Gravitational Lenses

[Charles Bishop]

Forgive the lack of details and any incorrect ones; it was a week or so
when I read the story in the LAT.

The Los Angeles Times reported that a distant galaxy has been discovered
that existed 500 million years (this is one of the facts that I may have
misremembered) after the big bang. This of course has significant
importance for the state of the universe after the big bang.

What I wanted to ask about was a sentence in the article that dealt with
gravitational lensing somehow allowing us to see the faraway galaxy better
(or at all?). It mentions that when light passes by a large mass the light
is bent. Fine so far, something I knew.

However, it started me thinking about the lensing aspect of this. When
light passes through a glass lens, the light is bent and can be focused on
a "spot" depending on the design of the lens. So there is a focus to light
and earth lenses.

Shouldn't there be a focus for gravitational lenses as well? If so, How
likely is it that the earth is at the focus? Or is the focus for
gravitational lenses so large in 3D that the earth falls well within the
focus for this particular galaxy?

--
charles

[Andrew Plotkin]

Here, Charles Bishop <ctbi...@earthlink.net> wrote:
> Forgive the lack of details and any incorrect ones; it was a week or so
> when I read the story in the LAT.
>
> The Los Angeles Times reported that a distant galaxy has been discovered
> that existed 500 million years (this is one of the facts that I may have
> misremembered) after the big bang. This of course has significant
> importance for the state of the universe after the big bang.

Maybe
http://hubblesite.org/newscenter/archive/releases/exotic/gravitational-lens/2011/12/ ?

> What I wanted to ask about was a sentence in the article that dealt with
> gravitational lensing somehow allowing us to see the faraway galaxy better
> (or at all?). It mentions that when light passes by a large mass the light
> is bent. Fine so far, something I knew.
>
> However, it started me thinking about the lensing aspect of this. When
> light passes through a glass lens, the light is bent and can be focused on
> a "spot" depending on the design of the lens. So there is a focus to light
> and earth lenses.
>
> Shouldn't there be a focus for gravitational lenses as well? If so, How
> likely is it that the earth is at the focus? Or is the focus for
> gravitational lenses so large in 3D that the earth falls well within the
> focus for this particular galaxy?

A glass lens is shaped to focus to a sharp image. A gravitational lens
doesn't have that geometry; there's a lot of distortion. (Indeed, the
first gravitational lenses discovered were not enlarged images of
distant objects, but distant objects that appeared in multiple copies.
Wikipedia has some images.)

Also, of course, the image of an infinitesmal dot will remain an
infinitesmal dot when seen through a lens -- whether you're at the
focus or not.

So, looking at this and similar articles, they're not talking about
sharp magnifications, but dots that are brighter than they otherwise
would be. E.g. "Though the diffuse light of the faraway object is
nearly impossible to see, gravitational lensing has increased its
brightness by nearly 10 times, making it bright enough for Hubble and
Spitzer to detect."

Things that are behind the lens, but not so cosmically distant as to
be a point, will be distorted -- that's how the lens is detectacle.

Your general idea that this is a lucky break is correct.

--Z

--
"And Aholibamah bare Jeush, and Jaalam, and Korah: these were the borogoves..."
*