Not specifically to cosmological assumptions except for absolute rate, because the ratio flux in various bands is independent of assumed distance. The absolute star formation rate will always be sensitive to the assumed galaxy distance; there are relative measures (H-alpha equivalent width as indicator of star-formation timescale, so-called birthrate parameter, which compares star formation in the last 300 Myr or so to its long-term average, or e-folding timescale) which are not.I wonder how much this approach depends on the calculated absolute luminosity of e.g. H II emissions versus comparative luminosity in different bands of a given galaxy? The later analysis should be less sensitive to cosmological assumptions at z>1.
The various tracers of (massive) star formation are differently sensitive to dust extinction, mix of stellar masses and thus recent changes in SFR, and relative distributions of stars and gas or dust. Now that UV, mir-IR, and emission-line data are available for large numbers of galaxies, there are recipes which use a weighted combination to try to minimize the disadvantages of each tracer.
How come? By virtue of Einstein's theory of General Relativity; specifically, by 'gravitational lensing'.
Let's say that M31's SED peaks at 500 nm, and is 1 magnitude fainter at 250 nm (ditto at 1 Ám). Let's assume we can image it, from above the Earth's atmosphere, at z = 0.
If M31 were at z = 1, its SED peak would be observed to be at 1 Ám, and it would appear 1 magnitude fainter at 500 nm (ditto at 2 Ám). In other words, to obtain an image of a z = 1 M31, to directly compare with how it appears to us here, we'd need to do our imaging in appropriately redshifted bands. This is a challenge: while the observed size (area) of an M31 changes only slowly with increasing z (beyond a certain value - do you know what? - and assuming '737 cosmology'*), the Hubble's resolution declines with increasing wavelength. On the other hand, the real M31 image we'd have to compare a distant one to, if we observe it in wavelengths Hubble's vision is sharpest in (i.e. the UV), is one we're not used to (e.g. this GALEX view of M31). Further, as M31's SED drops rather rapidly with decreasing wavelength (blueward of ~300 nm), it'll appear a lot fainter.
Yes, it would. And that's rather the point, isn't it? After all, your original question asked about detectability, didn't it?No offense please, but why should I care? Isn't surface brightness more useful to talk about. I would imagine that compressing the luminosity into a point would much improve visibility/detectability.More generally, how do you convert the observed luminosity (brightness) of a z ~1 galaxy in an HST image into its estimated total luminosity if it were concentrated into a point, located 10 pc from us? If you don't know, just say so; I'll be more than happy to explain (HINT: the answer is in that 8-page thread).
If the pixel-size of the instrument (assumed to be comparable to its areal resolution) you're using to observe highly redshifted M31's is larger than the (redshifted) outer isophote of your image of the real M31, that's the best you can do, isn't it?
I started a new thread on this: What kinds of galaxy has a jet (or pair of jets) coming from its nucleus?That's interesting and something I was not aware of. What sort of galaxies are found with jets then? Are AGNs and quasars thought to be the same phenomenon, the result of central SMBHs in galaxies?As I understand it, spiral galaxy AGNs rarely, if ever, have jets. Certainly not ones visible in the x-ray part of the spectrum.
I'll return to this later; it's getting close to some of the central issues at play, wrt your OP question.In this, I think we are in much agreement. The various sorts of detection discussed don't necessary tell us that much detail about what we have detected at cosmic distances. Again I think this is the whole point of the D&L paper. I'm not sure I see the basis for your severe criticism. I guess I could read that thread again but it really lost me in the details.But here comes a possible problem: for a source (object) which is just barely resolved, how do you tell if it's a spiral (and not an elliptical)? And even if you can tell it's a spiral (by some magic), how can you tell it's a spiral like the Milky Way?
Fortunately, M31 does have some nice emission lines in its spectrum. However, if you had asked about an elliptical galaxy, one without an AGN, your new criterion would have made it extremely difficult to answer in the affirmative for distances greater than ~0.1 (i.e. requiring that an elliptical galaxy must have several emission lines in its spectrum, in order to determine its z, before attempting to address the rest of the question, concerning detectability).Definitely emission, who knows where the absorbing medium is relative to the emitter? Of course you could also argue about re-emission but that seems much less likely to be a problem.Emission or absorption? Or you don't care?You are a stickler, but of course you would need many more lines to identify the elemental source of any measured line.And if there's only one, how do you know which one it is? If you make a mistake, you'll get the estimated z completely wrong, won't you?
In any case, as ngc3314 has addressed in his later post, quite a number of independent observables are used to assign a type to a distant galaxy. And it is pretty rare that an absorption line system, in the spectrum of a distant galaxy, is misleading (in terms of the z of that galaxy). There are some very nice papers based on SDSS on this topic; would you be interested in reading some?
Actually, I'm not (continuing to make the D&L paper's point). If you think so, I recommend that you re-read that thread, and perhaps ask some questions on bits you don't quite follow.You are continuing to make the D&L paper's point! Why did you get so upset about it?Assuming the redshift is confident (by some magic), wouldn't that simply result in you having identified an extended source, of redshift z? How would you know it's even a galaxy?
In any case, I'll also return to this later; it's getting close to some of the central issues at play, wrt your OP question.
The thing about determining redshift from lines (especially absorption lines) in the spectrum of an object, in the ~ UV to ~mid-IR, is that you need a lot more photons, than you do to merely image the objects in some broad waveband or other (if the galaxy has extremely strong emission lines - as many starburst and AGN's do - then you don't); if you need a lot more photons, detectability becomes more difficult.(I wasn't aware that magic was required to be confident about a red-shift. Are you referring to my failure to mention the need for multiple lines?)
I'll present some (simplified) calculations, in a later post.I'm very interested in what you can tell me about the above very specific question about an HST image of M31 at z~1.2.
* Hubble constant 70 km/s/Mpc, matter content 30%, 'dark energy' content 70% - this is a simplification
Once again, this is bringing us closer to getting answers to the question in the OP; specifically, from observations made using all available current instruments, at their limits, what criteria can be used to estimate the type of object (galaxy) is detected?
Yes, and the fact that the bulge seems too small, that the arms are quite unlike those of our own galaxy (see this APOD, for example).Are you referring to the absence of a bar in that galaxy?
The images in that paper illustrate the limits of current instruments, in terms of imaging.That helps answer my question(s). Interesting that the authors don't highlight how this population might substantially differ from the current population. The featured conclusion is that the merger theory of large galaxy formation is probably wrong. However they do mention star formation rates as much higher. This makes me wonder how star formation rates can be measured.At greater distances what do spiral galaxies look like? Well, even with the Hubble, not many look like spirals, but many are clearly disk galaxies. For example, look at the postage-stamp images in this recent paper: CANDELS: Correlations of SEDs and Morphologies with Star-formation Status for Massive Galaxies at z ~ 2.
To actually start answering your question(s), we need to get quantitative, as ngc3314 has done (and as a later post - or two - of mine will do too).
Another would be to look at the galaxies detected, with current instruments, in their deepest/most sensitive views of random bits of the sky (well away from bright stars and the galactic plane and bulge), and decide which - if any - look like redshifted M31s.
I'll write a post (or two) on the former, later.
For the latter, let's look at some of work done - both already published, and also underway - and proposed.
GOODS (The "Great Observatories Origins Deep Survey") is one such effort (note: also now published are deep 1.4 GHz VLA observations of GOODS-North):
Building on GOODS is CANDELS ("Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey"):GOODS unites extremely deep observations from NASA's Great Observatories, Spitzer, Hubble, and Chandra, ESA's Herschel and XMM-Newton, and from the most powerful ground-based facilities, to survey the distant universe to the faintest flux limits across the electromagnetic spectrum. GOODS data track the formation and evolution of galaxies across cosmic time and map the history of universal expansion using high redshift supernovae. The survey covers a total of roughly 320 square arcminutes in two fields centered on the Hubble Deep Field North and the Chandra Deep Field South.
Another example is COMBO-17 ("Classifying Objects by Medium-Band Observations - a spectrophotometric 17-filter survey"). And there are several others ...The Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS; Grogin et al. 2011; Koekemoer et al. 2011) is designed to document the ﬁrst third of galactic evolution from z = 8 to 1.5 via deep imaging of more than 250,000 galaxies with WFC3/IR and ACS. It will also find the first Type Ia SNe beyond z > 1.5 and establish their accuracy as standard candles for cosmology. Five premier multi-wavelength sky regions are selected; each has multi-wavelenght data from Spitzer and other facilities, and has extensive spectroscopy of the brighter galaxies. The use of ﬁve widely separated ﬁelds mitigates cosmic variance and yields statistically robust and complete samples of galaxies down to 109 solar masses out to z ~ 8.
This, from the stated objective of CANDELS, would seem to be directly relevant to TooMuch's question: "... yields statistically robust and complete samples of galaxies down to 109 solar masses out to z ~ 8" ... if M31's mass is >~109 solar masses.
AFAIK, M31's estimated mass is well above a billions sols (for example), and even if only the stars, gas, and dust in the disk are counted, M31 is still well above the 109 solar masses limit (for example).
Among the CANDELS science goals, grouped by theme, Cosmic Dawn and Cosmic Noon would seem to be directly relevant (same link as above). For example, it would seem that an M31 out to z~8 will be robustly detected in CANDELS, at least in some statistical sense, and robustly imaged (i.e. as a clearly resolved extended object) out to z~4 (again, at least in in some statistical sense).
Of course, for CANDELS to succeed it must obtain the images (actually data) described in the HST (etc) proposals, and those images/data must be as good as expected. Also, the interpretation of the data will rely on a great deal of published astrophysics and astronomy, so the relevant conclusions from those papers will have to be robust too.
Does this answer your question to your satisfaction, TooMuch (at least this version of your question)?
Yes, astronomers know this, which is why there are extensive (and expensive) surveys at a variety of IR wavelengths and going as deep as possible.. This is really the name of the game in study of galaxy evolution. Are you suggesting that such selection effects are routinely ignored?
I tried to reduce this question to a question about how far away we could detect galaxies if they were as they are now in the local universe. Now it's a valid answer to say well we could detect them at some very high z if this or that gave us some evidence of their presence, e.g. x-rays, lensing or absorption of background quasar light etc. But that's not what I'm really asking. I'm asking what can we really be sure about at those distances. In particular can we already exclude the existence of "mature" galaxies similar to those in the local group? That's really what I meant by detection.
I'm quite skeptical about current cosmology but it is this theoretical lens through which nearly all astronomers view the universe. A problem with this is the human tendency to see just what we expect to see and downplay or ignore that which contradicts what is expected. I see a tendency to explain away evidence that challenges theory. Certainly it's healthy that astronomers recognize things which conflict with their theory and focus on those issues. However the tendency seems to be work hard to hunt for something (anything?) to explain the discrepancy. As soon as a possible explanation is found, the discrepancy seems to be forgotten. The problem is that the explanation for the discrepancy may also be pretty theoretical and quite possibly wrong (e.g. inflation). However because it resolves a discrepancy with a popular theoretical model, it is readily accepted as correct.
Just a quick response to your last post, TooMuch (I'll write at greater length later).
As ngc3314 pointed out, and as the many COMBO-17, GOODS, etc papers explain*, an enormous amount of work has been done on the topic of galaxy evolution. And an even greater amount of work has been done on cosmological models.
Perhaps the strongest take-away is that most conclusions are pretty robust, in the sense that they've been tested six ways to Sunday, by independent teams, with data gathered independently, in different parts of the electromagnetic spectrum, etc, etc, etc.
In the case of your question - re-stated, and concentrating on its key intent - perhaps it's worth spending some time on understanding what a distant Local Group spiral (the Milky Way, M31, M33) would look like, in terms of the actual data (and not just its visual appearance)?
Quick side question: strongly lensed background (distant, high-z) galaxies are certainly rare, but there are now, what, dozens (hundreds?) reported in the literature. To what extent do you think conclusions from studying these would be (should be?) directly relevant to galaxy evolution? cosmology?
* as well as those which they reference; hundreds of papers, at least
But that's not much help, is it? Perhaps you could re-phrase your question?
You might find this recent paper of interest: "Sizes and surface brightness profiles of quiescent galaxies at z ~ 2" (arXiv:1111.3361)
Totally.Further, how much do the conclusions depend on cosmological models.
For example, how bright (= total emission of electromagnetic radiation) a z~2 galaxy 'really' is (or was) depends not only on the values you plug in to your LCDM cosmological model, but also on the fact that you're using an LCDM cosmological model to begin with.
Not so.It is common in the literature to talk about galactic evolution. Astronomers believe that the age of the universe in finite and they have settled on a specific value (13.7Gy). They use their GR cosmological model to interpret what they are seeing at z > 1 because at these distances it matters. Locally (say within 1 Gly) cosmological models have small effects.
Changing the values of parameters in an LCDM cosmological model can make an enormous difference. For example, suppose you use a value for H0 (the Hubble constant) of 700 km/s/Mpc (instead of 70): this rather dramatically changes your understanding of just about all galaxies beyond the Local Group*.
Of course not.So I wonder. At say z=2, do we really know what the galactic population is like?
And we don't even really know what the galactic population is like in the Virgo cluster (the nearest galaxy cluster)!
After all, everything is inference built upon inference built upon analysis of electromagnetic radiation detected by 'telescopes' on Earth and above it**.
No, we don't; we haven't been there.Do we really know that it is very different from the local universe?
Where, in the long set of interlocking chains of evidence and applied physics do you want to start, in your quest for 'really'? For example, how do we know that the bright green lines, at ~500.7 and 495.9 nm, seen in so many astronomical objects, are "[OIII]" lines? After all no one has (AFAIK) ever seen this pair of emission lines in any lab, here on Earth.
I think you are the one who needs to do the hard work to answer this, to your satisfaction. Perhaps you could start by attempting to answer ngc3314's question ("Are you suggesting that such selection effects are routinely ignored?").Or is the selection effect so strong that we measure only the odd ball cases (e.g. star burst galaxies) and potentially draw incorrect conclusions.
Really sure? Really, truly sure? Really, truly, absolutely sure?I tried to reduce this question to a question about how far away we could detect galaxies if they were as they are now in the local universe. Now it's a valid answer to say well we could detect them at some very high z if this or that gave us some evidence of their presence, e.g. x-rays, lensing or absorption of background quasar light etc. But that's not what I'm really asking. I'm asking what can we really be sure about at those distances.
We can be really, truly, absolutely sure about nothing. At any distance.
The "maturest" galaxy in the Local Group is probably what we call the globular cluster Omega Centauri (currently thought to be the remnant of the bulge of a dwarf whose other stars have been tidally stripped by repeated close encounters with our own galaxy). Omega Cen is faint, and compact; it's unlikely to be detected even out to z ~ 0.3, much less z ~ 2. The Local Group has quite a few "mature" dwarf galaxies, none of which could be detected beyond z ~ 0.3.In particular can we already exclude the existence of "mature" galaxies similar to those in the local group?
But perhaps you had something different in mind, with respect to "mature" galaxies?
How else could you do astronomy (other than as a form of stamp collecting)?That's really what I meant by detection.
I'm quite skeptical about current cosmology but it is this theoretical lens through which nearly all astronomers view the universe.
May I ask if you regard papers (published in relevant, peer-reviewed, journals) as the primary source material for your understanding of astronomy? Or is it, perchance, press releases, articles in New Scientist, that sort of thing?A problem with this is the human tendency to see just what we expect to see and downplay or ignore that which contradicts what is expected. I see a tendency to explain away evidence that challenges theory.
If that's what you really (truly, absolutely) think, then may I suggest you attend a conference, or symposium, or workshop, attended by a bunch of working astronomers and astrophysicists?Certainly it's healthy that astronomers recognize things which conflict with their theory and focus on those issues. However the tendency seems to be work hard to hunt for something (anything?) to explain the discrepancy. As soon as a possible explanation is found, the discrepancy seems to be forgotten. The problem is that the explanation for the discrepancy may also be pretty theoretical and quite possibly wrong (e.g. inflation). However because it resolves a discrepancy with a popular theoretical model, it is readily accepted as correct.
To make sense of what you hear at such an event, you'll likely need to become pretty au fait with the relevant sub-branch of astronomy or astrophysics that the conference (etc) is about ... if you aren't very familiar with the topic, most of the discussion will go right over your head (and you'll certainly miss some of the sharpest disagreements, except if they're conveyed in emotional tones).
Stay tuned!That's closer to what I'm looking for. It's cool that there are ongoing projects to look deeper with the instruments we already have (GOODS, CANDLES etc.) and I look forward to reading about the results. I'm seriously disappointed that space exploration missions like Webb have been cut or delayed (to pay for shuttle flights to maintain humans in metal cans in low orbit). Hence it's particularly nice to know that there will be other sources of new info in my life time. (When is the Planck data coming?)Another would be to look at the galaxies detected, with current instruments, in their deepest/most sensitive views of random bits of the sky (well away from bright stars and the galactic plane and bulge), and decide which - if any - look like redshifted M31s.
Well, I'm certainly learning a lot, and I see how my original question is vague. But still I wonder if you can answer the specific question I asked about a visual spectrum image with HST of M31 at z~1.2. The paper I referenced (that you pointed out) had a picture of what seems to be a face on spiral at that distance so I wonder what would M31 look like. Would it show up as a spiral in a similar image? Or is the galaxy pictured much brighter and/or larger allowing an image to be made that would not be possible for M31.Does this answer your question to your satisfaction, TooMuch (at least this version of your question)?
* or maybe a bit further; understanding of some nearby groups would not necessarily change dramatically
** strictly speaking there are also inferences from detections of very high energy cosmic rays and a tiny number of neutrinos (SN1987a)
As far as I'm concerned, atomic emission lines are a fact and we know that visible matter in the universe is just like the visible matter on earth. As I said before, our ability to observe the cosmos in detail fails off rapidly at high z. I'm just trying to get a sense of what can be taken for granted as correct and what is truly speculative or entirely dependent on cosmology.
Distributions of DM as it is understood in LCDM should be cuspy. No cusps found in the most profoundly DM dominated galaxies. There should be a ton of dwarf satellites around local galaxies but there are not. In fact I just read a paper claiming that most of the dwarfs in the MW (and perhaps M31 as well) are the result of a merger or near miss.
It's like the tail is wagging the dog, theory is ruling observation. Remember I said before I wish astronomers would be more humble about their conclusions? This is what I mean. The whole house of cards is built on the CMB interpretation and BB theory and it keeps running into serious problems. Still suggesting that the LCDM interpretation is wrong is heresy (to some).
I think the debate about current cosmological assumptions is heating up, especially in the last few years. If the current theory is in fact wrong, it will fall eventually so no worries there.
Clearly I'm playing devil's advocate here. Maybe all is fine with BB theory. Challenging things helps me learn more. Skepticism is important in a study like astronomy where inferences are stacked up many levels deep. Most of the regulars here seem to be staunch defenders of the "mainstream" theory. That begs for challenge.
For instance, there are many that come here, complain about DM and have their own little idea about not needing DM for galactic rotation curves. Someone will then ask how that affects, say lensing, or the observations of cluster movements or the CMB, and the answer comes back, HUH? A lot of times, presenters here are simply not aware of the interlocking nature or some of these observations. If a majority of the presenters were better versed in the theories they were trying to overturn, there wouldn't seem to be as many defenders of the mainstream here.
And besides, that's how it works. If someone presents a new idea, if it can't pass all the questioning, it's not going to get very far anyway. It's the mainstream theory because it matches the most observations. Are there anomalies? Sure. Are there open questions? Sure. But, there simply isn't anything else that provides as many answers. But the scientists with keep working on those open questions and anomalies and they may find something that overturns the current mainstream theory.
I will give you credit. You are asking questions and not just blowing off the answers. I get the feeling you may not agree with some of the answers, but it seems you know you don't have enough background to actually object rationally to the answers. So, you're looking for more information. That does get kinda rare around here. So, keep up the good work.
Or matter in those galaxies is not like matter here on Earth, or light (electromagnetic radiation) does funny things as it travels through that much space, or the IGM (intergalactic medium) is filled with a substance which does odd things to light, or ...Right, if those z formulas are wrong, those massive compact galaxies could actually be large galaxies.
Actually, my point was serious: we can't really have a rational discussion unless and until you make clear what you accept and what you are sceptical about.Unlikely that the error is that big and besides, the Local Group is so close... Well, you admit a possible exaggeration.
Perhaps the central assumption is that GR rules the universe, at all scales that matter to astronomers.Now you're going off the deep end. On the one hand, there is the claim that we are measuring the evolution of galaxies and that we "know" they were different in the past than they are now and now your saying we don't really know anything. What I'm trying to get a handle on is how reliable is that assessment. I know that cosmology assumptions play a role in the conclusions, but how much?
If you drop that assumption, then pretty much every extra-galactic astronomical observation needs to be re-evaluated*.
Of course, GR has been tested pretty extensively, and by many independent means, so maybe we should start our conversation there?
OK, GR it is then (the physics theory we need to discuss, first).As far as I'm concerned, atomic emission lines are a fact and we know that visible matter in the universe is just like the visible matter on earth. As I said before, our ability to observe the cosmos in detail fails off rapidly at high z. I'm just trying to get a sense of what can be taken for granted as correct and what is truly speculative or entirely dependent on cosmology.
There certainly are some galaxies whose most prominent stars seem to have formed more recently than ~10 billion years' ago. And some for which we infer that the majority of stars (in those galaxies) are 'young'.I was thinking about galaxies like our own and M31 which are thought to be nearly as old as the universe.
However, nearly all galaxies seem to have significant populations of stars which are as old as the hills (i.e. Population II stars).
Separating episodes of star-formation from galactic mergers is a hot topic in extra-galactic research today.
Are you sure? I'd say, off the top of my head, every paper mentions the values of the LCDM parameters it uses ('737' is common, but perhaps the use of h is more common)!Well you have a point there. However, the cosmological theory is so ingrained in the literature that use of the formulas in conclusions aren't even often mentioned.
And of course the conclusions depend upon the assumptions (fortunately, those assumptions are nearly always specified).
You mean that hypotheses have been developed, and are being tested? That different models give different predictions?And yet there are all these puzzles that result from this theory, like early galaxies are compact and massive and produced stars at 100 times the rate we see locally (according to (1+z)^4). Galaxies existed surprisingly early (say 700 My). Galaxies spin like there's more matter than seen around them, but that can't be because the CMB would be different. So there must be a new mysterious kind of matter comprising 80% of the matter in the cosmos. The CMB couldn't possibly be that smooth. An incredible expansion (of unknown cause) must of have happened when the universe was 10^-35 seconds old (inflation).
Isn't that what science is about?
The (really) large-scale properties of DM - that affect things like BAO, the CMB, the formation of galaxy clusters, etc - are somewhat independent of its small(er)-scale properties (e.g. its tendency to form constant density cores, ~10 pc in size).Distributions of DM as it is understood in LCDM should be cuspy. No cusps found in the most profoundly DM dominated galaxies. There should be a ton of dwarf satellites around local galaxies but there are not. In fact I just read a paper claiming that most of the dwarfs in the MW (and perhaps M31 as well) are the result of a merger or near miss.
And one thing SDSS showed - rather dramatically - is that the Milky Way has, in fact, rather a lot more dwarf galaxy satellites than we knew about before.
You say potato, I say potato; you say tomato, I say tomato.It's like the tail is wagging the dog, theory is ruling observation.
Imagine you had, at your personal disposal, 10 million seconds of time on the Hubble, ditto Herschel, Spitzer, Chandra, XMM-Newton, the VLT, the Kecks, the VLA, LOFAR, ... what would you observe?
I think you need to do some more reading.Remember I said before I wish astronomers would be more humble about their conclusions? This is what I mean. The whole house of cards is built on the CMB interpretation and BB theory and it keeps running into serious problems. Still suggesting that the LCDM interpretation is wrong is heresy (to some).
The foundation of "the LCDM interpretation" is GR; what would you replace it with?
Why not?Who do I believe? I can't check the math much less the quality of the observations.
How did you form that opinion, if I may ask?I think the debate about current cosmological assumptions is heating up, especially in the last few years.
Sure thing.If the current theory is in fact wrong, it will fall eventually so no worries there.
Clearly I'm playing devil's advocate here. Maybe all is fine with BB theory. Challenging things helps me learn more. Skepticism is important in a study like astronomy where inferences are stacked up many levels deep.
Well, as Tensor already noted, BAUT has many, many threads on challenges to "the "mainstream" theory". Most are pretty appalling, you don't know whether to laugh or cry at the profound ignorance displayed (sadly, willful ignorance seems rather common though).Most of the regulars here seem to be staunch defenders of the "mainstream" theory. That begs for challenge.
What I, for one, would like to see, here, is some serious challenges. But I expect that those who are sufficiently familiar with both the theory and observation are busy writing papers, putting them on astro-ph, and submitting them to MNRAS, ApJ, etc ...
* beyond the Local Group, and a few other nearby groups of galaxies
You can solve one problem without solving all problems. You do not have to simultaneously solve all problems because they may in fact have very different solutions even though you would like one that fits all. We have a pretty good idea how the sun works and we don't need to know how the universe started to figure that out. Why must we interpret the cause of galactic rotation curves based on measurements of the CMB?
I get somewhat put off by this mainstream line. "Well, our theory explains many cosmic things (CMB, expansion, supposedly ratios of primordial elements etc.) and therefore it is right even if it fails in several places." (no detection of non-baryonic dark matter, no cusps, no dark matter evident close by, two few dwarf galaxies, wild variations in the mix of DM and normal matter locally, little dark matter in ellipticals etc)."
It seems that the mainstream cosmological theory is having most trouble in explaining the part of the universe that we can observe best.
You shouldn't be too surprised that people (we the uneducated masses) are skeptical about non-baryonic dark matter. It is motivated primarily by a theory of cosmic creation that is very dear to the current generation of astronomers; it said to make up 80% of all matter, but nobody has detected any (yet)!
In an earlier post of mine, I recommended that you do more reading.You can solve one problem without solving all problems. You do not have to simultaneously solve all problems because they may in fact have very different solutions even though you would like one that fits all. We have a pretty good idea how the sun works and we don't need to know how the universe started to figure that out. Why must we interpret the cause of galactic rotation curves based on measurements of the CMB?
This part of your post illustrates a crying need for you to do just that.
First, you have the "history of DM" completely messed up. Then you invent some connecting logic, not found in the relevant textbooks and papers, and finally ignore independent nature of the confirmation (that it's all the same DM)!
There's a good book, by a "DM hunter" (Ken Freeman; he's spent a great deal of his long, professional career as an observational astronomer, testing hypotheses concerning DM), on this topic - In Search of Dark Matter; while it's now several years' old, it gives a good overview of the topic.
Perhaps you need to reconsider what the nature of astrophysics is then? Or even scientific theories (at least those closely related to physics)?I get somewhat put off by this mainstream line. "Well, our theory explains many cosmic things (CMB, expansion, supposedly ratios of primordial elements etc.) and therefore it is right even if it fails in several places." (no detection of non-baryonic dark matter, no cusps, no dark matter evident close by, two few dwarf galaxies, wild variations in the mix of DM and normal matter locally, little dark matter in ellipticals etc)."
If you're interested, there's an excellent author I'd recommend, Imre Lakatos (I think he was the first to point to "research programmes" as a key feature of science).
Huh?!?It seems that the mainstream cosmological theory is having most trouble in explaining the part of the universe that we can observe best.
We can easily (or not) observe that the Sun is composed, predominantly, of hydrogen, followed by helium, with a smattering of metals. The "mainstream cosmological theory" explains this rather well, doesn't it?
Even more pertinent: the night sky is dark, an easily confirmed fact explained by this theory.
Shall I go on?
Also it often seems that you see contradictory findings. In a recent paper some authors analyzed data concerning movements of 400 local red giants and concluded there was no evidence of substantial DM in our part of the galaxy (to sigma 3 or 4). A day or so later antoniseb finds a paper about a study of blue halo stars that he says contradicts those findings (which I haven't read yet but will). Some say rotation curves are explained by a huge spherical distribution of unseen non-baryonic dark matter and some say a smaller amount of ordinary dark matter in the disk explains these curves. Many say that there is a strong consensus about LCMD. Well maybe so, but is this theory born out by observations? What predictions does it make and how do they line up with observations?
I asked Nereid if LCMD predicted compact massive early galaxies. He says "who knows". Well, what does it predict?
If you read almost any popular summary of what we know about the universe, you get pretty much the same story. About 13.7 billions years ago an unimaginably (infinitely?) dense volume (maybe finite, maybe infinite) of energy expanded, leaving the CMB as proof, 80% of matter is made of particles we don't understand and have not detected, 70% of all content of the universe is "dark energy" and last (but not least) some time between 10^-35 and 10^-33 seconds into this expansion, the universe suddenly expanded by 78 orders of magnitude in volume and that's why the CMB is so smooth.
Can you see how assertions like this can attract skepticism? And then you hear about the new age of precision cosmology! This when no firm conclusion has been reached about why our own galaxy rotates as it does.
It's definitely not a nice neat package, but that makes it fun. I look forward to learning more and reading more papers.
BTW, on the Planck results I keep checking the ESA mission site but never find very much new. I'll try what you suggested. Thanks.
This thread has gone beyond the intent of Q&A and into a discussion. Let's do that in S&T.
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I moved the asterisked item out from bolded bracketed ellipses, to make my answer a bit more clear.
Ok, where do we start? Observations? We better have some sort of theoretical underpinning to the observations, right? Since we're starting with a clean slate, I guess we ignore Quantum Field Theory(QFT), as interpretations based on it have verified the mainstream model. How about Chemistry, no that has to go also, for the same reason. Since we are starting over, what gravity theory are you going to use? What mechanical theory (since QFT is out)? What should we start with that has less trouble explaining the part of the universe we can observe best? Or, maybe you want to do something different with current theories. I'm open to suggestions. What can you give us for a better starting place than the model that explains the most things the best, the current mainstream model.
[QUOTE=Tensor;2011938] What should we start with that has less trouble explaining the part of the universe we can observe best? [QUOTE]
How about the physics we already know a lot about? If we need additional mass, let's look for the kind of matter that we know exists. Let's not get bent out of shape trying to ensure that the CMB makes sense as a relic of the creation of the universe. Do you see how far out on a limb we have gone to make the CMB be that?
A theory is not proven because no better theory exists. Once there was a theory that the world was flat and supported by a tortoise. It sure looked flat and no one knew how to detect the tortoise.
On the other hand, even if someone doesn't understand all the esoteric details they might still smell something strange. Sorry but I cannot help harping on this 10^78 factor expansion of the universe proposed to have happened at 10^-35 seconds after creation in order to explain the uniformity of the CMD. Does that not strike you as "far fetched"?
Last edited by TooMany; 2012-Apr-27 at 08:12 AM.
You really think no one has done that?How about the physics we already know a lot about? If we need additional mass, let's look for the kind of matter that we know exists.
The problems with cold baryonic matter is that it is hard to hide. Because it interacts electromagnetically there are ways to detect it. There is still room for some of it but the amount required to make rotation curves work is really pushing things. Plus it generally has to be added in by hand because it does not fall out of the models for the evolution of the universe. LCDM distributions generally do.
Maybe the reason you feel that alternative ideas are dismissed by haughty and unhumble astronomers is simple. They keep getting proposed in the same way:
"Dark matter? Sounds, like, totally ridic! Have you guys not thought that it could just be, like, gas and stuff?"
"Yes." (sound of teeth gritting) "In fact we thought of that first, something like thirty years ago and looked for that matter for ten years. Then we spent another five years trying to work out why we were not seeing it and coming up with new tests. Then we spent five years trying to work backwards, asking what this mysterious stuff had to be like to produce the results we have seen. Then we spent ten years working out what having this stuff around meant for all the other areas of astrophysics and testing those ramifications out."
"Well, you sure you looked in the right places? Gas can be cold and thin right, you might miss it."
In general people expressing a strong dissatisfaction with the current mainstream are not aware of the sheer number of alternatives looked into, the huge variety of tests and observations made. The reason the current mainstream is the current mainstream is not because us lazy, haughty scientists want to pretend we understand the universe or trample on people we look down on - it is because it has stood up better than any other models to the rigorous tests we have made of it.
I also think that there is often a communications barrier between scientists and people in other professions (this is not unique to scientists, by the way). We use a lot of terms and jargon that carry with them a lot of subtle meanings. Then this is grabbed by the press and translated, often in a way that doesn't carry across the meaning that a piece of work has to other scientists. The fact that everything we say about the world is model dependent is well known and accepted among scientists. When someone says that they have evidence for dark matter I know what they mean. It does not mean that dark matter is out there. It means that there is a bulk of observations that have been made that are consistent with the predictions made by a model that makes use of dark matter. When someone reports that they have found out what the shape of the universe from WMAP data is I know what that means. It means that they have predicted some previously unexplained features in the CMBR by constraining the geometry of the universe in the standard cosmological model. It is all model dependent, all interpreted and all 'best guess' - because it has to be. That is inherent in science - otherwise why would we continually test, probe and refine our models? it is because we want to break things, we want to see new stuff.
When you read those papers, do you also read all the ones referenced in the Introduction section? And all the ones in Introduction sections of those? And ...? I didn't think so.
By not having a good grasp of how the authors of a particular paper got to where they started from, in that paper, you seem to have missed the enormous amount of work done beforehand (I note that Shaula has already addressed this, in a different way; yes, gritted teeth is all too common).
Two things:I asked Nereid if LCMD predicted compact massive early galaxies. He says "who knows". Well, what does it predict?
One: unless and until you define - clearly - where you want to start from (in terms of a discussion of astronomy and astrophysics), I don't really see the point of responding to you in any other way*. So, how about it? Do we start with the cosmic distance ladder? With the physics of processes which give rise to the emission and absorption of electromagnetic radiation? With tests of GR? Or should we start even further back?
Two: 'Nereid' is the name of a moon of Neptune. Like Triton (another moon of Neptune, discovered before Nereid) it is taken from Greek mythology; and all Neptune's moons are similarly named. The Nereids, in that mythology, are sea nymphs.
Yet quantum stuff is far, far more incredible!If you read almost any popular summary of what we know about the universe, you get pretty much the same story. About 13.7 billions years ago an unimaginably (infinitely?) dense volume (maybe finite, maybe infinite) of energy expanded, leaving the CMB as proof, 80% of matter is made of particles we don't understand and have not detected, 70% of all content of the universe is "dark energy" and last (but not least) some time between 10^-35 and 10^-33 seconds into this expansion, the universe suddenly expanded by 78 orders of magnitude in volume and that's why the CMB is so smooth.
Can you see how assertions like this can attract skepticism? And then you hear about the new age of precision cosmology!
Yet I don't see you expressing any scepticism about that; why not?
A similar sentence, concerning a "why" to do with quantum weirdness, should be easy to write; are you as fascinated by the lack of "firm conclusions" concerning quantum mind-twisting things?This when no firm conclusion has been reached about why our own galaxy rotates as it does.
You might find this paper of interest, for all sorts of reasons. Note the date it mentions, concerning the expected next data release.It's definitely not a nice neat package, but that makes it fun. I look forward to learning more and reading more papers.
BTW, on the Planck results I keep checking the ESA mission site but never find very much new. I'll try what you suggested. Thanks.
* other than 'what would M31 look like at z~1.2', and similar questions
It'd be fantastic if the rotation curves of spiral galaxies could be accounted for without the need for any non-baryonic dark matter! All of them (not just in one or two spirals; not just one or two sets of observations). As far as I know, no one has made such a claim (though various authors, using MOND or something similar, have come close). Certainly no one has made such a claim without invoking some really weird forms of baryonic matter (e.g. Rydberg atoms, or was it molecules?), bizarre new theories of electromagnetism (or gravity), or exotica like ~10^5 sol-mass primordial black holes.
But how do you know how humble (or not), and how closed-minded (or not) "the mainstream people" are?...
Good one. What the heck do I want? Mostly I want the mainstream people to be bit more humble about what they think they know and be more open minded.
Let's say you assume you are interacting with some of "the mainstream people" here in this BAUT thread; how did you conclude they are insufficiently humble and/or open-minded?
It surely doesn't matter what you, or I, think, does it?I think theory has gone far beyond what can truly confirmed by observation.
I mean, I could say I think you have displayed gross misunderstanding of LCDM cosmological models, but that doesn't cut much ice in terms of moving a constructive, meaningful, science-based discussion forward, does it?
I believe the term for this is "strawman"....
This sounds a lot like Nereid's recent post. Should we discard GR, shall we dump QFT? As if the only possible conclusion given well established theories like GR and QFT is LCMD.
There are, almost certainly, an extremely large number of possible cosmological models using QFT and GR as foundations. And several have been proposed, as alternatives to the ones summarised as LCDM (e.g. 'void cosmology'). So far, none work (at least not anywhere near as well as LCDM ones do). Doesn't stop "mainstream people" from developing, and testing, new ones ...
Good luck with that....
How about the physics we already know a lot about? If we need additional mass, let's look for the kind of matter that we know exists. Let's not get bent out of shape trying to ensure that the CMB makes sense as a relic of the creation of the universe. Do you see how far out on a limb we have gone to make the CMB be that?
After you've spent some time in the library, I think you may conclude "been there, done that, got the T-shirt" ...
While you're in the library, check out the books by Imre Lakatos. He had a far better grasp of the nature of science than the philosopher Karl Popper ever did.Explaining "the most things best" is nice, but completely failing to explain some quite apparent things is supposed to be fatal to theories.
And because something better came along ......
Right, very successful predictions. But may I add that phlofgiston is gone, flat earth is gone, aether is gone, the geocentric universe is gone, spontaneous generation is gone, planetary orbits defined by perfect solids are gone, Newton's laws are gone (at relativistic speeds). Why are they gone? Because they made incorrect predictions and another solution had to be found.
Clearly I'm grating on everyone with my lack of knowledge and excessive skepticism concerning the results of the hard work of very smart people. I do think astronomers are trying very hard to figure things out. I do believe they have made an effort to find additional baryonic matter. But you can choose to be skeptical that there is more normal matter out there and I can choose to be skeptical that the non-baryonic matter is actually there.
I'll tone it down and learn more. We'll all see what unfolds as science keeps digging. I very much appreciate the links and suggestions that have been provided by everyone who has contributed.
It's a very exciting time for this field. The uncertainties are substantial and new information is constantly appearing. I enjoyed reading this paper today: Sizes and surface brightness profiles of quiescent galaxies at z ~ 2.
Something particulary interesting was this conclusion:
That is, since z ~ 2 galaxies have grown by a factor of almost exactly (1 + z). Is that just a coincidence?Galaxies at z ∼ 2 are signiﬁcantly smaller than those at z = 0. We ﬁt a power law of the form
re ∝ (1 + z)^α and ﬁnd α = −0.94 ▒ 0.16.
However, it's also important, in a science-based discussion, that you try to express your sceptical views within a framework that is shared with your intended audience. In this case, it would be very helpful - in terms of having a productive discussion - if you could state, clearly, where your scepticism starts, and ends. For example, you've said you're OK with atomic physics and nuclear physics (or something like this), but haven't said a word about GR.
Who knows? Who can tell (in light of today's collective understanding and observations)?[...] Is that just a coincidence?
Suppose you are the one doing the astrophysics; can you suggest how you might go about formulating a testable hypothesis on this? Doesn't have to be one that's immediately testable, but 'in principle' testable.
I'll guess detect at 13 billion light years, but we may never be able to prove that as that galaxy is much too dim to get the specra and thus the distance is a guess, not even an educated guess. With CCD = charged coupled display, you can turn up he contrast until you see planely stars that don't exist = artifacts. Neil