Seed Question: What will happen when a ray of light passes near a massive object? Sketch it!

Exploration: Tape the Einstein Ring image into your PJ.

Identify the distant galaxy and the lensing galaxy.

Sketch a side view of the set-up showing the distant galaxy, lensing cluster and us, in our own galaxy.

According to Fermat, what is special about the path takes?

Gravitational Lensing and Dark Matter

Us Lensing Galaxy Distant Galaxy

Tape galaxy cluster Abell 1689 into your PJ.

Abell Galaxy Cluster 1689 is 2.2 billion ly away. Yellow galaxies belong to the cluster itself. Red and blue streaks are the background galaxies that are being gravitationally lensed.

One of these very faint background galaxies (A1689Z-zD1) is 13 billion ly away. Since the universe is only 13.7 billion years old, light left this galaxy when the universe was only 700 million years old!

ESA/Hubble [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)]

What else can we learn from lensing events like this?

This is the most massive cluster of galaxies we have found. It contains over 100,000 globular clusters. Yet the visible mass (stars, etc.) only account for 1% of the mass that must be there. 99% of the mass is inferred to be in the form of mysterious Dark Matter represented by the purple haze.

Big Idea: Gravitational Lensing involves the bending of light and brings together the ideas of Fermat’s Principle of Least Time, Refraction and Einstein’s General Relativity.

Based on gravitational lensing, we think that most of the mass in the universe is in the form of Dark Matter.

Discussion: ?

By NASA, ESA, E. Jullo (JPL/LAM), P. Natarajan (Yale) and J-P. Kneib (LAM). - http://www.spacetelescope.org/images/heic1014a/, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=11561821