

What is the current understanding of the Universe's ultimate fate given the presence of dark energy?

The current understanding, supported by measurements of dark energy, suggests that the Universe will continue to expand forever, with the expansion rate accelerating over time.

What is the observable Universe and how is its size affected by the expansion?

The observable Universe is the part of the Universe from which light has had time to reach us, currently about 45 billion light years away. Its size is affected by the expansion of space, which, due to dark energy, is accelerating, making the observable Universe effectively shrink over time.

Why is the cosmic horizon approaching us and what are its implications?

The cosmic horizon is approaching due to the accelerating expansion of space caused by dark energy. This implies that in the future, we will only be able to see galaxies within our local group, as all others will recede beyond the cosmic horizon, making the night sky darker.

How does the concept of the Universe's geometry relate to its fate?

The geometry of the Universe, which describes its overall curvature and the shape of space on the largest scales, was once thought to determine whether the Universe would expand forever or eventually recollapse. However, the discovery of dark energy has shown that the Universe will likely expand indefinitely, regardless of its geometry.

Outlines

00:00

🌌 The Expansion and Mystery of the Universe

This paragraph delves into the concept of the expanding Universe, which was once thought to be denser and has been expanding since the Big Bang 13.82 billion years ago. It explains how galaxies are carried along by this expansion, with gravity playing a role in local motion, such as the Andromeda galaxy moving towards us despite the overall expansion. The expectation was that the Universe's expansion would slow down due to gravity, especially with the consideration of dark matter. However, the discovery in the 1990s of distant Type Ia supernovae, which were dimmer than expected, indicated an accelerating expansion, contradicting previous theories. This unexpected acceleration led to the hypothesis of 'dark energy,' an unseen force that permeates space and accelerates the expansion of the Universe.

05:01

🌟 The Prevalence of Dark Energy and Its Impact

The second paragraph discusses the composition of the Universe, highlighting that 95% of it is made up of dark energy and dark matter, which are not directly observable. It emphasizes the significant role of dark energy in shaping the Universe's fate, particularly its effect on the expansion rate. The paragraph explains the concept of the Universe's geometry and how it relates to the amount of matter present, which was initially believed to determine whether the Universe would continue expanding or eventually recollapse. However, the discovery of dark energy suggests that the expansion will continue indefinitely, overriding the effects of gravity and the Universe's geometry. The paragraph also touches on the idea that as space expands, light loses energy, leading to a cosmological redshift, and that galaxies moving away at the speed of light are observable because the Universe was smaller and denser in the past, allowing their light to reach us despite the current rapid expansion.

10:03

🌠 The Shrinking Observable Universe and Its Implications

The final paragraph explores the implications of the Universe's accelerating expansion due to dark energy, which suggests that the observable Universe is paradoxically getting smaller over time. It explains that as space expands at an accelerating rate, the light from distant galaxies is increasingly hindered by the expansion, making it more difficult for us to observe them. The paragraph introduces the concept of the cosmic horizon, which is the limit of what we can observe, and notes that this horizon is effectively moving closer to us as the expansion outpaces the light's ability to reach us. The conclusion is that while the Universe itself expands, our view of it diminishes, and we may have only a few trillion years left to observe and understand it before our observable Universe shrinks to a point where only our own galaxy is visible in the sky.

Keywords

💡Universe Expansion

The concept of 'Universe Expansion' refers to the ongoing increase in the scale of the observable Universe. It is the central theme of the video, explaining how galaxies are moving away from each other due to the expansion of space itself. The script mentions this phenomenon with the statement 'the Universe is expanding, space is expanding,' and further discusses its implications on the cosmic scale.

💡Singularity

In the context of the video, 'Singularity' is defined as the point in time when all matter, space, and time were infinitely dense and occupied a single point. It is the starting point of the Universe as per the Big Bang theory, which the script describes as 'all of space, time, matter, and energy was compressed into a single, infinitely dense point.'

💡Big Bang

The 'Big Bang' is the prevailing cosmological model for the beginning of the Universe. The script explains it as the event when the singularity 'suddenly let loose, expanding violently, cooling, and forming the Universe we see today.' It is foundational to understanding the origin and evolution of the Universe.

💡Galaxies

Galaxies are vast systems of stars, dust, and dark matter bound together by gravity. The script uses 'galaxies' to illustrate the observable phenomena of the Universe's expansion, noting how 'all the galaxies we see are moving away from each other as space expands between them.'

💡Dark Matter

'Dark Matter' is a hypothetical form of matter that does not emit or reflect light, but is inferred from its gravitational effects on visible matter. The script introduces it with the expectation that its presence 'meant the Universe should be slowing down even more than we first thought!' However, the discovery of an accelerating Universe challenged this expectation.

💡Type Ia Supernovae

A 'Type Ia Supernovae' is a specific type of stellar explosion that occurs in binary star systems and is used as a standard candle in astronomy due to its consistent peak luminosity. The script discusses these supernovae as crucial to the discovery that 'the expansion of the Universe was accelerating,' contrary to the expectations based on gravity and dark matter.

💡Dark Energy

'Dark Energy' is a term used to describe the unknown force that is causing the acceleration of the Universe's expansion. The script introduces it as 'an energy suffusing space, pushing on the expansion' and notes that it 'seems to be a property of space itself,' suggesting it is responsible for the Universe's ever-increasing rate of expansion.

💡Cosmological Redshift

The 'Cosmological Redshift' is the increase in wavelength of light emitted by objects moving away from the observer, which in this context, are galaxies in the Universe. The script explains it as 'the more distant the galaxy, the faster it recedes, and the more energy light loses as it travels to us,' which is a direct result of the Universe's expansion.

💡Observable Universe

The 'Observable Universe' refers to the part of the Universe that we can currently see or detect. The script discusses the concept, stating 'the most distant galaxies we see are now about 45 billion light years from us. We call this the radius of the observable Universe.' It is the limit of our current cosmic horizon.

💡Escape Velocity

In the script, 'Escape Velocity' is used as an analogy to describe the speed needed to overcome the gravitational pull of a celestial body. It is extended to the concept of the Universe's expansion, where if the expansion is not fast enough, gravity could eventually halt and reverse it, similar to an object not escaping Earth's gravity.

💡Cosmic Horizon

'Cosmic Horizon' is the boundary of the observable Universe, beyond which objects are moving away from us faster than the speed of light, and thus their light will never reach us. The script mentions that 'the cosmic horizon is approaching us,' indicating the shrinking of the observable Universe due to the accelerating expansion.

Highlights

The Universe is expanding, with space carrying galaxies along with it.

The singularity, an infinitely dense point, existed 13.82 billion years ago before the Big Bang.

Galaxies' motion is influenced by both the expansion of space and local gravitational effects.

The Andromeda galaxy is moving towards us due to mutual gravity overcoming the expansion.

Astronomers expected the Universe's expansion to slow down due to gravity but found it accelerating instead.

The discovery of dark matter suggested an even greater deceleration of the Universe's expansion.

Observations of distant Type Ia supernovae revealed the unexpected acceleration of the Universe's expansion.

Type Ia supernovae serve as standard candles for measuring cosmic distances due to their consistent luminosity.

The acceleration of the Universe's expansion contradicts the expected effects of gravity from matter.

Dark energy, an unknown form of energy, is hypothesized to be causing the accelerated expansion.

Dark energy is considered a property of space itself, with a small amount of energy in every cubic centimeter.

The Universe's composition is estimated to be 95% dark energy and dark matter, with normal matter being a minority.

Dark energy's influence suggests the Universe will expand indefinitely, regardless of its geometry.

The geometry of the Universe, describing its overall curvature, was once thought to determine its fate.

The observable Universe has a cosmic horizon, currently about 45 billion light-years away.

Due to dark energy, the observable Universe is actually shrinking, with the cosmic horizon approaching.

In the far future, only our own galaxy may be visible as the cosmic horizon recedes to a few hundred thousand light-years away.

The Universe's expansion and the shrinking of the observable Universe present a paradoxical scenario.

Transcripts

The previous episode of Crash Course Astronomy was a bit of a brain-stretcher. We saw that

the Universe is expanding, space is expanding, and it’s carrying galaxies along with it.

That means it was denser in the past, and at some point — 13.82 billion years ago,

to be fairly precise — all of space, time, matter, and energy was compressed into a single

infinitely dense point. Astronomers call this the singularity, which is as good a name as any.

Something caused this singularity to suddenly let loose, expanding violently, cooling, and

forming the Universe we see today. Coming to grips with this idea took a while for astronomers,

but nowadays the current working model for how the Universe started is with a Big Bang.

All the galaxies we see are moving away from each other as space expands between them.

That is, on large scales. Remember the ruler analogy, where on small scales the expansion

is small, and on bigger scales the expansion is faster. That’s why distant galaxies appear

to be rushing away from us faster.

On small scales, the expansion is small enough that gravity can overcome it. The Andromeda

galaxy, for example, is about 2.5 million light years away. That means it should be

moving away from us as at about 50 or so km/sec. But because of our mutual gravity, it’s

moving TOWARD us; its motion locally through space is more than enough to overcome the

expansion of space between us.

It’s like running up the down escalator. Run fast enough and you can make it to the top.

But every galaxy has gravity, and there are a lot of galaxies in the Universe! That adds

up, and should affect the expansion rate. It’s a lot like the idea of escape velocity:

Throw a rock hard enough and, even though gravity will slow it down, it will escape.

But if you don’t throw it fast enough, it’ll slow, stop, reverse course, and fall back down.

Astronomers fully expected to see this effect on the expansion of space. If you looked on

the very largest of scales, you’d expect to see the Universe slowing down, the gravity

of the matter in the Universe itself putting the brakes on the expansion. And with the discovery

of dark matter, that meant the Universe should be slowing down even more than we first thought!

But when they went looking for evidence of this, what they got instead was probably the

single biggest shock in the history of astronomy.

In the 1990s, two teams of astronomers were using the world’s biggest telescopes to

peer as deeply as they could into the Universe. They were looking for incredibly distant supernovae.

And not just any kind, but special ones called Type Ia’s.

I talked about these before. They occur when a white dwarf increases in mass until electron

degeneracy pressure can no longer sustain it against its own gravity. It collapses,

undergoes a catastrophic wave of thermonuclear fusion, and explodes.

The beauty of these types of supernovae is that they all occur when the mass of the white

dwarf gets to about 1.4 times the mass of the Sun; that’s the magic number where pressure

overcomes gravity, and they go kablooie.

That makes them good standard candles: objects whose intrinsic brightness, whose luminosity,

is known. Knowing that, plus measuring how bright they appear to be in a telescope, lets

you calculate their distances.

Then those can be compared to the supernovae redshifts, which is a different way of getting

their distance. This then tells you how fast the Universe is expanding on really big scales.

But the results they got didn’t make sense.

Time and again, the supernovae were all fainter than they expected. It was as if the predictions

based on the redshifts were underestimating the distances to the exploding stars. The

astronomers did everything they could to see if maybe they had made a mistake somewhere,

including making a literal list of things that can make stars look fainter — intergalactic

dust, different chemical compositions for the stars that blew up — all kinds of things.

But in the end, both teams independently came to the same conclusion: The supernovae were

farther away than expected.

And that meant something truly shocking: the expansion of the Universe was accelerating.

Now remember, that’s nuts. We were expecting the expansion of space to be slowing down

due to the gravity of all the matter in the Universe. But instead, it was speeding up.

It’s hard to overstate how shocking this is. It’s like tossing a rock in the air,

and instead of it slowing down and falling back down into your hand, it shot upwards

faster and faster, defying Earth’s gravity.

Of course, scientists were skeptical. Many still are. But in the end, several other independent

measurements have verified this result. The Universe is not only expanding, but that expansion

is getting faster every day.

What could possibly cause such a thing?

To be flatly honest, we don’t know. Well, not exactly. But whatever it is acts like an

energy suffusing space, pushing on the expansion. And we can’t see it, so it’s invisible.

We already have dark matter, so naturally this got tagged “dark energy.”

It seems to be a property of space itself, a tiny bit of energy in every single cubic

centimeter of the Universe. The amount per cc is incredibly small, but there are a lot

of cubic centimeters in the Universe. It adds up.

And we can add it up. Now that we have measurements of this, we can take an inventory of the Universe!

We can total all the matter and energy in the Universe, making a sort of budget of stuff

in the cosmos. When we do, it looks like this:

95% of the universe

is made of stuff we can't directly see.

Normal matter is outnumbered 20 to 1. Maybe we should rethink calling it “normal.”

So if 2/3rds of the cosmic budget is made up of dark energy, it must have some pretty big effects, right?

Yeah. Like, changing the eventual fate of the entire Universe.

A big question – one of the biggest – is, “Will the Universe expand forever?”

Well, astronomers have a framework to answer this question: We call it the geometry of

the Universe. Matter has gravity, and gravity bends space, so is there enough matter in

the Universe to stop the expansion? The geometry of the Universe mathematically describes its

overall curvature, the shape of space on the largest scales of all.

To be clear, this concept is important to cosmologists, but it can be weird and confusing

to someone who is just learning about all this. Still, a lot of astronomy classes teach

it, so I’m going to go over it very briefly, and if you want more information, we have links in the dooblydoo.

The idea behind the geometry of the Universe is that if there’s enough matter in the

Universe the expansion will slow, stop, and then everything will recollapse; a sort of

Big Bang in reverse. If there’s not enough matter then the Universe will expand forever.

And in between the two there’s just enough matter that the expansion slows, but never

quite reaches 0 until an infinite amount of time in the future. Conceptually, it’s a

lot like escape velocity.

It was once thought that the geometry of the Universe, tied to the amount of matter in

it, determined its destiny. But dark energy threw a monkey in the wrench for that, and

geometry alone doesn’t determine the eventual fate of the cosmos.

We think there’s enough dark energy in space to ensure the expansion will continue forever,

despite the geometry. Dark energy is just too powerful, and will always drive the expansion

of the universe ever-faster.

So, for now, the answer as far as we can see, is: Yes, the Universe will expand forever.

OK, there’s one more brain-melty thing we need to talk about. For this part you might want to sit down.

Space itself is expanding. As light travels from one galaxy to the next, it fights that

expansion, losing energy – just like you use up energy climbing a staircase. When light

loses energy its wavelength gets longer – that’s what cosmological redshift is.

The more distant the galaxy, the faster it recedes, and the more energy light loses as

it travels to us. But wait. At some distance from us, space would be expanding so quickly

that a galaxy in that part of the Universe would be moving away from us at the speed

of light. Anything farther away would be swept away from us faster than light.

Now, before you start complaining, yes, this IS possible. The speed of light is the ultimate

speed limit in the Universe – if you’re traveling through space. But space itself

is exempt from that rule; it can expand at whatever speed it wants. The matter in it

– galaxies, stars, and such – is swept along with it, so they’re not traveling

so much through space as with it.

When you solve the equations to calculate distance and redshift, the distance a galaxy

would have to be from us to be moving away at the speed of light is about 13.8 billion light years.

Here’s the fun thing: We can still see galaxies that far away. We can even see them farther than that. How?

It’s because that distance is how far the galaxy is from us now. When it emitted that

light the Universe was much younger, smaller, and the galaxy closer to us. It would’ve

been about 4.5 billion light years away at the time, and the light took over 9 billion

years to get here. Back then, the space between us and the galaxy wasn’t expanding as rapidly,

so the light could keep pace. now, after all this time, the space between us and that galaxy

means we’re moving away from each other at lightspeed. But back then we were close

enough to see each other.

For that same reason we can see galaxies that are moving away from us faster than light,

because when they emitted that light there was less space separating us. The most distant

galaxies we see are now about 45 billion light years from us. We call this the radius of

the observable Universe. It’s essentially the cosmic horizon.

Mind you, the actual Universe may be far larger than this… who knows, maybe even infinite.

But we can only see galaxies that are, by now, 45 billion light years away. That’s our horizon.

If space were simply expanding, the size of the observable Universe would expand as well.

But we have dark energy, and space expands more rapidly every day. That means a truly

distant galaxy’s light is fighting more and more expansion all the time – it’s

like climbing an ever-steepening staircase. Compared to a constant expansion, a galaxy

in an accelerating Universe has to be closer for us to see it. It’s losing energy faster.

Worse, that fight gets harder every day. A galaxy just at the cosmic horizon, right on

the edge of the observable Universe now might be visible, but in the future the space between

us and it will have expanded even more. The light can’t beat that expansion, and it’ll

never reach us. The galaxy, over time, disappears.

This has a weird and unnerving effect: The observable Universe is getting smaller. The

cosmic horizon is approaching us. Eventually it’ll be so close that every galaxy in the

sky except our own will lie beyond it. At that point our own galactic gravity may overcome

the expansion of space, so we’ll remain intact, but the sky beyond will be black,

the edge of the Universe hanging over us just a few hundred thousand light years away instead

of tens of billions.

It’s the ultimate irony. The Universe itself is expanding, but we see less of it every

day. So we’d better study it while we can; we

may only have a few trillion more years left to figure this all out.

Today you learned that the majority of the Universe is made up of dark energy, a currently

mysterious entity that pervades space. We don’t know exactly what it is, but we can

understand what it does – it accelerates the expansion of space. We think this means

the Universe will expand forever, even as our view of it shrinks as space expands faster all the time.

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

YouTube channel to catch even more awesome cool 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.