Question about Expanding Universe

Chanakya said:
eta: Here's a weird thing. If we're actually in a black hole, our whole universe, then what you have is a number of black holes within a giant black hole!
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Yes. And each of those black holes has a universe inside, which can contain other black holes, which have their own universes inside, and so on. Of course, those universes, as seen from the inside, would be much smaller and much shorter lived (as measured from the inside.)
 
Chanakya said:
Dissent from the position of an expanding universe? Didn't know that! It was my impression that everyone's kind of agreed, at this time, about an expanding universe.
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No, dissent from a model involving an initial inflationary period, not from an expanding universe as a whole.
 
Chanakya said:
Haha, how quaint that sounds, the idea that the Milky Way might be the whole universe. And yet these were apparently, as you say, prominent astronomers debating this, both of them, and not a couple of cranks. And this isn't even all that much of a long time ago!

Shows how much of what we know today we've only discovered so very recently, relatively speaking!
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Chanakya said:
That's ...mind-bending, that thought, that idea.

Of course, any such hypothesis raises many more questions than it answers! But then I guess that's true of most hypotheses, including our usual (as in no-giant-black-hole) Big Bang theory.

eta: Here's a weird thing. If we're actually in a black hole, our whole universe, then what you have is a number of black holes within a giant black hole!
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It's so weird contemplating these things. I don't believe we've proven The Big Bang. I also believe that The Big Bang theory best explains the available data. But I'm afraid we only have a tiny fraction of the data required. We have to stay open to other possibilities.
 
HansMustermann said:
Yes. And each of those black holes has a universe inside, which can contain other black holes, which have their own universes inside, and so on. Of course, those universes, as seen from the inside, would be much smaller and much shorter lived (as measured from the inside.)
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Makes your head spin just to think about something like that!
 
Mike Helland said:
That's exactly what I meant.

Radial velocity can be determined by spectral analysis.

Tangential velocity cannot.
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First, to avoid confusion about radial vs. tangential: you seem to be using these terms in relation to the viewer. But what we really want to know is velocity in relation to the galaxy center. If the galaxy is stable, the radial (relative to the galaxy) velocity will be close to zero, the tangential velocity (relative to the galaxy) will not be.

And in point of fact, we actually CAN measure both the radial and tangential velocity of a galaxy. Take Andromeda, for example. We're seeing it from an angle, so its profile is an oval with a long axis and a short axis, even though its actual shape is close to circular. I'll refer to the long axis as side-to-side, and the short axis as top to bottom.

The tangential (relative to the galaxy) component of velocity can be obtained by comparing the red/blue shift on the left side of the galaxy and the right side of the galaxy, because tangential velocity will cause matter here to move towards us on one side and away from us on the other.

The radial (relative to the galaxy) component of velocity can be obtained by comparing the red/blue shift on the top of the galaxy and the bottom of the galaxy, because radial velocity will cause matter here to towards or away from us on the top/bottom of the galaxy.

So we actually can check whether or not the assumption that galaxies are stable holds true. And while I haven't found references to people looking for that specifically, I suspect it wouldn't escape astronomer's attention if it was. It should jump out of the data for a close galaxy like Andromeda pretty dramatically.
 
Ziggurat said:
First, to avoid confusion about radial vs. tangential: you seem to be using these terms in relation to the viewer. But what we really want to know is velocity in relation to the galaxy center. If the galaxy is stable, the radial (relative to the galaxy) velocity will be close to zero, the tangential velocity (relative to the galaxy) will not be.

And in point of fact, we actually CAN measure both the radial and tangential velocity of a galaxy. Take Andromeda, for example. We're seeing it from an angle, so its profile is an oval with a long axis and a short axis, even though its actual shape is close to circular. I'll refer to the long axis as side-to-side, and the short axis as top to bottom.

The tangential (relative to the galaxy) component of velocity can be obtained by comparing the red/blue shift on the left side of the galaxy and the right side of the galaxy, because tangential velocity will cause matter here to move towards us on one side and away from us on the other.

The radial (relative to the galaxy) component of velocity can be obtained by comparing the red/blue shift on the top of the galaxy and the bottom of the galaxy, because radial velocity will cause matter here to towards or away from us on the top/bottom of the galaxy.

So we actually can check whether or not the assumption that galaxies are stable holds true. And while I haven't found references to people looking for that specifically, I suspect it wouldn't escape astronomer's attention if it was. It should jump out of the data for a close galaxy like Andromeda pretty dramatically.
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We might find out that galaxies are actually dynamic things rather than static.
 
HansMustermann said:
Yes. And each of those black holes has a universe inside, which can contain other black holes, which have their own universes inside, and so on. Of course, those universes, as seen from the inside, would be much smaller and much shorter lived (as measured from the inside.)
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Its black holes all the way down!
 
acbytesla said:
I don't believe we've proven The Big Bang.
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What have we proven? Proof is for mathematics and alcohol. All we can do is see if the evidence supports the hypothesis. So far as our best cosmologists have been able to determine, the evidence does support the Big Bang hypothesis.
 
acbytesla said:
It's so weird contemplating these things. I don't believe we've proven The Big Bang. I also believe that The Big Bang theory best explains the available data. But I'm afraid we only have a tiny fraction of the data required. We have to stay open to other possibilities.
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arthwollipot said:
What have we proven? Proof is for mathematics and alcohol. All we can do is see if the evidence supports the hypothesis. So far as our best cosmologists have been able to determine, the evidence does support the Big Bang hypothesis.
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Yep, I too was kind of surprised to see this POV here, considering that I've always found your views, that your present here, acbytesla, to be wholely completely pro-science.

(That is, it is my understanding that the "experts" are not in disagreement about the essentials of the Big Bang theory. Not going to swear by that "understanding", and am happy to be corrected; but provided that understanding itself isn't wrong, I don't see how we can see the Big Bang as some kind of "unproven" "hypothesis". After all that's what Evolution amounts to, as well.)

But maybe it's only how you've expressed yourself, or at least, how I (and apparently arthwollipot as well) parsed it? In general terms, agreed absolutely, all of these things are weird indeed, and if you really think hard about them they sometimes actually give you a feeling that is, for want of a better word, vertiginous! And agreed absolutely, about staying open to newer developments maybe changing our entire understanding about everything, in broad general terms I mean to say.
 
Chanakya said:
That's interesting. Not that it's likely to mean much/anything to me, but still, out of curiosity: what is this number, then, that's expected to have just one value but ends up having two such widely differing ones?
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Vacuum energy. The enormous difference between the vacuum energy inferred from observation and the theoretical value calculated using quantum electrodynamics is known as the cosmological constant problem.
 
W.D.Clinger said:
Vacuum energy. The enormous difference between the vacuum energy inferred from observation and the theoretical value calculated using quantum electrodynamics is known as the cosmological constant problem.
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Interesting. Those two numbers for vacuum energy, 10^(−9) joules/m^3, and 10^113 joules/m^3, it's difficult to imagine a more massive difference, either in absolute terms or, even more, in proportionate terms.

Not that I understood much of the technical stuff both those links were talking about, TBH, but how the "propsed solutions" read suggests that "cosmological constant problem" is nowhere close to actually being solved so far!
 
arthwollipot said:
What have we proven? Proof is for mathematics and alcohol. All we can do is see if the evidence supports the hypothesis. So far as our best cosmologists have been able to determine, the evidence does support the Big Bang hypothesis.
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The importance of this cannot be overstated.

There is no "proof" in science. The best we can ever do is to show such overwhelming evidence in support of a hypothesis, theory or proposition that the degree of uncertainty is reduced almost to zero. A great example of this is evolution. It hasn't been proven and it never will be. It always will remain a theory, even though evidence that evolution is correct - that single celled life forms evolved over billions of years to become all the other extant species we see today (including humans) utterly overwhelming.

The same cannot be said of its religious competitor, Creationism, for which there is not the slightest evidence at all, other than the inane, contradictory ramblings of a bunch of 1st C goat herderers
 
Chanakya said:
About this increasing rate of expansion, though:

Why is that a thing, do we know? That is, it is my (vague) understanding that right after the (alleged) Big Bang, there was a (very brief) period of super-massive expansion. But apparently that super-massive expansion rate then suddenly slowed down to a very feeble rate of expansion ("feeble", in terms of, for instance, what you quantify in your comment). And apparently this "feeble" rate of expansion is increasing, slowly but surely, so that one day, in the far far distant future, it will (or might) become 'strong' enough to override the other gravitational etc forces.

I was wondering, do we know why/how this rate of expansion, that was so super-massively-rapid to begin with, slowed down to this far more sedate rate; and/or why/how that rate now keeps on increasing?
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Ever since the expansion of the universe was first discovered it had been assumed that it was slowing down due to gravity. The only question was whether there was enough matter in the universe for gravity to eventually slow it down to the point where the expansion stopped and reversed, or if it would keep expanding ever more slowly forever.

Then in 1998 two teams of scientists who had set out to answer that question by observing lots of supernovae in distant galaxies to measure how the rate of expansion was changing over time announced their results - which was that the expansion was not slowing down at all, it was speeding up. It was an absolute bombshell. I still remember how shocked I was when I read about it; I initially thought it must be a mistake.

We still don't have an explanation. We do have a name for the explanation - dark energy.
 
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