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I'm studying Cosmology Lectures of Prof. Leonard Susskind (no GR involved at the moment). Vacuum energy density was introduced as $\rho_0$ and related to the Cosmological Constant $\Lambda$. I know this is also connected to Quantum Mechanics results, which does not particularly cease to amaze me.

I understand that this term of energy when modeling the Universe has an attractive effect (or a negative pressure) that is trying to "hold" Universe's expansion. I commented this to my younger brother and he immediately asked: "Ok. but, can it be used?".

Question: What would you answer to that? Cosmological effects can be seen (indirectly) from Hubble law at large scales, which is not something you could measure on Earth, to say something. Anyways, this energy term actually exerts pressure and does work so, only for pedagogical (and self-learning) purposes, could you think of an experiment (not necessarily one that could be done) in which Vacuum Energy could be related or converted to any other form of energy, in a way it can be understood for someone who is not familiarized with the expansion of the Universe? In other words: how would you attempt to explain Vacuum Energy more palpably, not necessarily going through the mathematics of it?

Might be related: Vacuum energy conversion to heat

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The vacuum energy does have an observable effect on the Universe, in that it drives the acceleration of the Universe's expansion.

However, we can't use it to do anything, assuming the vacuum energy is due to a cosmological constant, which is the current best model. You can think of it as an intrinsic, fixed energy density associated with empty space. Because it is constant, we can't convert the energy into a different form. Therefore we can't use vacuum energy to do anything.

There are other models of what the vacuum energy could be, where the energy is really due to some dynamical field. If that were the case, then in principle we could try to interact with this field to extract the energy. But, (a) there is no evidence in support of there being a field as opposed to a constant energy, and (b) if there is such a field, at the moment we don't know how we could interact with it directly, except that the interaction strength must be very small. So even in this very speculative scenario, it would be very hard to interact with it at all, much less harness it to do something.

Even if we could, the amount of energy density is quite small -- about $10^{-9}\ {\rm J}$ per cubic meter -- so you would need to perfectly extract all the energy in a cube $1\ {\rm km}$ on each side in order to have enough energy to raise a pineapple by one meter.

Having said that, there is a closely related phenomenon called the Casimir effect, which is not the energy of empty space, but an energy between surfaces that arises due to quantum fluctuations. In the simplest case of two parallel, conducting plates, it leads to an attractive force. At least theoretically, this could be useful in nanotechnology; see https://doi.org/10.1515/nanoph-2020-0425

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