The "observable universe" is everything we can see right now. The key point is, depending on how the universe expands, something that is now part of the observable universe may not have been so in the past, and may not be so in the future. (Similarly, something that was not once part of the observable universe could be so today.) What is "observable" depends on whether light can reach us at a given time, and that depends on what the geometry of space itself does. There is a nice paper which discusses various misconceptions about "the observable universe", particularly as it pertains to FTL expansion, although it's aimed at a physicist audience. I don't think it addresses this particular point, but it may still be worth reading.
The point is that van Flandern thinks he can prove the speed of gravity isn't c in general relativity, using pure mathematics. He's not making a statement about the universe per se, he's making a statement about the mathematics of the theory. His derivation is just wrong, and you can find correct mathematical derivations anywhere.
Furthermore, independent of van Flandern's mathematical errors, the experimental evidence is also that the speed of gravity is c, as I also mentioned in my post.
Maybe you read the actual arguments instead of making trite platitudes.
I think you are still wrong. Inflation happened very early on in the universe - between 1x10-36 seconds and 1x10-32 seconds, which is a very, very short time span. There were no galaxies present to influence at that time. No atoms of any sort as well.
Yes, that's what the authors here are proposing. Differential inflation introduced relative velocities between the particles in the very early universe, which later in turn seeded the primordial density perturbations that formed galaxies. Go read their paper. They state that their theory is about the influence of pre-inflationary inhomogeneities on the motion of current matter. The "dark flow" occurs prior to and during inflation, and persists to this day.
In an earlier paper, the lead author states this more explicitly: "In this scenario, the observed Universe (roughly, the present Hubble volume) represents part of a homogeneous inflated region embedded in an inhomogeneous space-time. On scales beyond the size of this homogeneous patch, the initially inhomogeneous distribution of energy-momentum that existed prior to inflation is preserved, the scale of the inhomogeneities merely being stretched by the expansion".
The point of inflationary theory is not that there were huge parts of the universe causally connected.
Yes, that is the point of inflationary theory (or rather, one of the points): to solve the horizon problem (why was the early universe so uniform on super-horizon scales)? Inflation's answer: because opposite ends of the universe, although they cannot now interact with each other directly, were once in close causal contact. Guth's book is a nice history of this idea. (His original motivation was to solve the monopole problem, but he quickly realized it also solved the horizon, homogeneity, and flatness problems.)
The thick outer coating helps a lot in keeping the spore contents isolated. Normal cells that are preserved in organisms don't have that, and they don't necessarily shut their metabolisms down ahead of time, so they're biochemically not "prepared" for preservation the way spores are. I'm not saying spores are immortal, but they're much more robust to environmental perturbations than just about anything else we know.
Not really disagreeing with you, 45 million years is still impressive, but perhaps it's not as terribly surprising as one might think.
What's going to degrade it? The contents of the spore are almost totally isolated from the outside environment. They're just chemicals. The spore state shuts down all the normal chemical reactions. If they're stable, they sit there, inert, pretty much forever — just like the amber itself. Other living things decay because they're smashed open, because other chemicals leech in, because other bacteria come along and eat them, etc. If that doesn't happen to the spore, its contents are safe for a long, long time, unchanged.
According to the terminology used by cosmologists, "universe" usually means "all of spacetime", and "observable universe" means "observable universe". (Sometimes they will use the term "universe" to refer to some kind of "bubble universe" inside of spacetime which may include, but is not limited to, the observable universe.)
According to general relativity, it definitely propagates at c. Whether that's true in reality is harder to answer. The best we have is indirect evidence, based on the rate at which energy is radiated away gravitationally in a binary star system. The answer is close to c. If we ever detect gravitational waves with LIGO, we'll have a direct measurement. (The LIGO detectors are thousands of kilometers apart, large enough that the difference can be timed.)
The real breakthroughs don't require advanced math. Einstein said that with all of his work the breakthroughs happen very quickly at the conceptual level and then only after that he worked out the math.
Real breakthroughs often require advanced math. Einstein is rather exceptional compared to how a lot of modern physics has played out. That's why he was a genius. Frequently results pop out of the mathematical formalism before people really understand what is going on physically. Then they go back and construct an elegant physical explanation.
I think the next big revolution will be easy too. My guess is that space-time is discrete and non-continuous.
That idea has been around for over 60 years. It's damn hard to come up with a workable discrete theory of spacetime. It's easy to come up with ideas. It's hard to come up with right ideas, and even harder to show that they're right.
It is a simple idea but has the potential to unite quantum and cosmic scales. Easy idea but the math is well past what we poor humans can handle.
The average person not working in the feild inly needs to understand the concept and some of it's implications and does not need the math tho understand them.
The original poster was somewhat correct. The Hubble radius is just an approximation to the actual location of the cosmological horizon. Something at the horizon is indeed at infinite redshift, much like something at the event horizon of a black hole. However, we can't see anything at the cosmological horizon because we can't see farther (earlier) than the cosmic background radiation, because the universe was opaque to light before that. Also, objects that are on the horizon at one time will not remain on the horizon at another time. They may pass behind it, or they may enter into our observable universe, depending on whether the expansion is accelerating or decelerating.
A lot of the "proof" for the Big Bang however it's supported mostly by vague theories.
Big Bang cosmology makes rather concrete predictions for all of the phenomena I mentioned above, and more. It is not vague. It's now specific enough that we are able to rule out different versions of Big Bang theory based on higher-order details in the CMBR angular power spectrum like the acoustic peaks.
No, this article is about a part of the universe too far away to see affecting the part of the universe we can see. It doesn't have anything at all to with our universe being created inside some pre-existing medium.
UT I've never been convinced that COBE and other projects offer definitive proof the single-origin universe.
They don't. They can't rule out explanations that invoke processes outside the observable universe.
But the more abstract that multiverses and extra boundries get, the more LIKE steady-state the description of our universe gets.
Well, you can try to cook up eternal universes that way, but they're very much not "steady state". If anything, they're even more dynamic (multiple bangs, etc.)
Consider that an electron is bounded oscilating energy. Likewise, a galaxy is bounded/cohesive energy.
I don't know if that's a good descriptions of either electrons or galaxies.
Why not consider even larger scales, might E&M radiation act differently on even larger scales - curving in on itself like we speculate happens in a black-hole (in constrained scales).
By what mechanism?
Might gravity have tremendously different characteristics on these larger scales?
Maybe, but there are all kinds of things you have to be able to explain. For instance, the luminosity-redshift relation obeys a certain relationship in an expanding universe. If you want a non-expanding universe with "weird gravity", can you construct a theory which explains Hubble redshift while also honoring the observed luminosity-redshift relation? Can you explain the blackbody CMBR spectrum? The CMBR anisotropies? The atomic spectra in distant galaxies which indicates they were once bathed in a much hotter CMBR? Light element nucleosynthesis ratios?
It's really easy to say "Well maybe it could be this..." if "this" is very vague. But as Feynman said, it's impossible to prove a vague theory wrong. Once you start writing down details, 99% of theories so proposed end up being wrong. A theory like the Big Bang which passes so many different tests is a rare phenomenon.
You're right that this isn't an explanation of dark energy. However, it's not correct to say there is no evidence for any earlier accelerated expansion. The inflationary phase of the universe was just such a period (with a much, much greater rate of expansion than dark energy today). Inflation does have evidential support, although much of it is indirect.
It's theoretically possible to exploit this "no restriction on how fast spacetime can expand" idea for FTL travel, but you can prove that to do so would require the existence of "exotic matter" with negative mass and other crazy properties. That's not very encouraging.
There are huge comment threads here arguing over the same question.
The answer is that their gravitational fields do not now have any effect on our observable universe. They used to. Pre-inflation, they could gravitationally influence us, and we could see them too. (Or we could, except for the fact that the universe was opaque back then.) Now we can't, because they expanded too far away.
So it would seem interesting that the bodies outside our observable bubble are transmitting information (they exist, they are moving this way, etc) faster than their light itself would reach us.
The article is a little unclear on what's going on, and I think I perpetuated that by my wording.
According to the theory in TFA: Objects beyond our cosmological horizon are not currently tugging on those galaxies. They tugged on them long ago and changed their velocity. Then the universe inflated so that those objects are now incredibly far away from us or the galaxies in question, and no longer interact with them. So no interaction, information, gravity, light, etc. is currently traveling faster than light from those distant objects to our observable universe.
Doesn't appear mainly because there's only one or two physicists with the guts to study it.
That's an idiotic statement. There are a number of people who have studied MOND, for instance, and you can easily find talks and papers which compare and contrast the two theories. (Look up Sean Carroll's talk from a few years ago. He takes modified gravity quite seriously, but has ultimately concluded it doesn't hold up.)
And regardless of how many people have studied it, the fact remains is that modified gravity just doesn't work to explain all the phenomena that dark matter does, and is virtually impossible to reconcile with specific phenomena like the Bullet Cluster without giving up and appealing to dark matter.
The rest would rather just make up fairy particle that can't be observed aside from there supposed mass and attendant gravity.
That's wrong. Some kinds of dark matter can be observed in principle, either from direct detectors or in particle accelerators.
Moreover, such particles were not made up to explain astrophysics. Plenty of dark matter candidates have been proposed to explain ordinary phenomena in particle physics.
Matter and energy cannot travel through space at greater than c. But general relativity doesn't place a restriction on how fast space itself can expand. That can happen at an arbitrarily large rate.
This new theory supports the standard inflationary picture. It doesn't introduce huge new amounts of mass that can collapse the universe. It just suggests that different parts of the universe pulled on each other during inflation.
If the speed of gravity is equal to the speed of light, no measurement no matter how precise will ever tell us if the former is equal, above, or below the latter. The error bars will always include above c and below c, even if they're incredibly small.
Our indirect measurements indicate that the speed of gravity is the speed of light, to within about 1% accuracy.
In Big Bang theory, the universe is not inside of anything else; there is no other "medium". Whether you can "create anything out of nothing" is a different question from whether the universe is inside of anything else pre-existing.
Both of those are assumptions. If they were true, there wouldn't be a logical explaination for tachyons.
Who cares if there's a logical explanation for tachyons, since we don't have evidence for any?
Anyway, even if tachyons existed you'd never actually observe anything traveling faster than light; see this FAQ.
In other dimensions things DO move faster then light.
Says who? In any relativistic quantum field theory or string theory, c is the limit in any dimension.
Furthermore, the speed of gravity is much greater then c.
van Flandern's website is a bunch of crackpot nonsense. He was pretty notorious on Usenet for years. He misapplies perturbation theory; if you apply his same arguments to electromagnetism, you "conclude" that light travels faster than light too (see here). In fact, you can rigorously prove in general relativity that the speed of gravity cannot exceed c (see here, assuming that the gravitational waves aren't produced by weird things like negative mass). The 1993 Nobel prize in physics was awarded, in part, for an observational determination of the speed of gravity. (You can deduce it by the rate at which gravitational energy is radiated by orbiting bodies.) The measurements indicate that the speed of gravity is c, to within a few percent accuracy.
Slashdot Mirror
The "observable universe" is everything we can see right now. The key point is, depending on how the universe expands, something that is now part of the observable universe may not have been so in the past, and may not be so in the future. (Similarly, something that was not once part of the observable universe could be so today.) What is "observable" depends on whether light can reach us at a given time, and that depends on what the geometry of space itself does. There is a nice paper which discusses various misconceptions about "the observable universe", particularly as it pertains to FTL expansion, although it's aimed at a physicist audience. I don't think it addresses this particular point, but it may still be worth reading.
The point is that van Flandern thinks he can prove the speed of gravity isn't c in general relativity, using pure mathematics. He's not making a statement about the universe per se, he's making a statement about the mathematics of the theory. His derivation is just wrong, and you can find correct mathematical derivations anywhere.
Furthermore, independent of van Flandern's mathematical errors, the experimental evidence is also that the speed of gravity is c, as I also mentioned in my post.
Maybe you read the actual arguments instead of making trite platitudes.
I think you are still wrong. Inflation happened very early on in the universe - between 1x10-36 seconds and 1x10-32 seconds, which is a very, very short time span. There were no galaxies present to influence at that time. No atoms of any sort as well.
Yes, that's what the authors here are proposing. Differential inflation introduced relative velocities between the particles in the very early universe, which later in turn seeded the primordial density perturbations that formed galaxies. Go read their paper. They state that their theory is about the influence of pre-inflationary inhomogeneities on the motion of current matter. The "dark flow" occurs prior to and during inflation, and persists to this day.
In an earlier paper, the lead author states this more explicitly: "In this scenario, the observed Universe (roughly, the present Hubble volume) represents part of a homogeneous inflated region embedded in an inhomogeneous space-time. On scales beyond the size of this homogeneous patch, the initially inhomogeneous distribution of energy-momentum that existed prior to inflation is preserved, the scale of the inhomogeneities merely being stretched by the expansion".
The point of inflationary theory is not that there were huge parts of the universe causally connected.
Yes, that is the point of inflationary theory (or rather, one of the points): to solve the horizon problem (why was the early universe so uniform on super-horizon scales)? Inflation's answer: because opposite ends of the universe, although they cannot now interact with each other directly, were once in close causal contact. Guth's book is a nice history of this idea. (His original motivation was to solve the monopole problem, but he quickly realized it also solved the horizon, homogeneity, and flatness problems.)
The thick outer coating helps a lot in keeping the spore contents isolated. Normal cells that are preserved in organisms don't have that, and they don't necessarily shut their metabolisms down ahead of time, so they're biochemically not "prepared" for preservation the way spores are. I'm not saying spores are immortal, but they're much more robust to environmental perturbations than just about anything else we know.
Not really disagreeing with you, 45 million years is still impressive, but perhaps it's not as terribly surprising as one might think.
What's going to degrade it? The contents of the spore are almost totally isolated from the outside environment. They're just chemicals. The spore state shuts down all the normal chemical reactions. If they're stable, they sit there, inert, pretty much forever — just like the amber itself. Other living things decay because they're smashed open, because other chemicals leech in, because other bacteria come along and eat them, etc. If that doesn't happen to the spore, its contents are safe for a long, long time, unchanged.
According to the terminology used by cosmologists, "universe" usually means "all of spacetime", and "observable universe" means "observable universe". (Sometimes they will use the term "universe" to refer to some kind of "bubble universe" inside of spacetime which may include, but is not limited to, the observable universe.)
Some references in this post.
According to general relativity, it definitely propagates at c. Whether that's true in reality is harder to answer. The best we have is indirect evidence, based on the rate at which energy is radiated away gravitationally in a binary star system. The answer is close to c. If we ever detect gravitational waves with LIGO, we'll have a direct measurement. (The LIGO detectors are thousands of kilometers apart, large enough that the difference can be timed.)
The real breakthroughs don't require advanced math. Einstein said that with all of his work the breakthroughs happen very quickly at the conceptual level and then only after that he worked out the math.
Real breakthroughs often require advanced math. Einstein is rather exceptional compared to how a lot of modern physics has played out. That's why he was a genius. Frequently results pop out of the mathematical formalism before people really understand what is going on physically. Then they go back and construct an elegant physical explanation.
I think the next big revolution will be easy too. My guess is that space-time is discrete and non-continuous.
That idea has been around for over 60 years. It's damn hard to come up with a workable discrete theory of spacetime. It's easy to come up with ideas. It's hard to come up with right ideas, and even harder to show that they're right.
It is a simple idea but has the potential to unite quantum and cosmic scales. Easy idea but the math is well past what we poor humans can handle.
The average person not working in the feild inly needs to understand the concept and some of it's implications and does not need the math tho understand them.
The original poster was somewhat correct. The Hubble radius is just an approximation to the actual location of the cosmological horizon. Something at the horizon is indeed at infinite redshift, much like something at the event horizon of a black hole. However, we can't see anything at the cosmological horizon because we can't see farther (earlier) than the cosmic background radiation, because the universe was opaque to light before that. Also, objects that are on the horizon at one time will not remain on the horizon at another time. They may pass behind it, or they may enter into our observable universe, depending on whether the expansion is accelerating or decelerating.
In the theory discussed in TFA, the answer is:
D) Occurred long ago and no longer influences anything in the observable universe.
A lot of the "proof" for the Big Bang however it's supported mostly by vague theories.
Big Bang cosmology makes rather concrete predictions for all of the phenomena I mentioned above, and more. It is not vague. It's now specific enough that we are able to rule out different versions of Big Bang theory based on higher-order details in the CMBR angular power spectrum like the acoustic peaks.
No, this article is about a part of the universe too far away to see affecting the part of the universe we can see. It doesn't have anything at all to with our universe being created inside some pre-existing medium.
UT I've never been convinced that COBE and other projects offer definitive proof the single-origin universe.
They don't. They can't rule out explanations that invoke processes outside the observable universe.
But the more abstract that multiverses and extra boundries get, the more LIKE steady-state the description of our universe gets.
Well, you can try to cook up eternal universes that way, but they're very much not "steady state". If anything, they're even more dynamic (multiple bangs, etc.)
Consider that an electron is bounded oscilating energy. Likewise, a galaxy is bounded/cohesive energy.
I don't know if that's a good descriptions of either electrons or galaxies.
Why not consider even larger scales, might E&M radiation act differently on even larger scales - curving in on itself like we speculate happens in a black-hole (in constrained scales).
By what mechanism?
Might gravity have tremendously different characteristics on these larger scales?
Maybe, but there are all kinds of things you have to be able to explain. For instance, the luminosity-redshift relation obeys a certain relationship in an expanding universe. If you want a non-expanding universe with "weird gravity", can you construct a theory which explains Hubble redshift while also honoring the observed luminosity-redshift relation? Can you explain the blackbody CMBR spectrum? The CMBR anisotropies? The atomic spectra in distant galaxies which indicates they were once bathed in a much hotter CMBR? Light element nucleosynthesis ratios?
It's really easy to say "Well maybe it could be this ..." if "this" is very vague. But as Feynman said, it's impossible to prove a vague theory wrong. Once you start writing down details, 99% of theories so proposed end up being wrong. A theory like the Big Bang which passes so many different tests is a rare phenomenon.
You're right that this isn't an explanation of dark energy. However, it's not correct to say there is no evidence for any earlier accelerated expansion. The inflationary phase of the universe was just such a period (with a much, much greater rate of expansion than dark energy today). Inflation does have evidential support, although much of it is indirect.
It's theoretically possible to exploit this "no restriction on how fast spacetime can expand" idea for FTL travel, but you can prove that to do so would require the existence of "exotic matter" with negative mass and other crazy properties. That's not very encouraging.
There are huge comment threads here arguing over the same question.
The answer is that their gravitational fields do not now have any effect on our observable universe. They used to. Pre-inflation, they could gravitationally influence us, and we could see them too. (Or we could, except for the fact that the universe was opaque back then.) Now we can't, because they expanded too far away.
So it would seem interesting that the bodies outside our observable bubble are transmitting information (they exist, they are moving this way, etc) faster than their light itself would reach us.
The article is a little unclear on what's going on, and I think I perpetuated that by my wording.
According to the theory in TFA: Objects beyond our cosmological horizon are not currently tugging on those galaxies. They tugged on them long ago and changed their velocity. Then the universe inflated so that those objects are now incredibly far away from us or the galaxies in question, and no longer interact with them. So no interaction, information, gravity, light, etc. is currently traveling faster than light from those distant objects to our observable universe.
Doesn't appear mainly because there's only one or two physicists with the guts to study it.
That's an idiotic statement. There are a number of people who have studied MOND, for instance, and you can easily find talks and papers which compare and contrast the two theories. (Look up Sean Carroll's talk from a few years ago. He takes modified gravity quite seriously, but has ultimately concluded it doesn't hold up.)
And regardless of how many people have studied it, the fact remains is that modified gravity just doesn't work to explain all the phenomena that dark matter does, and is virtually impossible to reconcile with specific phenomena like the Bullet Cluster without giving up and appealing to dark matter.
The rest would rather just make up fairy particle that can't be observed aside from there supposed mass and attendant gravity.
That's wrong. Some kinds of dark matter can be observed in principle, either from direct detectors or in particle accelerators.
Moreover, such particles were not made up to explain astrophysics. Plenty of dark matter candidates have been proposed to explain ordinary phenomena in particle physics.
No, there's no debate about that. It's in every relativity textbook. van Flandern is just wrong.
That's a FAQ.
Matter and energy cannot travel through space at greater than c. But general relativity doesn't place a restriction on how fast space itself can expand. That can happen at an arbitrarily large rate.
This new theory supports the standard inflationary picture. It doesn't introduce huge new amounts of mass that can collapse the universe. It just suggests that different parts of the universe pulled on each other during inflation.
If the speed of gravity is equal to the speed of light, no measurement no matter how precise will ever tell us if the former is equal, above, or below the latter. The error bars will always include above c and below c, even if they're incredibly small.
Our indirect measurements indicate that the speed of gravity is the speed of light, to within about 1% accuracy.
In Big Bang theory, the universe is not inside of anything else; there is no other "medium". Whether you can "create anything out of nothing" is a different question from whether the universe is inside of anything else pre-existing.
Both of those are assumptions. If they were true, there wouldn't be a logical explaination for tachyons.
Who cares if there's a logical explanation for tachyons, since we don't have evidence for any?
Anyway, even if tachyons existed you'd never actually observe anything traveling faster than light; see this FAQ.
In other dimensions things DO move faster then light.
Says who? In any relativistic quantum field theory or string theory, c is the limit in any dimension.
Furthermore, the speed of gravity is much greater then c.
van Flandern's website is a bunch of crackpot nonsense. He was pretty notorious on Usenet for years. He misapplies perturbation theory; if you apply his same arguments to electromagnetism, you "conclude" that light travels faster than light too (see here). In fact, you can rigorously prove in general relativity that the speed of gravity cannot exceed c (see here, assuming that the gravitational waves aren't produced by weird things like negative mass). The 1993 Nobel prize in physics was awarded, in part, for an observational determination of the speed of gravity. (You can deduce it by the rate at which gravitational energy is radiated by orbiting bodies.) The measurements indicate that the speed of gravity is c, to within a few percent accuracy.