What is the nature of inflatons being the particles of the inflaton field?

It is thought that the inflationary expansion of the universe is driven by an inflaton field. A scalar field with negative energy. Today, the observed accelerated expansion of the universe is thought to be caused by dark energy. It’s an energy of spacetime itself, which is thought to be caused by the virtual sea of particles and anti-particles. But what is the nature of the inflatons? They are no fluctuations like the virtual particles-anti particles. They are scalar particles with negative energy. Are they massive? Are they necessary for spacetime to expand (can spacetime have negative energy without anything in it?)? And how can they decay into positive energy particles? I know these are three questions but they are closely related.

A comment made below made me think. If it’s not the energy density that is negative but the pressure density, what does this mean? Energy and mass are represented by the time-like component of the stress-energy tensor $T_{munu}$. that is $T_{00}$. The energy and (four)momentum of a particle are connected by $E^2=m^2+p^2$. If $E^2$ is positive and the pressure is negative, does this mean that mass is imaginary (we can’t imagine indeed what mass would be like!)? If the mass is zero, doesn’t this mean that $E^2$ (in the case of negative pressure) is negative too (contrary to, say, photons)?

Now, in defining the stress-energy tensor for the inflaton field, the squareroot of the negative of the determinant of the metric is involved: $sqrt(-det g)$. Now. the determinant of a metric corresponds to a volume. But what is a negative volume? Of course you can take the negative of a positive number, but what does it mean in this case?