Many people have heard of the Doppler effect; as a matter of fact, without this staple of physics, radar (RAdio Detection And
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Ranging), sonar (like radar, but employing sound rather than radio frequencies), and lidar (or laser radar) would not exist. Despite this familiarity with the term, far fewer can actually explain what it is.
At its simplest level, the Doppler effect is demonstrated every time you hear an emergency siren while driving. As the emergency vehicle approaches, it compresses the sound waves, which in turn increases the pitch, or frequency of the sound; when it starts moving away the sound waves are stretched and the frequency drops. In the case of light, a closing object has its light shifted towards the blue end of the electromagnetic spectrum (shorter wavelengths), or blue-shifted, while a receding object undergoes red-shifting (longer wavelengths). The resultant shift in the frequency of the sound or light can then be used to determine the velocity of the vehicle (if you know the original frequency of light or sound emitted, you can figure out how fast the object is moving towards or away from you). In astronomy, redshifts are assigned a z-value; this number indicates the percentage shift of the lines in the star’s electromagnetic spectrum.
How does this relate to the universe as a whole? It has long been accepted that the universe is expanding, and that the expansion is uniform (that is to say, there is no discernible center of the expansion, viewed at a “universal” scale). With the exception of our galactic companions in the Local Group, most other galaxies are moving away from us; only in the case of nearby galaxies can the force of gravity overcome the expansion of space and keep galactic clusters intact. Edwin Hubble, the man who gave his name to the Hubble Space Telescope, investigated the redshifts and distances of galaxies and realized that there appeared to be a linear relationship between the redshift (which, based on what we discussed above, we know to represent velocity) and the distance (or, rather, the luminosity) of Cepheid variables, a class of variable stars in which the period of variability is related to their absolute luminosity, allowing us to easily determine distance. The graph of this data confirms that the universe is expanding , and even goes so far as to say that the universe’s expansion gets faster the further from your “center” you move. The proportionality of this relationship is given by Hubble's constant, and, far from being inconsequential, this revelation has allowed for the mapping of the structure of the universe.
