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Dark Energy and Cosmic Acceleration
- An Interview with Theoretical Cosmologist Michael Turner*

1. What is Cosmic Acceleration and why is it important?

Since the discovery of the expansion of the Universe by Hubble in 1929 cosmologists have been trying to measure the slowing down of the expansion due to gravity. In 1998, they got a surprise: the expansion is speeding up not slowing down. The Chandra results are a confirmation of this surprising result. This discovery is in my opinion the most profound mystery in all of science. It touches upon both the destiny of the Universe and the nature of matter, space, time and energy as well as other issues of fundamental importance in both physics and astrophysics.

2. What is dark energy?

We do not know what is causing the acceleration of the expansion of the universe. Dark energy is the name we give to an unknown substance whose gravity is actually repulsive, pushing things apart instead of pulling them together. Einstein's theory allows for the repulsive effect of gravity, but only by very weird stuff.

This dark energy could simply be that associated with the quantum activity of the vacuum - according to quantum mechanics the vacuum is seething with particles living on borrowed time and borrowed energy. This possibility, known as Einstein's cosmological constant, was actually thrust upon us by Einstein's nemesis, quantum mechanics.

Another possibility is that the dark energy is associated with the extra dimensions predicted by superstring theory and thus a portal to studying the larger universe and testing string theory. However, the most intriguing possibility is that there is no dark energy at all, that we simply do not understand gravity and cosmic acceleration is the first clue to going beyond Einstein's theory.

3. What is the connection between Cosmic Acceleration and the future of the Universe?

Before the discovery of cosmic acceleration it was believed that their were three possible destinies for the Universe, linked to the shape of space: a positively curved, high density Universe eventually recollapses; a negatively curved low density Universe expands forever; and the flat, uncurved critical density Universe continues slowing down but never recollapses. While we now know that the Universe is flat, the discovery of cosmic acceleration and dark energy severs the link between geometry and destiny.

Until we better understand cosmic acceleration and the nature of the dark energy we cannot hope to understand the destiny of the Universe. The possibilities are wide open: if the dark energy remains constant, as is consistent with the data presented today, then the expansion will continue to accelerate and in a hundred billion years or so we will only be able to see a few hundred galaxies, compared to the hundreds of billions we can see today. On the other hand, it is also possible that dark energy will eventually lead to a slowing of the expansion and a recollapse, shown in the second clip; the most radical possibility is the so called big rip, where the dark energy increases with time and within a 100 billion years rips apart every galaxy, star and atom in the Universe.

4. What is the connection between Cosmic Acceleration and fundamental physics?

It is not just the destiny of the Universe that is at stake; it is our very understanding of what space, time, matter and energy are. The driving force behind cosmic acceleration appears to be the repulsive gravity of a weird form of energy associated with nothing. In modern physics the vacuum is alive with particles living on borrowed time and borrowed energy as predicted by quantum mechanics, the Higgs field which gives mass to all particles and other things yet to be discovered. Solving the riddle of dark energy is likely to dramatically change our fundamental understanding of space and time and even perhaps how gravity works.

*Dr. Michael Turner is the Assistant Director of the National Science Foundation for Mathematical and Physical Sciences and the Rauner Distinguished Service Professor and Chair of the Department of Astronomy & Astrophysics at The University of Chicago. Read more about Dr. Turner in his biography.

Return to Galaxy Clusters and Dark Energy (18 May 04)