# The Naked Scientists Forum

### Author Topic: Can this reduce the amount of needed Dark matter?  (Read 4795 times)

#### Zimblerigor

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##### Can this reduce the amount of needed Dark matter?
« on: 04/05/2015 08:19:26 »
The main evidence for the existence of dark matter is the flat rotation curve of galaxies, and high peculiar velocities of galaxies in galaxy clusters.

The mass eclosed in a radius R is derived from the Keplerian equation M(R) = v2R/G .

Also we know about the existence of gravitational lensing.

My question is this: was gravitational lensing taken into account when galactic rotation curves were plotted? Because here is an effect of gravitational lensing that I didn`t find mention of:

The photon emmited by the star can`t travel directly to the observer because it is gravitationally pulled to the center of the galaxy, so its trajectory is bent. And because of this bending effect the observer will see the star as being farther away from the galactic center. This in turn will push the observer to the conclusion that there is more mass enclosed in the observed radius. If the orbital speed of the star is not calculated from it’s red shift then it will also appear to be bigger then it actually is, since v = w*r, where w – angular velocity, r – observed radius.

Consequences:

1) All massive celestial objects appear greater then they actually are.

2) The farther away the celestial object from the observer the stronger the magnification effect.

3) Most of the distant celestial object that we observe are not actually in the direction that we observe them.

P.S.: The fish is also distorted
« Last Edit: 04/05/2015 14:43:44 by Zimblerigor »

#### acsinuk

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##### Re: Can this reduce the amount of needed Dark matter?
« Reply #1 on: 05/05/2015 17:57:58 »
The real problem is that the outer stars in a spiral galaxy should rotate much slower than the inner ones.  If we magnetise a galaxy from a central hub then all stars would be forced to move around together.  But we have not been able to find that magnetic field yet.   But we have found plenty of other magnetization and Faraday rotation??

#### chiralSPO

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##### Re: Can this reduce the amount of needed Dark matter?
« Reply #2 on: 05/05/2015 19:12:18 »
The main evidence for the existence of dark matter is the flat rotation curve of galaxies, and high peculiar velocities of galaxies in galaxy clusters.

The mass eclosed in a radius R is derived from the Keplerian equation M(R) = v2R/G .

Also we know about the existence of gravitational lensing.

My question is this: was gravitational lensing taken into account when galactic rotation curves were plotted? Because here is an effect of gravitational lensing that I didn`t find mention of:

The photon emmited by the star can`t travel directly to the observer because it is gravitationally pulled to the center of the galaxy, so its trajectory is bent. And because of this bending effect the observer will see the star as being farther away from the galactic center. This in turn will push the observer to the conclusion that there is more mass enclosed in the observed radius. If the orbital speed of the star is not calculated from it’s red shift then it will also appear to be bigger then it actually is, since v = w*r, where w – angular velocity, r – observed radius.

Consequences:

1) All massive celestial objects appear greater then they actually are.

2) The farther away the celestial object from the observer the stronger the magnification effect.

3) Most of the distant celestial object that we observe are not actually in the direction that we observe them.

P.S.: The fish is also distorted

That's an interesting theory. I am no expert in this field, but I am always interested in possible explanations of the phenomena that indicate the presence of dark matter.

A few thoughts:

Would the mass of the galaxy itself cause the type of gravitational lensing that you are describing, or would there need to be a massive body between the galaxy and the observer? Many of the galaxies (and clusters of galaxies) that were observed to show this anomalous velocity distribution do not have massive objects (that we can see) between them and us...

I believe that all of the angular velocities are calculated from red/blue shifts (I don't think we can really observe the motions directly on our meager, mortal timescale), so the distortion you describe might artificially inflate the apparent radius of the orbit, but should not increase the apparent velocity.

#### PmbPhy

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##### Re: Can this reduce the amount of needed Dark matter?
« Reply #3 on: 05/05/2015 22:13:57 »
Quote from: Zimblerigor
My question is this: was gravitational lensing taken into account when galactic rotation curves were plotted? Because here is an effect of gravitational lensing that I didn`t find mention of:
The effect of the gravitational deflection of light from stars by the galaxy is so insignificant that it's virtually undetectable.

#### RD

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##### Re: Can this reduce the amount of needed Dark matter?
« Reply #4 on: 05/05/2015 23:38:10 »
1) All massive celestial objects appear greater then they actually are.

2) The farther away the celestial object from the observer the stronger the magnification effect.

If the gravitational-lensing-effect was noticeable the far-half of a galaxy would appear bigger than the half closest to us ...

In reality no conspicuous distortion : looks symmetrical to me ...

http://en.wikipedia.org/wiki/Andromeda_Galaxy
« Last Edit: 05/05/2015 23:52:52 by RD »

#### Zimblerigor

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##### Re: Can this reduce the amount of needed Dark matter?
« Reply #5 on: 06/05/2015 15:23:27 »
Thanks a lot for your replies, guys. I was wrong apperantly.

#### RD

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##### Re: Can this reduce the amount of needed Dark matter?
« Reply #6 on: 06/05/2015 20:11:32 »
Thanks a lot for your replies, guys. I was wrong apperantly.

The effect you've described is real , it's just not big enough to be an explanation for the flat rotation curve.

I'm a bit sceptical about dark matter myself : there are other explanations for flat rotation curve ... http://en.wikipedia.org/wiki/Galaxy_rotation_curve#Alternatives_to_dark_matter

#### Le Repteux

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##### Re: Can this reduce the amount of needed Dark matter?
« Reply #7 on: 20/10/2015 20:51:22 »
Hi Zimblerigor,

I posted a similar idea here http://www.thenakedscientists.com/forum/index.php?topic=60307.0 but about the suns' light, and RD redirected me to yours when I began applying it to galaxy rotation. I think you have let this idea down too fast Zim, so I will continue to discuss it, and I hope you will be back to discuss it with me. Its not so frequent that someone has the same idea as ours, lets see how we can push it further! Cheers!

Quote from: RD
If the gravitational-lensing-effect was noticeable the far-half of a galaxy would appear bigger than the half closest to us ...
Thanks for the redirection RD. If curving occurs at the far side of the galaxy, then it should occur at the close side too, so your bottom line of sight should also be curved, which would change the apparent position of the bottom star down from its real one, and which would tilt the galaxy plane towards the observer a bit. But this is not the way we measure the rotational speeds. For that, we take the stars that are on a line which is at a normal to our line of sight, because they produce more doppler effect. For those stars, providing they are at the same distance from the galaxy center, they will suffer the same bending, which will be more important if they are closer to the center, as it is for their speed, so if we attribute that speed to the real distance, we should get closer to the predicted curve.

You say the effect would be too small to be observable, but how could it be less important than the bending of the sun's light, which would necessarily be observable since it would be close to the one for that specific starlight?

#### RD

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##### Re: Can this reduce the amount of needed Dark matter?
« Reply #8 on: 21/10/2015 02:43:37 »

... If curving occurs at the far side of the galaxy, then it should occur at the close side too, so your bottom line of sight should also be curved, which would change the apparent position of the bottom star down from its real one, and which would tilt the galaxy plane towards the observer a bit.

The apparent position of the near [bottom] star would not be deflected as much as the far star : the light from the far star has to pass the centre of the galaxy where most of the mass is. The far half of the galaxy would be more magnified by gravitational lensing than the near half.

The observed speed is 2-3 times the expected speed. If that was due to gravitational-lensing expanding radial measurements the far half of the galaxy would appear conspicuously bigger than the near half.
« Last Edit: 21/10/2015 02:59:44 by RD »

#### jeffreyH

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• The graviton sucks
##### Re: Can this reduce the amount of needed Dark matter?
« Reply #9 on: 21/10/2015 02:59:58 »
The nearer a star is to the dense mass at the centre of a galaxy the more intense the time dilation. This is not enough to account for the rotation but will be a factor. How much of a factor depends upon how gravitation exactly affects mass in regions of space with weak time dilation. Time will be running faster. The force of gravity may die away with distance from the source but it still has an action. Dark matter is really the best hypothesis to make up the missing piece of the puzzle.

#### Le Repteux

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##### Re: Can this reduce the amount of needed Dark matter?
« Reply #10 on: 21/10/2015 16:02:21 »
Quote from: RD
The observed speed is 2-3 times the expected speed. If that was due to gravitational-lensing expanding radial measurements the far half of the galaxy would appear conspicuously bigger than the near half.

Here is a post from another forum where a moderator named Janus made an approximate calculation to verify that possibility:
Quote from: Janus"
let's work out an example and see if this is even feasible. We'll assume a galaxy 100,000 light years across with a mass of 100,000,000,000 solar masses which is 1 billion light years away.

The light from a source behind this galaxy and just skimming its edge, would by gravitational lensing be deflected by 0.258 sec of arc. Since 1/2 of this deflection occurs on the inbound path, the most we can expect light coming from a star at the edge to be bent on its outward path to us is 0.129 sec of arc.

At a distance of 1 billion light years, this equates to an apparent displacement of ~625 light years ( meaning the galaxy would appear to be 50,625 light years in radius instead of 50,000.)

Calculating the difference in orbital speed at 50,000 vs. 50625 light years produces orbital velocities of 167 km/sec vs. 168 km/sec.

If we work out how much extra mass it would take to make that 1 km/sec difference at a fixed radius of 50625 light years, it works out to a difference of ~1.2%, or far short of the amount of dark matter needed to make up for the missing mass according to the actual orbital velocities we measure.

In addition, gravitational lensing in fact gives us additional evidence for dark matter. Because of the extra mass due to DM, the light passing galaxies bends more than and differently from what we would expect from just the matter we see.

On top of that, we have the case of the Bullet Cluster, in which dark matter has been "knocked loose" from the visible matter in a collision between galaxy clusters. In this situation we see gravitational lensing of objects behind the cluster where there is no visible matter to cause it.

Galaxy rotation curves may have set us on the road to dark matter, but there has been a great deal of other supporting evidence uncovered since the first step on that path.

And here is what I answered to him:

Quote from: Le Repteux
Thanks for the calculations Janus. So you came to the conclusion that this kind of bending cannot account for dark matter, but it sure can account for the flatness of the curve though, because both the exaggerated speed and the apparent bending are caused by the same importance of the curved space at the same distance from the center of mass, so if we do the calculations for different distances, we should get a rotation curve which would be similar to the predicted one, but a bit higher on the graph. Am I right?
http://www.thescienceforum.com/astronomy-cosmology/49499-can-dark-matter-optical-illusion-caused-gravitational-lensing.html#post658865 [nofollow]

Unfortunately, I have just been banned for two weeks from this forum, so I can't see the answers anymore. If I am right for the curve though, I have an explanation for its speeds being respectively higher than the predicted ones: redshift. When we take for granted that redshift is due to motion, we consider that the rotation periods that we observe are also red shifted, so we accelerate them in the same proportion, which gives higher speeds than putting them as they are in the equations. My explanation for gravitational redshift not being due to motion is linked to the idea about mass that I posted here: http://www.thenakedscientists.com/forum/index.php?topic=53171.msg446148#msg446148.

Hey, Jeffrey, you never told me what were the points that you found interesting in that former thread!
« Last Edit: 21/10/2015 16:19:55 by Le Repteux »

#### evan_au

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##### Re: Can this reduce the amount of needed Dark matter?
« Reply #11 on: 24/10/2015 00:40:30 »
Slightly off-topic for this thread: A video about the LUX experiment, one of the most sensitive attempts to detect hypothetical Dark Matter particles. It takes you 4850 feet underground to their hidden lair, deep in a disused goldmine (17 minutes).
http://www.sciencefriday.com/videos/4850-feet-below-the-hunt-for-dark-matter/

Spoiler alert: They haven't discovered Dark Matter particles, yet!

#### The Naked Scientists Forum

##### Re: Can this reduce the amount of needed Dark matter?
« Reply #11 on: 24/10/2015 00:40:30 »