
A new Einstein cross is discovered - arunc
https://phys.org/news/2019-03-einstein.html
======
McKayDavis
WMAP measurements put the age of the universe at ~13.77 billion years, so I
was initially perplexed by this part of the article:

> The object acting as a lens turns out to be an elliptical galaxy located at
> a distance of approximately 7 billion light years (z = 0.556), while the
> source is at least 20 billion light years away (z = 3.03).

...but I found this explanation in the Wikipedia article on the most distant
known galaxy (GN-z11), which is 32 billion light-years away [1]:

> At first glance, the distance of 32 billion light-years (9.8 billion
> parsecs) might seem impossibly far away in a Universe that is only 13.8
> billion (short scale) years old, where a light year is the distance light
> travels in a year, and where nothing can travel faster than the speed of
> light. However, because of the expansion of the universe, the distance of
> 13.4 billion light years traveled by light from GN-z11 to Earth, called the
> light-travel distance, has expanded to a distance of 32 billion light-years
> during the 13.4 billion years it took the light to reach us.

[1]
[https://en.wikipedia.org/wiki/GN-z11#Notes](https://en.wikipedia.org/wiki/GN-z11#Notes)

see also:

[2]
[https://en.wikipedia.org/wiki/Expansion_of_the_universe#Meas...](https://en.wikipedia.org/wiki/Expansion_of_the_universe#Measuring_distances_in_expanding_space)

~~~
teekert
Thank you, went to the comments to learn this. So the universe actually
expands pretty fast then! Faster than the speed of light? Strangely...

~~~
UlisesAC4
Space still expands lower than light speed, the moment expansions goes faster
night skies will be completely dark because other objects' light won't be able
to reach our field of vision. I do not remember how that phenomenon is called.

~~~
caprese
Or the night skies light up completely because there was another big bang
somewhere else and the light from that universe cluster just reached us

our observable universe may be around an unobservable area whose light hasn't
reached us yet

~~~
martin1975
so a parallel universe somehow interferes with ours? I don't think even most
enthusiastic multiverse theorists will be able to grasp that.

~~~
caprese
not really parallel like alternate dimensions, maybe adjacent to not get
distracted by colliqualisms and scifi tropes?

just another area in the vast void that also had a big bang which is outside
of our observable area right now, but whose light may reach us one day.

------
btilly
My favorite thing about Einstein crosses is that the images do _not_ appear at
the same time. So a supernovae will appear at a different time in each image.
See [https://www.space.com/31418-hubbles-einstein-cross-
supernova...](https://www.space.com/31418-hubbles-einstein-cross-supernova-
strikes-back.html) for a case where we actually saw that.

~~~
sandworm101
What is really odd is that we are alive to see both. At billions of light-
years even a slight extension of the travel time, say a few hundred years,
would exceed human perception and probably go unnoticed. For a short-lived
object like a nova, we may only ever have one of the two images visible at any
one time.

~~~
btilly
It may be surprising, but if you do the math, this is what we should expect.

According to
[http://www.marmet.org/cosmology/einsteincross/](http://www.marmet.org/cosmology/einsteincross/),
the distance between images is on the order of 1.6 arcseconds. That's a bit
under 10^(-5) radians. The difference in time here is proportional to the
difference in cosines of the angles taken. An cos is proportional to the
square of the angle in radians. Which means we'd expect a time difference on
the order of 10^(-10) of the total time taken. If the distant object is
several billion light years away, we would therefore expect time gaps in
arrival time that can be measured in no more than months.

~~~
waynecochran
I don’t understand the geometry and why they occur at these four positions 90
degrees from each other?

~~~
btilly
There is a long explanation at
[https://physics.stackexchange.com/questions/14056/how-
does-g...](https://physics.stackexchange.com/questions/14056/how-does-
gravitational-lensing-account-for-einsteins-cross) that gives all of the
details.

Here is a simplified version.

First you need to understand Fermat's principle. Light will follow a
particular path from A to B when all nearby paths take approximately the same
time as that one. Distant paths may be faster or shorter - consider a straight
line vs a mirror. But if nearby paths are different lengths there is
destructive interference and no light travels.

Now suppose that we have a spiral galaxy between us and the distant source,
but tipped on its side. And that galaxy is most of the way to us. From our
view it is somewhat elliptical. There are five paths from there to here that
meet the description of Fermat's principle. They are a straight line through
that galaxy, a bent line to either side of the galaxy, and a bent line over
the edges of the galaxy. At all other angles and directions, you don't meet
Fermat's principle, and therefore light doesn't reach us that way.

However the central image gets blocked out by the lensing galaxy. Therefore
you only see the other four.

Somewhat counterintuitively, the short side of the ellipse represents a
greater gravity gradient, which bends the light more. Therefore those two
images are farther apart and we don't get a perfect cross.

Also the lensing object is never perfectly lined up. This will also affect the
size and placement of the images. Plus the length of time that light takes to
get here.

Your understanding matches mine if you follow the following explanation. In
the 2015 supernova, the lensing object was slightly off center. This meant
that light that passed to the side of the lensing galaxy more or less straight
to us had a fairly short route. Likewise light that passed by the long ends of
the ellipse were bent less (because less gravitational gradient) so were also
short. Therefore the light of the supernova came fairly close in time along
those three routes. The fourth image, which went on the far side of the center
of the lensing galaxy, bends the most and therefore had a longer route. Which
is why it arrived with a significant delay from the other three.

~~~
waynecochran
Wow. Thanks for the detailed answer.

------
ycombinateur
Why does gravitational lensing produce four copies of the star and not a whole
circle of copies? I would have expected the light to bend evenly around the
intermediate star, giving a halo-like effect.

~~~
lainga
I was also interested. It seems to be a combination of the intermediate galaxy
being elliptical, and the star behind it being off-center from our point of
view:

[https://physics.stackexchange.com/questions/14056/how-
does-g...](https://physics.stackexchange.com/questions/14056/how-does-
gravitational-lensing-account-for-einsteins-cross/14631#14631)

~~~
quakeguy
Not a Star behind it, but another galaxy!

------
panarky
_> This study, which has combined images from the Hubble Space Telescope with
spectroscopic observations from the GTC, has confirmed the existence of a new
example of a gravitational lens_

What is the "GTC"? The article doesn't say.

It appears to be the Gran Telescopio Canarias.

[https://en.wikipedia.org/wiki/Gran_Telescopio_Canarias](https://en.wikipedia.org/wiki/Gran_Telescopio_Canarias)

~~~
happy-go-lucky
The article is provided by the Instituto de Astrofísica de Canarias and this
institute operates the Roque de los Muchachos Observatory. One of the
telescopes at the site is Gran Telescopio Canarias (GranTeCan or GTC).

[https://en.m.wikipedia.org/wiki/Roque_de_los_Muchachos_Obser...](https://en.m.wikipedia.org/wiki/Roque_de_los_Muchachos_Observatory)

------
okket
Link to paper on arXiv:
[https://arxiv.org/pdf/1902.10964.pdf](https://arxiv.org/pdf/1902.10964.pdf)

Abstract: [https://arxiv.org/abs/1902.10964](https://arxiv.org/abs/1902.10964)

------
Grue3
>One of the most striking conclusions of Albert Einstein's theory of general
relativity is that the trajectory of light curves in the presence of matter.

A good example of "garden path sentence" [1]. "The trajectory of light curves
in the presence of matter is what?". It took me a while to realize that curves
is supposed to be a verb and not a noun.

[1] [https://en.wikipedia.org/wiki/Garden-
path_sentence](https://en.wikipedia.org/wiki/Garden-path_sentence)

------
nvusuvu
Why wouldn't it be a halo around the lensing galaxy instead of four distinct
points?

~~~
SiempreViernes
Not all lenses are good lenses, especially not the ones nature makes by
accident. It also matters where the background source is relative the centre
to the crappy lens, see third picture here for how the same basic shape can
make different images of the same source:
[http://inspirehep.net/record/850223/plots#](http://inspirehep.net/record/850223/plots#)

------
Causality1
Why does the lense effect produce four separate images instead of a single
image in the middle or a smeared ring all the way around the outside?

------
amelius
> With GTC it was possible to isolate and disperse the light of objects ABC,
> demonstrating that they belong to the same light source.

That's pretty amazing.

------
Krokku
and it is not a kvasar, which is unusual

~~~
cdelsolar
what is a kvasar

~~~
jbf1001
I had to look it up too, it's the German word for Quasar.

[https://en.wikipedia.org/wiki/Quasar](https://en.wikipedia.org/wiki/Quasar).

~~~
bhaak
That's wrong, Quasar is also the German word for Quasar.

But it looks like that the Scandinavian and the Slavic languages use the form
"kvasar".

~~~
oska
Wiktionary is good for finding what languages a particular sequence of
characters appears in.

[https://en.wiktionary.org/wiki/kvasar](https://en.wiktionary.org/wiki/kvasar)

According to that entry, it appears in both Swedish and Czech (as an alternate
spelling).

Given the commenter's username is krokku and the fact that "krok ku" means
"(a) step forward/towards" in Czech, I think we can assume that the commenter
is Czech.

~~~
bhaak
Two entries are not that much. On Wikipedia you can look at the list of
articles in other languages.

Or go directly to Wikidata
[https://www.wikidata.org/wiki/Q83373](https://www.wikidata.org/wiki/Q83373)

~~~
oska
Ah, interesting. So I see it also appears in Danish, Estonian and Norwegian.

But with the additional clue from the username I'm still betting on Czech.

------
samstave
ELI5: How can the universe be "infinitely big" yet have a measurable "finite
age" and the velocity arc of the 'expanding' universe not be some known rate?
Such that we can estimate the actual farthest distance.

And WHAT THE HECK is the universe 'expanding into'?

The answers to these questions make me dubious on tiny-human understanding of
the nature of reality.

Mechanical Elves, anyone?

~~~
TaylorAlexander
Normally the term I hear is “finite, but unbounded”, but I suppose it’s hard
to grok that either.

I also really wonder what is the universe “inside of”? Like why is spacetime
here in the first place?

I agree that our explanation must be impoverished compared to the true nature
of nature. It really boggles my mind. I enjoy the public physics lectures
scattered around YouTube, but it all still leaves me realizing how limited our
understanding is compared to the mundane goings ons of our universe. Our
current understanding is that we’ll never communicate with distant galaxies,
meaning there’s likely a trillion inhabited worlds full of beings we will
never know.

~~~
plazmaphyujin
There isn't really an "inside of". There is no other universe that our
universe is expanding into. The universe itself just "is", it is made of
space-time and that metric is expanding and accelerating. Why it exists is
philosophy. Maybe it was made just for us to exist in. Maybe it doesn't give a
shit about us and we're simply hydrogen left to own devices long enough to
futilely question its own existence.

When we talk about the universe we're rarely talking about what do not know
yet, but rather what is possible to know.

Whether there is another universe "outside" of ours is not just something we
don't know. It's something we cannot know but it's also something that cannot
matter. The observable universe is, by definition, the total encompassment of
everything that can possibly ever matter to us in even the strictest
mathematical sense.

------
acct1771
Universe is expanding faster than the speed of light? Convenient. Simulation.

------
BenMorganIO
> in four separate images in the form of a cross, the light of another galaxy
> located 20,000 million light years away.

So that's 20 billion light years away.... Interesting. I think something's
wrong here given the universe is 13 billion years old.

~~~
BenMorganIO
Ah, see McKayDavis for the explanation.

------
sizzzzlerz
Space is big. You just won't believe how vastly, hugely, mind-bogglingly big
it is. I mean, you may think it's a long way down the road to the chemist's,
but that's just peanuts to space.

20 billion light-years! Truly mind-boggling.

------
gmm1990
How can the further Galaxy be 20 billion light years away?

~~~
McKayDavis
see my comment here:
[https://news.ycombinator.com/item?id=19483785](https://news.ycombinator.com/item?id=19483785)

