[HN Gopher] Gravitational-lensing measurements push Hubble-const...
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Gravitational-lensing measurements push Hubble-constant discrepancy
past 5s
Author : digital55
Score : 86 points
Date : 2020-02-18 16:20 UTC (6 hours ago)
(HTM) web link (physicstoday.scitation.org)
(TXT) w3m dump (physicstoday.scitation.org)
| mirimir wrote:
| 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 LCDM value by
| 5.3 standard deviations.
|
| So what is it about LCDM 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 L ...
|
| That is, reducing L would mean no discrepancy?
|
| But what other discrepancies would that create?
| astro123 wrote:
| > Is it simplistic to conclude that it's this? > Dark energy,
| the model presumes, takes the form of a cosmological constant L
| ...
|
| 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-t...
| [2]
| https://ui.adsabs.harvard.edu/search/q=title%3A%22time%20dep...
| mirimir wrote:
| 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 L basically a constant term in the gravity
| equation? So then you argue that L isn't constant. Maybe it
| depends on time. Or on distance, which I guess just makes it
| a polynomial. Something like that?
| astro123 wrote:
| > That sounds a little like parameter fitting.
|
| You're exactly right. We have a model (Lambda CDM + GR + a
| few other details). We have ways to generate descriptions
| of how the universe would look given certain parameters
| (H0=70, Omega_Lambda=0.73, etc, etc) and we basically just
| see what range of parameters gives a universe that looks
| like (quantified using some statistics) the one we see
| through our telescopes.
|
| But this is just phenomenology. The next step is working
| out the physics. For example, let's say we know there is X
| amount of something that looks like a cosmological constant
| - but what is that. This is what e.g. the search the dark
| matter particle is about - we know there is something that
| is cold + collisionless but what particle is it.
|
| > So then you argue that L isn't constant. Maybe it depends
| on time. Or on distance, which I guess just makes it a
| polynomial. Something like that?
|
| Yup, I'm pretty sure the only models we have tested are
| wCMD which allows w (the equation of state of DE) to be
| something other than -1 (which is what the cosmological
| constant is). There is also w(a) which parameterizes the
| equation of state of dark energy as a linear function of
| scale factor (just think of it as time, a=1 now a=0 at the
| big bang). So linear rather than constant. We haven't gone
| to higher order than that.
|
| The downside to adding parameters though is that, while you
| can always fit your data better (or at least as well) with
| more parameters,
|
| 1: Your error bars often blow up
|
| 2: Getting from phenomenology to physics might become hard.
| There are some models people have proposed that might allow
| us to fit the data, but then you need to explain why w
| changed in a very particular way at a very particular time.
| Basically it starts to look a little like overfitting.
| mirimir wrote:
| Thanks again.
|
| Overfitting a model for global climate change, for
| example, isn't an issue, because you're not interested in
| something like physics. I mean, it's based on physics,
| but that's buried way down in the model.
|
| But physics has different goals. Closer to math, I guess.
| cultus wrote:
| It's actually very easy to overfit climate models. They
| are fit to observed data with statistical inverse problem
| techniques (the same as I imagine they do with
| astronomical data). Climate change models are just
| directly discretized physical equations. Just like
| astronomy, the decisions are made on what physics are
| represented in the model and what are parameterized.
| mirimir wrote:
| > Climate change models are just directly discretized
| physical equations.
|
| I'm no expert, but it's my understanding that they're
| hugely more complicated than that implies. Sure, there's
| lots of physics there. But also chemistry and biology.
| The best ones are general circulation models,[0] and the
| outcomes will never fit some pretty theoretical
| structure.
|
| 0)
| https://en.wikipedia.org/wiki/General_Circulation_Model
| lisper wrote:
| > Overfitting a model for global climate change, for
| example, isn't an issue, because you're not interested in
| something like physics. I mean, it's based on physics,
| but that's buried way down in the model.
|
| It's not so much that as that the goals are different. We
| want to understand cosmology for its own sake. We want to
| understand climate change because that knowledge drives
| policy. For that purpose, it doesn't really matter that
| we're unable to predict the exact weather in Denver at
| 11:23 AM on October 27, 2091. What matters is that we are
| able to predict in broad brushstrokes that the
| consequences of business as usual will probably be bad,
| and so we ought to seriously consider doing something
| about it. There is no conceivable outcome of cosmological
| modeling that will drive policy changes like that.
| mirimir wrote:
| I agree that models for the overall development of the
| universe are a lot like models for global climate change.
| The scale is vastly different, of course. But I bet that
| the relative cell sizes in our models are similar.
| Because they're running on similar machines.
|
| But the goals for a theory of gravity, and its
| integration with QM, are totally different. Or at least,
| that's my perhaps naive opinion.
|
| Edit: That is, relative cell sizes and total cell counts.
| lukasb wrote:
| "There is no conceivable outcome of cosmological modeling
| that will drive policy changes like that."
|
| We hope
| Florin_Andrei wrote:
| > _5 sigma_
|
| Sounds like there's new physics to be gleaned from the
| phenomenon.
| 8bitsrule wrote:
| There'll need to be a few more funerals first.
| astro123 wrote:
| 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 wrote:
| Perhaps I should have used the conditional - "there might be
| new physics".
|
| But yeah, that's the long and short of it.
| tyfon wrote:
| 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 wrote:
| 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/pla...). And a paper discussing how they
| will measure the CMB dipole using planck
| https://ui.adsabs.harvard.edu/abs/2002A%26A...393..359P/abst...
| tyfon wrote:
| 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
| mturmon wrote:
| 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.
| the8472 wrote:
| Another recent item that raises questions about dark energy:
| https://news.ycombinator.com/item?id=21974117
| timtylin wrote:
| > 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 wrote:
| "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!'""
| wiredfool wrote:
| 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 wrote:
| 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
|
| [2] https://arxiv.org/abs/1308.4150
| sounds wrote:
| 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.
| astro123 wrote:
| Nope, that's total garbage. H0 is defined as the rate of
| expansion now.
| DiogenesKynikos wrote:
| 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).
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