  We show that Cosmological Constant (CC) is not optional in GR (General
Relativity) because it is required by SR (Special Relativity). This completely
unexpected result is obtained by introducing a minimal acceleration (Milgrom),
square root of CC, in Einstein boost with Lorentz Transformation (LT). We prove
that hyperbolic rotation (LT) is an hyperbolic motion with a centrifugal
acceleration. In SR with CC (CSR or CR), the universe is not only in expansion
(with the law of Hubble) but even in accelerated expansion (cosmological
parameter is minus one). Given that the structure of space-time in Einstein's
GR is determined by the presence of matter and CC is associated to the absence
of matter, we associate CC not to "quantum" vacuum but to classical "vacuum" of
Minkowski's space-time (with a renormalization of Minkowski's metric). Finally
we show that 1917 Einstein's CC corresponds to 1906 Poincare's
non-electromagnetic negative pressure.
