Abstract.
The 1998 discovery that the universe is accelerating set off an enormous amount of activity in the field of cosmology, both theoretical and observational. The original result, from two groups observing the Hubble diagram of Type Ia supernovae, has since been verified by a variety of independent types of observations. It seems clear that our universe is really accelerating; what remains a mystery is why. The most straightforward explanation for the universe's acceleration is the presence of a dark energy component comprising 70% of the universe. In order to fit the data, dark energy must have two features: it should be smoothly distributed, and its density should be nearly constant as the universe expands. The simplest candidate for dark energy is vacuum energy, equivalent to Einstein's cosmological constant. Simple estimates from quantum field theory indicate that vacuum energy should exist – indeed, in an amount larger than what we observe by a factor of $10^{120}$. This discrepancy, the 'cosmological constant problem', led to a widespread assumption that some mysterious mechanism worked to set the vacuum energy precisely to zero. If the dark energy really is a cosmological constant, we must find a mechanism to suppress its natural value without driving it all the way to vanishing. In the present talk we review some of the basic principles and problems in cosmology, as well as the fundamental problem if dark energy is a modern version of Einstein's cosmological constant, or another form of gravitational/quantum energy that changes with time. Either conclusion is an enigma that points to gaps in our fundamental understanding of gravity.
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發表於 2014-8-11 16:04:43
發表於 2014-8-19 11:15:20