Hubble telescope in another tight spot: Between astrophysicists sparring over a 'dark matter deficient' galaxy Never mind memory errors from radiation. Another deteriorating part of the decades-old Hubble Space Telescope has found itself in a jam. This time its camera unit is once again in the middle of a clash between scientists over whether or not the galaxy NGC 1052-DF2 contains any dark matter. When DF2 was written up in a Nature …
"scientists rarely agree 100% on something"
When a new discovery is made the evidence is not necessarily convincing. When the Michelson–Morley experiment results were published someone had a theory that the earth dragged the aether with it so they performed experiments at various elevations. They got a smooth curve with altitude showing that, indeed, the earth was dragging the aether with it.
For delicate experiments you have to repeat them carefully to be sure - remember the EM drive?
EVERYTHING is still theoretical. The current theory holds until something bigger and better disproves, or (usually) improves upon it, like Einstein's relativity improved on Newton's gravity.
Even the facts are at conflict in this story. Just how far away is the galaxy... a simple fact, no?
Dark matter is something convenient until it's better supported (AKA "proved") or disproved like phlogiston.
You don't need an alternative to reject a flawed hypothesis. Dark matter models are already contradicted by observations, but rather than drop them and focus on developing alternatives, astronomers are adding the modern equivalent of epicycles to keep them alive.
The default position of science is "I do not know". Not "I assume I'm right", as is the case with this failed hypothesis.
Dark matter models are already contradicted by observations,
Citation needed.
The theory of Dark Matter is actually pretty well agreed upon, as in there does not appear to be enough Mass in galaxies (based on current understandings) to maintain their current forms and shapes. That's pretty much universally agreed upon.
What isnt agreed upon is why? Or in what form that missing matter takes? Thats where you get into the hypothetical. Is it just that were bad at estimating the mass of galaxies? Is it WIMP's (weakly interacting massive particles)? Neutrinos? Fuzzy Dark matter? Axions?
There are a lot of hypotheses, and only some very vague evidence at the moment to support one or the other theories, but the fact remains most scientists agree something is missing from our understanding. Adding Mass to galaxies, explains with extreme precision their movement, shapes and make-up. As such, by observation we can say a galaxy should weigh this much, but our calculations only add up this much. Hence why dark matter is added. Maybe we are just bad at estimating the mass in a galaxy but there's a lot of very smart people doing very detailed calculations and they all seem to be finding mass missing.
Discussions like this are therefore highly intriguing because if these galaxies are really further away and light on dark matter, than that opens up possibilities for more study opportunities, more things to look at to say why are these galaxies different compared to others we have studied. Anomalies are great for advancing science and I look forward to hearing more on this topic in the future.
Dark matter is a beautifully simple explanation. I worked on it as a student. It's missing one crucial thing: the bloody particle(s). And since GR isn't renormalizable, you start to think maybe what's missing is our understanding of gravity at scale. With the right tweaks maybe we can solve two problems with one theory. And we have form on getting gravity wrong.
Which is not to say the dark sector couldn't be out there. But for many years, now, I've felt dark matter resembles epicycles or the new aether and in a hundred years time students will look back and think "why did people ever believe that crap?" and think we're all idiots. Clearly, we're not idiots. But maybe we should be looking harder at alternatives. They aren't perfect but that's because they are mathematical challenging and under-researched. We've gone for the easier solution; the universe may have other ideas.
(For readers unfamiliar with the details, Sabine Hossenfelder has a recent blog which runs through some of the arguments and some of the problems.)
"Maybe we are just bad at estimating the mass in a galaxy but there's a lot of very smart people doing very detailed calculations and they all seem to be finding mass missing."
But there are also a lot of very smart people trying to work out what form the missing mass takes and so far have come up with a variety of possible explanations as per your list but some of them are hypothetical and those which aren't, such as neutrinos so far haven't been shown to exist in sufficient quantities.
Sometimes science predicts things which are yet to be discovered. Predictions based on gaps in a theoretical framework have often been successful, as in particle physics. Predictions based on a gap in observations, such as phlogiston or luminiferous aether less so.
So, another conveniently anonymous 'all the scientists are conspiring to keep this dead theory alive' comment. Why so anonymous: are you worried that 'they' will come to get you? Does your tinfoil have pinpricks in it through which the mind rays might leak?
Well, the funny thing about this is that it's the diametric opposite of the truth.
The problem with dark matter is that it can explain almost anything: it's really, really hard to disprove. And that makes it unsatisfying to everyone.
As an example of the opposite of this take General Relativity: this is a theory with a single adjustable parameter (it's normally thought of as having three, but geometrically it has only one). So once you've decided what that parameter is (on smaller scales it's safe to say it's close enough to zero that it doesn't matter) you can then just crank the handle on the theory (which is hard, OK) and it will make predictions for you. And those predictions will be either right or they'll be wrong, and if they're wrong you have exactly one parameter to adjust, and if you can't make them right by tweaking it, GR fails. That's great: that's what a theory should be.
Now take dark matter: if you look at some galaxy & measure its rotation curve then you find it doesn't agree with what you think it should be. So you throw in some dark matter and you fix up the rotation curve to be right. And within some rather minor constraints you can fix up any rotation curve you find by adjusting the distribution of dark matter. There are an uncountable infinity of adjustable parameters (lterally: there's a continuum of them).
So until people actually observe dark matter somehow this is a bit unsatisfactory.
So much nicer, really, if you can come up with some modified gravity theory: this won't be as pretty as GR because it will necessarily have more parameters (GR is arguably the unique theory with only one parameter), but perhaps only a few. And it's nice because you can then try and fit that theory to the observed rotation curves.
And people do that, and so fat their theories fail to match the data unless they start throwing in absurd numbers of parameters. So dark matter, unsatisfactory as everyone finds it, is the only idea really left standing at this point.
And, of course, as I've commented elsewhere, if it really is the case that DF2 has no dark matter this is a huge bit of evidence that dark matter is in fact the right answer, because gravity should work the same for all galaxies.
> you can fix up any rotation curve you find by adjusting the distribution of dark matter
And that's precisely the intellectual (I'm no astrophysicist!) problem I have with it: It looks like a simplistic way to make contradictions go away: Expected 5 but it's actually 8? No problem, add a cheat factor of 3. Expected 7 but it's actually 9? No problem, add a cheat factor of 2. Now why is it "3" there and only "2" here? Well, simply because we needed 3 there and 2 here!
That's way too convenient, especially since (correct me if I'm wrong), the only point and reason of Dark Matter is that it fixes our faulty gravity calculations. It doesn't seem (AFAIK) to have any proof or use beyond fixing our apparently faulty assumptions about gravity, which makes it look like sort of a band-aid.
Now I definitely don't pretend to know better, but that does create some intellectual discomfort I suppose I'll have to live with.
I think it's more complicated than that. See the commend by Cuddles but there are really quite a lot of independent observations which indicate that there is a lot of mass in the universe which we don't see. You might be able to deal with galactic rotation curves with MOND say (In fact I think it doesn't work very well), but then you will have a whole bunch of other things which you also need to tweak your theory, in different ways. Or you could say that well, there actually is a lot of missing mass.
I'm sorry I didn't mention this in my earlier comments: this is because galacic rotation curves is the only thing I ever really learned about, most of the others are past my time and I've not kept up.
Like much of science, it really comes down to Occam's razor. We know matter exists, we know gravity exists, and we have a theory that explains how they behave with incredible accuracy in the vast majority of cases. We then make a new observation, and our existing theory doesn't quite seem to work for some unknown reason. We now have two options. One, we can throw out all our existing theories and come up with a new, significantly more complicated, one with a whole pile of unknown or arbitrary parameters that need to be carefully chosen specifically to explain this one new observation. Or two, we can assume that there's a bit more stuff out there than we previously thought, at which point the existing theories continue to work perfectly. And then we make another new observation and this time we need yet another even more complicated theory to replace everything, or we can just assume that there's exactly the same amount of extra matter and see that our existing theories still continue to work. And so on.
People love to complain that it looks like endlessly adding complications like epicycles, or arbitrary fudge factors to fix things, but it's actually the exact opposite. By far the simplest explanation for a whole raft of different observations is that there just happens to be a bit more matter out there than we can easily see. All other attempted explanations are far more complicated, contain far more arbitrary assumptions made specifically to explain one or two observations while also contorting themselves to explain why all the observations made previously failed to see anything unusual, and ultimately fail to actually explain the vast majority of other observations anyway.
Specifically to your complaint, the thing about adding an arbitrary fudge factor is that you'd expect it to be different every time. But what we actually see is that we have to include exactly the same factor for every single observation no matter how its made or at what scale. Every individual galaxy we look at seems to have about 5 times more mass than expected. Every cluster we look at seems to have about 5 times more mass than expected. The CMB says the entire universe has about 5 times more mass than expected. At some point it stops being a factor added just to make things work, and instead becomes a ton of different lines of evidence all telling us that maybe there actually is 5 times more mass out there than we expected.
A final thing worth considering is that when you have a bunch of complicated equations describing how thing work, there are relatively few ways things can be added or changed while having the whole thing continue to work in a consistent manner - you can't just chop bits out or add new parameters in wherever you like and have it all still make sense. For example, Einstein famously called the cosmological constant his greatest mistake, because at that point he thought it wasn't necessary and therefore looked like he'd just added a fudge factor. But even before we discovered the universe was expanding he was absolutely correct to include it, because it's actually the only thing that could have been added to his equations. In fact, from a certain point of view he'd have been wrong not to include it, even if it had turned out to be zero in practice (that's a bit of a mathematcian vs. physicist argument).
The same is true for dark matter. It's not just adding a random number to make things work. It's changing one of the very few parts where it actually makes sense to change anything. Things like modified gravity theories invariably end up horrifically complicated precisely because you can't just change a couple of bits in general relativity and still have it make sense, instead you have to come up with a whole new theory that mostly still gives the same answers except in a few specific cases. Total mass, on the other hand, is essentially a free parameter as far as theory is concerned, set only by our epirical observations of how much we can see. So changing it really isn't arbitrary at all, it's one of the parts that exists specifically to make things work. We pick a value that makes things work based on our observations. Previous observations gave one number, newer observations say it's actually a different number. That's far simpler and less arbitrary than trying to come up with convoluted explanations for how things actually work in a completely different way but just happen to behave how we thought in every observation up to a certain point.
> Every individual galaxy we look at seems to have about 5 times more mass than expected
Isn't that article precisely about a galaxy which doesn't (seem to) have the prescribed 5x dark matter content?
I get your point, but it still seems artificial and inelegant to me. Sorry.
We currently do have many seemingly good theories, which unfortunately don't really work together (relativity and quantum is a good example), so I wouldn't be surprised if we eventually discover that, while our current collection of ill-assorted theories managed to explain lots of things, they were utterly simplistic or even partially wrong. I'd just love to be still alive when they find a theory which explains the universe's workings flawlessly, from elemental particles to superclusters. (Yes, I know it's the physics' grail and we might have to wait a century or two for it, but one can dream, no?...)
You need more upvotes for this comment.
I have always liked the idea that Einstein's quote about the cosmological constant being his greatest mistake meant that the mistake was not adding it in the first place, but rather assuming it would allow a static universe, and that everyone else then read him incorrectly. I'm probably wrong, but when I solve the whole closed-timelike-loop problem (really close now) I am definitely going to use my time machine to retcon the universe so that is what he meant. Because there has never been a good reason to assume Λ=0.
(And further: I don't think this should be a maths-person / physics person disagreement: no maths person should assume it was zero, and it is shameful that physics people did for 80 years instead of going out and measuring it.)
> And that's precisely the intellectual (I'm no astrophysicist!) problem I have with it: It looks like a simplistic way to make contradictions go away: Expected 5 but it's actually 8? No problem, add a cheat factor of 3. Expected 7 but it's actually 9? No problem, add a cheat factor of 2. Now why is it "3" there and only "2" here? Well, simply because we needed 3 there and 2 here!
But using your fudge factor analogy, what we have with DM isn't what you have written. It's more like:
Expected 5 but it's actually 8? sure, add a fudge factor of 3. Now in another observation, we expected 19, but got 22, so we need a fudge factor of 3 to make it work. In this other observation we expected 42 but got 45, so we ned a fudge factor of 3. In yet another observation we got -11 when we were expecting -14, so using a fudge factor of 3 works.
So we have multiple different situations, using the same underlying theories, that all work fine when the same fudge factor, 3, is used. And we have a consistent (but not proven) explanation of where that 3 came from and why it only seems to matter in those specific cases, but not in others.
A single explanation, DM, makes General Realtivity work at large scales (and explains other things as well). All other alternatives offered to date required X, and Y, and Z and, in some cases, also needed DM to work (MOND theories that worked great for galaxy rotation curves didn't work for galaxy clusters, but if you added DM - or something analogous to DM - to them, they also worked for galaxy clusters).
Is it 'simpler' to toss out all these underlying theories that have been developed and refined over deacdes, that work in every other situation except these other half dozen, with new theories that require several new (also unproven) things, or is it simpler to say "we've missed some constant of nature that is 3, that if we add to all of our different equations and situations makes them all work, and doesn't stop the existing situation where they worked already from working"?
So much nicer, really, if you can come up with some modified gravity theory: this won't be as pretty as GR because it will necessarily have more parameters (GR is arguably the unique theory with only one parameter), but perhaps only a few. And it's nice because you can then try and fit that theory to the observed rotation curves.And people do that, and so fat their theories fail to match the data unless they start throwing in absurd numbers of parameters. So dark matter, unsatisfactory as everyone finds it, is the only idea really left standing at this point.
And, of course, as I've commented elsewhere, if it really is the case that DF2 has no dark matter this is a huge bit of evidence that dark matter is in fact the right answer, because gravity should work the same for all galaxies.
Beyond that, most of the promising MOdified Newtonian Dynamics (MOND) theories were all blown out of the water with the LIGO detection of merging neutron stars in 2019? I think it was.
Most MOND theories modify the speed of gravity, such that it doesn't propogate at the speed of light. Doing this modifies many of the results of gravitational equations, in effect changing the strength of gravity at different distances, thus explaining the different galaxy rotation curves and so on. Without this factor, a different propogation rate for gravity, most of these theories fail entirely.
When LIGO detected the gravitational waves of the neutron star merger, optical (including radio - x-ray, gamma-ray, etc.) observatories also detected the light of that collision. The light and gravitational waves arrived within 2 or 3 seconds of each other over a distance of hundreds of millions of lightyears. The light was a tad slower because light - unlike gravitational waves - can be slowed, warped and scattered by intervening objects, such as the expanding shell of matter resulting from the merging of the neutron stars whch the light had to emerge from first (same as how light generated in the heart of our sun takes a million years to escape to the surface). Based on this merger, light and gravity do travel at the same speed. Therefore all those MOND theories became so much toilet-paper, as the key pillar they relied on - Einstein was wrong about the speed of gravity - collapsed.
So while Dark Matter might not be a very ... satisfying ... theory, with the collapse of the most promising alternatives - MONDs - it is the only 'fleshed-out' one left standing at the moment.
This doesn't mean DM is right, it's the "rightest" we have at the moment. Cosmologists are looking for alternatives, because some are becoming unhappy with the lack of progress in finding DM candidates - WIMPs etc., but those alternatives still don't hold a candle to DM right now.
Hmm. If a theory is "really hard to disprove" then it starts to flirt with being non-scientific (in Karl Popper's terms*), and starts to become philosophy/religion/superstition (delete according to taste) instead.
However, I agree that we're probably between theories at the moment, both with Cosmology and Physics. The Dark Matter problem needs some testable assertions, and Quantum Physics has gaps that we're filling arbitrarily (even though it makes very successful predictions). So, I wait with eagerness for the new theories to come along, with better explanations.
* Interestingly enough, according to Popper's principles, Astrology can be called a scientific theory, as it makes falsifiable assertions ("you will meet a tall handsome stranger..."). It's just that it' keeps getting proven wrong.
I wouldn't say wrong as such, it just depends what you mean by hard to disprove. Like any scientific theory, dark matter is very simple to disprove in principle - just find enough contradictory evidence, or an alternative theory that explains things better. The reason it's really hard to disprove in practice is the same reason it's difficult to disprove general relativity or Boyle's law - we already have a huge amount of evidence saying things work the way we currently think they do. No single anomaly is ever going to be enough to justify throwing all that out, so you either need enough contradictory evidence that things start looking really suspicious or, again, a better theory. Lots of people have worked on the latter and so far failed to come up with anything that works at all, let alone is better than we already have. And while occasional things like the case this article covers pop up, the vast majority turn out not to be interesting after all on further investigation, and the remainder are few enough to be odd anomalies and outliers rather than anything that conclusively points to a specific problem or new solution.
So it's not hard to disprove because it's not really scientific, but simply because there's already been a lot of work done on the subject, so you'll need to do a lot more work again before you're going to be able to come up with something with a similar amount of support.
Ah I see - I sit corrected, thanks.
Yes Popper's stuff is about theories being at least theoretically disprovable. So, GR, Newton's/Boyles Laws etc are all "scientific", and still withstand all efforts to disprove them - which is what I now understand you were getting at.
Unlike, say, conspiracy theories, which will just mutate every time they are challenged, and therefore can never be disproven..
Well, what I meant by 'hard to disprove' was specifically tied to the rotation curve thing, and what I meant was that if you have some rotation curve for a galaxy then you are pretty much at liberty to invent some distribution of unseen mass which will cause that curve to make sense. Then when you see another galaxy with a different one, you can invent a different mass distribution to make that one work. And there's nothing particularly wrong with doing that: galaxies have different distributions of visible mass so why shouldn't they have different distributions of dark matter? Except that now you have a theory which has a continuum of free parameters (the distribution of dark matter) and can fit (almost) any rotation curve.
That's different than GR, say. If it turns out that antimatter falls upwards (it won't, but bear with me) then GR is dead. It's not damaged in some way that can be fixed: it's dead along with any theory which pretends to explain things by spacetime geometry, really. There's this lovely single experiment you could do which could just kill GR. Well, of course, it's not going to kill GR because antimatter will behave as matter does under gravity (and since photons do, we kind of know this already), but it could. In much the same way, if the FTL neutrino thing had been real it would have left us in a situation where either we could make time machines or special relativity was wrong.
But I think I was wrong about it being hard to disprove for at least two reasons: first of all there are lot of other, independent, indicators of missing mass, and DM explains all of them. Secondly I think that you must be able to model what you expect the distribution of mass which only interacts gravitationally would look like in/around a galaxy and then look at what rotation curve you get for that, which agrees with the curves you see and is much more constrained than the 'you can just fit any curve' idea I originally had.
Dark matter is 'theoretical' in the sense that we don't have direct observations of it – in the same sense, say, that gravitational waves were theoretical until LIGO heard the first one. As with gravitational waves pre GW150914 we have very good indirect evidence for something: for gravitational waves that was things like the spin-down of Hulse-Taylor, for dark matter / modified gravity it's pretty much every galaxy we look at.
In particular the discovery of galaxies without dark matter would be an enormous boost to the the idea that there is actually missing mass: in other words that what we're looking for is indeed dark matter and not a modified theory of gravity.
The reason for that is that it's a pretty safe assumption that gravity works the same everywhere. Indeed, if this isn't true – if the laws of physics vary from place to place – then so many assumptions about pretty much everything fail that it's hard to describe: momentum is not conserved, for instance. Under that assumption then if there is no dark matter but instead a modified theory of gravity then all galaxies should obey this modified theory of gravity. If we observe that some don't obey it we can say with very high confidence that a modified theory of gravity can't explain what we observe: we need missing mass, aka dark matter, as well. And Occam's razor would then say that, since we need dark matter then we should not assume modified gravity until we are sure that dark matter alone cannot explain what we see. And there's very little evidence that it can't.
I didn't realise that dark matter could be explained away so easily. It just needs to be shown that all inter-galactic distances are wrong, presumably due to an inaccurate assumption about something that is used in those calculations.
This science stuff is easy - although I'll leave it up to someone else to find what that something is.
Galaxy distances are normally definitively measured by the yardstick of Type 1a supernova. This is meant to be the best way and yet it has pretty massive error bars, similar to the ones mentioned above. My Granddad was a pretty keen amateur astronomer (I believe the Carver scope in my barn inspired an Astronomer Royal in his hands (the scope!)) and he had a spectrometer that attached to this thing and he would use that to try and calculate stars redshift. That was hard enough for local stars. I've love to see in detail how Hubble try this on ones buried in distant galaxies. But not as much as getting hold of the fuck that stole the spectrometer.
The requirement for dark matter or a modified theory of gravity doesn't generally depend on getting distances correct. The reason you need dark matter is that the velocity curves of galaxies make no sense otherwise.
If you consider a spiral galaxy then it's really a large collection of stars orbiting around their common centre of mass. Well, you can estimate the mass within some radius from the common centre of mass simply by looking at how many stars there are and their types and looking for dust & gas &c. And you can use that mass estimate to compute what the orbital velocity of a star should be at some radius from the centre of mass. If you're confused about the distance all of this will be off by some constant factor perhaps, but it will only be off by a constant factor. The variation of orbital velocity with radius is the velocity curve of the galaxy.
Then you can measure the velocities of the stars from spectrographic data (Doppler shifts in spectral lines really). And that data will also be offset by recessional velocity and whatever the peculiar velocity of the galaxy is, but you can correct for those offsets because you know where the spectral lines really are because you can measure them locally.
And that data gives you the actual velocity curve of the galaxy. And it's different than the predicted curve, and critically it's not different by a constant factor: the curve is a different shape (it's flatter than it should be, with distant stars orbiting faster than predicted). That's something you can't fix by simply adjusting the distance.
I think the case with galaxies like this one, which are ultra-diffuse, is different. I'm not sure but I suspect you can't get a good measurement of the velocity curve so you rely on a rather small number of observations to give you a mass estimate. And in this case things do depend on knowing the distance.
But you can't fix the dark matter / modified law of gravity problem in general by any fix to distance scales.
"The reason you need dark matter"
To be clear, there is no the reason you need dark matter. This is the big problem all the crackpots complaining about it miss - there are multiple different, entirely independent observations all saying that there is a lot more mass out there than we can see in stars and other baryonic matter. Galactic rotation curves was one of the first lines of evidence and remains one of the best known, but it's far from the only one. And since the different bits of evidence come from many different places at very different scales, no single issue like an argument over the distances to a couple of galaxies can ever be a reason to just throw the whole thing out. Whatever turns out to be the answer for this particular galaxy, it will have no bearing whatsoever on measurements of galactic cluster masses, measurements of the cosmic microwave background, direct observations of gravitational lensing, or a wide variety of other observations.
This is why dark matter remains the overwhelmingly favoured theory among the people who actually know what they're talking about. Plenty of alternative theories have been proposed, and some of them even do a decent job of explaining one specific set of observations (most are just mindless crackpottery and conspiracy theories that don't even manage that). But I'm not aware of a single one that manages to cover even just two separate lines of evidence.
So no, dark matter definitely can't be explained away that easily. Even if you come up with a brilliant explanation that fits galactic rotation curves perfectly, if it doesn't also explain galactic clusters, the CMB, the Lyman-alpha forest, the shape of the universe, the large scale structure of the universe, and a bunch of other stuff, you have done basically nothing in terms of explaining away dark matter.
Thank you: I was wrong to say 'the reason'. I think that the galactic rotation curve thing was the big one when I was an undergraduate so it's the one I still remember, but as you say there are just a mass of things which are now saying 'there is a lot of mass we can't see'.
OK, If I understand this correctly:
The distance to the galaxy matters because the actual diameter of the galaxy is worked out from the angle the galaxy subtends in the sky. If the galaxy is closer then it is more compact, further away means larger in actual diameter. (I had to draw a diagram for this bit, with a constant angle for the viewer.)
We can estimate the mass of the galaxy by looking at basically how many stars it has and what sorts (how bright they are, their spectrum indicates how old they are, stars with more elements are younger generation).
We can calculate the velocity of the stars around the centre of the galaxy based on red / blue shift of spectral lines in the stars' light, and compare these with the velocity they ought to have based on the actual distance from the centre and the observed mass of the galaxy and General Relativity.
If the velocities are what the stars should have based on the calculated mass of the galaxy we don't need dark matter, but if they are travelling too fast but still in orbit, then we do need dark matter.
Am I even close?
No, for rotation curves it really doesn't matter. at all. Let's say you're looking at a galaxy and you have no idea how far away it is and thus no idea how big it is. Now you measure the orbital velocities of stars at various distances from the centre, where you just measure the 'distance' relative to how big the galaxy appears to you, since you don't know how big it is. (In fact you don't need to know the velocities even: you just need to know the velocities up to some constant factor.)
Once you know that data (which is the velocity curve) then you can use it, and Newtonian gravitation, to compute, up to a constant factor, how the mass of the galaxy goes with distance from the centre (again, distance a fraction of the radius).
So now you can measure how the mass of the galaxy goes with distance (as a fraction of radius) by actually looking at the stuff you can see and estimating its mass.
So now you have two curves which describe how the mass of the galaxy goes with distance (as a fraction of radius). And those curves should be the same up to a constant factor which is because you don't know how far away the galaxy actually is. And they're not the same.
If on the other hand you only can meauure the velocities of a very small number (or just one) thing, then you do need to know how big the thing is to weigh it.
[Disclaimer: may be wrong. At least one other commenter here knows much more than me about this. Never underestimate the deviousness of astronomers.]
I think because they didn't measure (and I presume cannot measure) the velocity curve. Instead they measured the velocities of ten compact objects (assumed, I think, to be globular clusters), all of which have the same velocity within a pretty small margin (a great chunk of which may be observational errors). So, I think, they have really just one data point to weigh the galaxy, and in that case size does matter.
[Disclaimer: have read the paper only extremely casually and I also was never an experimentalist so all those error bars and confidence i tervals mean 0 to me.]
Being a faint dwarf galaxy means it is very hard to see since it is tiny and, well, faint. That would preclude many - perhaps all? - ground observatories from being able to see it.
However, using more observable galaxies as a calibration for Hubble's camera should help determine the accuracy of Hubble's camera. But I think this has been done, as the team who are saying there is little or no DM in it say (according to this article) that they have accounted for that problem.
The reason Hubble really helps with measuring distance isn't because it has some special ability all other telescopes lack, it is mainly because of how good a telescope it is - and it isn't affected by atmosphere or Starlink satellite trails.
The James Webb should improve things once it is available. While it isn't designed for visible light, it will pick up red and orange light so it'll be useful for the "tip of the red branch" type observation/measurement, even if it won't be able to produce the spectacular visible light photos Hubble can.
> building three Hubble class scopes
Wishful thinking... It would be great already if we even managed to get the James Webb off the ground at last. It was supposed to be launched 15 years ago...
As about launching other, additional space telescopes, that era has clearly passed. Too expensive, no profit, and without the Shuttles we can't service them any more, which reduces their expected lifespan drastically. Nobody is capable right now to reach those altitudes with a crew and the required tools and spare parts. That might change in the future, but it's definitely not their priority, since the private sector is profit-driven and there is little profit in "space telescope repairing".
Not quite: https://en.wikipedia.org/wiki/Nancy_Grace_Roman_Space_Telescope
The Nancy Grace Roman Space Telescope (the telescope formerly known as WFIRST) is still on the books and has been approved to move ahead. Basically an NRO spy satellite converted for Infra-red space telescope use, very similar in size and capability to Hubble (though wider field of view iirc).
The hope/plan is that this new scope will be capable of robotic servicing.
> is still on the books and has been approved to move ahead
Given that the James Webb, which had not only been approved but also been built, has accumulated 15 years of delay (so far...), I don't expect that one will get launched anytime soon...
I hope I'll be proven wrong.
Compared to JWST and it's origami mirror system and sun shield, the design of WFIRST/NGRST is relatively straightforward. It's optics and satellite bus are based on known designs and much smaller scale so most of the challenges will be in it's science packages and (potentially) in making it serviceable. I have more hopes of this getting off the ground in a slightly more timely schedule than JWST.
Isn't it the case that WFIRST/NGRST is/was planned to use one of the NRO-donated telescopes? If that's right it really shows how much simpler it is than JWST.
I really hope JWST flies and works: it will be a victory for science if so as it's way less glamorous than Hubble but does actually help enormously doing new things that Hubble cannot do.
> While we'll make good use of the James Web scope, building three Hubble class scopes and putting two of them in Saturn's Lagrange points and the other 180 degrees out in Saturn's orbit would be of greater use.
No, absolutely not at all. They are very different things, different capabilities. In no way, shape or form can Hubble replace what JWST can do, it just isn't designed to do the same job.
For starters, JWST has 6-times the light-collecting power of Hubble. Therefore you'd need to launch six Hubbles.
Secondly, the location of Hubble, Earth orbit, Saturn, wherever (assuming you aren't putting it in orbit around other stars entirely that is) is irrelevant for Hubble's instruments.
Thirdly, JWST operates at different light wavelengths (0.6–28.5μm) than Hubble (well, apart from a small overlap, 0.115-2.4μm across 3 different instruments), infrared, therefore it can 'see' further back in time - even ignoring the difference in light-collecting power - than Hubble, because the light from the early universe has been red-shifted outside Hubble's (and normal human vision's) capability to detect. So even if Hubble was up-scaled to the same 6.5m diameter, or JWST was down-scaled to Hubble's 2.4m, JWST would still 'see' light that Hubble just isn't capable of seeing, the 'old' light of the early universe. It is for this reason that JWST is in the L2, for 'cooling' of its instruments so it can see in infrared. Since Hubble can't see in the infrared, it doesn't need the cooling requirements of JWST, and it is for those cooling requirements - being able to keep the same side, the sunshield side, always pointing at the sun - of putting it in the L2, which doesn't apply to Hubble.
The Register 
Biting the hand that feeds IT
Copyright. All rights reserved © 1998–2024
