Is the expanding universe tearing galaxies apart?

08 January 2012

Question

Hello there,

A question that I would love answered is this: The Universe is expanding and accelerating in it's expansion. Galaxies and other cosmic objects are moving away from each other. Why then are the individual galaxies not being torn apart by this expansion?

Why is the solar system not affected by this expansion? Or is it and if so, how? Many thanks. I love all the podcasts. Manoo Golden
(South Africa)

Answer

Dominic - Well, the universe is expanding because it formed out of the Big Bang which was this massive explosion at the beginning of the universe which imparted momentum and inertia to all the material in the universe and this was initially a smooth distribution of material for the first half million years or so.

After about half a million years, you enter an era that we call the Dark Ages when the first structure started to form in the universe. And what happened at that point was that in some parts of the universe where there was an over density, so where there was a lot of material compressed together, gravity was strong enough to pull that material together and for it to become gravitationally bound into a stucture we call the galaxy.

So, the initial velocity given to that material out of the Big Bang was overcome by gravity and so, the movement of material in that galaxy is entirely determined by the gravitational force in that galaxy. Individual galaxies will be moving apart because they're not gravitationally bound. Their velocity is just still determined by what they were given by the Big Bang. But inside the galaxy itself, it's all gravity now.

Chris - So, even though the universe is getting bigger as a whole, if I were to measure the distance between the Earth and Pluto, even though there's a lot of space in our solar system between the Earth and Pluto, 6 billion kilometres or so, that space isn't actually getting any bigger?

Dominic - Yes, and whether that distance is changing, it will be entirely down to the gravitational forces of all of the planets in the solar system.

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