Universe of Learning: Science Briefing 12/5/19 – of the Early - 1 - Dr. Quyen Hart 0:00 [Slide 1] Hello everyone, this is Dr. Quyen Hart from NASA's Universe of Learning. Welcome today's science briefing. Thanks to all of you for joining us and to anyone listening to the recording in the future. For this December science briefing, our panel will talk about how astronomers use novel observations to understand the formation of the first stars and of primordial . Slides for today's presentation can be found on the Museum Alliance and NASA Nationwide sites, as well as NASA's Universe of Learning site. All the recordings from previous talks should be up on the websites as well. As always, if you have any issues or questions now or in the future, you can email Jeff Nee at [email protected] Again, that's [email protected] Or Museum Alliance members can contact him through their new team chat app and the info is on the website.

Dr. David Neufeld 1:02 So, thanks everyone very much for joining us. Today we're going to be talking about a long ago. First of all, before there were any stars, in the first briefing, and then, in the second part, we'll hear about the formation of the very first stars.

Dr. David Neufeld 1:23 [Slide 2] If we could advance the slides, please.

Dr. David Neufeld 1:26 [Slide 3] And one more. So, I'm David Neufeld, and I'll talk to you about what we believe was the very first type of ever to form in the history of our universe. And I'll discuss how this molecule has been detected recently in the present-day universe in a nebula that is shown on the left of this slide, and using NASA's, and the German space agency, DLR's, SOFIA Airborne Observatory to observe this very first molecule, the .

Dr. David Neufeld 2:09 [Slide 4] So we could go to the next slide, please, slide four. Let me just say a few words about the very early history of our universe. And in fact, for the first, roughly, 100 million years after the universe began with the , there were no stars or galaxies at all. Initially, the universe was very uniform, smooth, and very hot. And there were no , there were just atomic nuclei, which formed within the first few minutes, and then free , filling up the universe. In fact, there were only three chemical elements at that time: , helium, and lithium. And as you may know, all of the other elements including, of course, the main constituents of the and our bodies, were produced only later by the nuclear reactions that power the stars, or the explosions that end the of stars. Before there were any stars, we just had these three chemical elements.

Universe of Learning: Science Briefing 12/5/19 – Chemistry of the Early Universe - 2 - Dr. David Neufeld 3:27 [Slide 5] If we can go now to slide five, the next slide. We can continue the story by saying that as the universe expanded, it also cooled as it grew older. And eventually, about 100,000 years after the Big Bang, the first atoms formed. So, what I mean here is that the negatively charged electrons and the positively charged nuclei of hydrogen, helium, and lithium, started to combine together to form atoms. In fact, first helium atoms formed, and then, a little later, hydrogen atoms.

Dr. David Neufeld 4:09 [Slide 6] So if we could go to the next slide please, number six. We can now see that the very first molecules to form actually involved the combination of the helium atoms that had formed in the cooling universe with the hydrogen, still a nuclei. And the hydrogen nucleus is just a proton. So, this was the first molecule to form, and so the first chemical bond in the history of our universe was between a helium and a hydrogen ion to form the helium hydride molecule, which is sort of shown in this picture on the right where you'll see it has two electrons, sort of in between the two nuclei pulling them together in a molecule. And then there were subsequent chemical reactions produced other molecules, and in particular, hydrogen molecules, H2, so two protons and two electrons shown in the bottom right hand panel. And the formation of these molecules was actually quite important, we believe, because molecules are very efficient, more so than atoms, in radiating away energy. And that allows the to cool faster. And once the gas became cool enough, it could collapse under its own weight to form the first stars and galaxies, and this will be discussed in the next part of the briefing.

Dr. David Neufeld 5:48 [Slide 7] So if we could move on now to seven Thank you. Sorry, back one. Yeah. Helium hydride , they contain the very first bond in the universe. We consider this as the very first step on a pathway leading to more and more complexity in our universe, starting with the formation of atoms, and then molecules, and then of course, more and more complicated molecules and more and more complicated things in the universe, like DNA, and our brains, but this was sort of the first step on the path to complexity. Now, the story I've just told you, is based on very well-informed theoretical models, but we're actually very far from being able to detect helium hydride ions, this first type of molecule in the early universe that I'm talking about now.

Dr. David Neufeld 6:47 [Slide 8] But it is possible, and it's recently happened that we've detected helium hydride ions in our own galaxy in the present-day universe, in a nearby known as NGC 7027. And a Hubble Space Telescope image of this is shown in the right panel of slide eight. These planetary nebulae actually have nothing to do with , or nothing directly to do with planets, they were so named, because in a small telescope, they looked a little bit like the disk of the . But they're obviously not planets, what they are, in fact, are stars that are at a more advanced aged stage of evolution than our Sun that have shed an envelope, and all that's left in Universe of Learning: Science Briefing 12/5/19 – Chemistry of the Early Universe - 3 - the middle of the star is a very hot white dwarf star that is then illuminating, with very strong radiation, this envelope, which is lit up and glows as you can see.

Dr. David Neufeld 8:00 [Slide 9] We could go to slide nine please. We can say that this shell, then, has very abundant helium ions and hydrogen atoms. The helium ions are produced when the ultraviolet radiation from the central star strips away the electrons, and in that shell that surrounds the star, reactions can take place to form the helium hydride ion.

Dr. David Neufeld 8:27 [Slide 10] Next slide please slide 10. So, the helium hydride ions emit radiation at a very characteristic wavelength. It's about 0.149 millimeters, and this is known to many more decimal places. But unfortunately, this radiation cannot be detected from the ground. It's in a part of the spectrum we call the "far infrared." It's actually blocked by vapor in Earth's . So, in order to observe it, we need to get either out of the atmosphere with a space observatory, or at least above most of the . And in fact, we can do the second of those things with an airborne observatory. And so, my colleagues and I, and particularly, this effort was led by Rolf Güsten at the Max Planck Institute for Radio Astronomy in Germany, we used the SOFIA Airborne Observatory, which is a heavily modified Boeing 747 operated by NASA, and its German counterpart, DLR. And you can see in the upper right panel, a picture of Sofia, and you can see there's a door there on the left side of the aircraft and sticking out, or pointing out ,is a two-and-a-half-meter diameter telescope. This is an incredible technical feat to get this thing to work. And then inside the aircraft, there's a pressurized cabin where various different instruments can be used to analyze the infrared radiation that this telescope can see. And we're above about 99% of the water vapor. And so, we have a clear view of some of the wavelengths that would be completely blocked, even from a mountaintop observatory. And one of the instruments is a high-resolution spectrometer capable of splitting up the infrared radiation into its constituent wavelengths, with very great precision. This is called GREAT which is an acronym for the German REceiver for Astronomy, at Terahertz frequencies, and this is perfectly suited for looking for the helium hydride ion in this nearby planetary nebula.

Dr. David Neufeld 11:10 [Slide 11] We could go to the next slide please. This will be my last slide. So, we carried out these observations and reported the results this spring in an article in , and what we found is that the helium hydride ions showed up as a of exactly the expected wavelengths. So, the plot on the right-hand side shows how much radiation we see as a function of its frequency, with the gray histogram, or bar chart, showing helium hydride's radiation. And the frequency is sort of interpreted as a Doppler shift either towards or away from us. You see there's this peak, and it lines up perfectly with similar observations that were carried out for radiation from , which is a well-known molecule in this nebula. And you can see that they line up exactly, which basically makes it really an unequivocal detection of the helium hydride . Not again in the early universe, but in a nearby planetary nebula. Actually, Universe of Learning: Science Briefing 12/5/19 – Chemistry of the Early Universe - 4 - what the emission we saw was somewhat larger than what our theoretical model predicted, and we're still trying to figure out why, and whether that has any implications for our understanding of the formation of the first molecules in the early universe. But finally, I'll just end by saying that, to me, this is a really very dramatic demonstration of nature's tendency to form molecules. This is a molecule that is formed out of what you might say are very unpromising ingredients. We have hydrogen, but we also have helium, which as you may know, is a element from the last column of the . So that's a that contains helium, , , , etc. And those are called noble , because they have a very low tendency to form molecules. They have a full shell of electrons, but yet, they can form these molecules. It's a stable molecule, although, a fragile one. And remarkably, we can see it form in interstellar space and can detect it now with our instruments. So that'll finish up the first part of the presentation.

Dr. Quyen Hart 13:59 Thank you very much, David. We'll save questions until after the next speaker. We’re on slide 12 now and Jordan, it's all yours.

Dr. Jordan Mirocha 14:07 [Slide 12] Okay, well, thanks for being here, everyone, virtually anyways. The image you're seeing on my title slide is a rendering of a numerical simulation of structure formation in the early universe, focused in particular on the formation of the first stars, and how they impact their surroundings. So, at the center of the frame, stars are forming rapidly in the simulation. These are these tiny specks of light that you're seeing. And they're shining brightly in the optical and ultraviolet part of the spectrum. And that radiation gives rise to this sort of butterfly shaped bubble of hot gas around them. And so, what I want to talk about today is how we can, in principle, use these bubbles as tracers of star formation in the early universe. And this field, which is now known as 21-centimeter cosmology, is uniquely suited to probe these first stars, which may be too faint to ever see directly.

Dr. Jordan Mirocha 14:58 [Slide 13] So if we move on to the next slide, we can see kind of the bigger context here. So, this is a cartoon picture of structure formation in the early universe, where time is proceeding from left to right, sort of the Big Bang all the way on the left, and then present day all the way on the right. And time is not linear in this plot. So, I'm sort of inflating the first billion years of cosmic evolution. And depicting this process that we now refer to as cosmic reionization. And so, the bubbles that you're seeing in this cartoon are the same sorts of bubbles that you saw in the title slide. There are bubbles of hot, ionized gas forming around the first galaxies. And as time goes on, these bubbles grow larger as more and more galaxies continue to form. And eventually they overlap, something like a billion years after the Big Bang. And we know this empirically. We know that this process was over about a billion years after the Big Bang, but we know very little else about this process. So, because reionization is driven by galaxies, our hope is that by trying to measure reionization, we can learn about galaxies. Universe of Learning: Science Briefing 12/5/19 – Chemistry of the Early Universe - 5 -

Dr. Jordan Mirocha 16:09 [Slide 14] And so if you advance to the next slide I just added in the title slide picture for context. This is a very small region cosmologically speaking, giving rise to one of these early bubbles of hot ionized gas.

Dr. Jordan Mirocha 16:23 [Slide 15] So, if you advanced one slide further, you can see the outline of my talk for next 10 minutes or so. So, I will go through the properties of the hydrogen atom that are relevant in this context, and how we expect the first stars to behave, and how we expect to observe them with this 21-centimeter transition to neutral hydrogen. And then I'll wrap up with a recent result from EDGES [Experiment to Detect the Global Epoch of Reionization Signature] collaboration.

Dr. Jordan Mirocha 16:46 [Slide 16] So now on slide 16, you're seeing a cartoon picture of the hydrogen atom that depicts an in orbit around a proton, much like planets orbit the sun. We know that's not really what's going on, but it's a useful framework to think about these things. And so, this particular configuration is the lowest energy configuration of the hydrogen atom, which we call the ground state.

Dr. Jordan Mirocha 17:07 [Slide 17] So on slide 17, I just indicated this ground state with the variable n. So, n = 1 corresponds to this ground state configuration. And what's really special about this ground state is it is actually two states.

Dr. Jordan Mirocha 17:21 [Slide 18] So if you're advanced in the next slide, I've tried to depict this with little arrows. And so what quantum mechanics has taught us is that protons and electrons have angular momenta that are intrinsic to the particle. And that's what's being depicted by these arrows here. And the fact that these spin vectors, the spins of the proton or electron, can either be aligned or anti- aligned gives rise to this splitting in the ground state of a hydrogen atom. And so, this is really important because it means even neutral hydrogen atoms in the ground state can be seen either by absorbing 21-centimeter radiation or emitting 21-centimeter radiation. So that that wavelength at 21 centimeters is corresponds to the energy difference between these two, what we call, hyper fine states. So, it turns out that the structure of the atom, the hydrogen atom, both defines how we are trying to constrain the first stars but also qualitatively helps us predict what the properties of the first stars might be like, and David alluded to this a little bit of in his talk.

Universe of Learning: Science Briefing 12/5/19 – Chemistry of the Early Universe - 6 - Dr. Jordan Mirocha 18:25 [Slide 19] So, if you advance to the next slide, we're onto lesson number two about the hydrogen atom for this talk, which is about cooling. So, in order for stars to form in the early universe, we need clouds of gas to cool efficiently, and one way of doing that is to cool radiatively. You can imagine in a cloud of hydrogen gas. Sufficiently energetic collisions can bump electrons into excited states, which I'm depicting here with n = 2. And when those electrons decay back down to the ground state they release a photon, and that photon, if it can escape the system, will carry away energy, effectively helping to cool the cloud, which may result in the formation of stars. Now, collisions energetic enough to excite this transition aren't happening very much in clouds colder than a few thousand Kelvin, which basically means that the first clouds in the universe are not hot enough to cool via this mechanism. And there are no heavy elements around either. And so, the first clouds likely have to rely on molecules, for their cooling, as David pointed out.

Dr. Jordan Mirocha 19:32 [Slide 20] If you advance to the next slide, you can see the results of the simulation exploring this idea in more detail. So, in a primordial gas, clouds cool very inefficiently, and in fact, they do so through the hydrogen molecule, H2. And because this cooling isn't very efficient, we think that only the most massive clouds are able to collapse under their own weight. And so, the qualitative expectation for a long time is that the first stars to form in the early universe are quite a bit more massive than typical stars are today. And so, if this is true, the consequences are numerous. Massive stars are prodigious sources of energetic radiation, which can modify their surroundings, and of course, these stars explode a supernovae. At the end of their lives, they will inject energy and heavy elements into the surrounding media, potentially change dramatically the chemistry of all clouds that form thereafter, and thus affecting how star formation proceeds for all times afterward. Very interestingly, these sites could also be the birthplaces of the first supermassive black holes that now live in the centers of all galaxies. And so, there's really been a lot of interest in trying to understand how the first stars formed in the early universe.

Dr. Jordan Mirocha 20:50 [Slide 21] If you advance to the next slide, I just want to say one more thing about this excitation that I didn't say before, which is particularly relevant to 21-centimeter cosmology: the absorption and reemission of a photon that excites electrons from the n = 1 to the n = 2 stage can actually result in a spin flip. So not only is this a cooling channel that's inaccessible for the first clouds in the universe, but the absorption and emission of photons with this particular wavelength can actually directly influence the strength of this spin flip radiation background that I'll talk about in more detail next.

Dr. Jordan Mirocha 21:39 Universe of Learning: Science Briefing 12/5/19 – Chemistry of the Early Universe - 7 - [Slide 22] Okay, so on the next slide, I have the third and final lesson about a hydrogen atom for this talk, which is that if hydrogen atoms are subjected to a sufficiently strong source of radiation, the electron can be completely removed from the proton. So, this is known as . And for us, this is important because ionized hydrogen atoms cannot absorb or emit 21-centimeter radiation. So, we expect that signal we're looking for will eventually be extinguished once all hydrogen in the universe has been fully ionized, about a billion years after the Big Bang.

Dr. Jordan Mirocha 22:17 [Slide 23] So on the next slide, I just have a little cartoon.

Dr. Jordan Mirocha 22:21 [Slide 24] And you can click once more to see the full slide all at once. So, what's going on here, in principle is simple. And it's analogous to trying to measure the brightness and the color of a light bulb, in the presence of some absorber or emitter between you and the light bulb. So, for us that light bulb is the cosmic microwave background, a really convenient light bulb, because it's behind everything. And we think we understand its temperature, and so its color, very well. And so, we have this hydrogen gas between us and the light bulb that absorbs or intense radiation due to the spin flip transition in the ground state.

Dr. Jordan Mirocha 22:58 [Slide 25] So on the next slide, I've depicted a slew of experiments that are trying to measure the signal in various ways. There are kind of two distinct classes of experiments here: the three on the upper left are single element receivers, PRIZM [Probing Radio Intensity at high Z from Marion], SARAS [Shaped Antenna measurements of the background RAdio Spectrum], and EDGES. So, these experiments are essentially single radio antennas. And so, they see a big patch of the sky all at once. So, they're not really trying to measure the detailed spatial structure in the 21-centimeter background. Whereas the other three experiments, as you can see, have many elements, and are, in fact, trying to make maps of the 21-centimeter background in the early universe.

Dr. Jordan Mirocha 23:36 [Slide 26] So to give you a sense of what that might look like on the next slide, I'm showing predictions from a theoretical model for what this radiation background might look like if we could map it in detail. So, time is proceeding clockwise from the upper left panel to the lower left panel in this picture. And so blue colors and green colors a corresponding to regions that are quite cold, and warmer regions are indicated in orange. And fully ionized regions are indicated in black because these are the regions where the gas is fully ionized. And so, there's no longer any way to produce 21-centimeter photons. And so, the basic idea here is that at very early times in the top left panel, the universe is quite a cold place. But as the first stars form, they start Universe of Learning: Science Briefing 12/5/19 – Chemistry of the Early Universe - 8 - to heat up their surroundings, giving rise to these oranger colors, more orange colors, in the color bar. As time keeps going on, the ionizing sources grow stronger and stronger and eventually carve out these large bubbles of ionized gas around them, giving rise to this Swiss cheese-like picture on the lower left-hand panel.

Dr. Jordan Mirocha 24:46 [Slide 27] So on the next slide, this is, in principle, what one could see with an imaging experiment, but for these single element receivers with poor angular resolution, that are essentially taking a big average over the universe. So, they're kind of measuring the average properties of hydrogen in the early universe over time. And this signal that they're going after is something we call the "Global" 21-centimeter signal, or the sky average of the 21 centimeters signal, which is what you're seeing in the bottom panel of this plot. So, the same series of events is expected to play out here, but now rather than occurring with interesting spatial structure as well, all that spatial structure has washed out, we're just seeing a single line that's telling us about the average properties of hydrogen as a function of time. And you can see because the 21-centimeter line, and the rest frame of the gas comes out at 21 centimeters, but because the universe is expanding, those wavelengths get stretched with time. And so, we actually observe the signal at very low radio frequencies which you're seeing on the x axis here, between 40 and 200 megahertz or so.

Dr. Jordan Mirocha 25:55 [Slide 28] So if we advance to the next slide, we come to March of last year when the EDGES collaboration reported detection of a signal at a frequency of 78 megahertz. Now, this is roughly in the range we expected for the global 21 centimeters signal, but its amplitude was twice as strong as expected. And I'm not going to talk about this in too much detail. But this has led to a flurry of very interesting ideas about the nature of or perhaps some modification to our understanding of the cosmic microwave background.

Dr. Jordan Mirocha 26:29 [Slide 29] But if we just set aside the issue of the signal's amplitude for a moment, we can advance to the next slide, and just point out a few simple features of the signal that tell us maybe about what's happening with star formation in the early universe. So, as I said, photons with this very particular wavelength can give rise to excitation and de-excitation of hydrogen atoms, which causes spin flips. So, it turns out that this first feature of the signal on the left requires a very strong radiation background at this frequency. And so, this gives us a very rough constraint on when the first stars must have formed. Because we need them to be forming in sufficient numbers to generate a radiation background with photons at this wavelength. And similarly, on the right-hand side of the signal we need - in order to produce a signal like this, the hydrogen gas has to be heated. And we think generally, that this is done by X-ray sources like black holes, which are perhaps the remnants of the first stars. And so, the time at which this feature occurs tells us something about when the first black holes are formed. Universe of Learning: Science Briefing 12/5/19 – Chemistry of the Early Universe - 9 -

Dr. Jordan Mirocha 27:39 [Slide 30] So if you advance to the next slide, I'll skip through this very quickly. We're just starting to do this work now. It's trying to whittle away the possibilities of how the first stars are forming in the early universe using this EDGES signal as a guide. And so, I'm not going to go into any detail on this plot. But this is basically a slew of predictions for how much star formation should be happening as a function of time in the early universe, And the punchline here is our models are wildly unconstrained, and we're just trying to use this new information to whittle down the possibilities here, and come to a better understanding of how star formation in the first clouds works.

Dr. Jordan Mirocha 28:12 [Slide 31] So with that, we can advance to the final slide. And I just like to say there are a lot of puzzles remaining with regards to the EDGES signal, and a lot of work still to be done in trying to understand it. And then new measurements coming out of the multi element experiments like MWA [Murchison Widefield Array], HERA [Hydrogen Epoch of Reionisation Array], and LOFAR [Low-Frequency Array]. So, it's an exciting time to be thinking about these things. And I guess we just all need to stay tuned for what may happen next. Thank you for your attention.

Dr. Quyen Hart 28:40 Thank you, Jordan. So, we have a little time now for some questions. Feel free to unmute yourself, say your name and ask your question. Just remember to mute yourself again when you're done. For WebEx, you can click the microphone icon. Red means you're muted and grey means you're not. For those of you on the phone press *6 to mute and unmute. You can also type in the chat and we'll try to get to your questions.

Adrienne Provenzano 29:06 Hi, this Adrienne Provenzano, I'm a ambassador. And I was wondering if you could talk about the size of the teams that are working on this kind of research?

Dr. Jordan Mirocha 29:15 [Slide 25] Sure, I can address that one first. So, you can scroll back actually to an earlier slide, to the experiments. So, in general, the global signal experiments are very small teams. To give you a sense of scale here. EDGES is about the size of a picnic table. And PRIZM and SARAS are very similar, and each of these experiments has only about a handful of people working on them, at any given time. The interferometric experiments like MWA, LOFAR, and HERA, are bigger collaboration, but by the standards of I'd say physicists or cosmologists, these are still relatively small teams. We're talking about, you know, 10s of people, maybe 60, or 70, all in all. Universe of Learning: Science Briefing 12/5/19 – Chemistry of the Early Universe - 10 - That's always been the case of HERA and MWA. The LOFAR, I think is quite a bit bigger. But certainly, nowhere near the scale of the LHC or something like that, or LIGO.

Adrienne Provenzano 29:24 Thank you. And are these international teams?

Dr. Jordan Mirocha 29:58 They are, yes. All of these experiments, have people working on them from a variety of places.

Dr. Quyen Hart 30:29 We have a question in the chat window. Why are the stars larger in the early universe?

Dr. Jordan Mirocha 30:34 Yeah, so this gets back to this argument about cooling in the first clouds after the Big Bang. So basically, the process of star formation is really a battle between gravity of the cloud that's trying to crush it inward and pressure of the cloud that pushes back. And so, if a cloud can't cool, very efficiently, there's lots of pressure resisting the inward pull of gravity. The reason we think the first stars were more massive than usual in the early universe is simply because the clouds from which they formed, had to have been massive enough to overcome a substantial amount of pressure within the cloud. And so, this is born out to some degree in numerical simulations that try to track this in more detail. And what they find is basically that when you have a cloud that's just made of hydrogen and some helium, as it collapses, it doesn't fragment into many pieces, like clouds do today. And so instead of getting lots of little fragments, you usually get just one or a few, much larger fragments that eventually then become stars. Hopefully that clears it up a little bit.

Dr. Quyen Hart 31:43 This is Quyen. I have a question for David. Are there other places where you would expect to find the helium hydride ion besides planetary nebulae?

Dr. David Neufeld 31:58 We've been thinking about other possible places we could look. I certainly think that in the present-day universe, it's the planetary nebulae that represents our best chance for detecting the helium hydride ion. And we're planning to look, again with SOFIA at a number of other planetary nebulae in the coming year to see if this is a widespread phenomenon, that we can detect this molecule. Universe of Learning: Science Briefing 12/5/19 – Chemistry of the Early Universe - 11 -

Dr. Quyen Hart 32:34 Great, thank you. We have another question in the chat window that I'll read: about how long after the Big Bang is it speculated that these first massive stars formed?

Dr. Jordan Mirocha 32:44 That's an interesting question. So, we don't know for sure. In simulations, this - and I'm maybe not up to date on this- this happens anywhere from 50 million years to 100 or 150 million years after the Big Bang. So, if the EDGES signal is real and the measurement is correct, it tells us that there's star formation happening at least as early as about 170 million years after the Big Bang. But unfortunately, the signal can't tell us precisely when the first stars formed. It really just tells us when there were enough of them to have generated a strong enough ultraviolet radiation background to trigger the spin flip signal. And so, it's really hard to pin down precisely when they form empirically anyways, but I'd have to check in with some more theorists to get the most up to date figure for when we expect them to be forming.

Dr. Quyen Hart 33:42 Okay, with a little bit of our remaining time here, I'd like to highlight some of our resources from various locations, including NASA's Universe of Learning, that you might find helpful in either using with your audiences or developing new materials directly or in further developing your own understanding of these topics. I just saw another question in the chat and we'll get back to that at the end of these slides here.

Dr. Quyen Hart 34:08 [Slide 32] So on slide 32, here, is our first set of resources, which is a collection of a lot of background information for you. In the two images on the left, we have hyperlinks that connect you to resources to learn about the Big Bang, and the conditions of the early universe. Specifically, you can learn about how the light from the first stars reheated the surrounding neutral hydrogen gas. The top center and far right resources connect you to information on the radio part of the electromagnetic spectrum, how large radio telescopes work, and some basic physics of 21-centimeter radiation, and how that works for neutral hydrogen atoms. At the bottom center resource is a SOFIA factsheet on the helium hydride ion discovery. This would make a great handout for visitors to your venue.

Dr. Quyen Hart 34:58 [Slide 33] If you can move to slide 33, next slide. Thank you. Here we have links to various articles for some more background research, so that you can delve deeper into the science here. The one on the left is an NPR news piece on SOFIA's unique capabilities. The link on the right provides an acceptable review of the predictive properties of the first stars in the universe, Universe of Learning: Science Briefing 12/5/19 – Chemistry of the Early Universe - 12 - and how they differ from stars in the current universe. This resource includes some great graphics related to these topics as well.

Dr. Quyen Hart 35:33 [Slide 34] Next slide. And on slide 34, here we have two articles from Scientific American, which discuss the dark ages of the early universe and the EDGES radio result that you just heard a little while ago.

Dr. Quyen Hart 35:48 [Slide 35] Moving on to slide 35. Here we've curated a collection of activities that are relevant to today's science briefing content. We have two low-cost, interactive activities for participants to visualize the expansion of the universe. The activity on the left uses balloons, sticky dots, and markers to visualize that expansion. The resource on the right has a few activities in it, including ones which use rubber bands and images that you can compare to each other.

Dr. Quyen Hart 36:17 [Slide 36] Moving to slide 36, there's a kinesthetic activity to act out the Big Bang and expansion and the of hydrogen/helium. That's the upper left-hand side there. All of these activities are linked in the PowerPoint. We also have a resource guide that you can access which also has the links as well as the internet addresses for this content. The collection of "Elements and You" on the lower left-hand side and "What is your Cosmic Connection to Elements?" activity in the middle, those activities center on the origins of the elements in the universe. The activities introduce the participants to the concepts of atoms elements, the periodic table, and elemental abundances in the universe using a variety of materials such as colored beads or beans or clay. So great hands on activities in those resources. On the far right, this is a collection of activities from the SOFIA Science Center, which contains activities on infrared light, including the role of infrared in astronomy, using filters, and understanding infrared detectors like photo cells.

Dr. Quyen Hart 37:38 [Slide 37] Moving on to slide 37. We've also compiled several videos and an animation resource related to today's science content. On this slide, we have two short videos between a minute and a half to two and a half minutes long, that are on the SOFIA detection of the first molecule.

Dr. Quyen Hart 37:58 [Slide 38] Onto the next slide, slide 38. The first two videos on the left here. One is from the National Science Foundation, which describes the early universe, the first stars, and the low frequency radio observations discussed in today's science briefing. The video in the center Universe of Learning: Science Briefing 12/5/19 – Chemistry of the Early Universe - 13 - talked about the early universe as well and the emergence of light from the cosmic microwave background radiation. And then the visualization on the far right displays a simulated piece of the universe with the first stars as they reheat the surrounding gas around them, ending the period of the dark ages in the universe. We hope that you find these resources helpful as you plan your activities and talks and professional development with your staff. So, we still have some time for questions. So, feel free again, to unmute yourself, say your name and ask your questions. And just remember to remute you yourself when you're done.

Unknown Speaker 38:52 The question: is the main ingredient in determining when things change in the early universe the temperature?

Dr. David Neufeld 39:00 Yes, so I mean, I think that's a good way of thinking about it. As time goes on, I mean two things are changing very rapidly: one is the of the universe. As the universe expands, of course, the density gets smaller, and intimately related to that is the fact of the temperature reduces, and starts to drop. And this may be familiar when you let air out of a tire, as the density of the air decreases, it gets cooler or conversely, if you compress air with a bicycle pump it gets hotter. But it is the temperature really that is the key determinant of whether you will have ionized gas or atomic gas or molecular gas because, at above a certain temperature, molecules can no longer exists. They get broken into atoms. And above a higher temperature, atoms can no longer exist, they get broken up into nuclei and electrons.

Dr. Quyen Hart 40:11 Alright. Let's have one more, huge thank you to Dr. Neufeld and Dr. Mirocha for spending their time with us today. A final thank you to all of our listeners out there. Remember that all of our talks are recorded and posted on the member websites and you're encouraged to share this presentation as professional development with your colleagues, including your education staff and your museum docents. If you have further questions about this topic, either now or in the future, you can reach us via the Museum Alliance team chat, but the email's fine as well. Our next Universe of Learning telecon will be at the January 2020 meeting of the American Astronomical Society meeting, so stay tuned for that and we hope to see you online, and as always, the most up to date information of our science briefings and resources is on the websites, have a wonderful weekend and we hope to hear from you.

Transcribed by https://otter.ai and Jeff Nee