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