Gravity’s Lens

Einstein_Ring_from_HubbleIn 1919, the great British physicist Arthur Eddington led an expedition to the island of Principe off the west coast of Africa.  His mission?  To test an astonishing prediction made by the physicist Albert Einstein, a prediction that massive objects warped the very fabric of space through which they traveled.

Eddington measured the position of stars just adjacent to the Sun, a feat made possible by the solar eclipse expected in Principe on May 29, 1919.  If Einstein was correct, the measured position of the stars should deflect slightly from their expected position as starlight followed a path through the slightly curved space near the Sun.  Eddington made the measurements, and a year later, after careful consideration, declared the position of the stars shifted by exactly the amount predicted by Einstein’s theory.  The news spread quickly, and Albert Einstein became the most famous physicist in the world.

Gravity from a foreground object bends light from a more distant object.

Gravity from a foreground object bends light from a more distant object.

All well and nice you might say, that mass bends light.  But is it a useful idea? Quite useful, actually.  Einstein himself worked out the details of his theory and predicted in 1936 that if light from a distant object passes through curved space around a massive foreground object, then light from the more distant object appears to follow a curved path to form images of the background object, somewhat like light curves when it passes through a glass lens.  This effect where a large mass bends the path of background light is called gravitational lensing.  Once again, Einstein was ahead of this time. The predicted effect was not observed until 1979 when multiple images of a quasar, the active nucleus of a young galaxy, were detected around a massive foreground galaxy.

The image above shows roughly how gravitational lensing works.

Since then, gravitational lenses have been detected all over the sky. Depending on the alignment of the observer on Earth with a distant background object such as a galaxy and a massive foreground object, which is often a galaxy or cluster of galaxies, all sorts of distorted images can be observed: rings (see top of page), arcs, or even multiple images of the same background object.  In some cases, background objects are made brighter because otherwise diverging light of the more distant object is focused into our line of sight by the gravity of the foreground object.  So gravitational lensing sometimes enables us to see faint objects we might not otherwise see.

The image below shows an example of gravitational lensing of extremely distant background galaxies by the massive foreground galaxy cluster Abell 2218.  The foreground cluster itself is some 2 billion light years away.

abell2218

Distorted images of background galaxies caused by gravitational lensing by the foreground galaxy cluster Abell 2218.  (credit: NASA)

The image below from the Hubble Space Telescope shows another example. Here you can see multiple images of a quasar and another background galaxy lensed by a cluster of foreground galaxies.  Each image looks a little different because light from each section of the background object travels a slightly different path through the curved space around the foreground galaxy cluster.  Astronomers can tell if multiple images came from the same background object by examining the spectrum, a type of optical “fingerprint”, from each image.  If the spectrum of each image is identical, they likely came from the same background galaxy.

Multiple images from a gravitational lens of a distant quasar and galaxy (credit: NASA)

Multiple images from a gravitational lens of a distant quasar and galaxy (credit: NASA)

A word to backyard stargazers: because gravitational lensing is most obvious with very distant objects, objects which are apparently faint, there are none bright enough to see visually in a small telescope.  This sort of work is for big scopes only.

Astronomers can do more than take pretty pictures of gravitational lenses. Using the ideas developed by Einstein and others, if the distance to the foreground and background objects can be determined using Hubble’s Law, and if the degree of deflection by the “lens” is measured, then astronomers can calculate the mass of the foreground object.  This is, of course, simply amazing.   That we can determine the mass of, for example, a cluster of hundreds of galaxies billions of light years away by measuring some light with a telescope and applying some inspired mathematics.  And yet science enables such things.

After measuring gravitational lensing by dozens of galaxy clusters, astronomers made an unexpected discovery.  The masses of galaxy clusters are much larger than can be explained by the visible light in the clusters.   There are simply not enough stars in these galaxies to account for the amount of gravitational lensing observed.  The conclusion?  Most of the mass of these clusters consists of matter which emits no light, a type of “dark matter”.

This was the same conclusion made by astronomers studying the rotation of relatively nearby galaxies.  These galaxies, it seems, are surrounded by large clouds of dark material.

In the next article in this series on basic cosmology, we look into the nature of this dark matter…