Full disclosure: this post is inspired by and draws much of its material from an article in a recent issue of Sky & Telescope.
Looking Out Is Looking Back
Telescopes are time machines. When we view a beautiful sunset, the finite speed of light means that we see it as it was a little over eight minutes ago. Light from the relatively nearby star Sirius is eight and a half years old. Light from the Orion Nebula takes 1344 years to reach us.
As we look farther and farther out into space, we see the universe at increasingly younger epochs in its 13.8 billion year history. More distant objects are of course dimmer. The Andromeda Galaxy, our nearest spiral galaxy neighbor at 2.5 million light years away, is bright enough to be visible to the naked eye under dark skies. But to see very distant, and therefore very dim, galaxies requires a telescope, and the bigger the telescope, the more distant the galaxy it can see.
Brightness or dimness is not a precise measure of distance. Some galaxies are bigger and therefore brighter than others, and galaxies evolve over cosmic time. As stars create elements and spew them into the interstellar medium, the very nature of the stars making them up changes.
Most of you reading this are probably familiar with the expansion of the universe, and with the fact that this detectable by measuring the red shift of an object. I’m going to explain that in a different way than what you have probably heard. This is often (and incorrectly) described as a Doppler red shift, one caused by the motion of an object receding from you. You observe the sonic analogue of this as a race car approaches you, with its sound waves being compressed and therefore of higher pitch, and then as it recedes with its sound waves being stretched out and therefore pitched lower.
But the cosmological red shift is actually due to the expansion of space-time itself, the very space-time through which a light wave passes as it moves from that distant galaxy to our telescope.
The further that light wave has traveled—the longer time it has spent being stretched by the expansion of the universe—the greater the red shift will be.
Determining the Red Shift
How do you measure red shift? First you have to collect the light, and then you have to spread it out into its constituent wavelengths. Here are two idealized galactic spectra, the one on top from a galaxy at rest with respect to us, the other receding due to expansion of the universe. All the wavelengths of the latter are red shifted, stretched to longer (redder) wavelengths.
This works fine for relatively nearby and therefore bright galaxies. Even a telescope of modest size can gather spectra from galaxies of the Local Group, those that are part of the galactic cluster that includes our own Milky Way. But as the distance to galaxies increases, and as the light we can gather from them dwindles to fewer and fewer photons, we need ever larger telescopes to obtain useful spectra.
We need a new trick to see galaxies at very high redshifts.
Galaxies contain a lot of neutral hydrogen gas, hydrogen being by far the universe’s most abundant element. This gas strongly absorbs light of wavelengths less than a certain value (91.2 nm), and the light from a galaxy drops off sharply below this value which is in the far ultraviolet (UV). If you were to look at a galaxy through a filter that blocked light of wavelengths longer than this, even a relatively bright galaxy could dim to invisibility. This sharp drop off is called the Lyman break.
Here’s the trick. While lots of light is needed to capture a really good galactic spectrum, we can use much cruder methods to determine where the Lyman break occurs. In a nearby galaxy at rest with respect to us, it of course occurs at 91.2 nm. In a very distant galaxy with a large redshift, it will occur at longer and longer and longer wavelengths. Rather than being dim in the far ultraviolet and bright in visible wavelengths, a really distant galaxy will be visible only in the long wavelength infrared (IR).
The images above are from the Hubble Deep Field, showing the same small patch of this larger image through four different filters. Each of these is centered on a particular wavelength of light. The arrow points to the exact same spot in the sky in each of the four images.
From left to right, the central wavelengths are 300 nm (ultraviolet), 450 nm (blue), 606 nm (yellow/orange; visual), and 814 nm (infrared). The arrowed galaxy is detectable only in the infrared image, an indication of just how distant it is. Even the Hubble was only able to gather a sparse few photons from this galaxy, and that only after exposure times measured in tens of hours. A full spectrum isn’t achievable, but this method lets us make at least a rough estimate of this object’s red shift.
For good measure, here is the full Hubble Deep Field image, taken in 1996.
The James Webb Space Telescope scheduled to be launched in 2019 (see below) will be both larger than the Hubble and better able to see at infrared wavelengths. If we are fortunate, we will be able to see galaxies all the way back to their initial births—all the way back to cosmic dawn. The photons that arrive in our solar system will have spent most of the history of the universe traveling to deliver their secrets to us.
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