  Arithmetic operations (addition, subtraction, multiplication, division), as
well as the calculus they imply, are non-unique. The examples of
four-dimensional spaces, $\mathbb{R}_+^4$ and $(-L/2,L/2)^4$, are considered
where different types of arithmetic and calculus coexist simultaneously. In all
the examples there exists a non-Diophantine arithmetic that makes the space
globally Minkowskian, and thus the laws of physics are formulated in terms of
the corresponding calculus. However, when one switches to the `natural'
Diophantine arithmetic and calculus, the Minkowskian character of the space is
lost and what one effectively obtains is a Lorentzian manifold. I discuss in
more detail the problem of electromagnetic fields produced by a pointlike
charge. The solution has the standard form when expressed in terms of the
non-Diophantine formalism. When the `natural' formalsm is used, the same
solution looks as if the fields were created by a charge located in an
expanding universe, with nontrivially accelerating expansion. The effect is
clearly visible also in solutions of the Friedman equation with vanishing
cosmological constant. All of this suggests that phenomena attributed to dark
energy may be a manifestation of a miss-match between the arithmetic employed
in mathematical modeling, and the one occurring at the level of natural laws.
Arithmetic is as physical as geometry.
