Powerful observatories like Hubble and the Very Large Telescope have pushed our vision billions of light-years into the Universe, allowing us to see further and further back in time. But there are regions which we still haven’t seen: the Cosmic Dark Ages. What’s it going to take to observe some of these earliest moments in the Universe?
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- Peering toward the Cosmic Dark Ages (Earthsky.org)
- Chronology of the universe (Wikipedia)
- The Universe’s Dark Ages: How Our Cosmos Survived (Space.com)
- Planck pins down the end of the cosmic ‘dark ages’ (Physicsworld.com)
- Epoch of Reionization (MIT)
- First Light & Reionization (JWST)
- LOFAR Radio Array (Main site)
- LOFAR (Wikipedia)
- Square Kilometre Array (Wikipedia)
- The HST Key Project to Measure the Hubble Constant (STSCI)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast, Episode 550, Missing Epochs – Observing the Cosmic Dark Ages. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, Dr. Pamela Gay, senior scientist for the Planetary Science Institute and the Director of Cosmo Quest. Hey, Pamela, how are you doing?
Pamela: I’m doing well, Fraser. How are you doing?
Fraser: Good. I hope you had a festive and fun Thanksgiving time with all your friends and/or family.
Pamela: I did Friendsgiving. I was with some good friends of the show. David Joseph Wesley, you did our theme music, I crashed at his house with a bunch of other friends. And oh my goodness, we ate way too much. But we also arted a lot. So you can now find my art up on Society Six. So societysix.com/starstrider. Go get yourself a planet.
Fraser: I still recommend our version of this is Cakesgiving. So we just get cake.
Pamela: I like that idea.
Fraser: Yeah, we just make cake. And so I’m trying to figure out how to turn Christmas into just pie. Piesmas?
Pamela: Rooksandra, whose last name I’m not going to destroy, one of the friends of our show, she, for her Friendsgiving, made a round meatloaf that had layered mashed potatoes and a parsnip puree, bright purple frosting, and then she used other fruits – not fruits, other vegetables, well tomatoes are a fruit, as garnish. And it looked like a cake-cake, but savory.
Fraser: Yeah, shepherd’s cake. As opposed to a shepherd’s pie.
Fraser: That’s awesome.
Pamela: This is a thing that I strongly, for your Cakesgiving and perhaps for Christmas, recommend.
Fraser: Yeah. Powerful observatories like Hubble and the Very Large Telescope have pushed our vision billions of light-years into the universe allowing us to see further and further back in time. But there are regions which we still haven’t seen. The Cosmic Dark Ages. What is it gonna take to observe some of these earliest moments in the universe? Are we about to have a conversation with the James Webb space telescope? Maybe? Just like a little bit?
Fraser: Just a tiny –
Fraser: No, all right. Fine. It’s gonna happen. All right. So the…I’m trying to think of how to set this up for people. So I mean, I guess when we look – I mean, obviously we’re here in the galaxy, we look around, we see the stars, we can see other galaxies, a couple with our unaided eye, maybe two. With powerful telescopes we can see many. But as we go farther and farther, the view just gets dimmer and more red-shifted and there’s less and less that we can see. So how far back and how far away, and I know this is two different things, can we see right now?
Pamela: Answering that with numbers would require me to know the Hubble constant. And we recently did a show that explains why I don’t know the Hubble constant.
Fraser: Yes, nobody does.
Pamela: What I can tell you is we can see the cosmic microwave background radiation, which we believe was released 400,000 years after the Big Bang. So there’s this one epic wall of light that we see in all different directions. And then we start to see things that cropped up a few million years after the Big Bang. And there’s this gap between the release of that cosmic microwave background and that – well, so there’s two gaps. There’s the Big Bang and then 400,000 years later cosmic microwave background was formed. Haven’t seen anything within that first mysterious epoch.
Fraser: And that’ll be the topic for next week’s episode is that first part that we can’t see. The time when the entire universe was like the interior of a star. It’s hard to see.
Pamela: And then we had a second epoch between when the cosmic microwave background was released and when the universe lit up with stars and galaxies. And we’re just starting to be able to get hints of those early days of the universe during a period that we refer to as the Epoch of Reionization or Epoch of Reionization. I don’t care. It’s that time at which the universe again became transparent.
Fraser: So I mean, I’m gonna need a more specific understanding. So I mean, we’ve got the Big Bang, pull universe is very dense, it’s getting less dense over time, it’s cooling down, eventually it cools down to the point that it becomes transparent, that light can finally escape. And we see this as the cosmic microwave background radiation. And the universe was red at that point. And then – but that’s just all of these photons of light that are left over from the universe being this hot star. And then you’ve got all of this hydrogen and helium gas and it is warm because it was kind of like the Big Bang, cooling off from the Big Bang, able to give off this light. And so then it kept cooling down?
Pamela: So let’s unpack this a little bit.
Fraser: Yeah, yeah. I mean, there’s a bunch of little phases in this idea of reionization and [inaudible] [00:06:09] lit up again. Yeah, so take us very carefully from – everyone understands cosmic microwave background. Let’s move forward from there to what we can see again so we can understand that missing part.
Pamela: So in the moment before the release of the cosmic microwave background the universe was this hodgepodge of atomic nuclei, of electrons, and of photons. So we had light, we had nuclei, and we had electrons. The universe was ionized. Then that next moment, the electrons glommed onto the nuclei. And when this happened, suddenly there were fewer things for all of that light to be interacting with. And light was able to go from the tiniest of distances that it was able to travel before being absorbed and re-emitted to being able to just keep going until it made it all the way to our detectors.
So in that moment, the universe became a mash of neutral atoms and all the photons that had been previously generated were set free. Now, there weren’t new photons being generated at this point in time because the universe is just like hydrogen, helium, trace amounts of lithium and beryllium sitting there going, hi, we’re atoms. Mostly. This is where we start to get interesting things like the 21-centimeter hydrogen forbidden line. And it wasn’t exactly lighting up the whole darn universe, but it was there, occasionally giving off a photon here, a photon there.
And in addition to that, we also have a universe that isn’t completely smooth. There are places that have higher densities and there are places that have lower densities. And that meant that we had an unequal pull of gravity. And so these higher density regions were able to pull material into them, eventually with some of that material forming high enough densities that nuclear reactions ignited in the cores of the first giant stars. We’re talking stars 30 times the mass of the sun to 300 times the mass of the sun.
Fraser: To tens of thousands of times the mass of the sun maybe. I said maybe.
Pamela: I’m gonna go with hundreds.
Fraser: Yeah, yeah.
Pamela: I’m more comfortable going with hundreds of times the mass of the sun. But when this occurred, the light coming off of these incredibly hot stars, ultraviolet light in a lot of cases, that energy is capable of ionizing all of that hydrogen gas. And taking it from being neutral to being, again, free electrons and free nuclei.
But now the universe is a whole lot larger. It’s also a whole lot cooler. So unlike before the release of the cosmic microwave background, when it was a bad thing to have an ionized universe, the universe has now expanded and cooled and expanded and cooled and expanded and cooled to the point that when we re-ionize it, all we’re doing is making it possible for the light from newly forming galaxies to spread out and be seen. And so it’s a good thing to re-ionize the universe.
Fraser: Right. So let me just make sure I’ve got this straight. So you’ve got the cosmic microwave background and so all this light has been bound up, bouncing around, inside from atom to atom and finally it cools down to the point that this light can just escape. And then the universe goes dark again after that first flash of light goes past everything. And the universe goes dark again and it’s all of this hydrogen and helium. And then this stuff collects into stars, the first monster stars. Those stars give off radiation and that radiation re-ionizes, re-illuminates the clouds of hydrogen gas that were everywhere, creating a time that you can see again.
Pamela: And the first stars probably weren’t alone in doing this illumination, this re-ionization of the universe. We also had at the same time, in the most massive galaxies, turbulent inflow of material that formed super massive black holes as well as the accretion disks around those black holes. And accretion disks, if they’re big enough, if they’re dense enough, if they’re hot enough, can ignite and also give off light. When we’re looking at quasars, when we’re looking at active galaxies, that bright central core, that’s not the black hole. Black holes don’t give off light, people. That’s the accretion disk that’s giving off all that light.
Fraser: Right. And so the part that is the dark ages then, is that time from after the cosmic microwave background was released to before the light from all those first stars started to light up all of the clouds of gas and dust. That was the – or gas. That was the – that’s the dark ages.
Pamela: That’s the dark ages. And then it takes time for those first stars and first galaxies to ionize the entirety of the universe. So you have this period of re-ionization, and we don’t know exactly how long it lasts, during which these first sources of ionizing radiation, bright light, are turning on. And they initially, each star ionizes the bubble around it. And then the bubbles start to overlap. And all of this pushes outward, illuminates outward, and clears up our universe. And exactly how that happens – we’ve got computer models, but seeing it, if something were just on the edge of potentially being able to do.
Fraser: Okay, so what’s it going to take to be able to see that then?
Pamela: Well, if we want to see the 21-centimeter radiation, we need telescopes that can see at some of the longest wavelengths of light. 21-centimeter light in our local galaxy requires a radio telescope. And we’re now looking at light that is red-shifted from the beginning, almost, of the universe into the modern day. We’re looking at like 2-meter-long radiation according to one paper I was looking at.
Fraser: Right. So this is the same thing where, say, the cosmic microwave background today is microwave, several millimeters long. Originally it was red light and it’s – over the expansion of the universe it has been stretched out to be this size. And so this 21-centimeter…this hydrogen radiation that was being emitted at originally 21 centimeters is now meters across. And to have a radio telescope that is capable of detecting that at that level of sensitivity is a pretty hard job.
Pamela: And this is where looking at telescopes like LOFAR, which is being built in northern Europe, looking at the future Square Kilometer Array, these massive new radio arrays that have, in some cases, quite large dishes as well, this is how we’re going – it’s not necessarily large dishes. Let me rephrase that. That have antennas sensitive to the longer wavelengths of light. You start building your antennas fundamentally differently. They’re not just big dishes anymore. When you start looking at things that are this long of wavelength. They start looking more like spiky bits coming up out of a field.
So when you start building larger spiky bits rising up out of a field, this is going to allow us to be sensitive to the extraordinarily faint because 21-centimeter radiation, this is generated by the spin flip within a hydrogen atom. There’s different states for the alignments of the particles within the neutral hydrogen. And when you change that alignment, that’s what gives off the 21-centimeter radiation. This isn’t an electron bouncing between layers, this is simply the atom going, huh, this other state might be a little more stable. I’ll flip. And it’s very rare, it’s very temperature dependent. It only occurs in low density environments where you don’t have collisions taking place.
In order to see that, we need massive arrays with huge collecting area and we need sensitivity to this really long wavelength. But we can do this.
Fraser: I mean, this 21-centimeter is great because it’s just – it shows you where all the gas is and where all the repositories of future star-forming gas is located around the universe. No matter how cold it is, it emits this very specific kind of radiation. And if you can see it, you can map out where all that stuff is. So then we’ve identified this time and it’s the first, what, few million years after the cosmic microwave background was emitted. I guess, what do astronomers wanna know about that time? And how could that then help them better understand the universe?
Pamela: Well, it gets down to constraining factors. We have ideas. We always have ideas. We have these theories about –
Pamela: Yeah, we have lots of models, simulations, that describe okay, so this is how the universe started, this is the age of inflation, this is the CMB, then we have gas. And then we start to form galaxies in two different ways, through little tiny things building up and merging, and through giant things turbulently and falling. Stars turn on. We don’t understand these stars at all. These stars were in an icy universe. So we can at least see the Swiss cheese of the universe having bubbles of ionized material getting blown up with the 21-centimeter line.
The next thing that we wanna look at, and this is where your JWST hint came in. The other thing is the first stars that turned on would have been giving off massive amounts of light, specifically in the ultraviolet. And that ultraviolet light gets red-shifted into the infrared which is where we start to see the use of the JWST when and if it ever launches.
Fraser: Yeah. It’ll launch. It’ll launch.
Pamela: I don’t count my telescopes till they’re functional. You know this.
Fraser: I know, I know.
Pamela: And it has the potential to begin to see the ultraviolet light of these early stars in its red-shifted into the infrared form starting to resolve the earliest galaxies. It’s not going to resolve individual stars.
Pamela: But we can start to put together patterns of this is how hot the stars appear by looking at these galaxies and figuring out what kinds of spectral energy distributions, what kinds of distributions of light coming off of different numbers and different temperatures of stars are necessary to reproduce the light we see coming from these earliest galaxies. And that’s kind of what we’re hoping to do to map out these still neutral hydrogen gas by tuning our 21-centimeter detections to ever increasing red shifts and by looking for those first stars by looking in the infrared and seeing what their added up light looks like and how we can match it with our models.
Fraser: Again, I know for you James Webb doesn’t exist, but for me – so a couple of the cool things about James Webb is the Hubble Space Telescope, right now, when they use a gravitational lens – normally Hubble can see out about 5 billion – the light from galaxies that are about 5 billion light-years away if it really tries. But it can go a lot farther than that when they did their Hubble ultra-deep field survey. And they were able to see to just a few hundred million years after the Big Bang. And when they wanna be able to – when the galaxies line up perfectly, then they can see with gravitational lensing they can see some galaxy that’s maybe only 500 million years after the Big Bang.
James Webb will be able to see those anywhere it wants, in any direction, at any time. Just like, do you wanna see galaxies over there that are 500 million years after the Big Bang? No problem. But also –
Pamela: You may have to wait for the Earth and the James Webb to orbit to where it needs to be to not have to look through the sun. But yes.
Fraser: Yes. Yeah, yeah, yeah, yeah, you can look at the stuff in the hemisphere of the sky that you want to today. But then also, you’re gonna get – when it does its deep-field with 100 hours of collecting for each of its filters, then it’s gonna be able to go all the way to 250 million years after the Big Bang, which is farther back than anything has been able to see. So we’re gonna get the James Webb version of the Hubble ultra-deep field. And that is just a mind-bending amount of capability. And yet, as you said earlier, that’s not good enough. That’s not enough to see those first stars. That’s enough to see those first galaxies, but you still can’t see those first stars.
Pamela: And this is where we have to remember that even with the Cepheid Key Project to measure the distances to as many of the nearby galaxies as possible using Cepheid pulsating variable stars, that was not able to see much beyond our local group. We are constrained in what we’re able to see. And an, 8-millimeter, an 8-meter dish isn’t going to get us individual stars at the beginning of the universe kinds of resolutions. I haven’t done the math, but my gut tells me a solar system sized telescope still wouldn’t get us the needed angular resolution to see individual stars at the beginning of the universe.
This is where luckily, by spending so much of our scientific history crippled by tiny telescopes, we’ve gotten really good at figuring out what’s going on in galaxies. But only being able to look at their cumulative light, by sticking an aperture on that whole darn thing and summing it up and figuring out what stars must be there. And this is what we’re gonna have to do to figure out the early universe.
Fraser: So now, have you looked into the Murchison Widefield Array? Is this one the observatories that you were gonna talk about?
Pamela: No, that one’s still new. Yeah, you know the future things better than I do because – so for those of you who are new and don’t understand why I’m so reticent to discuss things that don’t yet exist, LOFAR exists, it’s just getting upgraded. SKA will exist, it’s on the ground, it has precursor projects.
When I was working on my dissertation, I had an x-ray satellite I needed not exist the way it was supposed to after launch. There was an engine failure. And the big telescope I was supposed to use, the Hobby-Eberly, didn’t actually start to fully function until many years after my dissertation was complete. So I ended up using the historic McDonald Observatory telescopes, which while fine, meant that I only did a fine dissertation and I remain bitter.
Fraser: Yeah. So it’s farther along than you might think. The Murchison Widefield Array is in Australia. And it consists – and this is like those spiky things you were talking about. It consists of – the original one is 2,048 radio antennas arranged into 128 tiles. And it was built in 2013. And then the number was doubled to 256. And the goal, as we talked about earlier, is to look for that neutral hydrogen. And then all that data, all those data, is fed into a super computer and then they are crunching the information.
And now they have just done a paper just in the last couple of weeks about what they’ve been able to find. And it looks like – and this is going back to what you said, which is that there is a – you can’t resolve the individual stars, but you can see the collective radiation that is coming from all of them all at the same time. And in this case, they’re looking for this collective signal from the neutral hydrogen that was being released during that period during the dark ages. And so they’re getting a lot closer.
Pamela: And what I’m really enjoying is watching the white papers for the 2020 decadal survey starting to come out where we’re seeing people say we need to pay attention to these longer radiation wavelengths. We need to start thinking about how do we use the 20-centimeter line to explore the earliest parts of the universe. People are starting to get tired of waiting to get more infrared on orbit and saying let’s figure out how to do this from the ground.
And it’s really amazing to see all the science that people are figuring out new ways to do, whether it be through looking more closely at gravitationally lens distant galaxies, whether it be by looking in new pockets of the spectrum that makes it through the atmosphere, or just considering building telescopes, well, on the backside of the moon.
Fraser: Yeah, yeah. Now, did you know that there is now an operational radio telescope on the far side of the moon.
Pamela: China has one, doesn’t it?
Fraser: Yeah, it’s a space-based telescope, but it is in the shadow of the moon, is able to observe from that position.
Pamela: It’s a bit tinier than I think people are thinking. I think a lot of people really want to go and take one of these big old craters, buff out the boulders and other bits and turn it into a radio dish. And one of the really cool things is there’s some research I saw a few years ago coming out of Tennessee, I don’t remember if it was University of Tennessee or Tennessee State, where the composition of standard lunar regolith is such that if you nuke it with microwaves – I’m talking like microwave oven nuking it, like we talk about with food, not like nuclear bombs. Bad use of language on my part. If you microwave it, it will solidify.
So you can take regolith powder, shape it into whatever you want, hit it with the right color of light, and now you have a solid. And this gets talked about in terms of this is a great way to make roads, this is a great way to smooth out areas, make landing pads. It’s also a great way to, well, make the support structure that we build a radio dish onto.
Fraser: But, considering the fact that it’s, say, $50,000.00 per pound to put something onto the moon compared to walking over and just building it here on Earth –
Pamela: I know, but instead of having von Neumann probes, can I just have lunar von Neumann robots that just build things on the moon instead of putting people there?
Pamela: Okay. I just want lunar factories for building telescopes and stuff.
Fraser: So next week we’re gonna talk about that other period of dark ages, right after the Big Bang, which is also very difficult to study, but there may be some ideas. So, Pamela, do you have some names for us this week?
Pamela: So if you would like to support Fraser, Universe Today on Patreon is a great place to do. If you would like to support my artwork, Starstrider on Patreon is a great place to go. And this show is supported through Astronomy Cast on Patreon. All right. So here are the names for this week. I would like to thank for their patronage, Robert Johnson, Jason Kusterat, Jordan Young, Burry Gowen, Ramji Enamuthu, Andrew Polestra, Brian Cagle, David Truog, The Giant Nothing, Chauncey Wilson, Laura Kittleson, Robert Palsma, Jay Kidd, Corey Davoll, Les Howard, Jos Cunningham, Paul Jarman, Emily Patterson, and Warp Factor Nine.
Fraser: Thank you everybody. We really appreciate it. And we’ll see you next week.
Pamela: You make this possible. Bye-bye.
Female Speaker: Thank you for listening to Astronomy Cast, a nonprofit resource provided by the Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can email us at firstname.lastname@example.org, tweet us @AstronomyCast, like us on Facebook, and watch us on YouTube. We record our show live on YouTube every Friday at 3:00 p.m. Eastern, 12:00 p.m. Pacific, or 1900 UTC. Our intro music was provided by David Joseph Wesley, the outro music is by Travis Serle, and the show was edited by Susie Murph.
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Duration: 30 minutes