A new Einstein cross is discovered | Hacker News Reader

A new Einstein cross is discovered

phys.org

259 pointsarunc102 comments

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

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.

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

The shape of the universe currently prevents this. For what we can tell the universe is flat, which means at no point will it collapse on itself, unless something major changes. That said, major changes have happened, around 8 billion years ago expansion appears to have sped up.

I'm no expert (so correct me if I'm wrong), but from what I'm read, flat, concave, and convex universes are all possibilities and they would all look flat to us because our measurements take place on such a small scale.

Our measurements occur across billions of light-years. By comparing redshift agaist the standard candles we measure relative speeds of vastly distant objects. And given the speed of light, we are also measuring what those speeds were billions of years ago. It is a very big ruler.

We've measured the flatness of the universe on the scale of the cosmic microwave background, which is essentially as big a ruler as could possibly exist.

https://map.gsfc.nasa.gov/mission/sgoals_parameters_geom.htm...

https://wmap.gsfc.nasa.gov/universe/uni_shape.html

Those darn flatspacers

Space itself is expanding. At small scales, it isn't really that "fast" (gravity can/does overcome the effects within our galaxy and the Local Group). However, at large scales, it can exceed the speed of light (google "Hubble volume") so at some point, light emitted from sources outside this range will not longer reach us because the space between here and there expanded faster than light travels.

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 expanding away from each other" at ~10^90 meters per second.

0) http://curious.astro.cornell.edu/physics/109-the-universe/co...

1) https://www.forbes.com/sites/startswithabang/2017/03/24/how-...

Edit: Upon reflection, I get that "~1 meter in diameter" makes no sense. Maybe just say that the "edges" were ~1 meter apart.

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.

> Belated edit: More like ~10^81 times the speed of light.

I have no grounding in physics on which to place this hunch but... doesn't this sound like a wee fudge factor to make the Big Bang theory fit evidence?

I am reminded of the concept of "aether" and "land bridge theory" where people had a workable hypothesis except for the niggly little problem that the hypothesis was contraindicated by some of the evidence.

I wonder about that too.

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?

There is no edge. It defies the normal way of thinking but every point in the universe is also the center of the universe. The Big Bang happened at every point and space-time is expanding at every point.

There could be an edge, but it's definitely not within the distance we can see out. The main mind-bender is that there's no need for an edge.

Well, not really. The distance we can see out is measured in time and we can very nearly see the Big Bang. We can observe the Universe's Recombination event and it is in every direction we look as the Cosmic Microwave Background. This happened everywhere in existence.

If there is an edge to the Universe it is in the fourth dimension when the Universe was a singularity.

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 surprisingly exact.

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.

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

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 :)

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

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

It's not strong enough to overcome the forces keeping us together.

But that might not be the case for forever: https://en.wikipedia.org/wiki/Big_Rip

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.

Not really. The far edge of our perception can be at lightspeed, but then everything closer would remain slower. So night wont suddenly become dark. We will first loose distant galaxies, but 99.9999 of starlight comes from our own galaxy. By the time that is moving away at lightspeed, the view will be the least of our concerns.

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.

You might be referring to the Big Rip?

Yup! There is light moving towards us from the other side of the universe, but we'll never see it because the space between that light is stretching faster than thr light can travel (or something like that). Check out this page on the Observable Universe... https://en.wikipedia.org/wiki/Observable_universe.

I think it expands uniformly, so that, after time t, a distance of x becomes a distance of x * k, with k being very slightly greater than 1. Then there will always exist large enough x to outstrip the speed of light.

To illustrate, imagine the expansion factor is, say, 10^-20 per second. Then a distance of 10^30 meters will increase to a distance of 10^30 + 10^10 meters in one second, while light travels at 3 * 10^8 meters per second, so expansion would outstrip lightspeed at that distance. If the expansion factor were 10^-40 per second, then a distance of 10^50 meters would be increasing faster than lightspeed.

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

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.

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.

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.

Wow. Thanks for the detailed answer.

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.

"While gravitationally lensed light sources are often shaped into an Einstein ring, due to the elongated shape of the lensing galaxy and the quasar being off-centre, the images form a peculiar cross-shape instead." [1]

[1] https://en.wikipedia.org/wiki/Einstein_Cross

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...

Not a Star behind it, but another galaxy!

Einstein rings exist, too. A cross happens when the lensing object has a certain shape and orientation.

https://en.wikipedia.org/wiki/Einstein_ring

https://en.wikipedia.org/wiki/Einstein_Cross

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.

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

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#

> 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

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...

Link to paper on arXiv: https://arxiv.org/pdf/1902.10964.pdf

Abstract: https://arxiv.org/abs/1902.10964

>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

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

[1] https://en.wikipedia.org/wiki/Einstein_Cross

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#

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?

> 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.

and it is not a kvasar, which is unusual

what is a kvasar

It's quasar - OP is probably not a native speaker.

I can guarantee that OP is infact a native speaker. English however might not be OP's native language. =)

But given that the rest of the OP's comment was in English, it is natural to interpret the GPs comment as meaning "not a native speaker of the language they wrote the comment in (English)".

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

https://en.wikipedia.org/wiki/Quasar.

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".

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

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.

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

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.

"Kvasar" is Czech i think.

Sometimes I go to google translate to hear words pronounced, e.g. https://translate.google.com/#view=home&op=translate&sl=de&t...

(I hear "kvasar" and "cantoom" for quasar and quantum)

It's a type of supermassive object formed in the early universe from the fermentation of rye bread.

> It's a type of supermassive object formed in the early universe from the fermentation of rye bread.

Stop being kvazy!

Assuming you mean quasar here.

It's unusual, but only because there's such a small set of Einstein Crosses -- it's not particularly significant that the lensed object is another galaxy.

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?

The entire universe is infinitely large, (we think) but the observable universe is finite, limited by the age of the universe, the speed of light, and the portion of the early universe when space was opaque. (which is what the cosmic microwave background, or CMB is) (note that terminology is confusing; when people use the word "universe" you have to determine base on context whether they're talking about the infinite entire universe, or the finite observable universe)

Imagine an RTS game on an infinitely large map with fog of war. Sure, there's something out there beyond the fog of war, but we can't see it. (don't try to go too far with this analogy though)

The universe isn't expanding into anything. It just is. This is the key to your misunderstanding I think; the universe isn't expanding through or into some other, extra-dimensional medium. The universe itself is expanding.

Infinitely large?

My understanding was that the universe originated as a point, inflated a great deal, then began expanding at a rate that has changed over time. As a result, the universe would be significantly larger than the visible universe, but would have a finite size.

(A finite size, but no boundaries. The universe is a sphere whose circumference is everywhere but whose center is nowhere. Cue Borges music.)

It’s a common misunderstanding of the Big Bang. There is not a point where the universe originated.

Rather everything was just closer together. Imagine traveling back in time - you would see galaxies move closer and closer - until everything was super dense and hot - like the center of the sun but everywhere.

Isn’t the natural extension of things being “closer together” that they were at a singularity at some point (the Big Bang)?

Nope. The universe is infinite, according to the model. We see observe galaxies speeding away from us in every direction - and it seems unlikely that the earth was the exact center of the Big Bang.

Wouldn't that also be consistent with a finite universe as long as the distribution of galaxies is not biased towards or away from the edge, as long as the edge itself is beyond the observable part of the universe? We wouldn't have to be in the exact center, just not too close to the boundary.

>Wouldn't that also be consistent with a finite universe

It would. It is not an unanimous conclusion that "stuff" is found everywhere in the universe and that the universe is infinitely big. However, that doesn't necessarily imply a center or an edge.

The universe might be infinite and unbounded.

The universe might be finite and bounded.

The universe might be finite and unbounded.

If the universe is finite and unbounded, we can go back to the ants on a balloon analogy. As expansion continues (and indeed speeds up) the balloon will continue to inflate, for forever. It is not possible to ever make the trip around the balloon to get back to your starting destination due to the expansion of the universe. The universe and spacetime are the 'surface' of the balloon - there is nothing inside of it, the center is not the middle of the balloon, and you cannot tunnel from one side of the balloon through to another. That isn't to say the observable universe is likely to be the extent or even close to the extent of the actual universe - it is likely still just the tiniest of fractions of the actual universe.

We're probably finite and unbounded, and expanding into nothing. As the universe expands, it creates space where it did not exist.

There's other theories that are reasonable, however. Some posit that our universe is a bubble in a true vacuum, and that there could be other bubbles containing other universes. Those theories are largely driven by the theory of inflation, which is the most accepted explanation as to how the universe expanded so far and so quickly in its early stages. (That's not to imply that the bubble multiverse theory is the most accepted explanation, though - just that most theories relating to bubble multiverses stem from looking at vacuum energy)

We probably won't know for certain in our lifetimes. Intelligent lifeforms might never know for certain.

| and it seems unlikely that the earth was the exact center of the Big Bang.

Wouldn't it be more accurate to say that everything was at the exact center of the Big Bang? Assuming it started from a singularity (which can't really be known), everything was in the center.

We see stuff in all directions because the stuff extends much further than what we can observe. There may well be a 'middle' of the universe but there's no way to tell how close we are to it (or if it even exists at all.)

But...if it is infinite in extent and (nearly) uniformly dense, how would the density decrease to the current value?

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.

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.

> there’s likely a trillion inhabited worlds full of beings we will never know.

And by the time our radio signal gets to them, the ones there now will all be long dead, interpreted by some future generation. By the time we get the response, humanity may be long extinct. So for all intents and purposes, we may as well be alone in the universe.

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

> 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.

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.

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

see my comment here: https://news.ycombinator.com/item?id=19483785

The universe began 13.8 billion years ago. However the universe is not expanding at the speed of light. It's expanding faster than that, such that the observable universe from earth's perspective is about 47 billion light years in radius.

Reply on news.ycombinator.com