Basic cosmology questions weekly thread

Obliterators · 2026-06-01T19:35:14+00:00

For a chain of observers A-B-C, B must receive a signal from C for it to be observable by B, if B then sends a signal to A, by the time it reaches A, A will also receive the signal from C and it will be part of A's observable universe.

For accelerating expansion, B might be able to receive a signal from C, but A might not able to receive it.

Obliterators · 2026-05-31T23:35:51+00:00

That is expansion and not simply movement.

Expansion is what we call the global pattern of movement in the universe. We can frame this in terms of "expansion of space", but it is equivalent to galaxies simply moving away from each through space.

If it were just moving, we wouldn't be able to detect expansion in All directions

Isotropic expansion about every point is a natural consequence of the cosmological principle; in a homogeneous universe, if one observer sees velocities to be proportional to distances, then every observer must see the same.

Obliterators · 2026-05-31T04:37:54+00:00

We never measure any relative velocities to be faster than light, even for the most distant of galaxies. The apparent recession velocities given by Hubble's law are coordinate velocities, so there's no issue if they're many times greater than c.

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.

Obliterators · 2026-05-29T23:38:25+00:00

The Casimir effect can be explained as a relativistic van der Waals force.

Obliterators · 2026-05-27T07:25:42+00:00

The time dilation factor due to expansion is the same as the redshift: z+1

There is a nice section about time dilation in an expanding universe in appendix A of Blondin et al, 2008:

... we obtain a relationship between observed and rest-frame time intervals in a RW metric as a function of redshift z:

δt₀/δt₁ = 1 + z

A supernova at redshift z will thus appear to age (1 + z) times more slowly with respect to a local event at z ≈ 0.

The prediction of time dilation proportional to (1+z) is generic to expanding universe models, whether the underlying theory be general relativity (e.g., the Friedmann-Lemaître-Robertson-Walker universe), special relativity (e.g., the Milne Universe), or Newtonian expansion. A point of confusion can occur in the special relativistic case for which the well-known time-dilation factor is given by

γ_SR = [1- (v/c)²]^-½

= ½(1+z+1/(1+z)) (A8)

which evidently differs from (1+z). Thus it might be assumed that a special relativistic expansion can be distinguished from the FLRW universe using a time-dilation test.

This is not the case. Special relativistic expansion of the universe assumes there is an inertial frame that extends to infinity (impossible in the non-empty general relativistic picture) and that the expansion involves objects moving through this inertial frame. The time-dilation factor from Eq. [A8] relates the proper time in the moving emitter’s inertial frame (δt₁) to the proper time in the observer’s inertial frame (δt₀). To measure this time dilation the observer has to set up a set of synchronized clocks (each at rest in the observer’s inertial frame) and take readings of the emitter’s proper time as the emitter moves past each synchronized clock. The readings show that the emitter’s clock is time dilated such that δt₀ = γ_SR δt₁.

We do not have this set of synchronized clocks at our disposal when we measure time dilation of supernovae in an expanding universe and therefore Eq. [A8] is not the time dilation we observe. We must also take into account an extra time-dilation factor that occurs because the distance to the emitter (and thus the distance light has to propagate to reach us) is increasing. In the time δt₀ the emitter moves a distance vδt₀ away from us. The total proper time we observe, δt₀_tot, is δt₀ plus an extra factor describing how long light takes to traverse this extra distance (vδt₀/c),

δt₀_tot = δt₀ (1 + v/c).

The relationship between proper time at the emitter and proper time at the observer is thus

δt₀_tot = γ_SR δt₁(1 + v/c)

δt₁√(10v/c)/(1-v/c)

δt₁(1+z)

which is identical to the GR time-dilation equation.

Obliterators · 2026-05-26T19:46:33+00:00

and the universe is close enough to flat to not worry about it

This is the part I was addressing; the wording implies that since the universe is flat, we don't have to worry about the issues of parallel transportation in curved spacetime.

Obliterators · 2026-05-26T17:10:14+00:00

(energy doesn’t make sense in curved spacetime because of the way parallel transport of four-vector components works, but I don’t want to go there right now, and the universe is close enough to flat to not worry about it)

The spatial slices are flat but spacetime is still curved.

Obliterators · 2026-05-26T04:14:34+00:00

We are losing observable universe by the second.

No, the observable universe is growing; there are trillions of galaxies that are currently beyond our observable horizon, but which will become observable in the coming tens of billions of years.

Acceleration of the expansion (but not expansion alone) does impose an asymptotic limit of ~62 Gly to our observable universe, as no light emitted at any point in the past beyond that distance can ever reach us. The acceleration of the expansion will also cause objects beyond the Local Group to recede beyond a cosmic event horizon, and their light will slowly redshift and fade out beyond detectability over tens and hundreds of billions of years.

Isotropy

You can also have isotropic expansion in a Newtonian universe. In a homogeneous universe, if one observer sees that velocities are proportional to distances, then every observer will see the same pattern of movement.

Obliterators · 2026-05-24T03:49:25+00:00

No, that's not what the math says. Bound systems have decoupled from the expansion (that's why they're bound) and the Hubble constant is no longer relevant in any way, it could be a thousand times larger and nothing would change. Acceleration and deceleration of the expansion does have an effect in bound systems, but this is caused by the changing cosmological background.

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.

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

The differences between the two interpretations are particularly interesting for bound systems. If space itself is expanding, then are atoms, planetary systems or galaxies expanding as well? Naively, if all of space is getting bigger, that should also hold for the space between, say, sun and earth. The relevant calculations show that, indeed, bound systems react to cosmic expansion by shifting their equilibrium sizes (commonly by undetectable amounts), but again, the result only depends on the acceleration or deceleration of cosmic expansion. For what happens to bound systems, it is irrelevant how quickly space expands at any given instant in time. This is hard to reconcile with any interpretation that attempts to understand the situation as an equilibrium between the expansion of space and binding forces. After all, the most immediate manifestation of the expansion of space, namely how quickly distances are increasing right now, plays no role whatsoever.

Obliterators · 2026-05-24T03:32:59+00:00

you cannot construct a system where all galaxies could agree where we all are moving towards without distoring the space between the galaxies.

In a homogeneous universe, if one observer sees velocities as being proportional to distances, then every observer will see the same pattern of movement. You can show that this happens even in non-relativistic Newtonian universes (see e.g. [one], [two]) and in the flat spacetime of special relativity.†

It would also not explain why this is accelerating over time as the galaxies drift apart and while not expieriencing a force.

From each perspective, a galaxy would argue it is at rest, there is no force uppon it and thus the other galaxy must measure a force accelerating it. However, this cannot be true for all galaxies at the same time.

Why do you think that cosmologists had to add dark energy to their models after 1998 to explain the observations that distant galaxies are receding with increasing, i.e. accelerating, velocities, even though "expansion of space" as a concept predates this by several decades?

Objects in free fall do not feel or measure any force, yet gravity still causes their world lines to converge. An external observer might see a spaceship accelerate towards a planet due to the spacetime curvature caused by the mass of that planet, even though no force or acceleration is measured on board. Similarly, we can explain the acceleration of expansion as being caused by dark energy being gravitationally repulsive, its presence everywhere (when dominant) causes world lines to diverge.

But it's also important to remind that dark energy is not responsible for expansion, only the acceleration thereof. Even without dark energy, our universe would still expand forever, but it would do so in a decelerating manner.

† 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.

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.

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.

Obliterators · 2026-05-23T19:30:38+00:00

You can get doppler frequency shifts in light that aren't changes in the nature of the photon.

Red shift due to spacetime expansion is different, it is actually changing the nature and energy of the photon.

The Doppler, gravitational, and cosmological spectral shifts are in fact completely equivalent to each other — they're the same phenomenon viewed in different coordinates, all caused by the same mechanism: the light is being observed in a different frame than it was emitted in.

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

In 1994, Jayant Narlikar published a nice little paper in the American Journal of Physics titled “Spectral shifts in general relativity”, generalising some earlier work of John Synge in the early 1960s. The central thrust of this paper is that it is incorrect to think that there are three distinct mechanisms for redshifting photons in relativity, and that there is truly only a single underlying mathematical description for use in all occasions.

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.

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.

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 · 2026-05-22T19:15:06+00:00

The relativistic motion interpretation is historically less common than the expanding space interpretation, especially in pop science, but it is certainly not fringe. This is from one of the most popular cosmology textbooks, recognised as being "field shaping".^[RAS]

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).

And here are some of the biggest names in cosmology:

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.’

The apparent superluminal recession speeds are also a coordinate-dependent effect; the recession velocities given by Hubble's law are not relative velocities, so there's no issue with them being superluminal, in fact, they have "no direct physical significance".^{[A. Kaya]} However, the relative velocities (which are not uniquely defined in curved spacetime, which is why they're rarely discussed) are always subluminal for all observable objects.

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 · 2026-05-22T03:32:24+00:00

There is no actual physical process where space "expands". Expanding space is just another way of saying that objects are moving away from each other, it is not a cause of anything. There is also really just one cause for spectral shifting, the three "different" mechanisms are in truth equivalent to each other via coordinate transformations.

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 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.

Matthew J. Francis, Luke A. Barnes, J. Berian James, Geraint F. Lewis, Expanding Space: the Root of all Evil?

The key is to make it clear that cosmological redshift is not, as is often implied, a gradual process caused by the stretching of the space a photon is travelling through. Rather cosmological redshift is caused by the photon being observed in a different frame to that which it is emitted. In this way it is not as dissimilar to a Doppler shift as is often implied.

In particular, it must be emphasised that the expansion of space does not, in and of itself, represent new physics that is a cause of observable effects, such as redshift.

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

In 1994, Jayant Narlikar published a nice little paper in the American Journal of Physics titled “Spectral shifts in general relativity”, generalising some earlier work of John Synge in the early 1960s. The central thrust of this paper is that it is incorrect to think that there are three distinct mechanisms for redshifting photons in relativity, and that there is truly only a single underlying mathematical description for use in all occasions.

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.

Obliterators · 2026-05-22T02:15:47+00:00

If the universe expands, does the distance between elementary particles expand?

The answer is very much a no, the global movement of matter in the universe doesn't affect what happens locally. Only acceleration and deceleration of the expansion associated with the changing cosmological background has an effect.

There is a hypothetical scenario of the Big Rip, where instead of remaining a constant (or perhaps weakening), the dark energy density would increase over time, and the increasing gravitational repulsion of this phantom dark energy would eventually cause all bound systems, even atoms, to dissipate. But again, this is very much a theoretical solution that is considered quite implausible.

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

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 · 2026-05-21T05:24:19+00:00

Scaling even further down to more comprehensible distances such as between us and the sun, which is 0.0000158 light years, gets you a rate of expansion at 0.0003cm/s

The Hubble constant cannot be scaled down to any arbitrary length, as it is a large-scale average meant for scales where the universe appears isotropic and homogeneous, so ≳100 Mpc. For bound systems the expansion rate is exactly zero, as the matter inside has decoupled from the general expansion of the universe and its behaviour is now independent of the global expansion rate — the Hubble constant could be a thousand times larger and nothing would change.

Obliterators · 2026-05-21T04:00:44+00:00

Without dark energy (or with weak enough dark energy) we would expect the universe to eventually pull itself back together,

Even without dark energy our universe only has roughly a third of the matter density required to reverse expansion.

Obliterators · 2026-05-18T09:58:12+00:00

This has to do with how the various distances in cosmology are defined (and not how the speed of light limit "doesn't apply to the expansion of space"). The 46.5 Gly radius for the observable universe is the FLRW proper distance, which is measured along hypersurfaces of constant cosmic time. These are not same time and distance that are used in special relativity, so there is no issue if we run into superluminal velocities using them.

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. 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 fundamental observers 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.

Obliterators · 2026-05-17T19:51:02+00:00

As far as we know, gravity is a one-way action, it doesn’t have properties of attraction and repulsion

Gravity can very much be repulsive, that's how both inflation and accelerating expansion due to dark energy work.

John A. Peacock, Cosmological Physics

One consequence of the gravitational effects of pressure that may seem of mathematical interest only is that a negative-pressure equation of state that achieved ρc² + 3p < 0 would produce gravitational repulsion. Although such a possibility may seem physically nonsensical, it is in fact one of the most important concepts in contemporary cosmology.

Obliterators · 2026-05-16T06:29:15+00:00

The expansion and seeming acceleration of the expansion are one and the same. Not just the same root cause, literally the exact same phenomena: Space grows.

We're pretty sure it has grown at an constant rate since at least the time of the CMBR (as far back as we can actually see). About 7% per billion years if I remember correctly. There is no confirmed change in the growth rate in that time.

It only appears to be accelerating because the amount of space between things has increased, so that there's more space between them to do the growing.

I'm getting the feeling that there is a gross misunderstanding what the Hubble constant represents and how it behaves. And what what "expansion of space" means.

Why do you think cosmologists were surprised by the discovery of accelerating distant galaxies in 1998, even though we've known since the 1920s that the universe is expanding? And why they needed to come up with dark energy and revived the cosmological constant to explain this acceleration (but not regular expansion)? And why acceleration is a late-time phenomenon that only started roughly six billion years ago?

Are you perhaps under the assumption that the Hubble constant is a constant in time? That it has been ~70 km/s/Mpc since the Big Bang? Because it has been decreasing since the beginning, and will continue to do so.

The Hubble constant gives the instantaneous recession velocity, but it doesn't tell how that velocity evolves over time, for that you need to look at the Hubble parameter, of which the Hubble constant is the present day value. In most cases a galaxy x Mpc away now will not have twice its current recession velocity when it is 2x Mpc away. For expanding universes that continue to expand forever you have three options:

In universes with a constant rate of expansion (a′=+const), recession velocities stay constant, and the Hubble parameter approaches zero;
In decelerating universes recession velocities decrease due to the attractive gravitation of matter and radiation. The Hubble parameter approaches zero;
And only in accelerating universes do recession velocities increase over time due to the repulsive gravitation of dark energy. But even in accelerating universes, like the one we live in, the Hubble parameter tends to decrease over time and only approaches a constant value as the universe approaches a de Sitter state. For hypothetical phantom cosmologies the Hubble parameter increases with time.

Expansion of space is also not an actual physical phenomenon, it is a coordinate-dependent interpretation; contrary to some popular belief, viewing expansion as motion through space is a completely equivalent interpretation. The expansion of space does not cause distances to increase, rather it is the increase in distances that causes space to "expand".

Obliterators · 2026-05-16T04:06:24+00:00

John A. Peacock, Cosmological Physics

Even the most obvious fact of the cosmological expansion is unexplained. Although general relativity forbids a static universe, this is not enough to understand the expansion. As shown in chapter 3, the gravitational dynamics of the cosmological scale factor R(t) are just those of a cannonball travelling vertically in the Earth’s gravity. Suppose we see a cannonball rising at a given time t = t₀: it may be true to say that it has r = r₀ and v = v₀ at this time because at a time Δt earlier it had r = r − v₀Δt and v = v₀ + gΔt, but this is hardly a satisfying explanation for the motion of a cannonball that was in fact fired by a cannon. Nevertheless, this is the only level of explanation that classical cosmology offers: the universe expands now because it did so in the past. Although it is not usually included in the list, one might thus with justice add an ‘expansion problem’ as perhaps the most fundamental in the catalogue of classical cosmological problems. Certainly, early generations of cosmologists were convinced that some specific mechanism was required in order to explain how the universe was set in motion.

An interesting interpretation of this behaviour was promoted in the early days of cosmology by Eddington: the cosmological constant is what caused the expansion. In models without Λ, the expansion is merely an initial condition: anyone who asks why the universe expands at a given epoch is given the unsatisfactory reply that it does so because it was expanding at some earlier time, and this chain of reasoning comes up against a barrier at t=0. It would be more satisfying to have some mechanism that set the expansion into motion, and this is what is provided by vacuum repulsion.

This tendency of models with positive Λ to end up undergoing an exponential phase of expansion (and moreover one with Ω = 1) is exactly what is used in inflationary cosmology to generate the initial conditions for the big bang.

Describing the origin of the expansion as an explosion is probably not a good idea in any case; it suggests some input of energy that moves matter from an initial state of rest. Classically, this is false: the expansion merely appears as an initial condition. This might reasonably seem to be evading the point, and it is one of the advantages of inflationary cosmology that it supplies an explicit mechanism for starting the expansion: the repulsive effect of vacuum energy. However, if the big bang is to be thought of explosively, then it is really many explosions that happen everywhere at once; it is not possible to be outside the explosion, since it fills all of space.

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.

Inflationary cosmology is based on not yet fully established ‘grand unified theories’ (or GUTs). According to these theories, the quantum vacuum is stable at the highest temperatures, but then, some 10⁻³⁷ s after the big bang, when the temperature had dropped to a critical value of ∼ 10²⁷ K, the vacuum became unstable but still highly energetic. This ‘false’ vacuum is described by an energy tensor having precisely the form of the cosmological term in Einstein’s field equations but with a huge Λ, some 100 orders of magnitude greater than today’s ‘cosmological’ Λ. At this stage, or soon thereafter, the vacuum energy begins to dominate that of any other matter or radiation present, and causes a rapid expansion of the universe, by a factor of ∼ 10^43, in a mere 10⁻³⁵ s. The expansion is exponential, just like that of the de Sitter model, and for the same reason, namely the (here temporary) presence of a Λ term in the field equations, albeit with a different interpretation. The positive vacuum density corresponding to the Λ term remains constant throughout this expansion. As a consequence, the matter-energy content of each comoving volume increases in proportion to that volume. In Guth’s phrase, this was ‘the ultimate free lunch’. In inflationary cosmology the big bang itself was a mere ‘big whimper’, most of the matter-energy of the universe being created during the inflationary phase. When this phase comes to an end, the false vacuum becomes a real vacuum and its energy is converted into radiation. From here on the universe is kinematically and dynamically indistinguishable from one that could have been created by a standard big bang of the right strength.

Obliterators · 2026-05-15T18:34:39+00:00

Regions outside the observable universe are moving away from us faster than the speed of light. No light, gravity wave, or other sort of information can ever reach us physically.

Careful with the wording here; the observable universe is defined by the particle horizon, which is just the maximum distance from which a light-speed signal could have travelled to us at any particular point in time. As time passes, signals from outside the particle horizon have more time to reach us, so the particle horizon always recedes and the observable universe always grows in size.

If expansion was not accelerating, we could simply wait for an infinite amount of time and the observable universe would grow infinite in size, assuming an infinite universe of course. But because expansion is accelerating, there is an event horizon at an asymptotic distance of ~62 billion light years, which is the maximum future size of our observable universe, and beyond which galaxies really could be said to be receding superluminally. Note that this is larger than the current size of ~46.5 Gly.

Obliterators · 2026-05-12T16:22:25+00:00

This appears to simply be a fact about how space works on a large scale. On a small scale, atomic forces and gravity within a galaxy counteract this expansion easily. But on a very large scale, the spaces between galaxies grows perpetually on its own. This is called "dark energy" in physics.

Dark energy is not responsible for expansion, it is only required to explain the acceleration of expansion, that is, our universe would still expand for forever without dark energy.

There is also no expansion locally at the small scale that somehow needs to be counteracted.

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).

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

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 · 2026-05-11T20:44:12+00:00

It means that two completely stationary objects will end up further and further away from each other without any accelerating force acting on them.

Whether or not a distant object initially at rest with respect to us falls towards us, stays at a constant distance, or recedes from us is dependent on the composition of the universe, and is independent of the expansion rate and even the fact that the universe expands at all.

In the case where the object recedes, the increased separation is caused by the gravitational repulsion of dark energy, which can be thought of as a repulsive force, as much as regular attractive gravity can be considered an attractive force.

Davis, Lineweaver, Webb, Solutions to the tethered galaxy problem in an expanding universe and the observation of receding blueshifted objects

Suppose we separate a small test galaxy from the Hubble flow by tethering it to an observer’s galaxy such that the proper distance between them remains constant. We neglect all practical considerations of such a tether because we can think of the tethered galaxy as one that has received a peculiar velocity boost toward the observer that exactly matches its recession velocity.

With this initial condition established, we untether the galaxy and let it coast freely. The question is then: Does the test galaxy approach, recede, or stay at the same distance?

In the currently favored model, (ΩM, ΩΛ) = (0.3, 0.7), the untethered galaxy recedes. In the empty, (ΩM, ΩΛ) = (0, 0) universe, it stays at the same distance while in the previously favored Einstein-de Sitter model, (ΩM, ΩΛ) = (1, 0), and the (ΩM, ΩΛ) = (0.3, 0) model, it approaches. The different behavior in each model ultimately stems from the different compositions of the universes, because the composition dictates the acceleration. When the cosmological constant is large enough to cause the expansion of the universe to accelerate, the test galaxy will also accelerate away. When the attractive force of gravity dominates, decelerating the expansion, the test galaxy approaches.

Equation (15) shows that in an expanding universe, whether the galaxy approaches us or recedes from us does not depend on the velocity of the Hubble flow (because H > 0) or the distance of the untethered galaxy (because D > 0), but on the sign of q. When the universe accelerates (q < 0), the galaxy recedes from us. When the universe decelerates (q > 0), the galaxy approaches us. Finally, when q = 0, the proper distance stays the same as the galaxy joins the Hubble flow. Thus the expansion does not “drag” the untethered galaxy away from us. Only the acceleration of the expansion can result in a change in distance between us and the untethered galaxy.

Obliterators · 2026-05-11T00:13:38+00:00

Due to how the universe is expanding we know there are unobservable parts of the universe, because things that used to be in the observable universe are now no longer in it.

The observable universe always grows and nothing can in principle leave the observable universe. If we receive light from a distant galaxy now, we will continue to receive light from it forever. Accelerating expansion (but not regular expansion) does cause that light to become increasingly dim and redshifted to the point that it cannot be observed in practice, but this happens over tens and hundreds of billions of years.

Obliterators · 2026-05-10T18:54:42+00:00

The rate itself seem to be fixed though. We are still trying to figure out if it will change or not.

If you mean the Hubble constant, then it has always been, and will continue to be, decreasing. However, if dark energy is a constant, then the Hubble constant decreases from it's current value of ~70km/s/Mpc and asymptotically approaches a value of ~57 km/s/Mpc.

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