In Section (1.3) we discussed the large
difference between the magnitude of the
vacuum energy expected from zero-point
fluctuations and scalar potentials, _{}^{theor}
~ 2 x 10^{110} erg/cm^{3},
and the value we apparently observe,
_{}^{(obs)} ~ 2 x
10^{-10} erg/cm^{3}
(which may be thought of as an upper limit, if we wish
to be careful). It is somewhat unfair to characterize this
discrepancy as a factor of 10^{120}, since energy density
can be expressed as a mass scale to the fourth power.
Writing _{} =
*M*_{vac}^{4},
we find *M*_{vac}^{(theory)} ~ *M*_{Pl}
~ 10^{18} GeV and
*M*_{vac}^{(obs)} ~ 10^{-3} eV, so a more fair
characterization of the problem would be

Of course, thirty orders of magnitude still constitutes a difference worthy of our attention.

Although the mechanism which suppresses the naive value of the vacuum energy is unknown, it seems easier to imagine a hypothetical scenario which makes it exactly zero than one which sets it to just the right value to be observable today. (Keeping in mind that it is the zero-temperature, late-time vacuum energy which we want to be small; it is expected to change at phase transitions, and a large value in the early universe is a necessary component of inflationary universe scenarios [21, 22, 23]. If the recent observations pointing toward a cosmological constant of astrophysically relevant magnitude are confirmed, we will be faced with the challenge of explaining not only why the vacuum energy is smaller than expected, but also why it has the specific nonzero value it does.