Comment 114 by Jollyroger : Geraint - are you now saying that space is fixed and objects, such as galaxies, move apart because they received an initial push from the big bang (possibly augmented by dark matter etc)? That is the only conclusion I can reach from your balls and gravity analogy.And I think it is absolutely wrong.

Great. So if you already know the answer, why are you bothering to ask questions about it on a web forum?

Now Greyman says it is the metric of space that is expanding. By metric I assume he means the unit of measurement, the yardstick that we use to define lengths, et.al.

The metric is a tensor describing the structure of spacetime in general relativity.

I don't disagree with Greyman. I'm trying to give you a good mental picture of why atoms don't expand but distant galaxies move apart, without just saying 'go away and learn general relativity'. The dynamics of particles work out as I've said. The reason they work out that way is because of how the metric expansion of space works in GR. No, it's not the same as Newtonian dynamics in Euclidean space, but in certain restricted situations, viewed in certain ways, it works out similarly and so it can be used as an analogy.

I'm not going to bother replying to any more sarcasm, if you don't like the way dynamics works in an expanding Universe in GR, take it up with the Universe, not with me.

Comment 111 by Greyman :

Comment 109 by Geraint I'm saying that the expansion of the Universe consists of the contents of the Universe moving apart. They only continue to move apart because there is no force between them strong enough to pull them back together

No, it is the metric of space itself that is expanding.  That's why it appears that distant galaxies are moving directly away, at a rate proportional to their distance, from ours.

Yes, this is why I've kept on hedging my explanations. I think I've been pretty careful to do that. But talking about the expansion of space easily gives rise to misconceptions about what this means for bound objects.

If you consider the dynamics of a couple of bodies in the FLRW metric, the way I've described things in terms of an initial velocity gives a more accurate mental picture than thinking of space pushing things apart. There's a good discussion of this in Peacock's book, for example.

Comment 106 by raytoman : They postulated that at one time the increasingly seperating galaxies must have been together at a point (a singluarity which must have projected our universe. The math did not work so that had to invent inflation to explain the difference?

It's a fair bit more interesting than that. The math worked, but our Universe seemed to have fairly special properties that weren't required by the math. They weren't ruled out either, however: there was no discrepancy as such. Inflation was postulated as an extra process which explained those properties, and turned out to make some predictions that have been borne out quite well. The main problem is that further tests of inflation from this point require some very difficult observations.

A hot Big Bang is supported by a lot more evidence than some regression analysis on galaxies, but there are a lot of good resources out there which explain that much more thoroughly than anyone can manage on a web forum (for example the WIkipedia articles on cosmology are pretty good, in general).

Comment 103 by Jollyroger : Not being a physicist, I can't quite see your connection with throwing balls up in the air.

I'm saying that the expansion of the Universe consists of the contents of the Universe moving apart. They only continue to move apart because there is no force between them strong enough to pull them back together. Usually, if you throw a ball up in the air, the force of the Earth's gravity is enough to pull it back to Earth. But if you throw it hard enough, it will be going fast enough that it never gets pulled back to Earth: it will keep on moving away from the Earth forever. That doesn't mean that it has a 'tendency' to move away from the Earth, just that the Earth's gravity was insufficiently strong to stop it moving away.

Now, instead of the Earth and a ball, think of two galaxies given a large relative velocity at the Big Bang. The situation is the same, they keep on moving apart not because of some 'tendency', but because no force was strong enough to overcome the initial velocity. But, if they started off close enough together at the Big Bang, their mutual gravitational pull would be enough to pull them back together and they would merge. They have now forgotten about their initial velocities moving them apart. They don't know they 'should' be moving apart. There is no 'tendency' for them to separate from each other and move apart again.

(Note that I've used 'galaxy' here as a shorthand for ' dense region that will eventually become a galaxy').

What I don't understand is your 'explanation' - which explains precisely nothing, IMO.If the size of particles is fixed with respect to the remainder of the universe, as you say it does, (and I don't disagree with this) then it follows that any and every such fixed region could be taken as a fixed frame from which velocity - in particular the velocity of expansion - can be referred.That is my conundrum, because it contradicts just about all the other known laws of physics.

It doesn't contradict anything. Relativity doesn't say that no frames of reference exist. If I want to perform a calculation, I have to choose a frame of reference in which to make that calculation. What it says is that there is no 'preferred' frame of reference. I can't say that the rest frame of an atom in the Milky Way is a special, unique, more correct frame of reference from which to measure the expansion than any other, and I can't do it for any atom in some distant galaxy which is moving away from us either. The rest frame of either atom would be a valid frame. Neither is privileged. I could also use the rest frame of the centre of an expanding void in the galaxy distribution to be a valid rest frame. The void is expanding, but so what, I can still do it. Whether an object is expanding is neither here now there when it comes to choosing a reference frame.

Why should the space within a particle be exempt from otherwise universal expansion? Does the strong force or whatever exactly balance the tendency to expand?

What I'm saying is that there is no tendency to expand.

What makes that inner space so special?

Nothing.

And if we place such spaces end-to-end to form a rod - at what point does the agglomeration exhibit expansionist properties?

Never. The expansion is a property of the initial state, not space. (Or at least, I'm saying you have to think of it that way to have the correct picture of expansion in mind in this respect, unless you want to deal with general relativity and/or dark energy).

Comment 98 by Jollyroger :

Are you saying that the size of nuclear and sub-nuclear particles does not change together with the expansion of the larger universe?

Yes.

If so - and I agree that observation appears to confirm this hypothesis - then surely this constitutes a frame of reference from which the speed of expansion can be calculated.

Why? Atoms in the Milky Way aren't expanding. Atoms in distant galaxies aren't expanding. However, they are moving apart from each other because of the expansion of the Universe. Which of these non-expanding atoms would you take to be at rest in your universal frame of reference?

Any observer sees all sufficiently distant galaxies receding from him at the velocity predicted by Hubble's formula (ignoring relatively small local velocities). None of these observers is privileged or can claim to be in a special frame of reference.

It is not an explanation - its more like a fudge - to claim that other forces, such as the strong force, somehow prevent the expansion of particles.

It's no more of a fudge than to point out that if you throw a ball up in the air, it will come back down, because gravity 'somehow' prevents the ball travelling away from the Earth forever. It's just a perfectly normal and consistent application of general relativity and the other laws of physics.

If you threw the ball hard enough it would never come back down. The distance between the ball and the Earth would expand forever. This is analogous to the case of the Milky Way and distant galaxies. They were thrown apart hard enough at the Big Bang that they'll never 'come back down' to each other. I want to reiterate that that analogy isn't perfect, but it's the right way to think about how the Universe is expanding while bound objects within it don't expand. 'Expansion' just means 'the distance between things which are sufficiently far apart keeps increasing because they started off moving apart too fast for gravity to pull them back together'.

Comment 87 by Alex, adv. diab. :

I am still wondering about the question of angular size from earlier, and you seem to know the stuff. You've stated that far away galaxies don't have the angular size that would correspond to their actual distance nor the distance naively suggested by the travel time of the light. I suspect it isn't the angular size it would have if the universe had never expanded either. Is there a rule of thumb what it is?

The most commonly used distance measure is the 'comoving radial distance', which is the distance that goes into the Hubble relation. This is what I've described as the distance away that a galaxy seen at high redshift is 'now'. This has to be computed with an integral except in a few special cases.

In the absence of curvature, i.e. in a flat Universe (and ours seems pretty close to flat) then the 'angular diameter distance' is just the comoving radial distance divided by 1+z, where z is the redshift of the object we're looking at. 1+z is just the inverse of the scale factor, which is why I described this distance as the distance away the other galaxy was when its light set off.

This angular diameter distance is the distance you want, in that if you multiply it with the angle (in radians) that an object subtends on the sky, that tells you the physical size of the object in the plane of the sky. To put it another way, if you knew the object's physical size, and you measured the angle it subtends in radians, then this is the distance away which you would infer it to be in Euclidean space. I'll reemphasise that this very simple relation is only true if space is flat; the general expression is more complicated.

It actually behaves in a slightly peculiar way in that beyond a certain redshift it doesn't keep on increasing: a more distant object the same size can actually appear larger in angle. 'Objects in telescope may be more distant than they appear.'

Ned Wright's javascript cosmology calculator can compute all these things for you if you put in your desired redshfit; he also has a version of the calculator that turns light travel times into something more respectable, if you come across them in popular accounts. His site also has a link to a paper summarising all these distance measures.

http://www.astro.ucla.edu/~wright/CosmoCalc.html

A couple of technical links for those who are interested. The various distance measures are also summarised here:

http://arxiv.org/abs/astro-ph/9905116

That doesn't really give derivations, it's more of a collection of various useful results from numerous papers and textbooks.

Cosmological distance measures can be confusing enough that even the professionals have problems. For those who are interested in clearing them up, this is a good resource:

http://arxiv.org/abs/astro-ph/0310808

          Comment 80 by Jollyroger:

On the other hand if, as has been hypothesised earlier on this thread, inter- or intra-nuclear distances and thus our yardsticks do not expand to the same degree as the space between galaxies, then surely this implies a preferred frame of reference?

Firstly, I'd call that an observation rather than a hypothesis. Secondly, why would it imply a preferred frame of reference (it doesn't, but I want to know what your picture is of why it would)?

Expansion in the absence of dark energy can really just be pictured as things moving apart because of their initial velocity. If you think of any two normal objects moving apart from each other, you wouldn't be tempted to say that because the distance between them is increasing, the objects must be increasing in size. You shouldn't be tempted to say this about objects moving apart from the expansion of the Universe either. Unfortunately this picture of galaxies etc. flying apart because of their initial velocity puts over a picture of the Big Bang as something like an explosion, which is incorrect and which is why we say instead that the space between things is expanding. Really the situation isn't quite captured by either the 'velocities from the initial explosion' or 'space expanding everywhere' picture, so you just have to bear in mind which parts of each explanation are valid in different cases.

Dark energy makes things a little more awkward because it's like an extra repulsive force that gets bigger the further apart two objects are. The reason the expansion of the Universe accelerates, in this picture, is that the expansion is moving things further apart, so the repulsive force between them increases, so they move apart even faster, and so on in a sort of feedback loop. But if you have two objects which aren't moving apart from each other (the Earth and the Sun, say), then the repulsive force between them isn't changing with time. They're in equilibrium, and dark energy won't do anything to disturb that equilibrium. Bound objects remain bound, unless the properties of dark energy change with time.

I know atoms and molecules definitely take up a great deal what we homo sapiens refer to as physical space.

No, they don't. A nitrogen molecule in air travels about 1000 times its own length on average before hitting another molecule. And the mean number of atoms per cubic metre in the Universe is smaller than the number in air by a factor of approximately 100,000,000,000,000,000,000,000,000.

Even an atom is mostly empty space. The comparison between the size of an atomic nucleus and the size of an atom has been likened to a fly in a cathedral.

With all due respect, I can't pretend to think of raisins as "particle matter", when I know raisins have a physical molecular structure called physical mass.

In that case, you're just failing to understand what an analogy is. It's hard to progress from that point.

While terms, such as particle matter, might fit well the hypotheses or theories of quantum, that does nothing to validly explain physical atomic and molecular processes aka physics concerned more with what has been proved from the laws of nature aka laws of physics.Are you contending that atomic weight of atoms increase as atoms increase in size merely because we're told the universe is expanding? Or are you saying the atoms increase their size mass without increasing their weight because we‘re only told the universe is expanding? I’m sincerely not certain what you do mean.

I have no idea what you think the atomic weight of atoms has to do with the expansion of the Universe. I really don't have the first clue why you might think they'd be connected. And what is 'size mass' meant to mean?

No-one has mentioned anything about atoms increasing in size or mass because of the expansion of the Universe. The expansion of the Universe is an expansion of space.

PatW: So what happens when an additional mass, say a new star, occupies space already occupied by other atoms and molecules?

It's hard to answer this sort of question because it's very hard to understand what mental picture you have of these processes. There's clearly some misconception here, but it could take a while to get to the bottom of it. Stars, for example, form from tenuous clouds of atoms and molecules which are able to cool and collapse under their own gravity. Stars form from atoms and molecules, they don't suddenly pop into existence and have to push things out of the way.

In any case, the expansion of the Universe isn't creating new matter. The average volume that each atom has to itself increases as the Universe expands. This is one of the things to take from the cake analogy, if you think of the raisins as matter particles and the rest as the space (it also helps picture the raisins all moving away from each other, rather than moving away from a common centre). The raisins (matter) don't have to push other raisins (other matter particles) out of the way as the cake expands. And the risen cake doesn't contain any more raisins than the raw cake. But of course you can't take an analogy too far. Schrodinger's Cat introduced it to help you visualise a specific point, you shouldn't be trying to infer unrelated things from that analogy.

If you insist, it might be better to picture an infinite cake, so there's no top to worry about. A finite Universe might be better pictured with another well-known analogy of the surface of the balloon. The 2D surface of the balloon is meant to represent the 3D volume of the Universe (reducing the number of dimensions helps you picture things better, but it breaks down in different situations from the cake analogy). Stick some little paper stars or something on the balloon and inflate it. The stars get further apart because the space (balloon rubber) in between them stretches. The surface of the balloon has no centre (remember, we're only considering the surface of the balloon in this analogy), just as the expansion of our Universe has no centre. What's more, the surface has no edge, but it has a finite area. The lack of an edge gets away from the 'what about the cake tin?' part of the cake analogy, but the balloon analogy has its own problems, for example people are always drawn to thinking about the inside of the balloon, which isn't part of the analogy.

          [Comment 36](/discussions/643701-stupid-and-clever-questions-for-people-who-understand-the-physics/comments?page=2#comment_886143) by  [PatW](/profiles/181384)          :


                 I have to ask these questions when people are discussing an expanding universe.  Do they actually mean the universe is physically expanding as we expand as we grow from infancy to adulthood?  

It's physically expanding. The analogy with a person growing isn't really a very good one for various reasons, but people are talking about a real, physically meaningful and well defined expansion.

Or do they mean that through the artificial means of extremely high powered microscopes and spacecraft, such as Hubble, we now have the capability to physically observe what we could not possibly observe prior to the manufacture of artificial "eyes", and it was only an illusion there was expansion taking place?

Like when people say that the world is shrinking as we become able to travel and communicate faster? Obviously new instrumentation lets us see more and learn more, but when people talk about the expansion of the Universe, they really mean that it is expanding. Our understanding of it has expanded too, of course, and new instruments allow us to see further and in more detail, but I'd never really thought anyone might confuse this metaphorical 'expansion' with the real, physical expansion. It's something to bear in mind when I'm talking about it in future!

          [Comment 22](/discussions/643701-stupid-and-clever-questions-for-people-who-understand-the-physics/comments?page=1#comment_886065) by  [Schrodinger's Cat](/profiles/105285)          :

Comment 17 by Alex, adv. diab. Assume that you observe light from 12 billion years ago. When it was sent off, the source was maybe much closer to us, say 2 billion light years (I'm not sure about the correct number, but it is good enough to get the principle). So, back in the day, the light got sent from this 2 billion light year away object, and while it was traveling, the space in which it travels got stretched all the time, making the light redder and redder. Finally, after 12 billion years, it reaches us, and is redshifted by the amount the universe was stretched in these 12 billion years. The object which was initially 2 billion light years away, is now more than 12 billion light years away, and the 12 billion year travel time is some kind of average between 2 billion light years and the current distance.

Well now that gives rise to my own puzzlement.If the light was emitted when the galaxy was only 2 billion light years away, then the galaxy would have had a specific angular size....which the expansion of space should not affect, after all the light from each edge of the galaxy still travels in a straight line towards the center of an imaginary sphere with us at the center.But we don't see 12 billion year old galaxies with the angular size of galaxies 2 billion light years away.....we see them with the angular size of a galaxy 12 billion light years away.

Actually your galaxy at a light travel time of 12 billion light years does indeed have an angular size distance of about 4.9 billion light years, which is one of the numbers I mentioned in my first reply above. Its angular size is not the angular size a galaxy 12 billion light years away in Euclidean space would have.

          [Comment 10](/discussions/643701-stupid-and-clever-questions-for-people-who-understand-the-physics/comments?page=1#comment_885997) by  [Labyrinthos](/profiles/13044)          :


                 Here's a couple that's been bothering me for a while: - If the Universe is expanding would an observer on the "edge" of the Universe only see stars and galaxies in one direction, and not the other? 

There's no edge.

If an "edge" of the Universe doesn't make sense, then how can it expanding possibly make sense?

Expansion means everything is moving away from everything else (well, almost; objects which are close enough together can become gravitationally bound so that the distance between them doesn't expand with the Universe; what expansion 'really' means is the growth of the scale factor that Jos talked about).

There has to be something that is closer to the "border" than other things are, correct? - If we look in any direction, the farther away we look, the further back in time. Apparently, if I see a galaxy that is 12 billion light-years away, what I'm actually seeing is what that galaxy looked like 12 billion years ago.

Well, not quite. Light travel time isn't the same as 'distance away now', as it's usually defined. If we see light from a galaxy from which the light set off 12 billion years ago, it's now something like 23.3 billion light years away from us, and it was about 4.9 billion light years away from us when the light set off.

Having moving sources of light and moving observers should destroy that linear correlation, but everybody seems to be using it still. Why?

For some reason, popular articles always like to use light travel time as a distance measure. It's an awkward one to use if you're actually doing cosmology, and in practice it's not really used much by astronomers in their work.

Further, let's assume I'm continuously observing that far-away galaxy. After I have taken the first photograph of it as it appeared 2.7 billion years after the Big Bang, I can only see more recent versions of it. The image of the galaxy as it appeared 2.7 billion-years after the Big Bang is forever lost to me, since its' light has passed me, never again to return. Why isn't this applicable to all visible structures? How come we can still look back to shortly after the Big Bang? Wouldn't that light have already passed us, never to return again?

The reason is that the Big Bang happened everywhere in the Universe at once. When we talk about 'looking back to shortly after the Big Bang' we're often talking about observing the cosmic microwave background (CMB) radiation, which was generated about 380,000 years after the Big Bang. This radiation was generated everywhere in the Universe at (almost) the same time. So as time goes on we see the CMB radiation that was generated from points further and further away from us.

Comment 44 by glenister_m :

Just curious. Have they found galaxies in earlier stages of formation that support their simulation?

Qualitatively speaking, yes. If you look at the most distant galaxies in the Hubble Ultra-Deep Field, for example, you see 'train wrecks' of the sort you would expect from the messy merger process you see at the start of the video. But of course they don't come with a label saying they're going to turn into a nice spiral galaxy: you only ever get frozen snapshots of galaxies, so to link them together at different epochs and 'see' their evolution you need to use simulations.

Also, no-one need worry about astronomers simulating a Universe which matches all available data and thus giving up looking for improvements in their simulations for a very long time yet.

This reminds me of one of the first programs I wrote long ago. It produced a graph. I showed it to my boss, who was expecting me to do the analysis by hand. He said "cool... hmmm, but is it right?".How do you verify the accuracy of the program? If it doesn't produce the expected output, then you look for a bug. Forget about proving correctness, that fad simply showed it was probably more than 10x harder to prove a program correct than to write the program.I concur with mmsood99. I bet if the result didn't produce what they knew was the result, they'd be looking to see what is wrong. But if it does produce the expected results, then what is the motivation to look for flaws?

Are you talking about showing that the code correctly solves the equations you're asking it to solve, or talking about whether you put in equations which correctly capture what's going on in the first place? The former can certainly be verified with analytical test cases and comparisons between independent codes. The latter is what you're trying to learn about.

With all the different shapes and types of galaxies, can we really expect that some "butterfly" effect billions of years ago does not change the result considerably?

Well, one of the tests you might think of is to run with the same parameters in a cosmological volume and see if you get the right proportions of different types of galaxies. That's certainly interesting because you can then compare their histories and try to understand what causes the differences. There are many simulations aimed at doing that.

Anyway, just wanted to post a link to the arXiv version of the paper, which is freely available: http://arxiv.org/abs/1103.6030

The tweaking need not be willful. For example, given two different models of star agglomeration, it is tempting to take the one that most closely delivers the expected end point.

Well, how problematic this is depends on what you're trying to find out. The fact that we see spiral galaxies is an observational datum, and our models should be able to produce them or else they must either be wrong or incomplete.

If, for example, it was impossible to make spiral galaxies in a LambdaCDM cosmology whatever you assumed about star formation, that would be interesting to know. It's not interesting to know that 'LambdaCDM + the first model you thought of with some parameter values you chose arbitrarily because they can't be derived from first principles' can't produce spiral galaxies. That doesn't help you to isolate or test individual parts of the theory.

If you have a variety of star formation recipes which can make spiral galaxies, you can try to come up with tests to choose between them, and the simulations can help generate the predictions which you then test. Some may then be ruled out whatever parameters you assume, or might require inconsistent parameter choices to match different observations. But I don't really think the aim of these simulations is to fit the parameters of effective physical recipes, since that isn't really very informative about the smaller-scale processes which underlie the recipes. If it was possible to derive the parameters from smaller scale physics, or to simulate the smaller scales directly, you'd do that. In the face of genuine uncertainty, you don't want to abandon hope of learning about one part of a theory because another part is incomplete.

Comment 19 by mmsood99 It's a beautiful simulation, but I wonder if the results have been corrupted because the researchers knew their endpoint? The galaxy is known to have spiral arms, so the simulation has spiral arms. How much did knowing the endpoint affect the simulation?

Some more details are in the article. Apparently they use about 60 million particles, which isn't all that many compared to large cosmological simulations which can now contain tens of billions, but what's important here is the physics that is included in the modelling.

The formation of individual stars can't be tracked in a simulation of a whole galaxy, so you need some effective prescription which realistically accounts for where and when stars form, and the effects of star formation on the galaxy: the energy the stars emit during their lifetime and when they go supernova, the chemical enrichment of the interstellar medium, etc. One of the aims of this sort of simulation is to see just which physical processes are important in order to end up with a galaxy that looks like the sort of galaxies we see around us.

Knowing the endpoint doesn't really help at all. I'd have to look in the paper, but most simulations of this type start off with simple initial conditions motivated by observations of the cosmic microwave background: small fluctuations in density with the correct amplitude and scale. Typically, you run a simulation of a very large volume of the Universe, find an interesting region (e.g. one forming a galaxy), then go back and rerun the simulation at higher resolution, zoomed in on that interesting region. That ensures that the initial conditions aren't cooked and that the galaxy forms in the correct cosmological context. One can try to guess which regions are more likely to end up with nice disk galaxies (typically fairly quiet regions where this disk doesn't get disturbed by mergers with other galaxies etc.), run many simulations, and see what you end up with.

As for the underlying theory and assumptions, no doubt this simulation is an advance over previous ones, but (as far as I can tell from this little article, to know more I'd have to read the full paper properly) it's taking the next step on an established path, not doing anything dramatically different. Many groups are running simulations of this type.

Comment 36 by Duff :

I'm a little confused. I thought "precession" had to do with the earths progression around the orb of the galaxy. Not that it was caused by the wobble imposed by the moon. Anyone know for sure?

Nope, nothing to do with the Galaxy, everything to do with torques from the Sun and Moon.

You have my condolences, Richard.

Comment 35 by plasma-engineer :

Yes, I heard that cosmologists were prepared to accept an error of 300% too. But that was a while ago, and today perhaps their target is 295%. That's the way science works. Meanwhile the probability of the existence of a man who walked on water and turned water into wine is in a different league altogether. (Yes lower!) Must admit though - dark matter is beginning to look less necessary thanks to more and more new discoveries like this one. Previously undetected intergalactic gas clouds were in the news not long ago - now that we realise that one such cloud is colliding with our galaxy at relativistic speeds we know it is there. Isn't it fascinating?

10 years ago I nearly accepted a job in one of the world's leading organisations looking for Dark Matter. I regret it less and less every year.

Well, the total amount of baryons isn't really measured from that sort of thing, it's inferred from things like CMB observations and nucleosynthetic constraints. We already knew there were 'dark baryons' which hadn't been directly detected, and those direct obervations of intergalactic gas (and, to a much lesser extent, these stars) help fill that gap. But they don't have any bearing on the amount of nonbaryonic dark matter that's needed to explain other data.

Or, to put it another way, they'd already been accounted for in the big picture, but not directly observed.

Comment 11 by AtheistEgbert :

I don't think so. According to this diagram:

http://upload.wikimedia.org/wikipedia/commons/a/a5/080998_Universe_Content_240.jpg Dark matter makes up about 23% of all matter while dark energy makes up 72%.

So total atomic matter in the universe is only a staggeringly small 4.6%.

And maybe around 10% of the baryons are tied up in stars, so this news doesn't really have any bearing on dark matter (or, for that matter, on multiverses or inflation, so I don't know why those were brought up in other comments). No doubt it has implications for models of galaxy formation, though.

What's interesting isn't so much the absolute number of stars, though I suppose that was an easy thing to explain in a press release. Instead, it suggests that the ratio between the number of large stars and the number of small stars is different in the Milky Way and in elliptical galaxies, so presumably there's something different about the environment in which stars are forming in different types of galaxies. Galaxy formation and star formation are both complicated processes with lots of different things going on, so the physics gets messy (but interesting!).

I don't think this is the sort of cosmology that's 'fantastical', in Letsbereasonable's words. It's grounded in known physics, not speculative physics. But there's enough going on that observations and theoretical modelling are both difficult.

3) Literature: I admit to not being extremely well read but I think George Orwell deserves a nod. Lord of the Flies and 1984 are classics that everyone should read. Shakespeare, of course, is definitely the greatest playwright to have graced this planet.

Lord of the Flies is by William Golding.

I think the article is confusingly written. The paper is more to do with our estimation of the rate of star formation than it is to do with how much 'stuff' there is out there in total.

Observations of distant star formation are difficult, so you have to work hard to try and reconcile different ways of measuring star formation. One of the big problems is to do with how much of the UV light emitted by young stars is absorbed by gas and dust, which is affected by the distribution and properties of dust within a galaxy.

Spectral lines (such as Lyman alpha or Balmer alpha mentioned here) and different wavelength regions, such as the infrared, are affected differently and give different estimates of the star formation rate unless you know how to correct for all the other confusing factors. We can also try and estimate the star formation rate in the early Universe by looking at the number of old stars in nearby galaxies.

The paper's really nothing to do with e.g. dark matter. It's more to do with our understanding of awkward observational effects.

There's a page eplaining CMB physics here, which might be useful to people trying to understand the polarization stuff:
http://background.uchicago.edu/~whu/

It does get quite technical though. The scattering of light from free electrons does depend on polarization. The reason you end up with a net polarization at some point rather than everything cancelling out is because radiation reflected from different directions ends up with a different polarization. A region of the Universe will receive different amounts of radiation from different directions (because the Universe isn't completely homogeneous).

The signal is weak though: polarization measurements are much harder to do than total intensity measurements.

Regarding the amount of 'ordinary matter' in the Universe, current estimates are that it makes up about 4.6% of the critical density. Dark matter makes up about 22.8%, so you end up with a total of about 27% of the critical density coming from matter. The rest comes mainly from dark energy, with a very small contribution from radiation. It's thought that the total comes to something very close to 100% of the critical density.

Comment #427676 by Sally Luxmoore

I am still reading that paper you referenced - but one quick question - I see that K-T means "Cretaceous–Tertiary" (something I had not known before) but why 'K'? Is it because 'C' is taken by 'Cambrian'?

I think the Carboniferous was the first to bag 'C'.

I can't read this sentence without hearing that deep, gravelly American voice which does the voice-over on adverts for trite, one-dimensional money-spinners.

Unfortunately, the guy you're probably thinking of died last year:
http://news.bbc.co.uk/1/hi/entertainment/7595352.stm

Spawned a thousand imitators though I suppose.

If I recall correctly, there have been cases in the courts in which it had to be determined whether bridge was a game of chance (and so whether it came under gambling regulations). I know the result was that it was found not to be a game of chance, but I don't remember the details.

Well, quite a few people come to cosmology from a particle physics angle; many also come having specialised more in astronomy as undergraduates. That applies especially to cosmologists who don't focus on the very early Universe. Sometimes it seems to me that when someone says "cosmology", a lot of people (even physicists outside astronomy) think straight away of very early Universe cosmology and all the relatively speculative things that go with it.

A great deal of what's considered to be cosmology is instead to do with the subsequent evolution of the contents and structure in the Universe, which depends far less on speculative physics, and more on 'standard' physics but complicated astrophysics (the details of galaxy formation, for example). This is why I have a problem with the generalizations you sometimes see about 'problems' with the subject. To assess a lot of the ideas and results in modern cosmology, a good knowledge of astronomy would be more important than detailed knowledge of particle physics. I'd agree with bethe123 that general relativity is required, of course. I only quibble because I think that the average student who's taken physics to a high level is more likely to have missed out on advanced astronomy courses than advanced particle physics or GR courses. It's certainly the former that I had to catch up on when I started research in cosmology, having come from an applied maths background.

Quantum Flux
All observations for black holes are based on telescopic images thousands of light years away, in fact the images show a lot of gamma radiation eminating from them which means they're not "black". Astronomers claim that this is due to violent ripping apart of particles entering "the event horizon".....none of that has been directly observerd, it is merely assumed that on the quantum scale things are behaving that way. An assumption of quantum behaviours is by no means a direct observation of behaviours in my book. There are plenty of other ways that X-rays and gamma rays can be generated (just rapidly peel scotch tape or utilize a van-de-graph generator make lightning and you'll find X-rays being emitted), the truth is that astronomers don't know for sure what is going on around these high intensity radiation emitting zones because they don't have the resolutions to make the observations directly. It is tantamount to trying to explain the lines on Mars away as Martian Canals, from a low powered terrestial based telescope when there are much better explanations upon closer examination.

That's not how astronomers explain the emission from black holes; it doesn't depend on the sort of assumptions on 'the quantum scales of things' that you seem to think. Even if all the proposed mechanisms were totally wrong, though, you still have to explain where all the energy's coming from, and it would take a hell of a lot of scotch tape!

Of course it's not all from images either; for most of the real science you're talking about analysis of spectra over a vast range of observational wavelengths, and there's also high-resolution interferometry. We know a lot about the temperature, ionization state, magnetic field etc. of material near the 'compact object'. We don't just have snapshots either, but detailed observations of variability over many time scales (and, in the case of the Milky Way's centre, observations of the dynamics of nearby stars).

Similar mechanisms hold for accretion discs around neutron stars - I don't know if you have a problem with those too - which of course don't have a horizon. In fact you can see the difference between emission from an object where there is a surface for material to hit, and one where there's an event horizon. It's all rather fascinating and I don't think you do it justice.

Ah, um, ok I've just realised I can right click and select new tab.. doh!

Or Ctrl-click. Or click with the middle mouse button.