In the News: Dark Energy & Lensing

[CraigS]

In the news today:
"Astronomers use galactic magnifying lens to probe elusive dark energy."
http://www.physorg.com/news201427857.html

"A team of astronomers has used a massive galaxy cluster as a cosmic magnifying lens to study the nature of dark energy for the first time. When combined with existing techniques, their results significantly improve current measurements of the mass and energy content of the universe. The findings appear in the August 20 issue of the journal Science."

The rest of the article contains a general description of how gravitational lensing works and how scientists are using this phenomenon to determine the nature of 'darkness' (ie: Dark Matter & Dark Energy).

I'm sure we're likely to have some threads in the future on this 'little beauty'!

Cheers

[renormalised (Carl)]

Would be interesting to get the article. Might see if I can find it.

[Robh (Rob)]

Very interesting research.
Abell 1689 (+ dark matter) causes the gravitational lensing. The greater the mass, the more the light path is curved. The deviation of light from this expected path is a measure of the expansion of space. If space is expanding, the light path needs to bend more to still reach Earth. This effectively requires an increase in the mass of Abell 1689 as compared to a static space.
Correct or not?

Regards, Rob.

[renormalised (Carl)]

Good question, Rob.

The deviation of the light from the expected path is due to the changing geometry of spacetime as it expands. The lens the cluster generates still bends the light towards the observer, but the total path is altered because of the expansion. To compensate for the changed path and bend the light sufficiently so the observer can see it, the cluster would have to be heavier than it otherwise would need to be.

[CraigS]

Originally Posted by Robh

Very interesting research.
Abell 1689 (+ dark matter) causes the gravitational lensing. The greater the mass, the more the light path is curved. The deviation of light from this expected path is a measure of the expansion of space. If space is expanding, the light path needs to bend more to still reach Earth. This effectively requires an increase in the mass of Abell 1689 as compared to a static space.
Correct or not?

Regards, Rob.

Well, dangerous as it might be Rob, I'll have a go at this one.
Lensing is predicted by Einstein's GR. The light is bent when it passes close to a large mass. The effects of Grav Lensing are due to the curvature of spacetime. It is not a change in the material that the light passes thru that causes the bending, but a change in the strength of the grav field. Light does slow down in the presence of gravity but this effect is completely independent of the colour of the light. All colours are bent by exactly the same angle. This angle depends on how much spacetime has been warped and the incident path of the light - the closer it comes to the centre of the dimple in spacetime, the larger the deflection.

From this deflection they can determine the mass that should be contained within the 'lens'. And in most cases, this calculated mass far exceeds the mass measured by other means (of intervening galaxy causing the lens). So the mass must be coming from 'dark matter' stuff that doesn't appear in the visible spectrum but which effects gravity.

I reckon I'll be shot down for that explanation ... (I think its pretty OK, though).
Cheers

[renormalised (Carl)]

Right so far, but the question was would the mass of the cluster (+ its dark matter) have to be effectively greater in an expanding space in order to compensate for the change in the light path taken through the lens. Being that the change in the light path might not necessarily bend the light enough so the observer can see it....as compared to the same object in a static space. In an expanding space, you not only have to take into account the space expanding between the objects and the observer, you also have to take into account the space expanding at right angles to the light path and the objects (remember, space is expanding in all directions not just away in one direction from the observer). The path the light takes is actually a curved path through spacetime, not a straight line, so you also have to take into that into account. In order to bend the light in to focus towards the observer, the object in an expanding space needs to compensate for that curved path and there's only one way that can happen. It has to compensate not only for a lengthening of the light path due to expansion but also a widening of the path and that becomes increasingly difficult the further the lensed galaxy is away from the central optical axis of the lens.

[CraigS]

Originally Posted by renormalised

Right so far, but the question was would the mass of the cluster (+ its dark matter) have to be effectively greater in an expanding space in order to compensate for the change in the light path taken through the lens. Being that the change in the light path might not necessarily bend the light enough so the observer can see it....as compared to the same object in a static space. In an expanding space, you not only have to take into account the space expanding between the objects and the observer, you also have to take into account the space expanding at right angles to the light path and the objects (remember, space is expanding in all directions not just away in one direction from the observer). The path the light takes is actually a curved path through spacetime, not a straight line, so you also have to take into that into account.

Agreed.

In order to bend the light in to focus towards the observer, the object in an expanding space needs to compensate for that curved path and there's only one way that can happen. It has to compensate not only for a lengthening of the light path due to expansion but also a widening of the path and that becomes increasingly difficult the further the lensed galaxy is away from the central optical axis of the lens.

I think I get what you're saying - the angle between the expected light path, (predictably curved due to expansion), and the actual, observed deflected angle, allows us to calculate the mass of the lens, right ?

The image we receive from a 'lensing' observation is merely distorted. The different types of distortion tell us different things about the intervening object's mass density/distribution. I'm not sure I get this 'focusing' concept. Are you saying that the expansion of spacetime causes problems in detecting the deflection angle ? The type of distortion we see depends on the relative positions of the 'distorted galaxy' and the 'lens galaxy', also, but I'm not sure I get the 'focus' thing ?

Cheers
PS: Just trying to understand ...

[renormalised (Carl)]

http://en.wikipedia.org/wiki/Gravitational_lens

Right at present my head's in a bit of a fog for various reasons. This should explain it better than me, right now

[Robh (Rob)]

Given that the degree of lensing will be both a function of the mass and the expansion of space, how on Earth do you differentiate between the two? I would have thought that the mass of the dark matter would be estimated from the lensing but if this has an expansion component then the mass estimate is going to be somewhat unknown. Do we know enough about the visual galaxy components of Abell 1689 (e.g. relative motions) to estimate the mass of the dark matter?

Regards, Rob.

[CraigS]

Originally Posted by Robh

Given that the degree of lensing will be both a function of the mass and the expansion of space, how on Earth do you differentiate between the two? I would have thought that the mass of the dark matter would be estimated from the lensing but if this has an expansion component then the mass estimate is going to be somewhat unknown. Do we know enough about the visual galaxy components of Abell 1689 (e.g. relative motions) to estimate the mass of the dark matter?

Regards, Rob.

Good questions, Rob.
In a simple model (flat space), I do know that with no intervening mass, the light travels in a straight line. In the same space, the amount of bending is the angle between the original direction of the light (far from the massive object), and the direction in which it travels after it is past the mass. The angle is calculated from Einstein's equations and depends only on the mass of the object and the distance of closest approach between the object and the path of the light (this distance is called the impact parameter).

{The bending angle is the mass of the object times G divided by the impact parameter times c (squared)}.

I'm not sure what you mean "the degree of lensing". There is only light bending. The only variables are the angle, the lens mass and the impact parameter. I presume you could still use the redshifts of both the lensing and the lensed galaxies to eliminate the spacetime expansion factor (for the long hop), from the measured data? I'm not sure about the curvature bit, though.

The impact parameter may be another issue, though. I would have thought that this could be measured directly from the image itself. If this is the case, then expansion effects of this parameter could be eliminated as well?

I'm happy to defer to Carl on this one, though .. (guess we'll have to wait).

Cheers
PS: I also notice that the lensing calculations that I've seen seem to make lots of use of the parameters coming from the CMBR measurements and the Lambda-CDM model. I'm interested in probing this seeming dependency/inter-relationship aspect as well. Cheers

[renormalised (Carl)]

Originally Posted by Robh

Given that the degree of lensing will be both a function of the mass and the expansion of space, how on Earth do you differentiate between the two? I would have thought that the mass of the dark matter would be estimated from the lensing but if this has an expansion component then the mass estimate is going to be somewhat unknown. Do we know enough about the visual galaxy components of Abell 1689 (e.g. relative motions) to estimate the mass of the dark matter?

Regards, Rob.

It's a matter of geometry. You model what the lensing effect will be with matter + dark matter alone and then overlay that with the observed lensing effects. Any difference that can't be accounted for by either the modeled lens and the variations due to uncertainties in the modeled values would then be most likely due to the expansion of spacetime being generated by dark energy. However, as you have pointed out, it's predicated on having a good handle of the mass/DM distribution and relative motions of the galaxy in the cluster.

[Robh (Rob)]

In the article, it says the distances have been measured precisely with large ground-based telescopes. I wonder what method they are using to get this precision?

Regards, Rob.

[renormalised (Carl)]

Tully-Fisher, redshift, fundamental plane, Faber-Jackson...all sorts of methods.

[Robh (Rob)]

Originally Posted by renormalised

Tully-Fisher, redshift, fundamental plane, Faber-Jackson...all sorts of methods.

Thanks Carl.
Some pretty quick responses there!

Regards, Rob

[CraigS]

Hmm .. cool... interesting ... as an aside, (but still related to lensing calcs) ..

I found this one (albeit, another journo's article), dated Jan 2010 but the quality seems to be OK ...I'll try finding the scientists paper, when I get a chance.

Weak Lensing Gains Strength
http://www.physorg.com/news183144919.html

It contains some good words explaining how they're using X-Ray data to calculate where the centre of the lens is, for weak lensed objects, thereby improving the mass calculations...

"Berkeley Lab cosmologists were part of an international team that has extended the relationship between the x-ray luminosity and the mass of galaxy clusters as measured by gravitational lensing, improving the reliability of mass measurements of much older, more distant, and smaller galactic structures. These refined measurements will benefit both the understanding of dark matter and the nature of dark energy as well."
....
“By confirming the mass-luminosity relation and extending it to high redshifts,” Leauthaud says, “we have taken a small step in the right direction toward using weak lensing as a powerful tool to measure the evolution of structure.”

The article gives more details on how they go about these calculations ... (Note: these calcs only apply to weak lensed objects .. not in the same class of Abell 1689 . which is a massive object).

Interesting.

Cheers
PS: 'Lensing' seems to have taken off over the last couple of years or so, with different classification types, methods, etc and seems to be the primary tool being used in the hunt for darkness. Lots more to learn, in order to understand the outcomes ! Rgds.

[CraigS]

Found the paper mentioned below ..

http://arxiv.org/abs/0910.5219

"A WEAK LENSING STUDY OF X-RAY GROUPS IN THE COSMOS SURVEY: FORM AND EVOLUTION OF THE MASS-LUMINOSITY RELATION"
Alexie Leauthaud & a cast of tens ..

Pretty heavy on the number crunching theory side.
Also, as Carl said, they're using "all sorts of methods".

I'll stick to the journo's commentary about these types of calcs, though ...

... There ya go ... journos are helpful sometimes ...



Cheers

[renormalised (Carl)]

If you're interested in learning some more, here's a good book on high z galaxies....

http://www.fishpond.com.au/Books/Sci...s=astrophysics

and here's another on dark matter/energy....

http://www.fishpond.com.au/Books/Sci...s=astrophysics

[CraigS]

As another footnote, from probably the most up-to-date, comprehensive, paper I could find on lensing, (the Lensing Bible, I think), an update on the data processing techniques for investigating 'weak lensing', (the up & coming DM characterisation method), going forward into the future:

Source: "The Dark Matter of Gravitational Lensing" by Richard Massey, Thomas Kitching (Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh). (Submitted on 12 Jan 2010 (v1), last revised 11 May 2010 (this version, v2))

Summary follows:
" .. it is a challenging inverse problem to start from the observed data and extract the underlying weak lensing signal. Many image analysis techniques have been developed to measure galaxy shapes and correct them for these effects. To make the challenge more difficult still, the most distant galaxies, which contain the largest integrated lensing signal, are very faint and noisy."

The methods used to extract the signal are: KSB Quadrupole Shape Moments, Lenshift and Shapelets, the Shear Testing Program (STEP), GRaviational lEnsing Accuracy Testing scheme (GREAT), the HOLICS estimator and the FLexion Improvement Programme (FLIP).

All have their strengths and weaknesses, ins & outs. (Over to the mathematicians to extract the useful data from the image).

Cheers
PS: I added this snippet for anyone wanting to investigate further. Cheers.