What is remarkable about the evidence for dark energy is how perfectly uniform it is. There is no evidence that there’s more or less dark energy in the space occupied by rich galaxy clusters than in the voids of empty space. There is no evidence that dark energy correlates with density, direction, location, or epoch of the universe. It appears to be perfectly uniform, perfectly homogeneous, and perfectly constant: unchanging throughout space and time. And yet, despite its simplicity, it behaves fundamentally differently from all other known forms of energy.
As the volume of the universe increases — as it expands — the energy density does not change; it remains constant. It’s as though there is something present through all of space that isn’t dependent on anything else: matter density, radiation density, temperature, changes in volume, etc. Although we can measure and quantify its effects on the universe, we cannot say that we understand dark energy’s nature.Ethan Siegel, “Dark energy might be neither particle nor field” at Big Think (September 21, 2021)
Then what can we say?
It is time to take seriously the idea that dark energy might simply be a property inherent to the very fabric of space. Until we learn how to calculate the zero-point energy of empty space itself, or gain some bizarre, surprising, and unanticipated evidence, this will remain one of the biggest existential questions in all the universe.Ethan Siegel, “Dark energy might be neither particle nor field” at Big Think (September 21, 2021)
So this is existentialism for physicists, right?
First things first, what is dark energy? Dark energy is what causes the expansion of the universe to accelerate. It’s not only that astrophysicists think the universe expands, but that the expansion is actually getting faster. And, here’s the important thing, matter alone cannot do that. If there was only matter in the universe, the expansion would slow down. To make the expansion of the universe accelerate, it takes negative pressure, and neither normal matter nor dark matter has negative pressure – but dark energy has it.
We do not actually know that dark energy is really made of anything, so interpreting this pressure in the normal way as by particles bumping into each other may be misleading. This negative pressure is really just something that we write down mathematically and that fits to the observations. It is similarly misleading to call dark energy “dark”, because “dark” suggests that it swallows light like, say, black holes do. But neither dark matter nor dark energy is actually dark in this sense. Instead, light just passes through them, so they are really transparent and not dark.Sabine Hossenfelder, “What is dark energy?” at BackRe(Action)
Even Hossenfelder sounds sort of existential on this one.
See also: Rob Sheldon: Are “multiple measurements ”closing in on dark energy? Nope.
Researchers: Either dark energy or string theory is wrong. Or both are. But dark energy is so glitzy! Isn’t it a line of cosmetics already?
Researchers: The symmetrons needed to explain dark energy were not found
Rob Sheldon: Has dark energy finally been found? In pop science mags?
Are recent dark energy findings a blow for multiverse theory?
Science at sunset: Dark energy might make a multiverse hospitable to life… if it exists
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