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To know if the universe is expanding, we need to know the following: how far away things are, and whether they're moving away from us. In an accelerating univer

To know if the universe is expanding, we need to know the following: how far away things are, and whether they're moving away from us. In an accelerating universe, there's evidence that things were moving away from us slower in the past. The evidence that things are moving away from us is that as the universe expands, the wavelength of any light travelling through it also expands, shifting it towards the red end of the spectrum: redshift. (Think of the wavelength of a wavy line drawn on a piece of rubber that's then stretched out.) We've known for nearly 100 years that the universe is expanding. (Edit: not just that the objects in it are moving apart but that space itself is expanding.) The evidence of accelerating expansion is that based on the redshift of nearby objects, we would expect distant objects to have a higher redshift than they actually do. That means they were moving away from us slower in the past, so something must be accelerating them. We use the brightness of Type Ia supernovae to measure distance. Even though supernovae aren't identically bright, their brightness follows a curve which lets us calculate their peak brightness (they are standardizable). This paper argues that the calculations cosmologists use to standardize supernovae brightness fail to take into account the age of the progenitor stars, as far as I can tell. If true, this means our distance measurements are inaccurate and these stars are actually closer than we thought, enough to restore to linear relationship between distance and redshift that one would expect in a universe expanding at a constant rate. In other words, their redshift is lower not because the expansion rate of space was lower in the past, but because they're not as far away as we thought they were.

mirimir 7y ago
That's how I understand it. But maybe there are other standardizable objects. The sense I got from TFA is that there aren't.
- kiliantics 7y ago
  There are actually. There is a class of star in the "tip of the red giant branch" (TRGB) which have also been used to calibrate luminosities of type Ia supernovae[0] to form a "cosmic distance ladder". I believe they have been used to confirm the accelerated expansion results (though less strongly). I am not super well versed in stellar evolution, but I believe that the TRGB should make this calibration method independent of progenitor age? Hopefully someone could chime in on this. I was formerly a cosmologist and would be very interested to know more about this result from any experts that might be reading this. [0] There is an explanation of this calibration here: https://aasnova.org/2019/07/26/tldr-the-trgb-gives-us-another-h0ttake/ https://aasnova.org/2019/07/26/tldr-the-trgb-gives-us-anothe...
  - ncmncm 7y ago
    Of course these could have the same, or a related, problem.
  - sanxiyn 7y ago
    Nope, the link is proposing TRGB-SNIa as an alternative to Cepheid-SNIa, not TRGB as an alternative to SNIa.
    - mirimir 7y ago
      Yeah. In my reading, it almost characterized SNIa as fact.
wallace_f 7y ago
Thanks for the explanation, but I'm stuck on the last sentence of this part: >evidence of accelerating expansion is that based on the redshift of nearby objects, we would expect distant objects to have a higher redshift than they actually do. I think what you mean is "than we'd expect them to."
- ncmncm 7y ago
  Right. They have the red shift we would expect to find in more distant objects than they seem to be, just based on how bright their supernovas are. But if the brightness is off, the anomaly might disappear. Or get bigger, all bets are off.
- deleted 7y ago
  [deleted]
brylie 7y ago
I'm not an astronomer, but "tired light" makes more intuitive sense to me to explain the redshift. In effect, we already know that matter can affect light through gravitational interaction. Since light is travelling so fast and far it seems intuitive thst interstellar matter, as fleeting as it is, could act to slow light down over these great spacetime distances.
- brylie 7y ago
  According to the Wikipedia article, "tired light" has been discredited due to lack of observational evidence and assumptiins about light scattering: https://en.m.wikipedia.org/wiki/Tired_light https://en.m.wikipedia.org/wiki/Tired_light
  - shpeak 7y ago
    Light scattering assumption is invalid, this is true, so photon losses energy because of an other mechanism. However, newer telescopes discovers more stars and galaxies than predicted by theories of expanding universe, thus favors Tired Light Theory. Also, discovery of gravitational waves confirmed that (speed of light in pure vacuum) c≠const, so physical vacuum is not an empty space, thus favoring TLT again.
- CoryG89 7y ago
  This doesn't make much sense to me. The speed of light is a constant, right? Gravitational interactions may change the directional component, but the speed would remain constant, no?
  - earthboundkid 7y ago
    Constant in a vacuum. Read about https://en.wikipedia.org/wiki/Cherenkov_radiation https://en.wikipedia.org/wiki/Cherenkov_radiation
  - ncmncm 7y ago
    It was expressed clumsily. The idea would be that photons can lose energy over time or space. One problem is that they would all have to lose energy at exactly the same rate as neighboring photons, so be a property of the space they are going through, not the photons themselves.
snowwrestler 7y ago
If I am reading this correctly, in plain English what it seems like they are suggesting is that old Type Ia supernovae worked differently from newer supernovae; e.g. that a Type Ia supernova 5 billion years ago had a different brightness curve than a Type Ia supernova from 50 million years ago. On its face this seems like an extraordinary claim; I can’t think of how to square it with the (fundamental) assumption that the laws of the universe have not changed during its lifetime.
- enkid 7y ago
  You're not reading this correctly. It's questioning the assumption that the Supernova are of the same brightness no matter the age of the star. Because the supernovaing stars are on average older now then they were in the early universe, there's a systemic bias in the data making it seem like there is red shift when there is not. No new physics required outside of a new understanding of how Type Ia supernova work.
- mannykannot 7y ago
  Do the current models of dark energy not have it changing over time? Or at least allow for the possibility? The idea that fundamentally, physical law does not change over time, is either a metaphysical or methodological assumption (or a bit of both), but it is certainly not an empirically-derived result, and it clearly does not rule out manifestations of those fundamental laws changing over time.
  - PopePompus 7y ago
    The issue is much more prosaic than the fundamental laws of physics changing over time. On average, very distant supernovae have a different composition than nearby supernovae. As the universe ages, more light element atoms are fused into heavier element atoms and expelled into space. That material is incorporated into the next generation of stars, so stars begin their lives with less and less hydrogen, as the universe ages. If that influences the light curves of supernovae, then it greatly complicates the use of supernovae as standard candles for distance measurement.
bananabreakfast 7y ago
Speaking from the pov as a former astrophysicist, that definition of redshift is not quite accurate. While we have observed the universe expanding since Edwin Hubble published his findings 90 years ago, it was in fact the Friedmann equations produced 10 years earlier that established how it would work within the context of general relativity. The Universe is expanding at every known point at the same time at the same rate. However, this is only an observable effect over > 10 Mpc of distance (e.g. extra-galactic distances). This is far too large to effect a point particle like a photon no matter how far it travels so I'm afraid your analogy of a rubber sheet is incorrect. The lengthening of said photon's wavelength comes instead from doppler expansion due to the nature of differing relativistic reference frames. According to our local reference frame, doppler effects of nearby stars and galaxies are only due to local motion such as Andromeda being blue shifted as it approaches us for our eventual merger. However, as you go further and further out from our galaxy the metric expansion of spacetime begins to dominate the relative motion of all bodies, leading to observing progressively redder and redder light curves. A galaxy billions of light years away is perceived to be moving away from our reference frame at a significant percentage of the speed of light! However, if you were to change our relativistic reference frame to match a distance galaxy's you would not see its photons "stretched out" but in fact they would look completely unshifted despite having traversed nearly all of spacetime, expanding as it went.
- igravious 7y ago
  One thing I've never understood is this. How do we tell the difference between the expansion of space and the dilation of time? Given that space and time are crudely specking intertwined (spacetime) how do we know that the effect we see -- redshifting -- is due to the spatial expansion at every known "point" and not the temporal expansion (contraction?) at every known moment? Could not the spatial size of the universe be constant and its "clock" be being distorted?
  - fennecfoxen 7y ago
    If this temporal expansion or contraction produces redshift in the exact same way, and you can fit it into Relativity with the appropriate math, well, six of one, half a dozen of the other — until such point as we understand some discrete mechanism underlying the operation of the universe that would be affected by the expansion of one rather than the other: minimum time steps, minimum distances, proceeding from individual bits, increments of information. (Such a thing is theorized, but observing it is on no one's radar today, so good luck.) (If it doesn't fit, then it's just wrong.)
alex77456 7y ago
>To know if the universe is expanding, we need to know the following: how far away things are, and whether they're moving away from us May be a stupid question but wouldn't this apply only if we knew that the universe is uniformly dense? And we only see a tiny portion of it.

mirimir 7y ago

That's how I understand it. But maybe there are other standardizable objects. The sense I got from TFA is that there aren't.

kiliantics 7y ago
There are actually. There is a class of star in the "tip of the red giant branch" (TRGB) which have also been used to calibrate luminosities of type Ia supernovae[0] to form a "cosmic distance ladder". I believe they have been used to confirm the accelerated expansion results (though less strongly). I am not super well versed in stellar evolution, but I believe that the TRGB should make this calibration method independent of progenitor age? Hopefully someone could chime in on this. I was formerly a cosmologist and would be very interested to know more about this result from any experts that might be reading this. [0] There is an explanation of this calibration here: https://aasnova.org/2019/07/26/tldr-the-trgb-gives-us-another-h0ttake/ https://aasnova.org/2019/07/26/tldr-the-trgb-gives-us-anothe...
- ncmncm 7y ago
  Of course these could have the same, or a related, problem.
- sanxiyn 7y ago
  Nope, the link is proposing TRGB-SNIa as an alternative to Cepheid-SNIa, not TRGB as an alternative to SNIa.
  - mirimir 7y ago
    Yeah. In my reading, it almost characterized SNIa as fact.

wallace_f 7y ago

Thanks for the explanation, but I'm stuck on the last sentence of this part: >evidence of accelerating expansion is that based on the redshift of nearby objects, we would expect distant objects to have a higher redshift than they actually do. I think what you mean is "than we'd expect them to."

ncmncm 7y ago
Right. They have the red shift we would expect to find in more distant objects than they seem to be, just based on how bright their supernovas are. But if the brightness is off, the anomaly might disappear. Or get bigger, all bets are off.
deleted 7y ago
[deleted]

brylie 7y ago

I'm not an astronomer, but "tired light" makes more intuitive sense to me to explain the redshift. In effect, we already know that matter can affect light through gravitational interaction. Since light is travelling so fast and far it seems intuitive thst interstellar matter, as fleeting as it is, could act to slow light down over these great spacetime distances.

brylie 7y ago
According to the Wikipedia article, "tired light" has been discredited due to lack of observational evidence and assumptiins about light scattering: https://en.m.wikipedia.org/wiki/Tired_light https://en.m.wikipedia.org/wiki/Tired_light
- shpeak 7y ago
  Light scattering assumption is invalid, this is true, so photon losses energy because of an other mechanism. However, newer telescopes discovers more stars and galaxies than predicted by theories of expanding universe, thus favors Tired Light Theory. Also, discovery of gravitational waves confirmed that (speed of light in pure vacuum) c≠const, so physical vacuum is not an empty space, thus favoring TLT again.
CoryG89 7y ago
This doesn't make much sense to me. The speed of light is a constant, right? Gravitational interactions may change the directional component, but the speed would remain constant, no?
- earthboundkid 7y ago
  Constant in a vacuum. Read about https://en.wikipedia.org/wiki/Cherenkov_radiation https://en.wikipedia.org/wiki/Cherenkov_radiation
- ncmncm 7y ago
  It was expressed clumsily. The idea would be that photons can lose energy over time or space. One problem is that they would all have to lose energy at exactly the same rate as neighboring photons, so be a property of the space they are going through, not the photons themselves.

snowwrestler 7y ago

If I am reading this correctly, in plain English what it seems like they are suggesting is that old Type Ia supernovae worked differently from newer supernovae; e.g. that a Type Ia supernova 5 billion years ago had a different brightness curve than a Type Ia supernova from 50 million years ago. On its face this seems like an extraordinary claim; I can’t think of how to square it with the (fundamental) assumption that the laws of the universe have not changed during its lifetime.

enkid 7y ago
You're not reading this correctly. It's questioning the assumption that the Supernova are of the same brightness no matter the age of the star. Because the supernovaing stars are on average older now then they were in the early universe, there's a systemic bias in the data making it seem like there is red shift when there is not. No new physics required outside of a new understanding of how Type Ia supernova work.
mannykannot 7y ago
Do the current models of dark energy not have it changing over time? Or at least allow for the possibility? The idea that fundamentally, physical law does not change over time, is either a metaphysical or methodological assumption (or a bit of both), but it is certainly not an empirically-derived result, and it clearly does not rule out manifestations of those fundamental laws changing over time.
- PopePompus 7y ago
  The issue is much more prosaic than the fundamental laws of physics changing over time. On average, very distant supernovae have a different composition than nearby supernovae. As the universe ages, more light element atoms are fused into heavier element atoms and expelled into space. That material is incorporated into the next generation of stars, so stars begin their lives with less and less hydrogen, as the universe ages. If that influences the light curves of supernovae, then it greatly complicates the use of supernovae as standard candles for distance measurement.

bananabreakfast 7y ago

Speaking from the pov as a former astrophysicist, that definition of redshift is not quite accurate. While we have observed the universe expanding since Edwin Hubble published his findings 90 years ago, it was in fact the Friedmann equations produced 10 years earlier that established how it would work within the context of general relativity. The Universe is expanding at every known point at the same time at the same rate. However, this is only an observable effect over > 10 Mpc of distance (e.g. extra-galactic distances). This is far too large to effect a point particle like a photon no matter how far it travels so I'm afraid your analogy of a rubber sheet is incorrect. The lengthening of said photon's wavelength comes instead from doppler expansion due to the nature of differing relativistic reference frames. According to our local reference frame, doppler effects of nearby stars and galaxies are only due to local motion such as Andromeda being blue shifted as it approaches us for our eventual merger. However, as you go further and further out from our galaxy the metric expansion of spacetime begins to dominate the relative motion of all bodies, leading to observing progressively redder and redder light curves. A galaxy billions of light years away is perceived to be moving away from our reference frame at a significant percentage of the speed of light! However, if you were to change our relativistic reference frame to match a distance galaxy's you would not see its photons "stretched out" but in fact they would look completely unshifted despite having traversed nearly all of spacetime, expanding as it went.

igravious 7y ago
One thing I've never understood is this. How do we tell the difference between the expansion of space and the dilation of time? Given that space and time are crudely specking intertwined (spacetime) how do we know that the effect we see -- redshifting -- is due to the spatial expansion at every known "point" and not the temporal expansion (contraction?) at every known moment? Could not the spatial size of the universe be constant and its "clock" be being distorted?
- fennecfoxen 7y ago
  If this temporal expansion or contraction produces redshift in the exact same way, and you can fit it into Relativity with the appropriate math, well, six of one, half a dozen of the other — until such point as we understand some discrete mechanism underlying the operation of the universe that would be affected by the expansion of one rather than the other: minimum time steps, minimum distances, proceeding from individual bits, increments of information. (Such a thing is theorized, but observing it is on no one's radar today, so good luck.) (If it doesn't fit, then it's just wrong.)

alex77456 7y ago

>To know if the universe is expanding, we need to know the following: how far away things are, and whether they're moving away from us May be a stupid question but wouldn't this apply only if we knew that the universe is uniformly dense? And we only see a tiny portion of it.