It’s not the prettiest of telescope images. It doesn’t have the majesty of spiral galaxies, nor the sharp chic of Saturn’s rings. The color scheme is not exactly appealing, and there’s nowhere obvious to focus. Yet it's one of my favorites, a snapshot of the universe as it was in its infancy.
Some 13.7 billion years ago, a mere 380,000 years after the Big Bang, photons of electromagnetic radiation were set loose to journey across the universe. Up until that time, photons couldn’t travel far without being scattered by free electrons, in the same way that photons of light on Earth get scattered by fog. As a result, the universe itself was opaque, one giant fog. That changed when the universe cooled enough that electrons could no longer roam free, but were bound to protons as hydrogen atoms. With the electrons tied up, the photons were on their way.
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| Planck CMB / Image: ESA and the Planck Collaboration |
Those photons are travelling still, some of them are passing us now, but the universe has expanded in the intervening 13.7 billion years, and the wavelength of the radiation has stretched with it. They are now mostly in the microwave part of the electromagnetic spectrum, the sluggish low-energy end of the spectrum. Nevertheless, it is this “cosmic microwave background” (CMB) that keeps the temperature of outer space 2.7 degrees Kelvin (or Celsius) above the theoretical lowest possible temperature, absolute zero.
The CMB was predicted by Big Bang theory in 1948, sending astrophysicists racing to find it. In the end however, it was found accidentally in 1964 by radio astronomers searching for something else entirely, and wondering what was causing the noise coming from the sky in every direction. In their efforts to eliminate that noise, they even tried scrubbing the antenna clean of pigeon poop. It didn’t work, but the researchers Arno Penzias and Robert Wilson did later receive the Nobel Prize in Physics for discovering the CMB. Many of us have seen the effects of this background radiation ourselves without any fancy radio telescopes, since it contributed a small portion of the static we endured while channel surfing on old analogue TVs.
The CMB was predicted by Big Bang theory in 1948, sending astrophysicists racing to find it. In the end however, it was found accidentally in 1964 by radio astronomers searching for something else entirely, and wondering what was causing the noise coming from the sky in every direction. In their efforts to eliminate that noise, they even tried scrubbing the antenna clean of pigeon poop. It didn’t work, but the researchers Arno Penzias and Robert Wilson did later receive the Nobel Prize in Physics for discovering the CMB. Many of us have seen the effects of this background radiation ourselves without any fancy radio telescopes, since it contributed a small portion of the static we endured while channel surfing on old analogue TVs.
More recently, microwave telescopes mounted on spacecraft have been sent into orbit to capture the CMB. The most detailed image comes from the Planck spacecraft (top), which was launched by the European Space Agency in 2009 to scan the entire sky.
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| Planck scanning the sky / Image: ESA (image by C. Carreau) |
Since the image is a map of a sphere, it looks stretched out for the same reason maps of the Earth aren’t round. But instead of looking down at what we’re standing on as for a map of the Earth, this image places us in the center of the sphere and looks out, looks out at the “light” that was the very first to be set free on a journey across the universe.
If we could see microwaves as we can see the visible light part of the electromagnetic spectrum, the CMB would appear the same all over. To achieve the patchy effect of the image, tiny differences have been magnified. The different colors represent areas of slightly different temperatures and therefore slightly different densities in the young universe. Thanks to gravity, areas of slightly greater density attracted more matter. Consequently it is here that stars and galaxies formed as the universe came of age, and here that we find stunning images with our other telescopes today.
Further reading:
Electromagnetic Spectrum, NASA Imagine the Universe
Planck, ESA
The Cosmic Microwave Background, Big Bang Central
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