What would the universe look like if there was no gravity?

Answer

The universe without gravity would be a very different place, and possibly even couldn’t exist at all!

First, we need to think about what gravity is, and the various ways that it is described by scientists. Perhaps the simplest description of gravity is as a force that pulls stuff together. Any two physical objects — a football, a car, the Sun, a human being — will attract one another in the same way that the south pole of a magnet attracts the north pole of another magnet. The strength of the pull is related to how much stuff there is in each object, and how far apart the two objects are: the pull is stronger if objects are close together and also stronger if they contain a lot of stuff. You may be surprised to realise just how weak gravity is: an entire planet’s worth of rock is needed to hold you to the ground, whereas one small magnet can lift a nail up off the floor despite gravity trying to pull it down! This description of gravity as a force pulling objects together is called Newtonian gravity — after Isaac Newton — and we will use this description for this article. There is also a second, much more complicated description called general relativity, which is needed to understand extreme phenomena such as black holes; I will mention the differences between Newtonian gravity and general relativity at the very end.

Now that we have described Newtonian gravity, we can imagine what the Universe would like if gravity were not to exist. First, let’s pretend that in our current Universe — with gravity — we decide to switch off gravity completely. You could throw a ball into the air and ... it would keep going. It would be slowed down as it hits air molecules, but it would never come back because there is no force trying to pull it back. The same would be true for you if you were to jump into the air: you would sail off into the sky. Everything that is held together by gravity would slowly disintegrate. Planet Earth would come apart, and its former rocks and water would fly away from the Sun. The Sun could explode as there is no longer anything to balance the outward force of the nuclear engine in its core. The remains of the Sun would cease orbiting the centre of the Milky Way galaxy, with all the other, similarly disintegrating stars, slowly expanding outwards. The galaxies themselves, held in orbit around one another by the mysterious dark matter, which I describe using an image of a computer-simulated cluster of galaxies in Fig. 1, will also drift apart. In this scenario, the Universe would look completely dark, with no stars to illuminate it and blocks of rock and dust — those small enough to be held together by electrostatic forces alone — travelling endlessly across space.

Fig. 1. A supercomputer simulaton of a cluster of galaxies. In the left hand panel we show the stars of the galaxies that make up the cluster, and in the right hand panel we show the dark matter. We choose to colour the dark matter by how quickly it’s moving — purple for slow, yellow for fast — in reality is is completely dark to us. Its gravity holds stars into galaxies and then holds galaxies close to another.

This is what we would see if the Universe had gravity but all of a sudden that gravity were to be switched off. One could also ask what would happen if there was never any gravity to begin with. At the Big Bang, the Universe started off expanding rapidly, and was very hot and dense; it was full of fundamental particles of light — photons — that have much more energy than the light our eyes can see, and also fundamental particles of matter called quarks and electrons, plus the mysterious dark matter we mentioned earlier. As the Universe cools, the quarks will stick together into the proton and neutron particles that — in our own "gravity-on" Universe — make up stars, planets and people, but in the the gravity-less Universe will quickly fly apart with the continued expansion. The Universe would therefore be full of individual particles of matter and light, and would never be able to form stars, galaxies or human beings. Whereas the "switch off gravity" scenario would generate a transparent Universe populated by rocks and dust, if gravity never existed we would instead have a thin fog of basic atoms that would never have the chance to be fused together in stars and to form molecules.

Finally, throughout this article we have assumed Newtonian gravity as described above, where gravity is simply a force. We mentioned there is a second version, general relativity, which is required to explain how objects move when the gravitational force is much stronger than on Earth, for example next to the Sun or around black holes. In general relativity, gravity is intrinsically connected to time and space in such a way that gravity emerges from the warping of space and time by the presence of matter. Therefore, in this model our very experience of time itself would change if there was no gravity.

So what would the Universe look like now if there was no gravity? If we switched off gravity at this moment: a dark cloud of rocks and gas. If there was never Newtonian gravity: a dark cloud of atoms. If general relativity did not exist, there might not even be a now...

Image:

• Figure 1: Simulaton credit: Y. Bahé, D. Barnes et al. (DOI: 10.1093/mnras/ stx1403, DOI: 10.1093/mnras/stx1647). Image credit: M. Lovell.