Assisted dark energy

Cosmological scaling solutions, which give rise to a scalar-field density proportional to a background fluid density during radiation and matter eras, are attractive to alleviate the energy scale problem of dark energy. In the presence of multiple scalar fields, the scaling solution can exit to the epoch of cosmic acceleration through the so-called assisted inflation mechanism. We study cosmological dynamics of a multifield system in details with a general Lagrangian density p=(n)(i=1)X(i)g(X(i)e(i)(lambda)phi(i)), where X-i=-(del phi(i))(2)/2 is the kinetic energy of the ith field phi(i), lambda(i) is a constant, and g is an arbitrary function in terms of Y-i=X(i)e(i)(lambda)phi(i). This covers most of the scalar-field models of dark energy proposed in literature that possess scaling solutions. Using the bound coming from big-bang nucleosynthesis and the condition under which each field cannot drive inflation as a single component of the universe, we find the following features: (i) a transient or eternal cosmic acceleration can be realized after the scaling matter era, (ii) a thawing property of assisting scalar fields is crucial to determine the evolution of the field equation of state w(phi), and (iii) the field equation of state today can be consistent with the observational bound w(phi)<-0.8 in the presence of multiple scalar fields.