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Old October 20th 19, 01:52 AM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Default Is the Universe Younger than We Thought?

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