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Here is a quote from Brian Greene, The Fabric of the Cosmos: Space, Time, and the Texture of Reality, p. 273:

“The early universe provided an arena in which gravity exerted its repulsive side with a vengeance, driving every region of space away from every other with unrelenting ferocity.”

Relativity proves that spacetime is curved by gravity, but isn’t space expanding different from space curvature? In other words, how does negative gravity exert a force on space? If the universe stopped expanding, like in a “bounce” theory, would the expanse of space (containing the Higgs field and cosmological constant) contract as the density of mass and energy increased?

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The Friedmann Equations describe the evolution of the 'size' (described by a scale-factor $a$) of the universe over time. The equation looks a lot like one that would describe the collapse of a massive object---like a star, or gas cloud. Normal matter and energy cause space to contract, because of gravity. But dark energy (or whatever you interpret the cosmological constant $\Lambda$ as), causes a repulsive/expansive force. This is all to say that space itself seems to react to gravity in a very similar way to matter. We observe that not only is the universe expanding, but accelerating in its expansion, which means there must be a large amount of dark energy: resisting the collapsing force normal matter exerts.

The expansion of space is equivalent to space-time curvature. The time component allows us to describe the change of space: collapse or expansion. While observations suggest that space (i.e. 3D space) is flat, space-time is still curved because of expansion. Flat space-time (i.e. 4D, called 'Minkowski') would mean not only flat space, but also a space which doesn't evolve in time (i.e. the scale factor $a$ was constant).

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Well it seems like I am about to contradict an actual physicist, so I must be wrong. But the expansion of the universe isn't caused by gravity, but by what we call dark energy and dark matter, whatever they are... And gravity has a repulsive side? Like you said, gravity from objects bends space-time so other objects travel through that curvature and get affected by it. So for gravity to repel, it would have to be negative. Imagine the classic picture of an object interacting with space-time, like a pit in the fabric. A negative gravity object would be a bump instead. It is possible that dark energy acts in this way, but I think it is not fully known.

Another point, when he talks about expansion, does he mean the expansion that has been going on for billions of years, or the inflation that occurred fractions of seconds after the big bang? These are quite different things obviously.

Maybe if I had read the book you are quoting, I could better understand what he is talking about.

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  • $\begingroup$ Dark energy accelerates the expansion. The Friedman equations link the expansion and acceleration thereof to density and pressure, which in turn satisfy $p=\sum_i w_i\rho_ic^2$ where normal matter's $w$ differs from that of dark energy. $\endgroup$
    – J.G.
    Jul 5, 2017 at 11:48
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no, there is no gravity's force is avilabale in earth. beacuse,gravity has limit in earth and that is 400,000 kilomiter. so you can sey that gravity is not avilable in earth.

                                   Thanks your friend
                                   Name - arya rajput
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    $\begingroup$ wuut? I think you need to read a few physics books... $\endgroup$
    – Edward Rixon
    Sep 2, 2016 at 10:24
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    $\begingroup$ You ought to read the question as well. $\endgroup$
    – signus
    Feb 21, 2017 at 23:55

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