Skip to content

FloatHeadPhysics Physics Science

We never understood why the universe expands...until now!

Mahesh Shenoy retraces how astronomy actually figured out the universe is expanding, and restores the woman the standard story leaves out. The chain runs from Newton's unanswered question about why an eternal universe has not collapsed, to Einstein's cosmological constant invented to hold it static, to Friedmann's proof that the universe can simply move. Then the observers: Vesto Slipher measures spiral nebulae redshifting away at thousands of kilometers per second, and Henrietta Leavitt, an underpaid Harvard human computer, discovers that Cepheid variable stars obey a period luminosity law that turns brightness into distance. Harlow Shapley uses her ruler to move the sun off center, and Edwin Hubble uses it to prove Andromeda is a separate galaxy and that farther galaxies recede faster, the straight line that confirms an expanding universe. Einstein drops the constant and calls it his biggest blunder, though dark energy would later revive it.

Published Jan 16, 2025 21:41 video 21 min read Added Jun 16, 2026 Open on YouTube →

At a glance

Mahesh Shenoy of FloatHeadPhysics takes the story everyone thinks they know, that Edwin Hubble discovered the universe is expanding, and rebuilds it from the ground up to show how much got left out. The real engine of the discovery was a quiet, underpaid "human computer" at the Harvard Observatory named Henrietta Swan Leavitt, whose work on a strange class of pulsating stars handed astronomy the measuring stick it had been missing for centuries.

The video walks the chain link by link. Isaac Newton has no answer for why an eternal, gravity bound universe has not collapsed. Albert Einstein invents a repulsive term, the cosmological constant, to hold it steady. Alexander Friedmann shows on paper that the term is unnecessary because the universe can simply move. Then the observers arrive: Vesto Slipher measures impossible speeds in the spiral nebulae, Leavitt finds the law that turns brightness into distance, Harlow Shapley uses it to shove the sun off center, and Hubble uses it to prove the nebulae are whole other galaxies and that the farther ones flee faster.

The payoff is that two pieces snap together: Slipher's recession speeds and Hubble's distances form a straight line, and the only model that explains that line is Friedmann's expanding universe. Einstein, watching his "balancing" term become pointless, calls the cosmological constant the biggest blunder of his life. This page rebuilds the whole chain in the video's own order, with every number, every analogy, and every character kept in.

Newton's dead end: why hasn't everything clumped together?

The story does not start with telescopes. It starts with a question put to the man who started it all, Newton. By Newton's own law, everything attracts everything else. The dominant belief back then was that the universe was eternal, that it had simply always existed. Put those two ideas together and a problem appears immediately: if every mass has been pulling on every other mass for an infinite amount of time, why has the whole universe not already collapsed into one giant clump?

Newton's honest answer was: I don't know. He had no real solution. Gravity only pulls, never pushes, so an eternal universe full of matter should have crushed itself into a point long ago. The fact that it has not is a genuine paradox, and it sat unanswered until a better theory of gravity arrived.

Einstein's fix: a repulsive term pulled from the equations

Einstein had that better theory, general relativity, and when handed the same question he did have an answer. What if, at very large distances, gravity also carries a repulsive component, a push that counteracts the familiar pull, so the universe can hold itself in perfect balance?

He could write that idea directly into his field equations. The original equation has spacetime curvature on the left and matter and energy on the right. Matter and energy tell spacetime how to curve, and that curvature is the attractive gravity we know. Einstein then bolted on an extra term. Its negative sign means it acts on the curvature in the opposite way to matter and energy, producing an anti-gravity push. Crucially, this term does not depend on matter or energy at all. It is purely a property of space itself. He then tuned its value, dialing the push until it exactly cancelled the universe's tendency to collapse, and named it the cosmological constant. The universe was now balanced on paper.

Mahesh's reaction is the obvious one: Einstein, did you just invent this term out of thin air to force the universe to be stable? Essentially, yes. It was a fudge, an added constant whose only job was to deliver the static universe that everyone already assumed was correct.

Figure 1. Einstein's balancing act. Matter and energy curve spacetime and pull the universe inward, so to keep it static Einstein added a term that pushes outward and depends only on space, not on its contents. He tuned the constant until the two forces cancelled. The whole apparatus exists to deliver the static universe everyone assumed must be true.

Friedmann's counter: the universe is allowed to move

A Russian physicist named Alexander Friedmann had something better. He solved Einstein's field equation for the entire universe by making a few simplifying assumptions, and he reached a startling conclusion: you do not need the repulsive term at all. Einstein's own equations, read without prejudice, say the universe can be dynamic. It can be contracting, or it can be expanding. Either way, it does not have to be static, so the cosmological constant that was invented to freeze it is simply unnecessary.

Einstein's first reaction was flat rejection. The conversation, as Mahesh imagines it, runs roughly: give me your notes, let me check, hey, there is a mathematical error. Friedmann replies, no there is not. Einstein checks again: yeah, no there is not, but it still makes no sense. What do you mean, space stretching and expanding? Space has to be static, everything has to be balanced, and the constant stays. The static universe was the orthodoxy, and Friedmann's expanding universe was, at that moment, just a wild mathematical idea with nothing observational behind it. To turn the idea into science, somebody had to go out and measure the sky.

Vesto Slipher: the man hired to find Martians

The first observer is one of the great unsung heroes of the story: Vesto Slipher, an American astronomer. His official job was, no exaggeration, to look for Martians. Slipher was excellent at spectral analysis, decomposing light to read off which elements are present, so the hope was that he could point a telescope at Mars and find the chemical fingerprints of life: water, breathable air, and the like.

In his spare time, though, Slipher chased something more interesting: the faint spiral shaped smudges scattered across the sky. At the time nobody knew these were galaxies. The reigning belief was that the Milky Way was the entire universe, a single galaxy with us near its center, so these objects were filed under "spiral nebulae," presumed to be clouds inside our own galaxy. Mahesh pauses on how strange that is: relativity had already been discovered, yet people still thought there was exactly one galaxy. His point lands as a warning about our own era: we are very possibly in a similar position today, confidently assuming there is only one universe.

Slipher did what he did best and took the spectra of these spiral things, and he found something remarkable. The light from nearly all of them was redshifted.

Redshift and blueshift: reading motion in the colors

The reason a redshift matters is the Doppler effect for light. When a light source moves, the light heading in the direction of motion gets squeezed to shorter wavelengths, and the light going the opposite way gets stretched to longer wavelengths. Shorter wavelengths look bluish, which we call a blueshift, and longer wavelengths look reddish, toward the red end of the spectrum, which we call a redshift. The rule is simple: redshift means the object is moving away from us, and blueshift means it is moving toward us.

Figure 2. How Slipher read motion straight out of starlight. Light from a source moving away is stretched toward red; light from a source moving toward us is squeezed toward blue. Almost every spiral nebula he measured was redshifted, meaning almost all of them were rushing away, and the size of the shift gave their speed.

Slipher's results were striking. A few objects, Andromeda among them, were blueshifted, coming toward us at roughly 200 to 300 km per second. But most of the rest, about 30 to 40 of them, were redshifted and racing away, some at a staggering 2,000 km per second, read straight off the size of the shift. That was the fastest motion anyone had ever recorded, and the presentation reportedly earned him a standing ovation.

What did it mean? For some astronomers, nothing in particular, just an odd phenomenon with no larger significance. For Slipher and a few others, it was the first hint that these spiral objects might not belong to the Milky Way at all, that they might be separate galaxies. But proposing other galaxies back then was, in Mahesh's words, crazy talk, the way proposing other universes is crazy talk today. You need solid evidence, and a pile of speeds was not enough. The missing piece was distance.

The wall: parallax and the limits of the cosmic ruler

Why not simply measure how far away the spiral nebulae are? Because the only distance tool available at the time, stellar parallax, could not reach them. Parallax is the everyday effect you get by closing one eye, lining up two fingers, then switching eyes: the nearer finger appears to jump more than the far one. Whatever shifts more is closer. Astronomers did the same trick with stars, observing one from two different points in Earth's orbit and measuring the tiny angle by which it appears to move. Knowing that angle gives the distance.

The trouble is geometric. The farther the star, the smaller its parallax angle, and beyond a certain distance the angle shrinks below what any instrument can detect. Back then that range was roughly 100 light years. That sounds impressive, distances that light takes a century to cross, but the Milky Way was thought to span about 30,000 light years (an underestimate, today we know it is closer to 100,000), so a 100 light year ruler is essentially nothing against the size of the galaxy, let alone the nebulae beyond it. Try to measure a spiral nebula with parallax and you get no answer at all. It could be 200 light years away or 2 million; there was no way to tell. Astronomy needed a far longer ruler.

Henrietta Leavitt, the human computer who built a new ruler

The longer ruler came from Henrietta Leavitt, hired at the Harvard Observatory as a "human computer." Before electronic computers, the painstaking work of measuring and cataloguing thousands of stars from photographic plates was done by hand, and that labor was given largely to women, in plain part because they could be paid less. Working through plate after plate of stellar data, Leavitt made the discovery that would re-scale the cosmos.

Her starting point was the inverse square law of brightness. If you have a bulb and you know its true brightness, then an identical bulb that looks dimmer must be farther away, and you can turn the dimming into a distance. Compare a star's apparent brightness (how bright it looks from Earth) with its true brightness (how bright it actually is) and the gap tells you the distance. The relationship runs both ways: if you already know the distance and the apparent brightness, you can solve for the true brightness instead. That reversibility is the hinge the whole method turns on.

There is a catch. Using brightness as a distance proxy only works if you have a standard star whose true brightness you already know and which you can reliably recognize in the sky. Ordinary stars are useless for this, because they shine at every brightness imaginable, so a dim one could be a faint star nearby or a brilliant star far off. Leavitt's breakthrough was finding a class of star that solves exactly this problem.

Cepheid variables: a standard candle that announces itself

The special stars are Cepheid variables. A variable star is one whose brightness genuinely changes over time, pulsating brighter and dimmer. Mahesh asks the natural question: isn't that just twinkling? No. Twinkling is the atmosphere refracting starlight over fractions of a second; a Cepheid truly changes its own output, and it does so over days, not seconds. Among variable stars, Cepheids pulse in a very particular, signature pattern, which means you can pick one out of a crowd just by watching how its brightness rises and falls. That solves the recognition half of the problem: Cepheids announce themselves.

But it raises the next question. A standard candle is only useful if every example has the same true brightness, and Cepheids do not. This is where the real discovery happens, and where most retellings flatten it out. Leavitt was studying a batch of about 25 Cepheid variables clustered in one place, the Small Magellanic Cloud. The key move is that because they all sit in the same small patch of sky, they are all at essentially the same distance from Earth. So any difference in how bright they appear cannot be a distance effect. It has to be a real difference in the stars themselves: some Cepheids are simply intrinsically brighter than others.

With distance taken out of the picture, she could hunt for a pattern in those real differences, and she found one. When she plotted the logarithm of the pulsation period on one axis against the apparent peak brightness on the other, the points fell on a straight line. Cepheids with shorter periods (which pulse more quickly) are dimmer; Cepheids with longer periods are brighter. The brightness is tied to the rhythm.

Figure 3. Leavitt's law, the straight line that built the cosmic ruler. Because all 25 Cepheids in the Small Magellanic Cloud sat at the same distance, their differences in apparent brightness were real, and they lined up perfectly against pulsation period. Read a Cepheid's rhythm and you read its brightness, and brightness is distance.

There is one more step Leavitt is careful about. The line gives apparent peak brightness, not true brightness, so by itself it does not yet give absolute distances. But the fix is clean: find just one Cepheid close enough to sit inside the parallax range, measure its distance directly, and from that anchor you can convert the apparent brightness of every other Cepheid into its true brightness. Calibrate the line once, and forever after the period of any Cepheid hands you its true brightness, and the true brightness hands you the distance. That is Leavitt's law, the period luminosity relation.

The upgrade was enormous. Parallax topped out at a few hundred light years; Cepheid variables pushed the reachable range out to about a million light years. Astronomy suddenly had a ruler long enough to measure the things that had been hopelessly out of reach, and that ruler is what made every paradigm shift that followed possible.

Shapley: knocking the sun off center

The first to wield the new ruler was Leavitt's boss, Harlow Shapley, director of the Harvard Observatory. He took her published law and went hunting for Cepheids inside the Milky Way. Two results came out of it. First, the Milky Way turned out to be far larger than expected, roughly 100,000 light years across. Second, and more unsettling, by measuring Cepheids toward the center of the galaxy he calculated that the center lay about 30,000 light years away from us. We were not at the center of the Milky Way after all. The slow demolition of human centrality, begun by Copernicus, took another step, and it ran on Leavitt's law.

Hubble: a new galaxy, then an expanding universe

Now Edwin Hubble, a lawyer turned astronomer, enters. He found a Cepheid variable inside the Andromeda nebula, applied Leavitt's law, and measured its distance at close to a million light years. That single number broke the one galaxy universe. Andromeda was far too distant to sit inside the Milky Way, and judging by its apparent size at that distance, it had to be enormous, an entire galaxy of its own. This was humanity discovering a second galaxy, an event Mahesh rates as comparable to discovering a whole new universe today. The figure was actually an underestimate (Andromeda is about 2.5 million light years away), but the exact value hardly mattered. The point was made, and very soon it was clear the universe is teeming with billions of galaxies.

Then came Hubble's most important discovery. Slipher had already measured how fast the spiral nebulae were receding; what nobody had was their distances. Hubble supplied them, finding Cepheids and pinning down that these galaxies lie millions of light years away. When he plotted distance against recession velocity, the dots fell on a straight line: nearby galaxies recede slowly, distant galaxies recede fast, and the speed climbs in direct proportion to the distance. That is Hubble's law.

Figure 4. Hubble's law, the line that revealed the expansion. Combining Slipher's recession speeds with distances from Leavitt's Cepheid ruler, Hubble found velocity rising in lockstep with distance. The straight line is the fingerprint of an expanding universe, the observation Friedmann's theory had been waiting for.

Why the line means expansion, not just motion

The naive reading of Hubble's law is that galaxies are simply flying through space away from us. Mahesh shows why that reading fails on two counts. First, if everything is moving away from us, it looks like the Milky Way sits at the center of all the motion, and humanity has learned by now that we are not at the center of anything. Any honest model has to make every observer, anywhere, see the same thing: everything receding from everyone. Second, plain motion through space gives no reason why the more distant galaxies should move faster. The straight line demands an explanation, and "they are just drifting" does not supply one.

The model that does fit is Friedmann's. If space itself stretches, then every distance grows in proportion to its current size. A galaxy twice as far away has twice as much expanding space between it and us, so it recedes twice as fast, which is exactly Hubble's straight line. And there is nothing special about our viewpoint: stand on any galaxy, watch the same expansion, and you see everything else fleeing from you under the very same law. The theoretical idea Friedmann pulled out of Einstein's equations, the wild notion Einstein himself had dismissed, turned out to match the data perfectly. The universe is expanding.

The Big Bang, and a piece of homework

If the universe is expanding, then running the clock backward squeezes everything together, which points to a beginning, the Big Bang. Mahesh refuses to let that land too neatly. There is a serious counterargument: you can imagine a universe that expands yet never had a beginning, an idea called the steady state universe, in which new matter is continuously created to keep the density constant as space grows. He sets it as homework rather than spoon feeding the conclusion: look up the steady state model, and look up how it was eventually disproved, because the lesson of the whole video is that science is about the reasoning, not about taking anyone's word for it.

The blunder, or was it?

None of the headline breakthroughs, Shapley dethroning the sun, Hubble proving the expansion, would have been possible without Leavitt's law and the meticulous plate by plate work behind it. The men who made the famous discoveries knew it and credited her openly; Hubble himself said she deserved a Nobel Prize. When someone finally moved to nominate her, it was too late: she had died of stomach cancer, and the prize is not awarded posthumously.

And Einstein? Faced with an expanding universe, he accepted that Friedmann had been right all along. The universe was never static, so there had been nothing to balance, and the cosmological constant he had bolted on to freeze it was pointless. He dropped it and, the story goes, called it the biggest blunder of his life. Mahesh closes on the twist that opens the door to modern cosmology: or was it? The cosmological constant would return decades later as the leading description of dark energy and the accelerating expansion of the universe. The term Einstein threw away may have been one of his deepest insights.

Key takeaways

An eternal, gravity bound universe should have collapsed long ago. Newton had no answer; Einstein answered with a repulsive cosmological constant tuned to hold the universe static.
Friedmann showed from Einstein's own equations that the universe can expand or contract on its own, making the constant unnecessary. Einstein rejected the idea at first.
Vesto Slipher measured the spiral nebulae and found most were redshifted, racing away at up to 2,000 km per second, the fastest speeds recorded at the time, but without distances the speeds were just a curiosity.
Stellar parallax only reached about 100 light years, far too short to measure anything beyond the inner Milky Way.
Henrietta Leavitt discovered that Cepheid variables follow a straight line tying pulsation period to brightness, by studying 25 of them at equal distance in the Small Magellanic Cloud. Calibrated against one nearby Cepheid, the period luminosity relation extended the cosmic ruler to about a million light years.
Shapley used Leavitt's law to show the sun is about 30,000 light years from the Milky Way's center, not at it.
Hubble used it to prove Andromeda is a separate galaxy a million plus light years away, then combined distances with Slipher's speeds to find Hubble's law: farther galaxies recede faster, in direct proportion.
The straight line only makes sense if space itself expands, confirming Friedmann's model. Einstein dropped the cosmological constant and called it his biggest blunder, though dark energy later revived it.

Chapters

Timestamps are clickable. Click one and the player jumps there and keeps playing while you read.

0:00 The astronomer who made a startling discovery
1:07 Einstein's cosmological constant
2:54 Friedmann's counter argument against the constant
4:00 Vesto Slipher's puzzling discovery of the spiral "nebulae"
7:40 The best measuring methods until the 1920s
9:04 A serious upgrade
10:50 Henrietta Leavitt enters the story (Leavitt's law)
16:15 The paradigm shifts after Leavitt's law
17:50 The Hubble law
20:38 Einstein's blunder, or was it?

Notable quotes

Until the 1920s we thought our universe had just one galaxy, and that was the entire universe, and of course we are at the center of it. I mean, why wouldn't we be? Mahesh Shenoy, 0:00

We are always told an oversimplified story about how Edwin Hubble discovered that the universe is expanding, but when you look into the details, oh man, the details are a marvel of scientific thinking. Mahesh Shenoy, 0:35

Einstein, did you just pull out this term out of thin air just to make the universe stable? Mahesh Shenoy, 1:55

Thinking of other galaxies was crazy talk back then. It's kind of like today thinking about other universes. You better have solid evidence if you want to talk about that in science. Mahesh Shenoy, 6:50

Women were hired for that, mostly because you could pay them less. That's just how it worked back then. Mahesh Shenoy, 10:55

You give me the period and you will now figure out the true brightness of that Cepheid, and you can use that as a standard for measuring distances. Mahesh Shenoy on Leavitt's law, 14:50

They couldn't have done it without Leavitt's law. In fact, Hubble even mentioned that she deserved a Nobel Prize. Mahesh Shenoy, 20:00

He called it the biggest blunder he had made in his life. Or was it? Mahesh Shenoy, 21:20

Resources mentioned

Henrietta Swan Leavitt, the Harvard "human computer," and her period luminosity relation (Leavitt's law) built on Cepheid variables in the Small Magellanic Cloud.
Isaac Newton and the puzzle of why an eternal gravitating universe has not collapsed.
Albert Einstein, his field equations, and the cosmological constant he later called his biggest blunder.
Alexander Friedmann and the Friedmann equations describing a dynamic, expanding or contracting universe.
Vesto Slipher and his spectral measurements of redshift and blueshift in the spiral nebulae.
Stellar parallax and the inverse square law of apparent brightness.
Harlow Shapley, who used Leavitt's law to resize the Milky Way and move the sun off center.
Edwin Hubble, the distance to the Andromeda Galaxy, and Hubble's law.
The Big Bang and its rival, the steady state universe (set as homework), plus the later return of the constant as dark energy.
NordVPN, the video's sponsor.

Where it stands

The history here is solid and well told, and the central correction is fair: Leavitt's period luminosity relation really is the foundation that Hubble and Shapley built on, and her name belongs in the headline. A few details are simplified for the story, as Mahesh flags. The Cepheid distance scale needed later recalibration once astronomers learned there are two populations of Cepheids, which is part of why early numbers like Hubble's million light years to Andromeda came out low. The dialogue between Einstein and Friedmann is dramatized, not transcribed. And the "biggest blunder" line, while widely repeated, comes secondhand through George Gamow and may be embellished. None of that dents the core arc, which is accurate: theory said the universe could move, observation proved it does, and a meticulous human computer supplied the ruler that made the proof possible.

Full transcript

Until the 1920s we thought our universe had just one galaxy, and that was the entire universe, and of course we are at the center of it. I mean, why wouldn't we be? But then came along Henrietta Leavitt, a young, underpaid astronomer working at Harvard. She made a groundbreaking discovery that changed everything. Within just a few years we realized not only is the universe teeming with billions of galaxies, but that the universe itself is expanding, and apparently made Einstein say that he had made the biggest blunder of his life. But wait a second, what exactly was that blunder? And wasn't it Edwin Hubble who discovered that the universe was expanding? What exactly did she do? And most importantly, why didn't I ever hear about her? Well, it turns out that we are always told an oversimplified story about how Edwin Hubble discovered that the universe is expanding, but when you look into the details, oh man, the details are a marvel of scientific thinking, full of paradigm shifts, all centered around her groundbreaking discovery. And so I thought I should make this video to set the record straight. So if you're ready to rediscover the real story of how we figured out the universe is expanding, let's begin. So let's back up a little bit, because it all starts when we ask a fundamental question to the man who started it all: Newton. So Newton, according to you, everything attracts everything else in the universe. If that is the case, and if the universe has been there forever (that was a dominant view back then, that the universe was eternal), then the question was, why hasn't everything just clumped together? And Newton's answer to this is: I don't know. He really had no answer for this question. So we move on and we talk to Einstein, who has a much better theory of gravity. Einstein, same question. And Einstein says, I do have an answer for you. What if at large distances gravity is also having some kind of a repulsive force that could counteract the attractive forces, and the universe could stay balanced? In fact, you can even see this in his field equations. See, the original field equation had the spacetime curvature on the left side, a constant, and matter and energy on the right side. So matter and energy tells spacetime how to curve. That produces the attractive gravity. But then he added in another term. The negative sign represents that it has the opposite effect on the spacetime curvature as matter and energy. So this produces the anti-gravity, repulsive force, and it does not depend on matter and energy, it is purely a function of space. And he tweaked this constant to get the universe balanced, and he called it the infamous cosmological constant. So look, the universe was balanced. And at this point I'm like, Einstein, did you just pull out this term out of thin air just to make the universe stable? And I sense like, yeah, but do you have anything better? Yes I do, said Alexander Friedmann, a Russian physicist. Friedmann says he solved Einstein's field equation for the entire universe by making some assumptions, and realized you don't really need the repulsive term. In fact, he figured out, according to Einstein's equations, the universe itself can be dynamic, it can be contracting or it can be expanding. So look, he says, Einstein, the universe doesn't even have to be static, so we don't need this term at all. And Einstein's response to this was, wow, that makes no sense. And this is probably how the conversation went. Friedmann, give me your notes, let me check. Hey, there's a mathematical error. And Friedmann says, no there isn't. And Einstein's like, yeah, no there isn't, but still it doesn't make sense. What are you talking about, space stretching and expanding? That doesn't make sense. Space has to be static, everything needs to be balanced, and the cosmological constant stays. That was a dominant view back then, and Friedmann's idea was just that, a wild idea. To make any kind of progress we needed to start making some concrete observations, and that's exactly where our first unsung hero comes in: Vesto Slipher. Slipher was an American astronomer, and he was tasked with, I kid you not, finding Martians. Why Slipher? Because he was really good at spectral analysis, you know, where you look at the light, decompose it, and figure out what elements are present. So the hope was he could point telescopes at Mars and find evidence for life-giving stuff like water or breathable air, etc. Okay, so Slipher, what happened? Slipher says he was doing that, but in spare time he was really interested in figuring out what these spiral looking things in the sky were. And I'm like, Slipher, what are you talking about, these are galaxies. Oh yeah, back then we didn't know that there were other galaxies, we thought that there was only one galaxy. I find it a little weird that there was a time not too long ago, I mean relativity was discovered already, but we believed that there was only one galaxy. But if you think about it, we live in a similar time today where we believe there's only one universe. But anyways, coming back to this, Slipher was super interested in what the spiral looking things were. They called them the spiral nebulae back then. So Slipher, what did you do? Slipher says, I did what I do best, spectral analysis. So I looked at the spectrum, and he found something really, really interesting. He found that the light from all the spiral looking things were mostly redshifted. What does that mean? Well, you probably already know this, but remember, whenever a source of light is moving, then the light in the direction of the motion gets squeezed, making smaller wavelengths, and the light in the direction opposite to it gets stretched out, giving you longer wavelengths. And shorter wavelengths look to us bluish, and so we call that the blue shift, and the longer wavelength look to us as reddish, towards the red end of the spectrum, we call that the red shift. If something is redshifted, we know it's going away from us, and if something is blueshifted, we know it's coming towards us. Okay, he found there were some galaxies, like Andromeda for example, which were blueshifted, but most other galaxies that he noticed, about 30 to 40, were all redshifted. And so this meant that a few galaxies (which again, remember, back then we didn't know, so we thought there were spiral nebulae within the Milky Way), but anyways, this meant that some of these spiral things were hurtling towards us, and he could estimate the speeds. The speed was close to about 200, 300 km per second, and the rest of the others were just moving away at staggering speeds. In fact, he found some of them were moving at about 2,000 km per second, purely from the redshift. In fact, apparently he got a standing ovation when he presented this, because this was the fastest thing ever recorded back then. So the big question was, what did this mean? Well, for some people this didn't mean anything. This was just an unusual phenomenon. Maybe something is happening, maybe the redshifts mean something else, I don't know, but this did not have any broader significance. But for some other people, including Slipher, this was maybe beginning to tell us that these things are not part of the Milky Way at all, and maybe (they're questioning, just maybe) they're separate galaxies altogether. But remember, thinking of other galaxies was crazy talk back then. It's kind of like today thinking about other universes. You better have solid evidence if you want to talk about that in science, and this just wouldn't do. We needed something more. But I'm like, wait a second, why couldn't we just measure distances to these spiral looking things? I mean, if we knew how far they are, we would know. But we didn't have the tools to measure distances back then. The best tool to measure distances back then was by using parallax. You know parallax, where the usual demonstration is you close one of your eyes, align two fingers, and then switch the eyes, and you see the finger shifting. And what you notice is the finger that's closer to you shifts more compared to the one that's farther away. So that means whichever shifts more is closer, and if the shift is lesser it's farther away. And we did the same thing with the stars. We call this stellar parallax. We measure from two different vantage points and we figure out how much the shift is, in other words measure this angle, and by knowing this angle we could estimate how far the star or anything else was. But the problem is, the farther the star is, the smaller this angle becomes, and after a point this angle becomes just too small to measure anything. And therefore the parallax had a range, and the range was roughly, back then, about 100 light years. Now if you think about it, 100 light years, we could measure distances that took light 100 years, that is pretty incredible. But in the grand scheme of things, we believe that the Milky Way was about 30,000 light years (it's an underestimate, today we know it's about 100,000 light years), but because the Milky Way is so big, even with parallax of, you know, range of 100 light years, it is nothing. And so everything was pretty much out of range. So if you tried to measure distances to these spiral looking things, you wouldn't get anything. So you don't know whether it's at 200 light years or 2 million light years. There was just no way of knowing things. We needed some serious upgrades on our cosmic measurement systems. [Sponsor segment: NordVPN.] So we needed some serious upgrades in our distance measurement systems, and that's exactly where Henrietta Leavitt comes in. She was hired as a human computer at the Harvard Observatory. What does that mean? Well, back then we didn't have a lot of computing power, so we had to manually analyze the data, and women were hired for that, mostly because you could pay them less. That's just how it worked back then. And when Leavitt was meticulously analyzing thousands of, you know, data of thousands of stars, she came across a startling discovery that upgraded our measurement systems. So Leavitt, what exactly did you do? So Leavitt says, Mahesh, if you have a light bulb and you know exactly how bright it is, and then let's say you see a similar bright bulb but much dimmer, then that means that that bulb must be farther away, that's why it's appearing dim to you. So this means that if you know the apparent brightness of the bulb, you can compare it with the true brightness and you can estimate how far it is, because of the inverse square law. And this also works the other way around: if you know how far it is, and if you know the apparent brightness, well, you can find out the true brightness. You got that? This is going to be important. And I'm like, yeah, that makes sense, you're using brightness as a proxy to measure distances. So what you're saying, Leavitt, is that we can use the bulbs in the sky, the stars that we have in the sky, their brightness, as a proxy for measuring distances. But wait, this would only work provided we had some kind of a standard star whose true brightness we knew, and we could identify that we're looking at those particular kinds of stars. But stars shine with all kinds of brightness, so how does it work out? Well, Leavitt says, a normal star cannot be used for this, but there are certain particular kind of stars called Cepheid variables. And I'm like, Cepheid what? So variable stars are the ones whose brightness changes over time, it pulsates. And I'm like, that's just twinkling of stars, isn't it? And she says, no Mahesh, twinkling of stars happens because of the atmospheric refraction. Here the stars are truly changing their brightness, and this happens over days, not a few seconds. And of those variable stars, Cepheid variables are the ones whose brightness changes in a very particular way. It has a signature pattern, which means by analyzing how the star's brightness changes you can identify a Cepheid variable. And I'm like, that is awesome, now I have some kind of a standard star, the Cepheid variable. But wait a second, this would only work if all Cepheids had the same brightness. Is that the case? And Leavitt says, no, it's not. But, and this is where the story gets really interesting (most of the times this gets oversimplified because there's some subtle signs involved, but for you and I this is the perfect moment, this is the climax, because this is where the groundbreaking discovery happens), are you ready? Okay, while analyzing data she found about 25 Cepheid variables in all, in one spot, the Small Magellanic Cloud. What's interesting is that she found that some had a much higher peak brightness compared to the others, and since they are in the same spot, which means they're pretty much at the same distance from the Earth, this variation is not due to the distance, this variation is truly because some Cepheids are brighter than the others. And she could now study them to see if there's any pattern, and she did find a pattern. She found that if you plot a graph of the period of the Cepheid variables, the pulsation period, on the x-axis (actually the log of the pulsation periods, okay) and the apparent peak brightness (it's apparent because it's the brightness that appears to you when looking from the Earth), if you plot this graph, this is what she found, that is a straight line. What she had figured out is a relationship between the period and the peak brightness. The shorter ones, the ones that have shorter periods, which means they fluctuate much more quickly, they have a lower peak brightness, and the ones that have much longer periods, they have a much higher peak brightness. And I'm like, this is all we needed, now if I know the period of a Cepheid variable I know exactly how bright it is. And Leavitt says, no, my, almost, because this is the apparent brightness, this is not the true brightness. But that's okay, now all we have to do is find a Cepheid variable within our parallax range, then we know exactly how far it is, and then we can find its true brightness. And once we find its true brightness, we can use the Leavitt relationship, the period luminosity relationship, to find the true brightness of any other Cepheid. Long story short, because of Leavitt's law, as we call it today, we knew, we understood, what was the true brightness of any Cepheid. You give me the period and you will now figure out the true brightness of that Cepheid, and you can use that as a standard for measuring distances. Wasn't that incredible? So the big question is, how much of an upgrade was this? Well, remember, back then the range of the parallax measurements was slow, up to a few hundred light years. With Cepheid variables that range increased to about a million light years. That's right, this gave us a serious upgrade on our distance measuring, and now we had everything needed for discovering paradigm shifts in astronomy. So let's calm down, what happens next? Well, first of all, her boss named Harlow Shapley, who was the director of the Harvard Observatory, obviously got access to her publication, Leavitt's law, and he immediately started identifying Cepheid variables within the Milky Way. And he found that the Milky Way was actually much bigger than previously expected, roughly about 100,000 light years. But more importantly, he also found some Cepheid variables in the center of the Milky Way, and by calculating how far that was, he realized that it was about 30,000 light years away, which meant, again, we realized that we are not at the center of the Milky Way. And now we finally get to Edwin Hubble, a lawyer turned astronomer. He found a Cepheid variable in Andromeda, which means he could now find the distance to it. And what did he find? Well, he found, for the very first time, the distance to Andromeda to be close to a million light years. This means, for the very first time, we realized that Andromeda is not within the Milky Way, but it's much outside, and by looking at its apparent size, we now figured out that it must be much bigger, like a galaxy. This is how humanity discovered a new galaxy. This was huge. Remember, this is as huge as today discovering a new universe altogether. Now of course we know that this was an underestimate, the actual value is close to about 2 and a half million light years, but who cares, what matters is that pretty soon we realized that our universe is teeming with billions of galaxies. This now brings us to the most important discovery Hubble ever made. Remember how Slipher had already calculated the redshift values and he had figured out how fast these things were already moving, but we didn't know what that meant. Now Hubble said, hey, let me just go ahead and find some Cepheid variables, then I can figure out how far these things are. And he did that. In a few years he figured out that all of these galaxies are millions of light years away. But most importantly, he found a beautiful relationship between the distance and the recession velocity. He found that the ones, the galaxies, that are closer are moving away slower, and the ones that are much farther are moving away much faster. In fact, there was a beautiful relationship: when he plotted distance versus recession velocity, he found that they all lie pretty much on a straight line. This is called the Hubble's law. But what does that mean? Well, one interpretation was the most straightforward one, that the galaxies are indeed just moving through space. But there are two problems with that. Well, first of all, this means that our Milky Way comes at the center of all of this motion, and humanity has finally learned its lesson, we know that we are not at the center of anything. So this must be true from all perspectives, that everything must be moving away from everything else, as seen from any perspective. But more importantly, it doesn't also explain why things that are farther away are moving faster. So what model can fit this observation? Friedmann's model. Remember how Friedmann had suggested, theoretically, just by solving Einstein's field equation, that the universe itself could be stretching or contracting? But now let's see what happens if the universe itself stretches and expands. Well, see properly what we find: look, the ones that are closer move away at a shorter velocity, and the ones that are farther, they move away a lot more, exactly aligning with Hubble's law. And notice there's nothing special about the Milky Way. If I change the vantage point, let's say I look at the same thing, this expansion, from this perspective, I get the same result. Now, boom, you see that everything is moving away from this one according to Hubble's law. That is beautiful, isn't it? This is how we realized that the universe must be expanding. Friedmann's model perfectly fits the observations. And pretty soon we realized that if the universe is expanding, then if you wind back the clock, it must have had a beginning, and so the Big Bang hypothesis was put forward. Actually, not really. There is a strong counterargument put against the Big Bang. There's a way to imagine the universe expanding without ever having a beginning, and that's homework for you. Look up something called the steady state universe, and you can also see how we eventually disproved that, because at the end of the day it's about scientific thinking. So don't just take my word for it, do some analysis yourself. But anyways, the point of the story was that we made some groundbreaking discoveries, like Shapley discovering that the sun is no longer at the center of the Milky Way, or Hubble figuring out that the universe is expanding, but they couldn't have done it without Leavitt's law, without her meticulous analysis and discovery of Leavitt's law. This wouldn't have been possible. And they were openly crediting her for their work. In fact, Hubble even mentioned that she deserved a Nobel Prize. And so when someone actually tried to nominate her for a Nobel Prize, unfortunately she had suddenly passed away because of a stomach cancer. Anyways, Einstein finally realized that Friedmann was right, we did not need to force fit this particular term, because the universe is not static in the first place. So he got rid of the cosmological constant, stopped working on it, and apparently called it the biggest blunder he had made in his life. Or was it?