What Happened To My Duck?

[DRAirey1]

If me and my pet duck, who weighs next to nothing, were placed far away from any gravity source and in such a way that our mutual attraction was negligible, would I see my duck accelerate away from me?  If so, how fast?

[pzkpfw]

Do you mean from expansion of the Universe?

[sanctus]

Your mututal attraction is never negligible if far away from any gravity source. Like if you are in an empty universe with only you and your duck then independently how far you from each other eventually you will hug your duck, this because gravity decreases like r^2 but has not finite range.

[Vmedvil]

Ya, if your duck had no gravitational attraction toward you would see him slowly move further away at a velocity in meters per second equal to  V_Λ = (Hubble Constant)(change in Kiloparsec of distance)

Edited December 23, 2017 by Vmedvil

[spartan45]

 

If me and my pet duck, who weighs next to nothing, were placed far away from any gravity source and in such a way that our mutual attraction was negligible, would I see my duck accelerate away from me?  If so, how fast?

 

This is a significant question because it leads to something I’ve wondered about for some time, namely the strange closure of the Andromeda and Milky Way galaxies. First, figures for the Astronaut/Duck question:    

Let’s say the mass of Astronaut is 95kg and the Duck is 5kg giving a total mass of 100kg.

At 1,400 meter separation the Astronaut/Duck 100kg mass gives a gravity field attraction of 3.405 x 10^-15 m/s which at 1,400 meter separation is slightly greater than the universe expansion rate of 3.348 x 10^-15 m/s so would slowly move towards each other. If the separation distance is increased slightly, say another 10 meters to 1,410 meters, then the gravity field attraction is only 3.357 x 10^-15 m/s, while at 1,410 meter separation the expansion rate is 3.372 x 10^-15 m/s and the balance would be tipped and they would slowly move apart.

 

Now for the Andromeda/Milky Way galaxy mystery:

Andromeda Galaxy mass = 1.8 x 10⁴²kg, Milky Way Galaxy mass = 6 x 10⁴¹kg; combined = 2.4 x 10⁴²kg

At 2.36525 x 10²² meter separation the Andromeda/Milky Way 2.4 x 10⁴²kg total mass gives a gravity field attraction of 2.863076353 x 10^-13 m/s which at 2.36525 x 10²² meter separation is much less than the universe expansion rate of 5.656956875 x 10⁴ m/s so should be rapidly be moving away from each other, but the Andromeda galaxy is already closing the Milky Way at 1.116667 x 10⁵ m/s. This means the actual closing velocity before Universe expansion is taken into account must be (1.116667 x 10⁵ m/s) + (5.656956875 x 10⁴ m/s) giving 1.682366 x 10⁵ m/s. How this high closure velocity could have occurred seems a mystery.

[spartan45]

Now for the Andromeda/Milky Way galaxy mystery:

 

Oops, I’ve just googled ‘why do Galaxies collide?’ It seems all the matter and visible energy only accounts for 5 per cent of what is out there. Dark matter accounts for 25 per cent and dark energy for 70 per cent. One theory is that the dark matter surrounds galaxies like a halo, helping to pull them together. Both dark matter and dark energy are invisible and so far only their gravitational effect is evidence their existence. Using only the visible 5 per cent caused the mystery of the rapid closure of the Andromeda and Milky Way galaxies, but at least it was a good demonstration of why the existence dark matter/energy is necessary to make sense of what we observe in the Universe. 

[Vmedvil]

Yep, absolutely necessary for the universe to function. that 70% Dark Energy is what is causing that Hubble constant which is the expansion, while Dark Matter and Normal Matter along with energy cause attraction, the 30%

 

 

 

V_g - V_Λ = V_{Actual movement for that frame.}

 

Edited December 23, 2017 by Vmedvil

[spartan45]

Google ‘image showing dark matter between galaxies’ brings up top results showing the first such image, supplied by The University of Waterloo on 12 April 2017. The image was produced from the gravitational bending of light, using a technique called weak gravitational lensing. It shows the dark matter surrounding a pair of galaxies, pulling them together.  Nice evidence to why galaxies collide.