Is the Universe Younger than We Thought?

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.

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.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

In article , (Steve
Willner) writes:

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.

These effects, i.e. that the mass in the universe is at least partially
distributed clumpily (apart from the gravitational lens itself, which
is, essentially by definition, a big clump), also influence the
luminosity distance, which of course can be used to determine not just
the Hubble constant but also the other cosmological parameters.
However, it's not as big a worry, for several reasons:

As far as the Hubble constant goes, the distances are, cosmologically
speaking, relatively small, whereas the effects of such small-scale
inhomogeneities increase with redshift.

Whether at low redshift for the Hubble constant or at high redshift for
the other parameters, usually several objects, over a range of
redshifts, are used. This has two advantages. One is that these
density fluctuations might (for similar redshifts) average out in some
sense. The other is that the degeneracy is broken because several
redshifts are involved. (If the inhomogeneity is an additional
parameter which can also affect the distance as calculated from
redshift, with just one object at one redshift one can't tell what
effect it has, but since the dependence on redshift is different for the
inhomogeneities, the Hubble constant, and the other parameters, then
some of the degeneracy is broken.)

At the level of precision required today, simply describing the effect
of small-scale inhomogeneities with one parameter is not good enough.
It does allow one to get an idea of the possible size of the effect,
though. To improve, there are two approaches. One is to try to measure
the mass along the line of sight, e.g. by weak lensing. Another is to
have some model of structure formation and calculate what it must be, at
least in a statistical sense.

There is a huge literature on this topic, though it is usually not
mentioned in more-popular presentations.

I even wrote a couple of papers myself on this topic:

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

http://www.astro.multivax.de:8000/he...ons/info/etas=
nia2.html

On 02 Nov 2019, (Steve Willner) wrote:
3. while there have been several suggestion for new physics to fix
the problem, none of them so far seems to work without disagreeing
with other data. ... What fun!

How about a migrating space-time curvature? Can that connect the two
places? It would have the effect of making distant places dimmer.
Try it in a static universe also, its effects simulate FRW.

[[Mod. note -- I think this is on the edge of our newsgroup ban on
"excessively speculative" submissions, but clearly *something* odd
is going on.

I wonder if it could be "just" non-uniformity in the Hubble flow
in the region of the "local" measurements?
-- jt]]

In article ,
Jos Bergervoet writes:
Yes! So why are only 20 people attending?!

Attendance was far higher than that. The video shows only one side
of the main floor of the room, and the other side is far more popular
(perhaps because it has a better view of the screen). There's a
balcony as well, and quite a few people leave at the end of the talk
and before the question period. I didn't count, but I think the
attendance was close to 100. Anyway it was about the normal number
for a colloquium here.

The colloquium list for the fall is at
https://www.cfa.harvard.edu/colloquia
if you want to see what other topics have been covered.

To the question in another message, I don't see why some local
perturbation -- presumably abnormally low matter density around our
location -- wouldn't solve the problem in principle, but if this were
a viable explanation, I expect the speaker would have mentioned it.
It's not as though no one has thought about the problem. The
difficulty is probably the magnitude of the effect. I don't work in
this area, though, so my opinion is not worth much.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

[[Mod. note -- I apologise for the delay in posting this article,
which was submitted on Fri, 8 Nov 2019 21:15:25 +0000.
-- jt]]

In article , Steve Willner
writes:

To the question in another message, I don't see why some local
perturbation -- presumably abnormally low matter density around our
location -- wouldn't solve the problem in principle, but if this were
a viable explanation, I expect the speaker would have mentioned it.
It's not as though no one has thought about the problem. The
difficulty is probably the magnitude of the effect. I don't work in
this area, though, so my opinion is not worth much.

I'm sure that someone must have looked at it, but is the measured Hubble
constant the same in all directions on the sky? (I remember Sandage
saying that even Hubble had found that it was, but I mean today, with
much better data, where small effects are noticeable.) If it is, then
such a density variation could be an explanation (assuming that it would
otherwise work) only if we "just happened" to be sitting at the centre
of such a local bubble.

Of course, some of us remember when the debate was not between 67 and
72, but between 50 and 100, with occasional suggestions of 42 (really)
or even 30. And both the "high camp" and "low camp" claimed
uncertainties of about 10 per cent. That wasn't a debate over whether
one used "local" or "large-scale" methods to measure it, but rather the
deference depended on who was doing the measuring. Nevertheless, it is
conceivable that there is some unknown systematic uncertainty* in one of
the measurements.

---
* For some, "unknown systematic uncertainty" is a tautology. Others,
however, include systematic uncertainties as part of the uncertainty
budget. (Some people use "error" instead of "uncertainty". The latter
is, I think, more correct, though in this case perhaps some unknown
ERROR is the culprit.

In article , "Richard D.
Saam" writes:

The Ho data is tightening:

**
Testing Low-Redshift Cosmic Acceleration with Large-Scale Structure
https://arxiv.org/abs/2001.11044
Seshadri Nadathur, Will J. Percival,
Florian Beutler, and Hans A. Winther
Phys. Rev. Lett. 124, 221301 - Published 2 June 2020
we measure the Hubble constant to be
Ho = 72.3 +/- 1.9 km/sec Mpc from BAO + voids
at z2

and

Ho = 69.0 +/- 1.2 km/sec Mpc from BAO
when adding Lyman alpha at BAO at z=2.34
**

I guess it depends on what you mean by "tightening". If one
measurement is X with uncertainty A, and another Z with uncertainty
C, and they are 5 sigma apart, then someone measures, say, Y with
uncertainty B, which is between the other two and compatible with
both within 3 sigma, that doesn't mean that Y is correct. Of course,
if someone does measure that, they will probably publish it, while
someone measuring something, say, 5 sigma below the lowest measurement,
or above the highest, might be less likely to do so.

It could be that Y is close to the true value, but perhaps all are
wrong, or X is closer, or Z. The problem can be resolved only if
one understands why the measurements differ by more than a reasonable
amount.