APOD: Gravity's Grin (2015 Nov 27)

[APOD Robot]

Gravity's Grin

Explanation: Albert Einstein's general theory of relativity, published 100 years ago this month, predicted the phenomenon of gravitational lensing. And that's what gives these distant galaxies such a whimsical appearance, seen through the looking glass of X-ray and optical image data from the Chandra and Hubble space telescopes. Nicknamed the Cheshire Cat galaxy group, the group's two large elliptical galaxies are suggestively framed by arcs. The arcs are optical images of distant background galaxies lensed by the foreground group's total distribution of gravitational mass dominated by dark matter. In fact the two large elliptical "eye" galaxies represent the brightest members of their own galaxy groups which are merging. Their relative collisional speed of nearly 1,350 kilometers/second heats gas to millions of degrees producing the X-ray glow shown in purple hues. Curiouser about galaxy group mergers? The Cheshire Cat group grins in the constellation Ursa Major, some 4.6 billion light-years away.

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[Pkhabu]

How often has gravitational lensing been observed to date?

[Chris Peterson]

How often has gravitational lensing been observed to date?

It's very common. There must be hundreds of examples in the Hubble dataset alone.

[Boomer12k]

While there are many examples...it is really quite a rare phenomena, give the area of the universe... Gravitational Lensing is ALWAYS going on. "Weak" and "Strong". This is an example of the Strong... But what we see as Gravitational Lensing in these type of photographs are of denser clusters of galaxies, with galaxies behind them. "Strong Lensing".
There are Deep and Ultra Deep Fields of images by Hubble Space Telescope, and I observe no Gravitational Lensing even though the distance, and thus the amount of Dark Matter between us, is greater. One might expect more lensing at 13 billion light years, as 4.6 billion.

Thus, while there are lots of images and examples, it takes special circumstances, and those are rather rare.

:---[===] *

[Ann]

I agree with Boomer that although there are huge numbers of gravitational lenses in the universe, you can't expect to just see a gravitational arc simply by pointing the Hubble telescope at a random spot in the sky.

In today's APOD, I'm fascinated by the purple glow of hot intracluster gas in the two galaxy clusters. This hot turbulent gas that pervades the clusters may prevent gas and dust from cooling inside the galaxies in the clusters, thus impeding star formation in the individual galaxies.

Ann

[ta152h0]

How do you find these things ? incredible luck or a series of events ?

[Ann]

How do you find these things ? incredible luck or a series of events ?

The best place to look is in the direction of large galaxy clusters and huge dominant cD galaxies.

Click to view full size image

The Coma Cluster.
Photo: Justin Ng.

But even then you can't be sure you'll see a gravitational arc. The nearest really large galaxy cluster is the Coma Cluster, which may contain as many as 10,000 galaxies. But to my knowledge we see no gravitational arcs in the Coma Cluster.

Ann

[heehaw]

I looked at APOD, and then I had to check my calendar to make sure it was not April 1!

[Guest]

Such a cold scientific description for a 'happy face in space'.

As for the comments about the frequency of gravitational lensing. I think our perception is governed only by our position near the focal point of such a lenses, and it probably happens all over the place but we just can't perceive or appreciate it. I wonder, tho, if we were really near the focal point (as if that could really happen), would the image be really clear or still a bit fuzzy?

[Chris Peterson]

As for the comments about the frequency of gravitational lensing. I think our perception is governed only by our position near the focal point of such a lenses, and it probably happens all over the place but we just can't perceive or appreciate it. I wonder, tho, if we were really near the focal point (as if that could really happen), would the image be really clear or still a bit fuzzy?

There is no focal point as such, because gravitational lenses are optically different than the sort of lenses we are generally familiar with. They work "backwards": they deviate light most at their centers, and less as you move radially outwards. What this means is that the (distorted) object will be seen if the observer is anywhere along a line that passes through both the object and the lens.

[Chris Peterson]

Thus, while there are lots of images and examples, it takes special circumstances, and those are rather rare.

Of course, it depends on how you choose to define "common" (or "rare"). There are dozens (or even hundreds) of examples of visible lensed structures (but of course, take a narrow field image in any random direction and the odds are you won't have one in the image). But make any image near a galaxy or cluster of galaxies, and there's a good chance that you'll be able to measure lensing instrumentally- a technique that is used to map dark matter distributions. Lensing also shows up in photometric data, where single lensing objects briefly modulate the background light.

[bystander]

http://asterisk.apod.com/viewtopic.php?t=35395

[starsurfer]

seen through the looking glass

The man in the looking glass Is looking back at you at last.

[Guest]

As for the comments about the frequency of gravitational lensing. I think our perception is governed only by our position near the focal point of such a lenses, and it probably happens all over the place but we just can't perceive or appreciate it. I wonder, tho, if we were really near the focal point (as if that could really happen), would the image be really clear or still a bit fuzzy?

There is no focal point as such, because gravitational lenses are optically different than the sort of lenses we are generally familiar with. They work "backwards": they deviate light most at their centers, and less as you move radially outwards. What this means is that the (distorted) object will be seen if the observer is anywhere along a line that passes through both the object and the lens.

OK, got it. I see what you mean and now understand it better. Thank you Chris.

[Ron-Astro Pharmacist]

We could smile at Mona Lisa's moon trip? It's all about encryption and decryption. Gravity facilitates the process from far away but if the NSA could find a happy medium perhaps we could see the true picture to discover gravity's real nature and track down the missing pieces of the big puzzle.

the-big-picture.jpg

[epitalon]

Thus, while there are lots of images and examples, it takes special circumstances, and those are rather rare.

Of course, it depends on how you choose to define "common" (or "rare"). There are dozens (or even hundreds) of examples of visible lensed structures (but of course, take a narrow field image in any random direction and the odds are you won't have one in the image). But make any image near a galaxy or cluster of galaxies, and there's a good chance that you'll be able to measure lensing instrumentally- a technique that is used to map dark matter distributions. Lensing also shows up in photometric data, where single lensing objects briefly modulate the background light.

I understand that galaxies and clusters are aligned along filaments. If those filaments are pointing toward our galaxy... there is most chance to find a galaxy behind a cluster of galaxies.
But am I right when I write that all filaments point toward our galaxy ?
Do we happen see many streams of galaxy that are not aligned with our's ?

To me, the most interresting point is : Is our galaxy the center of the universe ?

[Ann]

epitalon wrote:
I understand that galaxies and clusters are aligned along filaments. If those filaments are pointing toward our galaxy... there is most chance to find a galaxy behind a cluster of galaxies.
But am I right when I write that all filaments point toward our galaxy ?
Do we happen see many streams of galaxy that are not aligned with our's ?

To me, the most interresting point is : Is our galaxy the center of the universe ?

Click to view full size image

A portion of the universe.
Where are we?

We humans are at the center of our own universe. From our own vantage point, everything seems to point in our direction.

If we lived in another galaxy in another part of the universe, we would experience the universe in exactly the same way as we do now. We would still be the center of our own universe. From our own vantage point in that other galaxy in that other part of the universe, everything would still seem to point in our direction.

So is the Milky Way the center of the universe? If we could "step back" and watch the observable universe in its current perhaps 96 billion light-years diameter, the Milky Way would be utterly insignificant and nowhere near a "center of the universe".

Ann

[Animal of Stone]

If the Milky Way is not the centre of the universe, is there a way to determine where in the scheme of matter we are placed ? And further can we pinpoint the direction of the centre of the cosmos ?

[Markus Schwarz]

If the Milky Way is not the centre of the universe, is there a way to determine where in the scheme of matter we are placed ? And further can we pinpoint the direction of the centre of the cosmos ?

On large scales (larger than about 400 Mpc, where even clusters of galaxies are just 'pixels') the universe is homogenous and isotropic, meaning that 'it is and looks the same everywhere'. Hence, there is no center or any preferred point for that matter.

[Chris Peterson]

To me, the most interresting point is : Is our galaxy the center of the universe ?

Viewed as a 3D structure, we are at the center of the Universe. Not just the center of the observable universe, but of the entire universe. As is every 3D point in the Universe.

Viewed as a 4D (spacetime) structure, the center of the Universe is the point were t=0, which is not accessible to us, being in the past.

[BMAONE23]

epitalon wrote:
I understand that galaxies and clusters are aligned along filaments. If those filaments are pointing toward our galaxy... there is most chance to find a galaxy behind a cluster of galaxies.
But am I right when I write that all filaments point toward our galaxy ?
Do we happen see many streams of galaxy that are not aligned with our's ?

To me, the most interresting point is : Is our galaxy the center of the universe ?

Click to view full size image

A portion of the universe.
Where are we?

We humans are at the center of our own universe. From our own vantage point, everything seems to point in our direction.

If we lived in another galaxy in another part of the universe, we would experience the universe in exactly the same way as we do now. We would still be the center of our own universe. From our own vantage point in that other galaxy in that other part of the universe, everything would still seem to point in our direction.

So is the Milky Way the center of the universe? If we could "step back" and watch the observable universe in its current perhaps 96 billion light-years diameter, the Milky Way would be utterly insignificant and nowhere near a "center of the universe".

Ann

So who's brain is that?

Micrograph of Neurons

[DavidLeodis]

It's a fascinating image, in which there also seems to be a lot of galaxies.

I wonder if there is a theoretical limit on the numbers of galaxies despite there being a lot of space in 'Space'?

[Chris Peterson]

It's a fascinating image, in which there also seems to be a lot of galaxies.

I wonder if there is a theoretical limit on the numbers of galaxies despite there being a lot of space in 'Space'?

Assuming that there isn't an infinite amount of space, we can safely say there are a finite number of galaxies. And in fact, we can reasonably estimate the number of galaxies in the observable universe (or in any particular volume of the entire universe).

[ta152h0]

actually saw the daytime moon today

[DavidLeodis]

It's a fascinating image, in which there also seems to be a lot of galaxies.

I wonder if there is a theoretical limit on the numbers of galaxies despite there being a lot of space in 'Space'?

Assuming that there isn't an infinite amount of space, we can safely say there are a finite number of galaxies. And in fact, we can reasonably estimate the number of galaxies in the observable universe (or in any particular volume of the entire universe).

Thanks Chris .

The shear enormity of the Universe (even if it is finite) is way beyond the comprehension of my brain. Just how many big expansions and subsequent big crunches there has ever been may never be known (I assume though that there cannot have been a big crunch before the 'first' big expansion). I think I need a and/or a better brain.