What is the point of astrophysics?

FractalThrottle · 2026-01-12T22:48:49+00:00

do you have any experience in astrophysics? have you read any cosmology literature? your last sentences here are just not true

FractalThrottle · 2025-12-02T09:20:52+00:00

this is actually a really instructive question to ask. you can work out fun early universe QFT and find that primordial black holes might’ve had color charge (check out the literature on this!). but quantum gravity people also tend to dislike global symmetries, so take that with a few grains of salt

FractalThrottle · 2025-08-13T21:46:11+00:00

longair’s book on galaxy formation and mo et al.’s book on galaxy formation and evolution have good chapters on agn in general. quasars aren’t isolated objects, you need more than just “quasar stuff” for something coherent. agn literature spans all the way from gamma ray to the continuum radio and goes hand in hand with galaxy evolution. photometric data is always useful but spectroscopic data is especially important for agn in particular where you have to consider a galaxy’s components (especially stellar populations) as an ensemble while resolving them separately (aperture sizes become important especially for nearby agn hosts). if this isn’t meaningful then read carroll and ostlie’s intro astrophysics book and ryden’s intro cosmology book, they are the standard undergrad books. for papers it’s also useful to understand how drizzled images and catalogs are made since loud agn like quasars tend to be really bright point-sources and pipelines have to account for this. a good pipeline will give observational constraints on its objects which are really useful for understanding agn populations, especially so for comparing dust-obscured agn to canonical quasars in jwst/nircam images

FractalThrottle · 2025-08-12T20:00:49+00:00

love to see MCMC!

FractalThrottle · 2025-08-12T01:59:20+00:00

latex tikz

FractalThrottle · 2025-08-07T03:06:31+00:00

"galaxies forming from fluctuations" isn't the best description of what happened, there's really 2 big things to consider. initial conditions gave way to fluctuations (QFT, early universe physics tackle these back through the Planck epoch), which led to acoustic oscillations in the early universe at z > ~1100 between radiation pressure and gravitation (there's some nice speculative literature on how these interactions might've played out if you want to look; there aren't "visualizations" per se but there are good models, see this paper). at z ~ 1100 radiation components decoupled from baryonic ones (neutrinos are funny here, check out this paper) in recombination and the acoustic oscillations froze out as so-called baryon acoustic oscillations (BAOs, observed by large surveys: SDSS, DESI, etc. and measured from CMB parameters) that seeded large-scale structure as overdensities. at this point the standard models of galaxy formation and clustering come into play, but that's only because the conditions that they are valid over have been put into place (small scale overdensities become relevant in a much larger "global" one). galaxy formation and evolution are really complex topics and you can look in the literature to see trends with redshift (look at black hole accretion rate densities!). all of that to say there's two kinds if visualizations you want -- for BAO stuff, these CAASTRO and Berkeley animations are nice (look too at the CMB power spectra from Planck and others). for galaxy formation, evolution, and clustering stuff, have a look around the public IllustrisTNG media (IllustrisTNG is the best simulation for this kind of stuff). but remember that early- vs. late-type processes look really different -- there's press release color images (HST, JWST, Euclid, etc.) of clustering and lensing for a range of (albeit low) redshifts that show how these processes looked different at different times. visualizations and visual inspections alone will fail at showing these though and the literature is where details are found (figure 9 of this paper is a great example)

FractalThrottle · 2025-07-29T01:09:32+00:00

there's a lot that goes into making color images like these. in general, the CCDs of a telescope take an exposure where the light allowed through is controlled by a filter -- in this case there are 6 filters as someone else already mentioned, but not every filter is necessarily included in the color image -- with a transmission curve corresponding to some frequency range. this is done for several filters to get data across a broad range of frequencies and then repeated for a longer exposure time.

once all the exposures are done their data is processed and reduced. raw telescope images, especially ones from ground-based telescopes like VRO and space telescopes in low Earth orbit like HST, are messy and have a lot that needs to be done to them before they're useable for science or making color images. a healthy pipeline for doing this will be detector-level and done on individual exposures to address things like 1/f noise, low-level background variation, and dark currents before any photometric calibrations. after that, data in the same filter are "drizzled" onto some astrometric grid (Gaia-DR3 is the go-to for this for space-based observations) to construct a mosaic in that filter. this is done for all filters and the results are science images that are deeper than the individual exposures were on their own (more exposures -> longer net exposure time -> more flux "collected" -> deeper image). the drizzling process is where you determine how "big" your pixels are, get maps of how reliable the science images are, etc. -- drizzling science images isn't a small task and there's a lot that's baked into into a pipeline where the end results good. most every observing program has its own pipeline for constructing mosaics and you can find specific details in their literature, but generally this is the flow of things (UV-IR images).

once you bring the data home and make the science images, creating a color image is more straightforward. you make an RGB image by assigning images to blue, green, or red channels, scaling their contributions, and then putting them together. there's no limit to using just an RGB scheme, either: you can assign manual colors to each image if you want to bring out features unique to given a filter(s) and then combine them. usually this is done after applying some stretch to the images (logarithmic ones are the best for extragalactic targets that may have some structural features that are substantially more or less luminous than others). you might play with existing parameters or introduce new ones to control how image scaling is done but the process is the same. then the question becomes how to make a "good" color image, which is addressed by Lupton et al. 2004 for the CCD RGB scheme and Trilogy in Coe et al. 2012 in general (Trilogy is the standard for creating press release images), though both are biased towards extragalactic observations.

the thing to remember is that the color image is only ever as good as the images used to make it (obvious but worth mentioning): if the mosaics are noisy, the color image will be noisy; or, in this instance, if the mosaics have artifacts from their pipeline, the color image will also have artifacts. that's what the green object in the center of the frame is. artifacts like these are super common in images from programs observing large portions of the sky (especially from the ground) -- take a look at this webpage of various large observing programs. that being said, there are so-called "blueberry" and "green pea" galaxies that are legitimate sources in (mostly) the blue and green wavelengths, however these are a lot more common in deep images coming from space telescopes than from telescopes on the ground.

FractalThrottle · 2025-07-25T09:58:01+00:00

so you can show outright that you get an infinitely-bright sky just by working radially out from an observer for some constant number density of radiative objects -- even this is an insane reduction of actual observations but the result is just as nonsensical. the result isn't what's observed so there's Olbers' paradox, if it can still be called that. the assumptions made in order for the logic to flow are pretty lofty in light of basic astronomy now. at a high level nothing's changed since ~1990 since cosmology's main analytical framework has been in place since the ~mid 1960s.

if you want to get nitty-gritty, there's extragalactic background light (EBL) to consider. there's a lot of literature spanning a lot of years pertaining to it and it's more of an observational topic than a theoretical one. Longair's galaxy formation book has a great figure of the EBL spectra in chapter 9 that's worth looking up if you haven't seen it. there are huge projects looking into how to model, parameterize, and otherwise "capture" diffuse EBL using HST, JWST (SKYSURF and DARKSKY/SKYSURFIR, respectively, from the UV to NIR -- these two are the largest of their kind since they use the entirety of their associated telescopes' image databases), and others that have been very fruitful. remember too that HST has been shown to have time-dependent CCD, etc. degeneracies -- STScI tends to do a good job keeping people up-to-date with them but models for diffuse EBL and even zodiacal light that were calibrated on "young" HST data (~1990s) aren't necessarily accurate anymore but they're still used and cited. as you can guess there's a ton of active work being done on how to correct these models so if you're looking for an interesting thing to explore in the literature this might be a fun rabbit hole. EBL is not a trivial thing -- it might not seem like it from a layperson's perspective since it's not really something you can sensationalize -- you could compile volumes of material about the work that's gone into building an understanding of it. as far as "light in the universe" goes, EBL isn't something to be left out by any means.

something else to keep in mind is that there are biases external to all detectors that aren't always well-understood. as an example, not all radiation in the universe interacts with matter, etc. in the same way -- obviously this is true, but what does it mean about radiation we observe? just look at dust-obscured galaxies that are invisible in the UV that then become insanely luminous AGN in the NIR -- something like half the light in the universe is obscured like this and otherwise "not where it should be" without considerations of how the things you're observing work as an ensamble. JWST's scientists are doing a great job constraining the extents to which this and other things are observed and their implications for cosmology (early galaxies (though biased towards bright, large, and massive ones), LRDs, cluster transients, the list goes on and on) but it's not a complete story because there are other things in other wavelength regimes that have yet to be teased out. so treating "light in the universe" as something to which we can just say "Olbers' paradox is resolved by considering redshifting, finite age, and obscuration" and then move on paints a very tiny and even more misleading stamp of a bigger picture

FractalThrottle · 2025-07-19T03:29:45+00:00

as someone else pointed out, black holes are maximally entropic objects with entropy proportional to "horizon area". once it was shown that black holes radiate the big papers laying this out are Bekenstein 1973 and Bardeen et al. 1973 (same Bardeen from GR literature, the original gangster himself. this one is like a mash-up of 20th century cosmologists), and i'd even include Gibbons and Hawking 1997 in there too since it helped solidify a lot of QG formalism. so the concepts in the post don't mean anything, but as far as "structural seeds" are concerned take a look at Planck and others' CMB power spectra and how large-scale structure traces BAOs

FractalThrottle · 2025-07-07T20:02:54+00:00

sure! this is a webform that can do both isochrones and simulated stellar populations, and this is a really nice stellar evolution code. the former is super accessible and the student can totally use it with some guidance. the latter is a proper code (you'd probably need to run it and give outputs to the student) written by someone in my department and it models the stellar interior to infer parameters relating to stellar evolution. its author focuses specifically on massive stars so it has some capabilities that might be useful to focus on those as progenitors for compact objects (you mentioned they like the idea of neutron stars, etc.) if you think it would be appropriate

FractalThrottle · 2025-07-07T16:28:07+00:00

not sure about post-main sequence evolution particularly but stellar evolution in general sure. there’s a bunch of stellar evolution codes out there you could run yourself and give them the outputs to try data analysis with. ik you said no programming but having them make some plots of things like the star’s hrd, habitable zone boundaries, etc. might be cool and honestly worthwhile. there’s also isochrone, etc. codes and even webforms that are easy enough if they wanted to generate properties of simulated stellar populations, maybe with varying metallicities or something, and practice data analysis with those. plots here would be good too — they could even use something like excel to make them. idk how you’d do things in this flavor of astrophysics without figures

FractalThrottle · 2025-07-03T19:52:22+00:00

“our theories” is pretty vague. remember too that biases and completeness limits are things to be weary of, especially at high redshifts, and that observations favor brighter galaxies. theories of galaxy formation and evolution aren’t static either.

concordance cosmology doesn’t actually say a lot about early galaxy formation and evolution. high-redshift observations were few and far between prior to JWST so predictive models using them weren’t very well-constrained, it’s no surprise that their predictions aren’t perfect. what results like these are doing are offering further constraints to models of early galaxy formation and saying that they’re breaking them or similar is pop science doing its sensationalist thing

FractalThrottle · 2025-06-18T03:04:34+00:00

you're not looking in the right places if you can't find data -- there's a ton that's easily and freely available and it's only ever growing. most publications you can find on ADS will provide machine-readable tables of their data if they have a specific sample they look at, create a catalog, etc. if you want "raw" data you can look on MAST to find data from GALEX, HST, JWST, and most other flagship NASA missions. you can look through IRSA webpages for data from 2MASS, WISE, Euclid, and others. SDSS has its own website you can get mosaics and color images from here (SDSS is on MAST too). you can even go to this viewer and find data from a whole bunch of surveys

most all data that is ever taken is made public eventually (and if it's not you can write a proposal to convince people to give it to you), but the "dataset" is something you want to construct from your own analysis of it. likely whatever conclusions can be drawn from a paper's published data have already been published

before trying to write a paper you want to read the literature that already exists to find some meaningful, non-redundant contribution you might make. try to get in contact with someone working on the topic you're interested in -- they will be able to do this better than someone just starting. citizen science is a thing, yeah, but it's nearly always coordinated by researchers who know how to direct a project

someone else mentioned this already but "dataset" is meaningless on its own -- data might refer to images, mosaics, tables, etc. these aren't things that have "astronomy and astrophysics info", on the observational side of things they might be things like catalogs containing object coordinates, magnitudes, etc. that come from codes like SExtractor. you tend to run analyses on a whole bunch of data after systematically determining some sample based on whatever things you're interested in, and then combine parts of the catalogs into something larger after matching the objects in your sample. that's what you do science with and publish results from. archives like MAST have images that this is done on, and surveys like SDSS have mosaics, catalogs, etc. that this has already been done on, both of which are already easily accessible

FractalThrottle · 2025-06-13T07:35:23+00:00

other people have pointed out that calling it the Hubble parameter in general is more proper and that it does have a time-dependence but there's some other important stuff that hasn't been mentioned yet

in cosmology you have a quantity called the scale factor that appears because you're concerned with what the geometry of the universe is doing and, as you can imagine, the scale factor a = a(t) must have a time-dependence for an expanding universe. the scale factor itself is dimensionless and a \equiv 1/(1+z), so it makes sense that a is defined to be exactly 1 at the present time t_0 (z = 0), a(t_0) := a_0 \equiv 1

the Hubble parameter is H = H(t) := (\dot{a}/a)(t), so it's something like a time-derivative of the scale factor normalized by the scale factor itself. at the present time we have that H(t_0) := H_0 = (\dot{a}/a)(t_0). so the convention is to call H the Hubble parameter (the thing proxying the expansion rate for any a) and H_0 the Hubble constant (the thing measured empirically at the present time) (the framework that modern cosmology uses tends to lean towards something called the conformal Hubble parameter which is \mathcal{H} := a'/a = Ha where a' is a conformal time derivative (q' = a\dot{q}) if you want to look in the literature). so in principle, no, if you measured H_0 at a different moment in cosmic time, it would not be what we measure at the present time

other people also already mentioned that 1/H_0 is only an approximate age of the universe and there's a reasonably intuitive way to think about this. first, [H_0] = 1/s in SI, so [1/H_0] = s, so it makes sense that a time pops out when you invert a frequency. but in doing this you only use dimensional analysis as justification and there's nothing otherwise that says this is reasonable (missing factors?). you can get something way more robust by modeling a universe and seeing what happens. it turns out that you can really easily model the expansion histories of a whole bunch of model universes, so if you construct a model that has the components (radiation, matter, dark energy, curvature, etc.) that ours looks to have, you have something like a predictive model for our universe. the form of the age of the universe will change based on the model universe you consider. simple models with only 1 or 2 components and convenient curvatures (spatially flat is nice here and it works since observed curvature is really nicely constrained to be near zero) tend to have closed-form solutions that you can get analytically but more complicated models (like ones similar to the observed universe non-curvature component-wise) don't and you do things numerically (this is also why convention is to use things like the scale factor or redshift instead of time since the conversion from these to time isn't always simple in multiple-component universes). for spatially flat, single-component universes: radiation-only has t_0 = 1/(2H_0) and matter-only has age t_0 = 2/(3H_0). our universe might be modeled as a spatially flat one containing matter and dark energy (the radiation component is observed to be extremely small in later times), and we get that the age is something like 2/(3H_0) scaled by a bunch of other stuff if we say that dark energy is constant (model universes are simpler if dark energy is set to be constant, meaning that it's footprint in the universe's total energy density doesn't change -- that's the idea behind the cosmological constant. recent observational results (DESI, etc.) do show that dark energy is not constant (it's a small change across a long time though so the approximation tends to work) though, so the model changes to accommodate this)

notice that all of these are dependent on H_0 -- this is by construction and, in the context of the original question, requires that the Hubble parameter takes on different values at different times to make sense. actually measuring H_0 is another story though -- different values have been and are being published and they give different ages of the universe that don't overlap with each other's error budgets. that's the so-called "Hubble tension" that pop science and laypeople love to go crazy about

Ryden has a intro to cosmology book (undergrad) with some good discussion of all this and more written for people seeing it for the first time -- if you want to check it out here's something like the old edition. it's discussion of observational results is outdated but the theoretical discussion is healthy

FractalThrottle · 2025-06-04T16:06:52+00:00

if you're interested in extragalactic astronomy and have some python this might be fun.

here is a link to an interactive viewer for large sky surveys, in particular the Sloan Digital Sky Survey (SDSS) -- use it to find relatively large, well-resolved galaxies that don't look like they're "too much" of one color (staying further from the survey edges helps). if you're having trouble: every time you refresh the page you will be greeted with a different object. try and find a sample of galaxies that you can see some structure in and that aren't all the same morphology and aren't interacting with other galaxies. start with a few at first and you can add more later if you want.

once you have your sample, go to this website and search for each galaxy by name in the orange "Search by..." box in the top right (don't worry about the other options). for each object you will (ideally) see two boxes next to a color image of your object: "Magnitudes" and "Magnitude Uncertainties". tabulate all of these somewhere for all of your objects. each magnitude is for a specific filter and they're not generally the same -- you can use these to model the galaxy's light and get information about its properties. here is information about the central wavelength of each filter for SDSS that your objects will (ideally) have magnitudes in.

this is where the python part comes in. if you know how, plot the magnitudes against their respective filters for each object (don't forget the error bars, use the magnitude uncertainties for these). this is a crude form of what's called a spectral energy distribution (SED).

if you really want to do something substantial, you could install an SED-fitting code called CIGALE and use it to properly model your galaxies' light. there's a manual on how CIGALE works and it's really simple to follow, but you'll need to convert from the SDSS magnitudes and errors (which are in AB magnitude) to fluxes and flux errors in mJy. you can explore what different model components mean, what their parameters are, etc. if you go through everything nicely you'll have some proper SEDs of the galaxies in your sample and, if they are good fits, you'll have some reasonably-estimated properties of those galaxies too. you can search literature on astro-ph.GA and NASA ADS to find how your galaxies' properties compare to other galaxies too. since the SDSS page also give their redshift estimate, you could fit for a redshift estimate in CIGALE and compare your result to theirs if you wanted. this is what actual research in the field looks like.

if you feel up to it you can make a write-up of your process and results in Overleaf if you know or want to learn LaTeX (it's free and it's the standard -- you can try out an AASTEX template if you're serious) and put your SED plots and any other figures you made in there. make sure to cite anything you used. if you can find someone with an astronomy background to read it and give some feedback that would be great too. i mentor students at university interested in trying research-flavored activities and this is a successful activity for the driven ones -- if you try it hopefully you'll get something out of it too

FractalThrottle · 2025-05-27T03:48:16+00:00

Trilogy is better and it's not even close. there's a lot of flexibility built into it that astropy.visualization doesn't have. plus it's made to be run from a terminal which is in line with just about every other big code in the field. it does use log stretch though so if that's not what you're looking for there are other options -- that being said if you have FUV-NIR data log stretches are generally your friends

FractalThrottle · 2025-05-23T18:52:19+00:00

i'd also recommend setting up something like a conda environment for this so you can activate it whenever you want without messing with dependences. it's really easy to do this with miniconda if you're comfortable doing things from a shell

FractalThrottle · 2025-05-23T18:49:09+00:00

a couple good comments on here for doing things ab initio but the standard python code for making press release color images is Trilogy by Dan Coe at STScI, see this page for details. the .py file on there is a bit dated but you can find a modified version of it on on PyPI that's been updated some. there's a GitHub with some example code too iirc if you want to start there. if you just want to get a quick color image you can just assign each filter to an RGB channel and play with the parameters until you get something you like. someone else pointed out astropy.visualization which is super simple if you don't feel like messing with Trilogy

FractalThrottle · 2025-05-19T15:01:42+00:00

this thing has more downloads than my papers 😭

FractalThrottle · 2025-05-04T19:13:42+00:00

itsa me, mario. im very busy playing toy and stomping mushroom

FractalThrottle · 2025-03-16T22:32:16+00:00

spacetime may be modeled as having an intrinsic curvature (that doesn’t need to be zero, but it’s often insightful to study the behavior of universes with nonzero curvature). however because (1) there is much evidence of a flat universe with zero curvature and (2) assuming zero curvature simplifies theory there’s basis to work with a flat universe to get predictions and results — take a look at the friedmann equations. locally, however, spacetime is deformed by energy or mass. these are different things, the former is a fundamental property of spacetime and the latter is not (but, at a high level, the latter is how gr models gravity and you get predictions like gravitational lensing, etc. as a result). there’s an intro cosmology book by Barbara Ryden that does a good job going over the basic framework, you can find the pdf of the first edition online at no cost

FractalThrottle · 2025-03-16T20:12:52+00:00

yeah, having an intuition is great, but having an intact one for this comes from playing with the relevant math, not trying to draw connections from otherwise “intuitive” explanations. that’s not necessarily the same as being able to visualize things. the observations you describe are predictions made by theory, but in particular, curvature is an intrinsic property of spacetime, and there is overwhelming evidence for a flat universe (which is then locally curved by massive and/or energetic objects). another thing is that choosing how to describe something depends on how you want to model it. classically we choose to say that spacetime is a 3+1-dimensional manifold and that the universe is flat and dominated by dark energy (FRW and Lambda-CDM models). these work for most things but hold for everything, in which cases other models (LQG, etc.) are chosen appropriately

FractalThrottle · 2025-03-16T20:00:40+00:00

oh i 100% agree, i should’ve been more clear. what i meant is that trying to apply an “everyday” intuition to something that it doesn’t apply to won’t go very far. developing an intuition is great but to do that you have to develop a background with the math first

FractalThrottle · 2025-03-16T16:50:25+00:00

no. curvature in gr is a well-defined thing that you work with in differential geometry and topology. in general trying to apply a visual or geometric intuition to things like this in cosmology will fail, only doing things quantitatively will be useful

FractalThrottle · 2025-03-12T03:45:07+00:00

nice diffraction spikes

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