© NASA, CXC, M. Weiss.
So far, our measurements of the cosmic equation of state are consistent with dark energy that behaves just like the cosmological constant. However, more precise measurements could reveal some variation and a different fate for our universe. The figure above shows various possibilities. If the dark energy is constant, the scale of the universe will continue to grow along the path defined by the red line. We now know that we are not headed for either the big rip or big crunch indicated by the blue and green lines, respectively, and that our fate lies between those two lines. While there is a lot of space between those lines in the future, at the present time they are close together. Detecting any deviation from a cosmological constant will require precision measurements. Any deviation from a constant would be very interesting. (Unit: 11)