Dynamical dark energy ( DE ) has been proposed to explain various aspects of the cosmological constant ( CC ) problem ( s ) .
For example , it is very difficult to accept that a strictly constant \Lambda -term constitutes the ultimate explanation for the DE in our Universe .
It is also hard to acquiesce in the idea that we accidentally happen to live in an epoch where the CC contributes an energy density value \rho _ { \Lambda } = \Lambda / 8 \pi G right in the ballpark of the rapidly diluting matter density \rho _ { m } \sim 1 / a ^ { 3 } .
It should perhaps be more plausible to conceive that the vacuum energy , \rho _ { \Lambda } , is actually a dynamical quantity as the Universe itself .
More generally , we could even entertain the possibility that the total DE is in fact a mixture of \rho _ { \Lambda } and other dynamical components ( e.g .
fields , higher order terms in the effective action etc ) which can be represented collectively by an effective entity X ( dubbed the “ cosmon ” ) .
The “ cosmon ” , therefore , acts as a dynamical DE component different from the vacuum energy .
While it can actually behave phantom-like by itself , the overall DE fluid may effectively appear as standard quintessence , or even mimic at present an almost exact CC behavior .
Thanks to the versatility of such cosmic fluid we can show that a composite DE system of this sort ( “ \Lambda XCDM ” ) may have a key to resolving the mysterious coincidence problem .