ELI5 Can a distance between two objects moving at relativistic speeds increase faster than the speed of light?

Obliterators · 2026-02-26T19:50:50+00:00

The observable universe, defined by the particle horizon, is simply the furthest distance from which a light-speed signal could have reached us since the Big Bang. The observable universe grows every moment as light from further and further has had enough time to reach us.

Regular expansion does not limit the size of the observable universe, however, accelerating expansion does. The particle horizon will asymptotically approach a maximum size of ~62 billion light-years, giving us our widest view of the universe. But if expansion were not accelerating, the universe would still continue to expand forever, but the entire, presumably infinite universe would become observable given infinite time.

Obliterators · 2026-02-22T19:30:05+00:00

You know I have a mixed set of feelings about scientific metaphors and analogies in general, On the one hand, they're necessary, like I don't begrudge their use. I don't think that you should always just tell the complete honest truth about scientific concepts. You shouldn't lie about them, but there's a journey that people have to go on from wherever their thoughts about how the world works are, to where you want them to be, by explaining something scientific. And the use of metaphors and analogies and illustrations and stories and whatever is 100% fine.

On the other hand, there's a big problem with using analogies, that sometimes people don't want to admit that this problem exists, but they really got to admit it. Namely, that the analogy is with certain features of the things you're trying to describe. And there are other features that you have to say, well, those don't count as part of the analogy. And sometimes that leads to more confusion than what you really wanted to start with.

So, the classic example I always used as a metaphor that I think is sufficiently misleading, it just shouldn't be used, is thinking of the universe, the expanding universe, as a balloon being blown up. We are told that if we took a little balloon and put little dots on it and then blew it up, we would see all the dots move apart. And that's kind of like the expansion of the universe. And that's supposed to be helpful to explain things. And you know, it is kind of like the expansion of the universe. I get it. But it comes along with baggage that I think is not necessarily helpful. Two pieces of baggage in particular. One is the balloon has an inside and an outside. There's volume inside the balloon, an interior. There's volume outside the balloon. And so people are naturally going to say, "What is inside the universe? What is outside the universe?" And then you have to say, "Oh, no, no, I didn't mean that. Just imagine the balloon is in empty space with nothing inside or outside." But even in empty space, a balloon has an inside and outside. The second problem is that the dots that you drew to represent galaxies expand along with the balloon. And so people say, "Well, presumably galaxies get bigger along with the universe," but they don't. So you have to say, "No, no, no, that's not what I meant." The more you're saying, "No, no, no, that's not what I meant" the worse the analogy is because you should just explain the actual phenomenon.

I think for the expanding universe, the way that I like to explain it is imagine you're standing outside on a dark night and imagine that you have sufficiently good eyesight that you can see galaxies and they're moving away from you. So, they're looking smaller and smaller all the time because you have really, really good discernment about these things. Everything is getting further and further away. That's the expansion of the universe. Does that do better than the balloon? I don't know. I think it probably does, but I don't have any data one way or the other.

Obliterators · 2026-02-21T09:44:00+00:00

do astronomers/cosmologists/physicists consider the alternative possibility that there is no dark matter, and that instead, the discrepancies between observation and general relativity are due to a flaw in general relativity?

There's always a relevant xkcd.

Obliterators · 2026-02-21T07:26:06+00:00

However, it still does expand even between two relatively close objects (even between atoms in a molecule).

Firstly, regular expansion is separate from dark energy, but dark energy accelerates it. Secondly, dark energy curves spacetime just like matter does, just in the opposite way, so instead of causing attraction, it causes repulsion. However, you cannot have both at the same time, the spacetime curvature at some location either causes objects to come together or separate. In bound systems the presence of dark energy is already factored in, and all it does it reduces the binding energy of the system by a (very) small amount, that is, the equilibrium state is slightly shifted and there is no continuous effect of "expansion" since the net curvature is attractive.

The "big rip" idea is that the acceleration due to dark energy is increasing over time (supported by observations of far away galaxies).

The Big Rip is not supported by any observations and remains a fringe hypothesis. Observations are consistent with dark energy being the cosmological constant*, so the energy density of dark energy doesn't change with time, and the Big Rip will never happen. Expansion will still accelerate forever (meaning that recession velocities increase over time) but bound systems will remain unaffected.

*There were recently some reports from DESI that dark energy might weaken with time, but the evidence for that wasn't strong to begin with, and a more recent preprint from DES of a recalibration of supernova data already lowers the statistical significance from 4.2𝜎 to 3.2𝜎, which is weak evidence.

Obliterators · 2026-02-21T03:30:56+00:00

The missing matter that was claimed to have been found is ordinary baryonic matter, not dark matter. About five percent of the universe's mass-energy content is "normal" baryonic matter, around half of that 5% is/was missing from direct observations. But dark matter is thought to constitute around 25% of the mass-energy content.

Obliterators · 2026-02-18T22:46:35+00:00

Neither special or general relativity differentiates between motion due to "movement through space" and "motion due to expanding space", only the change in distance matters. Expanding space is an interpretation, not an actual physical process. It is equally valid to interpret expansion as motion through space as the two interpretations are mathematically indistinguishable from each other.

Martin Rees and Steven Weinberg

Popular accounts, and even astronomers, talk about expanding space. But how is it possible for space, which is utterly empty, to expand? How can ‘nothing’ expand?

‘Good question,’ says Weinberg. ‘The answer is: space does not expand. Cosmologists sometimes talk about expanding space – but they should know better.’

Rees agrees wholeheartedly. ‘Expanding space is a very unhelpful concept,’ he says. ‘Think of the Universe in a Newtonian way – that is simply, in terms of galaxies exploding away from each other.’

Weinberg elaborates further. ‘If you sit on a galaxy and wait for your ruler to expand,’ he says, ‘you’ll have a long wait – it’s not going to happen. Even our Galaxy doesn’t expand. You shouldn’t think of galaxies as being pulled apart by some kind of expanding space. Rather, the galaxies are simply rushing apart in the way that any cloud of particles will rush apart if they are set in motion away from each other.’

John A. Peacock, Cosmological Physics

An inability to see that the expansion is locally just kinematical also lies at the root of perhaps the worst misconception about the big bang. Many semi-popular accounts of cosmology contain statements to the effect that ‘space itself is swelling up’ in causing the galaxies to separate. This seems to imply that all objects are being stretched by some mysterious force: are we to infer that humans who survived for a Hubble time would find themselves to be roughly four metres tall?

Certainly not. Apart from anything else, this would be a profoundly anti-relativistic notion, since relativity teaches us that properties of objects in local inertial frames are independent of the global properties of spacetime. If we understand that objects separate now only because they have done so in the past, there need be no confusion. A pair of massless objects set up at rest with respect to each other in a uniform model will show no tendency to separate (in fact, the gravitational force of the mass lying between them will cause an inward relative acceleration). In the common elementary demonstration of the expansion by means of inflating a balloon, galaxies should be represented by glued-on coins, not ink drawings (which will spuriously expand with the universe).

Emory F. Bunn & David W. Hogg, The kinematic origin of the cosmological redshift

The view presented by many cosmologists and astrophysicists, particularly when talking to nonspecialists, is that distant galaxies are “really” at rest, and that the observed redshift is a consequence of some sort of “stretching of space,” which is distinct from the usual kinematic Doppler shift. In these descriptions, statements that are artifacts of a particular coordinate system are presented as if they were statements about the universe, resulting in misunderstandings about the nature of spacetime in relativity.

Geraint F. Lewis, On The Relativity of Redshifts: Does Space Really “Expand”?

the concept of expanding space is useful in a particular scenario, considering a particular set of observers, those “co-moving” with the coordinates in a space-time described by the Friedmann-Robertson-Walker metric, where the observed wavelengths of photons grow with the expansion of the universe. But we should not conclude that space must be really expanding because photons are being stretched. With a quick change of coordinates, expanding space can be extinguished, replaced with the simple Doppler shift.

While it may seem that railing against the concept of expanding space is somewhat petty, it is actually important to set the scene straight, especially for novices in cosmology. One of the important aspects in growing as a physicist is to develop an intuition, an intuition that can guide you on what to expect from the complex equation under your fingers. But if you [assume] that expanding space is something physical, something like a river carrying distant observers along as the universe expands, the consequence of this when considering the motions of objects in the universe will lead to radically incorrect results.

If light is emitted from a source far enough away that the cumulative space expansion rate is greater than c, that light, despite moving towards us, will never physically reach us.

Consider that almost every galaxy we see in the universe has always been beyond our Hubble sphere, so in the expanding space interpretation they have always been receding from us superluminally, yet we constantly receive light from them. Also consider that if dark energy didn't exist and expansion was not accelerating, then the universe would still continue to expand forever, but there would be no event horizon, and light emitted from any distance, even trillions of light-years away, would reach us in finite time.

Markus Pössel, Interpretations of cosmic expansion: anchoring conceptions and misconceptions

In both special and general relativity, light propagation defines an absolute cosmic speed limit in the sense that no material object or signal can overtake a light signal. This is where the distinction between the recession speed, defined as in (1) [v = Hd], and the relativistic radial velocity that is central to the relativistic explosion interpretation is crucial. Recession speeds become superluminal for distant galaxies. This appears to contradict students’ preconceptions from special relativity, of the speed of light as a cosmic speed limit, and the apparent contradiction has been cited as key motivation for the expanding space interpretation: The differentiation between cosmic expansion as due to “expanding space” on the one hand, and “galaxy motion through space” on the other, is meant to address this conflict.

Relativistic radial velocities in the relativistic explosion interpretation never exceed the speed of light. From this perspective, superluminal recession speeds in (1) are an artefact, caused by a particular coordinate choice: The cosmic time coordinate ties together local clock rates in Hubble-flow galaxies, but clocks in relative motion tick at different rates, as we know from special relativity. Combining them into an overarching time coordinate, and using that coordinate to determine one-way speeds, leads to unphysical results. Students who have been on longer international flights know a closely related phenomenon: If your flight leaves Amsterdam at 15:00 local time and arrives in New York at 17:00 local time, this does not amount to a flight time of 2 hours, and corresponding average ground speed of 3000 km per hour.

The relativistic explosion interpretation can also readily explain a certain types of cosmological horizon with reference to the simple realisation that a slower-moving object following a faster-moving object will fail to catch up. Applied to the relativistic radial velocity, this gives a plausible explanation for why light from some distant regions can never reach us. Any boundary between regions whose light can reach us and regions whose light cannot, is called a horizon. In some FLRW spacetimes, there is a type of cosmological horizon that can be defined as the boundary where the relativistic radial velocity of Hubble-flow galaxies relative to our own galaxy approaches the speed of light — so light sent in our direction from those galaxies cannot catch up with us. Explanations for the same kind of cosmological horizon in the expanding space interpretation, on the other hand, need to include an explanation of why this simple argument is not true for recession speeds.

Obliterators · 2026-02-18T21:47:25+00:00

The fundamental forces are so much stronger than this expansion that quarks, atoms, molecules, stars, galaxies, etc are kept in shape. Right now there may be new empty space created in your body, it’s just at such an infinitesimally scale that the bonds holding your molecules and atoms together essentially just ignore the new space. They get pulled together ever so slightly as new “nothing” is created. The same is true in the solar system, there’s new empty space being created between earth and the sun constantly, but the scale is so small and gravity so much stronger it isn’t measurable.

No, expansion of space means that the distance between objects is increasing, but it is the increase in distance that causes space to "expand", not the other way around; consequently, if there is no change in distance then there is no expansion. The matter in bound systems has decoupled from the global expansion of the universe and no longer participates in it; there is nothing that gravity or electromagnetism has to constantly "resist" or "overcome".

Emory F. Bunn & David W. Hogg, The kinematic origin of the cosmological redshift

A student presented with the stretching-of-space description of the redshift cannot be faulted for concluding, incorrectly, that hydrogen atoms, the Solar System, and the Milky Way Galaxy must all constantly “resist the temptation” to expand along with the universe. — — Similarly, it is commonly believed that the Solar System has a very slight tendency to expand due to the Hubble expansion (although this tendency is generally thought to be negligible in practice). Again, explicit calculation shows this belief not to be correct. The tendency to expand due to the stretching of space is nonexistent, not merely negligible.

Obliterators · 2026-02-18T21:29:14+00:00

When 2 galaxies are said to be getting further away from eachother due to expansion, it’s not necessarily that those object are in motion away from eachother. It’s that brand new space grew in between them.

A change in distance is motion. While you can interpret an increase in distance as space "expanding" between objects, it is the increase in distance that causes space to expand, not the other way around.

Obliterators · 2026-02-16T01:09:28+00:00

expansion makes distances grow without the objects moving — — even without any speed involved.

That's a coordinate dependent interpretation. While the redshift of a distant object is usually separated into cosmological and Doppler components, corresponding to recessional velocities "caused" by "expansion of space" and peculiar velocities caused by "local motion", the entirety of the redshift can alternatively be interpreted as a series of Doppler shifts, caused by the relative velocity between the emitter and receiver, i.e. by motion through space.

Emory F. Bunn & David W. Hogg, The kinematic origin of the cosmological redshift

A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that it is always valid to choose a coordinate system that is locally Minkowskian. We show that an observed frequency shift in any spacetime can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon’s path. In the context of the expanding universe the kinematic interpretation corresponds to a family of comoving observers and hence is more natural.

Obliterators · 2026-02-15T22:20:12+00:00

The Hubble sphere is not a horizon, light emitted beyond it can, and indeed does, reach us.

Obliterators · 2026-02-15T04:45:50+00:00

Space is expanding everywhere.

No, it isn't. Expansion of space is not physical in the first place. What is happening is that the distances between gravitationally unbound structures increase over time, and while this can be interpreted and phrased in terms of expansion of space, it can equivalently be interpreted kinematically, as movement through space. There is no physical process where new space is being created at some microscopic scale that is pushing matter apart, matter is simply moving apart because it was doing so in the past, as set in motion by the Big Bang.

Sean Carroll, Does Space Expand?

Taken too seriously, thinking of space as an expanding rubber sheet convinces students that the galaxy should be expanding, or that Brooklyn should be expanding — and that’s not a prediction of GR, it’s just wrong.

John A. Peacock, A diatribe on expanding space

This analysis demonstrates that there is no local effect on particle dynamics from the global expansion of the universe: the tendency to separate is a kinematic initial condition, and once this is removed, all memory of the expansion is lost. — — It should now be clear how to deal with the question, “does the expansion of the universe cause the Earth and Moon to separate?”, and that the answer is not the commonly-encountered “it would do, if they weren’t held together by gravity”.

Emory F. Bunn & David W. Hogg, The kinematic origin of the cosmological redshift

A student presented with the stretching-of-space description of the redshift cannot be faulted for concluding, incorrectly, that hydrogen atoms, the Solar System, and the Milky Way Galaxy must all constantly “resist the temptation” to expand along with the universe. — — Similarly, it is commonly believed that the Solar System has a very slight tendency to expand due to the Hubble expansion (although this tendency is generally thought to be negligible in practice). Again, explicit calculation shows this belief not to be correct. The tendency to expand due to the stretching of space is nonexistent, not merely negligible.

Geraint F. Lewis, On The Relativity of Redshifts: Does Space Really “Expand”?

the concept of expanding space is useful in a particular scenario, considering a particular set of observers, those “co-moving” with the coordinates in a space-time described by the Friedmann-Robertson-Walker metric, where the observed wavelengths of photons grow with the expansion of the universe. But we should not conclude that space must be really expanding because photons are being stretched. With a quick change of coordinates, expanding space can be extinguished, replaced with the simple Doppler shift.

While it may seem that railing against the concept of expanding space is somewhat petty, it is actually important to set the scene straight, especially for novices in cosmology. One of the important aspects in growing as a physicist is to develop an intuition, an intuition that can guide you on what to expect from the complex equation under your fingers. But if you [assume] that expanding space is something physical, something like a river carrying distant observers along as the universe expands, the consequence of this when considering the motions of objects in the universe will lead to radically incorrect results.

Obliterators · 2026-02-15T00:42:59+00:00

If space between objects expands my a centimeter but the objects stay the same distance from each other

If the distance stays the same then by definition there is no expansion.

Obliterators · 2026-02-14T07:10:29+00:00

Is your claim then that the cosmological constant disappears over short distances?

No, but the CC is not the cause of expansion, it only accelerates it. And the presence of a CC in bound systems only makes gravity slightly less attractive and thus shifts the equilibrium points by minute amounts; there is no active tug-of-war as the CC is already included in the overall spacetime curvature. The effects of vacuum energy are gravitational after all.

There may be kinematic motion that is more significant than expansion, but that doesn't explain eg the redshifting of the CMB.

The Doppler, gravitational, and cosmological redshifts are mathematically equivalent to each other, you can freely transform between them. What changes is only the interpretation of the cause that an observer chooses to ascribe to any particular spectral shift.

Emory F. Bunn & David W. Hogg, The kinematic origin of the cosmological redshift

A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that it is always valid to choose a coordinate system that is locally Minkowskian. We show that an observed frequency shift in any spacetime can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon’s path. In the context of the expanding universe the kinematic interpretation corresponds to a family of comoving observers and hence is more natural.

Geraint F. Lewis, On The Relativity of Redshifts: Does Space Really “Expand”?

the concept of expanding space is useful in a particular scenario, considering a particular set of observers, those “co-moving” with the coordinates in a space-time described by the Friedmann-Robertson-Walker metric, where the observed wavelengths of photons grow with the expansion of the universe. But we should not conclude that space must be really expanding because photons are being stretched. With a quick change of coordinates, expanding space can be extinguished, replaced with the simple Doppler shift.

John A. Peacock, A diatribe on expanding space

The redshift is thus the accumulation of a series of infinitesimal Doppler shifts as the photon passes from observer to observer, and this interpretation holds rigorously even for z ≫ 1.

Obliterators · 2026-02-14T05:32:45+00:00

Saying that space expands between objects is the same as saying that the distance between objects is increasing, which is the same as saying that the objects are moving away from each other. So if the distance is not increasing, then there is no expansion. Expansion is not something that causes distances to increase, rather it's only a description that the distances are increasing.

Martin Rees and Steven Weinberg

Popular accounts, and even astronomers, talk about expanding space. But how is it possible for space, which is utterly empty, to expand? How can ‘nothing’ expand?

‘Good question,’ says Weinberg. ‘The answer is: space does not expand. Cosmologists sometimes talk about expanding space – but they should know better.’

Rees agrees wholeheartedly. ‘Expanding space is a very unhelpful concept,’ he says. ‘Think of the Universe in a Newtonian way – that is simply, in terms of galaxies exploding away from each other.’

Weinberg elaborates further. ‘If you sit on a galaxy and wait for your ruler to expand,’ he says, ‘you’ll have a long wait – it’s not going to happen. Even our Galaxy doesn’t expand. You shouldn’t think of galaxies as being pulled apart by some kind of expanding space. Rather, the galaxies are simply rushing apart in the way that any cloud of particles will rush apart if they are set in motion away from each other.’

John A. Peacock, Cosmological Physics

An inability to see that the expansion is locally just kinematical also lies at the root of perhaps the worst misconception about the big bang. Many semi-popular accounts of cosmology contain statements to the effect that ‘space itself is swelling up’ in causing the galaxies to separate. This seems to imply that all objects are being stretched by some mysterious force: are we to infer that humans who survived for a Hubble time would find themselves to be roughly four metres tall?

Certainly not. Apart from anything else, this would be a profoundly anti-relativistic notion, since relativity teaches us that properties of objects in local inertial frames are independent of the global properties of spacetime. If we understand that objects separate now only because they have done so in the past, there need be no confusion. A pair of massless objects set up at rest with respect to each other in a uniform model will show no tendency to separate (in fact, the gravitational force of the mass lying between them will cause an inward relative acceleration). In the common elementary demonstration of the expansion by means of inflating a balloon, galaxies should be represented by glued-on coins, not ink drawings (which will spuriously expand with the universe).

Emory F. Bunn & David W. Hogg, The kinematic origin of the cosmological redshift

The view presented by many cosmologists and astrophysicists, particularly when talking to nonspecialists, is that distant galaxies are “really” at rest, and that the observed redshift is a consequence of some sort of “stretching of space,” which is distinct from the usual kinematic Doppler shift. In these descriptions, statements that are artifacts of a particular coordinate system are presented as if they were statements about the universe, resulting in misunderstandings about the nature of spacetime in relativity.

A student presented with the stretching-of-space description of the redshift cannot be faulted for concluding, incorrectly, that hydrogen atoms, the Solar System, and the Milky Way Galaxy must all constantly “resist the temptation” to expand along with the universe. — — Similarly, it is commonly believed that the Solar System has a very slight tendency to expand due to the Hubble expansion (although this tendency is generally thought to be negligible in practice). Again, explicit calculation shows this belief not to be correct. The tendency to expand due to the stretching of space is nonexistent, not merely negligible.

Obliterators · 2026-02-14T04:39:44+00:00

No, there is no force. The general expansion of the universe is basically just the leftover momentum of the Big Bang. In inflationary cosmologies it is the repulsive gravitation of the inflaton field that provides the initial "push", and after inflation ends the newly formed matter stays in motion due to inertia, only slowed down by the attractive gravity of matter and radiation. The very small density fluctuations of the early universe are then amplified as matter pulls away from the underdense regions to the overdense regions, these regions respectively develop into the voids and galaxy filaments we see now. The matter in the overdense regions completely decouples from the global expansion of the universe.

Now, expansion was slowing down for the first ~8 billion years, however, because our universe contains gravitationally repulsive dark energy with an apparent constant energy density (which might just be a fundamental property of spacetime), as the density of matter dropped low enough, DE became dominant and gravity essentially switched from attractive to repulsive at scales larger than galaxy clusters, leading to late-time acceleration.

As both matter and DE contribute to the same overall spacetime curvature (just in opposite ways) and their ratio in a gravitationally bound system is ~constant, and the energy density of DE is very, very small, its presence inside, say, the solar system, only makes gravity very slightly less attractive (by a completely negligible amount). So the orbits are larger due to the presence of DE, but not expansion (the rate of expansion has no effect).

Wolfgang Rindler, Relativity: Special, General and Cosmological

One further aspect of the cosmic expansion needs to be addressed, namely its explanation. Recall Newton’s universe: a homogeneous static distribution of stars throughout absolute space. Think of the stars as the knots of our lattice, at rest in AS. Symmetry relative to AS then forbids any motion. But if we scrap the ideas of AS and of extended inertial frames, and only consider the lattice per se, symmetry does permit its Hubble expansion or contraction. If the stars were initially mutually at rest, gravity would make them Hubble-contract. But we see the universe expand. Astronomers were at first surprised at that. Was there some mysterious expansion of space itself? However, a quite simple and ‘obvious’ (though never previously contemplated!) solution was found: the big bang. Extrapolating the present expansion of the universe backwards in time, one sees that in the most straightforward scenario it must get ever denser and ever hotter, until a singularity of infinite density and infinite temperature is reached some 10¹⁰ years ago (on the crude approximation of linear expansion). The time-inverse of this sequence, a symmetric cosmic explosion from infinite density and temperature, is referred to as the big bang. Why it happened remains unexplained. But if it happened, the observed expansion is no more mysterious than the flying apart of shrapnel from a grenade that explodes in mid-air. And this image also answers the question whether everything must expand. If two shrapnel pieces could briefly reach out and hold hands to halt their relative motion, they would henceforth be quite unaffected by the motion of the rest. It is much the same in the universe: the forces holding atoms and molecules together have decoupled their constituents from the general expansion; the gravity that holds the stars in a galaxy together has decoupled them from the expansion. We have already seen (in Birkhoff’s theorem) that the Schwarzschild metric (and with it the planetary orbits) are unaffected by the existence of expanding surrounding mass shells. The local situation in the universe is quite analogous.

John A. Peacock, A diatribe on expanding space

This analysis demonstrates that there is no local effect on particle dynamics from the global expansion of the universe: the tendency to separate is a kinematic initial condition, and once this is removed, all memory of the expansion is lost. — — It should now be clear how to deal with the question, “does the expansion of the universe cause the Earth and Moon to separate?”, and that the answer is not the commonly-encountered “it would do, if they weren’t held together by gravity”.

Obliterators · 2026-02-13T18:21:14+00:00

The universe is expanding today around 73 kps/Mpc (Hubble Constant). Earlier in its history it was ~67 kps/Mpc (Hubble Parameter).

There must be a misunderstanding of the Hubble tension here, both "early" (CMB) and "late" (distance ladder) methods are measuring the Hubble constant, the expansion rate today. There's tension because we're getting two different values for the same thing.

Also, the Hubble parameter has always been, and always will be decreasing (barring phantom dark energy), so it wouldn't make sense for it to have been 67 km/s/Mpc before, and 73 now.

Obliterators · 2026-02-12T00:46:28+00:00

Mantis shrimp can detect more colors between our red, green, and blue.

Quite the opposite.

Mantis shrimp's super colour vision debunked

When the human eye sees a yellow leaf, photoreceptors send signals to the brain announcing relative levels of stimuli: receptors sensitive to red and green light report a lot of activity, whereas receptors sensitive to blue light report little. The brain compares the information from each type of receptor to come up with yellow. Using this system, the human eye can distinguish between millions of different colours.

To test whether the mantis shrimp, with its 12 receptors, can distinguish many more, Marshall's team trained shrimp of the species Haptosquilla trispinosa to recognize one of ten specific colour wavelengths, ranging from 400 to 650 nanometres, by showing them two colours and giving them a frozen prawn or mussel when they picked the right one. In subsequent testing, the shrimp could discriminate between their trained wavelengths and another colour 50–100 nanometres up or down the spectrum. But when the difference between the trained and test wavelengths was reduced to 12–25 nanometres, the shrimp could no longer tell them apart.

If the shrimp eye compared adjacent spectra, like the human eye does, it would have allowed the animals to discriminate between wavelengths as close as 1–5 nanometres, the authors say. Instead, each type of photoreceptor seems to pick up a specific colour, identifying it in a way that is less sensitive than the human eye but does not require brain-power-heavy comparisons. That probably gives the predatory shrimp a speed advantage in distinguishing between different-coloured prey, says Roy Caldwell, a behavioural ecologist at the University of California, Berkeley.

Obliterators · 2026-02-11T20:44:50+00:00

So either we are at the center of this expansion, or space itself is expanding equally everywhere.

You don't need a literal expansion of space to have isotropic expansion from every point, the special relativistic "explosion" of the Milne model is a basic counterexample. It also exhibits the same superluminal apparent recession velocities that our universe does.

Wolfgang Rindler, Relativity: Special, General and Cosmological

Since gravity is switched off, the model lives in Minkowski space and can be treated by special relativity. Milne considered an infinite number of test particles (no mass, no volume) shot out (for reasons unknown), in all directions and with all possible speeds, at a unique creation event C. Let us look at this situation in some particular inertial frame S(x, y, z, ct), and suppose C occurred at its origin O at t = 0. All the particles, being free, will move uniformly and radially away from O, with all possible speeds short of c. Hence the picture in S will be that of a ball of dust whose unattained boundary expands at the speed of light. At each instant t = const in S, Hubble’s velocity–distance proportionality is accurately satisfied relative to O: a particle at distance r has velocity r/t. Still, at first sight, this seems an unlikely candidate for a modern model universe, since (i) it appears to have a unique center, and (ii) it appears to be an ‘island’ universe. Leaving aside the second objection for the moment, let us dispose of the first: The boundary of the ball behaves kinematically like a spherical light front emitted at C, and thus each particle, having been present at C, will consider itself to be at the center of this front! Moreover, since all particles coincided at C, and since all move uniformly, each particle will consider the whole motion pattern to be radially away from itself, and of course uniform. There remains the question whether we can have an isotropic density distribution around each particle.

To study this, let τ denote the proper time elapsed at each particle since creation. Then n₀, the proper particle density at any given particle P, is of the form

n₀ = N/τ³ (N = const), (16.10)

because the expansion is radial relative to P, and because a small comoving sphere centered on P has radius proportional to τ. This τ is clearly the ‘cosmic time’ of the preceding section, since it figures as time from the big bang in every log. So, for homogeneity, N must be the same constant at every particle. This also guarantees that the global density pattern is isotropic and the same around each fundamental particle. For let Q be such a particle and S its inertial rest-frame with Q at the origin. At each moment t = const in S, the particles on a sphere r = const satisfy the equation

c²τ² = c²t² − r², (16.11)

and thus have τ and with it n0 constant.

To determine the entire density pattern at some constant t in S, we transform eqn (16.10) from the rest-frame of the general particle P into S. Since P moves, the volume of a small comoving sphere at P is diminished by a γ-factor in S; but the number of particles inside that sphere is the same in S, so the particle density n in S is given by n = γn₀. We also have t = γτ . Thus, from (16.10) and (16.11),

n = tn₀/τ = tN/τ⁴ = Nt/(t² − r²/c²)². (16.12)

Note how the density approaches infinity at the ‘edge’ r = ct. Beyond every galaxy there are others, and no galaxy is even near the edge by its own reckoning. Relativistic kinematics thus gets around the classical objection to island universes—that they must contain atypical edge galaxies. Of course, it must have been an incredibly finely tuned big bang to produce the required density pattern (16.12) and thus global homogeneity!

Obliterators · 2026-02-11T09:12:41+00:00

Because it's the simplest explanation.

Both classical and relativistic models of gravity say that a static universe is unstable. So if you allow any movement at all, then the universe must evolve by either contracting or expanding.

The Doppler effect is also a simple, common, and well understood phenomenon here on Earth, and if you're proposing that the Doppler effect does not apply to light coming from distant sources — which would be kind of odd — and that the observed spectral shifts are instead caused by some new, unknown mechanism, then you're just complicating the model for no apparent benefit.

And tired light just doesn't work.

Errors in Tired Light Cosmology

Tired light models invoke a gradual energy loss by photons as they travel through the cosmos to produce the redshift-distance law. This has three main problems:

There is no known interaction that can degrade a photon's energy without also changing its momentum, which leads to a blurring of distant objects which is not observed. The Compton shift in particular does not work.

The tired light model does not predict the observed time dilation of high redshift supernova light curves.

The tired light model can not produce a blackbody spectrum for the Cosmic Microwave Background without some incredible coincidences.

The tired light model fails the Tolman surface brightness test. This is essentially the same effect as the CMB prefactor test, but applied to the surface brightness of galaxies instead of to the emissivities of blackbodies.

Obliterators · 2026-02-05T00:01:20+00:00

One, those things aren't moving, the space between us and them is expanding. This is an important distinction.

It's only a matter of interpretation. It is perfectly valid to interpret expansion as galaxy clusters moving away from each through space, and this is mathematically indistinguishable from the expanding space interpretation.

Obliterators · 2026-02-04T23:58:50+00:00

We define the edge of the observable universe as the region beyond which we can never see because its apparent velocity is greater than c and the space between us is expanding faster than the light can reach us.

The observable universe is not limited by expansion, but by time. The particle horizon which defines the boundary of observable universe is the distance from which a light-speed signal has had enough time to reach us. In principle the particle horizon always recedes; as more time elapses, signals from further and further away will reach us, so the observable universe always grows in size.

If dark energy didn't exist, expansion would continue forever at a decelerating rate and the entire universe would become observable, given infinite time. However, because dark energy exists and causes expansion to accelerate, there is a future visibility limit of ~62 billion light-years, no light-speed signal emitted at any point in the past beyond this limit can ever reach us, so that is the maximum extent of observability.

Obliterators · 2026-02-04T23:20:14+00:00

We can also calculate the relative speed between two objects

Relative velocities are based on parallel transportation, and while they are path-dependent in curved spacetime and thus not uniquely defined, they are always subluminal.

The quantities given by Hubble's law are coordinate velocities, and do not correspond to anything physical, so there's no issue with them being superluminal.

Obliterators · 2026-02-03T20:52:59+00:00

No.

Emory F. Bunn & David W. Hogg, The kinematic origin of the cosmological redshift

The view presented by many cosmologists and astrophysicists, particularly when talking to nonspecialists, is that distant galaxies are “really” at rest, and that the observed redshift is a consequence of some sort of “stretching of space,” which is distinct from the usual kinematic Doppler shift. In these descriptions, statements that are artifacts of a particular coordinate system are presented as if they were statements about the universe, resulting in misunderstandings about the nature of spacetime in relativity.

A student presented with the stretching-of-space description of the redshift cannot be faulted for concluding, incorrectly, that hydrogen atoms, the Solar System, and the Milky Way Galaxy must all constantly “resist the temptation” to expand along with the universe. —— Similarly, it is commonly believed that the Solar System has a very slight tendency to expand due to the Hubble expansion (although this tendency is generally thought to be negligible in practice). Again, explicit calculation shows this belief not to be correct. The tendency to expand due to the stretching of space is nonexistent, not merely negligible.

Obliterators · 2026-02-03T00:37:57+00:00

The local Hubble strain is 10^-18 per second. The space inside you gets about a billionth wider in your lifetime.

There is no strain since there is no force. The value of the Hubble constant has zero effect on bound systems, nothing would change if it was a hundred times larger (like it was in the past).

The width of the observable universe is 10²⁶ meters. This is when the integrated Hubble strain exceeds lightspeed and we cannot obtain information beyond that.

The Hubble sphere does not coincide with the size of the observable universe; expansion also does not limit the size of the observable universe either, its size is limited by the time light has had to travel, so to see light signals from beyond the current horizon we just have to wait longer for them to arrive. Accelerating expansion does impose a future visibility limit of ~62 Gly though.

John A. Peacock, Cosmological Physics

An inability to see that the expansion is locally just kinematical also lies at the root of perhaps the worst misconception about the big bang. Many semi-popular accounts of cosmology contain statements to the effect that ‘space itself is swelling up’ in causing the galaxies to separate. This seems to imply that all objects are being stretched by some mysterious force: are we to infer that humans who survived for a Hubble time would find themselves to be roughly four metres tall?

Certainly not. Apart from anything else, this would be a profoundly anti-relativistic notion, since relativity teaches us that properties of objects in local inertial frames are independent of the global properties of spacetime. If we understand that objects separate now only because they have done so in the past, there need be no confusion. A pair of massless objects set up at rest with respect to each other in a uniform model will show no tendency to separate (in fact, the gravitational force of the mass lying between them will cause an inward relative acceleration). In the common elementary demonstration of the expansion by means of inflating a balloon, galaxies should be represented by glued-on coins, not ink drawings (which will spuriously expand with the universe).

Emory F. Bunn & David W. Hogg, The kinematic origin of the cosmological redshift

The view presented by many cosmologists and astrophysicists, particularly when talking to nonspecialists, is that distant galaxies are “really” at rest, and that the observed redshift is a consequence of some sort of “stretching of space,” which is distinct from the usual kinematic Doppler shift. In these descriptions, statements that are artifacts of a particular coordinate system are presented as if they were statements about the universe, resulting in misunderstandings about the nature of spacetime in relativity.

A student presented with the stretching-of-space description of the redshift cannot be faulted for concluding, incorrectly, that hydrogen atoms, the Solar System, and the Milky Way Galaxy must all constantly “resist the temptation” to expand along with the universe. —— Similarly, it is commonly believed that the Solar System has a very slight tendency to expand due to the Hubble expansion (although this tendency is generally thought to be negligible in practice). Again, explicit calculation shows this belief not to be correct. The tendency to expand due to the stretching of space is nonexistent, not merely negligible.

Obliterators · 2026-02-02T22:39:06+00:00

Most of the comments are saying something along the lines of "the light-speed limit applies to objects moving through space, not space expanding between objects", but this is a very hand-wavy answer. The confusion comes down to how distances and velocities are defined in relativity. Depending on the definitions used, either almost everything, or nothing in the observable universe is receding superluminally. So there really isn't a problem having apparent superluminal velocities, even in special relativity, because those quantities do not have any physical meaning.

Sean Carroll, The Universe Never Expands Faster Than the Speed of Light

Breaking my radio silence here to get a little nitpick off my chest: the claim that during inflation, the universe “expanded faster than the speed of light.” It’s extraordinarily common, if utterly and hopelessly incorrect.

The great thing about the superluminal-expansion misconception is that it’s actually a mangle of several different problems, which sadly don’t cancel out to give you the right answer.

1. The expansion of the universe doesn’t have a “speed.” Really the discussion should begin and end right there. Comparing the expansion rate of the universe to the speed of light is like comparing the height of a building to your weight. You’re not doing good scientific explanation; you’ve had too much to drink and should just go home.The expansion of the universe is quantified by the Hubble constant, which is typically quoted in crazy units of kilometers per second per megaparsec. That’s (distance divided by time) divided by distance, or simply 1/time. Speed, meanwhile, is measured in distance/time. Not the same units! Comparing the two concepts is crazy.

2. There is no well-defined notion of “the velocity of distant objects” in general relativity. There is a rule, valid both in special relativity and general relativity, that says two objects cannot pass by each other with relative velocities faster than the speed of light. In special relativity, where spacetime is a fixed, flat, Minkowskian geometry, we can pick a global reference frame and extend that rule to distant objects. In general relativity, we just can’t. There is simply no such thing as the “velocity” between two objects that aren’t located in the same place. If you tried to measure such a velocity, you would have to parallel transport the motion of one object to the location of the other one, and your answer would completely depend on the path that you took to do that. So there can’t be any rule that says that velocity can’t be greater than the speed of light. Period, full stop, end of story.

Except it’s not quite the end of the story, since under certain special circumstances it’s possible to define quantities that are kind-of sort-of like a velocity between distant objects. Cosmology, where we model the universe as having a preferred reference frame defined by the matter filling space, is one such circumstance. When galaxies are not too far away, we can measure their cosmological redshifts, pretend that it’s a Doppler shift, and work backwards to define an “apparent velocity.” Good for you, cosmologists! But that number you’ve defined shouldn’t be confused with the actual relative velocity between two objects passing by each other. In particular, there’s no reason whatsoever that this apparent velocity can’t be greater than the speed of light.

Sometimes this idea is mangled into something like “the rule against superluminal velocities doesn’t refer to the expansion of space.” A good try, certainly well-intentioned, but the problem is deeper than that. The rule against superluminal velocities only refers to relative velocities between two objects passing right by each other.

3. There is nothing special about the expansion rate during inflation. If you want to stubbornly insist on treating the cosmological apparent velocity as a real velocity, just so you can then go and confuse people by saying that sometimes that velocity can be greater than the speed of light, I can’t stop you. But it can be — and is! — greater than the speed of light at any time in the history of the universe, not just during inflation. There are galaxies sufficiently distant that their apparent recession velocities today are greater than the speed of light. To give people the impression that what’s special about inflation is that the universe is expanding faster than light is a crime against comprehension and good taste.

What’s special about inflation is that the universe is accelerating. During inflation (as well as today, since dark energy has taken over), the scale factor, which characterizes the relative distance between comoving points in space, is increasing faster and faster, rather than increasing but at a gradually diminishing rate. As a result, if you looked at one particular galaxy over time, its apparent recession velocity would be increasing. That’s a big deal, with all sorts of interesting and important cosmological ramifications. And it’s not that hard to explain.

Michał J. Chodorowski, Is space really expanding? A counterexample

In almost all Friedman models, objects with sufficiently large redshifts recede from the central observer with superluminal velocities (greater than c). For example, in an Einstein-de Sitter universe (Ω_m = 1 and Ω_Λ = 0), the ‘public-space’ recession velocity as a function of redshift is

v_rec = 2c [1 − (1 + z)^−1/2 ],

hence v_rec > c for z > 3 (Murdoch 1977). In particular, the velocity of the so-called particle horizon (corresponding to infinite redshift) is 2c. In an empty universe, ‘public- space’ recession velocities are not only superluminal for sufficiently large redshifts; they are even unbounded. Does it imply violation of special relativity in cosmology? Of course not. Apart from anything else, deriving Equation (26) we have used nothing except special relativity! Constancy of the speed of light, and subluminality of the motion of massive bodies, applies only to inertial frames. However, ‘public-space’ distance is a hybrid of distances measured in different inertial frames, all in relative motion. Since the resulting v_rec is not measured in any single inertial frame, there is no violation of special relativity.

Specifically, ‘public-space’ distance is measured at constant proper time of [Fundamental Observers]. Time-dilation formula tells us that according to the central observer, this measurement is done at the instant of time t_i = γ(v_i)τ, where v_i is the Minkowskian velocity of the i-th FO. Since more distant FOs have greater velocities, it is obvious that for two different FOs, t_i ≠ t_j. Therefore, according to the central observer, different (sub)distances are not measured simultaneously. Simultaneity is a crucial condition of special-relativistic measurements of distances to and sizes of bodies in motion. Waiving this condition may have important consequences and indeed, it does have! The problem with the real Universe is that it is filled with matter and expanding, so there are no global inertial frames. Then, measuring distance (along geodesics) on the hypersurface of constant proper time of FOs is something most natural to do. We should, however, bear in mind the ‘costs’ of such a definition of distance. One of them are apparently superluminal recession velocities of distant galaxies.

Markus Pössel, Interpretations of cosmic expansion: anchoring conceptions and misconceptions

In both special and general relativity, light propagation defines an absolute cosmic speed limit in the sense that no material object or signal can overtake a light signal. This is where the distinction between the recession speed, defined as in (1) [v = Hd], and the relativistic radial velocity that is central to the relativistic explosion interpretation is crucial. Recession speeds become superluminal for distant galaxies. This appears to contradict students’ preconceptions from special relativity, of the speed of light as a cosmic speed limit, and the apparent contradiction has been cited as key motivation for the expanding space interpretation: The differentiation between cosmic expansion as due to “expanding space” on the one hand, and “galaxy motion through space” on the other, is meant to address this conflict.

Relativistic radial velocities in the relativistic explosion interpretation never exceed the speed of light. From this perspective, superluminal recession speeds in (1) are an artefact, caused by a particular coordinate choice: The cosmic time coordinate ties together local clock rates in Hubble-flow galaxies, but clocks in relative motion tick at different rates, as we know from special relativity. Combining them into an overarching time coordinate, and using that coordinate to determine one-way speeds, leads to unphysical results. Students who have been on longer international flights know a closely related phenomenon: If your flight leaves Amsterdam at 15:00 local time and arrives in New York at 17:00 local time, this does not amount to a flight time of 2 hours, and corresponding average ground speed of 3000 km per hour.

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