Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

A new Einstein cross is discovered

phys.org

Post reply on HN
Print view View original

71–80 of 106 posts

Re: A new Einstein cross is discovered

#71

Post by [deleted] »

btilly wrote: post #30
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...

This is a well-known phenomena. The usual image is to consider ants traveling on an expanding balloon. The speed of light is the limit of how fast the ants can walk. The expansion of the universe is the expansion of the balloon. And now you see how two distant ants can be carried away from each other faster than they can walk towards each other. If you understand manifolds and general relativity, this analogy is surp…

[deleted]

Re: A new Einstein cross is discovered

#72

Post by btilly »

sandworm101 wrote: post #33
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.

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 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.

Re: A new Einstein cross is discovered

#73

Post by jandrese »

mehrdadn wrote: post #68
btilly wrote: post #30

Earlier quoted context omitted.

This is a well-known phenomena. The usual image is to consider ants traveling on an expanding balloon. The speed of light is the limit of how fast the ants can walk. The expansion of the universe is the expansion of the balloon. And now you see how two distant ants can be carried away from each other faster than they can walk towards each other. If you understand manifolds and general relativity, this analogy is surp…

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.

Re: A new Einstein cross is discovered

#74

Post by jandrese »

mirimir wrote: post #64
mirimir wrote: post #34

Earlier quoted context omitted.

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

Belated edit: More like ~10^81 times the speed of light. > According to the theory of inflation, the Universe grew by a factor of 10 to the sixtieth power in less than 10 to the negative thirty seconds, so the "edges" of the Universe were expanding away from each other faster than the speed of light [0] It seems that the universe was on the order of ~1 meter in diameter then.[1] So the "'edges' of the Universe were e…

So the universe is expanding at greater than 10^90 m/s now, since the expansion is accelerating?

Re: A new Einstein cross is discovered

#75

Post by mehrdadn »

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.

That wasn't the point. The cupcake represents space. The things are the raisins which are generally not in the center despite moving away from each other.

Re: A new Einstein cross is discovered

#76

Post by btilly »

mehrdadn wrote: post #68
btilly wrote: post #30

Earlier quoted context omitted.

This is a well-known phenomena. The usual image is to consider ants traveling on an expanding balloon. The speed of light is the limit of how fast the ants can walk. The expansion of the universe is the expansion of the balloon. And now you see how two distant ants can be carried away from each other faster than they can walk towards each other. If you understand manifolds and general relativity, this analogy is surp…

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.

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.

Re: A new Einstein cross is discovered

#77

Post by mehrdadn »

btilly wrote: post #76
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.

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.

Good point, I guess they're good for illustrating different things :)

Re: A new Einstein cross is discovered

#78

Post by improbable22 »

jandrese wrote: post #74
mirimir wrote: post #64

Earlier quoted context omitted.

Belated edit: More like ~10^81 times the speed of light. > According to the theory of inflation, the Universe grew by a factor of 10 to the sixtieth power in less than 10 to the negative thirty seconds, so the "edges" of the Universe were expanding away from each other faster than the speed of light [0] It seems that the universe was on the order of ~1 meter in diameter then.[1] So the "'edges' of the Universe were e…

So the universe is expanding at greater than 10^90 m/s now, since the expansion is accelerating?

No, "inflation" refers to a specific episode in the very early history of the universe. Or rather, in some models of this.

Much slower expansion continued and continues today, and if you look really carefully is slightly accelerating not decelerating right now.

m/s, or comparing to the speed of light, isn't really a great way to measure this. Points far away from us are moving away, and faster the further you go; we know of no limits to this.

Re: A new Einstein cross is discovered

#79

Post by vanderZwan »

btilly wrote: post #76
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.

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?
Post reply on HN