Thursday, April 14, 2005

Theories of Cosmic Acceleration

I fly out to Tampa early in the morning to give an invited talk on Theories of Cosmic Acceleration in the Cosmological Constraints on Theories of Gravity and Fundamental Physics session at the April meeting of the American Physical Society. Sean (of Preposterous Universe note) is convening this session, and it'll be nice to see him and possibly even make some progress on our ongoing projects.

I'll be discussing a variety of ideas about what could be driving the recently observed acceleration of the universe. There are three broad classes of approaches. The first is that cosmic acceleration may be due to a pure cosmological constant. The second assumes that the true vacuum energy of the universe vanishes, and that the dynamics of some exotic matter component, such as a scalar field, might be driving acceleration. This approach is like having a version of cosmic inflation happen in the late universe, and goes by the name quintessence. The final approach is again to set the vacuum energy to zero, and then to have a long-range modification of General Relativity be responsible for cosmic acceleration.

At various times I've worked on aspects of each of these approaches, but most recently the modified gravity one. Sean Carroll, Michael Turner, Vikram Duvvuri and I had our own idea of how to modify gravity to make cosmic acceleration happen (we later elaborated on it with my student Antonio De Felice and my postdoc Damien Easson). We considered adding inverse curvature terms to the Einstein-Hilbert action so that when all matter dilutes away, the universe cannot approach flat space-time. Our initial model was constructed to affect the universe on large scales (the Hubble size). However, gravity is a tricky topic and it turns out that there is also an unavoidable effect at smaller, solar-system scales, which leads to critical constraints on our model. One such constraint arises from precise measurements of the timing of communications signals from the Cassini mission (man those robotic missions do some good science!). There have been some suggestions that modifications of our models (using more complicated versions of what we did, while trying to retain the main idea) can evade the constraints. However, I actually don't see how these can be correct and, at present, I don't know how to overcome the obstacles. My collaborators and I are fine with that though - this is how science is meant to work.

There are some other suggestions of how to modify gravity to obtain late-time cosmic acceleration. One belongs to the fiendishly inventive Gia Dvali and collaborators, who have an extra-dimensional brane-world model, known as DGP gravity, in which gravity becomes five dimensional at very large distances. This model can also exhibit shorter-range effects in addition to the large scale ones that provide cosmic acceleration. However, my understanding is that these models evade current bounds, but may be testable with precision tests of gravity in the near future. This has been worked on by, among others, my good friends Glenn Starkman and Arthur Lue.

I'll be discussing all these ideas, and more, in my APS talk, assuming that I finish writing it in time for the Saturday afternoon session. It's going to be a busy flight tomorrow.
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