The universe is immersed in a sea of lights, from the blue-white flicker of young stars to the deep red glow of hydrogen clouds. Aside from the colors that the human eye perceives, there are X-ray and gamma-ray flashes, powerful bursts of light, and the faint, ubiquitous glow of the cosmic microwave background. The cosmos is filled with colors that you see and do not see, old and new. But of all these, there was one color that came before everyone else, the first color of the universe.
The universe began 1
At first the temperatures were so high that there was no light. The cosmos had to cool down for a fraction of a second before photons could emerge. After about 10 seconds, the universe entered the photon epoch. Protons and neutrons had cooled down into the nuclei of hydrogen and helium, and the space was filled with a plasma of nuclei, electrons, and photons. At that time, the temperature of the universe was about 1 billion Kelvin.
But even though there was light, there was no color yet. Color is something we can see, or at least some kind of eyes that we can see. During the photon epoch the temperatures were so high that light could not penetrate into the dense plasma. Only when the nuclei and electrons have cooled to the point where they combine to form atoms will color appear. It took 380,000 years for the universe to cool down.
Until then, the observable universe was a transparent cosmic cloud of hydrogen and helium with a diameter of 84 million light-years. All photons formed in the Big Bang could finally flow freely through space and time.
This is the cosmic microwave background – the light of a time when the universe could finally be seen. For billions of years, the glow has cooled down to a temperature of less than 3 degrees above absolute zero. When it first appeared, the universe was much warmer, about 3,000 K. The early universe was filled with a bright, warm glow.
We have a good idea of what this first color was. The early universe had an almost uniform temperature and its light had a wavelength distribution known as the blackbody. Many objects get their color from the type of material they are made of, but the color of a black body depends only on its temperature. A black body at about 3,000 K would have a bright orange-white glow, similar to the warm light of an old 60-watt bulb.
People do not see colors very well. The color we perceive not only depends on the actual color of the light, but also on its brightness and whether our eyes are adjusted to the dark. If we could go back to the time of the first light, we would probably perceive an orange glow that resembles the light of the fire.
Over the next one hundred million years, the faint orange glow would fade with progressive expansion and redness of the universe cool. Eventually, the universe would fade to black. After about 400 million years, the first bright blue-white stars began to form and new light appeared. As stars and galaxies appeared and developed, the cosmos took on a new color.
In 2002, Carl Glazebrook and Ivan Baldry calculated the average color out of all the light we see today from stars and galaxies to determine the current color of the universe. It turned out to be a pale tan resembling the color of coffee with cream. They called the color "Cosmic Latte".
Even this color lasts only a while. When big blue stars age and die, all that remains is the deep red glow of the dwarf stars. Eventually, after billions of years, even their light will fade and the universe will become a sea of black. All colors fade over time, and time will carry us all into darkness.
But for the time being the colors of the universe are still painting us. And if you ever sit with a creamy coffee by the fire while you look into the darkness of the night, you should know that you are bathed by cosmic colors. Past, present and future.
When was the first light in the universe?
Ivan K. Baldry et al. The 2dF Galaxy Redshift Survey: Limitations of Cosmic Star Formation from the Cosmic Spectrum, The Astrophysical Journal (2002). DOI: 10.1086 / 339477
What was the first color in the universe? (2019, 21st October)
retrieved on 21 October 2019
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