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>> Hello, everybody. Welcome to After Dark, dark matter. I am a member of the programming team here at the Exploratorium. Is this anyone's first time at After Dark? Great. More than usual. Welcome. Do we have any After Dark members in the audience? Welcome to our members as well. For those of you on your first visit, we have After Darks every week and most weeks have special programming like this, so if you like what you saw, you can trade into night's ticket for your membership and it pays off after two visits. Before we begin, I want to of knowledge the land we are on. We are on unseated territory. We recently were able to develop a land acknowledgment with leaders Jonathan Cordova, Greg Castro. And in that land acknowledgment, they talk a lot about this landscape as well as land acknowledgments overall and their value. A Takeaway I appreciate from that is the need to observe the land you are on, and to think about the history of that land. To think about how the people before you treated it, and how the people currently treat it. And to think about the length of time, which relates a bit to what we are looking at tonight, and how to make sure that landscapes continue to exist in sustainable ways. And also, I do want to of knowledge that we have a thriving community here in the Bay Area and celebrate the leadership they bring to these conversations. For tonight's program, we're excited to be part of the Bay Area science Festival, which is an annual event that happens. It is in its 11th iteration. It is a celebration of science and technology organized by OSCC science and health partnerships. You can find more information on Bay Area science Festival.org or we have some brochures in the back. We are at the tail end of the festival, but they have some great event this weekend. They do two really wonderful every day events. There is one in the East Bay this weekend. And now, I am pleased to introduce your really fantastic speaker, Dr. Maria Elena Monzani . I really like the title of this talk, and also, I have seen a talk before. You're in for a treat. As someone who does not have expertise in physics, I found it to be a real breakthrough in understanding dark matter. Dr. Maria Elena Monzani is a lead scientist at the slack National accelerator laboratory. She leads the software computing effort for the LZ dark matter experiment and science operation teams for the satellite. She received her PhD from the University of Milan and University of Paris seven. Here is Dr. Maria Elena Monzani. [Applause] 

Dr. Monzani: Hi, everyone. It is great to be here. Thank you for the kind introduction. Tonight we are going to talk about dark matter, or everything you are about to see is about dark matter. I would like to say a few words about myself. I grew up in a town that looked like this. We had mountains, we had a very pretty river. My hometown is not very well known for physics, but you may recognize this. It is famous for water. So, when I was growing up, the boat as -- people asked me how do you get to physics. I barely remember because I was three years old, but the story is when I was growing up, the Voyager got to Jupiter, two fantastic photos. We had two Italian TV channels, and they were showing commentary about Jupiter. That is how I got into physics. So my parents got me a book. One of the pages of the book had these drawings of the solar system, except for here you have photos, and that had drawings, because it had been printed before the Voyager photos. So I spent my days staring at this type of picture. Then in 1984, I was in elementary school, something very dramatic happened in Europe, which was the explosion of the Chernobyl reactor. The cloud went all over Europe, went to Italy. It was kind of scary. We cannot play outside for a few months. So I picked back up my physics book and thought, OK, I want to see what the atms look like. And what they found was something like this. This is a drawing, an illustration of the carbon atom. Can see a kind of looks like the one above. This is on purpose. This is a way of representing the Adams -- atoms. When I saw this it really blew my mind. It was like the most intense experience are a member from my childhood. That the smallest thing I knew, the atom, and the largest thing I knew, the solar system, looked the same. This is a good reason why gravity and electromagnetism are central forces, so there is a different reason why they look the same. But at 10 years old, that was good enough. I am not going to tell you my whole life. This is 20 years later. This is my first post of my advisor from Columbia University, and this is my colleague. Here behind me, is my first dark matter detector, which is about this big. Basically I grew up doing what I wanted to do as a child, which is study things that are extremely small as a way to learn the universe, or the other way around, study properties of the universe to learn things that are extremely small, like particle physics. I am going to explain for the next 30 minutes what that means. So, fair question, what is dark matter and how do we know it exists? I am going to start from the second part and give you reasons why we think the universe is full of dark matter. So, spiral galaxies have a secret. Take five steps back, one step back, and talk about the solar system. This is a chart that shows the speed, how things faster going around the sun as a function of distance. The earth is one year by definition. One orbit of Mercury is more like three on -- three earth months. Neptune is more like 160 earth years. If you have a central system arranged by gravity, all the masses are in the center and things go slower you go out. So we can ask if we can make the same graph about the stars of the Milky Way. We can. We use the Doppler effect. Red shift and blueshift. If you have ever heard an ambulance, that is the same concept, how the sound changes coming toward you are going away. That is one way they measure the Doppler shift, bouncing back and forth from your car, not unlike radar signal. Anyway, they made the measurement. This is one of the original galaxies is the triangle. This is a photo of the galaxy superimposed to the curve. This is the mathematical curve that you expect to draw out the density you see in this photo. Most of the stuff is in here. Velocity is going to graph, and that it is supposed to go down because there is nothing out here. They made the measurement in the early 1970's and what they saw is the velocity not only did not go down, it went way up. So this thing be more like a solid object, a CD, or a disk on a record player when you went out. But like, way out you don't see anything. Not just a CD player, it goes faster when you go out, which is very bizarre. Sorry. If you did mechanics in college, you can apply here to this plot. If you don't know what that is, it does not matter. It is a factor of three, basically, between what you expect here and what you see. Three to the second is nine. We're missing 90% of the matter. This is what this plot shows you. The person who made the measurement, she died a few years ago, she was a grad student at the time. So, she's vastly credited with the first systematic measurement of dark matter. So, you could travel way out of the galaxy, I just told you, 90% of the stuff that is there is not a star. It does not shine, it does not make shade, it does not absorb light, you don't see it anywhere in this drawing. 90% isn't visible, and it goes out way farther. So if we could see the dark matter from the -- sorry, the Milky Way from the outside with dark sensitive light, would see something like a cuckoo form. If we consume out a little, moved from the doubts he a cluster of galaxies. I am going to skip that because it takes a couple minutes to explain. I am not going to explain this to you, because there is an exhibit in gallery three or four about these tings. But it's a galaxy cluster way back. Another galaxy right behind the galaxy cluster, and this guy is acting as a lens. You see two of them. And the lens of the galaxy behind. You don't understand what I just said because I went too fast. This is what really happens. You will have a very dense object such as a galaxy cluster. Light that's bent by the galaxy, so it's not the straight one, it goes in the Valley. If you have a foreground object it will be magnified and deformed by the supermassive object in between. So that is why this galaxy looks this way. It was supposed to be behind this massive cluster. We should not even see it, but we do because of the lens effect. So this photo, you take the measurement, you take the angles, you figure out the distance of the objects, and again, can see that this central cluster is lacking 90% of the mass if you measure what comes out of the light. Now we consume all the way out of the whole universe. How much dark matter is there in the whole universe? So, the entire history of the universe, the universe blew up, we call that the big bang, it was supposed to be making fun of the guy who came up with the theory. Big Bang was supposed to be insulting. Anyway, it was a rapid expansion, then now we are here. Something cool happened here about 400,000 years from the beginning, which is the universe cooled down enough that protons started capturing electrons and this created neutron atoms for the first time which meant light could escape from this superhot radiation and particles. So light could escape and sure enough, that light is still around. So this light which is almost 14 billion years old, if you look behind the scars -- stars, behind the galaxy, all the way in the back and you measure the temperature, you are going to see three degrees above absolute zero, which is -- it doesn't matter. And this is the light left over from the Big Bang. This is like a cloud. By definition it is the oldest light, the big. You can represent this light in different ways. In color coordinates. It is like we are rolling the universe up and looking at it. These dots represent minuscule temperature variations. These temperature variations are like two parts per 100,000. So it's very small. This is all the light in the universe. It was emitted 400,000 years after the Big Bang. This specific map was observed by the Planck telescope. Now you can represent this object in a type of -- you don't need to remember that word. It does not matter. But from starting a big the density fluctuations are and how they are spread out inm  -- in the space of the universe, you can tell a lot from it. And you can tell the density of the universe. That is super important because it tells us if the universe is going to expand forever, or eventually come back into a big crunch. So when I was starting grad school, which was 20 years ago -- although I am only 29, although that is a different story. We thought that was the only big question left in physics. There is one number, the density of the universe, we need to measure. We did not even know the organ of magnitude. It could be anywhere from four to 1000. The measurement came out would like 98% precision. Oh, and by the way, that was the last number. We were kidding. The matter that we know is 5% of the total energy matter density of the universe. 95% of the universe is unknown. First time we made the measurement was 96%. This was outside a newspaper stand in England when the predecessor of Planck. So this was a very cool thing to happen in my career, because it was deciding whether to go to grad school and I was told physics is almost over. Then they measured the number and figured out, oh no, we still literally don't know 96% of physics. It is very good to start a career like that. I call that job security. I am going to make that joke a couple times tonight. I told you how we know it is dark matter. I have not told you what it is. Spoiler alert, I don't know. But I can start with telling you what it is not. So, this is a chart of all the elementary particles that we know. It is called the standard monocle -- the standard -- It has six bows and. These are all the particles we have discovered. Cosmic rays, accelerators, collider's, all of them. We can compare the chart with the properties of dark matter that we know. First thing, it's gravitationally interacting because we see its effects on the scale of the universe because the galaxy together come etc.. It cannot be something that the case and a fraction of a second because otherwise it does not hold anything together. We get rid of two thirds of non-particles that way. Then it cannot be hot or light or slippery. It cannot be a neutrino. Neutrinos go through the rings all the time. They cannot pull things together. If I stick my finger out, my fingertip has 100 billion neutrinos going through it every second. Same with you. It is completely slippery, does not do anything. So neutrinos cannot hold the galaxy together. Then it does not emit or absorb light, so we have three particles left. Those things emit or absorb light, so, cannot do that. Very cool. I just got rid of all the known elementary particles. On ambiguous evidence for new particles. There is something we have not seen yet. Again, job security. There is a new particle or class of particles we need to look for. So what do we know about it? There is a lot of it. This is the iceberg metaphor. This is like dark matter, five times more dark matter, eight times, depending on how you count. When can you make a lot of anything? The Big Bang basically had infinite energy, so you could create a lot of anything from the Big Bang, out of an infinite amount of stuff. So if you wanted to move the big hang for a few seconds, you can see dark matter interact with regular matter and radiation. That gives an idea. Very cool. Can we make it into a friendly ghost and interact with dark matter again? So there are three ways to look for dark matter. One is you can smash particles of regular matter onto each other. It will create a micro Big Bang. Micro Big Bang, can create anything, particles, one can be a particle of dark matter. So this is happening at the hydronic collider. This is one of the detectors. So they are trying to make in a lab, basically, by collision. Spoiler alert, they have not found it. Secondly you can do the opposite, you can look for a dense space where maybe particles of dark matter collide and produce particles of regular medical -- regular matter or radiation or life. These can be found with telescopes on the ground. This is an example of a magic telescope. Or from space. This is the telescope I work on. Spoiler alert, we have not found it this way, although there are some controversial or specifically dubious evidence there might be something in the center of the Gav -- of the gravity potentially with dark matter. So we don't really know what it is, but they only hint that we have so far of like, seen something like dark matter comes from this guy. Finally, you can do this. You can make -- you can take a big target of regular matter, and like a billiard ball experiment. Wait for a dark matter particle to you. It mostly goes through you but maybe one particle will interact with your detector. I told you about the neutrinos earlier, how many go through us in a given second. In your life, maybe you would have one interaction from a neutrino in your body in your entire life. So, very, verfy rarely do these things interact. So this is an example of this type of detector that looks for dark matter. We call it the billiard ball experiment. I'm going to focus on this specific technique for the rest of the talk, because I have a lot of pretty pictures of a detector. We call this -- we look at exchange of energy between dark matter and another nucleus, some other particle in our detector material. So, here the recipe. You build a massive tank or a massive tower of nuclei. You hide it deep underground. You wait for a dark matter particle to hit one of the nuclei in your tank or in your tower. And then you look for tiny vibrations from nuclei that have been hit by a particle of dark matter. Here, you should stop me and ask me a question. Why underground? Why would I hide it? I'm looking for something that comes from the galaxy, right? The atmosphere is very bright, even at night. The atmosphere is full of charged particles, or cosmic rays. Can anybody tell me what this is? Aurora borealis as seen from space. This is how bright the atmosphere is a lot of the time. It's not this bright allow the time, so we don't see the glowing, but if I am making an ultrasensitive particle detector, it is going to be blind. So we put our detectors under a mountain. This is an example. It will interact with the atmosphere, make a lot of other particles, the mountain will absorb most of them, and if you put your detector down here, -- regular matter is transparent. So my experiment, I am going to apologize for you now. It is called LZ. LZ is an acronym of acronym. It comes from the Luz-Zeppelin experiment. I am sorry, I can barely say it. This is the experiment. It is one mile underground in South Dakota. The lab is called SURF, and it's built in a gold mine in South Dakota. The mine used to belong to the Hearst family if you know who those guys are, etc. A mile under found. It is a lot of fun to go to Homestake. They started digging from underneath. If you have seen Western movies at a child and wondered where that is, that is South Dakota. Wild Bill, calamity Jane, etc. This is all gold from the mine. So how does the detector work? This is very cool. We have liquid xenon containing 10 tons of this stuff. The quit xenon is very dense -- liquid xenon is very dense. 10 tons of it is basically my size. The interesting thing is we convert the vibration of a particle hit by dark matter into light. So this is how it works. We made an animation with very rudimentary tools. Dark matter particle comes here, hits a nucleus of xenon, kicks it, and then we have some light -- sorry, went too fast -- when the community is kicked, all -- when the  xenon gets kicked -- What we do is we apply an electric field to the body of the xenon tank. Bring the electrons to the top. We have a thin layer of gas and then the xenon in the gas will get excited and make more light. Those two flashes of light, more particles. Where it interacted in the bottom of the detector. If it was a xenon, electron, neutron, etc. The cool thing is we detect using light. Which is kind of fun because we are looking for dark matter. So this is the detector I showed you earlier. When we were building it. Columbia University in 2006. And this is the defector now -- detector now. 1000  times bigger in mass. And I think 100,000 times more sensitive, something like that, in 13 years. I should've looked up that number before I told you. So how do you build a massive tank of nuclei? First, take pictures, because then you can share. You want to detect the light, so you want photo sensors. These are like, two inches across. We have a lot of them. A lot of them. Like, a lot of them. 500 of these guys. To detect the light on top and bottom. This is before assembly. Then you stack a tank, which is made of teflon. It's a freakishly reflective material. We don't exactly know why, but this thing in liquid xenon is reflected 99%. Does that mean any tune -- By the way, there's an exhibit in the hallway where you can see pieces of the Teflon. Anyways, highly reflective material. Then you start stacking them. Sorry, went back instead of forward. A little bit more. Little bit more. So then, I told you earlier, need to apply an electric field over the body of the object. Now, the object is five feet across. And you want it to be transparent because you want to see the light. Nobody has metal above one meter wide, so we needed bigger. So we built a loom. Our mechanical engineer went to a weaving club to figure out how I loom was built, we built our own. And we made 1.5 meters of stainless steel mesh we use as a n electron. This is what it looks like. .5 inches, approximately. Then you carry it. It's very flimsy. It is 100 microns thick, so very flimsy. You bring students and make them carry it very carefully. Send it with students. Then you put it across. This is the photomultiplier's where you see the light. You book your electrodes on top of the photomultiplier's. This is what looks in front of the light sensors. Then what you do -- what is the word -- you assemble everything together. This is a movie. Took two days but I will show it in 30 seconds. And you mate the light detector sensor array to the back I showed you earlier. Everything is clean, the people are clean. Taking pictures, taking video. Everything is packed, including the people, to protect from radioactivity. This is about one million times cleaner than anything else you would find in nature. Otherwise you would say radioactivity from your material and the dark matter. You guys know you are little radioactive, right? That is why carbon-14 works to see when people die. Anyways, that was it. We opened it. Taking pictures, very important to take pictures. Light sensors here, let sensors here. 1.5 meters of plastic back. Then we need to put the plastic thing in a double wall titanium things with vacuum in between. Because liquid xenon is only liquid at -150 Fahrenheit, in a very short range. So we want to keep it cool. So then you do a job of extreme -- this also took two days, we got stuck a couple times. You put everybody in a hazmat so they don't fall in. See, people taking pictures most of the time. And you know, we want to share it with you, so yeah. And you lower it. This is stuck again. OK, there we go. So, we wanted it to be very precise because the area between the plastic and the matter that's filled with xenon, it has very few light collectors around it. So all that volume is dead and you don't know what happened and it is important -- it is not really completely dead, but you want a tight fit. So ta-da, that is the finished seal. Then we rolled it to the side and brought it to the mine. This here is the elevator. But we don't use that word in mining, they would be mad I used it. It is a cage going down a shaft. Anyway, you roll it to the side, wrap it very neatly, then send it down one mile. This is the end of the shaft and these are the technicians unloading. I was asleep when this happened. I was getting surgery. That was a very good day to like, not be awake and not know what was happening. I had hip replacement. So, both the mine and me got titanium that day. This is the detector going through the drift, not the tunnel, very important. If you ask us how did you come up with the size, this is why. We could safely get it through and install. And this is it end of the day or next day, very happy, ready to be lowered in the water tank. I am going to say a few words about the water tank. This is what the water tank looked like. My friend eagerly awaiting for the other piece of the detector. This is what it is going to look like in its final configuration. It is going to be full of liquid xenon. We have neutron detectors around it and water around it. This is to shield from everything that comes from the rock of the mine. So you have to shield everything from everything. I'm sorry, I don't have the result of this guy, because we just started taking data. Maybe in a couple years I can come back and tell you that we found dark matter, or that we did not find dark matter. But you know, like I said earlier, no, I will say it in a minute. OK. I forgot that I put this in. Dark matter is extremely elusive. There are about three dark matter particles per liter on earth. My tiny water bottle would have one dark matter particle in any given time. Your hand will have about 10 at any given time. They go through us very fast, 150 miles per second. Then the volume I showed you all contains almost 10,000 particles at any given time and they go through very fast. So basically, we see one billion of those particles go through the detector every second. If we are lucky in nature cooperates, we are going to see the interaction. The knocked off neutrons will be one per year. So out of one billion per second. And that is if nature is our friend. So we're going to have a massive problem of sifting through a huge amount of data. It's an extreme needle in a haystack problem, and this is us right now, basically. We just started taking data, we're overwhelmed with the data. We are looking for day one, two, three, five particles of dark matter. I am not going to tell you how we do this, because we are way out of time. And we have a lot of data, about one perabyute of data per year. And we need supercomputers. This is the supercomputer, we just installed another one. We do calculations at the national liberatory -- laboratory, where they had a scientific computing center. I made up one of those words, I am sorry. We have a lot of supercomputers we used to analyze the data that comes out of the experiment. Again, a lot of data in there. Zooming out, this is not done by computers, it is done by humans who use the computers. The team who works on this is about 250 people. This is maybe half of us in South Dakota in the before times. We have not seen each other in person for a couple years. There are approximately 30 experiments around the world looking for dark matter. Like I said, nobody has found dark matter yet with this technique, which again, I call job security. This is a photo of the Milky Way here from New Zealand. New Zealand is very cool because you can see the center of the galaxy, things you cannot see from the northern hemisphere. It is around here in this cloud of gas, gas of the galaxy. Most of what you see in this photo is dark matter, except you don't see dark matter, which is frustrating and exciting at the same time. Dark matter is hiding in plain sight. Most of the matter in the universe is dark. Maybe we will find it this time. If you don't know what you want to do for work in 20 years, maybe we will still be looking for it in a few decades. So, it's an exciting thing to do. And thank you for listening to me. [Applause] 

>> Thank you so much. And I think we have time for a few questions. If you can just raise your hand, I will bring a mic over. 

>> Thank you so much for sharing with us. That was really incredible. One thing I am curious about is how you guys got the estimate for the properties of dark matter if we know so little about it. Dark matter particles are flying through, I'm curious how you get those estimates. 

Dr. Monzani: This is a phenomenally good question. I could talk about this all night but I will give a short answer. I skipped through it, but one of the things I skipped when I said we were rewinding the movie of the universe, there is a temperature that corresponds to the density of dark matter that we observe now, and that temperature is if you order of magnitudes within the weak interaction. So we assume that dark matter has weak interaction and we look for it the way we would look for a neutrino. So, that's our main basic assumption. My colleague will be here in one hour to talk about other ways to look for it, other types of particles. I think I have it. This is 10 years old. It's a aset of models of dark matter. We're looking for one of them, because it is like a drunk person looking for their light, they put their keys under the streetlight. That is where we can look, and we know how to. So that is the easy answer. But there is a motivation that comes from big bang when we are looking for the particle. 

>> I was curious what the failed experiments were that failed to detect dark matter. Were there any learnings from those experiments? 

Dr. Monzani: Absolutely. Whenever you look for a dark matter particle in a certain range, you look for a certain type of mass, and you look for a certain probability of interaction. So you probe a set of parameters what dark matter could look like. When you don't find it, you know dark matter is not this type, not this type, and you can delete a lot of models. Which is very disappointing, but you still learn something about the property. Am I answering your question? I am not sure I heard you right because of the mask. OK, great. One of the predecessor experiments which was called LUX, they had an article in the New York Times finding nothing better than everybody else, because they were able to eliminate a range of theoretical models of what dark matter could look like. 

>> So, between the Big Bang and 400,000 years after the Big Bang there was no light in the universe? 

Dr. Monzani: There was light, but it was confined, because all the matter was iron iced. -- ionized. It was basically superlight charged particles and light was bouncing around. It was a box of exchanged energy, but the energy cannot escape. The one I showed you, the Big Bang photo, is the oldest observable light. It's the same thing as seeing the bottom of the cloud. You still see the light from the sun, but you don't see the sun. That is basically what happens if you have an ionized gas. Is this what you asked? OK, good. I should learn to repeat the question before answer. 

>> Can you talk about the relationship between dark matter and Black holes? If there is a relationship. 

Dr. Monzani: This is something we have been thinking about for 40 years, because it goes in and out of fashion. A Black Hall is also something that does not emit light. It absorbs light, but you will not see the effect. And it adds mass to the galaxy, the universe. Basically in the 80's they did a lot of experiments looking for massive objects and the theory of a Black holes that we have is they contribute to the mass of the universe but they are a small component. With the discovery of blackhole mergers there are more massive black holes than we thought. 30, 50 massive black holes. But it is still a small fraction of what we think we need. Black holes don't come from dark matter, they come from regular matter, because they are left over from the death of a star. They are not made of dark matter. They could be a component of what we are seeing. We think there is not enough based on what we know today. It goes in and out of fashion. They were looking for this massive halo objects and the theory was -- It was machos versus wimps, and the wimps won in the 1980's, so everyone was happy. 

>> We have time for two more. 

>> Do we know if dark matter is a single type of particle or could it be many? 

Dr. Monzani: I showed you earlier the standard monocle is 13 particles. There is five times more dark matter, six times more. So there is no good reason it should be one particle and not a spectrum of particles. We tried to do these in a way recall model dependent we don't make too many assumptions on what we are looking for, how they interact, how many they are. It is possible there is a whole spectrum of particles with interactions we are not sensitive to. If we go down that way we are never going to detect it, which is exciting and discouraging at the same time. 

>> Last question. 

>> The motion of galaxies, is there a strong evidence for matter? 

Dr. Monzani: Yes, the Big Bang. Galaxy clusters, it is not just a single galaxy. How much time do I have? 

>> Five minutes or so. 

Dr. Monzani: OK, I can do this in five minutes. Ooh, this one. This is the bullet cluster. It is something that we see very clearly. The bullet cluster was formed from a collision of two independent clusters of galaxies. So we can measure it in two ways. The blue is microlensing, it shows the distribution of matter. And red shows the hot gas that travels in the cluster. So this is what happened when those two clusters collided. This is all the hot gas that collides, and you know, bullet structure. You see how the blue stuff, most of the stuff goes away? This is the apology we see today. The blue stuff is something that collided and did not really talk to each other. These guys talk to each other, these guys did not. See something on a large scale of some type of matter that does not talk to itself. So that is one way to see it. And then the other one is when I take the full of the Big Bang light, I can translate into specular harmonics, and from that I can cite how dense is the universe in total, how much regular matter versus total matter there is, how much dark matter there is, which is the opposite of dark matter, something accelerating the expansion of the universe. So we have evidence of different scales, which is what I personally find convincing. You see in a galaxy, a cluster of galaxies, whole universe. It is not like -- it's across scales and it is a fixed fraction of like, seven to one. I should know the fraction but I don't. 

>> Thank you so much. A big question to end on, but a round of applause. [Applause] And in just 15 minutes, I encourage you to come back to hear about the dark matter candidate, the Axion. So, going to go real in-depth on one of the candidates. And also, enjoy several other ways of looking at dark matter out in our gallery one.