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Is this an accurate summary of the current status quo? 1. The universe is expanding 2. The velocity at which the universe is expanding is increasing, since th

by bmitc 3y ago

Is this an accurate summary of the current status quo? 1. The universe is expanding 2. The velocity at which the universe is expanding is increasing, since there is non-zero acceleration. 3. The acceleration is actually slowing down, but it is not heading towards zero. It is approaching some steady state acceleration. ---------- In Exploring Black Holes by Edwin Taylor and John Wheeler, published in 2000, I recently read the following passage by Wheeler: > An article by John Noble Wilford in the Science Times section of the New York Times for Tuesday March 3, 1998, reports observations by two separate groups of investigators which they interpret as showing that today the expansion of the Universe is speeding up rather than undergoing the slowdown expected for any approach to maximum expansion. Later that day I encountered a hard-bitten veteran gravitation physics colleague in the elevator of the Princeton physics building and asked him if he believed the purported evidence of accelerating expansion. "No," he replied. Neither do I. Why not? Two reasons: (1) Because the speed-up argument relies too trustingly on the supernovas being standard candles, (2) Because such an expansion would, it seems to me, contradict a view of cosmology too simple to be wrong. Such clashes between theory and experiment have often triggered decisive advances in physics. We can hope that some decisive advance is in the offing. Now, Wheeler was one of the most preeminent physicists of the 20th century and spearheaded several fields and was not afraid of wild ideas. I am curious if anyone knows if he had any inklings that further his thoughts here. Was he just wrong here, before new measurements came out? Or is there something missing in these calculations and the theory that we're missing? I'm also not sure what means in (1).

mr_mitm 3y ago
Yes to 1) and 2), however: > 3. The acceleration is actually slowing down, but it is not heading towards zero. It is approaching some steady state acceleration. No. The scale factor in LCDM (the current concordance model) is proportional to sinh^(2/3)(t/t_Lambda), which means it's asymptotically exponential. Acceleration will always increase. (Edit: The expansion rate, so a'/a, will go to a constant value, though.) Caveat: If the Hubble tension proves real, LCDM is wrong.
- bmitc 3y ago
  Thanks! So the currently accepted model says the acceleration will grow indefinitely but our observations are saying that the acceleration levels out to a non-zero, positive constant?
  - mr_mitm 3y ago
    No, I think the confusion here is the difference between "acceleration" and "expansion rate". It's less ambigious when using math: https://en.wikipedia.org/wiki/Lambda-CDM_model#Cosmic_expansion_history https://en.wikipedia.org/wiki/Lambda-CDM_model#Cosmic_expans... a is the scale factor, which goes as sinh^(2/3). a' and a'' will always increase, but the _expansion rate_ a'/a will approach a constant value from above. Besides the Hubble tension discussed in the article, there is no real discrepancy between observations and theory.
    - bmitc 3y ago
      > a is the scale factor, which goes as sinh^(2/3). a' and a'' will always increase, but the _expansion rate_ a'/a will approach a constant value from above. Thanks for that. I didn't realize there was such a definition of expansion rate and was going off the intuitive meaning of the terms. Thank you for the clarifications.
naasking 3y ago
> 2. The velocity at which the universe is expanding is increasing, since there is non-zero acceleration. Some physicists are contesting this: https://arxiv.org/abs/1808.04597 https://arxiv.org/abs/1808.04597
nonameiguess 3y ago
To the Wheeler question, I would say he was just wrong. Pre-eminent physicist of the 20th century he may have been, smarter that I'll ever be, but he was also nearly 90 at that point and had retired 22 years earlier. To (1), he's talking about the distance ladder. Distances to various galaxies were computed by the teams involved in developing the LCDM model by scouring the skies for instances of type IA supernovae in this galaxies. When these are called "standard candles," what that means is that the luminosity is known and thus the distance can be computed using the inverse square law by comparing the observed brightness to the known luminosity. Historically, this wasn't a super-reliable method and yielded some bounded error rate for various reasons, including dust causing some reddening of the light that could be mistaken for redshift, as well as the fact that type IA supernovae are not all collapsing stars of exactly equal mass and you might catch them at different times after the initial supernova. What Wheeler probably didn't realize at that point was the methods and frankly just downright tedious repeated measurement work over the course of decades that astronomers had been doing to make this more reliable and accurate. If you want to know all about this via first-hand account from one of the astronomers credited with the dark energy discovery, read The Extravagant Universe by Robert Kirschner. It's very accessible, not super-technical, but goes into painstaking detail on exactly how astronomers made the type IA work as a standard candle. It's also just a great testament to the work involved in experimental astronomy, down to the logistics of convincing a scheduling committee for the various telescopes capable of seeing that far that what you're looking for is worth looking for.
- bmitc 3y ago
  > To (1), he's talking about the distance ladder. Thank you for this! I didn't realize that "standard candle" is an actual term, and I thought it was Wheeler being playful with language, which he quite often does. I have some reading to do on this distance ladder stuff. Have the standard siren's in gravitational waves confirmed these expansion measurements? I realized Wheeler was older at the time of publication and given his death in 2008, he obviously couldn't know the measurements that have occurred in the last 15 years. I think by his last two sentences he wasn't outright rejecting it but rather being suspicious of both the measurements and the theory. There is a second edition of the book, published well after Wheeler's death, so I wonder if Taylor kept that section of the book. To Wheeler's credit, the section was in a specific box entitled: "Opinion: The Bang-to-Crunch Universe Too Simple to be Wrong!". Edit: I checked the second edition (again, Wheeler didn't appear to work on this edition), located here https://www.eftaylor.com/exploringblackholes/ https://www.eftaylor.com/exploringblackholes/, and it does indeed appear that there are now entire chapters dedicated to the expanding universe and cosmology and that the above quoted opinion section is removed.

by bmitc 3y ago

mr_mitm 3y ago

Yes to 1) and 2), however: > 3. The acceleration is actually slowing down, but it is not heading towards zero. It is approaching some steady state acceleration. No. The scale factor in LCDM (the current concordance model) is proportional to sinh^(2/3)(t/t_Lambda), which means it's asymptotically exponential. Acceleration will always increase. (Edit: The expansion rate, so a'/a, will go to a constant value, though.) Caveat: If the Hubble tension proves real, LCDM is wrong.

bmitc 3y ago
Thanks! So the currently accepted model says the acceleration will grow indefinitely but our observations are saying that the acceleration levels out to a non-zero, positive constant?
- mr_mitm 3y ago
  No, I think the confusion here is the difference between "acceleration" and "expansion rate". It's less ambigious when using math: https://en.wikipedia.org/wiki/Lambda-CDM_model#Cosmic_expansion_history https://en.wikipedia.org/wiki/Lambda-CDM_model#Cosmic_expans... a is the scale factor, which goes as sinh^(2/3). a' and a'' will always increase, but the _expansion rate_ a'/a will approach a constant value from above. Besides the Hubble tension discussed in the article, there is no real discrepancy between observations and theory.
  - bmitc 3y ago
    > a is the scale factor, which goes as sinh^(2/3). a' and a'' will always increase, but the _expansion rate_ a'/a will approach a constant value from above. Thanks for that. I didn't realize there was such a definition of expansion rate and was going off the intuitive meaning of the terms. Thank you for the clarifications.

naasking 3y ago

> 2. The velocity at which the universe is expanding is increasing, since there is non-zero acceleration. Some physicists are contesting this: https://arxiv.org/abs/1808.04597 https://arxiv.org/abs/1808.04597

nonameiguess 3y ago

To the Wheeler question, I would say he was just wrong. Pre-eminent physicist of the 20th century he may have been, smarter that I'll ever be, but he was also nearly 90 at that point and had retired 22 years earlier. To (1), he's talking about the distance ladder. Distances to various galaxies were computed by the teams involved in developing the LCDM model by scouring the skies for instances of type IA supernovae in this galaxies. When these are called "standard candles," what that means is that the luminosity is known and thus the distance can be computed using the inverse square law by comparing the observed brightness to the known luminosity. Historically, this wasn't a super-reliable method and yielded some bounded error rate for various reasons, including dust causing some reddening of the light that could be mistaken for redshift, as well as the fact that type IA supernovae are not all collapsing stars of exactly equal mass and you might catch them at different times after the initial supernova. What Wheeler probably didn't realize at that point was the methods and frankly just downright tedious repeated measurement work over the course of decades that astronomers had been doing to make this more reliable and accurate. If you want to know all about this via first-hand account from one of the astronomers credited with the dark energy discovery, read The Extravagant Universe by Robert Kirschner. It's very accessible, not super-technical, but goes into painstaking detail on exactly how astronomers made the type IA work as a standard candle. It's also just a great testament to the work involved in experimental astronomy, down to the logistics of convincing a scheduling committee for the various telescopes capable of seeing that far that what you're looking for is worth looking for.

bmitc 3y ago
> To (1), he's talking about the distance ladder. Thank you for this! I didn't realize that "standard candle" is an actual term, and I thought it was Wheeler being playful with language, which he quite often does. I have some reading to do on this distance ladder stuff. Have the standard siren's in gravitational waves confirmed these expansion measurements? I realized Wheeler was older at the time of publication and given his death in 2008, he obviously couldn't know the measurements that have occurred in the last 15 years. I think by his last two sentences he wasn't outright rejecting it but rather being suspicious of both the measurements and the theory. There is a second edition of the book, published well after Wheeler's death, so I wonder if Taylor kept that section of the book. To Wheeler's credit, the section was in a specific box entitled: "Opinion: The Bang-to-Crunch Universe Too Simple to be Wrong!". Edit: I checked the second edition (again, Wheeler didn't appear to work on this edition), located here https://www.eftaylor.com/exploringblackholes/ https://www.eftaylor.com/exploringblackholes/, and it does indeed appear that there are now entire chapters dedicated to the expanding universe and cosmology and that the above quoted opinion section is removed.