Gravitational Lensing: Mother Nature's Telescope
A near-perfect example of the gravitational lensing effect in action as seen by Hubble.NASA/ESA
I've owned a telescope of some variety for nearly twenty years. They've varied from small shop-bought ones to large home-made 'attempts.'
They have enabled me to see some incredible sights; from the beautiful rings of Saturn to the moons of Jupiter; from distant star clusters to an asteroid as it passed harmlessly through the Earth-moon system.
The largest telescope I've owned had a mirror 35.5 cm (14 inches) across, which is dwarfed by the largest telescopes on Earth measuring several meters across.
But that's nothing compared to the telescopes made and operated by Mother Nature that measure light-years across and allow us to probe the darkest depths of the Cosmos and the very nature of space and time.
Lets put this all into perspective. The telescopes that we use here on Earth are manmade instruments and have varying designs from the simple refractor composed of lenses to the more common reflector built mainly from mirrors.
The thing that all telescopes have in common are that they have something to collect light — either a lens or mirror — that brings incoming parallel beams of starlight to a point of focus for study. Having a bigger telescope essentially means you can collect more light and see fainter objects. You can also see a finer level of detail than a smaller telescope.
Our ground-based telescopes have given us great views of the Cosmos and the Hubble Space Telescope (HST) has revolutionized our understanding of astronomy. But even the HST, orbiting high above our turbulent atmosphere, has its limitations. It seems Mother Nature, like so many other things, is way ahead of us by building great beasts of telescopes in deep space — we just need to find them! I am referring to gravitational lenses.
As their name suggests, a gravitational lens is a lens created as a consequence of the force of gravity and their existence was predicted by Einstein in his famous theory of general relativity.
To understand how gravitational lenses work, it's necessary to delve into the very fabric of space (which is fiercely intertwined with time but for the purpose of this article, lets forget about the "time" in "spacetime").
Firstly, imagine space as an unending sheet of rubber with balls rolling around on the sheet. The balls (of various sizes) represent the galaxies and the rubber sheet represents the "fabric of space." Placing the balls on the sheet will cause deformation and dents will form as the balls' mass exert a downward pressure. The greater the mass, the deeper the dent.
Now, imagine getting some smaller ball bearings and rolling them past the 'galaxy' balls — their paths will be bent by the deformations in the sheet. In this model, the smaller ball bearings represent photons of light and the dents in the sheet are the force of gravity bending (or warping) space.
You can quickly see that the paths of the photons (ball bearings) have bent from their initial direction and it's this process that is known as gravitational lensing.
In real terms, the photons of light will have come from a more distant galaxy along the same line of sight as the intervening 'lensing' galaxy which bends the light granting us a new view on the Universe behind. These "lensing events" manifest themselves in the sky in a number of ways dependent on the size and matter distribution of the 'lensing galaxy' and the exact alignment of the three objects, including Earth as our focal point.
If the alignment is perfect, we will see a circular, magnified image of the distant object surrounding the intervening galaxy (as pictured top) or we may see a series of arcs or multiple miniature lensed versions of the galaxy behind. Lensing provides us with a natural means to see objects way beyond the capability of our most advanced space telescopes.
Gravitational lenses are great for helping us study distant objects that might otherwise be out of view, but they are also very useful for helping us measure the mass of the intervening galaxy or galactic cluster.
This was demonstrated beautifully recently when a team of astronomers led by Frederic Courbin of the Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland used HST observations to study distant quasars that were acting as lensing galaxies. Quasars are found in distant galaxies powered by massive black holes but because of their intense brightness, their host galaxies are often hidden in the quasars' glare.
This situation would normally dictate that we'd be unable to study the host galaxy and determine its mass, but by studying a lensing event we can measure the amount of distortion from the lensed image and determine the mass of the intervening object — the galaxy.
One of the things that attracted me to astronomy is the way that the Universe has the ability to constantly surprise. From the discoveries of complex chemicals on comets to finding water on the moon; from stars made of diamonds to planets orbiting distant suns, it all astounds me.
These may be incredible discoveries, but finding telescopes carved out of the fabric of space itself, aiding astronomers to see deep into the Cosmos? Well... I'll be darned.