REGINA BARBER, HOST:
Who loves astronomy history? This gal.
EMILY KWONG, BYLINE: You're listening to SHORT WAVE from NPR.
BARBER: Less than a hundred years ago, astronomers were intensely debating whether there were other galaxies, other places with hundreds of billions of stars beyond our Milky Way. The debate lasted years, with the two sides publishing conflicting data and conflicting conclusions about whether it was just us out there. All that changed with astronomer Edwin Hubble. Back in 1924, he published results showing that there was more to our universe than just our galaxy - a lot more. And once Edwin squashed this great debate and discovered that we weren't the whole universe, the field of astronomy got even more complicated. More questions were raised.
VICKY SCOWCROFT: So what he noticed is that when he looked at all these different galaxies that he'd now measured the distance to, the ones that were further away were moving away faster.
BARBER: Dr. Vicky Scowcroft is an astronomer who studies the same type of stars Edwin Hubble did to make his discovery. And she says his results show that not only were there other galaxies, those galaxies were traveling away from ours. The question was, how fast was the universe actually expanding? - which is ultimately a question of cosmology, a subfield of astronomy, a question of...
SCOWCROFT: Understanding how the universe is evolving and our place in it, as well.
BARBER: Today on the show, the third and final episode in our series on cosmic distances and our place in space. This time - the trajectory of our entire universe. I'm Regina Barber, and you're listening to SHORT WAVE, the daily science podcast from NPR.
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BARBER: So first, it's important to talk about that really famous physicist - you know, Einstein. Over the course of his career, he created many theories, including a theory on gravity. That theory says that objects with a lot of mass - like planets, stars, black holes - literally warp space-time. This theory flew in the face of everything scientists believe because it suggested the universe was always changing, expanding. And scientists had long believed the universe was static. This suggestion that the universe was expanding was alarming even to Einstein. So alarming that in his equation...
SCOWCROFT: He put in this kind of fudge factor to make it so the universe didn't expand 'cause all of his equations told him that the universe should expand. And he put in this thing called the cosmological constant to stop the result being that it expanded.
BARBER: But when Hubble came to the conclusion that further away galaxies were moving away faster, it also confirmed that Einstein's original math was right. The universe is expanding.
SCOWCROFT: And Einstein was like, this is my biggest blunder, adding the cosmological constant.
BARBER: More on Einstein's biggest blunder later. But for now, let's answer how Edwin Hubble knew galaxies were moving away. To understand that, think about an ambulance and sound.
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BARBER: Because light is a wave.
SCOWCROFT: If you think about, like, an ambulance driving away from you, the pitch of that sound changes because the wave gets stretched out and the same thing happens to light.
BARBER: These galaxies are moving away as the universe expands, and they're producing light, too. The movement shifts the light the same way a moving ambulance changes the way the siren sounds. When this happens with sound, it's called the Doppler shift. And when it happens to light of a galaxy moving away, it's called redshift. So why redshift? For that, I want you to picture a prism.
SCOWCROFT: Like one of those crystals that people hang from their rearview mirrors. When the sun hits it, you see all the little rainbows.
BARBER: With high-powered telescopes, astronomers can break up white light from galaxies.
SCOWCROFT: And split it into, like, a really long rainbow so you could see it really clearly. You'd see black lines in it.
BARBER: Those black lines are the key here. They tell you what the galaxy is made out of because all the elements - hydrogen, helium, oxygen - they exist at specific places on the rainbow. And as galaxies and stars move away from us...
SCOWCROFT: The black line that might normally have lived in the green bit might be shifted further to the red.
BARBER: To the red part of the rainbow spectrum of light.
SCOWCROFT: How much it's been shifted - that tells us how fast that thing must be moving. So if you have a galaxy moving far away from you, now, where those black lines are is going to shift, as well.
BARBER: Almost all galaxies are moving away. And Hubble - he's the one that figured all this out.
SCOWCROFT: He measured the redshift of them by looking at how the lines moved, and he saw that the further-away ones were moving away faster. And what that tells us is that everything is moving away faster in every direction. It doesn't matter which way we look. And that's telling us that the universe itself must be expanding.
BARBER: And if everything's moving away from us no matter where we look, kind of makes you wonder, are we the center of the universe? The answer is no, because...
SCOWCROFT: What that actually means is that the universe started in some tiny, tiny, single point, and everything is moving away from everything else. It's the space itself that is expanding. This is the weird thing.
BARBER: The space between us and other galaxies is getting bigger, stretching.
SCOWCROFT: And that's happening everywhere, which makes it look like everything is moving away from us. So if we went and sat on another galaxy, then we would see the same thing. We would still see everything moving away from us because the space itself is getting bigger.
BARBER: Let's do a science demo. I have a deflated balloon. Then I add a lot of dots on the balloon with a marker. The dotted balloon material represents the fabric of space-time, everything in our universe.
SCOWCROFT: Before you inflated it, the dots are all quite close together. But when you blow up the balloon, and the dots all move further apart from all of the other dots - the dots on two opposite side of the balloon will have moved further away from each other than two dots next to each other.
BARBER: Which is all good and cool, but where is the center then? It's nowhere. The science says it's nowhere.
The students yell at me and they're like, but the center of the universe is inside the balloon. And I say, no, reality is - and space-time is only on the surface of the balloon. So there's nothing in the center.
BARBER: It's hard.
SCOWCROFT: It is really hard. This is the master's course I teach, and it's really hard to get your head around.
BARBER: But we've got this. We know the universe is expanding now. We know that the further away something is, the more the light shifts to the red, and we can measure the rate of expansion, of objects speeding away quicker and quicker. Although this value of how fast the universe is expanding has created a debate of its own.
SCOWCROFT: In about the 1980s, 1990s, there was lots of experiments to try and measure this, and there were two camps.
BARBER: Each camp had expansion rates that were pretty far apart. Like, one number was nearly double the other one.
SCOWCROFT: There was one camp that thought it was - the number is 50 kilometers per second per megaparsec. Then there was the other camp who thought it was a hundred.
BARBER: Which is a difference of millions of light years for these distant galaxies.
SCOWCROFT: You had to be on one side or the other.
BARBER: But once scientists started getting data back from the Hubble Space Telescope in the mid-1990s, the debate died down because...
SCOWCROFT: Surprise, surprise, they measured it to be 75...
BARBER: Right in the middle.
SCOWCROFT: ...Or about 75. Right in the middle. So it kind of got resolved for a while. Everyone was happy at that point.
BARBER: But in the last decade, astronomers discovered other ways to get an even more precise number for the expansion of the universe. And that - well, it started another nerd match because they again got differing numbers from one another - closer together to that middle ground but still too different to agree.
SCOWCROFT: So there's a few reasons why that could happen. It could be that one of us has done something wrong. So there's been lots of work going back through everything, making sure we've done everything correctly.
BARBER: At this point, all the mistakes have been corrected. But maybe we just need more data, more starlight to check the distances.
SCOWCROFT: When we did it, we only had 10 stars with parallax measurements that we could use. And now we have Gaia.
BARBER: Gaia is a fancy space observatory that's mapping the Milky Way and lets us use way more objects as data for these measurements. But Vicky says these explanations are kind of boring, that the discrepancy about how quickly the universe is shooting away in all directions could be about something even more fundamental.
SCOWCROFT: The exciting reason could be that neither of us are wrong and there's no calibration issue, but it's that we don't understand the physics anymore.
BARBER: Changing physics as we know it? That's a huge endeavor, and this still hasn't been settled. Getting an accurate measurement for how quickly the universe is expanding is still one of astronomy's greatest prizes, which brings us back to Einstein's greatest blunder from the beginning of our story.
SCOWCROFT: He'd put in this fudge factor when he'd done all his equations about how the universe evolves, and he put in this fudge factor to stop it expanding, basically. But turns out, we did need it, and - but that - we can also use that fudge factor to explain the acceleration. And so he wasn't wrong, after all. He called it his biggest blunder, but actually, we did need it.
BARBER: We need it because it helps us understand how the universe is expanding and doing so more quickly, though what's causing the accelerating expansion is still a mystery today. Astronomers call it dark energy. And so at the conclusion of our series on cosmic distances, we're basically back to where we started, unsure of our place in the universe and in disagreement of what it all means. That's the joy and frustration of astronomy. You're welcome.
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BARBER: The mystery that is dark energy is a whole episode unto itself. We'll link to that and the other two episodes in this series on measuring cosmic distances in our episode notes. This episode was produced by Chloee Weiner and Rebecca Ramirez, who also edited the piece. It was fact-checked by Brit Hanson. The audio engineer for this episode was Robert Rodriguez. And I want to give a special thanks to James Davenport. Gisele Grayson is our senior supervising editor. Beth Donovan is our senior director of programming, and Anya Grundmann is the senior vice president of programming. I'm Regina Barber. Thanks for listening to SHORT WAVE, the daily science podcast from NPR.
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