• Foregrounds
  • map based work (soumen, belen, graca, Mathieu, …)
  • small patch analysis (Raphael, Stephen)
• Cluster analysis (Nick)
• Surveys (SH to include responses from Marcel, Alex, et al)
• Figure Upgrades

Comments removed from text which still need follow up:

• Sect. 2.4.1, end of last paragraph, %\comor{what about wfirst? jwst? DESI? CIB?}. Line 18 of complementarity.tex
• Sect. 2.2.2, Galaxy clusters, which high redshift. Line 125 of extragalactic.tex
• Sect. 2.2.2, Galaxy clusters, why is it an excellent cosmological probe? didn't we want to add forecasts on parameters such as neutrino mass, and dark energy? wasn't there a point about a sample that is not restricted to a particular location in the sky?. Line 127 of extragalactic.tex
• Sect. 2.2.2, Galaxy clusters. Double check number, masses above ∼ 3 × 10^14 M_sol line 137 of extragalactic.tex.
• STM: constraint on sum of m_nuetrino by PICO alone (Dan Green). Constraint on mu and M_A (David/Laura)

• What is the ask
• Align better with STM
• Highlight better need for simulations
• Too much technical information. Shorten technical section.
• Be clearer about the need for space relative to ground/balloon

Hi Shaul

Yes, feel free to quote me. It was off the cuff, but sometimes those are the most true things we say! (well unless you live in the White House!)

Here are some things that I think a) should be changed b) could be changed. I expect most in the latter category can't be changed because time is too short, plus I expect all are a matter of taste/style and so would need to be considered by the PICO team before implemented. In the unfortunate (and I hope unlikely) event that it isn't approved this time, then you could consider them for next time.

General comment: assuming PICO is affordable (i.e. costs less than LSST) then this is a no brainer, the science is varied, fundamental and compelling. The only issue I see is that people who do similar science (be it using CMB or not) will feel elbowed out and if they are on the panel then they might not be able to get over that self-preservation bias. So what would be great is if the proposal included some suggestions about how to build a large community around PICO in the years up to launch.

FYI I haven't read past page 34.

Best of luck with the bid. Thanks for asking me to review it, I learned a lot.

Kathy

a)

You didn't “big up” the papers already submitted about PICO. I am guessing they are in the list of 186 references, but could have more prominence. I see 3 on ADS (sadly a bit lost in the noise due to some bubble chamber with the same name).

Executive summary - I though it a bit odd that you didn't mention EUCLID, SO or SKA, given that WFIRST and LSST are mentioned.

Executive summary - I flagged up the protoclusters and noted “how - no ICM?” and only got an answer many sections on (via star formation). So you could add a note about that.

Table 1 - I couldn't read it at all: I printed 2 pages per side, so it came out tiny. Even on the .pdf version it looks too small. To read it I'd need to zoom in. So I didn't. You could try rotating it or printing it across two pages. [BTW I really liked the graphic on the cover page - that was very clear and persuasive]

Figure 1 - You probably said this somewhere, but I missed it: EE spectrum is not the same as TT spectrum: most people are very familiar with the latter by now, so would be puzzled by figure 1.

Section 2.1 - para 2 - should it be “Galactic science” or “Milky Way science” rather than “galactic science”? Or maybe you did mean galaxies in general?

Section 2.1 - para 4 - “Its measurements of the spectral index” → “PICO's measurements of the spectral index”

Section 2.1 - para 5 etc. - you refer to S05 but didn't refer to S01-04 in preceding paragraphs. As someone who couldn't read the table, it wasn't clear to me at first what SO* meant (unfortunate overlap with SO, see below)

Figure 3 - you have an “SO” annotation, but this isn't explained in the caption.

Section 2.2.1 - Dark Matter bullet - you say “interactions between dark matter and protons” and that puzzled me. What type of interaction, given that DM is usually thought of being WI at best?

Section 2.2.2 - Cosmic Birefringence bullet - when I read this I worried that this effect would be degenerate with CMB lensing and/or primordial B modes. This no doubt shows my ignorance of the field, but maybe some of the official reviewers might be similarly ignorant.

Section 2.2.2 - Formation of First Luminous Sources part - I highlighted “higher resolution than PICO”: that felt like a bit of an own goal (by suggesting that PICO isn't as good as other experiments). So maybe that could be phrased differently to sound less of a negative. Similarly the bit “while PICO alone may not enable high SNR…”. [Although the ending comment about complementarity was strong, so definitely keep that in.]

Section 2.2.2 - Formation of First Luminous Sources part - I don't think “Stage-III CMB” was defined. I know what you mean, but maybe not everyone would.

Section 2.2.2 - Structure Formation via Gravitational Lensing part - “would represents” → 'would represent“

→ Done

Section 2.2.2 - Structure Formation via Gravitational Lensing part- It wasn't clear to me how you could “effectively cancel cosmic variance”. [BTW I found this section compelling, it is one of the few where I have useful background!]

→ Marcel: Edited slightly: “For instance, one can use correlations between large scale structure tracers with different clustering bias factors and measure the relative difference between their clustering power spectra to effectively cancel cosmic variance~\citep{2009PhRvL.102b1302S,2018PhRvD..97l3540S}; this can constrain…”

Figure 11 - I don't understand what you are trying to show with the full sky and with the zoom ins (and why two zoom ins of the same region)?

Section 2.2.3 - Dust Physics part - you say “levels relevant for B-mode science” but don't refer back to the section where this was explained.

Section 2.3 - I think there needs to be something at the start to segue from 2.2 to 2.3. A couple of sentences as to why the Legacy stuff is different to what has already been talked about. Are you expecting other people to do this science?

Section 2.4 - I thought it was very noticeable that ground based (rather than balloon) based CMB experiments were not discussed. There are several kind things said about balloons, but nothing about SPT/ACT/SO. That seems risky given who'll likely be on the panel.

Section 2.6.4 - for some reason this section worried me. It seemed like this issue might be a show stopper. I'm guessing it isn't, so maybe you could soften the language a bit.

Section 2.6.4 - “future space mission as PICO” → “future space mission such as PICO”

b)

Why no table of contents?

Executive summary - not being an expert, I underlined “cosmic birefringence” and added ”?“. So for specialist terms like that you could add a link to the relevant section.

Executive summary - “future cluster redshift surveys” is a bit vague and probably inaccurate. These days we measure cluster redshifts from photometric data. LSST is not specifically a cluster redshift survey, but it will measure cluster redshifts to z~2 (i.e as long as there is a red sequence - it won't do so well with star forming cluster galaxies).

Sectioning - general comment: this is a very long proposal and the hierarchy does get a bit confusing. Section 2 is “Science” but I don't think 2.5 - 2.7 fit under that heading - maybe better as Section 2 “Science background: Cosmology and Physics”, Section 3 “Science background: Other”, Section 4 “Technical Requirements” (with current 2.5-2.7 in Section 4).

Section 2.1 - this section has a lot of detail in it, but not everything is explained (e.g. the meaning of “quantum gravity” or how “specific models of inflation” would be tested) so I think it would benefit from referencing ahead to the sections where more information is given. And, on the flip side, maybe this section could have been shorter and at a more general (“for dummys”) level because a lot of what is in it is repeated later on.

Glossary - a glossary might be useful if you have room within the page limits.

Section 2.2.2 - Formation of First Luminous Sources part - I got lost because it started talking about large scales and then talked about small scales. Probably me not paying close enough attention, but maybe the official reviewers might do the same.

Structure of sub-sub-sections: this is definitely a style/taste thing, and for a future bid (which hopefully won't be needed) but one thought I had was that you could structure the sub-sub-sections (sometimes sub-sub-sub-sections) to bring out the “elephant in room” e.g. why do we have fluctuations in the CMB, the “state of the art and why the elephant remains hidden”, e.g. Planck, SPT, balloons etc., “how PICO will unveil the elephant” (either on its own or with synergy with LSST, balloons etc.). Section 2.2.2 - Galaxy Formation via SZ… part - the ordering of the sections doesn't quite work, since you define kSZ and tSZ here, but have referred to them already in earlier sections. May not be a big deal since most people will know what they are anyway.

Section 2.2.2 - Galaxy Clusters part - similar point to above: you've already talked about using clusters to do cosmology and now you are telling us about the catalogue.

Section 2.2.3 - Formation of Stars within… part - I wasn't sure if it was 1000's of independent measurements in time or spatially. I figured out that it was spatially through the context. related: why do you need so many measurements?

Table 2 - I like the way this is broken down with “Current knowledge” summaries. But it only talks about Planck. What about ACT, SPT, SO, balloons etc.?

Section 2.3.1 - page 22 - I noted “why do we care about lensed high-z galaxies” in the margin. So maybe you need to stress the motivation for this section more? Section 2.6.2 - this section fizzled out. I wasn't sure if you'd done the predictions or not.

Section 2.7 - I found it a bit odd that the requirements are described here, when they have been alluded to throughout the preceding sections. But its not a big deal.

Dear Shaul,

This is an exciting project indeed, and the writeup is very good! I attach a PDF scan of it with my hand-written comments. Most are minor typo corrections and suggestions on wording. Just a few general comments:

• As you state fairly late in the text, PICO is the obvious successor to the legacy of COBE, WMAP and Planck. I think this basic theme should be stated explicitly from the beginning of the report.
• On a closely related note, the report makes multiple statements from the beginning about how much better PICO will do than Planck, by all sorts of metrics. But the reader doesn’t know what PICO actually *is* until quite late in the report: a 1.5m space-based telescope with a 5-year lifetime to do an all-sky survey, and 22 logarithmically spaced bands, designed to minimize systematics, especially for measurement of polarization. It would be good to have a clear statement of this from the beginning, to put all the claims about what it will *do* into the context of what it *is*.
• Given that the current state of the art is Planck, it would also be good to clearly state what distinguishes PICO from Planck. After all, they are both are ~1.5m telescopes carrying out full-sky surveys in multiple bands. I *think* the answer is:

1. Far more detectors on the focal plane (with better noise characteristics).
2. More than twice as many filters (although the range of frequencies is quite similar).
3. Far superior control of systematics in the design, especially for polarization.
4. Considerably better angular resolution.

This should be stated from the outset. In this regard, the requirements for the mission are somewhat spread out through the document; first appearing in Table 1, but then only being explained right at the end (Section 2.7), which is slightly confusing.

OK, more in the attached PDF. You’ll note I have only a handful of comments on the technical/instrumental part of the report. Do let me know if any of this is unclear.

This is a *really* exciting project; I do hope it goes ahead!

Cheers, Michael

Strauss Comments Karl and Shaul currently going through these handwritten comments (Nov. 28-??)

Shull Comments

• Douglas' first round

In any case, feel free to post my detailed comments. General issues include:

• it would be better to include pointers to SO1 and SO2 first, since otherwise this labelling in the text is confusing
• there are places where the text should say “PICO will” rather than “PICO would”
• I hope that later (which I haven't gotten to yet) there's a discussion of comparison with ground-based efforts (in particular CMB-S4) - plus is it clear whether to assume Litebird will happen or what?

More to come. Douglas

Here's another set of pages (up to p.19 now).

• Douglas' second round

More general comments:

• I think I agree with Martin that the structure doesn't seem optimal because it's driven by methods rather than science questions. But rearranging things at this point would be quite painful to do.
• Presumably we can claim that detailed modelling of the dust polarization stuff will ultimately help with removing Galactic polarization in ordet to get at primordial signals? That's probably not trivial to show woth simulations, because the simulations aren't complicated enough - but the only way we'll discover that dust behaviour is more complicated than we thought it was is by studying the dust signals.
• It's odd to motivate birefringence by saying that it might have something to do with an explanation for acceleration. Well maybe, but I don't think that's the usual story. Mostly people are just saying there are possiblities for this kind of coupling, so let's keep an open mind and see if you can constrain the signature. As well as the Polarbear paper, the relevant Planck paper (Intermediate XLIX) should be cited.
• Is f_NL=1 really well motivated by theory? Maybe this is the strongest statement that can be made, just because this is the value that's doable for PICO? But can someone spin this a bit more convincingly?

Here's the last set of detailed comments on the science part (pages 20-35).

• Douglas' third round

General comments on this part:

• It can't really be true (as stated in Table 2, that polarization has only been measured for one “dusty galaxy” - unless we have some restricted meaning (above some z and L?)
• The text says that Planck's resolution was too poor to detect individual proto-clusters - but I've written many papers on these, so that's just not true! The easiest way to fix is is to say “too poor to resolve individual proto-clusters” - certainly Planck's 5' beamsize wasn't optimal for studying such sources, and PICO will be much better.
• Is it worth saying “SPHEREx if selected” - isn't that just a red flag for any SPHEREx fans? The text doesn't use such language for other future projects.
• In the version I'm using, the text that straddles pages 23 and 24 is a HUGE red flag! It's about the relative costs of ground-based versus space-based, and I don't think the wording as currently written would be agreed by everyone! Please be VERY careful to reword this part - maybe there's a different way of spinning this whole thing to sound more positive about space without getting into the dangerous territory of making claims that not everyone agrees with? We're surely not claiming that space is now cheap?!
• And why is there no explicit mention of CMB-S4 at this point?
• In fact CMB-S4 is introduced (I might even say buried) in the middle of Section 2.5.2, which seems very odd. I think this comparison (and obviously the thing to do is stress the complementarity rather than the competition) needs to be called out more explicitly in a subserction of its own.
• Around the bottom of page 28 and top of page 29, I'd add some text (suggestions included on the pdf) about the possibility that the foregrounds will be more complicated than anticipated - this is a big issue in people's minds, and doing things properly with PICO is surely a major selling point.
• It seems odd that the discussion of systematics is a subsection of Section 2, rather than being a Section on its own.
• I became less and less of a fan of “IGW” as I read this. Why introduce this new acronym? Not everyone will know what it means, PLUS it assumes that the signal will be from inflation, which precludes the possibility that we'll measure something and find out that there's a non-inflationary eplanation (howwver unlikely that may seem, it seems unwise not to at least SOUND open-minded!). Why not just say “primordial GW” or just “GW”?
• Are you required to write out all the authors in the reference list (it looks sloppy to me, like no effort was made to reduce the list to the first N).
• Lastly - I see that the technical parts of the report are in a completely different format. I think that gives a very bad message, namely that the science and technology parts aren't fully integrated. Maybe it's a lot of work, but it would be much better to format all of it the same way (maybe the intention is to do this before submission?)

Dear Shaul,

My major comments follow:

• Assuming that PICO comes after LiteBIRD and CMB-S4, the improvement on r is not dramatic, factor 3 or so (sigma(r=0)~0.0005 both from CMB-S4 and LiteBIRD with external data). You may need to choose some other killer applications to sell PICO. Of course it could be the case that neither of them is accepted, but it is not a good strategy to ignore these proposals.

• The science goal of sigma(r=0) < 10^-4 is experimentally very ambitious. Since there is no quantitative result on foreground cleaning and systematic error estimations, I cannot judge if this goal is achievable at the moment.

• Sec.2.6.2 seems to say sigma(r=0) < 10^-4 is NOT achievable with using TB and EB. Is that your main message?

• Bandpass mismatch and its stability during observations is a large concern. Although Table 4 seems to say it is written in Sec. 2.6.2, no description is found.

• There is no description about interplays between foreground cleaning and instrumental systematic errors. To achieve sigma(r=0) < 10^-4, they could be the killer. So I suggest that you include them in your risk list.

• Have you considered effects of multiple-reflections and ghosting in your far sidelobe estimations? 80dB is very challenging.

• Lowest frequencies are on edge wafers. Have you checked that beams at the edge pixels are still OK for the lowest frequency?

• What is your plan to mitigate 1/f noise? What are your requirements? Detector 1/f is not an issue. 1/f is a system issue, coupled to energy deposits by cosmic ray hits, scan-synchronous instability, cooling system instability etc. It is hard to control. I suggest you add some more descriptions about 1/f noise requirements and how to deal with system issues related to 1/f noise.

• How do you deal with gain variation within 10 hours?

• There is no convincing argument that >450GHz is required for foreground cleaning. Foreground experts say that under the current foreground models considered, better sensitivity below 450GHz is equivalent to having >450GHz. Frequencies closer to CMB bands are actually better if you consider something unknown, because at higher frequencies you need to worry more about new components such as warm dusts. Somewhat related to that, COMMANDER results used to show the necessity of higher frequencies, but new COMMANDER-2 introduced this summer does not anymore. So if you happen to use COMMANDER, results should be updated.

• You quoted S4 results, but simulation results from S4 and SO are optimistic compared with the one for LiteBIRD (by Josquin and Radek arXiv:1811.00479) because they do not take into account possible spatial variation of the synchrotron curvature. The SO science paper (arXiv:1808.07445. Sec3.5 limitations of current forecasts) also mentions it as a limitation. Since the description of foreground cleaning in this document is short, I cannot judge if this is also the case in your study. Since the foreground cleaning is a key to the success, I suggest you describe more details.

• In general, space agencies appreciate if you give a thorough list of things that need to be addressed, such as the interplay I mentioned above, rather than ignoring them and giving an impression that you do not recognize them.

Minor comments

• 2 < ell, do you mean no sensitivity is required for ell=2?

• How do you avoid CO lines?

- Optical system: From fig.3.1 left and fig.3.2 it seems that there is a direct path from radiation diffracted from the lowest edge of the primary mirror to the focal plane (with a large incidence angle, but still after a single diffraction). Is there any way to baffle it, may be moving up a bit the focal plane ?

- Polarization modulation: There is no polarization modulator, and polarization modulation is achieved through an optimized sky scanning. According to the simulations mentioned in the proposal, accurate polarimetry can be achieved nevertheless. However, the reviewer would expected to see more details on the simulations, to evaluate how realistic and reliable they are. This is important since LiteBIRD, sharing some of the main goals of PICO, has implemented a very different strategy, stating that the presence of a polarization modulator is mandatory. The main problems to be considered more quantitatively are, in my view:

a) efficiency of the mitigation of the effects of 1/f noise (the discussion at the end of paragraph 4.1 is really short and not quantitative)

b) effect of beam ellipticity, coupling anisotropy to polarization

It would be good to add a column in Table 4 with upper limits to the bias/error affecting the determination of r, as estimated from end to end simulations, for each of the systematic effects listed. I understand that more detailed analysis will be carried out in phase-A, but the reviewers should be reassured that the evaluated risks are under control.

- In the caption of fig.5.2 the general layout is the Planck one, but the focal plane of Planck was at 100 mK while 250 mK is indicated.

I'll continue to go through, but wanted to transfer these.

Dear Shaul,

This is an exciting project indeed, and the writeup is very good! I attach a PDF scan of it with my hand-written comments. Most are minor typo corrections and suggestions on wording. Just a few general comments:

As you state fairly late in the text, PICO is the obvious successor to the legacy of COBE, WMAP and Planck. I think this basic theme should be stated explicitly from the beginning of the report.

On a closely related note, the report makes multiple statements from the beginning about how much better PICO will do than Planck, by all sorts of metrics. But the reader doesn’t know what PICO actually *is* until quite late in the report: a 1.5m space-based telescope with a 5-year lifetime to do an all-sky survey, and 22 logarithmically spaced bands, designed to minimize systematics, especially for measurement of polarization. It would be good to have a clear statement of this from the beginning, to put all the claims about what it will *do* into the context of what it *is*.

Given that the current state of the art is Planck, it would also be good to clearly state what distinguishes PICO from Planck. After all, they are both are ~1.5m telescopes carrying out full-sky surveys in multiple bands. I *think* the answer is:

• Far more detectors on the focal plane (with better noise characteristics).
• More than twice as many filters (although the range of frequencies is quite similar).
• Far superior control of systematics in the design, especially for polarization.
• Considerably better angular resolution.
  

This should be stated from the outset. In this regard, the requirements for the mission are somewhat spread out through the document; first appearing in Table 1, but then only being explained right at the end (Section 2.7), which is slightly confusing.

OK, more in the attached PDF. You’ll note I have only a handful of comments on the technical/instrumental part of the report. Do let me know if any of this is unclear.

This is a *really* exciting project; I do hope it goes ahead!

The following are from one pass through the current document. I've not made efforts on style, syntax, etc! It may be easiest to follow up by phone.

I have some editorial comments in places that I’ll send separately.

-r and tau limits should go in exec summary.

-the ground will get more strongly lensed galaxies. There are already more than ALMA can follow up although PICO may find some exotic ones. PICO will give the full spectrum though and that is good to emphasize. We still don’t know the dust index or the optical depth for DSFGs.

-my theory colleagues tell me that there is no real direct relation between “the energy scale of inflation” and a B-mode detection. There are a few possible field configurations that can produce B-modes and they are not be related to the inflation energy scale. Also, they tell me that n_s<1 is already a detection of gravity acting on quantum scales. For me the B-modes are a new and unique probe of gravity acting on a quantum scale. It would tie G-waves to quantum processes and that seems deeper than scalars to me. Ask Raphael.

-I was surprised not to see any mention of the large ell “anomalies” (quotes because they may not be anomalous). Is the l=20 dip in TT reflected in TE and EE ? I’m not sure Planck can tell us.

-Say that you will delens up front.

-Fig 1. Put polarization in the title so it is clear that this is well beyond T anis.

-Fig 2, chop of bottom factor of 10.

-For the magnetic field related elements: What limits Planck from doing better? Is it S/N or is it understanding the complexity of the ISM.

-I think SZ things will be done better from the ground on this time scale except for large low z clusters. An all sky map of y would be really neat.

-In Table 3, has 36 uK-s^{1/2} ever been achieved even in the lab? I don’t think so.

-You might emphasize that with the satellite you can get all this science with a consistent calibration for all the different science channels.

-For the angle calibration, if there is one rotation angle that that sets the power spectrum of bi-refringence to zero at all ell then I think it will be clear that there is no cosmic bi-ref. And, if there is a hint of cosmic-biref, then there will be a huge ground based push to develop a series of calibrators. So, ultimately I don’t think the angle calibration will be an issue. BTW, for coherent systems and probably for bolos, the pol angle varies quite a bit within band.

-Far sidelobe pickup. This one got me worried! The dB level is not what counts, as you know. It is dBi. I would quote in dBi to show that you know what’s going on. Also, I strongly recommend a ground based pre-flight measurement. For WMAP the moon was the best source in the end but the ground based sidelobe measurements were key to figuring out what was going on. Also, GRASP is only as good as what you put into it. Scattering from inside the cryostat is really important and is difficult to capture with GRASP.

-After reading 2.6.5 my take away was that a sub-orbital program was really important!

-Are you sure the DSN can handle this rate (sec 4.2)

-5.1.1 How much less sensitive to cosmic rays are the TES’s? SPIDER has a bunch of other noise.

-5.2.1 What efficiency did you assume for each of the three freq bands in the trichroic system? I did not catch how the filtering enters. As you know, the only groups to have demonstrated 100 mK TES CMB detectors are ACT and CLASS. No sinuous antenna coupled detector has come near to the sensitivity you require.

-Sec 6. WMAP, from proposal to launch, took 6 years. Almost every key element had been tested in the field already and the overall design was *much* more simple than PICO’s. So, I don’t think the schedule is realistic.

The case I see here is that there is an exciting future satellite that could tell us a lot of new things about the universe both near and far. However, there is a lot to be done to increase the TRL level of the key components. I see this as a strong case for continued support by NASA in labs and for balloons to increase TRLs. Indeed, in some ways the difficulties faced by the recent generation balloons thus far tells us more work is needed before a space mission gets the nod. I don’t know the best way to mesh this need with what you have. I hope NASA is aware that the detectors for space will need to be different than those for the ground.

(Note: Underlines were done by SH)

Nov. 4

In terms of science focus, I am concerned about the neutrino mass case. Given a realistic timeline for PICO, it's not clear to me how relevant this science will turn out to be. Third generation experiments will already take a big bite out of this space, though a better measurement of \tau is clearly a big deal here. If we had ground-based measurements and LSST, one could possibly do as well as 25meV. So in terms of differential reach this seems weaker than a headline science case would ideally be. Your argument would have to focus on systematic issues in the modeling of other data sets, which I always find a hard thing to write compellingly (especially since we're less likely to be experts on those data sets).

I feel somewhat similarly for the cluster case, if it is tied to dark energy. Much of that science will be done by pre-PICO experiments, and the unique reach of CMB lensing to very high redshifts doesn't make a cluster-based case as compelling as it could be. It would be nice to emphasize a different aspect of this science or to motivate a high-z extension more clearly.

Thirdly, the text is organized more around “technique” than “science” which I don't think is optimal. For example, fNL is only mentioned in the inflation section and then taken up again in S2.2.2. Similarly reionization's impact on kSZ is mentioned on p.4, not when you discuss constraints on reionization; and cross-correlations are separate from e.g. neutrino masses. If it had been me, I would have had the science cases mention each probe PICO provides, ordered by topic. I bring this up early because this kind of reorganization is tricky, especially late in the game. And maybe it's already too late.

Nov. 6

Meanwhile, here are my comments from having gone through the science section a few times now. I'm listing the comments more or less in the order they appear in the section, though some comments are more general. In my earlier E-mail I argued that the neutrino case may not be the best thing to hang your hat on, and that I think it would be better to reorganize the discussion more by “science goal” than “technique”. These are pretty big perturbations, obviously but I think the team may want to think about them.

One thing I noticed which is missing is a caveat about what you can and can't say regarding the achievability of sigma®~1e-4 given existing simulation capability. My reading of this is that right now we don't know whether 1e-4 is achievable in the presence of complex foregrounds (for PICO especially what if the lower frequency foregrounds have lots of small-scale structure?). Nobody can do the simulations with enough realism to say one way or the other. While this document is more aspirational than a proposal, I would still think you'd want to get out ahead of that and frame it on your terms rather than let the readers (and anybody else trying to ghost you) word this conclusion for themselves.

A relatively trivial thing is that Section 2.1 should contain forward references (ideally links) to the subsections later which discuss each claim in greater detail. This could be as simple as (\S X) at the end of a sentence.

The reionization case is tau-focused, it doesn't mention kSZ constraints until page 4 (and then in detail in S2.2.2). I would have moved that text up. Is this because you just don't have the angular resolution to really use kSZ? There is also no mention of 21cm experiments or LyA forest constraints which should be on a similar timescale.

On the topic of kSZ, you seem to be missing any discussion of velocity field reconstruction or “kSZ science” beyond reionization. I suspect this is because you don't have enough resolution? But you have excellent frequency coverage and you see (essentially) the whole sky. This would bring in more synergies with optical/IR experiments too and of course if eROSITA ever flies …

I don't know what kinds of graphics help you can get, but the Neff plot in the latest Planck parameters paper is really nice, and swapping out your current figure for something akin to that would be a big improvement I think.

Table 1 is very hard to read in its current format, the font is too small. Obviously rotating the Table is one option, but if you could combine two columns that would have a big impact on readability. Not really a science comment, but …

The discussion of inflation is very r-focused, and does not mention possible improvements in fNL until 2.2.2 (similarly discussion of isocurvature modes or primordial power spectrum “features”). I find this splitting of the inflationary science across subsections odd – it focuses on technique not science goal. Also, this would bring in a nice synergy with LSST, which is the premier ground-based optical facility of the next decade and I think you'd want to repeat this whenever possible. The text in S2.2 doesn't match the figures that are currently in the draft (what is shown in what panel etc.). The discussion in the inflation section about the models feels misleading. It states there are two classes of models, and gives some examples from the second class that could be constrained by PICO. It then says there are models such as G-L with a “somewhat” smaller r. It does not mention at all that there are models with significantly smaller r, which PICO could not probe, which feels like an important omission. If I were in a competition with you guys for a scarce mission slot, I would definitely use this to suggest you are over-claiming and insinuate the true impact of the mission has been oversold across the board. Just as a matter of “proposal politics”. I know this isn't really a proposal, but I think it's better if *you* frame the discussion of the models you don't rule out, rather than letting any “competition” do it. I thought Raphael Flaugher did an excellent job of this for the CMB-S4 science case. I don't know whether he could be persuaded to revisit that here.

The discussion of neutrino mass and the impact of \tau in S2.2.1 contradicts the discussion of neutrino mass from cross-correlation in S2.2.2. Again, it seems odd to me to separate these discussions – a more technique-centric than science-centric view.

When you talk about “halo lensing”, you are implicitly referring to measuring the projected mass of the (possibly stacked) halos. When you mention the cluster cosmology program you are referring to the virial mass. These are not the same thing (mass), and there is no discussion of the model-dependence in the conversion. The first published measurement from this technique didn't inspire huge confidence in the robustness of this method, so if it looks like you're over-selling you could again become open to ghosting. I'm not suggesting this case be removed or anything, but some carefully chosen words here will let the reader know that you know that life isn't quite as simple as the rosy picture the current text suggests.

On the scales probed by PICO, wouldn't you be better off cross-correlating your Compton-y map with galaxies than with the LSST lensing map? I don't really see what using LSST lensing is gaining you. I guess you'd clearly do both, so maybe it is the omission of the clustering measurement which is more jarring than the existence of the lensing cross-correlation. Since WFIRST is likely to cover very little sky, it seems really unlikely to me that cross-correlation between PICO and WFIRST is going to be compelling for almost any science goal. Unless the high-frequency channels are super-critical I can almost certainly do better from the ground. What am I missing?

Why are the PICO component-separated, CMB-lensing and CIB maps not items “1 .. N-4” in Table 2, “Cosmological Legacy Science”? I would have thought they would have immense legacy value. Do you mean “object catalogs” here? All three examples are of catalogs of objects.

In S2.4.2 you claim space has advantages “in every respect” except for repairability. I presume “repairability” also includes “upgradability”? Even then, I don't think that's true. The ground has the advantage in angular resolution, which for some cross-correlation science cases is important I think. You essentially say this later in your text – thus directly contradicting yourself.

Anyway, this section clearly represents a huge amount of work by many people and the science is strong and I would find it compelling. I think with a bit more work it could be even better though…

Hi Shaul – Thanks for providing copy of PICO report. Please find comments for consideration from a read of the document below. Hope that these observations and comments are helpful – feel free to reach out if questions arise. Cheers and Best, – Ed

1) “It will produce the first ever full sky polarization maps at frequencies above 350 GHz, and it will have diffraction limited resolution, giving it a resolution of 1’ at 800 GHz.” –> redundant statement – suggest –> “It will produce the first ever full sky polarization maps at frequencies above 350 GHz and will have a diffraction limited resolution of 1’ at 800 GHz.”

2) “The mission will have a deep impact on particle physics by measuring the expected sum of the neutrino masses in two independent ways, each with at least 4σ confidence, rising to 7σ if the sum is near 0.1 eV.” –> 7σ what? –> “The mission will have a deep impact on particle physics by measuring the expected sum of the neutrino masses in two independent ways, each with at least 4σ confidence, rising to a 7σ detection if the sum is near 0.1 eV.” or similar…

3) ”…The map will be cross-correlated with other next-decade galaxy surveys, such as LSST, to give strong, sub-percent accuracy constraints on structure growth parameters. An extraordinary amount of information about the role of ‘energetic feedback’ on structure formation will come from correlating PICO’s map of the thermal Sunyaev-Zel’dovich effect with WFIRST and LSST…“ –> suggest –> ”…The map will be cross-correlated with next-decade galaxy surveys to give strong, sub-percent accuracy constraints on structure growth parameters. An extraordinary amount of information about the role of ‘energetic feedback’ on structure formation will come from correlating PICO’s map of the thermal Sunyaev-Zel’dovich effect with WFIRST, Euclid, and LSST…“ or similar… (i.e., given anticipated reviewers it may be of value to explicitely convey synergy with LSST, DESI, Euclid, WFIRST, other…)

4) “These counts are factors of 100 to 1000 larger than available with catalogs today, and the window PICO provides because of its high frequency bands is entirely unique and not available to any other experiment.” –> ?as written text is unclear? –> “These counts are factors of 100 to 1000 larger than available with catalogs today because the observation window PICO provides is entirely unique and not available to any other experiment.” or similar…

5) Observation: The spectral coverage will not be “unlimited” in practice nor should it be – some readers will expect that this resource will be set by the experimental need – weather or not the thermal environment is benign or not is a matter of the orbit, observational strategy, and observatory design – this is not a given – it is something one will need to strive to experimentally realize. For these reasons suspect that the following sentence should be reworked to retain key points but reflect a well founded understanding of what is experimentally needed will be applied in specification of the baseline mission configuration, prudent margins, and potential descopes – this text does not really help tell this story. In its present form the text suggests unlimited resources are desired – which is not really the case: “This combined experience unambiguously shows that the unlimited frequency coverage and thermally benign environment aboard a space-based platform give unpar- alleled capability to separate the combination of galactic and cosmological signals and to control systematic uncertainties.”

6) Check type setting:

"...thus mixes the the power spectra..." (the the --> the) 
"...many of of the simplest models of inflation..." (of of --> of)
"The best current mass constraint arises from a combination of Planck and BOSS barion..." (barion --> baryon)
"...and/or isnÕt well understood..."  (isnÕt --> is not)
"(4) PICO’s survey of the Galactic plane and regions..." (4 --> 5)
other...

7) “see SO3” …had to search document to find uncover S03 = “Science Objective 3”… might help reader with more words, introducing notation to be used at this point of the narrative, or forward reference (note: science objective are not discussed until section 2.2)…

8) “PICO will find all the massive, virialized, galaxy clusters at any redshift.” citation to support this statement would be add value.

9) ”…for the foreseeable future only way to detect these gravitational waves.“ –>”…for the foreseeable future only way to detect gravitational waves at this scale.“ (i.e., this statement is really about long wavelength primordial gravitational waves not gravity waves in general – minor refinement would help clarify to the reader.)

10) ”…purely primordial (no dynamo) origin…“–> suggest –> ”…purely primordial (i.e, nom-dynamo driven) origin…“

11) Observation in reading the following text – one wonders what knowledge of the instrument polarization alignment is needed – stated another way – the constraint presumably requires realization of other experimental attributes than pure sensitivity: “Using the sensitivity of only the 155 GHz, PICO will improve current con- straints on cosmic birefringence (from POLARBEAR [67]) by a factor of 300.”

12) Subtitles unclear…

“Structure Formation via Gravitational Lensing” –> “Probing Structure Formation via Gravitational Lensing” “Galaxy Formation via the Sunyaev-Zel’dovich (SZ) Effects” –> “Constraining Galaxy Formation via the Sunyaev-Zel’dovich (SZ) Effect”

or similar. (i.e., one is probing structure and galaxy formation – the stated effects do not induce these processes)

13) “There is no known way to achieve any cosmological constraint on the sum of the neutrino mass σ (∑ mν ) < 25 meV without improving Planck’s measurement of the optical depth τ .” –> “Any refinement of the cosmological constraint on the sum of the neutrino mass σ (∑ mν ) < 25 meV will require improve Planck’s measurement of the optical depth τ.” or similar…

14) “In every respect, with the exception of repairability, space has the advantage.” –> “In every respect, with the exception of serviceability, space has an experimental advantage.”

15) “These advantages used to come with higher relative costs.” – it is not clear that this statement is necessarily true – the overhead for a given capability is quite likely higher for space than than the ground given the required reliability and environmental design considerations. The reason why the envisioned ground based experiment approaches that of a probe is due to the detector count required to carry out the desired science program. This is not an argument for or against either; however, would advocate are more careful comparison if text is retained. The several of the stated guidelines arguably held prior to the generation of experiments in question (i.e., this is essentially the same argument which motivated earlier space and suborbital mission designs – as such – it should be stated – this said – the cost argument is questionable).

16) In reading this text, ”..Several publications have demonstrated that fitting complicated temperature profiles using a simple one- or two-temperature model will bias the fitted CMB signal at levels δr 􏱴 10−3, large compared to the PICO goal [163–167]…“, the reader is left wondering how this will be mitigated to enable PICO to reach its science objective? Either moving this sentence to the next paragraph or having a stronger leading sentence in the mitigation paragraph could help here. Similarly, the final paragraph of this section, ”…We also provide forecasts us- ing other techniques that use analytic calculations to estimate the efficacy of foreground separation, or others in which the simulated sky map is assumed to have specific Galactic emission models, which are then being fitted…“, does not leave the reader with a clear picture that the stated simulation methods have reached the level of maturity to enable the mission. A statement supported by citation reflecting the state of the art in this area could potentially be used to convey this point. If the desired point is that the sections that follow are to convey this information – a forward reference or change in narrative order/structure is recommended to guide the reader to the desired conclusion regarding achievable foreground mitigation methodologies.

17) Unclear if risk levels help Table 4 – will intended audience want this detail (i.e., what was studied for the report is of value to state – beyond this it is unclear what value other designations provide)?

18) “Results of the simulation (neglecting foregrounds) are shown as power spectrum residuals in Fig. 17. We estimate the gain fluctuations to better than 10−4 solving for the gain every 40 hours (4 precession periods). The scanning strategy employed by PICO allows for a much better calibration than Planck, thanks to the much faster precession.” – question in reading – does the cross-linking use by PICO also help here or is this really just a question of precession rate to cover sufficient range of CMB dipole for calibration? (i.e., the discussion in instrumentation section 4.2 would suggest that this is the case.)

19) “Like the Planck-HFI instrument, PICO cools its focal plane to 0.1 K to enable detector operation (§3.4.1).” – some statement regarding heat sinking the arrays to minimize the influence of cosmic ray induces transients may be of value here (i.e., the reader may wonder if the lesson's learned from Planck experience are not clearly conveyed)…

20) ”…More than 60 people participated in-person in two community workshops (November 2017 and May 2018).“ –> ”…More than 60 people participated in-person in two community workshops (November 2017 and May 2018). As such, this study represents broad contributions and technical experience from the CMB theory, analysis and instrumentation communities.“ or similar.