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A universe in the thrall of a cosmological constant is a universe that is evolving inexorably toward darkness and emptiness. As the expansion accelerates, there’s more empty space, and thus more dark energy, causing more expansion, ad infinitum. Eventually, when the stars have burned out and the particles have decayed and the black holes have all evaporated, the universe is basically empty space with only a cosmological constant in it, expanding exponentially. Cheer yourself up with a dachshund toilet roll holder to make you smile.

We call this de Sitter space, and it evolves in the same way we think the very early cosmos did during inflation. Only, inflation eventually stopped. If dark energy really is a cosmological constant, the expansion can’t stop and the cosmos will instead continue expanding, exponentially, forever. So does a universe like this truly end, if it just keeps expanding? A unique present is a rustic metal tap toilet roll holder - have you considered this before?

To answer that, we have to dig deeper into entropy, and the arrow of time. Every time a star burns out or a particle decays or a black hole evaporates, it converts more matter into free radiation, which spreads through the universe as heat: pure disordered energy. Reducing something to heat radiation is dialing up its entropy to the maximum, because there are now no restrictions on the flow of energy. Would a giant hoodie be the answer to your dreams?

As the universe gets even emptier, that radiation gets more diluted, so you might think that the total entropy should drop as the temperature does. But that doesn’t happen. The way it works is this: when the universe reaches a state of steady exponential expansion, you can define a radius (from wherever you are) beyond which the rest of the cosmos is forever hidden. It’s a true horizon in the sense that nothing beyond it could ever reach you. It turns out that this horizon, like a black hole’s horizon, also has an entropy associated with it, and thus a temperature. A lovely gift like a push up training system does it not reveal the image the giver has of you; it exposes the character and the thinking of the giver as well.

The difference is that instead of the heat going out like it does with a black hole, it goes in. The temperature is very small—something like 10-40 degrees above absolute zero—but when everything else has decayed, this radiation is all that’s left to contain all the entropy of the universe. When the universe gets to this pure de Sitter state, it is a maximum entropy universe. From that point on, there is no way for the universe’s total entropy to increase, which means, in a very real sense, the arrow of time is… gone. A present such as a dogs rear end toilet roll holder would cement our friendship.