The fish tank analogy

28:48first one to use this movie to discuss strong lensing. >> Okay. >> This is just a a movie made on Earth of a fish tank. Uh there's a goldfish in the fish tank and you'll see that as it approaches the corner of the tank the fish becomes doubly imaged. So, we see two images of the fish. >> But there are not two fish. >> But there's only one fish, right? So, what's happening is the light is being deflected by the glass. >> Mhm. >> In this case, the corner of of the fish tank in such a way that there are two paths through space that connect our eyes with the fish. So, you know, light takes two different paths. It gets bent by the by the glass in the fish tank and

29:29then it comes to us. >> Right. So, it's like light is bending. So, we're looking at the corner of a fish tank. This sort of left side as the fish comes across, the light's coming into my left eye from the left side. And but as it's coming, as it gets closer to that corner light is now traveling down the glass from the right side and coming to my right eye. And so, it's not It is the way in which the the line of sight we have currently is what is driving this because it just happens to be at the the right angle to see it coming from both points. >> Yeah. So, those are two images of the same fish. Both images are just as

30:11valid, right? I mean, it's not like one of them's a fake image, right? It's just you know, we see two images of this background source, which in this case is a fish. >> Mhm. >> Uh because the foreground deflector, which in this case is a fish tank, is is bending the light in different directions. >> I see. >> So >> I see. >> In cosmology, you you replace the fish with the galaxy and the fish tank with another galaxy and then and then you understand strong lensing. >> I see it. So, the fish is the background galaxy. >> That's right. >> And then the glass of the fish tank is the foreground galaxy and we are and so the we can look at the image of the background galaxy being warped to be able to define like the glass of

30:53the fish tank for like to be able to better understand like the structure of the glass of the fish tank. >> Yeah, totally. So you could imagine that you know even if you couldn't see the fish tank for some reason you could infer that there's something there >> I see where you're going with the two images of the fish. So in this analogy if the glass of the fish tank was dark matter which we couldn't see we would be able to infer that there was a piece of glass that was at a right angle right here because we're seeing two fish like that. >> Yes, unless we can even take it a step further, right? If this fish tank had a bunch of defects in it like dents and stuff those would be the clumps and they would affect the small scale structure of the fish, right? So if we

31:33saw a fish that had a bunch of small deformations in a in addition to being >> Yep. >> doubly imaged >> Yep. >> we might conclude that whatever lens is there is clumpy on some scale and we can >> That is great. Okay. >> That was that's very helpful and so you know the the concept here is gravitational lensing >> Yeah. >> and it is a methodology by which um it's something that happens because of the way optics works and the way light and gravity interact. It it it is we can see these lenses throughout all of our different sky surveys and etc. etc. And we can use the structure of the

32:15lens to do a whole bunch of science around it. >> Yeah, that's right. So you can do lots of different cool science with lensing not just dark matter. >> Okay. >> You can also use them to measure distances which are sensitive to the expansion rate of the universe. >> Mhm. >> For example, you can use lensing to study the stars >> Mhm. >> uh because you know, regular matter also contributes to this effect, so you can use it to study uh stars around different galaxies. For us, we're interested in using it to study the dark matter, and in particular to study these clumps. And I think we actually have another picture that would be helpful here. It's the uh Let's see. It is number eight.

32:56>> Number eight. >> No, I'm sorry. No, no. Number nine. Number nine. >> Number nine. Let's go to number nine. Yes. >> So, yeah, this is another uh cluster of galaxies