what is the cause of big bang, beginning of universe?

[mathematicalproiectionofme]

^

[J.C.MacSwell]

No one knows.

[Phi for All]

Also, the BB wasn't necessarily the beginning of the universe. The BB theory describes only the development from an earlier very hot and very dense state. It's possible the universe has always existed.

[mathematicalproiectionofme]

  On 7/12/2019 at 8:18 PM, Phi for All said:

Also, the BB wasn't necessarily the beginning of the universe. The BB theory describes only the development from an earlier very hot and very dense state. It's possible the universe has always existed.

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good point 

[QuantumT]

  On 7/12/2019 at 8:18 PM, Phi for All said:

Also, the BB wasn't necessarily the beginning of the universe. The BB theory describes only the development from an earlier very hot and very dense state. It's possible the universe has always existed.

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But if the "hot dense state" was the universe (spacetime), then the nothingness around it was not, I assume.
Or do we count the "place" that spacetime is expanding into as a part of the universe?

[J.C.MacSwell]

  On 7/12/2019 at 8:38 PM, QuantumT said:

But if the "hot dense state" was the universe (spacetime), then the nothingness around it was not, I assume.
Or do we count the "place" that spacetime is expanding into as a part of the universe?

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No. I think we count it as part of our imaginations.

There is absolutely no evidence that it is there, and there is no requirement for it aside from the way we tend to think.

[QuantumT]

  On 7/12/2019 at 8:40 PM, J.C.MacSwell said:

No. I think we count it as part of our imaginations.

There is absolutely no evidence that it is there, and there is no requirement for it aside from the way we tend to think.

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So, you're saying that "everything" (spacetime) grows bigger, but there is no place to grow in?

Doesn't that defy logic?
 

Edit: Unless we name the place it's growing into: "Does not exist", of course.

Edited July 12, 2019 by QuantumT

[Strange]

  On 7/12/2019 at 8:38 PM, QuantumT said:

But if the "hot dense state" was the universe (spacetime), then the nothingness around it was not, I assume.

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There is no nothingness around it. The universe is all there is (nothing outside)

  On 7/12/2019 at 8:38 PM, QuantumT said:

Or do we count the "place" that spacetime is expanding into as a part of the universe?

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It is not expanding into anything.

If that is hard to get your head around, there are two things that might help:

1. Assume the universe is infinite. If it gets bigger, it is still infinite (see Hilbert's Hotel for a good analogy).

2. Just think of it as the distance between things increasing, rather than the universe getting bigger.

3. Or think of it as the average density decreasing.

Note that even if the universe is finite, there is no "edge" or boundary, so there is no "outside".

[J.C.MacSwell]

  On 7/12/2019 at 8:57 PM, QuantumT said:

So, you're saying that "everything" (spacetime) grows bigger, but there is no place to grow in?

Doesn't that defy logic?

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Yes. Based only on the set of assumptions we are wired with it certainly does. But there is no reason to believe that set of assumptions is correct. There is no evidence or requirement for it.

Edited July 12, 2019 by J.C.MacSwell

[Strange]

  On 7/12/2019 at 8:57 PM, QuantumT said:

Doesn't that defy logic?

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It might defy intuition, but that's not the same thing (and not really relevant!)

[QuantumT]

  On 7/12/2019 at 9:06 PM, Strange said:

There is no nothingness around it. The universe is all there is (nothing outside)

It is not expanding into anything.

If that is hard to get your head around, there are two things that might help:

1. Assume the universe is infinite. If it gets bigger, it is still infinite (see Hilbert's Hotel for a good analogy).

2. Just think of it as the distance between things increasing, rather than the universe getting bigger.

3. Or think of it as the average density decreasing.

Note that even if the universe is finite, there is no "edge" or boundary, so there is no "outside".

It might defy intuition, but that's not the same thing (and not really relevant!)

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  On 7/12/2019 at 9:06 PM, J.C.MacSwell said:

Yes. Based only on the set of assumptions we are wired with it certainly does. But there is no reason to believe that set of assumptions is correct. There is no evidence or requirement for it.

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I get it now. But it wasn't Hilbert's Hotel that helped me. It was a name I gave it. I call it: "Does not exist"

[J.C.MacSwell]

  On 7/12/2019 at 9:14 PM, QuantumT said:

I get it now. But it wasn't Hilbert's Hotel that helped me. It was a name I gave it. I call it: "Does not exist"

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Probably a pretty safe bet...

[StringJunky]

  On 7/12/2019 at 9:14 PM, QuantumT said:

I get it now. But it wasn't Hilbert's Hotel that helped me. It was a name I gave it. I call it: "Does not exist"

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Space is created between things, so, we don't need an 'outside'.

[QuantumT]

  On 7/12/2019 at 9:40 PM, StringJunky said:

Space is created between things, so, we don't need an 'outside'.

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There are two reasons I thought of an "outside". Mainly because I've seen universe simulation videos, where you actually look at it from beyond.
And secondly because of the multiverse model, where our universe is just one of countless neighbor universes in an endless vast vacuum.

Edited July 12, 2019 by QuantumT

[StringJunky]

  On 7/12/2019 at 9:47 PM, QuantumT said:

There are two reasons I thought of an "outside". Mainly because I've seen universe simulation videos, where you actually look at it from beyond.
And secondly because of the multiverse model, where our universe is just one of countless neighbor universes in an endless vast vacuum.

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Yes, it's a necessary visual device to imagine an outside but we should be mindful it is not actually part of that model. The MV  model is another different model but don't mix them up and it's not generally accepted as the mainstream model.

[QuantumT]

  On 7/12/2019 at 8:18 PM, Phi for All said:

Also, the BB wasn't necessarily the beginning of the universe. The BB theory describes only the development from an earlier very hot and very dense state. It's possible the universe has always existed.

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Are you suggesting a big bounce?

By asking that, I also bring this thread back on the intended track 

[Phi for All]

  On 7/12/2019 at 11:44 PM, QuantumT said:

Are you suggesting a big bounce?

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I was pointing out that assuming the BB was the "beginning of the universe" was incorrect. 

I think it's likely the universe is cyclic, but we can't assume it. We observe expansion, but what mechanism does the universe use to contract back?

[beecee]

  On 7/13/2019 at 12:54 AM, Phi for All said:

I think it's likely the universe is cyclic, but we can't assume it. We observe expansion, but what mechanism does the universe use to contract back?

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Particularly since we observe that the expansion is accelerating. I suppose though that that scenario would rest on whether the DE will always be there.

[swansont]

  On 7/12/2019 at 8:57 PM, QuantumT said:

So, you're saying that "everything" (spacetime) grows bigger, but there is no place to grow in?

Doesn't that defy logic?

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It defies your preconceptions

[Strange]

  On 7/12/2019 at 11:44 PM, QuantumT said:

Are you suggesting a big bounce?

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There are multiple speculative ideas for what might have happened: big bounce, continuous inflation, universe from nothing, infinitely old universe, and on and on.

There is no evidence for any of them currently, so pick your favourite. Dark energy would seem to make the big bounce unlikely, but maybe future discoveries will make it plausible again.