Which came first?

[Magical Realist]

The spacetime continuum? Or the quantum vacuum? I've read where spacetime was big banged from a singularity that came from a quantum fluctuation in the quantum vacuum. But I've also read that the quantum vacuum itself assumes spacetime, So which is correct?

 

[Sarkus]

My understanding is that, as "normally" used, the term quantum vacuum assumes the existence of spacetime. But there can be instances when it might make sense to talk about the universe arising out of a quantum vacuum. It might be dependent on what theory you're looking at, and the perspective it's taking, but those instances tend to be within the theories of quantum cosmology, and are where they're trying to conceptualise the mathematics of their theory in a realm that already makes little sense, and might have little bearing with reality.

 

[C C]

The spacetime continuum? Or the quantum vacuum? I've read where spacetime was big banged from a singularity that came from a quantum fluctuation in the quantum vacuum. But I've also read that the quantum vacuum itself assumes spacetime, So which is correct?

The idea of the universe springing from a quantum fluctuation came from Edward Tryon back in 1973, and is one of those "just-so" explanations that merely allows cosmologists (or whoever is applicable) to contingently have their own genesis story to plug into the blank placeholder that was offered before then. The original actually sounds as if it was engaging in a recursive fallacy of positing a space existing prior to the fluctuation (or arguably repeating something akin to the current situation).

Edward Tyron: In the early 1970s, most physicists believed that, within the boundaries of science, one could not speak about what came before the Big Bang. [...] Although Tryon's paper gives the impression that the mystery of where our universe originated is solved, it is not. His paper mentions how there is this "larger space in which our Universe is embedded," but this idea is given only a very vague and short description. Further, although Tryon proposed that our universe came into being from an accident allowed by the laws of physics, he does not indicate what created the laws of physics, leaving the mystery of the creation of the universe incompletely resolved.

Is the universe a quantum fluctuation: Tryon’s proposal falls into the category of universes with a beginning, but created out of nothing. However, nothingness here, as well as in all the other examples of quantum-created universes that followed Tryon’s inspiring idea, must be understood in terms of quantum mechanical nothingness, and not from an absolute nothingness that translates to complete emptiness. In physics, you simply cannot get something out of nothing. Creation ex nihilo is not the way of nature.​

Sans the hypothetical inflaton field (which seems necessary for the inflation process itself), the supposed fluctuating quantum fields of today may have only emerged during cosmic inflation -- rather than having existed "prior to spacetime", thus placing them within the universe's early stages rather than prior-in-rank to it. (And again, even Tryon's proposal vaguely sported space and change beforehand.)

https://www.forbes.com/sites/starts...get-its-first-quantum-fields/?sh=54df687829c9

EXCERPT (Ethan Siegel): If the quantum fields that we know of today emerged from some earlier state where they didn’t exist previously, that state must be confined to a realm before the hot Big Bang. Does that mean they could have been created during cosmic inflation?

It’s possible, but we don’t know. Based on the inferred limits on the energies achieved during inflation — which themselves come from the fluctuations generated during inflation that are imprinted in our CMB and large-scale structure today — inflation might not have reached sufficient energies for this to occur. Although successful models of inflation demand a multiverse, it’s still speculative to presume that the constants or laws are different in different “pocket Universes.”

[...] However, one thing that is certain is that quantum fields of some variety must have still existed during inflation. They may or may not be the same quantum fields that exist today, and there may have been additional quantum fields over and above the ones we know of and have today, but they had to exist. How do we know? Because the fluctuations that we see in the Universe, the ones that gave rise to the cosmic structure that eventually formed, match exactly the ones predicted to arise from fluctuating quantum fields that existed during inflation.

Those fluctuations, the ones that normally occur on tiny, microscopic quantum scales, get stretched across the Universe during inflation, get translated into temperature and density fluctuations at the start of the hot Big Bang, and imprint themselves irrevocably onto the Universe. The fact that we’ve observed these fluctuations and their consequences tell us, quite definitively, that those quantum fields did exist during inflation.

For as long as spacetime has existed, some version of quantum fields must have existed as well. But whatever occurred in our Universe prior to the final tiny fraction-of-a-second of inflation can never be observed or accessed from within our observable Universe. In the absence of evidence, we are bound to probe the limits of what’s known and match them up with whatever’s left as a possibility. However fun and instructive it may be to speculate, the truth is we simply don’t know.​

And this doesn't even touch on the confusing mêlée triggered by those who still contend that virtual particles are not ontologically valid, even in some half-sense (which the "virtual" adjective implies to begin with). Who discount the Casimir effect as being evidence via interpreting the results in other ways...

John Rennie: This is a common misunderstanding. The virtual particles used in calculating the properties of the QFT vacuum are a computational device and do not exist in real life. [...] The vacuum is not fluctuating and there are no violations of energy conservation - not even short lived ones.

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Arnold Neumaier: Thus it is misleading to interpret vacuum fluctuations as fluctuations in the common sense of the word, which is the traditional name for random changes in space and time. The vacuum is isotropic (i.e., uniform) in space and time and does not change at all. The particle number does not fluctuate in the vacuum state; it is exactly zero since the vacuum state is an eigenstate of the number operator and its local projections in space-time, with eigenvalue zero. Thus there is no time or place where the vacuum can contain a particle. In particular, in a vacuum particles are nowhere created or destroyed, not even in the tiniest time interval.

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Quantum fluctuations, the Casimir effect, and the historical burden(PDF): Both Julian Schwinger and Wolfgang Pauli cast doubt on the reality of vacuum fluctuations. [...] However, it has been argued since 1948, when it was experimentally demonstrated, that the Casimir effect [...] shows the reality of vacuum zero-point fluctuations.

[...] The way out of this conundrum is to reexamine whether or not the Casimir effect does prove that zero-point vacuum fluctuations are real. This was done in 2005 by Jaffe [...] Jaffe, while he does show that the Casimir effect cannot be used to prove the reality of the zero-point vacuum fluctuations, cautions that the reality of these fluctuations, should they exist, are beyond the scope of his paper and that the question of their reality remains open...​

Claims made otherwise:

Quantum fluctuations were experimentally proven way back in 1947
https://www.forbes.com/sites/starts...-were-experimentally-proven-way-back-in-1947/

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Physicists say they've manipulated 'pure nothingness' and observed the fallout (2017)
https://www.sciencealert.com/physicists-say-they-ve-managed-to-manipulate-pure-nothingness

EXCERPT: These are more like 'virtual' particles than physical matter, so ordinarily you can't detect them. But although they're invisible, like most things in the quantum world, they subtly influence the real world.

These quantum fluctuations produce randomly fluctuating electric fields that can affect electrons, which is how scientists first indirectly demonstrated their presence back in the 1940s.

For decades, that was all we had to go on. Then, in 2015, a team led by Alfred Leitenstorfer from the University of Konstanz in Germany claimed they'd directly detected these fluctuations, by observing their influence on a light wave...

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Sudden death' of quantum fluctuations defies current theories of superconductivity (Jan 2024)
https://research.princeton.edu/news...ons-defies-current-theories-superconductivity

EXCERPT: “What we found, by directly looking at quantum fluctuations near the transition, was clear evidence of a new quantum phase transition that disobeys the standard theoretical descriptions known in the field,” said Wu. “Once we understand this phenomenon, we think there is a real possibility for an exciting, new theory to emerge.”

PAPER: https://www.nature.com/articles/s41567-023-02291-1#citeas ​

 

[James R]

I've read where spacetime was big banged from a singularity that came from a quantum fluctuation in the quantum vacuum. But I've also read that the quantum vacuum itself assumes spacetime, So which is correct?

The fact is: nobody knows anything about what happened prior to about $10^{-43}$ seconds after the big bang. Therefore, any speculations about singularities or universes popping out of quantum vacuums are just that: speculations.

As others have said, some people have postulated the existence of a multiverse in which there is some kind of thing - call it a "quantum vacuum" if you like - that can (somehow!) spawn universes. Maybe there is such a thing; maybe there isn't. I'm not currently aware of any proposed experiments or tests that could possibly decide the matter one way or another, but maybe there are some proposals.

The term "quantum vacuum", when applied inside our current universe, just refers to whatever the default background state of spacetime looks like at the quantum level. The Standard Model of particle physics assumes the existence of various types of fields. It describes all known particles as excitations of the various fields.

Under suitable conditions, new particles can be created out of the "quantum vacuum" and 'old' particles can disappear back into the vacuum.

Quantum field theories assume the existence of spacetime. They are theories of the fields and particles that exist within spacetime. They do not attempt to explain where the space and time come from in the first place.