I Sachs-Wolfe effect in a Dark Energy universe?

AI Thread Summary
The integrated Sachs-Wolfe effect involves photons gaining energy (blueshift) when entering and losing energy (redshift) when exiting a gravitational well due to spacetime expansion, typically influenced by dark energy. In a hypothetical universe with only the Milky Way and dominated by dark energy, the Sachs-Wolfe effect would not be observed, as there would be no other gravitational wells to interact with. While dark energy contributes to accelerated expansion, it is not necessary for the integrated Sachs-Wolfe effect to exist; spatial density variations are sufficient. If only one gravity well exists, photons would only experience a minor blueshift as they fall into it. Thus, the presence of multiple gravitational structures is crucial for observing the Sachs-Wolfe effect.
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TL;DR Summary
Sachs-Wolfe effect in Dark Energy universe?
The integrated Sachs-Wolfe effect occurs when a photon goes through a gravitational potential that changes due to spacetime expansion (presumably caused by dark energy). For that reason, a photon going through a gravitational well would gain energy (blueshift) when entering and it would lose energy (redshift) when exiting it. If the universe expands, the well becomes "less deep" and therefore, the photon gets more energy and becomes blueshifted.

However, would this effect still occur if we had a photon traveling in a universe with only one galaxy (the Milky Way) and the rest of it dominated by Dark Energy (that is, the Universe once the expansion separates all non-gravitationally bounded structures to us)?
 
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Suekdccia said:
The integrated Sachs-Wolfe effect occurs when a photon goes through a gravitational potential that changes due to spacetime expansion
Yes.

Suekdccia said:
(presumably caused by dark energy)
No. Dark energy is not the cause of expansion. It's only the cause of accelerated expansion. But you don't need accelerated expansion to have a nonzero integrated Sachs-Wolfe effect. All you need is some spatial variation in density.

Suekdccia said:
would this effect still occur if we had a photon traveling in a universe with only one galaxy (the Milky Way) and the rest of it dominated by Dark Energy (that is, the Universe once the expansion separates all non-gravitationally bounded structures to us)?
If you postulate that there are no other gravity wells except the one we are in, then no, we would no longer observe any Sachs-Wolfe effect, just a (small) blueshift from photons falling into our gravity well.
 
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