# If the universe is expanding, what is it expanding into?

1. If the universe is expanding, what is it expanding into?

2. When the big bang happened where did it occur?

3. When the big bang happened how did it occur?

4. Where did the energy come from? Energy can not be created or destroyed does that mean, energy has existed before the universe was here?

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To my experience, it is not possible to read on this topics without some mention of this "questions" and why they are free of meaning. Not wrong, just free of sense! –  Georg Mar 21 '11 at 15:03
This question is bound to stay open. Even if you accept someone's answer, there is no way proving he is correct. –  ertx Sep 6 '12 at 11:54

There are much better people to answer this question, but I will answer it from the point of view of an experimental particle physicist, the way I have filed this knowledge in my head.

First of all the big bang is a particular solution within the theory of general relativity, which treats gravity as a deformation of space time around massive bodies.

This theory differs from the Newtonian and special relativity world we have explored and are familiar with. For these we have definitions of energy and also conservation of energy.

In the general relativity framework there exists energy but the concept of conservation of energy is not valid. Energy can appear according to the appropriate equations. General relativity is necessary in the description of observations of large dimensions in space and time and very massive bodies.

So the big bang in this theory happens from one point, as an explosion as shown in the links I gave you. All points in our universe are this point. Practically metaphysical but true. So there is no meaning on "what it is expanding to".

A useful for me analogy is the surface of a balloon. Our universe, in smaller dimensions, is like the surface of an expanding balloon. All points on the surface are receding from each other and as far as the surface is concerned, it just expands. Others like the analogy of a cake with fruit interspersed. As it rises in the oven each piece of fruit recedes from the others and if we could think that all of it could be originating from a point, all points in the expanded cake would be the beginning point.

As for when the big bang happened, read the links I gave you.Astronomy has measured this.

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Your assumption that there has to be something before, after, outside or even in between, some sequence of events, while natural to classical Physics, is not necessary. Google for “Lucretius arrow "edge of the universe"” finds various web references, none of which I particularly want to cite, to the fact that Lucretius asked what happens at the edge of the universe around 100BC. With various similar search terms, Google leads to various Philosophical forums that have had similar Questions asked.

Your Question also has slight similarity to the ancient Zeno's paradox of the Arrow.

AFAIK, there is no settled Answer to this. It's often convenient in mathematics to assume that there's a continuum, and such models can be quite effective, but there's no necessity. Taking things to be without end, infinite, in endless ascents or descents, etc., while often useful, also requires care when constructing mathematical models, because without care things can end up being badly defined.

To step into the detailed Question you put, you are asking about before, after, and outside what we can confidently infer from fitting experimental data to models of current physical theory. There does not have to be anything at all there. A different possibility, perhaps equally disconcerting, is that we may be using models of the scientific theories that we find to be very accurately consistent close to Earth and the Sun in a domain in which the the experimental data does not fit any single consistent model. In particular, there is no guarantee that there is a consistent atlas of the big bang (and yes, for those who know of these things, I also mean this technically, as of a manifold).

I fear that this is rather general, but insofar as I see the Question blows Philosophical so blow I.

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Energy conservation is funny with general relativity. To have a conservation law you need something which is a constant of the motion. In the case of energy in spacetime this requires there be a Killing vector that is constant along all timelike geodesics. This isometry is what defines the conservation of energy. Spacetime metrics for cosmologies do not admit a Killing vector which is constant along timelike geodesics.

The universe expands as a manifold. The spacetime is foliated by spatial surfaces, where points on that surface increase their distance along the time direction of the foliation. If the universe is flat these spatial surfaces are just $R^3$ spaces, and the curvature of spacetime is defined by their extrinsic curvature in the space plus time = spacetime. The universe is not particularly expanding into anything outside of itself.

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...anything outside of itself that we are experimentally or theoretically aware of. Occam razor cuts out the requirement for an embedding space. –  lurscher Mar 23 '11 at 18:28

Only a physics undergrad here, so I only know of this stuff cause ive done a bit of extra reading, perfectly possible that I misunderstood it. But you can "create" energy/matter, the catch being that you must also make a matching amount of anti matter/energy, resulting in a net gain of 0 energy/matter. Something I saw before I learned this that kind of started my understanding of it was this video on Hawking radiation http://www.youtube.com/watch?v=S6srN4idq1E

If I understood the reading correctly, this is a possible mechanism of creating the universe (particle/anti particle mechanism). This apparently happens all the time in Quantum Mechanics, but on a much smaller scale, and apparently the only way to use this effect to create something that lasts more then a fraction of a second is to have it create a universe worth of matter/energy and matching anti matter/energy. What makes this so hard to get a definite understanding is the fact that you have to apply general relativity as well as Quantum field theory, and the 2 can sometimes disagree (I don't have enough understanding of either to say exactly what the disagreement is, I just know there is apparently one)

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In this answer you erroneously suggest that anti-particles have negative energy. This is incorrect. Any third/fourth year undergraduate course on introductory QFT will teach you about how Dirac derived his equation by seeking a relativistic form of the Schrodinger equation. His interpretation of the negative energy solutions is called 'the Dirac sea' but they are now understood as positive energy anti-particles, travelling backwards in time. See the Dirac equation wiki page for more information. (en.wikipedia.org/wiki/Dirac_equation) –  qftme Mar 23 '11 at 11:35
@qftme there is a fairly long distance to be covered between first and third year, wouldn't you say? Also most physics undergrad courses are unlikely to cover relativistic qfts in any great detail. –  user346 Mar 23 '11 at 16:50
@qftme I can fairly safely say that QFT is not covered in an (at least my) undergrad degree, however, the correction is appreciated. Not sure I understand it, mainly because the negative energy solution is so much simpler in terms of energy conservation, so will have to give that wiki page a good read through. So these positive anti particles, can they be thought of as particles created when the 2 particles collide and anilate each other, and destroyed when the particle and antiparticle are made in their frame? –  CJB Mar 23 '11 at 21:11