The double smeared null energy condition

1- Derivation of central claim is clearly laid out and easy to follow
2- Comparison with previous literature to support central claim and compare similarities and differences are very clearly and explicitly explained throughout the whole paper
3-The DSNEC appears to have applicability for bounding other quantities beyond the energy-momentum tensor

1-Derivation only for free fields in Minkowski spacetime. Abstract claims "Our method allows for future systematic inclusion of curvature corrections." but this is only briefly mentioned in the discussion
2-Some of the intermediate results lack clear definitions
3- There is some confusion regarding UV divergences at the start of the central derivation
4- Boost optimization involves additional inequalities so the optimal bound is not found

The paper's main objective is to derive the double smeared null energy condition, which bounds how negative can a specific average over two null directions of the energy momentum tensor be. It appears to recover previous results in the literature and allow for generalisation for bounding other quantities. However, the methods laid out seem to be rely somewhat heavily on free fields (albeit generalised free fields) and Minkowski spacetime.

It is very well written and laid out in quite a pedagogical manner. Even readers not entirely familiar with this field ought to have no trouble following the narrative and understanding the central points of the manuscript. However, when following there were a couple of questions left unanswered and it was lacking a few definitions.

1- In footnote 1, despite attempt at clarifying notation it was slightly unclear whether $d^2x^\pm=dt~dx^1=\frac{1}{2}dx^+~dx^-$ or $d^2x^\pm=dx^+~dx^-=2dt~dx^1$. Clarification of this would help readability.

2- In section 2, the definition of ultra-regular domain is left to a reference, this generates some confusion. It would be better for the definition to be explicit in the text. Given it seems that the properties of these domains are not essential to the derivation, a quick intuitive definition should be sufficient.

3- In the paragraph before Eq. (18) exactly the same issue arises but with respect to the definition of $T^{ren}$. Explicit statement of its definition, or an intuitive explanation would help clear the confusion surrounding that paragraph.

4- Equations (26) and (27) are exactly equal, it is merely a re-writing of the same expression. However, the way the text is presented, talking of point-splitting the operators, suggests that in fact they are no longer equal. A remark emphasising that they are still exactly equal would be important to aid the reader.

5- After Eq. (30) it is claimed that the restriction to the half-space makes the point splitting real rather than fictitious, softening contact divergences. However, this does not seem to be correct.

Under Assumption 1 the integrand is symmetric so Eq. (30) and (27) are still exactly equal, we can perfectly carry the reasoning backwards to go back to the full space. Therefore if (26) is UV-divergent, then (30) would also have to be UV-divergent. It therefore does not seem correct to state this manipulation softens contact divergences.

6- In a related note, it seems unclear why is (30) a finite quantity. If I have understood section 2 correctly, then the two terms in (27) are separately infinite and it is only their difference which is finite. This would suggest (30) is also infinite. Later on in the paper (118) appears to suggest the smearing is taking care of UV-divergences and making both terms in (27) finite. If this is true this should be mentioned at the very start of section 3, right after (26) is written. If this is not true then it is a major flaw in the reasoning in the paper. An alternative argument as to why are both terms in (27) finite would then need to be provided.

7- Minor typo: before Eq. (40) it should read $Q_{T_{--}}[D_\eta]$ rather than $Q_{T_{++}}[D_\eta]$

8- Typo in Eq. (71), the domain of integration in the second line is written as $\mathcal{D}$ which is an operator, not a domain. It should be clarified what is the correct domain of integration.

9- Minor typo in the paragraph after Eq. (71) it should read "where $\mathcal{D}$ is a partial differential operator" rather than "where $\mathcal{D}$ is partial differential operator"

10- Minor typo in second to last paragraph of section 5.1, it should read "is not a linear functional" rather than "is not linear functional"

In summary, this is a very relevant paper which paves the way to new avenues of research in semiclassical gravity. There are many immediate follow-up questions one can pursue. If the points raised above are addressed I certainly recommend its publication.

1- Clarification of footnote 1
2- Present definition of ultra-regular domain, at least intuitively
3- Present axioms that define $T^{ren}$, at least intuitively
4- Make it clear that (26) and (27) are exactly equal
5- Remove statement that (30) softens divergences
6- Clarify if the terms in (27) are independently finite
7- Correct the typos presented in main report above
8- Remove statement "Our method allows for future systematic inclusion of curvature corrections." in the abstract or add a section on how to systematically include curvature corrections.

The authors propose a new bound on the stress tensor of quantum fields in gen-
eral spacetimes, known as the Double Smeared Null Energy Condition (DSNEC).
The stress tensor in question is smeared along null directions and such stress
tensor will generally scale with the smearing lengths simply based on symmetry,
so the non-trivial result that the authors obtain is a bound on the coefficient
multiplying the form obtained through symmetries. The paper’s relevance con-
sists of their method of obtaining such a bound, which will allow future work to
include curvature corrections to the energy bound (in a state-independent way).
However, there are several points which, if clarified correctly, can benefit the
reader of the paper greatly. Namely, some technical points include:

•The authors went through the trouble of writing out full equations and
many steps in the calculations, but failed to properly define several quan-
tities that appear throughout the calculations. For instance, in section
2, the authors mention the Hadamard parametrix, although they do not
define what it means, but leave it for section 4, which makes the section
2 unfollowable on its own. Likewise, the equation 16 is supposed to be
definitional for what one might mean by a certain expectation value of
the stress tensor, but they use a symbol o which is not defined on its own,
and so on. Please make sure all of the objects, variables and operations
are properly defined throughout the text.

•The optimization over boosts is interesting, although I’m not quite sure
I fully understand the logic. Namely, the equation 34 tells us to mini-
mize over the domains for the boost parameter. Why is minimization the
correct way and not extremization in general? As far as I understand,
the point of the optimization is to give you the tightest bound on the
stress tensor, so I’m not sure why that is implied by the minimization.
The footnote that provides some explanation regarding the optimization
procedure is likewise somewhat unclear: I understand that by performing
such a procedure, one is obtaining a local minima so to say, and therefore,
infinitesimal coordinate changes cannot change the domain and hence,
we obtain a coordinate-independent formulation. However, it is not clear
that, in this language, this is a global minima of D; could you please clarify
this point?

•Under equation 47, there is a sentence: ”Processing h_D a bit, we obtain...”.
Please elaborate.

•In the ”HSANEC” paragraph, does the acronym stand for Half-Sided
ANEC? Please define the term properly and elaborate on its relevance.

•Please expand on the last sentence on page 15: ”However, Ref. [4]...”,
since the sentence by itself does not contain any information.

•Please check equations 120 and 121.

Overall, the paper mostly consists of printing out the relevant calculations,
but the reader would benefit from some physical intuition on why some of these
bounds are relevant and what is their physical significance. Some conceptual
points include:

•Please elaborate on the physical relevance of the quantum worldvolume
inequality.

•Could you also elaborate on why AANEC is not ”enough” to consider
when it comes to semiclassical (self-consistent) solutions? One of the
reasons why AANEC is a natural bound also comes from the fact that
it can be derived from the GSL (this was proven by Wall in 0910.5751). Is
there any sense in which one can derive the DSNEC in a similar way? If
such a derivation is possible, then this new bound can be connected with
other known developments in the context of semiclassical gravity (such as
the QFC), which would then reaffirm its relevance. It is also not clear how
one would independently of AANEC obtain the QNEC bound, given that
this one is state-dependent (the authors also acknowledge this).

•It is mentioned that one possible application of this bound would be for
proving singularity theorems. However, there have been already several
different approaches to the singularity theorems, and especially if they
require the non-compactness of the Cauchy slice on which the trapped
surface resides, then that significantly weakens the usefulness of the new
bound (all one requires in that case is an argument why a geodesic must be
incomplete in some sense). If the singularity theorem shows new results,
then it would be quite significant: for instance, there was a recent paper
2201.11132 on a new take on singularity theorems which also leads to
unexpected results (such as a singularity in an island setting where it
wouldn’t be expected). It would be quite interesting to show that such a
singularity would be seen through a DSNEC bound. After all, a bound is
only as useful as its applications.

•Another point of interest is the proof of DSNEC in non-trivial situations:
it is already stated that the provided proof will go through for generalized
free fields and large N theories. Have you thought about obtaining a proof
through the holographic duality for the strongly-coupled CFTs, that is to
say, by looking only at the bulk causal/entanglement features?

Overall, the paper points to some sort of a bound that certainly exists at
least for free fields. If it can be used in an interesting way, the bound can pave
the way for new results within the regime of semiclassical gravity. Therefore, I
recommend the paper to be published, as long as the points mentioned above
are properly addressed.