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Astrophysicists Puzzle over Webb’s New Universe

quantamagazine.org

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Re: Astrophysicists Puzzle over Webb’s New Universe

#31

> Faced with observations of early black holes and galaxies that weren’t expected to exist, scientists have come up with a wealth of new theories to explain them. Now they just need to figure out which ones are true. This subtitle really bothers me. Science isn't about finding out what is true. Science is about finding out what is false and building models to explain the rest. We can never confidently say we know som…

I think you are confusing the scientific process, in particular Popper's falsification principle, with science's purpose, which is to find the truth, or at least sort things into true and false. It's a bit like saying the purpose of programming is to have a bunch of unit tests.

He's saying that what is believed to be the truth at one point in time often ends up being false from another point in time. And this is inescapable since we never know as much as we think we do. In the late 19th century it was believed that physics was basically done, and all that remained was refinement to ever more decimal points. Then came along the early 20th century when quantum mechanics and relativity completely revolutionized the field and largely overturned stuff that had been believed to be true for centuries.

Science can do a decent job of disproving a hypothesis because even a single contradiction should be enough to suffice in good science, but it's far less efficient at proving anything true even if it seems to always be true. For instance mathematical relationship describing the gravitational attraction between large bodies seemed to always work, but it turns out it was merely a rough approximation that completely fails in various cases such as when one body has a particularly large gravitational pull, or when very high relative velocities are present. And even modern understanding is, at best, another rough approximation because we can already see endless examples in the cosmos of examples that defy current understanding and require further refinements in a direction that's currently unknown.

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Basically at any point in history if you look at the bleeding edge science from a century before, it looks naive in many ways. In each era people always think they have finally moved beyond this, but we never have and it's entirely possible we never will since it's likely this universe has surprises awaiting us that we can't even yet imagine. Think about how utterly bizarre it is that time itself is a relative variable meaning with tech capable of reaching sufficiently high velocities you can literally travel into the future, relative to people at rest (such as all of Earth for example). It's nonsense, but it's completely real.

Re: Astrophysicists Puzzle over Webb’s New Universe

#32
post #2

As observations become too numerous, it seems like it can be summarized as there now being too many possible candidate explanations. As data increases and becomes clearer, more and more things don't fit the existing theories. What are the current theories explaining the early universe? What happened to the Big Bang? I only studied astronomy up to an undergraduate level, so I don't really know. I imagine that various…

> spatial distortions

Acoustic distortions. The universe was small and dense enough for sound to travel through ‘space’, which was filled with plasma. The theory is that inflation blew up these tiny distortions to the scale of the structure we see in the universe.

Re: Astrophysicists Puzzle over Webb’s New Universe

#33
post #30
post #22

The most exciting idea to me that JWST has bolstered is primordial black holes. Many models already predict them but JWST has provided the first good indirect evidence in the form of too-early galaxies. The models that predict PBHs predict that. If they exist, they would not be constrained to stellar mass and above. There could be a population of little black holes floating around. Anything under the mass of a decent…

Of course if we had a black hole in a lab (or one in a convenient orbit) we could run all sort of experiments, but which experiments exactly? We will start by throwing things at it and watch, obviously, but that's unimaginative. What are the smart experiments?

I'm not sure anything after the event horizon right? Since no light == no information.

Re: Astrophysicists Puzzle over Webb’s New Universe

#34
post #2

As observations become too numerous, it seems like it can be summarized as there now being too many possible candidate explanations. As data increases and becomes clearer, more and more things don't fit the existing theories. What are the current theories explaining the early universe? What happened to the Big Bang? I only studied astronomy up to an undergraduate level, so I don't really know. I imagine that various…

The evidence for the big bang is generally not that if you look far enough back in a telescope, the universe looks younger, which is somewhat the layperson's confusion.

Evidence for the big bang is about measuring redshift of galaxies throughout universal history, homgeneity and thermal equilibrium of the universe and CMBR, which could only be explained by it all having been in a compressed location where it could reach thermal equilibrium at some point in the distant past.

None of that is challenged by the Webb observations about very young supermassive black holes.

In fact, the existence of supermassive black holes themselves has basically always been an unsolved problem even before Webb. The only known possible explanation (stellar collapse -> accretion -> supermassive black hole) could be ruled out even before Webb on theoretical and experimental grounds, we just have stronger evidence against it now. (To wit: if supermassive black holes form from stellar black holes by growing, you would expect to see lots of intermediate mass black holes. We see almost none. Furthermore, the process of accretion is extremely energetic, so IMBHs would be the most visible objects in the night sky. The fact we see none is doubly damning)

The mainstream position now will be big bang + some kind of primordial black hole formation during the very early stages of the universe. Work of Hawking/Penrose shows that black holes can form under generic conditions in solutions to the EFE equations. We have a general understanding of how they could come about from certain dense matter layouts in a standard GR cosmological model.

Re: Astrophysicists Puzzle over Webb’s New Universe

#35

> Faced with observations of early black holes and galaxies that weren’t expected to exist, scientists have come up with a wealth of new theories to explain them. Now they just need to figure out which ones are true. This subtitle really bothers me. Science isn't about finding out what is true. Science is about finding out what is false and building models to explain the rest. We can never confidently say we know som…

I agree with you.

"True" has a connotation of absoluteness and finality. But I doubt humanity can ever know what is "true" about the universe. We can only model its phenomena with better theories, where "better" is always a temporary badge conferred for its prediction power and degree of agreement with known observations. Until an even "better" theory is figured out.

"Now they just need to figure out which ones are _better_"

Re: Astrophysicists Puzzle over Webb’s New Universe

#36
post #15

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?

> 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.

Re: Astrophysicists Puzzle over Webb’s New Universe

#37
post #30
post #22

The most exciting idea to me that JWST has bolstered is primordial black holes. Many models already predict them but JWST has provided the first good indirect evidence in the form of too-early galaxies. The models that predict PBHs predict that. If they exist, they would not be constrained to stellar mass and above. There could be a population of little black holes floating around. Anything under the mass of a decent…

Of course if we had a black hole in a lab (or one in a convenient orbit) we could run all sort of experiments, but which experiments exactly? We will start by throwing things at it and watch, obviously, but that's unimaginative. What are the smart experiments?

> which experiments exactly?

Put a bunch of charge into it to generate a naked singularity. Then look at it.

More usefully: perfect the Penrose process.

Re: Astrophysicists Puzzle over Webb’s New Universe

#38
post #22

The most exciting idea to me that JWST has bolstered is primordial black holes. Many models already predict them but JWST has provided the first good indirect evidence in the form of too-early galaxies. The models that predict PBHs predict that. If they exist, they would not be constrained to stellar mass and above. There could be a population of little black holes floating around. Anything under the mass of a decent…

My pet theory is that supermassive black holes are older than the universe and they didn't grew much.

Re: Astrophysicists Puzzle over Webb’s New Universe

#40
post #15

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?

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)

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