GRAVITATIONAL LENSES

Nature’s Long-Distance Telescopes

In 1979, astronomers using a telescope at the Kitt Peak National Observatory found something strange in one of their images. It looked like a pair of identical quasars sitting side by side and fairly close together. They were quickly dubbed the Twin QSOs. Quasars are bright, very distant point-source objects that put out prodigious amounts of light and are now known to be extremely active cores of distant galaxies. It seemed weird that there would be two so close together. So the astronomers looked at the Twin QSOs with radio telescopes to see if there really were two quasars out there.

It turns out the double quasars are really two images of one distant object that lies behind a massive galaxy cluster. The combined gravitational pull of all the galaxies in that cluster is enough to deflect light from the quasar as it passes by, and that gravitational lensing is what creates two quasar images.

Gravity’s Secret Role

Physical bodies all have mass. Each mass exerts an attractive force on other masses. The more massive the body, the stronger its gravitational force. Gravity and velocity keep planets in orbit around the Sun, moons going around planets, and galaxies orbiting other galaxies. So, how do they work?

We all owe our understanding of orbits and gravity to Sir Isaac Newton, an English mathematician in the seventeenth century who came up with a universal law of gravitation. Gravity, he said, is a force that acts on all objects in the universe. You can calculate the force of gravity between two objects if you know their mass and their distance from each other. The closer the objects are together, the stronger the force of gravity that attracts them to each other. The farther apart they are, the weaker the gravitational pull.

There are very detailed textbooks on gravity and its role in the universe. However, what’s most important to know about gravity in astronomy is that it influences everything from star formation and galaxy evolution to the orbital mechanics of objects in our solar system to the path that light takes through the universe.

Applying a Gravitational Lens

Any distribution of matter in space can act as a lens. The bigger the mass, the more gravitational distortion it creates. Astronomers knew from Einstein’s work that this was true, and some even predicted that this effect would be produced by galaxy clusters. For a gravitational lens to work, you need several things:

• A source—such as a quasar or a distant background galaxy
• Lensing material—anything from a star to a distant galaxy cluster
• An observer
• The images the observer detects

It’s All Relative

Space and time are interesting things. They can both be affected by matter—particularly large amounts of matter that have strong gravitational influences. This is the basis for the work that Albert Einstein did, spurred on by a solar eclipse that occurred in 1919. He predicted that light rays from distant stars would be bent as they passed by the Sun due to the Sun’s gravitational influence. The eclipse blocked sunlight, allowing observers to see stars they normally wouldn’t see, and they succeeded in measuring a tiny shift in light due to gravitational lensing. This observation led Einstein to publish work describing how the mass of an object curves local space-time, thus forcing light rays to bend ever so slightly. The 1919 eclipse produced the first experimental confirmation of gravitational lensing. Today, there are many gravitationally lensed objects that have been observed, ranging from stars to distant quasars.

Types of Gravitational Lenses

There are three types of gravitational lensing:

1. Strong lensing: The distortions are very obvious because the light from a distant object is passing by a very massive object that is fairly close by. The light follows more than one path past the lensing object, and the observer often sees what’s called an Einstein ring or even a set of multiple images of the same object all centered on the central mass doing the lensing. Most of the time the lensing object is a massive galaxy cluster.

2. Weak lensing: Here, the lensing object doesn’t have enough gravitational pull to create rings, arcs, or multiple images. What the observer does see are often sheared images of the background object. Even from distorted, sheared images and rings, astronomers can deduce something about the background object in order to figure out what it is.

3. Microlensing: This is an interesting use of lensing that has led to the discovery of planets around distant stars. The lensing objects can also be stars and even stellar black holes.

Lenses and Exploration

Gravitational lenses are equal-opportunity tools for exploring the distant universe. They act on light from across the electromagnetic spectrum, so they can be used to study the last faint quivers of light from the Big Bang, called the cosmic microwave background. This is a diffuse background of light that began its journey across space some 370,000 years after the creation of the universe. It was once very energetic, hot, and possibly as bright as the surface of a star. But the expansion of the universe has stretched the wavelengths of that light, and we see it today as microwave radiation. It’s faint and difficult to study. Gravitational lensing offers a way to observe changes and fluctuations in this remnant radiation that contains the last echoes of the Big Bang.

There’s another clever use for gravitational lensing: the search for dark matter. This mysterious “stuff” seems to be especially thick in galaxy clusters. Not only are their own gravitational influences holding them together, but a healthy distribution of dark matter (with its own gravitational influence) is helping, too. Dark matter adds to the gravitational lensing capability of a galaxy cluster. If all galaxy clusters have significant dark matter components, then gravitational lensing becomes a very important way to figure out the distribution of this mysterious stuff throughout the universe.