Is the Universe Younger than We Thought?

Accordingly to this article:
https://medium.com/the-cosmic-compan...t-e8a649a32ec8
"Is the Universe Younger than We Thought?", is the age of the universe,
not 13,8 billion years, but 11 billion years old.
This seems, to me, a rather big shift, specific because it is based on
gravitational lensing.

Nicolaas Vroom

[[Mod. note -- This article is based on this press release
"High value for Hubble constant from two gravitational lenses"
https://www.mpa-garching.mpg.de/743539/news20190913
which in turn describes this research paper
"A measurement of the Hubble constant from angular diameter distances
to two gravitational lenses"
https://science.sciencemag.org/content/365/6458/1134
which is nicely synopsized in this commentary
"An expanding controversy"
/An independently calibrated measurement fortifies the debate
around Hubble's constant/
https://science.sciencemag.org/content/365/6458/1076

Figure 6 of the /Science/ research article gives a nice comparison
of some of the recent Hubble-constant measurements, showing that the
choice of cosmological model (at least within the range of models
considered by the authors) makes rather little difference.
-- jt]]

Details at
https://www.iau.org/news/pressreleas.../iau1910/?lang

This one looks like a comet, but observations are just beginning.

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In article ,
Nicolaas Vroom writes:

Accordingly to this article:
https://medium.com/the-cosmic-compan...ger-than-we-t=
hought-e8a649a32ec8
"Is the Universe Younger than We Thought?", is the age of the universe,
not 13,8 billion years, but 11 billion years old.
This seems, to me, a rather big shift, specific because it is based on
gravitational lensing.

All else being equal, the age of the universe is inversely proportional
to the Hubble constant.

The headline doesn't deserve any prizes. There are many measurements of
the Hubble constant, and the field has a history of discrepant
measurement (i.e. measurements which differ by significantly more than
their formal uncertainties). Recently, the debate has shifted from "50
or 100?" to "67 or 73?" but since the formal uncertainties have also
gone down, one could argue that the "tension" is comparable to that in
the old days. There is more than one measurement supporting 67, and
more than one supporting 73. So, ONE additional measurement doesn't
mean "the textbooks will have to be rewritten" or some such nonsense,
but rather is an additional piece of information which must be taken
into account.

It should be noted that there are many measurements of the Hubble
constant from gravitational lenses. Not all agree. The biggest source
of uncertainty is probably the fact that the result depends on knowing
the mass distribution of the lens galaxy.

For what it's worth, I am co-author on a paper doing this sort of thing:

http://www.astro.multivax.de:8000/he...ons/info/0218=
..html

Our value back then, almost 20 years ago, was 69+13/-19 at 95%
confidence. The first two authors recently revised this after
re-analysing the data, arriving at 72+/-2.6 at 1 sigma, though this
includes a better (published in 2004) lens model as well. The papers
are arXiv:astro-ph/9811282 and arXiv:1802.10088. Both are published in
MNRAS (links to freely accessible versions are at the arXiv references
above). It's tricky to get right. As Shapley said, "No one trusts a
model except the man who wrote it; everyone trusts an observation except
the man who made it." :-)

The above uses just the gravitational-lens system to measure the Hubble
constant. Such measurements have also been made before for the two lens
systems mentioned in the press release. What one actually measures is
basically the distance to the lens. Since the redshift is known, one
knows the distance for this particular redshift; knowing the redshift
and the distance gives the Hubble constant. In the new work, this was
then used to calibrate supernovae of with known redshifts. (Determining
the Hubble constant from the magnitude-redshift relation for supernovae
is also possible, of course (and higher-order effects allow one to
determine the cosmological constant and the density parameter
(independently of the Hubble constant), for which the 2011 Nobel Prize
was awarded), but one needs to know the absolute luminosity, which has
to be calibrated in some way. Since they measure the distance at two
separate redshifts, the cosmology cancels out (at least within the range
of otherwise reasonable models). Their value is 82+/-8, which is
consistent with the current "high" measurements. There are many reasons
to doubt that the universe is only 11 billion years old, so a value of
73 is probably about right.

The MPA press release is more carefully worded: "While the uncertainty
is still relatively large" and notes that the value that that inferred
from the CMB. However, many would say that the anomaly is that the CMB
(in particular the Planck data) seem to indicate a low value.

Figure 6 of the /Science/ research article gives a nice comparison
of some of the recent Hubble-constant measurements, showing that the
choice of cosmological model (at least within the range of models
considered by the authors) makes rather little difference.
-- jt]]

In principle, the cosmological model can make a difference, but these
days we believe that the values of lambda and Omega have been narrowed
down enough that there isn't much room to move; measuring the distance
at two different redshift essentially pins it down.

In article ,
Nicolaas Vroom writes:
Accordingly to this article:
https://medium.com/the-cosmic-compan...t-e8a649a32ec8

which in turn describes this research paper
"A measurement of the Hubble constant from angular diameter distances
to two gravitational lenses"
https://science.sciencemag.org/content/365/6458/1134

The paper is behind a paywall, but the Abstract, which is public,
summarizes the results. Two gravitational lenses at z=0.295 and
0.6304 are used to calibrate SN distances. The derived Hubble-
Lemaitre parameter H_0 is 82+/-8, about 1 sigma larger than other
local determinations and 1.5 sigma larger than the Planck value.

As Phillip wrote, the observations have their uncertainties, but 50
or so lenses would measure H_0 independently of other methods.

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Help keep our newsgroup healthy; please don't feed the trolls.
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[[Mod. note -- I've now found the preprint -- it's arXiv:1906.06712.
Sorry for not including that in my original mod.note. -- jt]]

Steve Willner wrote:
which in turn describes this research paper
"A measurement of the Hubble constant from angular diameter distances
to two gravitational lenses"
https://science.sciencemag.org/content/365/6458/1134

The paper is behind a paywall, but the Abstract, which is public,
summarizes the results. [[...]]

In a moderator's note, I wrote
[[Mod. note -- I've now found the preprint -- it's arXiv:1906.06712.
Sorry for not including that in my original mod.note. -- jt]]

Oops, /dev/brain parity error. The preprint is 1909.06712
repeat 1909.06712. Sorry for the mixup. -- Jonathan

In article ,
"Jonathan Thornburg [remove -animal to reply]"
writes:

The preprint is 1909.06712

Two additional preprints are at
https://arxiv.org/abs/1907.04869 and
https://arxiv.org/abs/1910.06306
These report direct measurements of gravitational lens distances
rather than a recalibration of the standard distance ladder.

The lead author Shajib of 06306 spoke here today and showed an
updated version of Fig 12 of the 04869 preprint. The upshot is that
the discrepancy between the local and the CMB measurements of H_0 is
between 4 and 5.7 sigma, depending on how conservative one wants to
be about assumptions. The impression I got is that either there's a
systematic error somewhere or there's new physics. The local H_0 is
based on two independent methods -- distance ladder and lensing -- so
big systematic errors in local H_0 seem unlikely. The CMB H_0 is
based on Planck with WMAP having given an H_0 value more consistent
with the local one. "New physics" could be something as simple as
time-varying dark energy, but for now it's too soon to say much.

One other note from the talk: it takes an expert modeler about 8 months
to a year to model a single lens system. Shajib and others are trying
to automate the modeling, but until that's done, measuring a large
sample of lenses will be labor-intensive. Even then, it will be
cpu-intensive. Shahib mentioned 1 million cpu-hours for his model of
DES J0408-53545354, and about 40 lenses are needed to give the desired
precision of local H_0.

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Help keep our newsgroup healthy; please don't feed the trolls.
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In article , (Steve
Willner) writes:

Two additional preprints are at
https://arxiv.org/abs/1907.04869 and
https://arxiv.org/abs/1910.06306
These report direct measurements of gravitational lens distances
rather than a recalibration of the standard distance ladder.

The upshot is that
the discrepancy between the local and the CMB measurements of H_0 is
between 4 and 5.7 sigma, depending on how conservative one wants to
be about assumptions.

"New physics" could be something as simple as
time-varying dark energy

Now THAT'S an understatement! :-)

Also interesting on this topic: arXiv:1910.02978, which suggests that
the local Cepheid measurements are the odd ones. arXiv 1802.10088
re-analyses data on one lens system, resulting in a slightly longer time
delay and hence slightly lower Hubble constant, i.e. making this
particular system more consistent with the CMB value. Steve mentioned
how long the modelling takes. A modeller has the input data, though;
there is a huge amount of work just to get that far as well: observing,
reducing the data, and so on.

On 19/10/15 10:17 PM, Steve Willner wrote:
In article ,
"Jonathan Thornburg [remove -animal to reply]"
writes:

The preprint is 1909.06712

Two additional preprints are at
https://arxiv.org/abs/1907.04869 and
https://arxiv.org/abs/1910.06306
...
...
One other note from the talk: it takes an expert modeler about 8 months
to a year to model a single lens system. Shajib and others are trying
to automate the modeling,

You obviously do not mean that they do it by pencil and paper at this
moment. So why is modeling labor-intensive? Isn't it just putting a
point mass in front of the observed object, which only requires fitting
the precise position and distance of the point mass using the observed
image? (And if so, is the actual imaging with the point mass in some
place the difficult part?) Or is the problem that the lensing object
may be more extended than a point mass? (Or is it something worse!?)

--
Jos

[[Mod. note -- In these cases the lensing object is a galaxy (definitely
not a point mass!). For precise results a nontrivial model of the
galaxy's mass distribution (here parameterized by the (anisotropic)
velocity dispersion of stars in the lensing galaxy's central region)
is needed, which is the tricky (& hence labor-intensive) part.
-- jt]]

In article , Jos Bergervoet
writes:

On 19/10/15 10:17 PM, Steve Willner wrote:
In article ,
"Jonathan Thornburg [remove -animal to reply]"
writes:

The preprint is 1909.06712

Two additional preprints are at
https://arxiv.org/abs/1907.04869 and
https://arxiv.org/abs/1910.06306
...
...
One other note from the talk: it takes an expert modeler about 8 months
to a year to model a single lens system. Shajib and others are trying
to automate the modeling,

You obviously do not mean that they do it by pencil and paper at this
moment.

Right; it's done on computers these days. :-)

So why is modeling labor-intensive? Isn't it just putting a
point mass in front of the observed object, which only requires fitting
the precise position and distance of the point mass using the observed
image?

A point mass could be done with pencil and paper.

(And if so, is the actual imaging with the point mass in some
place the difficult part?) Or is the problem that the lensing object
may be more extended than a point mass? (Or is it something worse!?)

[[Mod. note -- In these cases the lensing object is a galaxy (definitely
not a point mass!). For precise results a nontrivial model of the
galaxy's mass distribution (here parameterized by the (anisotropic)
velocity dispersion of stars in the lensing galaxy's central region)
is needed, which is the tricky (& hence labor-intensive) part.
-- jt]]

Right.

In addition to the time delay, which depends on the potential, one fits
the image positions, which depend on the derivative of the potential,
and can also choose to fit the brightness of the images, which depends
on the second derivative of the potential. (Since the brightness can be
affected by microlensing, one might choose not to fit for it, or to
include a model of microlensing as well.) If the source is resolved,
then the brightness distribution of the source also plays a role.

Also, one can (and, these days, probably must) relax the assumption that
there is only the lens which affects the light paths. While in most
cases a single-plane lens is a good enough approximation, the assumption
that the background metric is FLRW might not be. In particular, if the
path is underdense (apart from the part in the lens plane, which of
course is very overdense), then the distance as a function of redshift
is not that which is given by the standard Friedmann model. At this
level of precision, it's probably not enough to simply parameterize
this, but rather one needs some model of the mass distribution near the
beams.

The devil is in the details.

Think of the Hubble constant as determined by the traditional methods
(magnitude--redshift relation). In theory, one needs ONE object whose
redshift (this is actually quite easy) and distance are known in order
to compute it. In practice, of course, there is much more involved
(mostly details of the calibration of the distance ladder), though this
is still relatively straightforward compared to a detailed lens model.

In article ,
"Phillip Helbig (undress to reply)" writes:
At this level of precision, it's probably not enough to simply
parameterize this, but rather one needs some model of the mass
distribution near the beams.

That's exactly right (at least to the extent I understood Shajib's
talk). In particular, one has to take into account the statistical
distribution of mass all along and near the light path and also (as
others wrote) the mass distribution of the lensing galaxy
itself. It's even worse than that in systems that have multiple
galaxies contributing to the lensing. Not only do their individual
mass distributions matter, their relative distances along the line of
sight are uncertain and must be modeled.

Presumably all that can be automated -- at the cost of many extra cpu
cycles -- but it hasn't been done yet.

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Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
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