Abell 2218: A Galaxy Cluster Lens (APOD 10 Feb 2008)

[Czerno]

<http://antwrp.gsfc.nasa.gov/apod/ap080210.html>

Could someone help me locate the "young, still-maturing galaxy (which) is faintly visible to the lower right of the cluster core" on today's splendid picture, please ?

[emc]

Czerno, Not sure but I think they are showing us through the link "faintly visible" http://oposite.stsci.edu/pubinfo/PR/200 ... /0132w.jpg

I don't understand how we can see two images of the same object... it seems to me that since we are looking through the "gravational lense" of the condensed galaxy cluster, that we would only see what the "lense" allows? But then I wonder, how would we know the light is bending without two images of the same thing?? I think I am too far out of my tree again.

[PrinsEdje80]

Why there are two images? That has to do with the way the light is bent around the mass. It's not like a normal lens you have in a camera or magnifier on Earth. The lens is somewhat more complicated and hence you get multiple images. http://en.wikipedia.org/wiki/Gravitational_lensing

A second note: this is not the furthest away galaxy ever measured. That is one at z (redshift) = 6.96. It might be that it is this galaxy is the one having the higher redshift using this method...

[emc]

PrinsEdje80, Thanks for the link! Wow! So Einstein understood this before there was any documented evidence.

If it is not intuitive, I seem to have difficulty wrapping my head around the phenomenon. Sometimes, science seems too fanciful for me

[copper5817]

If it is possible to bend light with gravity, might it also be possible that we are seeing the same galaxies many times like in a "kaleidoscope"? It appears that many of these galaxies appear to be the same ones repeating. Obviously there is no real evidence to support this on my part, but some of them seem so similar in nature that I wonder if it is possible that there are really that many galaxies with similar compositions at nearly the same distance so as to apeear to be so similar in size.

[rigelan]

Those arcs are the objects being telescoped around the galaxy. Its those bent lines. The image behind could possibly be seen from all sides around the galaxy doing the lensing. But for some reason, we only get part of an arc.

[NoelC]

The image appears to have an amorphous - i.e., random and scattered - collection of matter, right?

What I want to know is this:

Why is the gravitational lens refracting the light so perfectly? The refracted arcs are nicely circular. Yet a glass lens with even the smallest imperfection would not yield a circular pattern.

I keep seeing images like this shown and this question never seems to be addressed.

I'm betting there's still something we don't understand about gravitational lensing here.

-Noel

[JimOrlando]

Ya know, I have a quibble with the phrasing "Gravity can bend light", and I think Albert might have a quibble, too.

Matter curves space. The light is not being "bent", rather it's following the curvature of space in its travels from its origin to our sensors. I know "gravity bends light" is a nice shorthand, but since gravitational lensing is considered evidence of the validity of the General Theory, I like to try to think about what's really occurring from within that frame, not from one that sounds to me Newtonish.

I don't have a good alternative shorthand, though - any suggestions?

[henk21cm]

What I want to know is this:

Why is the gravitational lens refracting the light so perfectly?

Noel,

Light is bend around a havy object, since its mass deforms the space around it. Albert Einstein predicted it, and the solar eclips of 1919 was used to test his hypothesis. Eddington published the results, which are commonly seen as the first proof of Einsteins general theory of relativity. The stars of which the light passed the vicinity of the sun, were slightly shifted, as far as i remember, about 1 arc second (1/1800 of the solar disk).

The sun has a spherical shape, so the light of an object behind the sun is bend by the same amount in all directions around the solar disk. Well, the mass of the sun is far to small to produce noticable deviations. Suppose we make the sun a lot havier (without interfering the nuclear fusin processes). A million times havier as now. It will bend light far more. Rays of light originated by an object behind the sun are emitted in all directions. One particular direction is perfectly aligned so that light will be bend directly towards the earths telescopes. Since the sun is spherically symmetric, all rays lying on a cone are alligned to be bend towards the earth. The footprint of the cone on a plane perpendicular on the line through sun and earth is a circle. As a result, there will be a halo around the sun visible, caused by the light of the distant object. (Not to be mixed up with the halo around the sun, as caused by ice crystal in our atmosphere).

A cluster of galaxies is not spherically symmetric, so their gravitational field is not symmetric either. The random distribution of galaxies in the cluster breaks the symmetry. Light of objects behind the cluster is bend asymmetrically. The halo is broken into several segements.

Although i did not do the math for a random cluster of galaxies, for me it's plausible that small arcs will remain. These segments are an indicator that the gravitational field in that direction is rather homogeneous.

Regards,

[PrinsEdje80]

Think of it this way: have a trampoline (so, this is a 2D example, but that's how you should see it in 3D) and put a basketball on it. You'll see that the ball will form a dip in the fabric of the trampoline. Now take a marble and let it pass the basketball at long distance, you'll see that it does not deviate from its original path. Let the marble pass closely to the basketball (high speed to mimic the speed of light) and you'll see that the curvature has changed the direction of the marble.

Replace basketball with any heavy mass. In this case the cluster, in other cases a black hole or even the Sun. Replace the marble with photon and the trampoline with space-time. That's how gravitational bending of light works. And you'll only see the beautiful arcs (which might appear perfectly circular, but are not necessarily) when the alignment between lensed object, lens and observer is correct. Look on the internet for words such as caustic (http://en.wikipedia.org/wiki/Caustic_%28optics%29)

[emc]

I'm glad to see the discussion on "gravity lense" it becomes more intuitive when compared to something more 2D like the trampoline surface.

Is the universe (space) curved such that we could start traveling in a "straight" line any direction and end at the same point we started from? I have heard this somewhere before.

[zbvhs]

Since we can see the galaxies producing the lens, might it be possible to define the shape of the lens and reconstruct the image of the more-distant object - through a ray-tracing process perhaps?

[neufer]

Is the universe (space) curved such that we could start traveling in a "straight" line any direction and end at the same point we started from? I have heard this somewhere before.

The problem with that is that the curved universe is expanding (and now we even know that the expansion rate is increasing due to dark energy).

You can't go home!

[neufer]

Since we can see the galaxies producing the lens, might it be possible to define the shape of the lens and reconstruct the image of the more-distant object - through a ray-tracing process perhaps?

One can't define the shape of the lens based on the bright galaxies alone since there is also dark matter to consider. It might make more sense to guess at the shapes of the distorted galaxies and (knowing their red shifts) use a ray-tracing process to determine the gravitational field (and even, perhaps, reconstruct "an image" of the dark matter).

[emc]

Is the universe (space) curved such that we could start traveling in a "straight" line any direction and end at the same point we started from? I have heard this somewhere before.

The problem with that is that the curved universe is expanding (and now we even know that the expansion rate is increasing due to dark energy).

You can't go home!

What is the universe expanding into?

I used to think the universe was infinite, which is incredibly paradoxed since I can't comprehend infinity. Every time I try to wrap my head around infinity... my thoughts collapse. But now I understand that scientists think the universe is finite. I guess an expanding universe requires a finite state but then my thoughts collapse once again as to what the universe would be expanding into...??? I thought maybe the fourth dimension, but that is another collapse point for my 3D thinking.

[neufer]

What is the universe expanding into?

Nobody knows; it may not even be a valid question to ask.

I used to think the universe was infinite, which is incredibly paradoxed since I can't comprehend infinity. Every time I try to wrap my head around infinity... my thoughts collapse. But now I understand that scientists think the universe is finite. I guess an expanding universe requires a finite state but then my thoughts collapse once again as to what the universe would be expanding into...??? I thought maybe the fourth dimension, but that is another collapse point for my 3D thinking.

If you really want to blow your mind think of our universe being one expanding bubble among an infinite number of other bubbles each with it's own random set of fundamental physical constants [e.g., like the 1/(137.037..) fine structure constant].

Then consider that our own bubble universe may be constantly bifurcating into alternative universes with every quantum collapse of every wavefuntion.

Or, then again, maybe you shouldn't.

[emc]

If you really want to blow your mind think of our universe being one expanding bubble among an infinite number of other bubbles each with it's own random set of fundamental physical constants [e.g., like the 1/(137.037..) fine structure constant].

Then consider that our own bubble universe may be constantly bifurcating into alternative universes with every quantum collapse of every wavefuntion.

Funny, when my daughters were young and impressionable, they once asked my why we celebrated New Year's... I told them it was kind of a secret, but it was because the entire universe was mirrored... I further explained that this included us and our brains so that we percieved things the same as before...

I think you just collaborated my theory

[NoelC]

The presence of matter is warping not just space, but space-time (most notably time), which helps explain gravitational lensing. I find it most helpful to think of gravity more as an effect, rather than a cause.

A cluster of galaxies is not spherically symmetric, so their gravitational field is not symmetric either. The random distribution of galaxies in the cluster breaks the symmetry. Light of objects behind the cluster is bend asymmetrically. The halo is broken into several segements.

Problem is, I don't see it that way. I see a generally quite circular pattern around a point - assumedly the center of mass. Visually it seems way too circular and perfect...

With all those galaxies I might have expected more of a "olde tyme glass" (e.g., the type of glass window you'd find in a two hundred year old house) view of the universe beyond.

And I'm not yet convinced of the existence of "dark matter" nor that it somehow could be perfectly distributed where the visible galaxies are so randomly placed.

-Noel

[NoelC]

Then consider that our own bubble universe may be constantly bifurcating into alternative universes with every quantum collapse of every wavefuntion.

I prefer to think the alternative choices might be just thrown away. Seems an awful lot of work to duplicate an entire universe just because someone watched a photon do its thing.

-Noel

[Arramon]

On-topic:

Does the lensing from the mass of the galaxy clusters in the foreground affect the redshift of the galaxies in the background being distorted? They become warped, but are they also enlarged somewhat? And seeing multiple versions of the same background galaxy would be a good thing to know, so wouldnt checking the composition of each galaxy tell us if we are seeing duplicates from lensing?

Not to mention that over time, even the viewable galaxies can be totally different from what we see since it's taken that amount of time for the light to reach us, so we are seeing shapes that may not even be the same as they are to that galaxy's own inhabitants (if they exist) or to galaxies much closer than we are.

Off-topic:

Could the Dark Energy be like what causes cells in biology to multiply, grow and therefor require more room, so the entire entity containing all cells gets bigger? Kind of like we humans do when we grow. We feed, new cells grow, old cells die, the encoded lifespan of the entity dwindles and no new cells form while the entire entity slowly whithers away. Multiply this by whatever factor to equal the 'known' universal scale we see, and maybe we're just in an entity that's growing and getting bigger, and all things will slowly wither away after nothing new is formed (everything has a predetermined lifespan?).



Lifespans
Human = 77.5 - 80 years avg
Planet = <>Millions of years (Depends on parent star's size)
Star = Thousands - Billions of years (smaller is longer, bigger shorter)
Galaxy = Billions of years
Known Universe = 13-15 billion years

=/

[greatergood]

Arramon-

I'm an armchair astronomer, so take this with a grain of salt, but this is what I understand.

Shouldn't affect the red-shift. The speed of the light doesn't change with respect to the path it takes, but only with respect to the distance because the universe is expanding.

Also, yes the lensed galaxies can possibly be magnified for greater detail, although I don't think we've ever observed that. In order for lensing to result in greater detail the following must happen:

1) For greater detail the lensed objects must be close to the center of the lensing field and must not be blocked by the objects causing the lensing. That would require a cluster of a small number of colossal objects evenly spaced far enough so that the lensed field has enough open space to allow the lensed objects on the other side to be seen shining through the middle.

2) For magnification the cluster causing the lensing must only weakly bend the space around it. From what I gather images such as this one are the result of lensing strong enough that would cause severe aberrations for lensed objects shown close to the center of the field, and that is why most lensed objects show up smashed into arcs surrounding the center of the gravitational lens. Such objects are not enlarged but smashed tangentially to the radius, and stretched perpendicular to the radius.

I think these conditions that would magnify a whole galaxy to provide greater detail are extremely rare - and in fact, I'm not sure they've ever been witnessed in an ideal condition yet. Surely in the infinitude of space these conditions exist in some part of the sky but I doubt we've found it yet.

What we do observe however is aberrated smeared pieces of almost impossibly far galaxies mixed in among the lensing objects, and it is these pieces that we measure to determine redshift. Due to the imperfections of the lenses I don't think we've been able to say anything other than, "there's a smear of a galaxy farther away than anything else that's visible", instead of ""look there's a guy looking at me through a telescope on his planet". Though that would be really cool.

These observations I pointed out above are fairly intuitive if you've played with thick magnifying glasses or a large glass sphere.

[Arramon]

http://hubblesite.org/newscenter/archiv ... s/2008/09/

New images released of gravitational lensing.