Astronomers on Earth are limited in how they can look at the universe. We basically have three tools. We can detect light across a broad spectrum of colors. We can capture high energy particles – cosmic rays – that are flung at us from distant events. We can also potentially measure gravity waves (but we’re still sorting out that technology). In all three instances, we are limited by our technology’s sensitivity to an event. This means that faint, small, low energy stuff at any significant distance is invisible as far as our detectors are concerned. And stuff like dark matter… well… it can’t be directly detected at all. When direct detection of something is impossible, it becomes necessary to find indirect methods. We are like Plato, looking to understand reality but only able to see shadows dancing on a cave wall.
Two of the most well defined ways we have of studying the universe’s shadows are the Cosmic Microwave Background (CMB) and gravitational lenses. Both things are scientifically interesting in their own right, and each can be used to indirectly see otherwise invisible content in the universe. Recent papers have shown how the CMB may allow astronomers to study our own solar system’s Oort Cloud (the source of long period comets), and how gravitational lensing effects can be used to map dark matter. Rather than try and discuss both these topics in one post, I’m going to take on gravitational lenses today, and dig into the cosmic microwave background tomorrow. (image credit: Kneib & Ellis w/ Caltech Digital Media Center)
The concept of gravitational lensing is perhaps one of the most important in astronomy. Put very simply, just as the gravitational pull of a planet on a comet can cause the comet’s orbit to significantly change shape, so too can the gravity of an object cause the path of light to change. How much the light’s path changes is directly related to the mass of the object the light is passing near. If I shine a flash light past you, the beam will not deviate in a way any measuring apparatus can measure. If I shine a flash light past the Sun, the beam will bend a couple of seconds of arc (It will appear to move about two hair’s width on the sky). If I shine that same light past a black hole (making sure the beam doesn’t enter the event horizon!) the beam might get bent well over 90 degrees! (for mathematical discussion see this neat discussion)
Bending light isn’t a new concept. Lenses and mirrors can both be used to bend light to create magnified, shrunken, and/or distorted images. If I look at an image of myself in a circus mirror, I might appear as an arc of a human; stretched out and distorted in a long curve. This is because the photons that bounce off of me are having their paths distorted by the mirror. In a similar way, a large object (like a galaxy cluster) between us and a distant object (such as a single galaxy), can bend the paths of photons from that distant object and make it appear distorted.
Here is where the neat science comes into play. Dark matter is by definition stuff that exerts gravitational pull on other stuff but can’t otherwise be detected. This means that if we look for objects that are distorted and we can’t find what is doing the distorting, then we can probably blame dark matter. That sounds nice and straightforward, but because the term “distorted” is hard to define in a universe of seemingly (but not actually) infinite possibilities, it is hard to know what is distorted and what is just plan weird looking by nature. Here is where we have to start dealing in generalities. If you take the images of 100 or so galaxies and (after scaling them to have the same radius) average them together, you should get a disk. Some galaxies will be oblong smears pointed from 5 o-clock to 11 o-clock. Others will be boxy cigar shapes aligned from 8 o-clock to 2 o-clock. Others will be plan old circular face-on disks. Averaging all these differences together in a distortion free world gives you a circle every time. Now, if instead of a disk, you find yourself, after averaging, looking at a tear drop shape, or a crescent moon, or any other non-circular shape, then there is something distorting your view on the universe. If you don’t see what is causing the distortions, then what you don’t see is dark matter.
In the past year, there has been a new and exciting stream of results that have used this technique to map the distribution of otherwise hidden materials. The COSMOS survey imaged galaxies, figured out how far away they are, and then measured their average shapes as a function of their distances. This allowed them to say, these nearby galaxies have this type of distortion caused by even more nearby dark matter, while these more distant galaxies have that distortion plus a second distortion that came from dark matter at some intervening distance (think of it as looking through a whole series of distorting pieces of glass with objects randomly placed here and there between different pieces of glass – the distortions add up the more pieces of glass you look through). Their first ever three dimensional map of dark matter did two things: It showed that matter we can see (luminous matter) and dark matter are generally found in the same places but not always, and it showed that dark matter is at least in part, if not in whole, a real physical thing and not just an additional term in the equation for gravity. (credit: NASA / ESA / R. Massey (CalTech))
Other work, observations of both the Bullet cluster and the Zwicky galaxy cluster 0024+1652 were able to map the distribution of dark matter after a collision, and showed that the dark matter can form structures, but doesn’t interact (e.g. collide with stuff) like normal gas or dust. (credit: NASA / ESA / MJ Jee (John Hopkins))
Slowly, using indirect techniques, we are tracing out the features of dark matter. Perhaps Plato would be proud that we have figured out how to find truth in the firelight of the stars and the gravitational shadows dark matter casts on our detectors. Then again, Plato would probably find some existential reason to poo poo our results…
But still, what these people are doing is amazing and beautiful. And gravitational lenses aren’t only used to define dark matter of the “whatever it is, it just isn’t something I understand,” non-baryonic variety, they are also being used to find stuff that is simply too faint or distant to be identified.
Consider mirrors again. If a large mirror (one much bigger than I am) is used to focus light that is reflecting off of me, the image of me may be brighter than if you just looked at me with your eyes! This is because the mirror is capturing more light than your eyes alone would capture. In a similar way, gravitational lenses can also, in some cases, bend more light toward us then would otherwise come our way. This can do two things: It can make an otherwise invisible very distant object visible by lensing it, or it can illuminate the location of an otherwise invisible nearby object that is doing the lensing.
What was for a while the most distant galaxy to be observed was identified as a red, gravitationally lensed pair of smears in an image of the galaxy cluster Abell 2218. This distant galaxy is located more than 13 billion light years away at a redshift of z=5.58. The uneven distribution of galaxies, dust and gas in Abell 2218, split the light of the distant galaxy into two images that are magnified by a factor of about 30. Today, the MACHO and OGLE teams have each looked for the brief (defined on the order of days) brightenings of background stars that occur when a foreground star/planet/dead star orbits in front. These galaxy orbiting objects are generally too faint to otherwise find, and their planets are definitely beyond our abilities to otherwise detect. In this case, out indirect gravitational detections are helping us take a census of the faint non-findable objects hiding in our galaxy’s halo. These things will probably start to become visible as we build bigger infrared telescopes, but until then we can find and count them using gravitational lensing. So far scores of white dwarfs, brown and red dwarfs, black holes, neutron stars and even planets have been found this way. (credit: NASA / ESA / R. Ellis (CalTech) / J.-P. Kneib (Observaoire Midi-Pyrenees))
Unlike standard mirrors and lenses, we have no way to choose where our gravitational lenses will point. We can’t polish them to create perfect images. They only help us see the smallest fraction of what the universe has to show us – but that fraction us stuff that we otherwise wouldn’t ever notice. The invisible is made visible by looking for what isn’t in our images.