Big Bang theory vs. the universe as we know it

Low-Q

I have a question you might have a good answer to.

I hear astronomers keep saying that the universe as we know it was created by a big bang. One single point that exploded.
If every known matter was flying off from the same point, how can it happen that there are stars, meteors, planets going in all different directions? Our galaxy is on collosion course on another galaxy, so these galaxies would appearently not have its origin from the same big bang. Do gravity really pull that hard between them? Or is our galaxy have its origin from a different big bang than the other galaxy?

I know about gravity, but that cannot be the only reason why these objects are not following a stright path away from the Big Bang. In the beginning there was only very small particles created, all with very little mass. How did they lump together into atoms and later small lumps of matter?

I know that the universe expands (At least what is found with delicate instruments), but with all the different directions these objects are going in the universe, the theory would not seem logical - not to me at least.

Vidar

Whovian

Your mistake seems to be thinking that everything exploded from a single point. The current consensus is that the BB occurred everywhere.

mfb

That is not true. The explosion happened everywhere at the same time.
In addition, the big bang model does not include "the first moment" - it deals with the expansion and evolution of some initial, very hot and very dense universe.

Quantum fluctuations lead to small asymmetries in the early universe, and gravity enhanced them. Those relative motions are small relative to the overall expansion and come from local gravitational and other forces.
I would not call it "hard", but yes.

It is the main reason. Electromagnetism is there, too, and the weak and strong interaction are relevant for stars and other objects.
Nuclei: Strong+weak force
Atoms, dust particles and so on: Electromagnetic force

If you look at the universe on a large scale, all those local movements are insignificant, and everything expands nearly uniformly.

Low-Q

Thanks guys! Not that my survival depends on it, but I'm much wiser now - thanks :-)

Just one thing: How can the universe expand if the explosion was everywhere at the same time?

Vidar

mfb

There is no problem. General Relativity is just not very intuitive.

marcus

I agree with what Mfb has been saying and want to amplify something. Are you comfortable with this idea? Namely that the universe can expand with the expansion happening everywhere at the same time. Just changing one word, in what you said.

That is, can you understand expansion as something that is experienced from within space (by distances between stuff increasing) without there being "space outside of space" for space to "expand into". I have to say that "explode" is an awful word to use, gives exactly the wrong idea of the start of expansion.

The basic assumption is there is NO SPACE OUTSIDE OF SPACE, because there is no evidence of that so it is an unnecessary complication. So when we talk about space expanding it is not in the sense of it expanding "into" some larger surrounding. The expansion we mean is that process experienced internally by distances increasing according to a definite pattern. (They currently increase about 1/140 of one percent every million years.)

In case you or anyone is curious, here is a chart that shows the past expansion history
http://ned.ipac.caltech.edu/level5/M...s/figure14.jpg
The heavy solid line corresponds to what we think is the case---with the model parameters set to fit observational data.

Drakkith

There was no "explosion" like you are used to seeing. It was more like someone baking bread with radioactive super explosive-growth yeast. The dough went from a little ball the size of your fist to a big ball the size of your house in a millionth of a second. Except the ball may have been infinite in size both before and after. Keep in mind that the Big Bang Theory does NOT say that the universe came from a singularity, only that it was once very hot and very dense and expanded outward from there. Like baking.

Murdstone

Low-Q Your conception of the Origin of The Universe was originally what was postulated about 50 years ago. In its original form “The Big Bang” was a finite point expansion, not a field manifestation. There were estimates of the size and the energy contend of this “point”. The conundrum was that no center could be found from which The Universe could have emerged. Of course there was always the problem of – if The Universe is expanding, what is it expanding into?

Do not let others suggest that The Origin of The Universe has been solved by postulating that the distance between points is somehow expanding - stretching. This trick is meant to resolve the need for something outside The Universe for it to expand into. As elegant as it is made to appear, there is no definitive explanation for what causes this stretching.

A good start to something better would be to define what “Space” is. At present our best is that “Space” is the distance between two points. The inference being that Space is created by necessity.

Khashishi

Space doesn't expand. Everything just moves farther apart. You can play games with coordinates, but all you can measure is the distance between stuff like galaxies.

So, if you somehow glued every galaxy onto a grid, and you expanded the grid, then it would look like every galaxy was moving away from you, no matter which galaxy you stood on. But of course, the grid doesn't exist. Galaxies are just coasting apart via inertia, but also subject to attractive gravity and repulsive dark energy.

We don't really know what happened at the big bang, only some fractions of seconds later.