To be pedantic, we don't know this. Hawking radiation has never been observed.
If anything seems like due to dark energy total energy dissipation per area is increasing until death of universe.
Related, I posted a paper on William Sidis earlier today where he explored the idea of a black hole before they were theorized or discovered: https://www.sidis.net/animate.pdf
Maybe I'm just not a fan of strong language, making it appear we know something, where we don't.
This leaves you with spin 0 and spin 2 as the simplest alternatives. Spin 0 doesn't result in light bending and gives a wrong result for Mercury's perihelion precession. Spin 2 gives you General Relativity.
https://skullsinthestars.com/2009/03/06/michael-faraday-gran...
Hawking radiation is a semiclassical result: the curved spacetime is classical General Relativity and the scalar field (in which Hawking quanta arise near the central black hole) is quantum.
The dynamical spacetime creates -- through the equivalence principle -- an acceleration between past observers and future observers, and this acceleration corresponds with the Unruh effect. The Unruh effect rests on the definition of a vacuum as a state in which an observer sees no particles, and that when an observer accelerates a no particle state may be transformed into a state with particles. Equivalently, differently-accelerated observers will count different numbers of particles in a spacetime-filling quantum field. (A family of observers may count no particles, i.e., it's vacuum.)
The important part here is that a dynamical spacetime ("gravity") and a relativistic quantum field is needed for Hawking radiation.
So, "[can] spacetime alone ... create light?" No. There must be a matter field filling the spacetime. That matter field, if quantum, can look like it has no particles in it to some observers, but not all observers. The dynamical evolution of the spacetime can cause observers' counts of particles to evolve.
> gravitational waves ... carry energy, so can be transformed into light
The paper is about how, given:
* a massless quantum field theory proxying for light
* a quantum field theory in which gravitation is mediated by a massless spin-2 boson
* a dense medium with a (light-) refractive index greater than 1
* standing gravitational waves of significant amplitude occur in cases where gravitational radiation from widely separated sources converge within the dense medium and somehow [a] cancel out polariation and [b] are within a wide (compared to the wavelength) patch of flat spacetime
* the non-light massive and massless particles within the medium couple very weakly to the incoming gravitational radiation
* the particles of the refracting medium couple weakly to the "light" field, and generate practically no spacetime curvature even in bulk
then the light-proxying particles may be produced via a process which the authors compare with electron-positron pair production and Cherenkov radiation. (Although they do the latter comparison very very breezily, not delving into the cross section of light-by-light scattering).
There are weaknesses in this list of requirements, some of which the authors admit requires further study.
The key point though is that their mechanism cannot work in vacuum. It absolutely requires that the light travels significantly slower than the gravitational radiation (which in turn is assumed to travel at c, even in the non-vacuum in which light travels slower than that) and that a far-from-negligible momentum is lost by the incoming gravitational radiation as it passes through the refracting medium.
> great experiment here
The last paragraph in the Conclusions and Discussion section suggests there may be avenues for experimenting with the ideas in the paper.
I am not a physicist, but I understand we are talking about Universe so early after Big Bang that it wasn't yet transparent to light. There simply wasn't vacuum yet if by vacuum you mean electromagnetic waves being able to travel long distances.
In physical cosmology (and especially considering alternatives to General Relativity) it is very common to consider the possibility that some effect is strong in the very early universe and so weak as to be undetectable at present times (or even as early as the first galaxies or the surface of last scattering). Examples include auxiliary gravitational fields ("bimetric" theories, for example) that decay in the early universe, variable-speed-of-light/variable-Newton's-constant theories, and so forth.
Although one might think "hm, it's very convenient that an important effect only happens so early that we cannot use telescopes to see it", there is very good evidence for electroweak unification and cosmic inflation, both of which terminated (in different ways) in the very early universe, and are (or arguably were) too difficult to directly observe this late in the universe's history. Additionally there is ample indirect evidence that (if it exists) is within our reach.
That the hypothesized graviton-photon mechanism cannot work in vacuum makes it at least very difficult to test (or observe with telescopes) today, however the final section of the paper does suggest that if it happens in nature, where it happens is likely to become accessible to us in due course. This is not a theory that has a hard cut-off in the early universe; it is just a hypothesis that to be realized requires a configuration of e.g. binaries and molecular clouds that is not very close to what we commonly observe. (Double-binary compact objects in dusty environments might end up being commonplace though, and in those settings one could expect changes in "multimessenger" signals if the authors' ideas are correct. It's amazing how many star systems are turning out to be triples, and we know of triple-compact-star systems; there are a number of known quadruples like DI Chamaelontis; and Gamma Cassiopeiae is a system of at least seven ~stellar mass bodies.)
AFAIK there exists no popular belief that physics was different in early universe. The physics was the same, the only thing that was different was physical conditions. Meaning everything was densely packed together.
If you, even for a moment, assume that laws were different in early universe then you essentially lost any possibility to predict anything.
Spontaneous symmetry breaking has been at the root of at least three Nobel prizes, and is crucial to understanding the differences in physical systems at very high energies both in laboratories and in extreme astrophysical settings, at both early and approximately present times in the universe.
https://en.wikipedia.org/wiki/Spontaneous_symmetry_breaking
The early universe was in a high energy state, being very much hotter and denser than the later universe, as you say. There are several epochs -- notably the https://en.wikipedia.org/wiki/Electroweak_epoch -- where symmetry breaking is important, and using the lower energy theory (electromagnetism, in this example) simply does not work: results are (if even calculable) manifestly wrong, leading to a universe with a very different cosmic microwave background, and very different chemistry and nuclear physics.
I think at best one might say that theories with broken symmetries could still have those symmetries (i.e., the breaking may be reversible under "different ... physical conditions", like if our universe surprisingly evolved to a Big Crunch), however treating that as a denial of the possibility of different physics in the early universe is probably something you'd have to take up with philosophers or lexicographers for now.
Additionally, there is no reason to just assume (and refuse to trace out implications if wrong, or to validate) that physical constants are constants everywhere and everywhen. Putting some spacetime-location-dependent function on constants like G, k_B, \alpha, \Lambda, c has at the very least proven instructive in further understanding the concordance (standard) models of particle physics and cosmology, where those constants are taken as constant everywhere and at all times in the universe. Indeed paramaterizing apparent constants is outright productive science. See e.g. <https://en.wikipedia.org/wiki/Test_theories_of_special_relat...> for a scratch-the-surface set of details, and additionally <https://en.wikipedia.org/wiki/Variable_speed_of_light#Relati...> are at least [a] interesting [b] testable and [c] improves testability of the families of theories in which these constants are assumed truly constant (i.e, everywhere and everywhen).
> popular belief
Well, I guess your popular is could outweigh a literature search. But for scientists:
<https://duckduckgo.com/?q=%22spontaneous+symmetry+breaking%2...>
<https://duckduckgo.com/?q=%22new+physics%22+early+universe+s...>
etc.
Finally,
> lost any possibility to predict anything
It's been about half a century since Kenneth Wilson and Nikolay Bogolyubov explored rescaling and renormalization, and nowadays practically every physical theory is written down as, considered as, or is being adapted towards <https://en.wikipedia.org/wiki/Effective_field_theory> (EFT). It is common that different EFTs apply to the same physical configuration as some characteric scale is crossed, and it is possible that physical theories will be EFTs all the way down (and all the way up), with the concept of fundamental becoming a relation between families of theories. (For example, Newtonian gravitation is less fundamental than General Relativity, because the former can be derived from the latter (and not the reverse), not because General Relativity is known to be correct at all scales).
What does "gravity alone" mean here? Hawking radiation depends on a black hole that has energy to radiate. The radiation is not due to "gravity alone".
Any virtual particle pair which appears along that horizon can potentially be caught on the wrong side of it - i.e. one virtual particle is in a part of space which is now far enough way to be expanding faster then light, whereas the other particle is at a location close enough that it is not.
At the moment that happens, there's a particle or photon which is now inside the light cone of a distant object, paired with a particle that it will never meet again because it's outside of it.
As a result, you get Hawking radiation: because one particle can go off and interact with your universe, but it's partner will never be able to causally effect anything inside that horizon again. So the virtual particle has to become real.
Broadly Hawking radiation intensity goes with curvature - i.e. a more curved surface has a better chance of separating a particle pair then a flatter one. This is because a sharper curve means more vectors which carry you away from the event horizon.
This is also why small black holes evaporate faster then big ones - as the black hole shrinks, the Hawking radiation intensity increases because it's curving more and more (hence why microscopic black holes don't devour everything).
In 1976, Bill Unruh published "Notes on black-hole evaporation"[1], in which he showed that "an accelerated detector even in flat spacetime will detect particles in the vacuum" - now known as the Unruh effect This means that an observer in an accelerated reference frame will observe particles in the vacuum where an inertial observer will observe none. The presence of certain particles - the ones we call Hawking radiation in the context of a black hole - is a relative phenomenon. This is known as the Unruh effect. The equation for the Hawking temperature is essentially the same as the equation for the Unruh temperature, where the acceleration value is the acceleration due to gravity of the black hole.
Then in 1977, Gibbons and Hawking published "Cosmological event horizons, thermodynamics, and particle creation"[2], which showed that "the close connection between event horizons and thermodynamics which has been found in the case of black holes can be extended to cosmological models with a repulsive cosmological constant" and that "An observer with a particle detector will indeed observe a background of thermal radiation coming apparently from the cosmological event horizon." This is known as the Gibbons-Hawking effect.
There's a fairly complex relationship between the two effects which I won't try to describe, but if you're interested then [3] discusses it. The abstract itself gives some sense of the connection.
[1] https://journals.aps.org/prd/abstract/10.1103/PhysRevD.14.87...
[2] https://journals.aps.org/prd/abstract/10.1103/PhysRevD.15.27...