# Big Bang and Size of Universe

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I have a question about the big bang... How fast is everything moving? I've heard somewhere that the universe is 40 billion light-years across. If the universe is only 13.7 billion years old, and the big bang took place in what is now the center of the universe, then shouldn't the universe be less than 27.4 billion light years in diameter, assuming nothing can move faster than light? I thought I knew a lot about astronomy, but this one's confusing to me.

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There is no center of the universe in the same sense as there is a center of a conventional explosion. Current evidence indicates that space itself is expanding, much like a balloon's surface expands when it is inflated, sort of dragging matter with it, which makes it possible for the universe to have a diameter greater than 27.4 billion light years.

Edit: Severian's link covers the more technical aspects of this.

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I have a question about the big bang... How fast is everything moving? I've heard somewhere that the universe is 40 billion light-years across. If the universe is only 13.7 billion years old,...

You are asking questions about the standard cosmology model. It is in the context of that model that we have the age estimate of 13.7 billion years. And the estimate that the matter that emitted the CMB radiation that we are now seeing is currently at a distance of 45 billiion lightyears.

So basically you are asking to have some features of the standard cosmo model explained. After that you can make up your own model and deviate all you like. People can believe whatever they want. But it is really good if we can all understand what the standard consensus of mainstream professionals is first.

In standard cosmo there is a criterion of being at rest with respect to the CMB radiation or equivalently being at rest with respect to the primordial matter of the early universe which radiated the CMB. You are at rest if your CMB sky is roughly the same temperature all over---2.728 kelvin. If you are moving it will create a doppler hotspot in the direction you are going towards and a doppler coldspot in the opposite away direction. In fact we know the solar system is moving relative to CMB or early universe matter, so we correct for that motion (it is no big deal, a few hundred km/s)

All the galaxies we look at appear to be approximately at rest relative CMB. Moving this way and that with small speeds on the same order, a few hundred klicks of random individual.

In the standard cosmo picture the distances between widely separated stationary objects are increasing, according to a regular pattern called Hubble Law.

This increase in distance between stationary objects is allowed by Gen Rel, and its specialized form called the Friedmann equations. In fact you expect distances to not stay the same. That is what spacetime having a dynamic geometry is all about.

The increase in distance does not affect distances within gravitationally bound systems like our galaxy or our solar system. Nor does it affect distances within crystals, or rock, or pieces of metal. Their components are bound by chemical bonds. It affects only widely separated not bound together stuff.

The rate of increase in distance is not governed by special relativity---it is not ordinary motion---and it routinely exceeds many times the speed of light.

Hubble Law says it is a percentage rate of increase, that changes gradually over time. Currently the percentage rate is 1/140 percent per million years.

So a million years from now, largescale distances to things way outside our galaxy will each be 1/140 of a percent longer.

If you want to understand better, you should probably google

WRIGHT BALLOON MODEL

If you google that you will get Ned Wright's computer animation of an expanding universe showing how the galaxies get farther apart, and small wigglers representing photons slowly cover the (growing) distances between them. And yet each galaxy stays at the same latitude and longitude on the balloon. That is, it is not moving relative to the process as a whole, it is at rest relative to the CMB.

watch the animations for a while and it may not seem so strange that things can get farther away from each other while standing still

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watch the animations for a while and it may not seem so strange that things can get farther away from each other while standing still

That balloon looks like it's gonna pop. Hope nobody puts a space needle to it.

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Its a good question to which I did'nt really have an answer. So I had a quick look at Wiki and the answer has partly to do with the idea that the photons we observe were not produced until a time after the expansion of the universe started following the big bang. It also is partly to do with the curvature of spacetime at cosmological distances.

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martin, you seem to know what you are talking about

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martin, you seem to know what you are talking about

Thanks for the kind words. And welcome to SFN! I see you are a newcomer. There are several quite knowledgeable people that post here. I expect you will find your way about.

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