[6thbit]

That’s a beautiful article showcasing our predicament in having access to more information about the universe. Now i have to be the one to ask the dumb defensive question:

what makes us so certain that we can trust what we see on James Webb? Can we definitely discard a measurement problem?

[icegreentea2]

JWST has 4 different instruments on it. While they all share the same focusing mirrors, but otherwise are 4 different measurement devices.

For the red dot observations, I believe this things have been measured by at least 3 of the 4 devices on board - NIRCam (near infrared camera, has very limited spectral capabilities through its filter wheel), NIRSpec (near infrared spectrograph) and MIRI (mid infrared instrument).

I cannot pretend to have the actual expertise, but it does seem vanishingly unlikely that all 3 instruments could create consistent artefacts in the same location.

[wussboy]

Unless there was a flaw in the mirrors they all use. I’m not saying this is so, but the software developer in me would immediately try to figure out what was wrong with the component they shared.

[tux3]

A flaw in the mirrors wouldn't leave the anomaly in a consistent place, it would keep causing problems no matter where you look.

But I'm pretty sure they thought of all of this and many more objections already. It's not like this is a super advanced thread of skepticism that physicists would have overlooked

[consp]

Afaik they did that during calibration. Take known close by objects, compare results, make sure they are the same (up to the capabilities of your ground truth).

[tgarrett]

If you're worried about bad pixels or noise, it seems like there is an easy fix: point it in a direction specified by some angles theta & phi, wait long enough to accumulate light from distant faint objects (high redshift galaxies etc), then shift Webb's orientation by a small amount to theta+delta_1 & phi+delta_2, which will have a significant overlap with the original image, and after taking the 2nd image check to make sure that all the objects have shifted over together by the same amount...

[scotty79]

> what makes us so certain that we can trust what we see on James Webb?

We can trust what we see. We can't trust there's nothing where we don't see anything.

[neffy]

Some of the Hubble results were also raising questions. At the same time, I read one of the papers on the galaxy stuff, and what struck me was they were identifying galaxy shapes by counting the pixels each galaxy had, so there are definitely some question marks over how they do some of this.

[tetris11]

You would expect more background pixel fuzz when centering an image kernel over an artefact.

In Hubble, that fuzz was marked. With Webb, far less so.

I think these are real true positives

[rhesa]

Have a look at David Kipping's video on Cool Worlds: https://www.youtube.com/watch?v=QCKrUZeiTJo . It goes into a bit of detail on compensating for systemic issues in JWST.

[astro1234]

Not a dumb defensive question but you should know the nice thing about these experiments is the incredible amount of work that goes into calibration and understanding all error signals.

Messing up the data analysis has major precedents. If you aren't familiar you should look into BICEP data in 2014, they thought they had observed primordial gravitational waves which would have been earth shattering. Instead they just messed up the dust correction pipeline. I don't envy the day they came to that realization. I was in several conference rooms at Princeton where BICEP people presented their analysis and David Spergel (of WMAP, previous head of the department at princeton) and others were able to walk them through how they thought they had kind of messed things up. This is what routinely happens, ESPECIALLY when something unexpected is observed. Every possible explanation is looked into, and ESPECIALLY in cosmology, you can do that incredibly well. Cosmology is one of the most beautiful sciences in my experience, precisely because we have such good ways to model the observations to probe various models, and you can treat the observations with Bayesian stats with virtually no risk of misspecifying your model, or, if you do find its misspecified, you have discovered something new about the universe.

The process to go from raw observations to physics, correcting for all the crap in between early universe light and us (dust which also rotates light polarization -- this explained the BICEP issue, instrument systematics which are measured to incredible precision on the ground (e.g. point spread function -- what is the detector response to various intensities of light; e.g. you get electrons for bright sources that spill into neighboring pixels)

Everyone everywhere is looking to make a name for themselves by discovering the discrepancy -- be it a screwup of some other team (astro community is generally very supportive and positive but also competitive) or a problem with simulation assumptions, a genuine discrepancy in our understanding of the universe (i.e. the tension in the hubble constant -- you infer rate of expansion from cosmic background radiation / early universe observations, and then try it using an alternative method -- using local variable stars, and you get a statistically significant difference).

So I would say: if there's a screwup it will be found, and a genuine fuckup is possible and does happen, but when it does believe me we will know usually within a few months. You'll have a ton of people trying to reproduce the results, pouring over everything there is that could possibly explain these observations. The wheel of astrophysics grinds slowly but it grinds finely.

Edit: also shoutout to Jenny Greene -- one of the world's foremost experts on galactic astronomy and also a genuinely great person. She rented me her house for a summer for dirt cheap when I was a poor grad student with nowhere to stay. Also hosted the best graduate student parties (our idea of a party is beer and board games and complaining about our advisers)

[lioeters]

There's so much to love about this comment, especially this part:

> Cosmology is one of the most beautiful sciences in my experience, precisely because we have such good ways to model the observations to probe various models, and you can treat the observations with Bayesian stats with virtually no risk of misspecifying your model, or, if you do find its misspecified, you have discovered something new about the universe.

What a great example of the scientific process, collectively discovering, questioning, challenging, making mistakes, correcting them, and gradually refining our knowledge and understanding. Even as an outsider I can see the beauty in this earnest pursuit of truth.

Not to mention the subject matter is just mind-opening. The more you know, the more strange wonders and mysteries you find. It's incredible to get a glimpse of it.

[6thbit]

Thank you for taking the time to answer, I rest assured in the fine grind of the astrophysics wheel now.

All those screwup stories are amazing to know about! really help with staying humble and trusting the (scientific) process.

[CamperBob2]

astro1234, your account is dead for some reason - you might consider emailing the admins.

I vouched for your two posts in this thread, but that never works, and honestly it gets a little old trying to pick up the slack left by HN's inscrutable, unaccountable, and largely-broken filter. This has been happening a lot lately, unfortunately.

[BurningFrog]

This is one reason to dislike the NASA process of building one huge prestige telescope every few decades.

[astro1234]

its the only way to study certain things. Can't make up for it with smaller telescopes or cheaper projects.