What’s the deal with redshift? How can we actually interpret velocities when it comes to cosmic expansion? And what’s with the recent tension over measuring the expansion rate? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
How can you tell if something is moving? I mean, yes you can watch it move compared to the background, okay. What if that motion is towards or away from you? Right, let's make it a little bit harder. What, how can you tell if something is moving away from you?
Usually we tell that by if it's getting bigger or smaller. If something's getting bigger, something in our brain triggers, oh, I think that thing is getting closer, and if it's getting smaller we think, oh, it's getting further away. But what if that doesn't quite work? What if something's too far away and you can't really tell if it's getting closer or faster? Well, you wanna know how fast it's going.
Not just where it's going, but how fast it's going? Well, let's say you have a speed gun. You know speed guns are these handy little devices where it shoots out a beam of light, you know, radiation is invisible, but it's still light. It shoots out a beam of light, hits something, that light gets absorbed and then gets re emitted because that's what light does, and it comes back and there's a little sensor on the light gun that detects that light and it compares the light that sent out to the light that got back and see if there's any differences. And if the object is moving, there will be differences.
There'll be a shifting of the pattern of light. If something is moving towards you, then as it's emitting the radiation, these waves of electricity and magnetism, you can imagine the waves just like emanating off the object and as it's moving towards you, it squishes up those waves. Like it gets in the way of its own waves. It piles them up. It shifts them to higher energies.
It blue shifts them. This is a Doppler shift, the exact same kind of shift that causes stretching and compressing of sound waves, can do it to any wave including light waves. And the exact opposite of blue shift is red shift, if something's moving away from you, it's pulling on those waves. It's tugging at them as it's moving away. It's stretching them out.
That is a very, very physics based way of measuring how fast something is moving towards you or away from you. Good old fashioned speed gun isn't gonna work on distant galaxies, is it? If we just point, if we look at a random galaxy, we're like, wow, I wonder how quickly that galaxy is moving away or towards us. Well you could point a speed gun at it, wait for the light for millions of years to reach that galaxy, be absorbed by some random dust cloud in the galaxy and be re emitted and then millions of years later come back to you and you say, oh yeah measured it. We need a different trick.
Thankfully, nature's very very friendly sometimes, and we got a different trick. We have a way of measuring how quickly a galaxy is moving towards or away from us. Little bit harder to measure how quickly a galaxy is moving left or right on the sky, because then you'd have to just stare at the galaxy for a really long time. We don't get that. We can get that towards and away from us pretty easily, and that's through spectroscopy, perhaps the most important invention ever made that nobody understood.
Spectroscopy was discovered around the mid 1800s and what chemists and physicists realized is that elements give off very, very specific kinds of light. So if you take, say, a bunch of oxygen and put it in a bottle, and then heat it up, like just shoot it with electricity or whatever, and it starts glowing, and it has a very cool color to it. And then you wanna see, like, well, I wonder what what colors are there, what are represented by the light given off by say oxygen. So you run it through a prism or a diffraction grating, something that spreads out the light into a rainbow. In normal sunlight, you'll get like the full rainbow.
But with something like oxygen, you won't get the full rainbow. You only get bits and pieces of the rainbow. You'll get a set of very specific wavelengths. You'll get a fingerprint of light. And then if you do the same thing to say hydrogen, put it in a bottle, heat it up, get it all excited, let it glow, put the light through a prism or diffraction grating, see how it spreads out, you don't get a full rainbow there, you get a fingerprint of light.
You get very, very specific wavelengths of light emitted. And you repeat this exercise for every element, and you discover that every element has a unique fingerprint. These elements, the light pattern from the elements is unique and identifiable. And then you go on to molecules and compounds, and each molecule and compound has its own unique fingerprint. This was all worked out in the mid eighteen hundreds before we knew what atoms were, before we knew about what light was in terms of photons or even electromagnetic waves, before we knew about quantum mechanics and energy levels.
Like, it took another fifty, sixty years before we actually figured out this whole spectroscopy thing. But that doesn't stop us and it didn't stop anyone in the eighteen hundreds from making it useful. Like this is a thing, like okay, hydrogen looks like this. That's that's its light. So if I look at a hot glowing thing up in space, and I see the exact same fingerprints in the light that's coming off of that thing, I know there's a lot of hydrogen in it, or a lot of oxygen, or a lot of water, or on and on and on and on.
But some objects, especially galaxies, we could identify the fingerprints of light, like, okay, yo, that galaxy sure does have a lot of hydrogen and oxygen and nitrogen, like, you know, all the usual stuff, but all the fingerprints are there but shifted. They're just a few wavelengths away, or like a few, you know, a little bit away from where they should be. The patterns are the same, but just off by a little bit uniformly. What is that? That's a red shift or a blue shift.
If an object is coming towards us, then the light it emits will get blue shifted, and it means all the fingerprints of light emitted by all the stuff it's made of will all get shifted towards the blues. And if it's moving away from us, it will all get shifted towards the reds. Astronomers started using this almost right away even though they had no idea what was going on. It doesn't matter. It still works.
And in the early nineteen hundreds, Edwin Hubble was looking at distant galaxies and found that on average, all galaxies are moving away from us. By looking at the fingerprints of light, looking at the shift, measuring a velocity, everyone's moving away from us. Andromeda, our nearest neighbor, is coming close, coming towards us, but like every other galaxy is moving away from us. And that's not it. That's not just it.
There's something else too. Because Hubble knew how far away the galaxies were, so he's able to make a relationship. He was curious, like, okay, for a galaxy of a certain distance, it's moving away at a certain speed, and then a galaxy at a different distance is moving away at a different speed. Is there any relationship between distance and speed? Yes.
Hubble's big result. The further away a galaxy is, the faster it's moving away from us. And there's a very, very tight, close knit relationship between distance and speed. You go to a certain distance and there's a galaxy there and it's moving away at a certain speed. If you double the distance and find a galaxy there, that galaxy is moving away at twice the speed.
If you go three times the distance, you get three times the speed. You go 10 times the distance, you go 10 times the speed. You get a hundred times the distance, you get a hundred times of the speed. It's called Hubble's law. And this is befuddling because nobody expected this.
I mean, okay. A couple people expected it but basically nobody expected this. To be there, this this this relationship between distance to a galaxy and the speed at which it's moving away from us. Like, okay, first you have to wrap your head around the concept that galaxies move. These things that are hundreds of billions of stars are going around pretty fast, like hundreds of kilometers per second.
Okay, take a breath, I can accept that. Okay, some are moving towards us, some are moving away from us, big whoop, everyone's moving around. No, something's going on. Is there a conspiracy? Because it looks like we're the center of the universe, right?
But conspiracy seems weird, like how does a galaxy at a certain distance know how far away it is from the Milky Way and know how fast to fly away from us versus a galaxy that's 10 times further away. How does it know it's 10 times further away? Like, who cares about the Milky Way? We're not special. Conspiracy seems a little bit off, so we won't consider it.
Maybe it's not really motion. I mean, this looks like redshift. It looks like everything shifted to the reds. It looks like everything's moving away from us. But maybe, maybe it's not due to motion.
May maybe the red shifted due to something else. Maybe something's messing up the light. Maybe light as it comes from a distant galaxy, something happens to it to make it redshift. Well, it's not a bad idea. Not a bad idea.
The challenge is that any physics that you introduce to shift light like that as it travels does a lot of other things to the light. Like, it will scatter it, or it'll be depend on wavelength. Like like, blues will be shifted more than reds. Or in in just like anything you invent to make light redshift just as a universal law of the universe, if you try to make that happen, all the physics you introduce starts doing weird stuff to the light and weird stuff that we don't ever see. You know, a distant galaxy doesn't look any fuzzier than a closer galaxy.
We don't see any of the hints of this effect in our own Milky Way galaxy. The light isn't getting scattered. The red shifting is always uniform. It doesn't matter where these fingerprints are on the spectrum. You always get the same kind of shift.
This idea, by the way, is called tired light, and it just didn't work out. Edwin Hubble's result in the mid nineteen twenties, you know, people responded. They're like, oh, maybe it's this, maybe it's that, maybe this. And really, the only contender left standing, the only hypothesis, the only idea that was able to explain Hubble's result that there is a relationship between distance and velocity is that we live in an expanding universe where the distance between galaxies grows with time. So it's not just us.
We're not the center. Because if you go to any other galaxy, all the galaxies appear to be receding away from that one, and so on and so on and so on. Everybody is receding away from everybody else because the universe is expanding. Say a galaxy emits a bunch of light, and it's headed towards us. As that light is traveling, the space between our galaxy grows, and this itself will stretch out the light.
So this redshift that we're seeing from the galaxies isn't a Doppler effect. It's not like a siren on a car driving down the road and changing pitch. It's not like something you can measure with a speed gun. It's something different. It's due to the expansion of space itself.
It's stretching out underneath, quote unquote, the light, and it's making the light grow redder. And this perfectly explains Hubble's law because if you look at a galaxy that's twice as far away, that's twice as much space between us, which means that bit of light had to travel twice as far, and there's been twice as much expansion of the universe in that travel time, and so of course it will get red shifted twice as much. You look at a galaxy 10 times further away, there's 10 times as much space. So in the amount of time it took for that light to reach us, there's been 10 times as much expansion, which means there's 10 times as much red shifting. And it appears that the galaxy is flying away from us 10 times faster.
This redshift is not due to the actual motion of the galaxies. They don't have rockets attached to them, they're blasting around. No. It's due to the expansion of the wavelengths as the light travels from them to us. This gets a little bit confusing because I'm using terms like recession or moving away, flying away.
And we use words like velocity and the connection to redshift, even in the like, even in real astronomers, which some days I pretend to be one, will use terms like recession velocity. And this gets a little bit confusing because if you follow Hubble's law, where the farther you go in space, the distance, the faster the thing goes, double, triple, 10 times, a hundred times, a thousand times, eventually you get to a certain point, a certain distance, where a galaxy appears to be receding faster than the speed of light. And you're like, wait, I didn't think that was a thing. Hey. Wait a minute.
I thought nothing could go faster than the speed of light. You're right. So there are two answers to this of how could possibly a galaxy that's very far away recede away from us faster than the speed of light? There's two answers. Both are equally valid.
You can just pick either one. It's two different sides of the same math equations. Either you can say, well, oh, this idea of nothing can go faster than the speed of light, that appears in the special theory of relativity, which is a great theory. But it special theory of relativity is a local law of physics. It only tells you what happens right in front of your nose.
It doesn't tell you what things on the opposite side of the universe can do. They can do whatever they want. Special relativity just doesn't apply to objects that far away. It's not valid anymore. In which case, you can have whatever speed it wants.
Who cares? The other option is to say that the word speed only makes sense nearby. That distant objects are concept of the word velocity or speed only makes sense nearby. Basically, speed and velocity become meaningless at great distances. And so they can have whatever quote unquote speed they want because speed doesn't make sense.
You just get a red shift. That's what you're actually measuring is a shift in the light. It looks like something's moving away from you, but really the shift in light is due to the expansion of space between you and the object. You don't have to call it a speed if you don't want to. It's just a red shift.
And just the redshift is all you need to know that you live in an expanding universe. That's it. This Hubble's law, where you double the distance, you double the speed, you multiply the distance by 10, you multiply the speed by 10. This is only local to it breaks down or it doesn't break down search changing. Why?
Because the universe changes with time. If the universe always had the exact same expansion rate through all of its history, then Hubble's law would apply no matter how far out you go. Right? It's always the same expansion rate further and further and further away, which means more and more into the past. But that's not true.
The expansion rate of our universe has changed with time. Because throughout cosmic history, different things are in charge of the universe. Right now, dark energy is in charge of our universe and causing accelerated expansion, changing the rate of expansion of our universe right now, even today. Back in the day, matter was in charge, and it had a different expansion rate, different evolution. Way back in the day, radiation light was in charge, way, way, way back, like the first hundred thousand years.
As the universe evolves, its rate of expansion changes because the rate of expansion is governed by what kind of stuff is in the universe today and how much of it there is. We can measure this because we can measure distant distant distant galaxies and this Hubble's law, this this very nice tight relationship starts to curve, starts to bend, starts to get different shapes to it. We need to be able to connect distance to velocity though. Like all Hubble knew, all Hubble knew was this relationship between distance and speed. The answer to that, the resolution to that was, oh, yeah.
Space is expanding. And as long as space is expanding in the same way, at the same rate for a long time, then Hubble's law, this very tight relationship will always hold. But as soon as you start monkeying around with the expansion rate, for whatever reason, then you're gonna get a different relationship between how far away a thing is and how fast it's moving. Hubble's result was empirical. That's the fancy word.
It means it's just based on the data. Like, Hubble's like, I got no clue what's going on, but here's my pretty plot. Where's my Nobel Prize? He did put a footnote in the end of his 1924 paper. He's like, hey.
Maybe maybe the universe is expanding. I don't know, which is good enough. That was an empirical result. It was just based on the data without really understanding what's going on. But we need theory.
We need actual understanding. We need the underpinnings. We need a way to tie distance to speed or predict distance to be oh, yes. Yes. Yes.
If the galaxy is this far away, then it should be going this fast. That's a theory. Our theory comes from general relativity. General relativity describes how space time moves in response to the presence of matter and energy. That's the deal of general relativity.
This applies to objects in orbit around the Earth. It applies to the solar system. It applies to stuff inside the galaxy and it applies to the whole entire universe. Why? Because the whole entire universe is made of matter and energy and it looks like the whole entire universe is on the go.
Our universe is expanding. The rate of expansion is tied, influenced, directed, controlled by the stuff inside of it. And general relativity gives us the theory. General relativity is a snake pit of mathematics. If you ever want a headache, a migraine, just read the full equations of general relativity.
It's just surprised Einstein even came up with it. I mean, it's just wow. Wow. But you can simplify. You can you can chop down that forest of mathematics if you make some simplifications.
Like, if you assume our universe is isotropic, isotropic, which means it looks pretty much the same in any direction. And if it looks homogeneous, which means roughly at certain scales it's, you know, any random chunk of the universe looks pretty much like any other random chunk. If you make those two assumptions, then a lot of the goriness of general relativity gets cleaned out, and you're left with some nice pretty straightforward equations. A set of equations called the Friedman limit Robertson Walker metric, and and also specifically something called the Friedman equation, which governs, which tells us the expansion history of the universe based on what it's made of. Like, you this is the recipe.
You plug in what the universe is made of, and you get an expansion history. You get to see how the expansion of the universe changes with time. How quickly it expands. How fast galaxies recede away from each other based on what the universe is made of. That's the glue.
That's the theory. So you can run it in both directions. You can either take the ingredients, say you go out and you measure, okay, here's what the universe is made of, then you can plug it into the Friedmann equation, and this tells you how the universe grows with time. Okay. Today it's growing like this, five billion years ago it's growing like this.
And eight billion years ago it's growing like this fast. Etcetera, etcetera. It's your recipe. That's one way to do it is measure the ingredients and then get the growth rate, the growth history. Like if I figure out what I feed my kid, I can predict how how Big O gets, for example.
So, okay, okay, so we fed him macaroni and cheese seven nights in a row. How fast does he get a crow? Like, it's like that. That's one way to do it. Another way is to contribute to Patreon.
Go to patreon.com/pmsutter to learn how you can keep all of my education outreach activities going. This is my job, this is my life, is talking about science and having a good time, and I could really use your support. And I could I also deeply deeply appreciate your support, because I know you got bills made. I know you've got mouths to feed. I know you've got mac and cheese to buy seven nights a week.
So you go ahead and do that first. If you've got some leftover, I'd really appreciate it. That's patreon.com/pmcenter. Equations work in two directions. Right?
You can either take the ingredients and figure out the growth rate, or if you can measure the growth rate you can figure out the ingredients. Right? If you can somehow measure how the universe grows with time through whatever means, then you can run the equation the other way and you can figure out what it's made of. You can look at the growth chart of your kid and say, yeah. That kid ate mac and cheese seven nights a week.
Very obvious signs. You can run-in both directions. And we've done that, of course, because we're bored and we got nothing better to do. So we do we do all the things. Right?
We try every technique we can to tackle this problem, because we don't know what the universe is made of, we're trying to figure it out. And we have various ways of either trying to measure it directly, or to measure the growth rate and go back and forth. One of the best ways you can take the ingredients of the universe and tell how it's gonna grow with time is through the cosmic microwave background. Cosmic microwave background, this fossil radiation, this leftover life from the universe when the universe is just baby. 380,000 years old.
It was so cute back then, chubby cheeks that you could just pinch all day long. Cosmic microwave background. We can measure it. It's there. It's right there in the sky.
It's in the microwave, so you can't see it because your eyeballs aren't adapted to microwave light. If they were, you could. So instead, we build, like, antennas and satellites and all that kind of stuff to measure the microwave light. It's a picture. Like, wow.
That's what the universe looked like back then. From that picture, we can figure out what the universe was made of back then. And you say, okay, there's that much normal matter, that much dark matter, that much radiation, da da da da da. This is great. And then from there, once you know what the universe is made of, you plug it into the freeman equation and say, this is how the universe evolves.
Done. All the way to the present day. You can predict the present day expansion of the universe based on the cosmic microwave background days. Oh, yeah. When the universe was a baby, it looked like this, and then we have our growth model, the Friedman equation.
And then today, the universe is looking like this and acting like this and expanding this fast. The cosmic microwave background is one of the most pristine datasets we have in all of astronomy and potentially all of science. Like, it's up there. Like we have so well precisely measured the cosmic microwave background. Really good stuff here.
So the data are great, but I mean it is when the universe is a baby, right? Like there's a long stretch of history, thirteen point eight billion years between then and now. Like if I want to try to figure out how fast the universe is expanding right now, I have to go back to the CMB when it was just a baby, plug it into the Friedman equations, and then evolve it over 13,800,000,000, and that will tell me what's happening today. Like, you sure? Well, it's all it's what we got.
Might as well take it. And, yeah, from the Cosmic Microwave Background, we get a number for how quickly the universe is expanding today. That number is around 68 kilometers per second per megaparsec. Sixty eight kilometers per second. In megaparsec, you know, my, one parsec is like four light years.
A megaparsec is like 4,000,000 light years. It means if you pick a galaxy 4,000,000 light years away, it's receding away from us at 68 kilometers per second. If you go 8,000,000 light years away, it's gonna be double that. You go 10 times that, then you you get the picture. Yeah.
We get a number. It's a pretty good number. Okay, that's taking the equations one way of measuring the ingredients first and trying to figure out the growth rate from that. The other way is to just measure the expansion rate right right around us just like good old Hubble did. We do this with supernovas, specifically type 1a supernova in our local universe.
These are these are great objects, very very easy for us to tell how far away they are and how fast they're moving away from us. We use supernova a lot. Definitely some pros here. Like supernova are just right there. Like, we are literally measuring the expansion rate of the universe right now using the stuff around us.
Directly measuring it. Like, boom. There it is. There are some downsides to this. It's it's a little bit messy.
You know, you're not exactly sure how far away that supernova is. The redshift's easy to get. Pretty, pretty easy to get. You know, in astronomy terms, it's kind of easy. But the distance is a little bit harder and you need both.
You need the distance and the speed to really get a good solid relationship for the growth rate of the universe today. And that distance is it's kind of fuzzy. It's actually kind hard to get. And we don't have that many supernova, like, it's not as complete. It's not as pristine.
It's not as, oh, elegant as as the cosmic microwave background. So, you know, there's a trade off. Right? Yes. We're measuring it directly, but we're not measuring it that well.
But we get a number with this. We get a number of 72, plus or minus like one. But but but but but the cosmic background we got like 68 plus or minus one I'm giving very rough numbers here's don't numbers here don't quote me now this is interesting these two numbers should agree Right? Because it's the same universe. Right?
If I follow method one and look back at the baby picture, figure out what the universe is made of and then evolve it, bring it forward to the present day and predict what the number of the expansion rate ought to be today. It should be the expansion rate that we see today. These things should match. I should get the same universe today because the cosmic microwave background came from our universe. It's right.
It's it's there. It's ours. Nothing special about that. But we get this disagreement, and this disagreement cropped up a couple years ago. A few years ago once the Planck data came out, the Planck mission was a satellite to very precisely measure the Cosmic Microwave Background.
It did it, and it came out with a number that was a little bit lower for the expansion rate than everyone had guessed. Everyone's like, that's weird. And in the years since, that number has gotten even more firmly weird and firmly different, and people disagree. The people who measure the cosmic microwave background say, you know, we did a good job. This is it.
All you are wrong. And the people who are working on supernova on the actual, like, the measurement today are saying, oh, we we don't we're the ones that did a good job. We're the not ones that got the right number. Oh, y'all are wrong. There's a fundamental disagreement here.
Something funky is up because these two approaches ought to give us the exact same number. You know, within experimental uncertainty, but they're not. There's a tension here. Maybe the data from the cosmic microwave background are wrong. Maybe we screwed something up, like, whoops.
Forgot to check that part of the sky. No. Okay. No one who studies this cosmic microwave background is ever gonna admit that. Maybe we're getting the supernova data wrong.
Right? Like, oh, forgot to carry the two. Happens all the time. Sorry folks. No.
They've checked and rechecked and triple checked and quadruple checked and different teams to work on. They said, no. We're we're we're right. Our data are great. Good lucking anybody to budge on that.
Maybe they're both wrong. That'd be funny. It's what's left. If the data from the early universe are just fine and we're measuring it just fine, and the data from the present day universe are just fine or we're measuring it just fine, then the only thing left is the theory, the glue, the thing that takes the ingredients that we know from the early universe, plugs it in to the freeman equations, brings us the present day and tells us what the expansion rate is today versus the actual expansion rate that we see today. Something maybe something's going wrong in that glue.
We don't think general relativity itself is wrong because we test general relativity in other ways and it always passes, all those tests. So we think general relativity is legit. What's left is the ingredients because there's one specific thing that the cosmic microwave background doesn't tell us very much about, and that's dark energy. Because dark energy wasn't a major player when the universe was young. Dark energy came online about five billion years ago.
The cosmic microwave background tells us a lot about matter and radiation, all sorts of cool physics in the universe. It doesn't tell us a lot about dark energy. You have to make a guess as to what dark energy is like. Not just a guess. You take their other observations, other ways of measuring dark energy.
That seems to be the culprit? Here? Maybe. Maybe. That something funny is going on with dark energy?
This accelerated expansion of the universe? If you start monkeying around with it, if you start letting it change with time, grow stronger or weaker, or made of different stuff, or start talking to dark matter, you can potentially resolve this. Oh yeah, yeah, there's a reason the early universe measurements from the Cosmic Microwave Background don't agree with the modern day measurements of the supernova is because your model is too simplistic. If you assume that dark energy behaves exactly the same through all of cosmic history, then you get this tension. But if you allow it to flex and bend a little, you're like, oh, maybe dark energy is a little bit more slippier than we thought.
You can get their results to agree. Is that it? Well, we don't know. This is a big issue in cosmology right now as at the time I'm recording this. We honestly don't know.
Everyone's taken more data. Everyone's digging deep into the analysis. Everyone's accusing the other being idiots as usual. What's the resolution? You know, it could be that we just take more data and then this slowly goes away.
Happened before in astronomy. It could be that we take more data and this discrepancy, this difference cements itself. That's happened before and we have to come up with a way to resolve it. Our best guess at resolving it is start messing with dark energy, start start changing it. In that case, that would be huge because this would be a sign, this would be a way for us to test some exotic models of dark energy, which would be really, really cool.
We're just not quite there yet. If you have any ideas, let me know, and maybe we can go chasing after that Nobel Prize together. Thank you so much to Martin and Gary s, Vernon s, Jim k, Tim r at JamoSmith, Mike n, German for nine, and Cody for the questions that led to today's show. And please go to patreon.com/pmsutter to help keep all my education outreach activities going, keep my life going, keep my job going. And thanks to my top contributors this month, John, Matthew K, Helga B, Justin Z, Matt W, Justin G, Kevin O, Duncan M, Corey D, Kirk B, Barbara K, Nudadoo, Chris C, Eric M, Steve Z, and Digital Neo.
It's your contributions and everyone else's that keep me going with so much life. Go to askaspaceman.com for the episode archives. You can hit me up on social media, hashtag askaspaceman. You can also email askaspaceman@gmail.com. If you're not gonna do Patreon, go to iTunes and leave a review.
I really appreciate it. And I will see you next time for more complete knowledge of time and space.