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A new Einstein cross is discovered

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Re: A new Einstein cross is discovered

#81

Post by waynecochran »

btilly wrote: post #72
sandworm101 wrote: post #33

Earlier quoted context omitted.

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.

It may be surprising, but if you do the math, this is what we should expect. According to 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…

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

Re: A new Einstein cross is discovered

#82

Post by function_seven »

vanderZwan wrote: post #79
btilly wrote: post #76

Earlier quoted context omitted.

I haven't heard the cupcake version. It does have something to recommend it, though raisins don't crawl. So that may make it harder to visualize the difference between how fast you're expanding versus how fast you're moving versus local space. Which is the whole point.

What I find confusing is that the space inside the ants/raisins should expand as well. Why do we not experience this expansion locally?

Space is expanding everywhere, but the matter that resides within it is bonded to other matter, or gravitationaly attracted together, so it holds.

Back to the balloon analogy, the surface of the balloon is expanding, even under the ant's feet. But the ant doesn't grow.

Re: A new Einstein cross is discovered

#83

Post by acqq »

function_seven wrote: post #82
vanderZwan wrote: post #79

Earlier quoted context omitted.

What I find confusing is that the space inside the ants/raisins should expand as well. Why do we not experience this expansion locally?

Space is expanding everywhere, but the matter that resides within it is bonded to other matter, or gravitationaly attracted together, so it holds. Back to the balloon analogy, the surface of the balloon is expanding, even under the ant's feet. But the ant doesn't grow.

And since the even the whole groups of galaxies are gravitationally bound, it seems that the galaxies, including our own galaxy, are not "stretching" but remain the same size. And, based on the same principle, everything inside of them also, including us.

So back to the cupcake analogy, the raisins are the whole galaxy groups.

Also:

https://medium.com/starts-with-a-bang/this-is-why-we-arent-e...

"As physicist Richard Price once put it, “Your waistline may be spreading, but you can’t blame it on the expansion of the universe.”"

https://www.newscientist.com/article/dn8082-cosmic-expansion...

https://arxiv.org/abs/gr-qc/0508052

Re: A new Einstein cross is discovered

#84

Post by caprese »

martin1975 wrote: post #59
caprese wrote: post #49

Earlier quoted context omitted.

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

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

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.

Re: A new Einstein cross is discovered

#85

Post by 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.

Re: A new Einstein cross is discovered

#86

Post by caf »

function_seven wrote: post #82
vanderZwan wrote: post #79

Earlier quoted context omitted.

What I find confusing is that the space inside the ants/raisins should expand as well. Why do we not experience this expansion locally?

Space is expanding everywhere, but the matter that resides within it is bonded to other matter, or gravitationaly attracted together, so it holds. Back to the balloon analogy, the surface of the balloon is expanding, even under the ant's feet. But the ant doesn't grow.

Here "bonded to other matter" means "electrostatically attracted together".

Re: A new Einstein cross is discovered

#87

Post by BenMorganIO »

BenMorganIO wrote: post #85

> 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.

Ah, see McKayDavis for the explanation.

Re: A new Einstein cross is discovered

#88

Post by jmts »

jandrese wrote: post #73
mehrdadn wrote: post #68

Earlier quoted context omitted.

I've heard of raisins on a cupcake instead (I hate raisins on cupcakes btw), I assume since it's a 3D example. It's meant to show how it's possible for everything to be getting farther away from each other without anything being at the center.

Cupcakes have a center.

This is physics, so it is best to assume an infinite cupcake.

Re: A new Einstein cross is discovered

#89

Post by btilly »

waynecochran wrote: post #81
btilly wrote: post #72

Earlier quoted context omitted.

It may be surprising, but if you do the math, this is what we should expect. According to 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…

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

There is a long explanation at https://physics.stackexchange.com/questions/14056/how-does-g... 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.

Re: A new Einstein cross is discovered

#90

Post by v_lisivka »

McKayDavis wrote: post #20

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 know…

So, for object 13.4 b.l.y. away it took 13.4 b.y. to reach us, but for object 32 b.l.y. away, it took 13.4 b.y. to reach us only?

Nice match.

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