<p>I'm neither a quantum physicist, nor a cosmologist, so I'm going only from my laymans understanding.</p>

<p>The   framework for constructing theories to explain everything in the universe <em>except gravity</em> which has proved most popular and useful for the last 50 or so years, is <a href="https://en.wikipedia.org/wiki/Quantum_field_theory" rel="nofollow noreferrer">quantum field theory</a>. In this theory, there are multiple fields (conceptually similar to the electric and magnetic fields) suffusing every point in space and time. From the values of these fields, we can,  in principle, compute the probabilities of the outcomes of any experiment, including the probability of detecting a particular kind of particle at a particular place and time, and from the rules of these computations we can derive all the normal "laws of physics" such as the conservation of energy, the types of particles which exist and so on, as "emergent" properties. </p>

<p>I described QFT as a "framework" because there are many possible sets of fields and rules for their interactions, each of which gives different physics. From the experiments we can do, we can develop some constraints on the overall structure, but we also know that the fields and interactions that we know about cannot be the whole story. Over short distance and time scales and large energy scales they would give rise to non-sensical predictions, so they must be wrong, but we cannot yet do experiments to find out what really happens.</p>

<p>Anyway, the vacuum is the configuration of those fields which has the lowest possible energy, but that does not have all the fields being zero, and when we calculate that minimal energy (as far we can) using only the known particles and fields we get a ridiculously high value (that's one of the non-sensical predictions I mentioned). This is not taken as a sign that the vacuum energy <em>is</em> that high, but that the unknown high-energy fields must somehow contribute to lowering it.</p>

<p>None of this is very relevant to cosmology, which, except for the very earliest moments of the universe, does not really depend on the details of QFT, but only  on the "emergent" properties such as the behaviour of gasses, stars, light waves, neutrinos, etc. and on the one thing that (so far) does not seem to fit into QFT, namely gravity (or general relativity, if you prefer). For, sure, if the vacuum energy actually were the silly value computing from known forces using QFT, the gravitational effects of that energy would make anything like the universe we see impossible, but that is just taken as confirming that there is as yet unexplored physics on small scales. </p>

<p>So, the summary is "not essential at all". Cosmology and the expanding universe depends on GR and properties of matter and radiation on large scales and at relatively low energies. The Vacuum energy relates to the properties of the fields that underly that matter and energy on very small scales.</p>

CC BY-SA 4.0
2019-06-19T10:12:32.197Z
If it's non-essential and its very existence is in doubt (to put it mildly), why not abolish it altogether? You mention the types of particles said to exist, and I agree that many of them have been proven beyond doubt, but many more are purely imaginary. QCD was supposed to simplify the particle zoo, but instead has made it far more complicated and the zoo is as overpopulated as ever it was, often with fake particles.
Michael Walsby
CC BY-SA 4.0
2019-06-19T10:51:49.143Z
It's not essential *for cosmology* but no other theory does as well in explaining/predicting the results of particle physics experiments
Steve Linton
CC BY-SA 4.0
2019-06-20T08:25:26.957Z
It was reported in Scientific American that the claimed vacuum energy affects the cosmological constant and predicts a much greater redshift than is actually observed. I don't understand why something which significantly affects the cosmological constant would have no effect on our model of the expanding universe.
Michael Walsby
CC BY-SA 4.0
2019-06-20T09:36:01.703Z
I haven't seen that, but my guess would be that it is simply another reason to know that any of our current QFT based theories is just a low-energy approximation.
Steve Linton
CC BY-SA 4.0
2019-06-19T09:39:38.693Z
2019-06-19T09:39:38.693Z
Steve Linton
<p>How essential to modern cosmology is the vacuum energy concept, and what would happen to our model of the expanding universe if it were proved to be non-existent? </p>

CC BY-SA 4.0
2019-06-18T14:03:07.290Z
Do you mean what is commonly referred to as "dark energy" or do you mean the quantum mechanical vacuum energy which gives rise to Casimir forces and so on?
Steve Linton
CC BY-SA 4.0
2019-06-18T14:06:26.837Z
I have just edited to remove a lot of material which did not appear to be be part of the question, or relevant astronomical background. I haven't changed the sentence with the actual question. You can restore it if you want to, but I personally would then consider downvoting it as badly posed.
Steve Linton
CC BY-SA 4.0
2019-06-18T20:35:06.347Z
I mean the QM vacuum energy for which there is a 40-order-of-magnitude discrepancy with cosmological observations, the greatest order-of-magnitude discrepancy in the history of science!  As for the Casimir effect, Julian Schwinger & his CLA colleagues found a way to derive it using only fields arising from the material making up the sheets, not from any imaginary virtual particles in between.
Michael Walsby
CC BY-SA 4.0
2019-06-19T00:24:50.423Z
@MichaelWalsby I’ve noticed that many of your questions and answers here aim to challenge the mainstream cosmological model. Science relies on being challenged, but Stack Exchange is not designed for such critiques: we’re a Q&A site rather than a forum for debate.  We’re simply the wrong platform to use a question or answer to promote alternative models and/or dispute mainstream astrophysics and cosmology. If you could find the right forum, perhaps you could then use our site more effectively, to get an answer on a particular point for use in your argument on that other site.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2019-06-19T07:23:05.847Z
Another thing you may have noticed is that some aspects of science are becoming too much like a religion,& its senior advocates too much like high priests. One thing which could never be said about Richard Feynman is that he resembled a high priest. Read a biography of him.
Michael Walsby
CC BY-SA 4.0
2019-06-19T09:27:00.470Z
Seems too broad.  The SE sites are not designed to handle broad questions which cannot be answered reasonably definitively and what you need is a different site that works like a discussion forum.  SE is *not* a discussion forum, it's a simple Q&A forum.
StephenG - Help Ukraine
CC BY-SA 4.0
2019-06-19T11:56:43.903Z
On the topic of virtual particles, [ACuriousMind says](https://physics.stackexchange.com/questions/147096/are-virtual-particles-only-a-fictive-tool-in-equations#comment305973_147152): "Virtual particles are lines in a Feynman diagram, they do *not* correspond to any state of any field". Please see [Are virtual exchange particles real or just mathematical?](https://physics.stackexchange.com/q/206877/123208) and the questions it links to.
PM 2Ring
<cosmology><vacuum>
How Essential is the Vacuum Energy to Our Present Model of the Expanding Universe?
Michael Walsby
# How Essential is the Vacuum Energy to Our Present Model of the Expanding Universe?

(Asked by: username_0 on 2019-06-18)

How essential to modern cosmology is the vacuum energy concept, and what would happen to our model of the expanding universe if it were proved to be non-existent?

**Tags:** cosmology, vacuum

> **Comments:**
> **username_1**: Do you mean what is commonly referred to as "dark energy" or do you mean the quantum mechanical vacuum energy which gives rise to Casimir forces and so on?  
> **username_1**: I have just edited to remove a lot of material which did not appear to be be part of the question, or relevant astronomical background. I haven't changed the sentence with the actual question. You can restore it if you want to, but I personally would then consider downvoting it as badly posed.  
> **username_0**: I mean the QM vacuum energy for which there is a 40-order-of-magnitude discrepancy with cosmological observations, the greatest order-of-magnitude discrepancy in the history of science!  As for the Casimir effect, Julian Schwinger & his CLA colleagues found a way to derive it using only fields arising from the material making up the sheets, not from any imaginary virtual particles in between.  
> **username_2**: @MichaelWalsby I’ve noticed that many of your questions and answers here aim to challenge the mainstream cosmological model. Science relies on being challenged, but Stack Exchange is not designed for such critiques: we’re a Q&A site rather than a forum for debate.  We’re simply the wrong platform to use a question or answer to promote alternative models and/or dispute mainstream astrophysics and cosmology. If you could find the right forum, perhaps you could then use our site more effectively, to get an answer on a particular point for use in your argument on that other site.  
> **username_0**: Another thing you may have noticed is that some aspects of science are becoming too much like a religion,& its senior advocates too much like high priests. One thing which could never be said about Richard Feynman is that he resembled a high priest. Read a biography of him.  
> **username_3**: Seems too broad.  The SE sites are not designed to handle broad questions which cannot be answered reasonably definitively and what you need is a different site that works like a discussion forum.  SE is *not* a discussion forum, it's a simple Q&A forum.  
> **username_4**: On the topic of virtual particles, [ACuriousMind says](https://physics.stackexchange.com/questions/147096/are-virtual-particles-only-a-fictive-tool-in-equations#comment305973_147152): "Virtual particles are lines in a Feynman diagram, they do *not* correspond to any state of any field". Please see [Are virtual exchange particles real or just mathematical?](https://physics.stackexchange.com/q/206877/123208) and the questions it links to.  

---

## 1 answers

### Answer by username_1 on 2019-06-19. Score: 2

I'm neither a quantum physicist, nor a cosmologist, so I'm going only from my laymans understanding.

The framework for constructing theories to explain everything in the universe *except gravity* which has proved most popular and useful for the last 50 or so years, is [quantum field theory](https://en.wikipedia.org/wiki/Quantum_field_theory). In this theory, there are multiple fields (conceptually similar to the electric and magnetic fields) suffusing every point in space and time. From the values of these fields, we can, in principle, compute the probabilities of the outcomes of any experiment, including the probability of detecting a particular kind of particle at a particular place and time, and from the rules of these computations we can derive all the normal "laws of physics" such as the conservation of energy, the types of particles which exist and so on, as "emergent" properties.

I described QFT as a "framework" because there are many possible sets of fields and rules for their interactions, each of which gives different physics. From the experiments we can do, we can develop some constraints on the overall structure, but we also know that the fields and interactions that we know about cannot be the whole story. Over short distance and time scales and large energy scales they would give rise to non-sensical predictions, so they must be wrong, but we cannot yet do experiments to find out what really happens.

Anyway, the vacuum is the configuration of those fields which has the lowest possible energy, but that does not have all the fields being zero, and when we calculate that minimal energy (as far we can) using only the known particles and fields we get a ridiculously high value (that's one of the non-sensical predictions I mentioned). This is not taken as a sign that the vacuum energy *is* that high, but that the unknown high-energy fields must somehow contribute to lowering it.

None of this is very relevant to cosmology, which, except for the very earliest moments of the universe, does not really depend on the details of QFT, but only on the "emergent" properties such as the behaviour of gasses, stars, light waves, neutrinos, etc. and on the one thing that (so far) does not seem to fit into QFT, namely gravity (or general relativity, if you prefer). For, sure, if the vacuum energy actually were the silly value computing from known forces using QFT, the gravitational effects of that energy would make anything like the universe we see impossible, but that is just taken as confirming that there is as yet unexplored physics on small scales.

So, the summary is "not essential at all". Cosmology and the expanding universe depends on GR and properties of matter and radiation on large scales and at relatively low energies. The Vacuum energy relates to the properties of the fields that underly that matter and energy on very small scales.

> **Comments:**
> **username_0**: If it's non-essential and its very existence is in doubt (to put it mildly), why not abolish it altogether? You mention the types of particles said to exist, and I agree that many of them have been proven beyond doubt, but many more are purely imaginary. QCD was supposed to simplify the particle zoo, but instead has made it far more complicated and the zoo is as overpopulated as ever it was, often with fake particles.  
> **username_1**: It's not essential *for cosmology* but no other theory does as well in explaining/predicting the results of particle physics experiments  
> **username_0**: It was reported in Scientific American that the claimed vacuum energy affects the cosmological constant and predicts a much greater redshift than is actually observed. I don't understand why something which significantly affects the cosmological constant would have no effect on our model of the expanding universe.  
> **username_1**: I haven't seen that, but my guess would be that it is simply another reason to know that any of our current QFT based theories is just a low-energy approximation.  

---

