You misunderstand me. It's not a pile of semantics: the loop quantum cosmology framework has a very concrete picture of what the Big Bang is, and how "time" proceeds through it. My remarks concern the mathematical consistency and predictibility of the theory, which has nothing to do with semantics.
Determinism does not imply subjectivity of time, and non-determinism does not imply objectivity of time. It may well be true, however, that reconciling GR and QM will involve a better understanding of time, since spacetime is the heart of GR, and we do not have a full quantum theory of spacetime.
There are plenty of theories out there; brane collisions (the cyclic/ekpyrotic scenario) aren't even the only string/M-theory proposal (there is also, for instance, the so-called "pre Big Bang" model). The theory discussed here is formulated within the context of a competitor to M theory, loop quantum gravity. No one knows whether any of these explanations are right; no one knows whether the theories underlying them are right either.
This result is interesting within the context of loop quantum gravity, because it offers an approximation within which the Big Bang can be modeled directly. However, it's worth not losing sight of the fact that the LQG theory upon which it is based has serious issues with consistency. It is based on a non-standard quantization technique with no experimentally supported basis, its Hamiltonian constraint has never been solved (which renders any approximation based on that constraint suspect), and it suffers from potentially infinitely many quantization ambiguities (again, with no known and maybe no possible experimental method for singling out the correct quantization. Some of these concerns are summarized here. (Yes, it's written by string theorists, and yes, string theory has its own set of problems with experimentally selecting the "correct" solution. But the correctness of string theory aside, the objections raised in that article against LQG are valid.) It's very premature to suggest that LQG's picture of the Big Bang may be correct when the fundamental theory itself has serious unresolved problems.
I've always held that asking what came before the Big Bang is like asking what is North of the North Pole? That analogy was put forth by Hawking in A Brief History of Time. People use it as an example of how a singularity is a "boundary" to spacetime: it's not smoothly connected to any prior spacetime.
However, most of those people also miss the fact that Hawking meant it quite literally. He wasn't actually speaking of the usual Big Bang singularity. Rather, he was speaking of his No-Boundary Proposal in the (now disfavored) Euclidean quantum gravity theory. In the NBP, the Big Bang singularity in 3+1 dimensional spacetime is replaced by a (hyper)spherical surface in 4D Euclidean space (in imaginary time). In imaginary time, there is no Big Bang singularity, spacetime is just a smooth spherical surface.
Bojowald's original "bounce" solution merely has a single crunch which leads to a single bang. It also ignored the cosmological constant, which is what leads to the eternally accelerating expansion (dark energy) now favored. This does not by itself mean that loop quantum cosmology is incompatible with observation. It is possible (although maybe odd) that the universe could expand forever after the Big Crunch of a single progenitor universe. However, more importantly, the simple and highly symmetric LQG solutions so far considered are much more idealized than the actual universe, so it's quite probable that no truly realistic LQG solution has yet been written down. It's just a first step, to be able to write down any quantum gravity solution capable of describing the Big Bang.
how about, that even IF climate change is man made (that's a big IF) It's not really that big of an "if" anymore. The big "if" is how much of it will occur in the future.
how about, that even IF climate change is man made (that's a big IF) there is NO CREDIBLE way to link someone dieing in a storm to exxon. No one is linking any particular storm to climate change. You can, however, link changes in storm, crop growth, precipitation, etc. trends to climate change, and you can link storms, crop growth, and precipitation statistics to deaths.
I haven't done the research, but 150,000 climate change related deaths a year just triggers my bullshit sensor. I don't know why. It only amounts to ~0.25% of the deaths in the world. I don't think it's so incredible to believe that 1 out of 4000 deaths might be due to climate change, particularly given the fraction of the world population in the third world, far more susceptible than we are to drought, tropical disease, crop failure, etc.
While modern transportation technology saves lives (and also takes them), your logic doesn't really add up. It's possible to do all the things we currently do with oil but still use less of it, by improving the efficiency with which we use it. This isn't a "either we use up oil at a prodigious rate or we use no oil" false dilemma.
SUVs and light trucks in the U.S. (by which I mean, not 18-wheelers and such) do not contribute the majority of CO2 emissions in the world. The fetish for SUVs and gigantic pickups is mostly a U.S. phenomenon. But the U.S. contributes roughly 25% of the world's CO2 emissions (IIRC), passenger cars and trucks contribute maybe 20% of that, and SUVs and giant pickups contribute some even smaller fraction than that.
Getting rid of those kinds of vehicles would help, but it's ridiculous to attribute all putative climate change deaths to them, when they can be responsible for no more than a few percent of total emissions at best. And that's not even taking into account that they are a relatively recent phenomenon, and most of the manmade CO2 now in the atmosphere was put there before SUVs became popular. A small fraction of recent emissions is an even smaller fraction of total atmospheric concentration.
Your complaint about excessive air conditioning is more on point. Most of our CO2 emissions come from fossil fuel burning power plants (mostly coal plants, don't blame oil for everything). One of the easiest ways to reduce emissions is to reduce electricity consumption.
Except that the idea that 150K deaths are being caused by so-called "climate-shift" (I notice they don't like to call it global warming after all the SNOW storms last winter) is a bunch of crap. Climate shift (or climate change) is the more accurate term, and scientists have been using it for longer than this year. Snow storms are just one example.
"Climate change" is the better term, because warming is not the only climate change taking place. Indeed, there are regions which are predicted to get more snow storms in the future, because precipitation increases. Note that "warmer" doesn't automatically imply "less likely to snow", as it doesn't snow as much when it's too cold, either — the air can't hold as much moisture. (This is not the only way that precipitation patterns can change; regional weather patterns can shift too.) Warming can turn snow days into too-warm non-snow days, but it can also turn too-cold non-snow days into snow days. Which wins out depends a lot on where you are. (For this reason, global warming can lead to both increased droughts and floods — as well as decreased in different areas — depending on where you are.)
It is a complex subject. It is invalid to take a specific series of events occurring in one region in one year as evidence for or against climate change; you have to look at global trends over decadal time scales to see whether temperature, precipitation, or other changes are really taking place.
No...what's causing rates of malaria to rise is the banning of the use of chemicals that WORKED when it came to killing mosquitoes. I see you've bought the latest conservativetalkingpoint. Such chemicals were not banned in the countries with rising malaria problems. And in fact, overuse of DDT in countries such as Sri Lanka bred DDT-resistant mosquitoes. Chemicals are still an effective means of combating malaria, and they are most certainly still in use. But they must be used in moderation; when used excessively, there is a great short-term benefit but it's a long-term disaster. We're seeing the same sort of problem here with an overuse of antibiotics.
Malnutrition and diarrhea has more to do with poor drinking water and oppressive governments. That's almost certainly true. 150,000 deaths, for instance, is much smaller than the total number of deaths due to malnutrition and diarrhea. That doesn't mean that climate change can't produce 150,000 additional deaths, however — the idea is not "a bunch of crap". But it is good to put it in perspective, as you did.
The idea that every hurricane, flood or snowstorm is a sign of climate change is bad science. That's true. (Of course, you bring up this idea yourself.) But the story here makes no such claim. Why do you bring it up? It doesn't support any of your other points.
Your response isn't accurate. In fact, possibly most of the predicted global warming comes from water vapor. The carbon dioxide itself produces some warming, but the increase in water vapor as a result of that warming (i.e., more evaporation) increases the total warming by 2 or even 3x the CO2 warming alone. (There are other feedbacks too, some positive and some negative.)
You may be confusing water vapor with arguments that CO2 itself has saturated the adsorption spectrum. (This isn't true either, but is used by some to argue against any further CO2-based warming.)
It's important to realize, though, that the water vapor in the atmosphere doesn't go up by itself: if you try to put more water into the atmosphere at fixed temperature, it just precipitates back out. In order for the atmosphere to hold more water vapor, it has to get warmer due to some other reason. CO2 is a major reason. So your ultimate conclusion is sort of right, but for the wrong reason: water vapor is saturated in the atmosphere, and can't produce additional warming on its own. (But if something else causes warming, then water vapor can go up and add to that warming.)
You've missed half my argument, which is the part about getting it from space being cheaper than on Earth. Even assuming you have a space elevator to get into/out of LEO, mining asteroids is NOT CHEAP. (It's even less cheap once you factor in the cost of the space elevator, although that can be amortized over time and across projects.) Some metals are expensive, but I just don't see getting them from the asteroid belt being less expensive.
Space-based energy is a much better justification for getting to LEO cheaply. Though I think there would end up being other objections to beaming energy from space.
Anyway, I find much of the discussion moot; I don't think a space elevator is going to be technologically feasible within a timeframe useful for, say, replacing our energy dependence on fossil fuels. And seriously, I think replacing copper wires by carbon nanotubes is more likely to happen before anybody builds a practical space elevator out of them and then mines asteroids for copper with it.
By the way, it's kind of dishonest to compare asteroid mining, which has not provably passed a cost-benefit analysis, with, say, historical trade with India, which did. Or rather, it's simply begging the question.
We believe that the best solution to there not being enough resources on Earth for everyone is to go get resources off Earth. What resources off Earth are there not enough of on Earth? Serious question. Do you really think that lack of metal is going to be a serious constraint on human civilization? What resources does space have that we desperately need, and are cheaper to get from space (even with a space elevator) than from Earth?
That's a pretty juvenile vendetta there, mod. Did you really blow all your mod points in a pointless exercise to downgrade as many of my posts as possible? Did you really think anyone is going to not read those posts because of it? Or that my karma is going to be significantly affected?
Of course, the universe doesn't need to be expanding at the speed of light, or even at a constant speed, for there to be a functional relationship between the age of the universe and the diameter. Of course. Expansion at the speed of light is only necessary for the "radius" (in lightyears) to equal the age (in years). But in general, there is always some (nonlinear) relationship between size and age.
What about Hubble's Law? Hubble's law is an approximation. For an exact solution you need to look at the Friedmann equation.
If space is finite, you can define its size, which can be interpreted as a "diameter" if it is (hyper)spherical.
However, this size is not given by the age of the universe — that would only hold if the universe expanded at a constant speed equal to the speed of light, giving a linear relationship between size and age, which is not the case.
I don't know what it means to talk about space in the absence of matter, I can't see how it can even be defined. General relativity has no problem defining space in the absence of matter, and there are plenty of vacuum solutions of the Einstein field equation (Minkowski spacetime, the Schwarzschild solution, gravitational geons, a universe filled with gravitational waves, etc.). I don't know why the definition of space should have anything to do with matter; you can certainly define a geometric relationship between different points in vacuum.
For sure the big bang has broken the symmetry of space (if there was such a thing as space independent of the big bang), so I really don't understand your response. Of course space is not perfectly symmetric (there are stars and planets scattered everywhere), but on large scales, it is very close to homogeneous and isotropic.
You refer to 'space' as distinct from 'spacetime' ? There are infinitely many ways of slicing (foliating) "spacetime" into "space" and "time". Cosmologically, there is a "special" way of doing so, which is to choose the slicing that leaves space homogeneous and isotropic (the aforementioned CMBR-isotropic frame). The laws of physics themselves do not prefer such a slicing; it is a particular property of the homogeneous/isotropic Big Bang spacetimes.
Are you referring to some kind of broken Lorentz invariance, where space is translationally invariant but spacetime is not? Yes. Every curved spacetime breaks Lorentz invariance, which only holds exactly in Minkowski spacetime. (All spacetimes have local Lorentz invariance though, which is a fancy way of saying that spacetime looks flat if you look closely enough — ignoring quantum effects, of course.)
I don't understand how that can be, it would imply that the universe is evolving in the same way at all locations in space, which then implies (as far as I can tell, in my slightly drunk state) that there would be a preferred reference frame in which this takes place (otherwise the distinction between space vs spacetime could not be made). Yes, that's correct. The Friedmann-LeMaitre-Walker-Robinson (FLRW) spacetimes are homogeneous and isotropic, meaning that they have spatial symmetry that singles out a unique way of slicing spacetime into space-and-time. In that frame, the universe evolves the same way at all locations.
As I said, this is not exactly true: some locations have galaxies and some don't. But on the largest scales, you can treat the universe as a uniform noninteracting "gas" or "dust", with approximately the same density everywhere, expanding everywhere at the same rate. (These FLRW models neglect fancier inflation type theories in which specific patches of space inflate and others don't, as well as other inhomogeneous and anisotropic cosmologies. The observable universe, however, appears to be well approximated by the FLRW models.)
Also, as I mentioned before, the existence of such a frame does not make it a "preferred" frame in the sense of violating the relativity principle: the laws of physics themselves do not single out such a frame, and there are plenty of solutions of the Einstein field equation which do not have such a frame. Only these very symmetric solutions do (and, of course, our universe does not exactly possess this symmetry anyway; it's just an approximation, albeit a good one).
The infinite plane though, is not so clear. Ask a mathematician. An infinite plane has no center. It is translationally invariant: every point is like every other.
Is the universe really infinite, as opposed to finite but possibly curved? Inflation implies that it's either infinite and flat, or finite but so large that its curvature is unnoticeable. Observationally, it's very close to flat, but we can't prove it really is infinite and flat.
For another possible geometry, a Euclidean universe could certainly have a well-defined center of mass. Not an infinite and homogeneous Euclidean universe (that's the same as a plane). Newton wrote about that.
Are you just nitpicking? No.
If momentum has been conserved since the big bang, then there is surely a center of mass frame, and also a center of mass. Where do you think the center of mass of the universe is located? Consider the most popular geometries, planar and spherical. Where is the center of an infinite plane? Where is the center of a spherical surface? (Note: this is not "where is the center of a sphere", but rather, "which point on the surface of a sphere is its center"?) Both are ill defined: there is no unique point which is the center of a plane or a spherical surface.
The cosmic background isotropic frame is something analogous to a "center of momentum" frame, although it is not a "center of mass" frame. (Nitpicking, momentum is not globally conserved in general relativity, either, nor is energy; see here, noting the discussion of the unification of energy and momentum. Nevertheless, you can pick a particular frame in which there is spatial translation invariance — the cosmic background isotropic frame — which is what gives rise to momentum conservation in flat spacetime. In that sense, you can consider momentum to be conserved in that frame.)
There is no "center of the universe" or "center of mass of the universe", but there is still a "special" reference frame: the one in which the universe is isotropic (ignoring small anisotropies): it's the one in which the cosmic background radiation would not be blue/redshifted in opposite directions.
That being said, general relativity is largely irrelevant on non-cosmological scales for these purposes.
You think 50ppm is enough to doom the planet? It's not enough to "doom the planet", but it is enough to produce the warming which has been observed, and there will be more warming still, at a comparatively rapid rate.
Even with that change, CO2 only comprises 0.035% of the atmosphere. Compare this to the atmosphere of Venus, where CO2 comprises 96.5% of the atmosphere. Yes, the Earth has a much smaller greenhouse effect than Venus. Global warming will only amount to a few degrees of temperature change, but that doesn't mean that its effects are unimportant.
That's a monumental difference between the two planets, and shows just how much CO2 you need to have before things start warming up What are you talking about? The Earth has already warmed up due to the greenhouse effect even before humans started adding to it. It's responsible for about 50 F degrees of warming, in fact. Are you really denying the existence of the greenhouse effect on Earth?
And considering that the vast majority of the biomass of this planet breathes in CO2 and exhales O2, I would think any real increase in CO2 would lead first to more trees and greenery growing up before the temperature going up. That's the CO2 fertilization effect. More trees and greenery will grow and temperature will also go up; it's not a "one before the other" thing. While more vegetation leads to a larger terrestrial sink of CO2, it's still not enough to sink all of our emissions; CO2 concentrations (and temperatures) still go up, and there are other effects (such as higher temperatures increasing the rate of CO2 emitted by organic decay in soil).
Slashdot Mirror
You misunderstand me. It's not a pile of semantics: the loop quantum cosmology framework has a very concrete picture of what the Big Bang is, and how "time" proceeds through it. My remarks concern the mathematical consistency and predictibility of the theory, which has nothing to do with semantics.
Don't forget Bob the Angry Flower on loop quantum gravity.
Determinism does not imply subjectivity of time, and non-determinism does not imply objectivity of time. It may well be true, however, that reconciling GR and QM will involve a better understanding of time, since spacetime is the heart of GR, and we do not have a full quantum theory of spacetime.
There are plenty of theories out there; brane collisions (the cyclic/ekpyrotic scenario) aren't even the only string/M-theory proposal (there is also, for instance, the so-called "pre Big Bang" model). The theory discussed here is formulated within the context of a competitor to M theory, loop quantum gravity. No one knows whether any of these explanations are right; no one knows whether the theories underlying them are right either.
This result is interesting within the context of loop quantum gravity, because it offers an approximation within which the Big Bang can be modeled directly. However, it's worth not losing sight of the fact that the LQG theory upon which it is based has serious issues with consistency. It is based on a non-standard quantization technique with no experimentally supported basis, its Hamiltonian constraint has never been solved (which renders any approximation based on that constraint suspect), and it suffers from potentially infinitely many quantization ambiguities (again, with no known and maybe no possible experimental method for singling out the correct quantization. Some of these concerns are summarized here. (Yes, it's written by string theorists, and yes, string theory has its own set of problems with experimentally selecting the "correct" solution. But the correctness of string theory aside, the objections raised in that article against LQG are valid.) It's very premature to suggest that LQG's picture of the Big Bang may be correct when the fundamental theory itself has serious unresolved problems.
However, most of those people also miss the fact that Hawking meant it quite literally. He wasn't actually speaking of the usual Big Bang singularity. Rather, he was speaking of his No-Boundary Proposal in the (now disfavored) Euclidean quantum gravity theory. In the NBP, the Big Bang singularity in 3+1 dimensional spacetime is replaced by a (hyper)spherical surface in 4D Euclidean space (in imaginary time). In imaginary time, there is no Big Bang singularity, spacetime is just a smooth spherical surface.
Bojowald's original "bounce" solution merely has a single crunch which leads to a single bang. It also ignored the cosmological constant, which is what leads to the eternally accelerating expansion (dark energy) now favored. This does not by itself mean that loop quantum cosmology is incompatible with observation. It is possible (although maybe odd) that the universe could expand forever after the Big Crunch of a single progenitor universe. However, more importantly, the simple and highly symmetric LQG solutions so far considered are much more idealized than the actual universe, so it's quite probable that no truly realistic LQG solution has yet been written down. It's just a first step, to be able to write down any quantum gravity solution capable of describing the Big Bang.
Oops, 0.25% is 1 in 400, not 1 in 4000.
While modern transportation technology saves lives (and also takes them), your logic doesn't really add up. It's possible to do all the things we currently do with oil but still use less of it, by improving the efficiency with which we use it. This isn't a "either we use up oil at a prodigious rate or we use no oil" false dilemma.
SUVs and light trucks in the U.S. (by which I mean, not 18-wheelers and such) do not contribute the majority of CO2 emissions in the world. The fetish for SUVs and gigantic pickups is mostly a U.S. phenomenon. But the U.S. contributes roughly 25% of the world's CO2 emissions (IIRC), passenger cars and trucks contribute maybe 20% of that, and SUVs and giant pickups contribute some even smaller fraction than that.
Getting rid of those kinds of vehicles would help, but it's ridiculous to attribute all putative climate change deaths to them, when they can be responsible for no more than a few percent of total emissions at best. And that's not even taking into account that they are a relatively recent phenomenon, and most of the manmade CO2 now in the atmosphere was put there before SUVs became popular. A small fraction of recent emissions is an even smaller fraction of total atmospheric concentration.
Your complaint about excessive air conditioning is more on point. Most of our CO2 emissions come from fossil fuel burning power plants (mostly coal plants, don't blame oil for everything). One of the easiest ways to reduce emissions is to reduce electricity consumption.
"Climate change" is the better term, because warming is not the only climate change taking place. Indeed, there are regions which are predicted to get more snow storms in the future, because precipitation increases. Note that "warmer" doesn't automatically imply "less likely to snow", as it doesn't snow as much when it's too cold, either — the air can't hold as much moisture. (This is not the only way that precipitation patterns can change; regional weather patterns can shift too.) Warming can turn snow days into too-warm non-snow days, but it can also turn too-cold non-snow days into snow days. Which wins out depends a lot on where you are. (For this reason, global warming can lead to both increased droughts and floods — as well as decreased in different areas — depending on where you are.)
It is a complex subject. It is invalid to take a specific series of events occurring in one region in one year as evidence for or against climate change; you have to look at global trends over decadal time scales to see whether temperature, precipitation, or other changes are really taking place. No...what's causing rates of malaria to rise is the banning of the use of chemicals that WORKED when it came to killing mosquitoes. I see you've bought the latest conservative talking point. Such chemicals were not banned in the countries with rising malaria problems. And in fact, overuse of DDT in countries such as Sri Lanka bred DDT-resistant mosquitoes. Chemicals are still an effective means of combating malaria, and they are most certainly still in use. But they must be used in moderation; when used excessively, there is a great short-term benefit but it's a long-term disaster. We're seeing the same sort of problem here with an overuse of antibiotics. Malnutrition and diarrhea has more to do with poor drinking water and oppressive governments. That's almost certainly true. 150,000 deaths, for instance, is much smaller than the total number of deaths due to malnutrition and diarrhea. That doesn't mean that climate change can't produce 150,000 additional deaths, however — the idea is not "a bunch of crap". But it is good to put it in perspective, as you did. The idea that every hurricane, flood or snowstorm is a sign of climate change is bad science. That's true. (Of course, you bring up this idea yourself.) But the story here makes no such claim. Why do you bring it up? It doesn't support any of your other points.
Your response isn't accurate. In fact, possibly most of the predicted global warming comes from water vapor. The carbon dioxide itself produces some warming, but the increase in water vapor as a result of that warming (i.e., more evaporation) increases the total warming by 2 or even 3x the CO2 warming alone. (There are other feedbacks too, some positive and some negative.)
You may be confusing water vapor with arguments that CO2 itself has saturated the adsorption spectrum. (This isn't true either, but is used by some to argue against any further CO2-based warming.)
It's important to realize, though, that the water vapor in the atmosphere doesn't go up by itself: if you try to put more water into the atmosphere at fixed temperature, it just precipitates back out. In order for the atmosphere to hold more water vapor, it has to get warmer due to some other reason. CO2 is a major reason. So your ultimate conclusion is sort of right, but for the wrong reason: water vapor is saturated in the atmosphere, and can't produce additional warming on its own. (But if something else causes warming, then water vapor can go up and add to that warming.)
You've missed half my argument, which is the part about getting it from space being cheaper than on Earth. Even assuming you have a space elevator to get into/out of LEO, mining asteroids is NOT CHEAP. (It's even less cheap once you factor in the cost of the space elevator, although that can be amortized over time and across projects.) Some metals are expensive, but I just don't see getting them from the asteroid belt being less expensive.
Space-based energy is a much better justification for getting to LEO cheaply. Though I think there would end up being other objections to beaming energy from space.
Anyway, I find much of the discussion moot; I don't think a space elevator is going to be technologically feasible within a timeframe useful for, say, replacing our energy dependence on fossil fuels. And seriously, I think replacing copper wires by carbon nanotubes is more likely to happen before anybody builds a practical space elevator out of them and then mines asteroids for copper with it.
By the way, it's kind of dishonest to compare asteroid mining, which has not provably passed a cost-benefit analysis, with, say, historical trade with India, which did. Or rather, it's simply begging the question.
What would you do with a billion tons of nickel?
That's a pretty juvenile vendetta there, mod. Did you really blow all your mod points in a pointless exercise to downgrade as many of my posts as possible? Did you really think anyone is going to not read those posts because of it? Or that my karma is going to be significantly affected?
If space is finite, you can define its size, which can be interpreted as a "diameter" if it is (hyper)spherical.
However, this size is not given by the age of the universe — that would only hold if the universe expanded at a constant speed equal to the speed of light, giving a linear relationship between size and age, which is not the case.
As I said, this is not exactly true: some locations have galaxies and some don't. But on the largest scales, you can treat the universe as a uniform noninteracting "gas" or "dust", with approximately the same density everywhere, expanding everywhere at the same rate. (These FLRW models neglect fancier inflation type theories in which specific patches of space inflate and others don't, as well as other inhomogeneous and anisotropic cosmologies. The observable universe, however, appears to be well approximated by the FLRW models.)
Also, as I mentioned before, the existence of such a frame does not make it a "preferred" frame in the sense of violating the relativity principle: the laws of physics themselves do not single out such a frame, and there are plenty of solutions of the Einstein field equation which do not have such a frame. Only these very symmetric solutions do (and, of course, our universe does not exactly possess this symmetry anyway; it's just an approximation, albeit a good one).
The Big Bang implies that spacetime can't be infinite and translationally invariant. Space, however, can be.
The cosmic background isotropic frame is something analogous to a "center of momentum" frame, although it is not a "center of mass" frame. (Nitpicking, momentum is not globally conserved in general relativity, either, nor is energy; see here, noting the discussion of the unification of energy and momentum. Nevertheless, you can pick a particular frame in which there is spatial translation invariance — the cosmic background isotropic frame — which is what gives rise to momentum conservation in flat spacetime. In that sense, you can consider momentum to be conserved in that frame.)
There is no "center of the universe" or "center of mass of the universe", but there is still a "special" reference frame: the one in which the universe is isotropic (ignoring small anisotropies): it's the one in which the cosmic background radiation would not be blue/redshifted in opposite directions.
That being said, general relativity is largely irrelevant on non-cosmological scales for these purposes.