Quantum vacuum fluctuations and gravity Stephen Hawking predicted Hawking radiation by studying what effect gravity would have on quantum vacuum fluctuations near the event horizon of a black hole. I wanted to ask, does gravity really affect quantum vacuum fluctuations? Have scientists ever observed this or have they done any experiments on it? If they haven't, what are your thoughts on it?
 A: To this day, no experiments have been conducted to directly test whether Hawking radiation — or the related Unruh effect — does exist. Both effects are extremely subtle and would require quite extreme conditions to be observed experimentally. The Hawking temperature of a Schwarzschild black hole (in short, a black hole that is not charged and does not spin) is roughly
$$T_H \approx 6 \cdot 10^{-8} \textrm{K} \times \frac{M_{\text{Sun}}}{M}.$$
Since the temperature of the Cosmic Microwave Background is roughly $3 \textrm{K}$, this means usual black holes are colder than the Universe's average temperature, and hence it is pretty difficult to observe the Hawking effect in usual conditions.
However, there are reasons to believe in both predictions. I'll point out two of them.
Acoustic Analogues of Black Holes
While making experiments with actual black holes is astonishingly difficult, there is an approach which tries to make sense of some notions of gravitational Physics by studying analogue systems. For example, it can be shown that some models of fluid flow can be described quite well in terms of Lorentzian Geometry, so that they can be studied as if they were problems in General Relativity. A good review on these methods is, for example, arXiv: gr-qc/0505065.
In this framework, one may for example consider a spherically symmetric flow where the fluid is moving inwards to the center of the coordinate system. Think for example of a sink. If on a certain region the fluid can be made to flow faster than the speed of sound on the fluid itself, then no sound wave can escape from that supersonic region. We call it a sonic black hole (also known as an acoustic black hole or a dumb hole). One can then quantize the sound waves to describe phonons moving on the fluid. Since the fluid can be described as if it were a relativistic spacetime, the phonons can be described using Quantum Field Theory in Curved Spacetimes. Working out the very same computations used to derive the Hawking effect for usual black holes, one then gets an analogue prediction for acoustic black holes.
Acoustic black holes have been successfully produced in laboratory and recently a successful test of analogue Hawking radiation has been conducted (arXiv: 1809.00913 [gr-qc]). While this is not a direct test, it certainly improves on the confidence of the prediction.
Unruh Effect
While, as far as I know, the Unruh effect also has not been directly verified by experiment, there are also reasons to believe on its existence. Namely, consistency checks with the proton decay as predicted by standard Quantum Field Theory lead to the conclusion that "the Fulling–Davies–Unruh effect is mandatory" (arXiv: gr-qc/0205078). Furthermore, there are proposals for observing the Unruh effect, such as arXiv: 1701.03446 [gr-qc], which relates the Larmor radiation emitted by an accelerated charge as seen from an inertial reference frame to the interaction of the charge with the Unruh bath on the charge's accelerated reference frame. The latter proposal also allows for one to predict the outcome of the experiment using standard Classical Electrodynamics, which leads to the same prediction as the Unruh effect, further strengthening the belief in the conclusions taken from QFTCS.
In summary, at the present moment there is no direct evidence of the Hawking effect that I know of, but we do have indirect evidence by mean of consistency checks of the predictions of QFTCS and other well-established theories (QFT and Classical Electrodynamics) and by mean of experimental observation in analogue models.
