Physics Blog Number
5
- November 11, 2011
Cosmological constant
With
the
recent Nobel Prize in Physics being given to
Perlmutter et al., (Nobel
page) I’ve tried to understand, as a non-cosmologist,
exactly what is the meaning of L
and how much we know about it. A good
explanation of the supernova work was
in the April 2003 issue of Physics Today, where the following plot
shows the
experimental evidence for the scale factor of the universe as a
function of
time, along with the supernova observations.
The best fit model includes a non-zero cosmological constant with a value of 10-35 s2, which is interestingly small. A better way to see how small it is, is to express it not in units fit for humans (i.e., kg, m, s) but in so-called “natural units,” or Planck units. (See John Baez’s excellent explanation of Planck units here.) In this case it is equal to 3x10-122. This is even smaller! Or, as my friend Tristan Hubsch says, “hilariously teensy!”
Since
the
cosmological constant has the character of a
vacuum energy density in the universe, it is natural to ask what this
energy is
and where it comes from. One attempt to
explain its value can
be
made by the quantum field theory of the standard
model of particle physics, which, unfortunately predicts a value that
is 52 orders
of magnitude too large. (See Rugh and Zinkernagel’s
paper for more details, and
Abbott’s Scientific
American article for an overview.)
My question - a philosophical one - is this. Why do we need to 'explain' the value of the cosmological constant? Certainly it would be nice to understand all phenomena in terms of some simpler underlying theory (the stated goal of 'Theories of Everything'. However, to take an example from classical physics, the value of G (gravitational constant) is what it is. I haven't heard of a need to explain it in terms of anything else. It's just one of the parameters that define our universe. (Of course, there are the TOE-ists who want to explain all the fundamental constants from some underlying theory. And there are those that try to explain all the parameters in terms of the anthropic principle.) At first glance, I don't view the cosmological constant any different than G. They are just two fundamental constants whose value must be measured from observation.