  Physicists usually understand that physics cannot (and should not) derive
that $c\approx 3\cdot 10^8m/s$ and $\hbar \approx 1.054\cdot 10^{-34}kg\cdot
m^2/s$. At the same time they usually believe that physics should derive the
value of the cosmological constant $\Lambda$ and that the solution of the dark
energy problem depends on this value. However, background space in General
Relativity (GR) is only a classical notion while on quantum level symmetry is
defined by a Lie algebra of basic operators. We prove that the theory based on
Poincare Lie algebra is a special degenerate case of the theories based on de
Sitter (dS) or anti-de Sitter (AdS) Lie algebras in the formal limit
$R\to\infty$ where R is the parameter of contraction from the latter algebras
to the former one, and $R$ has nothing to do with the radius of background
space. As a consequence, $R$ is necessarily finite, is fundamental to the same
extent as $c$ and $\hbar$, and a question why $R$ is as is does not arise.
Following our previous publications, we consider a system of two free bodies in
dS quantum mechanics and show that in semiclassical approximation the
cosmological dS acceleration is necessarily nonzero and is the same as in GR if
the radius of dS space equals $R$ and $\Lambda=3/R^2$. This result follows from
basic principles of quantum theory. It has nothing to do with existence or
nonexistence of dark energy and therefore for explaining cosmological
acceleration dark energy is not needed. The result is obtained without using
the notion of dS background space (in particular, its metric and connection)
but simply as a consequence of quantum mechanics based on the dS Lie algebra.
Therefore, $\Lambda$ has a physical meaning only on classical level and the
cosmological constant problem and the dark energy problem do not arise.
