If what we’re seeing was 13.8B light years away 13.8B years ago and are now 46B light years away, wouldn’t that mean that those things traveled ~32.2B light years in 13.8B years. Wouldn’t that thus mean they travelled faster than the speed of light?
Maybe I’m misunderstanding though.
EDIT: I guess you also have to take into account the expansion of the fabric of space as well and not just the movement of the galaxies. Still a bit difficult to grasp because I still intuitively want to say that those galaxies are traveling faster than the speed of light with the assistance of space expansion like the old Hollywood trick of actors running on a moving rug to fake superhuman speed. They’re still moving faster, just not on their own. I guess the key difference is that an actor on a sliding rug is moving through space faster even if their legs are moving at the same speed, but in the case of the galaxies, space itself is moving thus they’re not technically FTL because the speed of light is relative to its movement through space.
In essence, it's 'okay' for things to be FTL due to expansion of the medium. Imagine an ant crawling across a balloon as we stretch it. The ant could get some pretty fantastic apparent speeds relative to another ant because the medium is expanding, but step by step its moving at regular ant speed.
It is interesting to think of the opposite: space fabric is shrinking, so light needs to travel "slower" so as not to break causality.
I know I know, it's not really slower because the fabric is shrinking.
Some of the confusion is, I think from referring to speed of light as "speed of light". When it really is speed limit of causality between 2 points in space. And light (em waves) just happen to be able to hit that limit. Now since expansion of space is itself not going to allow one point in space to cause any effect on another, it (the expansion) can be faster than speed of causality.
It is also peculiar that the energy conservation law is not applicable on cosmological scales, the light is redshifted due to expansion of the Universe and the energy difference between emitted and received light is "wasted", i.e. it doesn't feed the expansion in any way.
I don't know if 'feeding expansion' is necessarily the right way to think about this, but by the first Friedmann equation, loss of photon energy will in fact be balanced by the increased expansion rate...
Sure, though the first Friedmann equation, or any other equation, where terms are balanced, e.g. a + b = 0, doesn't tell you anything about causality, though redshifting of light is exactly caused by expansion, so it is not strange that both are "balanced".
Things moving in space/time are limited to the speed of light. Space itself isn't.
It's like a blowing up a balloon. There's no limit to how big it gets or how fast it grows, so objects on opposing sides can move apart at greater than the speed of light even if they're not moving themselves.
This rate of expansion is accelerating too, driven by something called "dark" (as in unknown) energy which is overpowering all the gravity that should be pulling things together. It's fascinating to think that in 100s of billions of years, our local galaxy will be the only thing we can see in space. Any civilizations then would believe that there's nothing else out there.
FTL travel of anything (matter, photons, EM fields, gravitational fields) would result in potential causality issues. I don't know if there are models of universes where GR doesn't apply; that would be interesting, but the causality problems might make such a universe uninteresting even if it were possible. I suppose that's using the anthropic principle as justification for GR?
Metric expansion does not result in causality issues because it's an isolating phenomenon (one that slows down interactions between things) rather than an interconnecting phenomenon (one that would speed up interactions between things). Therefore it's at least possible, and observations (which you can read a brief summary of in TFA) back it up.
> Metric expansion does not result in causality issues because it's an isolating phenomenon
No, in the specific case of the concordance cosmology (and more general expanding cases such as an expanding Robertson-Walker spacetime, or a 4-dimensional de Sitter spacetime), the manifold is still Lorentzian [1] and globally hyperbolic [2].
These features let one sort trajectories into spacelike, null, and timelike. If spacelike trajectories are forbidden to interacting matter and gravitational waves, then we get our usual idea of causality [3].
Collapsing Robertson-Walker spacetimes and anti-de Sitter spacetimes are defined on globally hyperbolic Lorentzian manifolds. They are defined as free of matter and gravitational waves. However we can perturb them slighly by adding a bit of matter, even matter that generates small amounts of gravitational radiation, and as long as that matter and radiation moves only on timelike trajectories or lightlike trajectories, we have the normal causality even though the interactions among separated bits of normal matter perturbing these collapsing spacetimes will generally become more frequent and stronger over time. Indeed, this remains the case even for very highly accelerated compactions, where at late times distances between objects contract faster than light between the same objects can move.
Other commonly encountered spacetimes -- the flat spacetime of Minkowski approximately describes laboratories on Earth and in the International Space Station, and the (exterior) Schwarzschild spacetime which approximately describes the spacetime in which Earth-orbiting satellites move -- are also globally hyperbolic and Lorentzian, and thus we only need to keep superluminality at bay to guarantee normal causality.
In short: our usual idea of causality comes from being able to forbid a certain class of trajectories, and in physically realistic spacetimes (i.e., ones which approximately match parts of our observed universe), those are all as far as we can tell free from things moving spacelike (i.e., nothing's faster than a massless particle, and light is made up of massless particles, so nothing is superluminal).
Taken the other way around: our local observations of causal interactions (both locally, like in laboratories, and astronomically) are strong evidence in favour of us inhabiting a manifold equipped with the requisite features [1][2]. Alternative explanations have generally either failed in some way (self-inconsistency, obvious incompleteness, or inconsistency with repeatable observations) or been more complicated than, but reduces to, General Relativity in some limit. None of the second variety of alternatives has yet to be demanded by actual observations that cannot be explained using General Relativity as the underlying spacetime theory.
[EDIT] Everyone please ignore this comment. It is totally wrong.
> If what we’re seeing was 13.8B light years away 13.8B years ago and are now 46B light years away, wouldn’t that mean that those things traveled ~32.2B light years in 13.8B years.
No. What we're seeing was 13.8BLY away (in our reference frame) when the light was emitted. Those things are 46BLY away now (again, in our reference frame) but we can't see them now, we can only see them as they were 138B years ago.
Still not quite right: 13.8 Gly is the light travel distance, whereas proper distance at time of emission was less than that.
For example, for redshift z=9, light travel time is 13.17 Gy, proper distance at time of absorption (aka comoving distance) is 30.72 Gly and proper distance at time of emission (which, in a spatially flat universe, corresponds to angular diameter distance) is 3.07 Gly. The factor of 10 between these two distances is no coincidence as the ratio of the scale factor is given by 1+z.
I'm sorry, but that's exactly what I said unless I'm misunderstanding your point.
I said what we're seeing was 13.8B light years away 13.8B years ago, since it took 13.8B years for the light to reach us from the point it was emitted. I never said we could see now where they currently are. What I failed to account for was the space expansion during the time that the light was traveling meaning that the light had to travel further to reach us than the point of emission actually was when it emitted.
So really, it wasn't 13.8B years ago; it was emitted later. I don't know by how much, but it seems like maybe the other commenter does.
Yes, I misread your comment. And then on top of that I screwed up the answer. I've edited it accordingly. Sorry about that, I guess I'm having a bad day.
Interesting ratio of 1:3.3141210374639769452449567723343 not exactly Pi.
But then that expansion ratio may of been different over time, but it is a bit like sampling the second hand on a clock in a photo and working out the entire motion and workings of the clock - at least on the scale of the data/measurements we have and the age of the universe. Currently we are still not 100% sure when midnight was, but as always moving in the right direction.
We have more recent data on closer objects. It’s a sequence of thin shells that expands as you go backward in time, or a 3D light cone aimed backward in 4D space time. https://en.m.wikipedia.org/wiki/Light_cone
I didn't read the article but I suspect the reason is that space is expanding.
If space wouldn't have expanded sufficiently fast in the beginning of time, all matter would be still clumped together as in a giant black hole. The fact that space expanded "faster" than the speed of light allowed this black hole to become free matter.
Yeah, the intuitive view of universe expansion is that it expands from the edges out.... but in reality, it expands everywhere at once, so really the space between us is expanding as well.
Indeed; the issue is covered much more clearly in the article from which the most relevant illustration was taken: https://calgary.rasc.ca/redshift.htm
It’s more complicated than that. It hasn’t moved that far because it can’t move faster than the speed of light. If you start 13B light years away and move away at the speed of light, you’d still only be 26B light years away by the time I saw your original light.
No, the object is now 46b light years away, but we can’t see what it looks like now at that distance. We see an image of it when it was much closer and younger.
If you are turning in bed and can't sleep I'll give you some questions to think about. Even though some are non-sensical in a way, they are a good starting point to ponder about:
- Is space really flat (infinite) or curved (finite)?
- If it is flat and the universe had the beginning (big bang) then does it just have "potential infinity", by ever-expanding?
- If it is curved is it implied that it is encompassed by something that has at lease 3+1 dimension into which it is curving?
- What would happen if you would happen to stand at the edge of space/universe and push a hand through the edge?
- Space could be infinite but at the same time with a boundary. Can you imagine that boundary without trying to imagine what it is bounded from?
- Where does new space, created by the universe expansion, come from?
- What is space fabric actually made from? Is it discrete or not.
- Are there other universes outside ours?
- If there are, do we live in an infinite Russian doll type of universes?
- What gives the initial conditions and laws in the universe?
- If the universe is cyclic or there is a multiverse, are there universes with totally different laws and if yes what generated/allowed that?
- Try to imagine nothing.
- Try to imagine infinite.
- You can't imagine how any of these 2 things would work (nothing or infinite), but one or both must exist. Either all existence existed in some way forever, or it had some genesis from nothing.
- Is there free will?
- Can you even imagine a proper mechanism that would allow for free will to exist?
- What would prevent you from "cutting" the smallest possible particle in 2 parts?
- Are there several dimensions of time?
- Is there a beginning of time?
- Is math invented or discovered?
- Can we ever, in theory, discover every law in the universe if we are looking at it only from the inside.
- Try to imagine 4.5 billion years of evolution (or more if life crashed to earth from a comet). How many organisms lived, and died, how many actions were made, how many found a partner and reproduced for you to be here and read this text at these exact moment. How many chances for failure at any point in that period were there?
> - What would happen if you would happen to stand at the edge of space/universe and push a hand through the edge?
That doesn't make sense if you say the universe is curved or infinite. There will be no edge.
> - Space could be infinite but at the same time with a boundary. Can you imagine that boundary without trying to imagine what it is bounded from?
No. That's what the definition of infinite is. There are no bounds.
> - What is space fabric actually made from? Is it discrete or not.
Probably numbers. But then I like Math, so...
> - Are there other universes outside ours?
I don't think we are any special. But I'd be happy to be proven otherwise!
> - Try to imagine nothing.
> - Try to imagine infinite.
You are having trouble with nothingness because you are linking it to your consciousness. Infinity is easy though. You keep pushing infinitely and there is no edge!
> - Is there free will?
No. But don't go and do stupid stuff because of that.
> - Can you even imagine a proper mechanism that would allow for free will to exist?
There isn't. You are the product of your environment and your parents.
> - What would prevent you from "cutting" the smallest possible particle in 2 parts?
Max Planck.
> - Are there several dimensions of time?
> - Is there a beginning of time?
There is no time. Try understanding the theory of general relativity.
> - Is math invented or discovered?
Math is very basic. It starts with set theory and then builds on top of that. It is invented since set theory is invented.
>No. That's what the definition of infinite is.. There are no bounds.
just a thought: it sounds like you are more describing the concept of an uncountable infinity in a mathematical sense, a set that is infinite and with no discernible end, as opposed to a countable one, where the set is certainly infinite but it can be iterated through and conceptually has some point where it transitions into a higher meta-set that encompasses it and far more. at least, that's the metaphor i use to interpret the concept of an infinity that is bounded; conceptually, everything that exists must be contained within something if you ask me. it could even be contained within a conceptual nothing, but that nothing itself acts as a boundary point for the infinite.
also: i am definitely not a cosmologist or a scientist in any rigorous sense, im just a person that likes to think a lot. my arguments or thoughts likely wouldn't hold their weight compared to one substantiated with support and such, so I just want to make it known that I don't exactly treat my views internally as the canonical explanation, just the one that makes the most sense to me given my limited scope of understanding. thanks for your time :)
The set of positive numbers is infinite but bounded by 0. The same concept could be applied to space.
I don't have trouble with nothingness because of that. Imagining it "visually" yes, but imagining how from something that would be nothing can become something that boggles my mind.
Max Planck would not prevent that. The smallest particle (that we know for know) is much larger than Planck length. But anyway even if it weren't, plack length have different implication.
My understanding is that there is time, just that it is not that simple as Newton imagined.
I can give one thing for you to ponder: what's the true difference of the world you experience in the waking state and the world you experience during dreaming state and the cause behind both of these? The puzzle of the universe, infinity and multiverse can become apparent if you ponder over the above question.
The world we experience in the waking state is persistent and objective. The world we experience during the dreaming state is subjective and temporary. There, I pondered, and it did not make “the puzzle of the universe, infinity and multiverse apparent”.
The objectivity you are assigning to the world of waking state is also subject to your mind. Different kinds of minds experience the world differently. Scientific investigation confirms us that the object we experience do not exist like that; it's just set of atoms which are empty by their nature. There's nothing solid or liquid in actual, it's our mind way of interpreting the input from senses that makes us experience solidity/etc.
Regarding the persistency argument, the waking world seems persistent to you because it has a longer duration relative to your dream. Sometimes we dream in a dream and experience continuity of the dream-world as well. There are stories of people experiencing years of life in dreaming [0] which confirms that continuity in dream is also possible.
That said, I'm not trying to prove that waking and dreaming state are literally the same. The idea is to ponder on the generality behind waking and dreaming states and question/contemplate on the true nature of reality. Do we not observe that it's the mind which seems to be the cause of us experiencing both these states the way we experience them? If so and if the dream seems to be our personal creation then the question arises what if the waking world is a our collective (cosmic) mind dream? If it's a cosmic dream then we can start deducing answers such as there cannot be any boundary to the universe because it's all in mind as a reflection. And multiverse is also possible as the same cosmic mind can dream N number of worls/universes as it's not bound by any real material quantity.
There have been interesting books [1] written on this subject going very deep into above idea which also talk about methods to realize it as experience rather than just as an intelectual exercise.
Our awake experience is also subjective. When we look at a car, we see how it slides on 4 wheels. A deeper look reveals that the wheels aren't sliding, but rolling. What we see in our mind is simplified models, while the objective reality is much more complex, messy and not comprehensible.
Sorry, can some explain the ‘universe is expanding’ concept? So far I thought it only meant that the space universe occupies is expanding, meaning the matter keeps going farther from its origin point, but I didn’t know the ‘fabric’ of the universe is expanding. I’m not able wrap my head around this concept. Can some please point me a resource that dumbs it down?
"The universe does not expand "into" anything and does not require space to exist "outside" it. Technically, neither space nor objects in space move. Instead it is the metric governing the size and geometry of spacetime itself that changes in scale."
This was confusing for me too. As time passes, the less the universe is made up of "something" (observable matter), so maybe a more accurate analogy would be saying the universe is "diluting" rather than "expanding"?
My understanding is along the lines of, the surface of a balloon is the "fabric" of the universe: As the balloon is blown up it can expand and push things on it further apart, but the matter itself is always in the same place on the surface/in the universe (or moving slowly along the surface/in the universe). One big difference of course being the surface of a balloon is 2-dimensional, while space is 3-dimensional.
What does it mean: "if we left today we could only reach one third of it"?
I couldn't find a the limiting factor... The stars would be dead before we arrived? My best guess. Of course, the qualification would be that we continue to be bound to the speed of light maximum.
The universe is expanding, at a rate which I believe is heading past the speed of light.
This limits our ability to reach galaxies further away from us as they are speeding away faster than we could conceivably travel since our engines would of course be bound by the speed of light.
Humans (assuming we are still around) will have to watch from another planet: the collision will happen in ~4.5 billion years [1], and Earth will be uninhabitable long before then [2].
That second link I'd read through before, but oh my, you're right there's a number of critical events ahead: an ice age around 50k years from now; in ~300mil years (oh cool never noticed this before), "All the continents on Earth may fuse into a supercontinent"..resulting in another glacial age; in 700 mil years, "The death of most plant life"..
Also realized, there won't be much "fireworks", since apparently the stars are far enough apart that the merging of two galaxies won't cause much collision.
If we can figure out how to solve our political issues and become a eusocial species, we may be able to build some generational starships. In 4.5 billion years we could potentially colonize many of the planets in the habitable zone of stars in the Milky Way.
Sure, it could take millions of years to cross the galaxy, but we could grow exponentially (like lily pads crossing a pond), at least until other limiting factors kick in.
If you point a laser pointer device to moon from earth (assuming it's a really strong laser), rotating it for a few degrees on earth will result a light mark kms long on moon. The build up time for the mark will even exceed the speed of light if your hand is fast enough. This doesn't mean you broke the law. It just means multiple objects (in this case, each photon hitting the moon) created the illusion of a single object, moving really fast. Background light trying to reach us from the edge of the universe mimics a similar behavior with the added extra spacial dimension. Locations in space aren't objects. They are some points objects can leave marks on. Tracking their speed can always create faster than light situations if you pick a bunch of them conveniently.
Everything that moves through space is bound by the speed of light. But space itself is expanding at a rate such that two distant points can move away from each other faster than the speed of light. Think of it like two ants on opposite sides of a balloon, one ant starts moving toward the other as fast as it can go, but then you start to inflate the balloon. The distance between the two ants expands faster than an ant can move and they'll never meet.
It is. Nothing can move through space faster than the speed of light. That's not what's happening with the expansion of the universe. Space itself is expanding.
Silly ananlogy that absolutely should not be taken too far: imagine somehow shimming an extra micron into every metre here on Earth every second. Or an extra nanometre into every millimetre. Locally, nothing much changes at a scale you can observe, but Los Angeles is moving away from New York City at about 4m/s. And getting faster because we're still shimming extra length into "old metres"; the metre itself hasn't changed, but now there are more of them. If there were something a thousand times farther away from New York than LA, it would be moving at 4km/s and accelerating. A million times farther and it's 4000km/s. A hundred million times farther, and you're at 400,000km/s - but light can't go that fast, so you can't see it as the light can never reach you. Nothing is "moving" (other than the light in this case), but the distances between things is getting bigger nonetheless.
Posted this to a point above, but probably more pertinent here, so reposting -
Some of the confusion is, I think from referring to speed of light as "speed of light". When it really is speed limit of causality between 2 points in space. And light (em waves) just happen to be able to hit that limit. Now since expansion of space is itself not going to allow one point in space to cause any effect on another, it (the expansion) can be faster than speed of causality.
PBS space time has a good video on this - https://www.youtube.com/watch?v=msVuCEs8Ydo
Roughly, anything that can be used as an information carrier is limited by the speed of light (a good rule of thumb is that such things have some mass or energy). Things that can not be exploited on their own for information transfer do not need to obey that limit (stretching of space, entangling of particles, maybe other things).
PS It is an open question how that does not lead to contradictions when you start considering energy density of the vacuum or the extremely small gravitational waves (which can in principle transmit information) created by the changes in the latter systems. But these effects are astronomically small and it is difficult to imagine even a thought experiment that can detect them.
Everything that moves is bound by the speed of light.
But when you create space between 2 points they can appear to move faster than the speed of light, despite not actually moving - it's just that there's new space created between them, so the distance between them grows faster than the speed of light.
Things can't move faster than light through space. The space itself has no such limitation. You can imagine an expanding balloon and ants walking on it that have a limit to how fast they can move relative to the surface of the balloon.
Not an expert but the speed of light limitation seems to be derived from special relativity considerations the constancy of that speed and the use of light to carry information about events. So a train cannot accelerate to beyond c because of this. But not every phenomenon is limited by this.
As an example if you shine a torch at a really far away rock the dot can move faster than c transversally when you wiggle the torch. But the actual light is going at c to the rock and back to you.
The limiting factors are the speed of light and the rate of expansion of the universe (same as the limiting factors of the observable universe).
"reach" is possibly the wrong phrase, since it is the corollary of the "observable universe", one might call it the "observing universe", which is the slice of the universe that light from our present day can ever reach due to the accelerating expansion of the universe and the limitation of the speed of light.
There are stars which we can "see now" (i.e. receive light that they released long ago) which are unreachable because the space between us is expanding faster than you can move - if you're moving at lightspeed towards them, every year the distance between you and that star is becoming larger; right now you're closer to it than you're ever going to be, no matter what you do.
So space expands itself at every point at a certain rate. There is an elephant in the room here: how do all the parts in space coordinate to expand at about the same steady rate? Why don't we see radically different expansion rates in different parts of the universe, e.g. stars on the left are getting further apart 10x faster than those on the right? We often use the air balloon analogy, but an air ballool is inflated by the same force which equally applies to every part of the balloon. My point is that, there is probably a simple underlying mechanism that drives space expansion and the clever nature of this mechanism would explain the relativity axiom used in GTR.
No, the evidence suggests that the rate is time-dependent; that is, the expansion is accelerating.
> how do all the parts in space coordinate to expand at about the same steady rate?
This is a good question!
The answer is: the coordination happens in the much earlier much denser universe in our past, just like the coordination that happened among the fields of the Standard Model that led to the earliest atoms and molecules.
Let's first take the case where there is no accelerated expansion (which is unlikely with current evidence). In this case, the mechanism can simply be inertia, with an enormous early impulse like those in the family of cosmic inflation theories. There must be a fine balance between the density of matter in the earlier universe and the impulse, because gravitational interactions will decelerate the expansion from the early impulse until the matter-density drops/dilutes-with-expansion to a critical point after which the rate of expansion will be constant.
Observations support an early deceleration and subsequent fixed-rate expansion, but about 8 billion years ago the expansion began to accelerate, as seen in images of galaxies with redshifts less than about z ~ 0.5.
> a simple underlying mechanism
This is what the study of dark energy is about: a simple explanation for the accelerating expansion of the universe.
Typically one starts with a field that fills the whole of spacetime, with enormous energy-density in the early universe, with that energy-density decaying at later times. In extremely high-quality vacuum and far from structures like stars and black holes, the decay of this field is faster than near those structures (too near and the field becomes locally stable; the solar system, for example, is not expanding). When the field decays, it gives a local increase to the inertial expansion rate described in the no-dark-energy case a couple paragraphs up. Because there is so much empty space outside galaxy clusters, there is a lot of local, and the amount of local increases over time, so larger regions of the field decay at faster rates as the unvierse gets older.
There are other approaches too, but the effect is the same: the amount of space outside matter structures increases at an accelerating rate in the late universe, and this corresponds in a frame-dependent way to an energy-density that remains constant as the energy-density of the fields of the Standard Model decreases. That is, in such a frame of reference, matter and radiation dilute away with the expansion MUCH faster than dark energy does.
Another approach is literally the cosmological constant, which if zero gives the inertial picture above, but if it takes on a small positive value, it produces a late-time acceleration of the expansion. No known fields behave like this: matter-density varies by location in spacetime (clumpier in space, tending to diffuse apart (especially light) or collapse gravitationally (especially baryons) over time), gravitational fields vary with position in spacetime and typically with the distribution of matter. By contrast, the cosmological constant is literally identically everywhere-and-everywhen. More likely we will find evidence for something which closely approximates the cosmological constant at large scales, with differences in the density of the "cosmological not-quite-constant field" lining up very closely with observed differences in the density of matter.
> clever nature of this mechanism
Cleverness shouldn't be expected of the mechanism itself, although it will probably take some cleverness to describe the mechanism in a tractable/scalable-yet-accurate-in-detail manner.
WRT your final sentence, General Relativity works just fine in the total absence of dark energy, and dark energy as the cosmological constant has been captured as the factor \Lambda in the Einstein Field Equations. That's like saying that all matter has been captured as a factor in the Einstein Field Equations, though: it has taken a lot of work including things like studying https://en.wikipedia.org/wiki/Deep_inelastic_scattering to work out many of the finer details of the stress-energy tensor. A comparable program likely is needed to work out the finer details of \Lambda, which might lead to a formal explanation along the lines of the informal one a few paragraphs above.
I feel like we are all idiots repeating "space is expanding" if humans could live 14billion years in age, and we were present when it started, but we left good old Bob on the part of the universe that was travelling away from us, he would have traveled FTL from us. Even if you said space expanded, the traveling has been accomplished. So now we know that space expansion is how you travel FTL.
In other words if humans ever invent warp, the space expansion work around we would effectively have FTL travel.
Just a minor technicality, it’s not that Bob travelled faster than the speed of light, it’s that the distance between you and Bob increased faster than the speed of light. Nothing can travel FTL through space, but space itself can be made smaller or larger.
I think we can use it to go back to a place. It is the same as going further just that now we “just” need to shrink the space.
Of course all is so far just an exercise of our imagination.
You can take a look at the curvature or more concrete to the density of the Universe. It is possible to be positive and in this case at one point in the future the Universe will collapse back on itself. Nothing will stop it to do this apparently FTL.
Unless we create a link between two points nearby ahead of time, wait for a billion years and then use this link to jump between the two points. In fact, I think that such links exist everywhere between any two points in space, but they remain latent in the form of quantum entanglement.
They are related (in fact there is no "time") but I think what the OP wanted to say is that you can have a universe that it is "that" big from the get go and so objects will be very far from the beginning.
That's an easy one. We hold the same chain and the distance between us is 100 links. The chain has a curious property: its links subdivide like living cells and on average 1 link takes a minute to split into 2. Thus, every minute the distance between us doubles and half hour later we'll be 1 billion times further from each other. We can replace the chain with a net or even with a 3d net.
I don't think it's just you. Maybe I can help; unfortunately developing intuition without going through the mathematics even at an introductory level [as in [1]] is a hard road.
The tl;dr is that the objects are certainly moving apart; the question is whether or not the distance between them is increasing, and that depends on the coordinates in which one measures distance.
In the cosmological case (and there are others in other areas of physics) the distant astrophysical objects aren't moving spatially in one particular set of coordinates, and that set is deliberately constructed to be comoving with the moving objects. Instead, the coordinates' spatial labels themselves are time-dependent, and so absorb the expansion of space. Schematically, an astrophysical object that is at (t=5,a,b,c) in the past will be at (t=4,a,b,c), (t=3,a,b,c) and so on, and in the future will be at (t=6,a,b,c), etc., where a=const,b=const,c=const. The bookkeeping of keeping a,b,c constant for all a,b,c is rolled into the metric, a factor in the Einstein Field Equations, and so one typically hears about the "metric expansion" of the universe in cosmological contexts.
(The astrophysical objects themselves are an idealization of the largest known gravitationally-bound objects: galaxy clusters. The idealization represents them as particles of an ideal dust, to underline that the metric expansion is adiabatic).
One is perfectly free to choose non-comoving coordinates; a change of coordinates does not change the physics of the expanding universe, merely how the physics are represented in calculations. For instance, one can apply spherical coordinates with the origin on you (or some idealization thereof) at all times. Jumping up and down moves everything else in the universe in that set of coordinates. The movement of the Earth, solar system, etc., all results in distant stars moving in that set of coordinates. In those coordinates, the metric expansion is a tiny movement of distant objects in those coordinates compared to you just turning 90 degrees to your left. Distant galaxy clusters can be held still in those coordinates by flying in an aeroplane.
I say an idealization of yourself, because what are you from moment to moment? Several kilograms of gas and liquid pass through you over the course of a day. Tissue turnover happens at scales of weeks to months. You weren't alive at all a couple hundred years ago, much less several billion, so where is the coordinate origin supposed to be then?
Likewise, galaxy clusters have internal motions, radiate, have gas outflows; some collide and merge. The earliest galaxies weren't around at the time the cosmic microwave background became the surface of last scattering, but idealizing on a dust (which is a type of fluid) lets one use the comoving cosmological coordinates sensibly even that long ago. And also into the far future when the universe is so large that there will be on average only one "dust" particle per Hubble volume, and galaxies may have disintegrated into radiation or collapsed into black holes or both.
If we were to switch to spherical coordinates on some well-chosen point within our own galaxy cluster, we'd note that distant galaxies' motions are overwhelmingly radially outwards. We'd still see the (least gravitationally lensed) distant galaxies in our sky holding to a pattern: angularly-smaller, redshifted quasar spectal lines, molecular gas spectral lines, and spectral lines of younger and younger-generation/lower-metallicity stars. It'd be reasonable to think about the implications for RADAR-like round trips, and the connection with RADAR-painting e.g. an aeroplane accelerating away from you during it's take-off and climb. But we also want to think about what two other galaxy clusters would see in their sky, and an expanding universe would give each of those a highly similar view. But they might not even be able to know about the spherical set of coordinates above, and certainly would not find them useful for describing their views of the cosmos.
One might pause here for a moment to consider how an Earth-centric astronomy, a Moon-centric and a Mars-centric astronomy, would each have epicycles: different ones for Mars and Earth with respect to the other planets, and very different ones for the Moon with respect to the other planets. Presumably independent civilizations throughout our solar system would eventually come up with a sun-centric astronomy.
The comoving coordinates seem very likely to be discovered by any comparable astronomers who discover and study in detail the cosmic microwave background, and eventually figure out how to remove the 10^-3 to 10^-6 local-motion multipoles from it [2]. That's at the root of the cosmological coordinates: in them the cosmic microwave background has only tiny deviations from an ideal blackbody radiator at a temperature that decreases with time according to an expanding ideal gas law (for massless gas -- a gas of light, or a photon gas if you like). And the coordinates are useful because later large scale structures (active galaxies and so on) also end up with only small deviations from an expanding ideal gas law for a massive gas, and those deviations are straightforwardly explained by the gas being self-sticky (it clumps, unlike the radiation gas).
It's precisely because general relativity applies that this is possible.
First, note that relative velocity and growth of distance between objects are distinct concepts: Shoot two bullets at 0.9c each in opposite directions, and from your perspective, the distance between them will grow at a rate of 1.8c, whereas their relative velocity (as computed from the 'angle' between their 4-velocities) will only be 0.994c.
In special relativity, that relative velocity will correspond to the growth of distance between the bullets as measured by an observer comoving with either bullet.
In general relativity, things are more complicated: Curved spaces lack distance parallelism, so you can only compare the velocity vectors of the two bullets when they are located in the same place at the same time. Also, there's generally no longer a canonical way to decompose spacetime into spatial slices across which you can measure distances.
Now, you can still define a relative velocity through parallel transport along a path connecting the two objects, and in particular the path of a photon emitted by a distant galaxy and observed by us. This will in fact recover cosmological redshift as a generalized Doppler shift. If defined this way, the relative velocity goes to c as you approach the cosmic event horizon, hence you'll never observe relative velocities larger than c.
However, while there's no canonical spatial slicing in general relativity, in Friedmann cosmology specifically, a preferred spatial slicing does exist: The one where the universe looks homogeneous and isotropic and the same amount of time will have passed since the big bang (as measured by an observer following the Hubble flow). This allows us to define the proper distance (at constant cosmological time) between astronomical objects, and the growth of this distance is what's known as 'recession velocities'. These velocities go to c at the Hubble sphere, which doesn't really have any particular significance: We can see galaxies that were located outside the Hubble sphere just fine (though as time goes on, the Hubble sphere will approach the cosmic horizon asymptotically).
Space is expanding everywhere and not just around the edges.
Space inside the edges has photons flying in it, while this photon travels, space is expanding in all of the photons directions.
This would mean that a photon from 5 billion years ago would take longer to get here, and its source is even further away than the speed between us and the source would make it be.
What I think about during all this:
If space is expanding everywhere, then it is sort of pushing things away from each other, meaning that objects are moved, like domino's everything is moved a bit so that everything has more space in between.
Do I have to separate expansion of the universe vs expansion of space?
Idk, but if we travel 13B light years one way, it moves 13B the other then were at 26, and space itself has to have expanded 20 billion light years within the time it takes light to travel 13... So yeah, sounds like something is moving faster than light.
Things can't move faster than light, that isn't possible, but we can think about abstract concepts that aren't things. Fairness for example. Infinity. These are not things. Expanding space isn't a thing, it's just a concept. So that can go however fast you want.
Let's try something much smaller and easier to reason about. Take a small fairly directional lamp (in my country we'd say a "torch" but words vary) and hold up a coin in front of it so that it makes a big obvious shadow on a far away wall, you may need to turn down ambient lighting. Relatively small movements of the coin result in the shadow moving quickly. Huh. How fast can that shadow move?
While the coin is a _thing_ the shadow is not, it's just your imagination processing the absence of light.
If you have a big enough room and a strong enough light source, you can do this trick so that a flick of the wrist appears to move the shadow much faster than the speed of sound. If a _thing_ did that in a room filled with breathable air you'd hear a sonic boom. But the shadow isn't a thing, it exists only in your imagination and so you hear nothing.
Large enough objects can occlude a star to have this effect on a cosmic scale. The shadow of a planet can pass over the face of another planet at faster than the speed of light. But no actual _thing_ moved, so no rules are broken by this happening, "faster than the speed of light" was only in your head, all the actual things (including the photons) obeyed the rules and stayed at or below the speed limit.
> Once objects are formed and bound by gravity, they "drop out" of the expansion and do not subsequently expand under the influence of the cosmological metric...
And... another author that doesn't get it or looks for a controversial, clickbaity title.
The correct way to state it would be to say: "We are seing things that are currently 46B light years away but since they are so far away we see them as they were 13.8B years ago when they were much closer than they are now."
I see absolutely nothing difficult about that statement unless you really need to create some piece of content and you figured out you will make something difficult so that you can then make it a little bit easier and be a hero.
Yet you can't account for them now being 46B light years away without breaching C, or encompassing the expansion of the universe, within which things are moving. I think he manages a clear explanation of that without scaring away too many.
Your "correct" version on the other hand, leaves more questions than answers, not least of which being how do we ever get 46B ly distance in something 13.8B years old. Then you have to cover the expansion, relativity, and hey presto you've written the article. Just skimming the other comments here makes it clear it's not a topic that's well understood or intuitive.
Clickbait it ain't, and the title is a very fair question.
I'm really disappointed to see this the top voted HN answer right now...
"(...) the notion of the universe expanding at a calculable rate was first derived from the general relativity equations in 1922 by Alexander Friedmann"
The space expands. It's been known for a long time.
We might not know why exactly this happens and there is still some discussion about ultimate fate of the universe although prevalent theory is that it will expand at an ever faster rate until every molecule will be torn from each other and matter will no longer be able to form.
Sure, but the high school, pop science and TV explanations of relativity and the universe rarely consider anything apart from the two trains travelling toward each other, nearing C, with an observer aboard one train. People can grasp that relatively easily, along with doppler explanations of red shift. Every decent attempt to take a step or two further is to be approved. :)
When tracks start moving as well it gets mind bending, and no longer intuitive. Now we have multiple scenarios where the combination of track movement (universe expansion), and train movement result in it being "obvious" you have to exceed C somewhere, which we're told is impossible. Otherwise how can the distances end up where they are. It's between these two points most non-physicists stop as it starts to feel deeply contradictory.
Right - It is not clear how we can see something more than 13.8B ly away since it would take more than 13.8B years for signals of its existence to reach us. From our frame of reference, that would mean the emission started before the big bang.
Your dismissal underestimates the qualifications and pedigree of an established and excellent populariser of "cool" scientific topics.
Sure he goes for clickbaity articles now and then, but he's pretty good at them in my experience, and your summary is both incorrect (there is no "current" over that distance scale) and leaves all explanation out.
It's at lesst a little confusing how something went from 14B light years away to 46B light years away (seems to be a distance of 32B light years total travel) in 14B years.
Except the galaxies do not travel. Galaxies are more or less stationary, it's the space inbetween that expands. Think in terms of raisins in a dough or points on expanding baloon. Neither of them could be said to travel, yet they can be seen to become more and more far away.
> In physical cosmology, peculiar velocity refers to the components of a galaxy's velocity that deviate from the Hubble flow. ....
> Galaxies are not distributed evenly throughout observable space, but are typically found in groups or clusters, where they have a significant gravitational effect on each other. Velocity dispersions of galaxies arising from this gravitational attraction are usually in the hundreds of kilometers per second, but they can rise to over 1000 km/s in rich clusters.
That's why I wrote "more or less stationary", which is describing 1000km/s compared to double the speed of light that would come from naive calculation.
Importantly, the expansion of the universe is necessary. Under special relativity two things could be moving at .9c toward one another and their velocities won’t somehow add to 1.8c in relative velocity. Light will still travel at c from one to the other.
FTL speed requires infinite amount of energy in this iteration of physics, except for case when vacuum is not empty, but even in this case medium cannot move at supersonic speed of medium, while still in one piece. If far away vacuum moves at FTL speed, then there must be "sonic" barrier somewhere.
There’s a huge difference between click bait and this. Click bait makes a crazy claim anyone would want to click on, “click here to learn the secret of immortality”, and delivers something mundane with little connection like “antioxidants are good.” This is choosing a compelling title for a topic or concept that otherwise might be a little boring or otherwise get viewer clicks - aka, marketing.
The way you state it pretty blatantly leaves the question of "if they're 46B light years away, then how is the universe not at least that old?".
This comment heavily reminds me of the mannerisms of professors that have the mentality of "I deeply understand this subject from years of past studies, so you should be able to understand it from my vague and half assed explanation alone".
It's a pop science article, and I'm sure the audience that's into pop science articles found it interesting and probably learned something. I'm not really sure what you expect.
Maybe I’m misunderstanding though.
EDIT: I guess you also have to take into account the expansion of the fabric of space as well and not just the movement of the galaxies. Still a bit difficult to grasp because I still intuitively want to say that those galaxies are traveling faster than the speed of light with the assistance of space expansion like the old Hollywood trick of actors running on a moving rug to fake superhuman speed. They’re still moving faster, just not on their own. I guess the key difference is that an actor on a sliding rug is moving through space faster even if their legs are moving at the same speed, but in the case of the galaxies, space itself is moving thus they’re not technically FTL because the speed of light is relative to its movement through space.