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Dark Matter & Gravitational Lensing

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  • 0:03 How Communities are…
  • 0:45 Gravity, Dark Matter,…
  • 1:57 Gravitational Lensing
  • 4:25 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will describe what dark matter is, how much of a cluster of galaxies is made of it, where it's located, and how all of this is related to gravitational lensing.

How Communities Are Held Together

Communities are held together by so many different things. Taxes help to finance projects, volunteer work helps the needy, police ensure safety, and so forth. Most of what goes on in a community is visible to at least someone. There's very little truly invisible work that goes into running a community.

Galactic communities, that is to say, clusters of galaxies, are a bit different. They are held together thanks in large part to something invisible. It's called dark matter for a reason and the way we know it's there, thanks in part to gravitational lensing, will be the subject of this discussion.

Gravity, Dark Matter, & Clusters

A cluster of galaxies has to be held together by gravity. Meaning, there has to be a large amount of matter in a cluster to prevent the galaxies from floating away from one another. The problem is that the visually-luminous matter in a cluster, which namely includes the stars of each galaxy, is way too small to glue the galaxies together gravitationally. Thus, there has to be lots of nonluminous matter, called dark matter, which is responsible for ensuring that galaxies remain in orbit around the center of their cluster.

For instance, astronomers have analyzed that for a standard rich galaxy cluster, a cluster that contains over a thousand galaxies, the total mass that's necessary to bind the cluster is roughly 10 times greater than the visually-luminous matter within it. But if this dark matter is invisible to us directly, how do we know it's there? There's actually more than one way to infer the existence of and location of dark matter, but we'll focus on gravitational lensing for this lesson.

Gravitational Lensing

Gravitational lensing is a process where a powerful source of gravity (a gravitational field) stemming from a massive object distorts the light from a distant object in such a way as to produce multiple images of the distant object or make the distant object appear brighter than it actually is. The source of gravity that actually does this is known as a gravitational lens.

Let's look at an example to help us understand this. Something like our sun can bend or deflect light rays by a few arcseconds. Logically, a more massive celestial object, like a galaxy, can obviously deflect light even more. The amount of light deflection can then be used to determine a galaxy's mass.

Take, for instance, a couple of magnets. Let's say you couldn't see the magnets as they are hidden from your view. However, you are allowed to pass a thin sheet of bendable metal by each one. If that piece of metal bends or deflects more towards one magnet than another, then you could reasonably assume that the larger magnet is the one responsible for the larger deflection. In space, it's light, not metal, that's deflected.

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