Podcaster: Chris Impey
Organization: University of Arizona
Description: This podcast examines first light in the universe. After the fiery heat of the big bang the universe expanded and cooled until the radiation dimmed to infrared waves. For about 100 million years, gravity gradually concentrates matter until pockets collapse within the overall expansion. This is “first light” and the stars that formed back then generated the first heavy elements in the cosmos. Stars aggregate into galaxies and for several billion years the universe undertakes an ambitious construction project, as galaxies like the Milky Way and put together from smaller pieces. Detecting first light is one of the goals of the James Webb space Telescope.
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 called First Light. I ‘m a University Distinguished Professor at the University of Arizona. My research is on the coevolution or parallel evolution of supermassive black holes and the galaxies they contain. I also like to work on educational technology, and in particular teaching massive online classes. At the moment I have a class of thirty four thousand students with Udemy and another of sixteen thousand students with Coursera and I hope some of you can check out these online courses. They’re free.
This year we’re celebrating International Year of Light with the U.N. raising awareness of the important of light and light technologies. I want to talk about light in the context of cosmology. In the first of this set of three podcast I will talk about the first time light was present in the history of the universe, when the entire universe was very small and very young and essentially glowed like a lightbulb.
The hot big bang describes an origin that was an iota of space-time 13.8 billion years ago, that would over time expand into the cold majestic universe we have now with a hundred billion galaxies spread over dimensions of forty six billion light-years in any direction. We pick up the story half a million years after the Big Bang, The universe has faded from view as the radiation present from the creation, now seen as the cosmic microwave background, has had its wavelength stretched by the expansion from intense gamma rays in the first few minutes to feeble red waves at the edge of the visible spectrum.
The universe is now in a time called the Dark Ages. Everything is almost perfectly smooth, matter and radiation not varying by more than a tiny fraction of a percent in any direction, or at any location. The radiation from this time, which we now see 13 billion years later to be stretched by a factor of a thousand into microwaves, is uniform in all directions in the sky. There is hydrogen and helium created in the first few minutes with a smattering of lithium and deuterium, but no heavier elements. The universe is a very simple place. As the universe silently expands it’s dark and quiet. There’s nothing going on and nothing to see, even if there were people there to see it and telescopes there to observe it. But gravity is exerting its essentially infinite reach to slowly amplify the tiny primordial variations in density. They were imprinted at the era of inflation in the first tiny fractions of a second.
Inexorably, matter migrates to slightly higher density regions and vacates the regions that are slightly lower than the average. This process continues. It’s linear and it’s very slow, so it takes tens of millions of years. After roughly a hundred million years or more the universe is now exceptionally cold, about thirty degrees above absolute zero. Then, in some of the densest spots of this expanding universe, a runaway process starts to occur. It happens when the density excess in a region exceeds the mean density by a factor of two. At this point gravity starts to pull mass in, which pulls more mass in and so on, in an accelerating process. This is called gravitational collapse. It happens in many places at once in the infant universe.
The result is something called First Light. In a blinding set of explosions, massive stars form and almost immediately detonate, polluting the universe with carbon and other heavy elements for the first time. These stars, called Population III stars, are the very first population of stars in the universe. We don’t know exactly how they work, but models suggest that very massive stars can form from this initial collapsing phase, as much as two hundred times the mass of the sun. They form out of pure hydrogen and helium, but they create by explosive detonation of nucleosynthesis all the elements of the periodic table, and in particular carbon, nitrogen and oxygen, the biogenic elements. Life would not be possible before 400 million years after the big bang because there were no heavy elements on which biology depends.
So the universe continues in this way, with a torrent of stellar detonations, as massive stars form and quickly die and send their heavy elements out into space. Meanwhile, lower mass stars are also forming and the smallest of them are still eking out their lives 13 billion years later as red dwarfs. The massive stars live lives that are less than a million years and so there many cycles of star birth and death in this early phase. They leave behind black holes and neutron stars, the first collapsed objects in the universe. Telescopes can’t quite reach back to First light. The Hubble Space Telescope came close, but it will take the James Webb space Telescope to see this important epoch in the history of the universe.
In parallel with this process of star formation, star clusters are forming as well because gravity works over large scales as well as small scales. Dwarf galaxies form and stars aggregate within regions of space defined by dwarf galaxies, then the dwarf galaxies aggregate to form galaxies like the Milky Way. Galaxy-building is an intense activity for the next five billion years. The universe has entered the era of stars and galaxies, the so-called “Stelliferous” era, and we’re on the fading edge of that glorious time thirteen billion years later.
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365 Days of Astronomy
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