What would happen if the gravitational constant of our universe were 34 times weaker? What effect would it have on the stars and planets?

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    $\begingroup$ Exactly 34 times? That's rather specific. $\endgroup$
    – Phiteros
    Commented Apr 25, 2017 at 23:38
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    $\begingroup$ What I remember from watching too many science shows, is our universe is in a precise balance. A change of that magnitude, and you might not have any planets, just dust clouds. A few of the most dense materials might form a very small number of planets. $\endgroup$
    – cybernard
    Commented Apr 26, 2017 at 0:25
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    $\begingroup$ what would happen (in the whole universe!) is the very exemplar of a too broad question. You should look over previous questions on this issue, and also get a feeling for what is “too broad”. $\endgroup$
    – JDługosz
    Commented Apr 26, 2017 at 1:12
  • $\begingroup$ Possible duplicate of What would the Universe be like if gravity was slightly stronger? $\endgroup$
    – Shalvenay
    Commented Apr 26, 2017 at 1:20
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    $\begingroup$ I'm voting to close this question as off-topic because it is pure what-if question $\endgroup$
    – MolbOrg
    Commented Apr 26, 2017 at 12:15

1 Answer 1


You're probably looking at a significantly less interesting universe for a number of reasons.

Cloud Collapse

I suspect that stars wouldn't form. The current leading explanation for the formation of the universe holds that after the Big Bang, the universe contained only thin hydrogen gas. Over time the gravitational attraction between the hydrogen atoms caused it to collapse into clouds that would form stars.

In order for a gas cloud to collapse under gravity, it must be subject to Jeans Instability. The calculation involves a lot of factors, but for our purposes the important thing to note is that the critical mass is inversely proportional to $G^{3/2}$. A gravitational force 34 times weaker would require a cloud to be about 200 times more massive in order to collapse.

Another fact to consider is that the universe is expanding at an ever-increasing rate, due to forces we don't yet fully understand. This expansion would increase the distance between the hydrogen atoms, counteracting the gravity that is trying to pull them together.

Solar Ignition

Assuming that the clouds still come together, the change could still disrupt star formation. Each star is a delicate balance between gravity trying to smash all of that mass together into a single point and the outward pressure produced by nuclear fusion trying to blast it apart. With a lower gravitational constant, it would take a much greater mass of hydrogen coming together to produce enough inward pressure to trigger the fusion reaction. Stars would be much rarer, and star formation might even be entirely impossible.

Any clouds of hydrogen that came together without sufficient mass to form a star in the new universe would form gas giant planets like Jupiter and Saturn.

Lack of Heavy Elements

Fewer stars means less nuclear fusion, which means that elements other than hydrogen make up a much smaller portion of this universe than they do in ours (and hydrogen is already by far the most common element in our universe). Those elements would also have a harder time spreading throughout the universe, because even if star formation is possible, supernovae might not be.

Stars die in one of two ways depending on their mass. Small stars like our sun (and most other stars in the known universe) will fuse up to the point of generating carbon, but don't burn hot enough to fuse carbon into higher elements - at a certain point, the core will collapse into a white dwarf and the outer layers will fly apart to form a nebula. Larger stars will continue fusing elements up to iron but will eventually hit a similar point of collapse; however, their greater mass produces so much pressure on collapse that it triggers a supernovae, a massive explosion believed to be the only way that elements higher than iron are ever created in nature. The explosion distributes those elements across space until gravity eventually pulls them together to create new stars and planets. Changing the gravitational constant would increase how big a star would have to be to trigger a supernovae on collapse, which could prevent elements higher than iron from ever being formed.

So this universe would contain much less carbon and silicon, the two most common elements with the greatest potential for forming chemical bonds, which most biologists believe is an important part of the creation of life. Combined with the fewer number of stars in the universe, this would make life much rarer or quite possibly non-existent. It would also contain little to no copper, silver, gold, lead, nickel, tin, uranium, or any of the other elements higher than iron on the periodic table (depending on whether supernovae are impossible or merely very rare).

  • $\begingroup$ This is interesting. Can you mathematically demonstrate if stellar ignition is possible with this reduced gravity? $\endgroup$
    – kingledion
    Commented Apr 26, 2017 at 1:25
  • $\begingroup$ Unfortunately, while I'm pretty confident about the physics of star formation in general terms, I'm not a professional astrophysicist and don't know the right equations. I can try to find them, but I figured I'd wait to see if this was enough of an answer for the OP first. $\endgroup$
    – Ben S.
    Commented Apr 26, 2017 at 1:33
  • $\begingroup$ Well, I like math, so let me encourage you to take a stab at it, if you find the time. $\endgroup$
    – kingledion
    Commented Apr 26, 2017 at 1:58
  • $\begingroup$ Makes perfect sense to me, I wonder if you could even have solar systems with stars that big assuming stars could even get started. Systems with rocky planets in the life zone I mean. Plus reducing gravity doesn't mean you're reducing the other forces to match so magnetism etc would be way out of kilter $\endgroup$
    – Kilisi
    Commented Apr 26, 2017 at 2:40
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    $\begingroup$ @Kilisi The relative strength of magnetism could be another issue, I'm not sure - part of the reason fusion is so difficult is that before the hydrogen atoms are moving fast enough to fuse on impact, the intense heat ionizes them, and as a result they have to be moving even faster to overcome the electrical repulsion between the ions. I think it's simply a question of adding more mass -> more pressure -> more heat -> moving faster, but it could be more complicated than that. $\endgroup$
    – Ben S.
    Commented Apr 26, 2017 at 3:14

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