Podcaster: Chris Impey
Organization: University of Arizona
Description: This podcast examines the first time we can meaningfully talk about light in the history of the universe. The universe began in a state of unimaginably high density and temperature 13.8 billion years ago. After the big bang the temperature cooled and the intense radiation that filled the universe was redshifted by the expansion to longer wavelengths. About 250,000 years after the big bang, a time before any stars and galaxies existed, there was a period of time when the universe glowed like a light bulb. By 500,000 years after the big bang, the universe had faded to red and then black.
Bio: Chris Impey is a University Distinguished Professor and Deputy Head of the Department of Astronomy at the University of Arizona. He has over 170 refereed publications on observational cosmology, galaxies, and quasars. He has won eleven teaching awards, and is currently teaching two online classes with over 35,000 enrolled. Impey is a past Vice President of the American Astronomical Society and he has been an NSF Distinguished Teaching Scholar, the Carnegie Council’s Arizona Professor of the Year, and a Howard Hughes Medical Institute Professor. He’s written over forty popular articles on cosmology and astrobiology, two introductory textbooks, a novel, and seven popular science books.
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Hello my name is Chris Impey and this is a podcast for 365 Days of Astronomy. I’m University Distinguished Professor of astronomy at the University of Arizona. In my research I work on supermassive black holes in their evolution with galaxies and I also have strong interest in education and educational technology, in teaching students online, and in writing popular science books. This year we’re celebrating the U.N.’s International Year of Light with a goal of raising awareness of the importance of light and light technologies.
Of course, astronomers know that light can also be the enemy in terms of light pollution and its contamination of the best observing sites on Earth. But for now let’s consider light in a positive cast. In this set of three podcasts, I want to talk about the past and future of light in the universe, in the broadest possible sense, in terms of cosmology.
Let’s start with the first time we can meaningfully talk about light in the universe. We go back to the big bang, the beginning of the universe of everything, the origin of space and time, a situation of infinite temperature and density, where the four fundamental forces of nature are melted together into a so-called super force. After this, the next major landmark is the inflationary era at an incredible ten to the minus thirty-three seconds after the big bang, and a temperature of ten to the twenty Kelvin. This event has not yet been confirmed, despite tantalizing evidence from the cosmic microwave background. This past year, that seems to have been refuted so the jury is still out on inflation.
The next landmark comes when quarks form into protons and neutrons at about a microsecond after the big bang, when the temperature was about ten trillion Kelvin. Then another major event after a few minutes with the formation of helium by fusion to form one quarter of the mass of the universe by weight, when the temperature was a hundred million Kelvin. It’s the same process of nucleosynthesis that takes place within the Sun, and similar stars. This early universe is dominated by radiation, an intense ocean of gamma rays. The universe in these early stages was monolithic and had no structure.
We can continue to follow the big bang model, which describes how the temperature goes down with time as the inverse square root of time. The big bang has a slow falloff of its radiation spectrum to long wavelengths. So an observer would see the steadily growing the light from the Big Bang, even when it is still very hot at a temperature of a hundred thousand Kelvin and the primary radiation is X-rays. We can visualize that as the universe expands and cools the locus of its radiation goes to longer wavelengths and eventually enters the visible spectrum. The universe begins to glow like a light bulb. It happens when the universe is about six thousand Kelvin, when it was two thousand times smaller than it is today, and about ten billion times denser than it is now.
When light starts to enter the visible spectrum, two hundred and fifty thousand years after the big bang, the universe is almost perfectly smooth. The largest fluctuations in density of either atoms or photons are about one part in ten to the five, a thousandth of a percent. The universe is glowing like a lightbulb, but its physical state is that of a plasma fog.
After four hundred thousand years the fog finally lifts. This major event occurs at a temperature of three thousand Kelvin when electrons combine with protons to form stable neutral atoms and light no longer ricochets but travels freely in the void of space. This transition is called recombination and as radiation decouples from the particles the weak force of gravity begins to sculpt structure. So the visibly glowing universe is now transparent, but it’s cooled to glow a dull blood-red color. Slowly and steadily the color leeches out of the world as the universe continues to expand and cool. Light has its wavelengths stretched due to the expansion of space. There are still no galaxies and no stars, no planets and no people. All that lies in the far future, and so we enter the Dark Age of the universe
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365 Days of Astronomy
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