What does the “redshift” of an object mean?
A person standing in place and listening to passing cars hears the engine sounds at a higher pitch than normal as they approach and at a lower pitch than normal as they recede. This is because the sound waves are compressed into shorter wavelengths as they
approach and stretched into longer wavelengths as they recede.
The same is true of light waves. The color red has the lowest wavelength of visible light, so light waves that are stretched will shift toward the red end of the color spectrum. This is known as redshift. Objects in space such as galaxies or exploding stars emit light in the form of light waves. As the light waves travels toward the earth over millions or billions of years, the universe continues to expand, lengthening the traveling waves as it does.
What does the “z” of an object mean?
The redshift of an object is quantified by calculating its value of “z“, which is the difference in the wavelengths of the emitted and detected light divided by the wavelength of the emitted light. This is written algebraically as z = Δλ/λ.
For example, if we know a particular electromagnetic wave was emitted at the star with a wavelength of λ=486nm but it was detected here on earth with a wavelength λ=520nm, we can say its redshift is z= (520nm-486nm)/486nm ~0.07. Since the wavelength of light stretches with the expansion of the universe, the redshift also tells us the difference in the size of the universe between emission and detection divided by the size at emission. For an object with redshift z=1, the light was emitted when the universe was only half its present size.
How will the Dark Energy Survey measure redshift?
The Dark Energy Survey will estimate the redshifts of distant galaxies by measuring their “photometric redshifts”. Astronomers measure the redshift of a galaxy or star by knowing the frequency spectrum of the light from an object when it is emitted and by looking at this spectrum when the light arrived on earth. By looking for shifts in the spectrum toward the long wavelength end of the spectrum (the “red” end), astronomers can determine the size of the universe at the time the light was emitted. In the standard “spectroscopic” method of determining redshift, astronomers point a telescope at a single star or galaxy and record the entire spectrum of light coming from the object. They then look for the shift in known peaks or dips (emission or absorption lines) in the spectrum.
While spectroscopy produces precise redshift measurements, it remains technically daunting to measure spectra for hundreds of millions of faint galaxies in a reasonable time span with current technology.
To get around this problem, DES will take advantage of the fact that the light spectrum of galaxies has a relatively sharp drop at a wavelength of 400nm, referred to as the 4000A break. The drop can be seen in the blue line on the graph to the right, which shows the relative amount of light in each wavelength that would be received from a typical galaxy. This blue line shows the light spectrum for a nearby galaxy with “z=0″. As the galaxy becomes more and more distant from earth and recedes at a faster rate due to the expansion of the universe, this drop will be observed here on earth at a higher and higher wavelength. This shift toward the longer wavelength (red) end of the spectrum can be been seen in the green and red lines on this plot, which are for galaxies with z=0.5 and z=1.0 respectively. DES will find the approximate position of this drop by taking five different images of each portion of the sky to be surveyed using five different filters to block out all the light in the spectrum except for that passing within a small window of wavelength. Four of these five windows are shown in the light green, red, purple, and black peaked lines (labeled g, r, i, z) in the plot to the right. By measuring the amount of light visible in each of these windows from each of the millions of galaxies it observes, that is, by determining their colors, DES will be able to estimate the redshift for each of them. This is referred to as finding each galaxy’s photometric redshift or its “photo-z.” While not as precise as a spectroscopic redshift measurement, photometric redshifts are sufficiently accurate to carry out the dark energy probes with DES.