From: Spudboy100@aol.com
Date: Thu Aug 15 2002 - 00:05:07 MDT
http://focus.aps.org/v10/st7.html
Phys. Rev. Lett. 89, 081301
(print issue of 19 August 2002)
12 August 2002
Title and Authors
Deflating Vacuum Energy
Treating the Universe as a hologram might solve one of the biggest problems
bedeviling modern physics. According to quantum field theory, the expansion
rate of the Universe should be accelerating wildly, much faster than it is. A
theorist publishing in the 19 August print issue of PRL points out that the
so-called holographic principle, which has arisen from quantum gravity
research, could resolve this decades-old quandary.
Since 1998, most astrophysicists have been convinced that the Universe's
expansion is accelerating because some "dark energy" in empty space is
pressing outward. One possible explanation is the cosmological constant, or
Lambda, an extra term allowed by the equations of general relativity but not
fixed by them. According to observations, Lambda is less than 1 eV/mm3. But
mix in quantum field theory, which demands the vacuum be awash with energy
from virtual particles, and the predicted Lambda value becomes 10120 times
the observed value.
Now Scott Thomas of Stanford University argues that a principle deduced from
the properties of black holes may bring the quantum prediction in line.
Technically, quantum fields have an infinite number of possible energy
states, all of which should contribute virtual particles to the vacuum.
Theorists normally assume that the number of states is actually finite
because they can't exceed the so-called Planck energy, says Thomas, which
corresponds to the smallest distance quantum mechanics allows.
But black hole physics implies a more drastic reduction. Researchers think
black holes possess entropy proportional to their surface area. They assume
the entropy measures the number of quantum states available to the fields
within. Because a black hole is the highest entropy object that can exist in
a given volume of spacetime, some have argued that the maximum number of
quantum states available in any region of spacetime is also proportional to
its surface area, instead of its volume, as in normal field theory. So
perhaps the ultimate quantum gravity theory is defined on this surface--a
concept called the holographic principle, after the way that 2D holograms
reproduce 3D images. A boundary surface takes up less space than the volume
within it, so "the holographic principle tells you that there are far fewer
quantum states in the theory" than a quantum field theory would suggest, says
Thomas.
Although holography has led a few others to propose solutions to the
cosmological constant problem [1], they introduced extra assumptions, Thomas
says. He estimates that the number and energy of the holographic states
should generate the observed value of Lambda, but there is no holographic
theory for a universe like ours to use for an exact calculation. The only
holographic theories so far constructed describe universes with different
properties than ours.
"It would take a lot to put this on a solid footing," says Sean Carroll of
the University of Chicago, but "certainly it's an important line of
argument." Holography seems to depart from the cherished principle of
locality--the idea that physical effects can only move from one point in
space to a neighboring point--because interactions on the boundary surface
affect observations on the volume within. "We are all now groping towards
just how this departure works, and using holography to understand the
cosmological constant is an important step in this direction."
--JR Minkel
References:
[1] A. Cohen, D. Kaplan, and A. Nelson, Phys. Rev. Lett. 82, 4971 (1999);
P. Horava and D. Minic, Phys. Rev. Lett. 85, 1610 (2000);
T. Banks, http://arXiv.org/abs/hep-th/0007146.
Holography Stabilizes the Vacuum Energy
Scott Thomas
Phys. Rev. Lett. 89, 081301
(print issue of 19 August 2002)
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