Upgrades performed on the Hubble telescope last year are now yielding images of objects that are 13 billion light years away. But how do astronomers know how far away those objects are?
It is helpful to understand some of the properties of light that are not immediately evident. Many experiments and observations demonstrate light has wavelike properties. Unlike water and sound waves, light requires no medium for propagation. It travels though empty space and all translucent media.
Waves have a natural periodic variation in their intensity. That is, some property of waves varies in strength at a regular frequency. The time or distance between crests in this variable property is called the wavelength. Wavelengths and frequencies are inextricably linked.
The frequency or wavelength of light determines its energy. That energy also determines the color of the light as you perceive it.
Light emitted from a star is characteristic of the elemental atoms making up the star. This is because light is emitted as electrons bounce from the shell of one atom to another, countless times over. The characteristics of that light are determined by what elements are involved and because elements are the same here as they are on the other side of the universe, the elements can be identified from their light signature.
Astronomers collect light from a star in a telescope and pass it through a prism. The prism separates the light into its various colors. Projecting the colors onto a screen shows the original light to have been composed of a number of distinct colors that appear as lines on the screen.
Lines of this sort are called emission spectra since they arise from light being emitted from excited atoms. Absorption spectra have a similar unique pattern of lines. They arise from light shining through gaseous material and being preferentially absorbed by atoms according to the binding energies of electrons they displace.
The principles behind the application of either of these line patterns for the measurements outlined below are the same.
When the energies of the colored lines are determined, they are found to correlate exactly with the energy emissions of elemental atoms. Hence, the elemental makeup of the star is revealed.
Since there are a number of spectral lines, the pattern of those lines proves to be as important as the exact color of any one line. In fact, for many celestial objects, the whole pattern of lines is typically shifted in position from what would be expected if the light was being emitted from a sample in the laboratory.
There are three phenomenon giving rise to shifts. The first provides a tool for astronomers to determine how fast an object is traveling relative to Earth. From the second, the distance to very far away objects can be inferred.
The implications of the last phenomenon, called the gravitational red shift, will not be discussed in this article. It arises when light is emitted from objects in extremely high gravitational fields. It is further evidence for Einstein's general theory of relativity.
The first of these is the most intuitive since it has a familiar analogy in sound waves. Sound waves are pressure waves propagating through air. A discreet tone, like a siren on an emergency vehicle, is a periodic sound wave having just one frequency or wavelength. Standing next to a stationary siren we would recognize its tone.
Should the vehicle speed away from us while sounding its siren, the tone would appear to shift toward a lower frequency. In this case, the distance between the generating source and a receiver increases as the wave is being produced. The resulting elongated wavelength yields a lower pitch. Speeding toward us, the tone of the siren would appear to have a higher pitch.
Similarly, as a celestial object recedes from us, the wavelength of its emitted light is elongated. When that light is broken down into its spectral lines, those lines are observed as being shifted toward the red end of the light spectrum. These are referred to as Doppler red shifts.
If the relative velocities of the observers and sources are well below the speed of light, the shifts in wavelengths are governed by the same principles as sound waves. However, velocities near the speed of light require adjustments taking into account special relativity.
Spectral lines will shift toward the violet end of the spectrum if the object is traveling toward us. Shifts in absorption lines in both directions are observed with binary stars. These are pairs of stars that orbit around one another. If we look at their orbital plane edgewise, during part of their circuit one star is moving toward us and the other away. Their velocities are revealed by the respective shifts in their spectral lines.
The second phenomenon gives rise to something called the cosmological red shift. It is used to measure distances to very far away objects. This feat is made possible through our understanding of space and time as revealed by Einstein's theory of general relativity. Consistency between these measurement and theory is strong evidence for the expansion of the Universe and Big Bang cosmology.
Einstein's ideas and astronomical observations came together in the early 20th century. In 1912 Vesto Slipher made extensive measurements of the red shifts of galaxies far beyond our own Milky Way Galaxy. He subsequently correlated these shifts with the recessional velocity of the galaxies.
Edwin Hubble (after whom the Hubble telescope is named) was able to show a relationship between increasing red shifts and ever greater distances. Hubble's Law, published in 1929, provided a mathematical framework explaining the data.
The framework used general relativity which suggested the Universe and space were expanding. As light waves travel through expanding space their wavelengths become stretched. And, as described above, the spectral lines of elongated light waves appear shifted toward the red end of the spectrum.
For decades these powerful measurement tools have been used. They were developed when extensive, meticulously collected data was shown to be consistent with mathematical theories characterizing space and time. Pretty cool stuff.
Einstein formulated relativity to be consistent with what had been previously discovered about light and other forms of electromagnetic waves and gravity. Ever since, it has been showing the way to understanding the entire cosmos.
Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.