Click here to read an original op-ed from the TED speaker who inspired this post and watch the TEDTalk below.
Honor Harger's TEDTalk is on radio astronomy, or, in some sense, the "sound" of the universe (even though radio waves are really electromagnetic radiation, just like light). Can we, however, say what the color of the universe is? To answer this question, we must first establish what we actually mean by the "color of the universe." A reasonable definition would be to add up all the visible radiation emitted by a very large number of galaxies in a huge cosmic volume, and to determine how all of that light might be perceived by the human eye. This is precisely what astronomers Karl Glazebrook and Ivan Baldry attempted to do in 2002. Using a survey of more than 200,000 galaxies (the "2dF Galaxy Redshift Survey") reaching to distances of a few billion light-years, they constructed the distribution of the colors (the spectrum) the eye would see if all that light were to be separated into its components by passing it through a prism. The resulting spectrum (going into the ultraviolet a bit beyond the actual sensitivity of the human eye) is shown in Figure 1, where the intensity of each color is proportional to its intensity in the galaxy survey.
Figure 1: The cosmic spectrum, as measured by Karl Glazebrook and Ivan Baldry; reproduced with permission from Karl Glazebrook.
Interestingly, just as Honor Harger's TED presentation is entitled "A History of the Universe in Sound," one can also talk about the history of the color of the universe.- Mario Livio
Since our universe is expanding, light from distant galaxies is stretched to longer (redder) wavelengths (a phenomenon known as redshift). The farther away the galaxy, the greater the amount of stretching that occurs. Glazebrook and Baldry removed this effect before combining all the light to form a smoothed-out average color. Then they took into account the mean response of the human eye to the various colors, eventually coming up with the final result seen in Figure 2. So the color of the universe is a conditionally perceived shade of beige!
Figure 2: The average color of the universe, as calculated by Karl Glazebrook and Ivan Baldry; reproduced with permission from Karl Glazebrook.
Several people who had read about the Glazebrook and Baldry result proposed names for the color and the one that stuck (suggested by Peter Drum) was "Cosmic Latte." Interestingly, just as Honor Harger's TED presentation is entitled "A History of the Universe in Sound," one can also talk about the history of the color of the universe. An examination of the evolution of the cosmic star-formation rate reveals that the birth rate of new stars reached its peak some ten billion years ago before starting to decline (see: When Did Intelligent Life Emerge in the Universe?). In fact, by about five billion years ago, most of the stars that would ever form in our universe had already formed. Since young stars are hot and blue while old stars are cooler and redder, the color of the universe was undoubtedly different in the past, when young stars were more abundant. Based on the observed behavior of the star-formation rate over time, the evolution of the color of the universe from about 13 billion years ago (when the first galaxies formed) to about 7 billion years into the future, looks something like Figure 3. So, if you continue to associate the color of the universe with the names of drinks, you might say that the universe evolved from a "Cosmic Blue Hawaii" to a "Cosmic Latte."
Figure 3. The evolution of the color of the universe from 13 billion years ago to 7 billion years from now, as calculated by Karl Glazebrook and Ivan Baldry; reproduced with permission from Karl Glazebrook.
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