  The nature of dark matter and of dark energy which constitute more than
$95\%$ of the energy in the Universe remains a great and unresolved question in
cosmology. Cold dark matter can be made of an ultralight scalar field dominated
by its mass term which interacts only gravitationally. The cosmological
constant introduced to explain the recent acceleration of the Universe
expansion can be easily replaced by a scalar field dominated by its potential.
More generally, scalar fields are ubiquitous in cosmology: inflaton, dilatons,
moduli, quintessence, fuzzy dark matter, dark fluid, etc. are some examples.
One can wonder whether all these scalar fields are independent. The dark fluid
model aims at unifying quintessence and fuzzy dark matter models with a unique
scalar field. One step futher is to unify the dark fluid model with inflation.
In the very early Universe such scalar fields are not strongly constrained by
direct observations, but Big-Bang nucleosynthesis set constraints on scalar
field models which lead to a modification on the abundance of the elements. In
this talk we will present a scalar field model unifying dark matter, dark
energy and inflation, and study constraints from Big-Bang nucleosynthesis on
primordial scalar fields.
