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The Cosmic Distance Ladder – How We Measure the Universe · Talking Science

0:00
So picture yourself for a second standing right
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on the edge of a wide, just incredibly deep river
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gorge. Okay, I'm picturing it. And you're looking
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out and there is this specific tree on the far
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bank. You really want to know exactly how far
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away that tree is. Right. But there's a massive
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problem. You can't just walk across and pace
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it out, right? And you can't, like, stretch a
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tape measure over open air and 100 meters of
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rushing water? Obviously not. You are completely
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stuck on your side of the cliff. Welcome to our
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deep dive for today, by the way. I'm your host,
0:32
and today we're looking at an article about how
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human beings actually measure the vastness of
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the universe. And I'm your resident astronomy
0:40
expert. Yeah, that feeling of being stuck on
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one side of an uncrossable gap. That is the exact
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position humanity is in every single time we
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look up at the night sky. Because we want to
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know how big the universe is. Exactly. We want
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to know how far away things are. But we are permanently
0:55
stranded on Earth. We just we can't stretch a
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tape measure to the stars. But if you know just
1:01
a little bit of geometry, you don't actually
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need to cross that gorge. You can just walk a
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short measured distance along your own side of
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the cliff. Right, which we would call a baseline.
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Right. So you measure the angle to the tree from
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the start of your walk. Then you walk, say, 10
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paces. And you measure the angle to the tree
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from the end of your walk. Suddenly you have
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a triangle. And simple trigonometry hands you
1:24
the distance across the water. No crossing required.
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It's so cool. It really is. And that simple geometric
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trick, using a known baseline and a couple of
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angles to find a distance you can't measure directly,
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that is the seed of one of the greatest achievements
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in modern astronomy. Which is what they call
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the cosmic distance ladder, right? Yeah, exactly.
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It's the exact system astronomers use to figure
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out the distance to almost everything in the
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cosmos. From, you know... our own moon out to
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galaxies so ancient their light has been traveling
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toward us since before the earth even formed
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wow okay let's unpack this because climbing that
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ladder is basically our mission for this deep
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dive if you want to understand how we measure
2:04
the universe without prior astronomy knowledge
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you really have to understand this ladder and
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it really is a ladder Because no single measurement
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method works at every scale of the universe.
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You literally have to climb rung by rung. Right.
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Each new technique you use is calibrated against
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the one right below it, which lets you reach
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further and further out. Exactly. I mean, if
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everything relies on the step below it, we have
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a chicken and egg problem. To measure the tree
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across the gorge, I had to walk 10 paces. I had
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to, like, establish a physical baseline. If we
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are stuck on Earth... How do we get that very
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first baseline to start measuring space? Well,
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we just bounce a signal off our neighbors. Wait,
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really? Yeah. The absolute bottom run of the
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ladder doesn't rely on starlight at all. It relies
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on radar. Oh, like air traffic control. Exactly
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like that. We use the exact same basic mechanism,
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but astronomers aim it at solid planets within
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our solar system. You send out a burst of radio
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waves into space toward Venus or Mars, and you
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just wait for the echo to return to Earth. So
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we are just timing the echo. Yep, because we
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know how fast radio waves travel. They travel
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at the speed of light, which is a fixed universal
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constant. It never changes. Okay, so if I send
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a radio burst to Venus and it takes exactly,
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say, four minutes for the echo to hit my receiver,
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I can just do the simple math to figure out the
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total distance of that round trip. Right. And
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because these measurements are direct and physical,
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they give us these rock solid distances within
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our own solar system. Like we can measure the
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distance to Venus down to a matter of meters.
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That's insane. Down to meters. Yeah. But if we
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connect this to the bigger picture, the real
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power of this radar trick isn't just knowing
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where Venus is. If we map out the distances to
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the other planets, we can use the laws of gravity
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to perfectly calculate the exact size of the
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Earth's orbit around the sun. OK, I see. So we
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use. the radar to figure out the distance from
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the earth to the sun and i know there's a technical
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term for that right yeah that specific distance
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is called an astronomical unit you got it And
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so that single number, that astronomical unit,
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that becomes the baseline for the next rung up
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on our ladder. Yes. We use the entire Earth's
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orbit to recreate your trick at the River Gorge.
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Which brings to mind a trick a lot of us played
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with as kids. Oh, the finger trick. Yeah, the
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finger trick. If you're listening, you can do
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this right now. If you hold your finger up right
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in front of your face and you close one eye.
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and then open and close the other eye, your finger
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seems to jump left and right against the background
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of the room. It's a great analogy. And the closer
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your figure is to your face, the bigger that
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apparent jump is. Right. And astronomers call
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that apparent shift parallax. It happens anytime
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you view the same object from two slightly different
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positions. For your finger, the two positions
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are your left eye and your right eye. And for
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a star. For a star, astronomers use the Earth's
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orbit. they look at a nearby star in january
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then they wait six months until july by july
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the earth has traveled hundreds of millions of
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kilometers to the complete opposite side of the
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sun so january is one eye and july is the other
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eye exactly and the background of the room is
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the field of much more distant essentially fixed
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stars when they look at the target star again
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in july it appears to have shifted slightly against
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that background. Wow. And because the radar measurements
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gave us the exact diameter of the Earth's orbit,
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we know the exact length of our baseline. We
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measure the angle of the star's shift, and geometry
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hands us the distance to the star. It's incredibly
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elegant. Because you don't need to know anything
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about the star itself. You don't need to know
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how bright it is or how hot it is. It's just
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pure math. It's just triangles, yeah. But stepping
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back for a sec, when you look at the sheer scale
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of what they're trying to measure, it seems kind
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of impossible. The shift they're looking for
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isn't like a finger jumping across a wall. It
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is staggeringly small. It is very, very small.
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The source mentions we are talking about a fraction
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of an arc second. To put that into human terms
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for you listening. Observing that shift is roughly
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equivalent to trying to measure the width of
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a single human hair viewed from several kilometers
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away. Yeah, it's wild to think about. Trying
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to do that from the surface of the Earth, looking
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up through our blurry, turbulent atmosphere,
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it seems like a nightmare. And it was, was a
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massive limiting factor for a long time. The
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atmosphere constantly distorts the light, which
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meant we could only use parallax for a few thousand
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of the very closest stars. To get past the blur,
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you have to leave the atmosphere. So in 2013,
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the European Space Agency launched a space observatory
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called Gaia. Oh, I've heard of Gaia. Yeah. By
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operating in the vacuum of space, Gaia has been
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able to precisely map the positions and parallax
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shifts of well over a billion stars. It pushed
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reliable geometric measurements out from a tiny
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local bubble to distances of many thousands of
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light years. A billion stars. That is just, I
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mean, that's incredible. But even with Gaia.
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In space, there has to be a physical limit to
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this, right? Yeah. The universe is massive. Yeah,
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there absolutely is a limit. Because if a star
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is millions of light years away, the angle of
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that shift eventually becomes so infinitesimally
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small that no telescope could ever measure it.
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The geometry math just breaks down. We need a
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new rung. We do. When geometry runs out, we have
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to start judging distance by brightness. Which
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means we run into the campfire problem. The campfire
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problem, exactly. Imagine you are standing in
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a dark field at night and you see a light in
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the distance. How do you know if you are looking
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at a massive bonfire 10 miles away or like a
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tiny matchstick just one mile away? You don't.
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Right, they might look exactly the same brightness
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to your naked eye. In astronomy, the solution
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to that campfire problem is finding what we call
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a standard candle. A standard candle, okay. Yeah,
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the whole trick of a standard candle is finding
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an object in space where we somehow already know
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its true intrinsic brightness. If we know exactly
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how many watts of light a star is pumping out
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and we measure how dim it appears from Earth,
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the laws of physics tell us exactly how far that
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light had to travel to fade by that amount. I
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see. But getting that intrinsic brightness is
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the hard part, isn't it? We can't just fly out
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to a star to check its wattage. How do we know
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the true brightness of a star we've never been
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to? Well, the breakthrough came from a rather
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unassuming type of star known as a Cepheid variable.
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It's named after the constellation Cepheus, where
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an early one was spotted. Okay, a Cepheid variable.
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What does it do? A Cepheid isn't a static star
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like our Sun. It rhythmically brightens and dims
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over a period of days or weeks. It is literally
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pulsating. Its outer layers physically expand
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and contract. Oh, wow. So it's basically a giant.
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cosmic blinking light. Pretty much. And the way
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we cracked the code on these blinking lights
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goes back to an astronomer named Henrietta Leavitt
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at the Harvard College Observatory in the early
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20th century. Right. She was studying photographs
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of thousands of these pulsating cepheid stars
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in a specific cluster called the Small Magellanic
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Cloud. And her choice to study that specific
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cloud, that is the brilliant part. And unlocked
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everything. Because the small Magellanic cloud
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is a dwarf galaxy orbiting our own. So since
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all the stars Leavitt was studying were packed
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together in that one distinct cloud, she could
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safely assume they were all roughly the exact
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same distance from Earth. Oh. Which isolates
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the variable. Oh, exactly. If she sees two blinking
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stars in that cloud and one looks brighter than
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the other, she knows it isn't because it's closer
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to Earth. It has to be genuinely intrinsically
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brighter. Right, and she realized the rhythm
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tells you the brightness. She discovered a mathematically
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consistent rule called the period -luminosity
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relationship. Basically, the longer a cepheid
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takes to pulsate, The slower its blinking rhythm,
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the more intrinsically luminous the star actually
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is. So a slow blink means a massive bright star.
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A fast blink means a smaller, dimmer star. That
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is such an elegant key to the universe. It's
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beautiful physics. But how do we put it on the
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ladder? Like, we know the rule now, but we still
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need a hard distance to calibrate it. We use
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the rung below. Astronomers looked for cepheid
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variables that were close enough to Earth that
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we could measure their distance using the parallax
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geometry we just talked about. The parallax gave
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us the exact distance. Knowing the exact distance
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allowed us to calculate the exact intrinsic brightness
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of those nearby cepheids. And that calibrated
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Henrietta Leavitt's rule. Wow. And once it's
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calibrated, parallax doesn't matter anymore.
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Not at all. We can look deep into space, find
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a completely new cepheid. Time its blink, use
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leave its math to find its true brightness and
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just calculate the distance. And this was the
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exact tool Edwin Hubble used in the 1920s to
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change our entire understanding of reality. It
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really was. Because before Hubble, people saw
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these fuzzy patches of light in the night sky
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and assumed they were just gas clouds inside
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the Milky Way. They thought the Milky Way was
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the entire universe. Which is funny to think
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about now. Right. But Hubble found cepheids inside
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those fuzzy patches, timed their blanks, and
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realized those patches were entirely separate
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galaxies, located millions of light years away.
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The universe got unimaginably bigger overnight.
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It was a profound paradigm shift. But just like
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with parallax, Cepheids eventually hit a wall.
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As we try to measure galaxies billions of light
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years away, individual pulsating stars just fade
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from view. Too far away to see the blink. Exactly.
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Even our best telescopes can't pick out a single
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Cepheid in a galaxy halfway across the observable
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universe. So we need a brighter candle. We need
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an explosion. Yes, we do. Which brings us to
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the next rung on the ladder. A specific kind
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of stellar detonation called a Type Ia supernova.
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Right. And these are some of the most violent
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events in the cosmos. A Type Ia supernova starts
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with a white dwarf. Okay, remind me what a white
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dwarf is. A white dwarf is essentially the dense,
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compact corpse of a star that was once similar
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to our sun. Imagine this dense white dwarf is
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locked in a tight orbit with a normal living
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companion star. Because the white dwarf has such
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intense gravity, it starts siphoning gas off
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its companion. Oh man, so it acts like a cosmic
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vampire just constantly feeding off the other
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star? That's a good way to picture it. But it
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can't just gorge itself forever, right? A star's
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core can only support so much weight. before
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gravity crushes it exactly there is a very specific
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physical threshold as the white dwarf pulls more
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and more gas onto itself its mass increases eventually
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it approaches a critical mass limit and then
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what the moment it hits that exact threshold
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the core can no longer support its own weight
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The white dwarf collapses inward, triggering
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a runaway thermonuclear explosion. The entire
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star detonates. The whole thing. And because
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that detonation is triggered at the exact same
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mass limit every single time, the resulting explosion
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is almost exactly the same size every single
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time. Right. The physics dictate that the bomb
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always has the exact same yield. It creates a
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thermonuclear flashbulb so bright it can briefly
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outshine an entire galaxy of 100 billion stars.
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And because they are phenomenally bright and
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incredibly consistent, Type Ia supernovae are
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the ultimate standard candles. We can spot them
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in galaxies billions of light years away, far
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beyond the reach of any individual cepheid star.
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But wait, so what does this all mean for the
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latter? Every rung has to connect. We know the
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supernova explodes at a specific mass, but we
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don't automatically know its exact brightness
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in watts. True. We have to calibrate the flashbulb.
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which means we have to wait for a type E supernova
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to go off in a galaxy that is close enough to
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us that we can also measure the Cepheid variables
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in that same galaxy. And that is the defining
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vulnerability of the entire cosmic distance ladder.
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The supernovae are calibrated by the Cepheids.
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The Cepheids are calibrated by parallax. Parallax
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is calibrated by radar. Nothing stands alone.
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It's a house of cards. Basically. If we made
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a tiny math error decades ago calculating the
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distance to Venus with radar, that error propagates
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up. It corrupts the parallax, which corrupts
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the cepheids, which means our entire map of the
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deep universe is wrong. Tell me cosmologists
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don't just rely on this one single fragile ladder.
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Oh, they don't. That house of cards scenario
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keeps scientists away at night. I bet. So they
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build independent backup methods to cross -check
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the wrongs. For instance, there is a principle
14:21
called the Tully -Fisher relation. What's that?
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This rule links how fast an entire spiral galaxy
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is spinning to how intrinsically bright the whole
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galaxy is. If you can measure the rotation speed,
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you can estimate the galaxy's brightness and
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therefore its distance without needing to isolate
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individual stars at all. Okay, that makes me
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feel better. And there is another backup called
14:42
the tip of the red giant branch. The tip of the
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red giant branch? Yeah. This looks at populations
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of aging stars. Right before an old star undergoes
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a specific internal change in how it burns helium,
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it hits a very sharp, predictable peak in brightness.
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That peak acts as an alternative standard candle.
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Ah, I see. Because these backup rungs generally
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agree with the suffades and the supernovae, astronomers
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have a high degree of confidence that the latter
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is fundamentally sound. That's reassuring. So
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we have our ladder. We can measure out to the
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deepest reaches of space. But what if we want
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to measure the expansion of the space itself?
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Then we need a new trick. Right. And to do that,
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we have to look at how light behaves when an
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object is moving away from us. If you are standing
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on a sidewalk and an ambulance speeds past you,
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the pitch of the siren Sounds higher as it approaches
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and then visibly drops to a lower pitch as it
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speeds away. The Doppler effect. Yeah, the Doppler
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effect. The sound waves get stretched out as
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the vehicle recedes. And light does the exact
15:45
same thing. When a galaxy is moving away from
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us, the light waves it emits get stretched out.
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In the visible spectrum of light, longer wavelengths
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look red. Okay. So as the light stretches, its
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color shifts toward the red end of the spectrum.
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Astronomers call this redshift. Redshift. And
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in the 1920s, Edwin Hubble. the same guy who
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used cepheids to discover other galaxies, he
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started measuring the redshift of those galaxies.
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He was a busy guy. Seriously. He compared how
16:10
far away a galaxy was to how stretched out its
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light was, and he found a direct link. He discovered
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what we now call Hubble's Law. The further away
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a galaxy is from us, the faster it is receding.
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To visualize why this is happening, imagine taking
16:25
a deflated balloon and drawing a bunch of dots
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all over it with a marker. Each dot represents
16:30
a galaxy. Now start blowing up the balloon. As
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the rubber expands, every single dot gets further
16:37
away from every other dot. The dots aren't actively
16:41
crawling across the surface. The rubber itself
16:43
is stretching, carrying the dots with it. That
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is what space is doing. It is expanding in all
16:49
directions. And because Hubble's law ties distance
16:52
directly to redshift, it becomes a powerful tool.
16:55
If you can measure how stretched out a galaxy's
16:58
light is, you instantly know how fast it is moving
17:00
and roughly how far away it is. Which sets up
17:03
one of the biggest plot twists in modern physics.
17:06
Oh, yeah, this is a big one. In the late 1990s,
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teams of astronomers were using our ultimate
17:10
standard candles, the type Ia supernovae, to
17:14
measure distances to very remote galaxies. But
17:17
the data came back weird. The distant supernovae
17:20
were noticeably dimmer than they should have
17:21
been. And everyone at the time assumed the expansion
17:24
of the universe was either coasting at a steady
17:27
speed or gradually slowing down due to the inward
17:30
pull of gravity. But the math didn't lie. The
17:33
supernovae were dimmer. But if they're dimmer
17:35
than expected, it means they're further away
17:37
than expected. Yes. What's fascinating here is
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the only way the math works is if the expansion
17:43
of the universe isn't slowing down. It is actively
17:46
accelerating. Space is stretching faster and
17:49
faster every single day. It's like throwing a
17:52
baseball up into the air. And instead of slowing
17:54
down and falling back to Earth, it suddenly rockets
17:56
upward into the sky. It defies gravity. Exactly.
17:59
This discovery proved there must be a mysterious
18:02
force driving this acceleration, a force we now
18:05
call dark energy. And the astronomers who figured
18:08
this out won a Nobel Prize, all because they
18:11
trusted the upper rungs of the distance ladder
18:12
enough to spot a discrepancy. And that brings
18:16
us to the ultimate prize in cosmology, the Hubble
18:19
Constant. The Hubble Constant. OK. Yeah, this
18:22
is a specific number that represents the exact
18:24
rate at which the universe is expanding today.
18:27
If we can pin down the precise value of the Hubble
18:31
constant, we can run the clock backwards to calculate
18:34
the exact age of the universe. Oh, wow. And we
18:37
can run the clock forwards to predict its ultimate
18:39
fate. Okay, here's where it gets really interesting.
18:42
This is the great mystery we promised at the
18:44
start of this deep dive. We currently have two
18:47
independent ways to measure the Hubble constant.
18:51
And they refuse to agree. They stubbornly refuse.
18:54
And this conflict has been tearing at the astrophysics
18:56
community for a decade. Let's look at the two
18:59
methods. First, you have the local or late universe
19:03
method. Right, which is everything we just talked
19:05
about. Building the ladder from the ground up.
19:07
Radar, parallax, cepheid, supernovae. Exactly.
19:10
Measuring the redshift of galaxies directly around
19:12
us. Teams have refined this ladder to a razor
19:15
sharp edge. And then you have the second method.
19:18
The top -down early universe approach. This method
19:21
completely ignores the latter. Instead, it looks
19:23
at the cosmic microwave background. What is that
19:25
exactly? When the universe was born in the Big
19:28
Bang, it was incredibly hot and dense. About
19:31
380 ,000 years later, it cooled just enough for
19:34
the first atoms to form, and light was finally
19:37
able to travel freely through space. So it's
19:40
the literal afterglow of the Big Bang, the oldest
19:42
light in existence. Yes. We use space satellites
19:45
like ESA's Planck satellite to map the tiny microscopic
19:49
temperature variations in that ancient radiation.
19:52
By looking at the physics of that baby universe,
19:54
scientists can plug that data into our standard
19:57
model of cosmic evolution, mathematically run
20:00
the clock forward 13 .8 billion years, and predict
20:03
exactly what the Hubble constant should be today.
20:06
Okay, so we have a bottom -up measurement using
20:08
exploding stars today and a top -down measurement
20:11
calculating forward from the oldest light. in
20:13
existence. Both methods have been checked, double
20:16
checked and triple checked by the smartest people
20:17
on Earth. They have. But when you compare the
20:19
final numbers, they don't match. They disagree
20:22
by about 10 percent. Ten percent. Yeah. And in
20:24
the world of precision cosmology, a 10 percent
20:27
gap is a massive, statistically significant chasm.
20:30
It is well outside the margin of error for either
20:33
team. This discrepancy is known as the Hubble
20:36
tension. The Hubble tension. So who messed up?
20:39
There has to be a math hair somewhere, right?
20:41
Maybe a smudge on a telescope lens or like a
20:44
flaw in how we calibrate the cepheid variables.
20:47
I mean, it could be. A lot of scientists are
20:50
desperately hunting for a hidden human error
20:52
or systematic bias in the instruments. But there
20:55
is another alternative, and it is what cosmologists
20:58
are taking very seriously right now. What if
21:00
nobody messed up? Right. What if both measurements
21:03
are perfectly correct? The bottom -up ladder
21:05
is accurately measuring the universe today. The
21:08
top -down calculation is accurately measuring
21:10
the baby universe. Okay, but if both are true,
21:13
it means our standard model of the universe is
21:15
broken. It means we are missing a completely
21:18
new piece of physics. There is a hidden ingredient
21:20
out there, perhaps something we don't understand
21:22
about how dark energy behaves over time, that
21:25
we haven't discovered yet. Man, that is wild.
21:28
So to bring this all together for you listening,
21:30
we started this journey... just trying to figure
21:33
out how to measure a tree across a river gorge.
21:36
And look where we ended up. Right. We built a
21:38
ladder of geometry and pulsating stars and exploding
21:41
white dwarfs, and we ended up standing at the
21:44
absolute edge of known physics. From bouncing
21:47
radar off Venus to mapping the oldest light in
21:50
existence, the cosmic distance ladder is the
21:53
ultimate detective story. It really is a living,
21:56
breathing mystery. And this Hubble tension isn't
21:59
a failure of science. It's a thrilling reminder
22:01
that the universe still has massive fundamental
22:05
secrets waiting to be uncovered in the dark.
22:07
The book of physics is far from finished. It
22:10
really is far from finished. And I want to leave
22:12
you with one final thought to mull over today,
22:13
building on that massive cliffhanger. If both
22:17
of these measurements, the baby universe and
22:19
the modern universe, are completely accurate,
22:21
what if the hidden ingredient we are missing
22:23
is that the fundamental laws of physics themselves
22:25
have actually changed over the last 13 billion
22:27
years? Ooh, I love that. What if the universe's
22:30
measuring stick is actively evolving along with
22:33
the universe itself? It definitely makes you
22:35
look at that rushing river gorge in a whole new
22:38
light.