Down The Rabbit Hole Through Gravity’s Lens

A simple convex (bulging outward) lens focuses an image of my wife's stained glass flower. Photo: Bob King

I played with lenses as a kid, fascinated by their seemingly magical ability to turn the world upside down, shrink and magnify it. The word lens comes from the Latin word for ‘lentil’ and refers to the biconvex shape of the familiar legume. Glass cut and finely ground into a curved surface can be used to gather and focus light. Lenses form the heart of a telescope, an instrument responsible for transporting the human eye and mind to places never before imagined. They also help us see the everyday world with clarity and drill down into the world of the microscopic.

Albert Einstein’s theory of relativity gave us a very different way of looking at lenses and gravity. According to Einstein, massive objects bend and warp the fabric of space. The more massive the object, the more severe the bending. You can imagine this by picturing a child standing on a trampoline. Her weight depresses the surface to form a little dip or bowl, while a 200-lb. adult creates a much deeper depression in the fabric.

Massive objects like the sun and even the planets warp the fabric of space. Here a planet orbits the sun but does not fall in because of its sideways orbital motion.

Similarly, the massive sun creates a deep, invisible wrinkle in the center of the solar system. The planets feel this ‘curvature of space’ and literally roll down toward the sun. What keeps them from falling all the way in is their sideways motion or angular momentum. At the same time Earth is plunging toward the sun, we’re also revolving around it. The two balance each other and we circle safely around the sun in a stable orbit year after year.

Curved space created by massive objects also bends light rays. Einstein predicted that light from a star passing near the sun would follow this invisible curved landscape and be deflected from an otherwise straight path. Scientists succeeded in measuring the tiny deflection during a total eclipse of the sun in 1919. The bending of light by the power of gravity is called gravitational lensing. Chalk up another amazing prediction-come-true to Einstein!

The giant cluster of galaxies called Abell 383 was taken by the Hubble Space Telescope. The cluster contains so much matter - regular and dark - that its gravity bends the light from distant background galaxies like a lens, magnifying and intensifying their light. Those arc-shaped smears you see are some of these remote galaxies, their light bent and distorted by the foreground cluster's gravity. Credit: NASA/ESA/J. Richard and J.-P. Kneib

Now imagine if you had a huge amount of matter glommed together in outer space. It would be capable of some serious space warping. Turns out there are lots of these gravitational ‘lenses’ out there in the form of massive clusters of galaxies. They contain regular matter as well as vast quantities of the still-mysterious dark matter that makes up 80% of the material in the universe. Rich galaxy clusters serve as probes of the distant universe, because their powerful gravity can magnify and intensify the light of galaxies billions of light years in the distance behind them.

This illustration shows how gravitational lensing works. The gravity of a large galaxy cluster is so strong, it bends, brightens and distorts the light of distant galaxies behind it. The scale has been greatly exaggerated; in reality, the distant galaxy is much further away and much smaller. Credit: NASA, ESA, L. Calçada

Galaxies that formed in the very early universe and otherwise too distant and faint to see come into clear view thanks to nature’s largest lens of all. Some of these clusters are 30 million light years or more across!

Using Abell 383, a team of astronomers recently identified a galaxy so far away we see it as it was less than a billion years after the Big Bang. It is visible as two tiny dots on either side of the bright cluster. Distant objects seen through gravitational lenses are typically multiply imaged and strongly distorted. Credit: NASA/ESA

Gravitational lensing not only brings the distant to the foreground, it often creates multiple images of the object. Astronomers studying the Hubble images of Abell 383 discovered twin images of a galaxy so far away, we see it when the universe was just 950 million years old. While not a record, the galaxy is different from typical young galaxies, which are packed with fresh, brilliant, blue stars. This one contains stars that are unusually old and faint. Astronomer Johan Richard, lead author of a new study on the galaxy, estimates its stars were already 750 million years old at the time.

That implies the galaxy and its stars formed much earlier than previously thought, just 200 million years after the Big Bang event 13.7 billion years ago. Nature apparently wasted no time in cooking up the first galaxies. Isn’t it amazing where playing with lenses can take you?