Will a Slow Decrease in Dark Energy Density Alter the Fate of the Universe?

[Ranku]

If dark energy density were to be very slowly decreasing, how will the fate of the universe be any different from the energy density being constant? Would not the energy density be completely dominant in the future even if it were to be very slowly decreasing?

 

[Drakkith]

Would not the energy density be completely dominant in the future even if it were to be very slowly decreasing?

I think that may depend on how quickly is decreases. If it decreases slower than the density of normal and dark matter, the outcome is likely to be the same as if it wasn't decreasing at all. But I confess I am not up to speed on all the details, so I could be incorrect.

 

[Chalnoth]

I think that may depend on how quickly is decreases. If it decreases slower than the density of normal and dark matter, the outcome is likely to be the same as if it wasn't decreasing at all. But I confess I am not up to speed on all the details, so I could be incorrect.

The critical question is whether it ends up decreasing faster than $1/a^2$ or not.

If the dark energy at some point starts decreasing faster than $1/a^2$ (it definitely isn't now), then the effect of the spatial curvature on the expansion will grow over time. Eventually the universe will either continue to expand forever or recollapse back on itself dependent upon whether the spatial curvature is slightly positive or slightly negative.

But as long as the dark energy density decreases more slowly than $1/a^2$, it will always dominate over the curvature and the universe will expand forever. It will expand more slowly, and the horizon will be much larger. But it will keep expanding.

 

[Ranku]

But as long as the dark energy density decreases more slowly than $1/a^2$, it will always dominate over the curvature and the universe will expand forever. It will expand more slowly, and the horizon will be much larger. But it will keep expanding.

Will it continue accelerating, even if expanding more slowly?

 

[Chalnoth]

Will it continue accelerating, even if expanding more slowly?

Yes, as long as the energy density reduces more slowly than this, objects within the universe will continue to accelerate away from one another.

 

[Ranku]

Thank you for clarifying.