Podcaster:  Chris Impey

Title: Last Light

Organization:  University of Arizona

Link :

Description:This podcast looks at the time when light disappears from the universe and it goes dark forever. We are currently in the latter stages of the stellar age of the universe, where new stars and galaxies are forming at a diminishing rate. Eventually the cycle of start birth and death will be broken as mass is trapped in dark stellar remnants and no gas is left to form new stars. At that point the universe will slowly fade to black, taking about ten trillion years to go completely dark. Meanwhile, all the other galaxies will have disappeared from view due to the accelerating expansion.

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 Last Light. I’m University Distinguished Professor at the University of Arizona working at Steward Observatory in the astronomy department. I do research on supermassive black holes and observational cosmology. I also have a strong interest in educational technology, and create websites, and also write popular books on cosmology and astrobiology.

We’re celebrating the International Year of Light, which was announced by the U.N. for the year 2015 to raise the awareness of the importance of light and light technologies. Astronomers don’t need any reminding of the importance of light, but I want to talk about light in a cosmological context. In the first of this set of three podcasts, I talked about the universe very early on, when it glowed in visible light; this was essentially the first time that visible light had any meaning in the infant cosmos. In the second podcast, I talked about a time somewhat later, a hundred million years after the big bang, when the universe had cooled and gravity had had enough time to form the first collapsed objects: stars and galaxies. That switching on of the first lightbulbs in the universe – stars and galaxies – was a momentous time which continues to the present day as stars are forming and evolving all around us in the Milky Way and in a hundred billion other galaxies in the universe. There was a long dark ages that came between these two epochs, when the lights went out because the universe cooled and darkened.

The last part of our story is about a more ominous form of the dark ages. A dark age that will essentially last forever. I don’t want to make this a gloomy podcast, but this part of the story is about the end of light in the universe as a whole. Prediction of this event is based on models of stars in the expanding universe. It’s inevitably uncertain at some level. And none of us or our descendants will be here to see what happens, so we need not take it to heart.

Let’s fast-forward through the previous thirteen billion years of stellar and galactic evolution and project into the future. After the end of the dark ages when the universe was ten times smaller and a thousand times denser than it is now, stars formed readily, galaxies merged to form larger entities and there was plenty of gas left around to make new stars and galaxies. Star birth and death is a cycle where stars are born, live their lives and die, recycling a third to a half of their gas back into the interstellar medium to become part of a new generation of stars. But over time, this process must end.

The overall peak of stellar formation and activity in the universe occurred about eight billion years ago and we’re on a declining slope with the glory days behind us. But the activity must one day cease, since more and more gas over time will be locked into stellar remnants and therefore be unavailable to recycle into the interstellar medium and be part of a new generation of stars. In addition, the universe is steadily expanding and thinning out and so the gas that is between galaxies and falls onto galaxies will diminish over time. Both of these trends point towards the end of the stellar era at some point in the future.

Our star, the Sun, will be gone in about five billion years, but the lowest mass stars last much longer. Due to dark energy, within ten to twenty billion years from now will be no more galaxies to see as they all recede beyond our horizon carried by the accelerating expansion of space-time. We’ll be left with the merger product of our galaxy and M31, Andromeda, joined into one huge stellar beast. Within this super-galaxy formed from M31 and our galaxy, stars will live their lives. White dwarfs will be the last cooling embers of low mass stars, but the very lowest mass stars operate stellar fusion at a miserly rate. These red dwarfs will eke out the last of their nuclear fuel over enormous time spans of hundreds of billions, and perhaps trillions of years.

The last light in the universe will be the glow from the galaxy, a dull red glow made up of the sum of hundreds of billions of red dwarfs; a virtual echo of the red glow when the fog lifted over thirteen billion years ago. If we can imagine the scene in our universe, ten trillion years from now, it’s almost perfectly dark. All the stars have lived their lives and exhausted all their nuclear fuel and there’s no new fuel available.

The last fizz – a punctuation point on the evolution of the universe – comes as mass transfer between low mass stars in a binary system kicks one of them into a last little burst of fusion. Don’t be sad if you imagine that the darkness of the universe is the end of all life; that’s not necessarily true. Although life on Earth depends on our star, all life really needs is an energy source, and gravitational energy provides plenty of power for life in the universe, even if it does eventually go dark.

End of podcast:
365 Days of Astronomy

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