A new Einstein cross is discovered • Hacker News Board

Re: A new Einstein cross is discovered

#31

Post by btilly » Mon, Mar 25, 2019, 4:50 PM UTC

ycombinateur wrote: post #14

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.

Please note that these are galaxies, not stars.

While stars can do gravitational lensing, due to both the fact that we can see more galaxies, and they have larger mass, it is easier for it to happen with galaxies.

Re: A new Einstein cross is discovered

#32

Post by will_brown » Mon, Mar 25, 2019, 4:54 PM UTC

ycombinateur wrote: post #14

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.

Not sure if it occurs around individual stars or galaxies, but halo lensing does occur around black holes.

Re: A new Einstein cross is discovered

#33

Post by sandworm101 » Mon, Mar 25, 2019, 4:59 PM UTC

btilly wrote: post #23

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... for a case where we actually saw that.

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.

Re: A new Einstein cross is discovered

#34

Post by mirimir » Mon, Mar 25, 2019, 5:26 PM UTC

jhoechtl wrote: post #28

teekert wrote: post #27

Earlier quoted context omitted.

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

Which makes it then effectivelly infinite?

IANAP, but I think so. I vaguely recall that early inflationary expansion was very fast. Like several times the speed of light.

Re: A new Einstein cross is discovered

#35

Post by teekert » Mon, Mar 25, 2019, 5:26 PM UTC

ryanseys wrote: post #29

teekert wrote: post #27

Earlier quoted context omitted.

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

I think it means that it's expanding and traveling in the opposite direction of us. If we start at the same spot, then both travel in opposite directions at 1 mile/hour, at the end of an hour we will be 2 miles away from each other.

There is not really a center, galaxies are more like raisins in a rising bread. They all move away from each other with the same relative speeds. At least... Any physicists here to tell me whether or not space-time density get less closer to the edge? I mean... Is there an Edge?

Re: A new Einstein cross is discovered

#36

Post by Latteland » Mon, Mar 25, 2019, 5:28 PM UTC

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…

I had to look it up, paraphrasing from the article below, z is the cosmological redshift, indicating the recession velocity of the object. So smaller z values mean closer, large values mean higher, and the farther away it is the faster it is moving away.

http://astronomy.swin.edu.au/cosmos/c/cosmological+redshift

Re: A new Einstein cross is discovered

#37

Post by SiempreViernes » Mon, Mar 25, 2019, 5:30 PM UTC

ycombinateur wrote: post #14

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.

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.

A favourite human made example seems to be the base of a wine glass, that manages similar levels of awfulness, see third picture here for how the same basic shape can make different images of the same source: http://inspirehep.net/record/850223/plots#

Re: A new Einstein cross is discovered

#38

Post by SiempreViernes » Mon, Mar 25, 2019, 5:31 PM UTC

nvusuvu wrote: post #13

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

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#

Re: A new Einstein cross is discovered

#39

Post by knappe » Mon, Mar 25, 2019, 5:32 PM UTC

mirimir wrote: post #34

jhoechtl wrote: post #28

Earlier quoted context omitted.

Which makes it then effectivelly infinite?

IANAP, but I think so. I vaguely recall that early inflationary expansion was very fast. Like several times the speed of light.

No, not really. One theory is that eventually things will slow and then collapse again. https://en.wikipedia.org/wiki/Big_Crunch

Taken to the extreme, this expansion and collapse has been happening over and over again in a cyclical fashion. https://en.wikipedia.org/wiki/Cyclic_model

Re: A new Einstein cross is discovered

#40

Post by UlisesAC4 » Mon, Mar 25, 2019, 5:34 PM UTC

teekert wrote: post #27

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…

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

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.