6 ms·

Gravitational-lensing measurements push Hubble-constant discrepancy past 5σ

155 points by digital55 7y ago

sounds 7y ago
There's a comment at the end of the article that poses a good alternate explanation: > Measurements that yield a value for H0 from the CMB amount to an average over the entire age of the universe, whereas the two other measurements described average over a relatively recent fraction of that age. Considering that high-z redshift surveys have given fairly solid indications that cosmic expansion is accelerating, and thus H0 should be increasing over time, the 5.3 sigma divergence between the two values could be a direct result of the different durations for each average.
- DiogenesKynikos 7y ago
  The comment doesn't quite make sense, but I think what it's trying to get at is that in order to compare the different measurements of H0, you have to assume a physical model. You can calculate the rate of expansion of the Universe in our vicinity using Type-Ia supernovae. One calls that a "local" measurement of H0. One can also do various measurements of the Cosmic Microwave Background (CMB), and using the standard model of cosmology (Lambda CDM, meaning a theory with a cosmological constant and cold dark matter), predict what H0 should be. In order to connect the local and CMB measurements, in other words, you need a theory of cosmology. If the local and CMB measurements don't match, then there are basically two possibilities: 1. There are systematic errors in the measurements. We just have to figure out who made a mistake in their measurements. (I think most people believe this to be what's happening). 2. We need to modify the theory, Lambda CDM, by adding in new types of matter (such as sterile neutrinos) or modifying the theory of gravity in some way (this is very difficult to do without violating other experimental results).
- astro123 7y ago
  Nope, that's total garbage. H0 is defined as the rate of expansion now.
Florin_Andrei 7y ago
> 5 sigma Sounds like there's new physics to be gleaned from the phenomenon.
- astro123 7y ago
  I don't know why this is downvoted (maybe I'm missing a bad joke)? But this is exactly why people are excited about this. Either, a) we have a systematic in our measurement and our model is right and eventually we'll work this out when we fix up everything, or b) our model isn't right and there is new physics (i.e. something not in Lambda CDM + GR) that explains this discrepancy.
  - Florin_Andrei 7y ago
    Perhaps I should have used the conditional - "there might be new physics". But yeah, that's the long and short of it.
    - diegoperini 7y ago
      It was obvious you were just guessing. No need to be politically correct about it.
- 8bitsrule 7y ago
  There'll need to be a few more funerals first.
tyfon 7y ago
I thought the CMB value for H0 (measured by the Planck satellite) was already known to be incorrect as the measurements for it did not take into account that the earth is moving through the CMB and only measured in one direction. This is the big discussion in the astrophysics field at the moment.
- astro123 7y ago
  I'm 99% sure this isn't true. Can you point to a single paper that mentions it? Edit: actually I'm 100% sure this isn't true. See for example the all sky map from planck (http://www.bbc.co.uk/news/special/2013/newsspec_5106/img/planck_976.jpg http://www.bbc.co.uk/news/special/2013/newsspec_5106/img/pla...). And a paper discussing how they will measure the CMB dipole using planck https://ui.adsabs.harvard.edu/abs/2002A%26A...393..359P/abstract https://ui.adsabs.harvard.edu/abs/2002A%26A...393..359P/abst...
  - tyfon 7y ago
    I stand corrected, the current Planck analysis seems to take in account that we are moving through the CMB rest frame although I need to read a bit more. It's a long paper [1] It was the earlier Planck measurements that did not account for this. [1] https://arxiv.org/pdf/1907.12875.pdf https://arxiv.org/pdf/1907.12875.pdf
    - mturmon 7y ago
      This effect seems to be treated and dismissed as insignificant w/r/t the cosmological parameters, in section 3.11 (page 78) of the paper you reference. There were a lot of new systematic effects in the Planck data, and a lot of the data analysis work amounted to identifying the most significant ones and modeling them out.
- deleted 7y ago
  [deleted]
timtylin 7y ago
> Led by Sherry Suyu, the H0LiCOW (H_0 Lenses in COSMOGRAIL’s Wellspring) collaboration uses gravitationally lensed quasars to independently measure H_0. OMG I love physicists so much
- dkural 7y ago
  "Its earliest known appearance was in a tongue-in-cheek letter to the editor: "A lover of the cow writes to this column to protest against a certain variety of Hindu oath having to do with the vain use of the name of the milk producer. There is the profane exclamations, 'holy cow!' and, 'By the stomach of the eternal cow!'""
mirimir 7y ago
OK, so TFA says: > Combining the H0LiCOW and standard-candle measurements gives an H0 of 73.8 ± 1.1 km/s/Mpc, which differs from the ΛCDM value by 5.3 standard deviations. So what is it about ΛCDM that could lead to the discrepancy? Is it simplistic to conclude that it's this? > Dark energy, the model presumes, takes the form of a cosmological constant Λ ... That is, reducing Λ would mean no discrepancy? But what other discrepancies would that create?
- astro123 7y ago
  > Is it simplistic to conclude that it's this? > Dark energy, the model presumes, takes the form of a cosmological constant Λ ... Yup quite possibly, and there are people investigating it! The extended model is "Time dependent dark energy" See [1] or many papers [2] > But what other discrepancies would that create? This is kinda the crux - modifying something to fix the current problems causes other problems. An example of this is the proposal that DE is just a result of us having the wrong model for gravity (GR) and that gravity is different at cosmological distances (note that this is not MOND which was proposed to not require dark matter and pretty universally unfavoured). However, gravity is really really well measured at solar system distances so you somehow need a theory of gravity that looks a lot like GR at small ranges and quite different at long ones, and that it hard. [1] https://www.forbes.com/sites/startswithabang/2017/05/30/is-there-really-a-cosmological-constant-or-is-dark-energy-changing-with-time/#4cad47ea78b5 https://www.forbes.com/sites/startswithabang/2017/05/30/is-t... [2] https://ui.adsabs.harvard.edu/search/q=title%3A%22time%20dependent%20dark%20energy%22&sort=date%20desc%2C%20bibcode%20desc&p_=0 https://ui.adsabs.harvard.edu/search/q=title%3A%22time%20dep...
  - mirimir 7y ago
    Thanks. That sounds a little like parameter fitting. But maybe that's ignorantly harsh. The fantasy of simple being beautiful and so more likely "true" (which itself is an iffy concept). Anyway, isn't Λ basically a constant term in the gravity equation? So then you argue that Λ isn't constant. Maybe it depends on time. Or on distance, which I guess just makes it a polynomial. Something like that?
the8472 7y ago
Another recent item that raises questions about dark energy: https://news.ycombinator.com/item?id=21974117 https://news.ycombinator.com/item?id=21974117
- chadcmulligan 7y ago
  Also a MOND simulation which is interesting https://news.ycombinator.com/item?id=22278215 https://news.ycombinator.com/item?id=22278215
wiredfool 7y ago
Redacted years ago as an undergraduate in an astro class, there were jokes about the "Hubble not so constant", as it was difficult to pin it down at the time. I see that we're continuing the tradition. (Astro was a bit of a rude awakening to an engineer -- anything that wasn't an order of magnitude could safely be shoveled into the "constant" part of the calculation.)
- astro123 7y ago
  Famously, Hubble's 1931 paper that detected the expansion of the universe found H0 ~ 500 km/s/Mpc (fig 5 in [1]). The distances that he was using were way off... Through most of the late 1900s the uncertainty was between 50 and 100 km/s/Mpc. Now we know it at least as well as most other things, but this history of uncertainty means it is treated differently. Most annoyingly, simulations often work in units of Distance/h (where h = H0/100). This causes anyone who uses them incredible annoyance as you need to get your factors of little h right. Someone even wrote a paper called "Damn you little h" [2]. It's a total pain... [1] http://articles.adsabs.harvard.edu/pdf/1931ApJ....74...43H http://articles.adsabs.harvard.edu/pdf/1931ApJ....74...43H [2] https://arxiv.org/abs/1308.4150 https://arxiv.org/abs/1308.4150
lifeisstillgood 7y ago
>>> the researchers studied quasars whose light is so strongly deflected by foreground galaxies that they appear as multiple distinct images, as shown in the figure So, I think I get the dummies guide, except this bit - I am surprised that a lens can produce multiple images from one light source - is this something i don't experience much on human scales or am I just not using the right lenses?
- dmitrybrant 7y ago
  Remember that these "lenses" are very irregularly shaped, not at all like the smooth lens of a camera. A good real-world analogy is to look through a wine glass, for example: https://laser.physics.sunysb.edu/_samantha/journal/ringpic.png https://laser.physics.sunysb.edu/_samantha/journal/ringpic.p...
- deleted 7y ago
  [deleted]
jobigoud 7y ago
The Hubble parameter. It's not defined to be constant in time, only in space.
- DiogenesKynikos 7y ago
  The Hubble parameter is a function of redshift (or time, equivalently): H(z). The Hubble constant is H_0 := H(z=0). The discrepancy is in H_0. That implies a discrepancy at other redshifts as well, though how you connect H_0 to H(z) depends on your cosmological model. That's why the discrepancy in H_0 measurements is model-dependent. Different experiments are sensitive to the expansion rate during different epochs of cosmic history.
swebs 7y ago
Quasars are very far away. What if the Hubble constant was just that much higher back then? Many models of the early universe show a period of rapid expansion in the first billion years.

155 points by digital55 7y ago

There's a comment at the end of the article that poses a good alternate explanation: > Measurements that yield a value for H0 from the CMB amount to an average over the entire age of the universe, whereas the two other measurements described average over a relatively recent fraction of that age. Considering that high-z redshift surveys have given fairly solid indications that cosmic expansion is accelerating, and thus H0 should be increasing over time, the 5.3 sigma divergence between the two values could be a direct result of the different durations for each average.

DiogenesKynikos 7y ago
The comment doesn't quite make sense, but I think what it's trying to get at is that in order to compare the different measurements of H0, you have to assume a physical model. You can calculate the rate of expansion of the Universe in our vicinity using Type-Ia supernovae. One calls that a "local" measurement of H0. One can also do various measurements of the Cosmic Microwave Background (CMB), and using the standard model of cosmology (Lambda CDM, meaning a theory with a cosmological constant and cold dark matter), predict what H0 should be. In order to connect the local and CMB measurements, in other words, you need a theory of cosmology. If the local and CMB measurements don't match, then there are basically two possibilities: 1. There are systematic errors in the measurements. We just have to figure out who made a mistake in their measurements. (I think most people believe this to be what's happening). 2. We need to modify the theory, Lambda CDM, by adding in new types of matter (such as sterile neutrinos) or modifying the theory of gravity in some way (this is very difficult to do without violating other experimental results).
astro123 7y ago
Nope, that's total garbage. H0 is defined as the rate of expansion now.

Florin_Andrei 7y ago

> 5 sigma Sounds like there's new physics to be gleaned from the phenomenon.

astro123 7y ago
I don't know why this is downvoted (maybe I'm missing a bad joke)? But this is exactly why people are excited about this. Either, a) we have a systematic in our measurement and our model is right and eventually we'll work this out when we fix up everything, or b) our model isn't right and there is new physics (i.e. something not in Lambda CDM + GR) that explains this discrepancy.
- Florin_Andrei 7y ago
  Perhaps I should have used the conditional - "there might be new physics". But yeah, that's the long and short of it.
  - diegoperini 7y ago
    It was obvious you were just guessing. No need to be politically correct about it.
8bitsrule 7y ago
There'll need to be a few more funerals first.

tyfon 7y ago

I thought the CMB value for H0 (measured by the Planck satellite) was already known to be incorrect as the measurements for it did not take into account that the earth is moving through the CMB and only measured in one direction. This is the big discussion in the astrophysics field at the moment.

astro123 7y ago
I'm 99% sure this isn't true. Can you point to a single paper that mentions it? Edit: actually I'm 100% sure this isn't true. See for example the all sky map from planck (http://www.bbc.co.uk/news/special/2013/newsspec_5106/img/planck_976.jpg http://www.bbc.co.uk/news/special/2013/newsspec_5106/img/pla...). And a paper discussing how they will measure the CMB dipole using planck https://ui.adsabs.harvard.edu/abs/2002A%26A...393..359P/abstract https://ui.adsabs.harvard.edu/abs/2002A%26A...393..359P/abst...
- tyfon 7y ago
  I stand corrected, the current Planck analysis seems to take in account that we are moving through the CMB rest frame although I need to read a bit more. It's a long paper [1] It was the earlier Planck measurements that did not account for this. [1] https://arxiv.org/pdf/1907.12875.pdf https://arxiv.org/pdf/1907.12875.pdf
  - mturmon 7y ago
    This effect seems to be treated and dismissed as insignificant w/r/t the cosmological parameters, in section 3.11 (page 78) of the paper you reference. There were a lot of new systematic effects in the Planck data, and a lot of the data analysis work amounted to identifying the most significant ones and modeling them out.
deleted 7y ago
[deleted]

timtylin 7y ago

> Led by Sherry Suyu, the H0LiCOW (H_0 Lenses in COSMOGRAIL’s Wellspring) collaboration uses gravitationally lensed quasars to independently measure H_0. OMG I love physicists so much

dkural 7y ago
"Its earliest known appearance was in a tongue-in-cheek letter to the editor: "A lover of the cow writes to this column to protest against a certain variety of Hindu oath having to do with the vain use of the name of the milk producer. There is the profane exclamations, 'holy cow!' and, 'By the stomach of the eternal cow!'""

mirimir 7y ago

OK, so TFA says: > Combining the H0LiCOW and standard-candle measurements gives an H0 of 73.8 ± 1.1 km/s/Mpc, which differs from the ΛCDM value by 5.3 standard deviations. So what is it about ΛCDM that could lead to the discrepancy? Is it simplistic to conclude that it's this? > Dark energy, the model presumes, takes the form of a cosmological constant Λ ... That is, reducing Λ would mean no discrepancy? But what other discrepancies would that create?

astro123 7y ago
> Is it simplistic to conclude that it's this? > Dark energy, the model presumes, takes the form of a cosmological constant Λ ... Yup quite possibly, and there are people investigating it! The extended model is "Time dependent dark energy" See [1] or many papers [2] > But what other discrepancies would that create? This is kinda the crux - modifying something to fix the current problems causes other problems. An example of this is the proposal that DE is just a result of us having the wrong model for gravity (GR) and that gravity is different at cosmological distances (note that this is not MOND which was proposed to not require dark matter and pretty universally unfavoured). However, gravity is really really well measured at solar system distances so you somehow need a theory of gravity that looks a lot like GR at small ranges and quite different at long ones, and that it hard. [1] https://www.forbes.com/sites/startswithabang/2017/05/30/is-there-really-a-cosmological-constant-or-is-dark-energy-changing-with-time/#4cad47ea78b5 https://www.forbes.com/sites/startswithabang/2017/05/30/is-t... [2] https://ui.adsabs.harvard.edu/search/q=title%3A%22time%20dependent%20dark%20energy%22&sort=date%20desc%2C%20bibcode%20desc&p_=0 https://ui.adsabs.harvard.edu/search/q=title%3A%22time%20dep...
- mirimir 7y ago
  Thanks. That sounds a little like parameter fitting. But maybe that's ignorantly harsh. The fantasy of simple being beautiful and so more likely "true" (which itself is an iffy concept). Anyway, isn't Λ basically a constant term in the gravity equation? So then you argue that Λ isn't constant. Maybe it depends on time. Or on distance, which I guess just makes it a polynomial. Something like that?

the8472 7y ago

Another recent item that raises questions about dark energy: https://news.ycombinator.com/item?id=21974117 https://news.ycombinator.com/item?id=21974117

chadcmulligan 7y ago
Also a MOND simulation which is interesting https://news.ycombinator.com/item?id=22278215 https://news.ycombinator.com/item?id=22278215

wiredfool 7y ago

Redacted years ago as an undergraduate in an astro class, there were jokes about the "Hubble not so constant", as it was difficult to pin it down at the time. I see that we're continuing the tradition. (Astro was a bit of a rude awakening to an engineer -- anything that wasn't an order of magnitude could safely be shoveled into the "constant" part of the calculation.)

astro123 7y ago
Famously, Hubble's 1931 paper that detected the expansion of the universe found H0 ~ 500 km/s/Mpc (fig 5 in [1]). The distances that he was using were way off... Through most of the late 1900s the uncertainty was between 50 and 100 km/s/Mpc. Now we know it at least as well as most other things, but this history of uncertainty means it is treated differently. Most annoyingly, simulations often work in units of Distance/h (where h = H0/100). This causes anyone who uses them incredible annoyance as you need to get your factors of little h right. Someone even wrote a paper called "Damn you little h" [2]. It's a total pain... [1] http://articles.adsabs.harvard.edu/pdf/1931ApJ....74...43H http://articles.adsabs.harvard.edu/pdf/1931ApJ....74...43H [2] https://arxiv.org/abs/1308.4150 https://arxiv.org/abs/1308.4150

lifeisstillgood 7y ago

>>> the researchers studied quasars whose light is so strongly deflected by foreground galaxies that they appear as multiple distinct images, as shown in the figure So, I think I get the dummies guide, except this bit - I am surprised that a lens can produce multiple images from one light source - is this something i don't experience much on human scales or am I just not using the right lenses?

dmitrybrant 7y ago
Remember that these "lenses" are very irregularly shaped, not at all like the smooth lens of a camera. A good real-world analogy is to look through a wine glass, for example: https://laser.physics.sunysb.edu/_samantha/journal/ringpic.png https://laser.physics.sunysb.edu/_samantha/journal/ringpic.p...
deleted 7y ago
[deleted]

jobigoud 7y ago

The Hubble parameter. It's not defined to be constant in time, only in space.

DiogenesKynikos 7y ago
The Hubble parameter is a function of redshift (or time, equivalently): H(z). The Hubble constant is H_0 := H(z=0). The discrepancy is in H_0. That implies a discrepancy at other redshifts as well, though how you connect H_0 to H(z) depends on your cosmological model. That's why the discrepancy in H_0 measurements is model-dependent. Different experiments are sensitive to the expansion rate during different epochs of cosmic history.

swebs 7y ago

Quasars are very far away. What if the Hubble constant was just that much higher back then? Many models of the early universe show a period of rapid expansion in the first billion years.