Bonus Episode: Dust with Dr. Paul Sutter


Recorded during the Astrotour to Costa Rica, Fraser talks to Dr. Paul Matt Sutter about the nature of dust and BICEP 2’s claim of discovering primordial gravitational waves.

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Fraser: Hey, everyone, Fraser here. This is a special bonus episode that I did with Dr. Paul Sutter when we were on our Astro Tours trip in Costa Rica. Paul and I got a chance to sit down and talk about the nature of dust and what impact it played on the BICEP2 claim of discovering primordial gravitational waves, and it was a fascinating conversation. Paul was on the team that helped disprove it, and so he had really a front seat to how this all went down.

And I also just wanted to give you an idea, this is sort of the kind of content, the kinds of things that we do, during our Astro Tours. The next cool one that we’re gonna be doing is of course our All-Stars Party, which is gonna be held in June at Joshua Tree. It’s gonna be me, Paul Sutter, Pamela, Skylias, John Michael Godier, and it goes from June 26-30. We’re gonna be staying at this really fancy hotel, and we’re gonna be looking at the stars every night, and we’re gonna be going on all kinds of interesting trips around the area.

So, if you’re interested, go to, and you can find out more information. The deadline to sign up is March 26, 2019. And if you went before, and the price seemed a little high, they’ve lowered the price, so it might be in your budget now. So, check that out, All right, onto the interview. So, I’m here in Costa Rica with a whole bunch of my best friends. And of course, another one of my best friends, Dr. Paul Sutter. Paul –

Paul: Just one of many.

Fraser: One of many.

Paul: There’s so many best friends.

Fraser: We just introduced all the other best friends.

Paul: You do realize that “best” is a superlative? There can only be one best friend. You can’t have many. You may have better friends –

Fraser: I just explained that I did.

Paul: Okay, never mind.

Fraser: Never mind. So, the purpose of this, of course, is whenever Paul and I are on one of the Astro Tours, this one in Costa Rica, I like to corner him and grill him on a bunch of astronomy questions, and this is no exception. And so, the topic that I wanted to talk about today, Paul, is dust.

Paul: Dust.

Fraser: And I talk about this all the time, and I usually – like, “gas and dust, gas and dust, gas and dust.”

Paul: Gas and dust. Or dust and gas.

Fraser: Dust and gas, yeah. So, over there, in the beginning, the universe was gas and dust, and then stars formed in regions of gas and dust.

Paul: Dust and…yeah.

Fraser: And planets are obscured by gas and dust.

Paul: Gas and dust.

Fraser: And the discovery of –

Paul: The disk of the Milky Way is full of gas and dust, yeah.

Fraser: And so, this dust, I have no personal experience with it. I’m assuming it’s small. When astronomers are talking about dust, what are they talking about?

Paul: So, are you familiar in general with the concept of dust?

Fraser: I think of smoke from a fire or like a dust storm.

Paul: It’s a bunch of tiny junk.

Fraser: Right.

Paul: All right, it turns out the universe if full of a bunch of tiny junk. We’re talking about little, little molecules of hydrogen, carbon, oxygen bound together. We’re talking about water molecules. We’re talking about carbon elements. It’s all the basic building blocks of you, me, planets, stars but just flung out there. And it’s everywhere. We see it in the solar system. You can actually literally see it. We call it zodiacal light. That’s the reflection of sunlight off of –

Fraser: Dust.

Paul: – dust in the solar system. And these are like micrometeorites, these are molecules, these are just little chains of hydrocarbons. It’s just junk, like someone needs to clean this place up.

Fraser: So, then where do I have to make that differentiation between gas and dust, right? Because again, I just that term “gas and dust, gas and dust”.

Paul: Gas and dust.

Fraser: So, which is the gas, and that’s different from the dust?

Paul: So, usually, the gas is going to be more elemental. It might be pure hydrogen or pure helium. And as soon as you start building more complex molecules, you start making dust because those complex molecules are gonna behave differently. They’re gonna react with light differently, they’re gonna react with each other differently, they’re gonna have different changes in temperature and density when exposed to heat and radiation than gas will. And so, it’s as soon as you start building those more complex molecular chains, you transition from just being a gas to being a dust.

Fraser: So, the idea that you have a dust means that some kind of process has happened. There’s been some intermediate step. And so, what are the sources of this dust that we’re gonna see out there in the universe?

Paul: This dust that we see in solar systems, in galaxies, and even between galaxies comes from everything. So, if you blow up a star, and you have a supernova, and some of those elements escape out into the galaxy or past the galaxy, and they start mixing with other elements, and they start bonding together and forming molecules, you get some dust. If you form planets in a solar system, and a bunch of those planets smash together and spread their bits all over the surrounding part of the galaxy, you get dust. If you process ingredients near a supermassive black hole as material is falling in and grinding against each other and subject to intense heat and pressures, then they get ejected, you get dust.

It’s just the accumulation of all the fun astrophysical processes that can happen inside of a galaxy.

Fraser: And so, I think where dust is quite difficult for astronomers is that it has made a lot of previous discoveries, ended up being wrong because astronomers didn’t understand what the impact of that dust was.

Paul: Yes, exactly. The fundamental problem is if you look at a star, you could pick a star out in the sky and you look at the star, you’re not seeing the pure unadulterated light from that star as it was emitted from its surface. You’re seeing that light after it’s passed through all those lightyears of dust between us and the star, and that dust affects the light. It can scatter it, it can shape it, it can change its polarization, it can change its wavelength, it can shift it around depending on what kind of dust, and how thick the dust, and where the dust is. And so, the light that you’re getting isn’t pure starlight. It’s starlight that’s been filtered through dust.

Fraser: And I guess will an astronomer not know that that dust is there? Do they have techniques to know which of the light is being obscured by the dust and which is the pure starlight that they should be seeing?

Paul: Yes. This is like 95% of astronomy. It is. It’s figuring out what is dust and what is not dust. Thankfully, we have some tools. One, we have physics. We know what the dust itself is made of because we can look at things like the zodiacal light where we know, wow, that light that is reflected light off of dust, so we can capture its properties, we can understand it, we’ve seen what it’s made of. We have laboratories where we try to recreate what interstellar dust or intergalactic dust might look and behave like and understand how light gets filtered through it, and we understand molecules, and molecular interactions, and interactions of molecules with light.

So, we bring all this to bear, and we look at a single star, and I have to say, we do our best to remove the effects of things like dust. Dust is just one of many things you need to remove, but it’s a big thing, and we just do our best to subtract out the effects of the dust. There are different ways to do this astronomically. Say you have two stars that are very, very identical or you know they’re identical, but they’re in different parts of the sky.

Then say one is in a direction perpendicular to the disk of the galaxy and one is in a direction along the disk of galaxy and maybe further away, then you know the one that’s further away in our disk is gonna have to pass through more dust compared to the light from the one that’s nearby and above us, and you can compare those differences and get a handle on what the dust is doing to the light. Once you have that handle, you can apply it to other stars.

Fraser: So, it’s kind of like when you’re looking at the sun at –

Paul: Which you should never do.

Fraser: No. You’re looking at the sun with proper eye protection –

Paul: Thank you.

Fraser: – and when you look at the sun directly up, it looks more yellowy-white, and when you’re looking at it down closer to the horizon, it’s going through more atmosphere, and you’re seeing it more red because more of the other light is getting scattered away.

Paul: That’s a great analogy.

Fraser: And you know that it’s still the same sun. It was the sun high up in the sky and it’s the sun as it’s getting towards sunset, and so it’s what’s going on in the atmosphere in between that’s changing your view.

Paul: Exactly. You’re not seeing the sun. You’re seeing sun plus atmosphere.

Fraser: And then making those two measurements will allow you to figure out how much of what you see of the sun is the atmosphere.

Paul: Exactly. Exactly.

Fraser: But it’s kind of strange that there’s this thing that, as you say, 98% of astronomy that haunts astronomers, that – and we’ll get to this in a little bit here – has ruined potential for winning Nobel prizes, and yet you’ve never actually even touched it. Are there any plans to try and pull some of this material out of the solar system and bring it home?

Paul: Some of the dust in our solar system is of interstellar origin, and we have missions on the International Space Station, I think there was a dust collector on New Horizons if I remember right where it impacted a little gel thing, and we can look at the dust in the outer solar system. And we know that some of it wasn’t born here. It just kinda filtered in because we’re swimming in it. But most of the work of understanding dust, believe it or not, there are some astronomer who are dusty astronomers, who are experts on dust. This is all they do is they like the dust.

That pesky starlight is so annoying they prefer the dust. It’s just by doing astronomy as it’s always done, which is looking at different objects in different wavelengths, in different directions, trying to put all the puzzle pieces together. One of those puzzle pieces is dust.

Fraser: And so, to kinda bring this around, there’s this really big discovery that was announced back in 2014 that they had seen direct evidence of primordial gravitational waves in the cosmic microwave background, and this was the smoking gun for inflation. We’ve talked about this on another episode with Pamela, so I don’t wanna go too deep into this idea, and I’m sure most people listening to this show – I hope – remember this time when it was like, “This is it, Nobel prizes for everybody because we’ve got this confirmation for inflation,” and then it turned out it was dust.

Paul: It was dust.

Fraser: So, can you tell me a little bit because I know you were on the team that detected the dust?

Paul: So, at this time in 2014, I was a member of the Planck collaboration. Planck was a satellite observing the cosmic microwave background in many, many frequencies and doing all sky maps, so doing relatively low-resolution compared to other experiments but doing the whole sky and doing a very, very good job of it. We’re very proud of ourselves.

And we are trying to understand this primordial light, this light that has been around for 13.8 billion years and is filtered through billions and billions of lightyears of dust to get to us. And the dust does a very, very specific thing to microwave light. The dust changes the polarization of the light. Every electromagnetic radiation has two polarizations, and it changes one of the polarizations of it, so it affects it in a very, very subtle way.

Fraser: In that if I looked at the microwave with my 3-D glasses, and my 3-D glasses happened to be space telescopes –

Paul: Yeah, very large.

Fraser: – and my eyes happened to be microwave sensors –

Paul: Microwave sensors, yeah.

Fraser: – then I would see the way the polarization was being changed.

Paul: If I could show you pure, unadulterated cosmic microwave background light and the light that actually reaches your sensors attached to your face, and you would see a little bit of difference because of the presence of dust between you and that light. So, we’re trying to get to that light. We’re trying to understand what the universe was like 13.8 billion years ago, which means we have to remove the effects of the dust. We have to get rid of that.

And us and the Planck collaboration, at the time this announcement was made, we were putting together our first round of results. We had a bunch of data. We had been collecting data for a year-and-a-half. We were doing all the hard work, and it’s a tricky, tricky thing to figure out what’s dust and what’s not dust, but there are many, many papers with very gory mathematical details that have all the recipes you need to pull the dust out of these maps [inaudible] [00:15:12].

Fraser: How to know dust from not dust.

Paul: Exactly. And the BICEP collaboration who made this announcement, they were trying to get at this primordial signal of inflation. That signal that they were looking for can also be generated by dust. The way the dust impacts the cosmic microwave background light mimics the way this primordial inflation thing affects the microwave light. So, if you just see it, you’re like, “Wow, I got a signal.” Okay, are you looking at inflation or are you looking at dust? They had an experiment at the South Pole that was very, very high resolution, super, super detailed but a very, very small patch of the sky because it’s just one telescope sitting down there.

So, they needed to know, in order to do their analysis, they needed to know in this patch of the sky that we’re staring at, what’s the dust in that direction. If I know for sure the dust in that direction, I can pull out the effects of the dust, and if there is any signal of this exotic thing left over, then I know that’s primordial inflation signal. But with their experiment by themselves, they couldn’t figure out the dust. They needed to rely on something like Planck to provide the dust map.

Fraser: Which was doing sort of an all-sky survey of where all the dust was.

Paul: All the dust, all the cosmic microwave background light.

Fraser: And then they could check their little area against the big map and then figure out –

Paul: Exactly. And the reason it needs to be done with Planck and not BICEP is various mathematical reasons. You need the whole sky in order to be able to pull out the dust. So, we were still working – in Planck – we were still working on what’s dust and what’s not dust, but someone gave a presentation at a conference showing a preliminary map of “hey, here’s the dust in the sky in the microwave band”. They technically weren’t supposed to give that slide. It was preliminary.

Someone from the BICEP collaboration took a picture of that slide, used that picture and said, “Aha, here is the dust map. Here is the dust everywhere in the sky, everywhere in our universe.” They took that, folded it into their own analysis, and said, “Okay, we’ve subtracted the dust –”

Fraser: And we’re fine.

Paul: “– and we’re fine, and here’s our primordial signal.” This is like fourth-hand information to me. I’m just repeating rumors, but I think the BICEP people would say this is how it went down. And then they made the big announcement, but 45 minutes later, there was an email from the leads of the Planck collaboration saying, “Okay, Step 1, do not talk to any media. Step 2, we are having a collaboration meeting right now to figure out what’s going on.”

And I remember in the collaboration meeting, the emails, and the conversations, we formed an emergency working group to just figure out what’s going on, like how are they claiming this, where did they get this dust map, everything, and we figured out the source, we figured out what was going on, and the dust map that the BICEP collaboration used was wrong because it was preliminary. We weren’t done yet. We were still fixing all our codes and cleaning everything up. We were cleaning up our dust.

Fraser: Cleaning up your dust, yeah.

Paul: And we weren’t done yet. So, internally in the Planck collaboration, we knew within I’d say a week that the result was wrong but because our analysis wasn’t finished, we couldn’t make that public because we weren’t done. We just knew that their dust map was wrong, but we didn’t have the final dust map. And so, it was another, I think, three or four months before our data and our analysis went public, but we made a special press release that was cleared by the – there was all sorts of hilarious internal politics that went into this saying, “No, sorry, it’s dust. They used the wrong map.”

And this is not the first time that dust has destroyed someone’s scientific discovery, and it won’t be the last. Dust is always there. It’s super annoying unless you’re interested in it, and then it’s the most exciting thing in the universe, but it’s pernicious because dust, like I mentioned, is all sorts of things. It’s all the stuff that’s not gas and not obviously a star or planet. So, it’s like that’s a pretty wide range of things, and all the different things are gonna have different effects on different kinds of light on different wavelengths on different bands in different ways at different temperatures, and it’s a mess.

Fraser: On that note, thank you so much, Paul.

Paul: Astronomy’s hard.

Fraser: I know, I know. Where can people find out what you’re working on?

Paul: They can visit That’s my website. It has links to everything I do. You can also follow me on social media, @PaulMattSutter, you can check out for my podcast, for my radio show –

Fraser: And if people wanna join us on tours like we’re doing right now –

Paul: You can go to

Fraser: I thought it was

Paul: It’s both.

Fraser: It’s both, okay.

Paul: I like saying

Fraser: I like too. I’m gonna use that from here on out. That’s great. All right, thanks, Paul. And thanks everyone for joining us.

[End of Audio]

Duration: 21 minutes

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