Peering Into the Gravitational House of Mirrors

What is it about galaxies that SO evoke our sense of space and distance?  Is it because they’re so big and magnificent? That they stretch across immense regions of space? The idea that these cosmic cities are thronged with stars? If you look at an image of a galaxy like the Milky Way, you see stars and regions where stars are born and die, and you see (if it has one) the central core with a black hole at its heart.

But, how did galaxies get started?  How were they born?  And what is their lifestyle like?  These are questions that astronomers are still working to answer. Understanding the origin and evolution of galaxies benefits from looking at galaxies in all stages of their lives.  And, so, astronomers look through billions of years of cosmic history to study some of the earliest galaxies.

There’s a little bit of a problem looking back that far. We have to peer through what amounts to a cosmic “house of mirrors” to see the youngest galactic objects in the universe. Everything we see in this house of mirrors is distorted by a phenomenon called gravitational lensing. This occurs when light from a distant object is distorted by a massive object that is in the foreground.

Astronomers have started to apply this concept in a new way to determine the number of very distant galaxies and to measure the amount of something called “dark matter” in the universe.

So, how does gravitational lensing work?

Albert Einstein showed that gravity will cause light to bend. The effect is normally extremely small, but when light passes close to a very massive object such as a massive galaxy, a galaxy cluster, or a supermassive black hole, the bending of the light rays becomes more easily noticeable.

When light from a very distant object passes a galaxy much closer to us, it can detour around the foreground object. Typically, the light bends around the object in one of two, or four different routes. This magnifies the light from the more distant galaxy directly behind it. What you get is a sort of “natural telescope”, called a gravitational lens. It provides a larger and brighter — though also distorted — view of the distant galaxy.

A very massive object — or collection of objects — distorts the view of faint objects beyond it so much that the distant images are smeared into multiple arc-shaped images around the foreground object. This effect is a lot like looking through a glass soft drink bottle at a light on a balcony and noticing how it is distorted as it passes through the bottle.

This is a very cool idea and I remember back in graduate school first learning about lensing, and we all thought it was almost too weird. At that time, all we could really see were the brightest, most obvious lensed objects.  Now, we can see many of these distortions. And, as we move toward fainter and more distant objects, many of the more recently observed ones pushing the limits of the Hubble Space Telescope.  Even fainter ones will need something with more observing power.  If all goes well, those next generation objects to be observed will be more effectively handled by a new space telescope on the drawing boards — the James Webb Space Telescope (JWST).

First Light and Lensing

When you look back to when the universe was young, you are seeing extremely early objects (also known as “first light” objects) that are very far away. The older and farther away the object, the more foreground universe there is to look through, which means the greater the chance that there will be something heavy in the foreground to distort the background image.  Dr. Rogier Windhorst of Arizona State University, is doing research suggests that gravitational lensing is likely to dominate the observed properties of very early galaxies, those that are at most 650-480 million years old The halos of foreground galaxies when the universe was in its heydays of star formation (when it was about 3-6 billion years old) will gravitationally distort most of these very early objects.  This leads to an effect called “gravitational lensing bias” where we are seeing many things whose light is stretched by lensing.  He reported on that work today at the AAS meeting, by way of pointing out just how useful future telescopes, especially the JWST, will be in extending our view out to the early universe and dealing with this house of mirrors effect.

JWST will have to take this bias into account. Scientists like Windhorst and his colleagues will need to design new ways of handling the data from those observations to really help them understand just what it is those early, distant gravitationally lensed galaxies are doing… and how they evolve to become the galaxies we see today.