He hopefully means “space” not “space-time”, because space-time includes time and has no further time to do anything, let alone absorb stuff. Let us nevertheless do a sympathetic reading and firstly show what he may mean with a simple example:

Imagine electromagnetic radiation (light) inside a box of volume *V = L*^{3}. Now remove one of the walls and let the radiation spread out into a larger volume, say *V’ *= 8*V*. The energy *U* of the radiation is conserved: *U’* = *U*. However, if you let the radiation pressure move a plunger instead so that at some point the volume is eight times larger, **half** the energy goes into the plunger: *U’* = *U*/**2**.

Just remember: **plunger = taking out half** the energy when expanding volume 8 fold.

**Straw-man:** “Why half the energy?”

**Sock-puppet:** “Consider a column with volume *V* = *L*^{2 }*x* with a plunger of surface *L*^{2} starting at *x* = *L* with energy *U*_{(L)}. Like in the picture, but note that Sascha was too lazy to draw it three dimensional. Moving the plunger slowly reduces the energy via d*U* = - *F* d*x* = - *P L*^{2} d*x* = - (ρ/3) *L*^{2} d*x* = - [*U*/(3 *L*^{2}*x*)] *L*^{2} d*x* = - [*U*/(3*x*)] d*x*. Thus *U* = *Constant * x*^{-1/3} and hence *U* = *U*_{(L)} (*L*/*x*)^{1/3}.

At *V’ *= 8*V* holds *U*_{(8L)} = *U*_{(L)} (*1*/*8*)^{1/3} = *U*_{(L)}/**2**.”

Sock, you scare away readers! Now to cosmology: Consider a cube of volume *V = R*^{3} expanding to a cube twice the length, resulting in the volume being again eight times more: *V’ = *(*2R*)^{3} = 8*V*. Radiation energy density ρ = *U*/*V* is proportional to 1/*R*^{4}, where *R*^{-3} is for the volume and the further factor *R*^{-1} takes care of the redshift, the stretching of the wavelength of the light.

The energy of the radiation is initially *U* = ρ *R*^{3}, but after expanding the universe, it is *U*’ = ρ’ (*2R*)^{3}. However, because of the redshift, it also holds that ρ’ = ρ/*2*^{4}.

And so we get *U*’ = ρ (*2R*)^{3}/*2*^{4} = *U*/**2**, i.e. the expansion takes out **half** the energy again.

In cosmology, space growing is the plunger, the “**space-plunger**”.

I will rip this argument above (and Sean's "solution") apart the next time. Here a more recent post on that energy is not a substance and how to most easily argue that fact.

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