10 ms·

Naive question: why should the expansion rate need to be uniform or constant everywhere? I'm likely misinterpreting the article, but it seems to frame things i

Naive question: why should the expansion rate need to be uniform or constant everywhere? I'm likely misinterpreting the article, but it seems to frame things in a way that first assumes expansion should be constant and it's a question of what the right constant value is between the measured/theoretical discrepancies. (*yeesh, editing all those spelling errors from typing on my phone)

BossingAround 2y ago
That's the first thing that occurred to me too. It could also not be constant even at the same place, i.e. could it not be speeding up and slowing down as the universe expands?
isolli 2y ago
Indeed. Some researchers have proposed quintessence, a time-varying form of dark energy [0]. > A group of researchers argued in 2021 that observations of the Hubble tension may imply that only quintessence models with a nonzero coupling constant are viable. [0] https://en.wikipedia.org/wiki/Quintessence_(physics) https://en.wikipedia.org/wiki/Quintessence_(physics)
itishappy 2y ago
It's not constant (the early universe inflated quite quickly), and it doesn't need to be uniform, but it sure does appear to be. We measure it via redshift, pulsar timing arrays, and the temperature fluctuations of the CMB, and it looks pretty much the same in all directions.
AnimalMuppet 2y ago
The issue here is that it's not constant depending on the type of star we use to measure it. It's not a discrepancy in location in space. Or at least that's how I read the article.
chongli 2y ago
The controversy is that we get 2 different numbers depending on which method (cosmic microwave background vs cosmic distance ladder) we use to calculate the present rate of expansion. These numbers used to have their error bars overlapping, so we assumed they would eventually converge to the true value. But as we get more data the opposite is happening: the numbers are diverging and their error bars are shrinking such that they no longer overlap. This tells us that either our model of the universe is wrong (therefore the cosmic microwave background method is giving us an incorrect answer) or that something is wrong with how we're calculating the distances along the cosmic distance ladder. The latter was originally the assumption that should be proven true with more and better data from newer telescopes. This is now turning out not to be the case: our cosmic distance ladder calculations seem to have been very good, so it now seems more likely that our model of the universe is wrong.
- detourdog 2y ago
  Thank you for this explanation. I would like to emphasize that our model being wrong and not our numbers sounds like progress. It also sound like progress that we seem to have 2 “scales” to play with to try to develop a consistently measurable distance.
- api 2y ago
  Distance ladder seems much more error prone than the CMB.
  - artemonster 2y ago
    there is no "seems" with provable error bars
    - feoren 2y ago
      ehh ... everything we measure relies on our understanding of the universe in some way. It's perfectly reasonable that our distance measurements could rely on a shakier foundation of assumptions than our understanding of the CMB. I don't know enough to say one way or the other, but GP's comment is not unreasonable on its face. Whereas talking about "provable" in cosmology, and certainly in this case, does seem unreasonable -- especially with error bars, which by definition have a small chance of not including the real value. Normally I'd say that we can generally refine our assumptions to be extremely good, and just take more measurements, and keep narrowing that error bar, until we hit a level of certainty that anyone reasonable would call "proven", but the entire point of TFA is that this isn't happening in this case. We seem on our way to "proving" two inconsistent things.
  - db48x 2y ago
    All of the cosmologists expected that too!
  - naasking 2y ago
    It's not just the CMB, it's the CMB + LCDM. You know, that theory that posits a form of matter that we have consistently failed to find and that has technically been falsified a number of times, but which was then adjusted to work around those problems, and yet problems still remain. You might have "slight" bit of unaccounted for error sneaking in there. I personally wouldn't trust the CMB calculation at all until the dark matter issue is resolved.
JumpCrisscross 2y ago
> what should the expansion rate need the be uniform or constant everywhere? It doesn't. "The simplest explanation for dark energy is that it is an intrinsic, fundamental energy of space" [1]. That's the cosmological constant. Dark energy is a thing because we don't assume that to be the case. Irrespective of your dark energy model, however, there will be a predicted global average. [1] https://en.wikipedia.org/wiki/Dark_energy https://en.wikipedia.org/wiki/Dark_energy
- tzs 2y ago
  There has been some interesting recent work that may get rid of the need for dark energy. Briefly, recent large scale maps of the universe suggest that the universe might not be as uniform as we thought it was. In particular we appear to be in a large region (something like a couple billion light years across) of low density. Dark energy is needed to make the solution to Einstein's field equations for the whole universe match observations. However that solution was derived based on a universe with matter distributed uniformly. At the time it was first derived that appeared to be the case--we thought the Milky Way was the whole universe. When we learned that the Milky Way was just a small galaxy in a vastly larger universe than had thought we were in and that there were bazillions of other galaxies, those galaxies appeared to be distributed uniformly enough the the solution to the field equations still worked. Later we found that there is some large scale structure in the distribution of galaxies, like superclusters, but those seemed uniform enough throughout the universe that things still worked. If that couple of billion light year low density region turns out to exist (large scale mapping of the universe is hard enough that it may just be observational error) the universe may not actually be uniform enough to for the field equations based on uniform matter distribution to actually work. Some researchers worked out the solutions to the field equations for a universe that has such large low density bubbles big enough to invalidate the uniform universe solution, and found that such a universe would have an expansion force without the need to invoke any kind of dark energy. There was a recent PBS Space Time episode that covered this: "Can The Crisis in Cosmology be SOLVED With Cosmic Voids" [1]. The above is my summary of what I remember from that. See the episode for a better explanation and references to the research. [1] https://www.youtube.com/watch?v=WWqmccgf78w https://www.youtube.com/watch?v=WWqmccgf78w
xhkkffbf 2y ago
Certainly when I look at convection currents in the ocean or the atmosphere, I see plenty of variation. Shoot, the earth's atmosphere constantly produces moving blobs of relatively high and low pressure.
diob 2y ago
So a lot of astronomy is based on the principle that we are not in a special pocket of the universe. See https://en.wikipedia.org/wiki/Cosmological_principle https://en.wikipedia.org/wiki/Cosmological_principle Basically, if this weren't to hold true, a lot of astronomy would fall over, even physics.
- XorNot 2y ago
  Though it is worth noting if this were the case you would expect to see boundaries: if the laws of physics change due to spatial position, the discontinuity should produce an effect of some sort where matter and light transitions between regions.
  - mcswell 2y ago
    I suppose there could be a gradual change over distance, i.e. the first derivative of this change never varies.
- deleted 2y ago
  [deleted]
- hindsightbias 2y ago
  Sophon is amused: https://en.wikipedia.org/wiki/Axis_of_evil_(cosmology) https://en.wikipedia.org/wiki/Axis_of_evil_(cosmology)
- eikenberry 2y ago
  Seems there are 2 ideas at odds. One is that the universe is infinite, in which case this is all localized and has no bearing on the universe outside of our small observable region. The other is that we are seeing enough of a bounded universe where the observations we make are of a significant enough chunk to make theories about it.
- beltsazar 2y ago
  Yes, cosmological principle is probably the most fundamental assumption in astronomy. Most people don't realize that science—and even everything in life—has to start from some axioms/assumptions, just like math. I first realized this fact when I was reading the Relativity book written by Einstein himself, who challenges the assumptions in classical physics. As time goes, some of the assumptions could be proved to be unnecessary or even wrong. There must be still some assumptions left, though—because without them, we can't talk about science, or anything, really.
photochemsyn 2y ago
Spacetime is apparently extremely rigid as it supports the transmission of gravitational waves originating billions of light-years away, as detected by the LIGO experiments. This suggests smooth and gradual uniform expansion, at least spatially. Temporal variation (speeding up and slowing down uniformly at all points) might be possible but seems hard to explain.
Jeff_Brown 2y ago
Seems perfectly possible. General relativity, after all, was precisely the discovery that the curvature of space, well spacetime, is not uniform.

BossingAround 2y ago

That's the first thing that occurred to me too. It could also not be constant even at the same place, i.e. could it not be speeding up and slowing down as the universe expands?

isolli 2y ago

Indeed. Some researchers have proposed quintessence, a time-varying form of dark energy [0]. > A group of researchers argued in 2021 that observations of the Hubble tension may imply that only quintessence models with a nonzero coupling constant are viable. [0] https://en.wikipedia.org/wiki/Quintessence_(physics) https://en.wikipedia.org/wiki/Quintessence_(physics)

itishappy 2y ago

It's not constant (the early universe inflated quite quickly), and it doesn't need to be uniform, but it sure does appear to be. We measure it via redshift, pulsar timing arrays, and the temperature fluctuations of the CMB, and it looks pretty much the same in all directions.

AnimalMuppet 2y ago

The issue here is that it's not constant depending on the type of star we use to measure it. It's not a discrepancy in location in space. Or at least that's how I read the article.

chongli 2y ago

The controversy is that we get 2 different numbers depending on which method (cosmic microwave background vs cosmic distance ladder) we use to calculate the present rate of expansion. These numbers used to have their error bars overlapping, so we assumed they would eventually converge to the true value. But as we get more data the opposite is happening: the numbers are diverging and their error bars are shrinking such that they no longer overlap. This tells us that either our model of the universe is wrong (therefore the cosmic microwave background method is giving us an incorrect answer) or that something is wrong with how we're calculating the distances along the cosmic distance ladder. The latter was originally the assumption that should be proven true with more and better data from newer telescopes. This is now turning out not to be the case: our cosmic distance ladder calculations seem to have been very good, so it now seems more likely that our model of the universe is wrong.

detourdog 2y ago
Thank you for this explanation. I would like to emphasize that our model being wrong and not our numbers sounds like progress. It also sound like progress that we seem to have 2 “scales” to play with to try to develop a consistently measurable distance.
api 2y ago
Distance ladder seems much more error prone than the CMB.
- artemonster 2y ago
  there is no "seems" with provable error bars
  - feoren 2y ago
    ehh ... everything we measure relies on our understanding of the universe in some way. It's perfectly reasonable that our distance measurements could rely on a shakier foundation of assumptions than our understanding of the CMB. I don't know enough to say one way or the other, but GP's comment is not unreasonable on its face. Whereas talking about "provable" in cosmology, and certainly in this case, does seem unreasonable -- especially with error bars, which by definition have a small chance of not including the real value. Normally I'd say that we can generally refine our assumptions to be extremely good, and just take more measurements, and keep narrowing that error bar, until we hit a level of certainty that anyone reasonable would call "proven", but the entire point of TFA is that this isn't happening in this case. We seem on our way to "proving" two inconsistent things.
- db48x 2y ago
  All of the cosmologists expected that too!
- naasking 2y ago
  It's not just the CMB, it's the CMB + LCDM. You know, that theory that posits a form of matter that we have consistently failed to find and that has technically been falsified a number of times, but which was then adjusted to work around those problems, and yet problems still remain. You might have "slight" bit of unaccounted for error sneaking in there. I personally wouldn't trust the CMB calculation at all until the dark matter issue is resolved.

JumpCrisscross 2y ago

> what should the expansion rate need the be uniform or constant everywhere? It doesn't. "The simplest explanation for dark energy is that it is an intrinsic, fundamental energy of space" [1]. That's the cosmological constant. Dark energy is a thing because we don't assume that to be the case. Irrespective of your dark energy model, however, there will be a predicted global average. [1] https://en.wikipedia.org/wiki/Dark_energy https://en.wikipedia.org/wiki/Dark_energy

tzs 2y ago
There has been some interesting recent work that may get rid of the need for dark energy. Briefly, recent large scale maps of the universe suggest that the universe might not be as uniform as we thought it was. In particular we appear to be in a large region (something like a couple billion light years across) of low density. Dark energy is needed to make the solution to Einstein's field equations for the whole universe match observations. However that solution was derived based on a universe with matter distributed uniformly. At the time it was first derived that appeared to be the case--we thought the Milky Way was the whole universe. When we learned that the Milky Way was just a small galaxy in a vastly larger universe than had thought we were in and that there were bazillions of other galaxies, those galaxies appeared to be distributed uniformly enough the the solution to the field equations still worked. Later we found that there is some large scale structure in the distribution of galaxies, like superclusters, but those seemed uniform enough throughout the universe that things still worked. If that couple of billion light year low density region turns out to exist (large scale mapping of the universe is hard enough that it may just be observational error) the universe may not actually be uniform enough to for the field equations based on uniform matter distribution to actually work. Some researchers worked out the solutions to the field equations for a universe that has such large low density bubbles big enough to invalidate the uniform universe solution, and found that such a universe would have an expansion force without the need to invoke any kind of dark energy. There was a recent PBS Space Time episode that covered this: "Can The Crisis in Cosmology be SOLVED With Cosmic Voids" [1]. The above is my summary of what I remember from that. See the episode for a better explanation and references to the research. [1] https://www.youtube.com/watch?v=WWqmccgf78w https://www.youtube.com/watch?v=WWqmccgf78w

xhkkffbf 2y ago

Certainly when I look at convection currents in the ocean or the atmosphere, I see plenty of variation. Shoot, the earth's atmosphere constantly produces moving blobs of relatively high and low pressure.

diob 2y ago

So a lot of astronomy is based on the principle that we are not in a special pocket of the universe. See https://en.wikipedia.org/wiki/Cosmological_principle https://en.wikipedia.org/wiki/Cosmological_principle Basically, if this weren't to hold true, a lot of astronomy would fall over, even physics.

XorNot 2y ago
Though it is worth noting if this were the case you would expect to see boundaries: if the laws of physics change due to spatial position, the discontinuity should produce an effect of some sort where matter and light transitions between regions.
- mcswell 2y ago
  I suppose there could be a gradual change over distance, i.e. the first derivative of this change never varies.
deleted 2y ago
[deleted]
hindsightbias 2y ago
Sophon is amused: https://en.wikipedia.org/wiki/Axis_of_evil_(cosmology) https://en.wikipedia.org/wiki/Axis_of_evil_(cosmology)
eikenberry 2y ago
Seems there are 2 ideas at odds. One is that the universe is infinite, in which case this is all localized and has no bearing on the universe outside of our small observable region. The other is that we are seeing enough of a bounded universe where the observations we make are of a significant enough chunk to make theories about it.
beltsazar 2y ago
Yes, cosmological principle is probably the most fundamental assumption in astronomy. Most people don't realize that science—and even everything in life—has to start from some axioms/assumptions, just like math. I first realized this fact when I was reading the Relativity book written by Einstein himself, who challenges the assumptions in classical physics. As time goes, some of the assumptions could be proved to be unnecessary or even wrong. There must be still some assumptions left, though—because without them, we can't talk about science, or anything, really.

photochemsyn 2y ago

Spacetime is apparently extremely rigid as it supports the transmission of gravitational waves originating billions of light-years away, as detected by the LIGO experiments. This suggests smooth and gradual uniform expansion, at least spatially. Temporal variation (speeding up and slowing down uniformly at all points) might be possible but seems hard to explain.

Jeff_Brown 2y ago

Seems perfectly possible. General relativity, after all, was precisely the discovery that the curvature of space, well spacetime, is not uniform.