TITLE: Alternative to Dark Energy
QUESTION [0 upvotes]: If Black Holes consume mass and re radiate it in the form of massless photons the mass of the universe is declining.  Could this explain the accelerating expansion of the universe without the necessity of invoking Dark Energy?

REPLY [0 votes]: This won't work. Make sure you understand what dark energy is.

In standard cosmology, there are three components of the universe: matter, radiation, and dark energy. Matter is anything whose energy density scales with the inverse cube of the scale factor, i.e., $\rho \propto a^{-3}$, while radiation is anything which scales to the inverse fourth power of the scale factor ($\rho \propto a^{-4}$). This can be understood intuitively: for an ordinary particle in a cube-shaped box, doubling the length of an edge of the box decreases the density (and hence energy density) by a factor of eight ($2^3$). For radiation, the decrease in energy density is greater, because an increase in spatial distance also causes a redshift.
The final component, dark energy, is an intrinsic property of space, and so has a constant energy density regardless of the volume under consideration ($\rho \propto a^0$). Thus, unlike ordinary matter, it does not get diluted with the expansion of space.

Photons are radiation - their energy density scales to the inverse fourth power of the scale factor. Dark energy is fundamentally different from photons, and cannot be made up of it.
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