Gravitational Lensing and Dark Matter - Meghan Gray - 11/6/12

posted 11.06.12 at 5:46pm

Today our guest is Meghan Gray and we're talking about Gravitational Lensing and Dark Matter. The universe is a pretty large place. Astronomers believe that the observable universe is a sphere, which measures 92 billion light years across. A light year is the distance light can travel in a year, which is roughly 6 trillion miles.

It is gravity, which governs the structure of the universe. Stars form into groups which orbit around a large mass in the middle – a bit like our atom. Our home galaxy, the Milky Way, is 100,000 light years in diameter.

Believed to be within all galaxies is a halo of a substance known as dark matter. This substance is incredibly dense and accounts for 80% of the matter in a galaxy.

Gravitational LensingGravitational Lensing However we have no idea where it came from or indeed what it looks like, as it doesn’t interact in ways like normal matter does, however we can passively detect dark from the effects it has on light and especially gravity.

A gravitational lens refers to a distribution of matter (such as a cluster of galaxies) between a distant source (a background galaxy) and an observer, that is capable of bending or lensing the light from the source, as it travels towards the observer.

The picture on the right is actually the STAGES Einstein Ring found by Prof. Alfonso Aragon-Salamanca and Meghan. Check more out about STAGES here:

Rotation curve of a typical spiral galaxyRotation curve of a typical spiral galaxy You might think that astronomers are making things up when they talk about Dark matter, if we can't see it, can't measure it, how do we really know it's there?

Well it's all down to the rotational speeds of stars around galactic centres. We observe stars orbiting at speeds far above that which we predict (see the handy graph on the left and click for a larger version) you need extra mass to account for these speeds and Dark Matter is the answer. The predicted speed follows line A, and the observed speed follows line B, there's quite a difference!

Gravitational Lensing

Galaxies aren't the largest thing out there, galaxy clusters far out weigh them. Well they should since they contain galaxies! Galaxy Clusters can be used to infer Dark Matter as well, in much the same way we can tell it with individual galaxies.

The Bullet Cluster below on the right is a great example of two colliding galaxies that can help show dark matter and other galactic constituents. Click the link for a trip to APOD, or Astronomy Picture of the Day, it's full of lots of pretty images.The Matter of the Bullet ClusterThe Matter of the Bullet Cluster

This image shows two clouds of hot x-ray emitting gas shown in red. Representing even more mass than the optical galaxies and x-ray gas combined, the blue hues show the distribution of dark matter in the cluster.

There are three classes of gravitational lensing:
1. Strong lensing,
2. Weak lensing
3. Microlensing

Strong lensing leads to the most obvious images, you may have seen them before. The first picture in this blog is an example of strong gravitational lensing, it can even lead to complete rings of light around an object, or a Einstein Ring.

Weak lensing isn't so obvious, it doesn't really lead to rings but is normally only noticeable within the statistics of galaxies. However it povides a way to measure the masses of astronomical objects.

Microlensing allows the study of objects that emit little or no light.

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