Why compact sources are better

36:57>> So imagine that you were at a football field >> Okay. >> and there's a gigantic floodlight that's shining down onto the onto the field. >> Yes. >> It's this huge lamp, basically, right? >> And someone held up a magnifying glass in front of that lamp. >> Mhm. >> Or in in front of that light. >> Okay. >> You probably wouldn't be able to tell that the magnifying glass is there >> Mhm. >> because the the this lamp is so enormous, right? So like you have this huge source and you put some little you know, lens in front of it, but it doesn't really change what you see when you look up at this giant floodlight, right? >> Right. >> So But if you do that same thing with a flashlight

37:38So if someone is shining a flashlight and you put the same little magnifying glass in front of the flashlight >> Yeah, yeah, yeah. >> Okay, then, you know, you're going to tell there's something there, right? >> Yes. >> Because all of a sudden this flashlight gets a thousand times brighter, right? >> And so it's actually really helpful to look at lenses where you have a really compact source that's being lensed and not like a whole galaxy that becomes lensed into an arc. >> Oh, okay. >> want to find lenses that have a really tiny source because then they're really sensitive to really tiny, you know, lensing perturbations >> from these smaller dark matter clumps. So actually, this is an interesting note. So when we say strong gravitational lensing, it doesn't necessarily mean visually when we look

38:19at these images, big arc. It just means the effect is strong, but we actually want tighter um from the against from the talking about it visually, we want a a tighter, more concentrated lens because we we are going to be more able to detect these sort of smaller perturbations from dark matter than we would if it was this giant and it's the it's the perfectly like the analogy you just said, you wouldn't be able to see the magnifying glass if it was a floodlight, but you would if it was a flashlight. And so we're looking for flashlights. >> Yeah, we're looking for flashlights. That's right. And there's actually, you know, nature provides flashlights for us. And sometimes they get strongly

38:59lens, which by the way, maybe we should have said this earlier. So it distinguishes strong lensing from weak lensing or some other type of lensing is the appearance of multiple images. >> Okay. >> You like you had multiple, you know, two fish, right? So that was a strong lensing by a fish tank. So >> That's a good Okay. >> We want We want to find strong lenses. This is the This is the the dream scenario for dark matter, right? You have a bunch of strong lenses where you have multiple images of a point-like background source. >> And so the in the in the fish tank example, when you say multiple images, we're talking about how when we're looking at the corner of the fish tank, we saw the fish twice. Is what part of what you're saying that

39:39the multiple images can also be more than two? >> Sometimes, yeah, they can be four. And