Jump to content


Senior Members
  • Posts

  • Joined

  • Last visited

  • Days Won


md65536 last won the day on December 15 2020

md65536 had the most liked content!


Profile Information

  • College Major/Degree
  • Favorite Area of Science

Recent Profile Visitors

The recent visitors block is disabled and is not being shown to other users.

md65536's Achievements


Primate (9/13)



  1. Yes, that seems valid. It doesn't matter whether they would measure it or find meaning in it, just that it's possible they could. It seems their meaning would basically be, "if there were other things not bound by gravity, the space between them is expanding." Just to add to what it all means: From https://en.wikipedia.org/wiki/Expansion_of_the_universe Expansion of our universe is stated in terms of comoving coordinates where general galaxies (or "comoving observers") that have not been accelerated by forces or gravity, have fixed spatial coordinates. Their distances would be considered at a common comoving time, where "The comoving time coordinate is the elapsed time since the Big Bang according to a clock of a comoving observer and is a measure of cosmological time." So for example if you had 2 galaxies not gravitationally bound to each other, on a line, one located at (0) in comoving coordinates and the other at (1), we say their locations are fixed and that the measure of distance between them is increasing. Say we call the distance between them "one intergalactic unit", which I just made up. Here, the meaning of "the measure of distance itself (in these coordinates) is increasing" is that an intergalactic unit is equal to an increasing number of lightyears, over time. The measure of a lightyear can remain fixed. It doesn't mean that every possible measure of distance is increasing, which would be meaningless (see several other threads) because a change in a measure of distance is relative to something else.
  2. I agree, but then the topic's question could be reframed by that. Our universe and this toy universe are expanding due to vacuum energy or dark energy, but it would collapse if gravity was increased (put everything closer together, and/or add more mass). So if there was still this vacuum "expanding" influence on everything, but the universe collapsed, is there any way in which it could be said that the space itself is still expanding? Or is the metric still expanding? I no longer think this is a scientific question because the definition of expansion (as far as I know it) just doesn't apply. I think the question might be philosophical, and similar to asking "what is the true nature of the universe beyond what can be measured?" Conversely, for example say you have a toy universe with vacuum energy but where everything is gravitationally bound and collapses, except for one particle that is distant and not gravitationally bound, and separates at an ever-increasing rate. This fits the definition of expansion and I'd say that the space between the particle and the rest of the universe is expanding. This might be a stretch because the definitions are based on a universe that appears homogeneous.
  3. Yes, but the effect of gravity would diminish as the energy was carried away. This is what I thought you were talking about when you first mentioned photons. In a Newtonian analysis, a symmetric "shell" of outbound photons (or an expanding shell with mass equivalent to the photons' energy) would stop attracting a mass to the center of the shell as soon as the mass was inside the shell (shell theorem, which also implies the gravitational effect would not diminish before that happened). Based on that, you could reason something like, the photons from the first mass that pass by the second mass stop attracting the second mass toward the first, and start attracting it away.
  4. But what is space without matter? Isn't it just distance, and specifically distance between things? Isn't the vacuum itself observer-dependent? Space isn't made of some "stuff" that is measurable independent of other things, I think. Anyway I think scientific answers don't depend on those questions, because science deals with definitions and measurements. If expansion is defined as "expansion of the metric", and not of "space", then it doesn't matter what space is. If expansion is defined only for gravitationally unbound objects, and defined as their increasing separation, then I think there's no expansion without that separation, and no way to apply it to gravitationally bound masses. But yes, it doesn't make sense that matter would "stop" expansion. For example, consider 2 comoving distant galaxies that are separating due to expansion. Then say there are 2 gravitationally bound small masses somewhere between them. There is metric expansion between the 2 galaxies. It seems fine to say that space is expanding everywhere throughout the distance between them. What is happening between the 2 small masses? The definitions can answer that! In comoving coordinates, the galaxies have fixed coordinates, and the measure of distance itself is increasing. In these coordinates, for the 2 smaller masses to remain a fixed distance from each other in their local coordinates, one or both must be changing location in comoving coordinates. That gives the answer, that space is expanding between the gravitationally bound masses and they're moving through that space (at the very least mathematically!). But then, we can also consider the local coordinates of the gravitationally bound masses, where in this example they're stationary. You really can say that the masses aren't moving through space (which means nothing more than that they're not changing location in these coordinates). The metric is covariant but I'm still not sure if expansion would be something that is an absolute part of the metric, or dependent on the coordinates. I think it's reasonable to say "space is still expanding here" but with the implication that it's referring to space generally, or in other coordinates, not space in local coordinates. Interesting, I hadn't thought of the physics of the model beyond being an example. I think that even a bunch of photons scattered in all directions would start separation because of decreased mass density.
  5. It doesn't violate GR. It is a prediction of GR given the values of several measured variables.
  6. Well I changed my mind again. Geodesics aren't some set of lines that remain fixed relative to absolute space, because there is no such thing. Whether something's moving or not will always (I think) involve a choice of coordinates. Also, whether 2 masses are gravitationally bound depends not just on their locations, but their relative velocity. So for example, two comoving fairly distant galaxies can be unbound and separating due to expansion, while two similarly located galaxies can be gravitationally bound just by giving them enough velocity toward each other (basically an "unescape" velocity to overcome expansion). So I think the answer to whether some specific effect is expansion or not, can depend on a choice, and doesn't have a single answer. To get around that, the definition exists only where it is applicable, eg. per wikipedia "The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time." As well, to say whether something is moving or not, "comoving coordinates" are used, not because that represents absolute motion or anything, but because most of the observable universe approximately fits them pretty well. I think whether expansion is occurring on smaller scales between gravitationally bound masses is literally undefined, and a definitive answer would require some extra definitions and that would necessarily involve making some arbitrary choices of what's moving and what's not.
  7. I think it's confusing because the expansion "breeze" would have to blow in all directions and not push on the ball bearing at all. But, the idea of expansion contributing and gravity contributing to the whole, makes sense. As for atoms expanding, what I've read is that that would only happen if the vacuum energy increased boundlessly, so that the scale at which expansion dominates gets smaller over time, approaching zero. But that would be speculative. I thought of 2 analogies that help me make sense of this. 1. Imagine the Earth is a perfect sphere and it's expanding uniformly. A rigid tectonic plate or island could represent a gravitationally bound region. This is analogous to universal expansion at all scales, since even though the island doesn't change in size, the underlying size of the Earth's surface does. Note that for the island to stay a fixed size, some other part of the crust would have to expand more than the average expansion, for the whole Earth to expand uniformly. 2. Imagine a patch of live skin cells that all slowly divide over time, so that any given area of skin doubles over some long time. A scab or patch of dead cells on it could represent a gravitationally bound region. In this case it's not needed for another area to expand faster to make up for the scab, because the whole isn't expanding with global uniformity and the distortion caused by the scab would be like curvature of spacetime. I think the second is a closer analogy to expansion in the universe, because it's attributed to local vacuum energy rather than some global expansion of the volume of space. I think the island analogy is wrong in that it confusingly represents space both as the crust (including the island) but also as some extra underlying thing that doesn't have a real analog in the universe. But beyond analogies as thought experiments, I think the technical answer needs to be based on the actual definition of expansion. Expansion refers to "metric expansion" which is a change in the measure of distance itself, rather than just a change in the distance between specific things. Well that's not helpful to think about, but what does it mean? The closest I've seen to a description of the technical meaning of expansion is that it involves a divergence of geodesics over time. I think in the case of a static gravitationally bound region, geodesics wouldn't diverge. It wouldn't matter how the rest of the universe was expanding, it would be like tattooed lines on a rigid scab. The geodesics would be like the fan and magnet contributing to a whole, or vacuum energy and gravity contributing to one metric. It would not be like the Earth analogy where one could imagine separate geodesics for an expanding Earth and a static island through the same place.
  8. I don't think anyone said it cannot fit? Just that it is meaningless. In fact there are published papers that propose exactly what you're saying, they're just not accepted as mainstream science, mainly because they don't account for everything that the accepted science does. Accepted science is always changing (usually slowly) but it changes to better account for actual observations, not imagined alternatives (unless it's a useful improvement). A lack of evidence doesn't prove your alternative doesn't fit, just that it doesn't improve anything. What it looks like to me, is that you're suggesting the possibility that instead of using GR to account for the universe as we observe it (where it fits very well for cosmology and large scales), we use it to account for something different than what we observe, and add in your proposals to account for the difference.
  9. The cosmological constant (CC) is associated with vacuum energy and its corresponding expansion, which is the accelerating expansion that dominates on the largest scales. There's also expansion (or contraction) in the Einstein field equation (EFE) without the CC (as in the original form of the EFE which didn't have the CC. The CC was first added to try to avoid expansion/contraction and allow for a steady-state universe). So, not all of expansion is an 'add-on'. That might provide the answer I was looking for. If the vacuum energy is contributing an expanding influence to the curvature of spacetime that is still present even at small scales, but the Einstein tensor overwhelms it and the net effect is no expansion or even contraction at small scales (eg. within a galaxy), then there's simply no expansion. So expansion is the end result, meaning space between comoving stuff is actually increasing, rather than just a component such as the influence of vacuum energy alone. I think my mistake is in thinking of space as a thing that is independent of the stuff in it, where for example the vacuum can expand without the stuff moving apart. I think I should consider it just as coordinates, but quickly get lost trying to think of expansion that way.
  10. I was about to reply to this saying basically that I think that space is expanding at all scales, but on smaller scales (even between Milky Way and Andromeda), gravity overwhelms expansion so that the galaxies are "moving through" space relative to each other faster than expansion can separate them. A google search shows only results that disagree with my view, and seem to suggest that if two things (in an otherwise empty universe, say) aren't separating, then I guess there's no meaningful way to say that that space is expanding? If two more-distant galaxies are separating at an increasing rate due to expansion, people use phrases like, "gravity has no effect at those distances" due to expansion, which is also not what I thought. Say for example you have 2 masses in an empty universe where their gravitational attraction exactly balances expansion of space between them, so they remain at a fixed distance. Would you say, space is expanding between them, but gravity accelerates them through that space at a rate that keeps them at the same distance? Maybe even there is a measurement that shows that gravity still applies and that expansion is also happening. Or would you say there is no expansion of space and no gravitational effect between the 2 masses in this system? It's nonsense to say the masses are moving through space or accelerating, because those only make sense relative to something else, and they're not moving relative to anything. There is no way to distinguish expansion and gravity, because any effect (like redshift) of expansion that would otherwise cause the 2 masses to separate, would be exactly cancelled out by an opposite effect of gravity that would otherwise cause the 2 masses to converge, and so no such effects are measurable. Is either correct? Could you also say that "gravitational effect" and "expansion" are just emergent effects of the metric, and are not fundamental and separate parts of the metric or the universe? It is the metric that has these two masses relatively stationary, and gravitational attraction and expansion are simply zero for these 2 masses? If so, then I think you could model or label the system so there is both a gravitational effect and expansion of space between the 2 masses, but you would only do that if you had a reason to separate them, otherwise it is simply an unnecessary complication. It seems, I shouldn't think of expansion as some intrinsic process that's happening throughout the universe, but is just a measurement that is a consequence of our universe's particular metric tensor?
  11. What is the time measured by a clock moving 0 m with velocity 0 m/s? How would you define an hour measured with a stationary clock? If time can be measured without motion, why would the "most basic" definition require motion?
  12. http://curious.astro.cornell.edu/about-us/102-the-universe/cosmology-and-the-big-bang/the-big-bang/586-did-time-go-slower-just-after-the-big-bang-advanced has the closest to an answer that I've found so far. From there: It seems everyone agrees there's not a lot of physical meaning to comparing such clocks. I don't understand "clocks were slower because mass density was so high" enough to believe it. A "shallow gravitational field" doesn't make sense to me, I think he means higher relative gravitational potential, but like in the first post I'm not sure that gravitational potential even makes sense here. It seems that to compare the "gravitational depth" here, it would be more correct to compare "the metric tensor of the universe in local coordinates of a clock in the past, to the metric tensor in local coordinates of a clock in the present." Is that the right way to say it? It still makes no sense to me, because if you could compare those, why can't you just compare the clocks? But basically it sounds like evolving from a high density uniform universe to a lower density uniform universe due to expansion of space, can be considered to be moving to a higher gravitational potential --- and when I write it like that it sounds just plain wrong because there's nowhere to fall to, where the gravitational potential is relatively lower. However it seems like it would be measured as if it was the same, for example light from an older clock would redshift as it traverses through expanding space, losing energy as if it were climbing out of a gravitational well. So, I'm still confused. Edit: Or is it, the metric in local coordinates of a modern clock, could tell you the gravitational (only) time dilation factor between the two clocks? Or is it pointless to talk about metrics without a better understanding first?
  13. Does this mean it could be done (compare a clock with itself in the past while accounting for any effects related to the location of everything in the universe relative to the clock)? The implication is that any expected difference in the rate of the clock over time can be attributed to a change in the universe relative to the clock, leaving nothing that could be attributed to a change in time itself, in accepted theory? If it's made up and can't be measured, how can you possibly draw a useful conclusion from it? What's stopping you from imagining it to be something where the rate of time varies wildly relative to what we measure, and someone else imagining it to be something where the two measures are the same?
  14. It could, but I think you'd have to remove any effects that depend on location, if you're trying to measure changes over time alone and not over space.
  15. Everyone in this thread (except AbstractDreamer) seems to be using the following: 1. Time is what clocks measure, by definition. 2. The only way to measure or reason about the rate of time, is to compare time measured by one clock to that of another clock. 3. A clock will always tick at 1 s/s compared to itself, there's simply no room for a clock to disagree with itself. AbstractDreamer, I can't make sense of what you're saying without knowing what definitions you're using or where you disagree with these 3 things. I suggested this before but it was split off and buried: To test for a changing rate of time you would compare observations of the rate of a clock from earlier in its history, relative to the clock now. You would need to account for any effects not attributable to time, and then any deviation that's left over would have to be due to changing rate of time. Or, compare a clock now to one in an environment that is identical to the environment in the first clock's past. Or, you could compare a clock at two different times, against a reference clock whose rate can be considered constant. I'm sure this is possible to reason about, because you could say "A clock speeds up as it climbs out of a gravitational well," and it makes implicit sense, I think, using any of these 3 methods. I think this is what AbstractDreamer is trying to ask about, specifically about expansion of space or other possible unknown effects in the history of the universe. What I still don't get is whether GR can and/or does predict that expansion would have an effect on a clock like this, or whether the rate will still be constant because there is always something else other than time to account for any observations (eg. redshift in older images of a clock would be due to expansion of space, not an increase in the rate of time), or if it still makes no sense because there is no reference clock that can be defined "outside the universe" to escape the effects of expansion, or no way to compare a space that's undergone expansion to one that hasn't, etc.
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.