I would like to know, how strong would the gravity be from the distant universe if we were somehow able to remove half of it. For example, if everything behind us did not exist, how strongly would the stuff in front of us be pulling? By distant, let us consider everything beyond 10 billion light years (e.g. remove all the closer stuff). How many G's acceleration would I experience (discounting the fact that I would be in free fall and wouldn't feel it)?
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1$\begingroup$ Not an answer, but it will be useful to consider that gravity falls off as 1/r^2. At 10 billion light years (much less than half the radius of the known universe), the effect would likely be negligible (maybe barring supermassive black holes). $\endgroup$– user15317Commented Apr 25, 2017 at 9:56
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$\begingroup$ @mikey Excellent starting point, yes, that factor will certainly come into play. Note that the universe used to be extremely dense, and the gravity from that should be reaching us from the region where the cosmic microwave background was emitted and beyond. We are effectively seeing and probably experiencing gravity from the past universe. $\endgroup$– JonathanCommented Apr 25, 2017 at 12:15
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$\begingroup$ Is this question sufficiently different from your other question? $\endgroup$– zephyrCommented Apr 25, 2017 at 13:40
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$\begingroup$ How abstract are we talking about this? Do we assume a uniformly dense universe (at sufficiently large distance scales)? $\endgroup$– Brian TungCommented Apr 25, 2017 at 23:00
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1$\begingroup$ @userLTK Although I assumed that the observable universe isn't expanding, I kind of included dark energy in my calculation because I used the full critical density, so my mass estimate is that of the total energy content of the observable universe - normal matter and energy as well as dark matter and energy. $\endgroup$– PM 2RingCommented Apr 26, 2017 at 2:12
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1 Answer
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Not very strong at all. I get a rough figure of $3.725\times 10^{-9} m/s^2$.
To perform that calculation I made a few simplifying assumptions.
- Assume that we can ignore everything outside the observable universe.
- Assume that the observable universe is a static homogeneous sphere with a radius of 46.6 billion light-years, and a mean density of $\rho = 0.85\times 10^{-26} kg/m^3$, that figure is the critical density needed for the global curvature of the universe to be zero (i.e., on the very large scale spacetime is flat).
- Assume that we can use Newtonian physics to calculate the acceleration due to gravity.
I didn't bother removing all the stuff closer than 10 billion light years.
We're standing at the centre of the flat surface of a hemisphere of radius $r$ and we want to know how strong the gravity is. We can pretend that all the mass of the hemisphere is concentrated at its centre of mass, which is $\frac{3r}{8}$ below us.
The Newtonian formula for gravitational acceleration is
$$a = \frac{GM}{r^2}$$
Mass is volume times density, i.e., $M=V \rho$ and the volume of a hemisphere is
$$V = \frac{2}{3}\pi r^3$$
Combining everything, not forgetting to use $\frac{3r}{8}$ as the distance term in the denominator of the acceleration formula, we get
$$a = G \rho \frac{128}{27} \pi r$$
Plugging in the values I mentioned earlier for $\rho$ and $r$, the Google Calculator says $3.72484086\times 10^{-9} m/s^2$. That's about $3.8\times 10^{-10}$ standard gravities.
