Slashdot Mirror
New Supernova Seen In Nearby Galaxy M82
The Bad Astronomer writes "A new and potentially bright supernova was just discovered in the nearby galaxy M82. This is a Type Ia supernova, the catastrophic explosion of a white dwarf. It appears to be on the rise, and may have been caught as much as two weeks before peak brightness. It's currently already brighter than magnitude 12, and may get to mag 8, easy to see in small telescopes. The galaxy is less than 12 million light years away, so this may become one of the best-studied supernovae in recent times. Type Ia supernovae are used to measure dark energy, so seeing one nearby is a huge boon to astronomy."
Just sayin'.....
I love Astronomers ... sure, 12 million light years away can be construed as 'nearby' on some scales.
Obviously galaxies tend to be a little further away, but it's definitely a relative use of the term 'nearby'.
Having said that ... go science! This is pretty cool.
Lost at C:>. Found at C.
More like huge boom! lol amirite?
Mostly random stuff.
Since no information can travel faster than light, for all intents and purposes and discussions of causality it is happening right now. Since we are just entering its light cone, anything outside of it is inaccessible to us - and always will be.
> I'm always stressing to people at our star parties the light you see is history.
I'm surprised they still invite you. They probably already know.
The galaxies M81 and M82 are only about 300K ly from each other. A decent telescope can image them both at the same time. Relatively easy to find in Ursa Major too. I look forward to viewing this during the next new Moon.
Technically, everything you ever perceive is in the past. More often than not, two simultaneous perceptions of the same thing are not even the same past.
How brains manage to correct for both the perception latencies and the action latency, so that we can interact with our environment, is pretty amazing.
Except, in our frame of reference, it's happening now, even though it happened then.
Which means in the future, we will would have seen this from before, but we won't have yet known if more stuff which will would have happened in the past will be happening in the present as the future unfolds.
So it is simultaneously not happening now, and happening now -- it isn't really happening now there, but here it is happening now, except it already happened there, and technically it has already happened here, but we're only now becoming aware of it now, but in the future, both will have happened in the past.
Which is why we stick with tenses which make sense to our poor little brains. it's just too damned hard to conjugate the verbs. ;-)
So, from what I've been able to tell -- we discuss it in the present tense, and then occasionally remind ourselves that we're seeing something which happened a long time ago. But then we try not to mix up the two, because it hurts more than an ice-cream headache.
Lost at C:>. Found at C.
I'm a casual and very interested follower of these post about astronomy, I'm always interested about what you guys are up to.
THE question I am sure many will think about is how many neutrinos will be detected.
For supernova 1987a at 168'000 light years 24 neutrinos have been detected.
At 12 mega light years M82 is 71 times further, which dilutes the neutrinos by a factor ~5000.
So the answer is 0 neutrino if the detectors were the same as in 1987.
I doubt that the present detectors have improved by a factor 1000 in the meanwhile,
but I would be glad to be disproved.
The information is new to us. Take your meds.
My God, it's Full of Source!
OUTSIDE_IP=$(dig +short my.ip @outsideip.net)
We're observing it some 12 million years after the fact. That hardly qualifies as "new".
Slashdot always lags behind the facts a few days, or more...
12 million years ago.
thanks for clarifying that, I mistaken from the article it was two weeks ago.
mfwright@batnet.com
Thank you, Dr. Streetmentioner.
Should we get out our sunscreen?
---- The above post was generated by the Turing Institute. Maybe.
> I'm always stressing to people at our star parties the light you see is history.
I'm surprised they still invite you. They probably already know.
I'm usually the one with the telescope, making peoples minds boggle. Explaining the speed of light and vast distances I usually have a rapt audience who aren't given to thinking about these things. Nothing quite like a Star Party to answer the questions the people themselves are asking. We don't have the great pictures of Hubble or Spitzer, but we have people who aren't watching TV, but are actually seeing things through the eyepiece and getting some of the science explained first hand.
A feeling of having made the same mistake before: Deja Foobar
So what you're saying is that you like flogging a dead horse with the same old science facts that most of the people here were well informed of by the time they reached the 8th grade. Got it.
Why don't you tell us again that a black hole has such an enormous gravitational field that not even light can escape it or that man and dinosaur didn't walk the earth at the same time? I just love hearing these trite facts over and over and over again.
When will then be now?
12 million years ago.
Well that depends on your frame of reference, doesn't it?
Give me Classic Slashdot or give me death!
Technically, everything you ever perceive is in the past. More often than not, two simultaneous perceptions of the same thing are not even the same past.
How brains manage to correct for both the perception latencies and the action latency, so that we can interact with our environment, is pretty amazing.
I try to convey to people that we are effectively, for the sake of explanation, a fixed point in the middle of an aquarium - with little floaty bits all around us in the water, moving at their own velocity on their own course. The light we see took some time to reach us and that floaty bit we think is there is since moved. If we got in a high speed star ship and traced the route back to it we'd find that route likely contains curves and even moves a bit as the gravity of other bodies have diverted it ever so slightly along its path. Due to the speed of light, and the passing nature of it along its way, if we sailed toward a galaxy at Warp speed we'd see it spin much faster than it actually does as we are seeing a fast-forward effect.
Daft as it may sound, we really get people's minds wrapped around some basic physics, even if for a short while, away from the other distractions of life.
A feeling of having made the same mistake before: Deja Foobar
12 million years ago.
thanks for clarifying that, I mistaken from the article it was two weeks ago.
12 million years + 2 weeks. News doesn't travel as fast as it once did.
A feeling of having made the same mistake before: Deja Foobar
I can't see through a monocular eye piece, you insensitive clod. ;-)
Actually, that part is true ... when I try to look through a telescope eye-piece all I see is a blob, but strangely, I can see through my DSLR (which seems to have a larger eyepiece than most telescopes do). But I've literally never seen anything through a telescope, which kinda bums me out, because I'd like to.
I strongly suspect that means I'm either an idiot, or looking through the wrong end of the telescope. Of course, the latter could be a symptom of the former.
Lost at C:>. Found at C.
This is just patently abs.. ALL GLORY TO THE GREAT HYPNOGALAXY!
(and a few words for the all-caps filter)
only here 12 million light-years could be implied to be near ^^
I can't see through a monocular eye piece, you insensitive clod. ;-)
Actually, that part is true ... when I try to look through a telescope eye-piece all I see is a blob, but strangely, I can see through my DSLR (which seems to have a larger eyepiece than most telescopes do). But I've literally never seen anything through a telescope, which kinda bums me out, because I'd like to.
I strongly suspect that means I'm either an idiot, or looking through the wrong end of the telescope. Of course, the latter could be a symptom of the former.
Years ago I invested in a motorized Reverse Crayford Focuser (from Jimsmobile) for my telescope. It's been the single best investment - no more motion on distant, high magnification objects from contact with the scope and I can hand the little control unit to anyone to adjust the focus best for their vision. Can't imagine doing these big star parties without one.
A feeling of having made the same mistake before: Deja Foobar
Do you know how many people in this world don't even get those basic concepts? Most. So while you're in your neckbeard haven (i.e. your mother's basement) not getting laid, bitching about repeatedly reading "the same old science facts", remember that if you want society to get a little better then you need to deal with simple factoids being repeated several times.
Never, as soon as it's then, then it will be now (but then), but then will refer to a different then than now, and a different now than now because it's then.
Then the now that will be used then is what we now refer to as then. Then we'll have another then from the now that is then, and the then then becomes now. The now we use right now will no longer apply, but the then we use now could still apply then if then was further out than then as of now and then.
And it's turtles all the way down. ;-)
Lost at C:>. Found at C.
Sure, but in our reference frame it happened approx. 12 million years ago. If the relative velocity between us and M82 was a significant fraction of the speed of light, then this number could vary quite a bit even for an observer at the same point in space, but I suspect that "12 million years ago" is a reasonably good approximation for anyone reading this article. ;)
> "The galaxy is less than 12 million light years away, ... so seeing one nearby ..."
We're talking galaxies here. Andromeda is the closest spiral galaxy to us, and that's 2.5 million light years away.
And it's HEADING STRAIGHT FOR US!!!
That isn't quite right. In principle, it could be happing "now" (or rather in the very recent past) for an observer in, say, our solar system, but only if that observer had a velocity (relative to M82) that was very close to the speed of light. For someone on Earth (having a relative velocity which is only a small fraction of the speed of light), it is correct to say that it happened about "12 million years ago".
I've heard gravity travels at the speed of light... Does time travel at the speed of light? If so, then it really is happening as "right now" as it can.
Except, in our frame of reference, it's happening now, even though it happened then.
Nope. In our frame of reference, it most definitely happened then. The light is reaching us now. It's too late to emit a beam of light of our own to meet the supernova light halfway, which it wouldn't be if it was happening now.
The only reference frame at this point in space in which it is happening now is that of the light which is reaching us.
systemd is Roko's Basilisk.
When will then be now?
Soon.
I might be bad at math, but 12 million years plus two weeks is still exactly 12 million years.
... would like to welcome our new supernova refugee overlords!
This sig is not paradoxical or ironic.
Optical devices, such as cameras with optical viewfinders, telescopes and binoculars are designed to to be used with the eye a certain distance away from the eyepiece's lens. This distance is known as "eyepoint", and pesons wearing eyeglasses often have difficulty using "low eyepoint" devices.
Since no information can travel faster than light, for all intents and purposes and discussions of causality it is happening right now. Since we are just entering its light cone, anything outside of it is inaccessible to us - and always will be.
Well, if you argue that, you have to give up concept of distance, or concept of speed of light. From our frame of reference, light traveled certain distance at certain speed, and simple calculation will tell how long time it took.
Or to put it another way, when you receive reflection of light you sent to a mirror, neither sending nor reflecting happened when you received the reflection back. It is in fact possible to determine distance of mirror by knowing how long ago sending and reflecting happened.
It's currently already brighter than magnitude 12, and may get to mag 8, easy to see in small telescopes.
That's a pretty optimistic statement given the rampant state of light pollution around the world!
The naked eye limit is Mag. 3 for most of us who live near any streetlights. Magnitude 8 objects require a 6-8" telescope, preferably with tracking if you want to find the Mag. 8 galaxy.
I don't think of telescopes above 4" as "small."
I type this not to be annoying, but because a lot of people are going to waste a lot of time at night trying to see this thing when it is likely beyond their equipment (or patience) limit.
Yes and pi is exactly 3.
The stars outside of the observable universe that we don't ever see explode. The ones that never interact with our part of the universe. When did those happen? Funny thing is. If you're anywhere 'near' my space-time reference then the event is happening now for us. Space and time are relative.
Well, if you argue that, you have to give up concept of distance, or concept of speed of light. From our frame of reference, light traveled certain distance at certain speed, and simple calculation will tell how long time it took.
Actually, if you want to be accurate: no, it is not a simple calculation, due to the metric expansion of space and all...
Except, in our frame of reference, it's happening now, even though it happened then.
Nope. In our frame of reference, it most definitely happened then. The light is reaching us now.
Well, in common language, there really are at least two "nows". There's "now"(1), as in what may literally be happening somewhere I can't perceive at the moment. "My friend is working at his office now." What I really am saying most of the time is "Based on past information, I predict that if we measured the position and activity of my friend at time T0 -- this instant -- we would be likely to find out at some point in the future (T1) that at T0 my friend had been working at his office."
But there's also "now"(2), as in what I actually can perceive myself at this instant. "The sun is shining now." Well, yes, I suppose -- except those light rays left it some minutes ago; we don't know what the sun is doing now(1). "The police siren is making loud noises now." Well, no -- the police siren made loud noises, perhaps a fraction of a second ago, perhaps even multiple seconds ago if I'm a mile or more away -- but I'm hearing them now. The police siren may actually have ceased sounding by the time I make that statement.
We commonly use language in this way, where "now" can refer to what is happening in our perception, rather than what actually is occurring somewhere else that we can't perceive. In effect, the only place where now(1) and now(2) come close to meeting is when we talk about present observation and perception... which is frankly all we know about anyway.
Light cones create a fixed boundary beyond which information about an event can be known. Talking about what is going on outside our light cone is speculative at best, meaningless at worst. It's not like we could place an intergalatic phone call to our friend and actually find out what's going on this star now(1). We can only know what's going on at now(2).
I understand for physics and mathematical purposes, we like to talk about the abstraction of now(1). But now(2) is actually a more human concept expressing something about our engagement with events as we perceive them. On a galactic or universal scale, it makes sense to describe a supernova as "new" according to the concept of now(2), as roughly applied to our planet. If at some future time, we have observation posts spread out over many light years, it may no longer make sense to have a collective now(2) for humanity. But since we all live on one planet at the present, I can see some usefulness (and common linguistic reasons) for talking about the first information arriving to us as what's happening "now" in our "frame of reference" (loosely defined).
In this case, what's "happening now" in our frame of reference is our perception of the supernova, just as what's "happening now" in my frame of reference may be that "the police siren is making loud noises." Whether or not in fact the siren has ceased making noise at now(1) is generally irrelevant to my statement; what's happening now(1) at the supernova site is not only irrelevant but completely unknowable. It barely even makes sense to define a "now" for that, from an epistemological standpoint.
Goon show what time is it
http://m.youtube.com/watch?v=-...
just goes to show everything has a life cycle even the galaxy its self.
Did you know that people like you existed alongside the dinosaurs?
Turd.
Over such distances the expansion of space is totally insignificant -- that's a large scale effect and is *only* active on large scales. Local structures are totally disconnected from it. (If the language doesn't sound intimidating, the expansion is a feature of the Robertson-Walker metric, which is assumed to be valid on very large scales. It is not a feature of Schwarzschild, Kerr, Lemaitre-Tolman-Bondi, Szekeres or other metrics that describe smaller structure, although it's true that you can find, say, an LTB that also has a cosmological constant. Since local structure will be described by something close to a Szekeres, it is not influenced by the "universal" expansion.) It's a bit like the universe is that old expanding rubber sheet, and local structures are pebbles rolling around on it. The pebbles aren't growing, even though the space between them is.
Extra points for anyone spotting an enormous logical flaw in this picture that is at the heart of one of cosmology's biggest (and unsolved) fundamental issues.
Time doesn't travel - that's as meaningless as saying "does left travel at the speed of light" or "does up move more slowly than diagonal?" It's just another dimension, and one the choice of which general relativity makes extremely arbitrary.
I work the star parties too and it was unanimous.... no one wants to here your elementary school bullshit.
Please tell me, precisely, the decimal value of Pi.
3
3.1
3.14
3.142
3.1416
3.14159
Saying "Precisely" and "Pi" is error in numeric values. 3 is no more wrong to say than 3.14159 is. Precision is relative at that point. How accurate do you need to be? a table with a circumference of 18 feet has a radius of just under 3. Saying pi is 3, radius is 3 is reasonable for getting a tablecloth, or knowing how much skirting you need or whatever else your measuring.
On the other hand, we now know the complete value of Pi to enough digits that we can encircle the entire Universe and be accurate to the NANOSECOND. How accurate is enough?
Agent K: A *person* is smart. People are dumb, stupid, panicky animals, and you know it.
On the other hand, we now know the complete value of Pi to enough digits that we can encircle the entire Universe and be accurate to the NANOSECOND. How accurate is enough?
You can quit when you reach the last digit of Pi. That'll be enough. Call me when you're done.
In the end they will lay their freedom at our feet and say to us, Make us your slaves, but feed us. - Fyodor Dostoyevsky
i'm genuinely curious, if there was an advanced civilization that sent an escape ship directly towards us 12 million years ago (but appearing to us as this very moment) at slightly less speed than the speed of light, we would only begin to see (in this theoretical case, the ship is somehow visible from start to finish) the ship right now, however it would be nearer than 12mil light years away by the time the image of its launch reached us. would the ship then appear to be moving at a speed greater than the speed of light? would there be "two" ships?
no, that ship can never outrun any of the light showing its history, so we can watch it launch and speed up and arrive.
the historical image of it would of course arrive first, but it would be impossible for an image of the traveling ship arriving at earth before the ship physically does. at what point does the speed of light image of the ship reconcile with the slower than speed of light ship itself on their respective (photons and ship) trips to earth?
Actually, if you want to be accurate: no, it is not a simple calculation, due to the metric expansion of space and all...
I don't want to be accurate to that degree. There's no point. The expansion at 12 million light year scale is who knows how many orders of magnitude less than the accuracy of our best measurements of the distance.
Well, if you argue that, you have to give up concept of distance, or concept of speed of light.
No, you just need to give up the concept of simultaneity: the idea that two events at different places can happen at the same time. And that's what you actually have to do to make relativity work. It sounds ridiculous ("counterintuitive" is the usual term), but it's the way the universe works.
It is in fact possible to determine distance of mirror by knowing how long ago sending and reflecting happened.
The time taken for the light to reach the mirror and come back, and hence the measured distance to the mirror, will actually depend on your reference frame (i.e. how fast you're moving when you measure it). Another counterintuitive result, but it happens, because of two different relativistic effects: length contraction and time dilation.
Isn't relativity fun?
Well, in common language, there really are at least two "nows".
When someone starts a sentence during a discussion related to astrophysics with "In our frame of reference..." I think it's safe to assume they're talking about now(1).
from an epistemological standpoint.
Okay, now you're just making up words! ;)
systemd is Roko's Basilisk.
I've always wondered about this one and it just hit me reading this particular story.
During a supernova explosion can light be propelled to travel faster than it's usual speed of 299,792,458 m/s?
You are needlesly introducing multiple reference frames. Here there's just on that is relevant: ours. If event happened 12 million light years away, it also by definition of speed of light happened 12 million years ago, when we see the light. When observing photons in vacuum, distance and time are same thing. "Now" would mean "here", and I don't feel the heat, so inverse square law must be in effect, which implies distance, which implies time.
"is *only* active on large scales"
Is only *significant* on large scales, and in particular in the weak-field limit, no?
Or do you think that a parsimonious everywhere-the-same negative pressure or everywhere-inertial models are dead for DE already?
This sounds like a video game perk. "Visible Supernova - Temporary 5% increase in research "
X
Just in time for the dupe.
"When you have eliminated the unacceptable, whatever is left, however improbable, must be the truthiness" - Holmes
On the other hand, we now know the complete value of Pi to enough digits that we can encircle the entire Universe and be accurate to the NANOSECOND. How accurate is enough?
You probably mean nanometre here. Nevertheless, I remember an Asimov essay saying that given the radius and even just a few dozen digits of pi, we could calculate the circumference of the universe to a small fraction of an electron. I think he said 70 digits would do.
"Is only *significant* on large scales, and in particular in the weak-field limit, no?"
Actually no, in the hard interpretation of what I'm saying -- that the acceleration predicted in cosmology is a result of assuming a negative pressure in a Robertson-Walker metric... but the universe is composed of a conglomeration of untold billions of different metrics -- there is no weak-field limit. In the softer interpretation where structure forms and "disconnects" in some way or another from the universal expansion -- basically, is modelled with something like an LTB patched onto a Robertson-Walker -- the acceleration still has little or no impact, although this depends on the exact model for acceleration, of course.
Something that was in vogue for a good few years was the idea that local structure could account for the observed dark energy, without the need for any negative pressure at all, by whacking Earth somewhere near the middle of a void around a gigaparsec across. This doens't seem particularly plausible anymore, although that's not least because the models studied so far have been staggeringly over-simplified, not because people are resistant (though some are) but becuase the problem rapidly grows impossibly difficult. Literally impossibly.
"do you think that a parsimonious everywhere-the-same negative pressure ... [is] dead for DE already"
I'm certainly not going to state that this is dead. The simplest model is a cosmological constant and there's no reason to state that there isn't a cosmological constant, and some good reasons to say that something that acts as a constant exists. (If nothing else, the low-energy limit of many generalised theories of gravity manifest in the action as basically the first few terms in a Taylor series for the Ricci scalar -- so where general relativity has R, genearlised theories can be written as C + R + (1/2)*c*R^2 + ... where C and c are constants, and those three dots hide a world of unhappiness. Interestingly, C here would act as a cosmological constant, and the R^2 term acts exactly as inflation and, in fact, is both the earliest studied inflationary model (Starobinsky in the late 1970s, a good few years before Guth, albeit with very different motivation) and is slap in the middle of the allowed parameter space. No other simple model is in as good agreement (though it must be pointed out that other simple models are within one sigma, so this isn't, strictly speaking, necessarily at all important.)
The point then is that the low-energy limit of a generic gravity has a cosmological constant, and therefore acts to accelerate the expansion of the universe. More significantly, this is a *screened* constant, meaning that there may well be a fundamental constant in nature, which we can call L, and a constant coming out of an approximate description of a better model of gravity, which we can call, I don't know, A. The observed constant in the action is C = A - L. (That negative sign arises for entirely tedious and unimportant reasons.) So even the naturalness problem, which queries why the observed constant is so small is, if not removed, at least bounced elsewhere.
What's even more, the low-energy limit of a generic gravity also gives us a theory of inflation that fits the data perfectly. *And* it gives us normal gravity. And it does all of this without any scalar fields.
Brilliant, right? Well, half-convincing at least, but there are the usual caveats and problems, not least that this is very definitely a phenomenological description - we don't have the theory this is meant to emerge from. Bummer.
Anyway, this digression was tho point out that there's no way I can say that an everywhere-the-same negative pressure doesn't exist... because I think it *does* exist, in the form of a screened cosmological constant. There may also be a negative pressure that is almost the same everywhere coming from some scalar field, probably an effective field rather than a genuinely physical field.
The presence of
One of the interesting notions that comes out of relativity is that light has no time. What you see happened at the moment the light touched your retina. It also happened at the moment it touched the camera pixel in the Hubble 100 miles above you and at the time it touched the retina of the guy 5 countries over. It also happened 12 million years ago by framing the question in terms of how long it will take to send a message back. Light is constant, the rest of spacetime can go bend itself.
Thank you. That was an unexpectedly deep answer.
Floundering around continuing to wonder what mechanisms generate the metric is pretty much the job description of a physical cosmologist, isn't it ? :-) I think that beats floundering around wondering how seventeen or twenty-four or whatever free parameters got fixed at such convenient values and whether that persists for a further hundred or so more...
"Floundering around continuing to wonder what mechanisms generate the metric is pretty much the job description of a physical cosmologist, isn't it ?"
Certainly one part of the job description, yes :) Most of them would disagree with almost everything I say about how to approach it, of course, which is what makes the whole thing great. In about ten years when all the data is in we'll be drowning in too much of it and I more or less expect more funding in cosmology to be channelled into more genuine theorists along with the statisticians and data miners we'll be needing. And this is something I'm very much looking forward to.
Wouldn't it be easier to work "bottom-up" trying to glue together Kerr or Fermat or whatever metrics with different timelike-separated sources, building up towards something like FLRW by way of some negligibility criterion for the more clearly localized sources as one glues more and more of them together?
It would probably be heuristic at best initially, and likely would raise questions like, "to what extent does it matter which solution one applied to each of the component (set of) objects as one glues them together?".
Or would you always prefer "top-down" and think about decomposing, say, a metric focused on a star into some set of metrics focused on ever-smaller-scale structures? Would you aim for exact solutions at each step from larger scale to smaller scale?
Should choice of metric be kept maximally free at each step?
"Limit-hopping" is likely to become tractable computationally in due course much in the same way that real work with 3+1 has become feasible, so I don't reject the ideas in your initial long message out of hand. If work along those lines were to identify specific problems with dust solutions generically, I think that would be important.
Definitely the bottom-up approach. It's almost intractible, but it's still easier than attempting to decompose a metric into constituents. A major issue is that general relativity is not linear, so if you've got two Schwarzschild metrics (which describe a spherical mass alone in the universe, edging towards flat spacetime inifinitely far from the mass) you can't simply add them together and get a new metric describing two spherical masses. In the weak-field regimes this will work OK but it's still only an approximation. Away from the weak field this simply doesn't work. While this does make it harder to attempt to connect up smaller-scale metrics to get to a larger one, it makes it basically impossible to go the other way.
So what we can have are models such as Swiss-cheese models, where the universe is built from LTB (or, increasingly, Szekeres) metrics each stitched onto a region of Robertson-Walker. Two sizes of LTB model spherical local voids and spherical local clusters, while if you use Szekeres you're modelling quasi-spherical voids and clusters, at the cost of computational complexity. Or you can use that complexity of a Szekeres, or related metrics, and build more realistic models of local structure than the LTB allows.
An alternative approach (as in papers by Clifton, Rosquist and Tavakol, http://uk.arxiv.org/abs/1203.6... and http://uk.arxiv.org/abs/1309.2..., or recently by Korzyski, http://arxiv.org/abs/1312.0494) is to literally take a universe composed of an arbitrary number of black holes. Surprisingly, you can solve this system exactly. It's not a viable model of the acual evolution, but it is a very good testbed for whether the type of effect I'm talking about is actually significant. The answers appear to be "A universe composed of a large number of black holes looks like Robertson-Walker" but "It is not clear that the dynamics are Robertson-Walker".
"Should choice of metric be kept maximally free at each step?"
In principle, yes, in practice, no. Without constraints all you'd end up with are the Einstein equations split into different scales, with no simplifications. That's not a realistic approach, much as it would be lovely.
Computationally we're definitely going to head towards numerical cosmology as a subset of numerical relativity. One of the major issues is dynamical range; theoretically, and using current perturbative techniques, we can cover a range of momenta from k->0 (edging towards infinite wavelength) all the way to around k~1 Mpc^{-1}, which is cluster scale. That can then be stitched onto n-body codes which can now map down to kiloparsec scales with a significant overlap with perturbation theory. Attempting to solve the whole problem you have to somehow cover at least that same five or six orders of magnitude, throwing at each step the full weight of GR, which is extremely costly. But it will definitely come, and sooner rather than later.
"Limit shifting" in principle, wherein one either tries to arrive at a new metric (perhaps with non-exact solutions) in a bottom-up style by composing two metrics sourced by two timelike-separated material objects is perfectly valid, whether those metrics are Kerr or Fermat or whatever. However, it should also work the other way around, where one starts with a dust solution and breaks that down into metrics sourced by smaller and smaller scale material objects, that is, top-down. The choice of "top" and "bottom" is of course arbitrary. :-)
Given the advances in numerical relativity and computational power, that sort of project should be tractable in due course. An interesting question is the extent to which artifacts are introduced when one composes or decomposes metrics, along the lines of issues that arise when one does a foliation of the manifold.
"while I'm certainly right that this is a fundamental issue and that cosmology is in principle totally wrong"
Totally wrong, or only unequipped with the investigative tools -- both observational and theoretical -- to make sense of the early boundary conditions? Or do you mean something as strong as being unable to predict the evolution of some arbitrary lower-dimensional hypersurface matching a real observation? Or alternatively, that predictions work but nevertheless we are liable to discover that the manifold is not smooth or there exists some field or fields that take real values at each point (or both)?
"Or do you mean something as strong as being unable to predict the evolution of some arbitrary lower-dimensional hypersurface matching a real observation?"
This may be the case. If nothing else, in cosmology we know we are not in a globally hyperbolic spacetime: the universe is riddled with geodesic crossing. That in itself makes a 3+1 split and any evolution based on it problematic. Cosmology is inherently built on a 3+1 split of some form, whether that's with respect to an observer comoving with the CMB or with respect to some timelike vector later associated with conformal time, or whatever. We also have, in principle, the issue that we don't have a good Cauchy surface to put initial date on. In reality these issues may or may not be all that significant. Certainly on the scales we're typically considering I'm not very concerned. I've spent most of my career in perturbation theory and I set my initial data on a constant redshift surface at about z=100,000 and integrate up to roughly megaparsec scales, and I don't really expect that we're going to hit horrible issues on such scales.
Except in principle, which might or might not be a point that interests you.
"Or alternatively, that predictions work but nevertheless we are liable to discover that the manifold is not smooth or there exists some field or fields that take real values at each point (or both)?"
We already know the manifold is not smooth; that part is without issue. The existence of a single black hole breaks any assumptions of smoothness. More importantly, the presence of any structure at all breaks it. What we don't know is whether the manifold can be assumed to be smooth for cosmological purposes. This is at the heart of the issues I'm talking about -- we derive cosmology assuming first that we can foliate spacetime with maximally-symmetric 3-surfaces (which it seems may very well be valid, although this is yet to be proven given we lack an averaging procedure that can be applied to tensors in a covariant manner), and then more damningly by assuming that the dynamics we recover by perturbing the resulting Einstein equations are equivalent to those that describe a perturbed matter distribution. This is categorically not the case. The Einstein tensor is non-linear, so it can't be the case.
Of course, the correction may be miniscule. Using spatial averaging and a mixture of different toy models has certainly suggested that the error is small, on the order of 10^{-5}. (This has its drawbacks: it's an unphysical procedure. Spatial averaging involves averaging over spacelike surfaces, which are obviously separated from an event by spacelike geodesics and therefore unobservable. If one takes the further step -- as some have -- and feeds the results of this averaging back into the evolution then one is breaking causality horrifically.)
In practical terms, I'm talking entirely on principle. In principle, Robertson-Walker cosmology is ill-defined until, and only until, we have clearly specified a consistent, coherent way to map from local solutions -- of the order of kiloparsecs if not parsecs -- up to cosmological scales. What emerges from such a procedure may or may not even behave like general relativity. We simply don't know. It seems unlikely that the result of a sane averaging *will* behave just like GR and the best approaches we have to it -- such as Zalaletdinov's averaging -- certainly introduce extra terms that crop up in the "Friedmann equations". (These typically appear as a "curvature" of spacetime.) There is no physical basis to them and they cannot be interpreted as though they were in a genuine Friedmann equation, for the simple fact that they're not in a Friedmann equation because the universe is not Robertson-Walker.
In practice, linear perturbation theory around a flat Robertson-Walker spacetime has been so successful that I fully expect it to continue being so. While -- again, in principle -- the parameters of the model are entirely phenomenological and it is dangerous to rush to ascribe physical meani
it doesn't matter that it's a ship, you could ask the same question about a baseball thrown at you. there is no "reconcile" point but a continuum, the object is seen to leave one place, travels and to arrive.