Question about Expanding Universe

[HansMustermann]

Originally Posted by Chanakya

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!

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.)

[sphenisc]

Originally Posted by Chanakya

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.

No, dissent from a model involving an initial inflationary period, not from an expanding universe as a whole.

[acbytesla]

Originally Posted by Chanakya

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!

Originally Posted by Chanakya

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!

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.

[Chanakya]

Originally Posted by HansMustermann

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.)

Makes your head spin just to think about something like that!

[Chanakya]

Originally Posted by sphenisc

No, dissent from a model involving an initial inflationary period, not from an expanding universe as a whole.

Ah, ok.

[Mike Helland]

Originally Posted by Ziggurat

No, that's not how it works. We never have to watch a star change position to figure this out.

That's exactly what I meant.

Radial velocity can be determined by spectral analysis.

Tangential velocity cannot.

[RecoveringYuppy]

Originally Posted by Mike Helland

That's exactly what I meant.

Radial velocity can be determined by spectral analysis.

Tangential velocity cannot.

That's only true if you're talking about a radius centered on the observer and we're not here.

[RecoveringYuppy]

..

[Ziggurat]

Originally Posted by Mike Helland

That's exactly what I meant.

Radial velocity can be determined by spectral analysis.

Tangential velocity cannot.

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.

[Mike Helland]

Originally Posted by Ziggurat

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.

We might find out that galaxies are actually dynamic things rather than static.

[theprestige]

Originally Posted by Mike Helland

We might find out that galaxies are actually dynamic things rather than static.

We already know galaxies are dynamic things. The same observations that tell us they're dynamic tell us their dynamics aren't as you hypothesize

[smartcooky]

Originally Posted by HansMustermann

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.)

Its black holes all the way down!

[Ziggurat]

Originally Posted by Mike Helland

We might find out that galaxies are actually dynamic things rather than static.

I didn't say static, I said stable.

[Mike Helland]

Originally Posted by Ziggurat

I didn't say static, I said stable.

That is true.

[arthwollipot]

Originally Posted by acbytesla

I don't believe we've proven The Big Bang.

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.

[Chanakya]

Originally Posted by acbytesla

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.

Originally Posted by arthwollipot

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.

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.

[W.D.Clinger]

Originally Posted by Chanakya

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?

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.

[Chanakya]

Originally Posted by W.D.Clinger

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.

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!

[smartcooky]

Originally Posted by arthwollipot

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.

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

[Pixel42]

Originally Posted by Chanakya

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?

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.

[Chanakya]

Originally Posted by Pixel42

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.

Oh, so is that what dark energy is about, then? I do know, vaguely enough, that dark matter and dark energy are postulated to account for actual observations that are contrary to accepted theory. Afraid I amn't really aware of what specific observations these refer to. Not quite sure how exactly that works, but apparently it is "dark energy", then, that is apparently the cause of this accelerated expansion. Good to know! (Although, as you say, and others have said here as well, we don't actually have an exlanation, not really.)

[HansMustermann]

Just to clarify some more, we don't really know WTH is either.

The only kind of dark matter that we know of is neutrinos, but there's BY FAR not enough of them to account for the gravity. As Neil deGrasse Tyson said at one point, even calling it dark matter might be misleading; it might be more apt to call it "dark gravity". SOMETHING is causing a LOT of gravity, and we don't really have a clue what it is.

I've already explained what the problem is, here:
http://www.internationalskeptics.com...5&postcount=14
http://www.internationalskeptics.com...8&postcount=15

The only thing we know of that causes gravity (unless GR is awfully wrong) in those circumstances is some kind of mass that we can't see. Hence our assuming it's matter. Our best assumption at the moment is basically just that the galaxy is filled with such matter we can't see.

It might be something else, but all other solutions have bigger problems of their own, so for the moment "dark matter" it is.

I would also add that it not interacting with photons has other implications too, rather than just being invisible. When you put your hand on the table, what keeps it from falling right through is that the electron shells of the atoms in your skin repel the electron shells of the atoms on the surface of the table. That's an electric field, i.e., an electromagnetic field, i.e., that happens via photons. There are photons from that field that bounce between the two. Matter that doesn't interact with photons -- and here again the neutrinos are a perfect example -- will just pass right through the table without even noticing it's there. This also means it won't accrete in the same way as normal matter, and will just go around in a blob.

"Dark energy", as was said, is just whatever pushes the universe apart. We don't have the foggiest idea what that is.

It's also WEIRD, whatever it is. Normal interactions are generally subject to the inverse square law. E.g., gravity or electric fields. If you double the distance between two things, the interaction drops to a quarter. Well, some also have a hard distance limit (e.g., the weak force needs the neutrino to be practically on top of a nucleus to be able to happen, because the particle carrying the interaction only lives an extremely short time) but even those are still subject to the inverse square law over that distance.

Dark energy is the opposite. The farther away two things are, the harder it pushes them apart. If you double the distance, you double how hard they're pushed apart.

This kinda rules out any particle or mechanism we know of. Or really, that we can even imagine at the moment.

[theprestige]

Originally Posted by Chanakya

Oh, so is that what dark energy is about, then? I do know, vaguely enough, that dark matter and dark energy are postulated to account for actual observations that are contrary to accepted theory. Afraid I amn't really aware of what specific observations these refer to. Not quite sure how exactly that works, but apparently it is "dark energy", then, that is apparently the cause of this accelerated expansion. Good to know! (Although, as you say, and others have said here as well, we don't actually have an exlanation, not really.)

("Dark matter" is the name for a gravitational effect we have observed, that is not accompanied by a visible concentration of mass that would cause such a gravitational effect. There are a number of such observations, One is the lensing of light with no visible intervening object to explain it. Another is the orbital speed of stars around a galactic center, where the speed does not correspond to the estimated mass of the visible matter in the galaxy. A third is the behavior of stars in colliding galaxies, where their trajectories are influenced by gravitational effects without a corresponding visible mass to exert that gravitational effect. Hence, "matter" because it has mass and exerts a gravitational effect, and "dark" because it doesn't absorb or radiate light.)

[Chanakya]

Originally Posted by HansMustermann

Just to clarify some more, we don't really know WTH is either.

The only kind of dark matter that we know of is neutrinos, but there's BY FAR not enough of them to account for the gravity. As Neil deGrasse Tyson said at one point, even calling it dark matter might be misleading; it might be more apt to call it "dark gravity". SOMETHING is causing a LOT of gravity, and we don't really have a clue what it is.

I've already explained what the problem is, here:
http://www.internationalskeptics.com...5&postcount=14
http://www.internationalskeptics.com...8&postcount=15

The only thing we know of that causes gravity (unless GR is awfully wrong) in those circumstances is some kind of mass that we can't see. Hence our assuming it's matter. Our best assumption at the moment is basically just that the galaxy is filled with such matter we can't see.

It might be something else, but all other solutions have bigger problems of their own, so for the moment "dark matter" it is.

I would also add that it not interacting with photons has other implications too, rather than just being invisible. When you put your hand on the table, what keeps it from falling right through is that the electron shells of the atoms in your skin repel the electron shells of the atoms on the surface of the table. That's an electric field, i.e., an electromagnetic field, i.e., that happens via photons. There are photons from that field that bounce between the two. Matter that doesn't interact with photons -- and here again the neutrinos are a perfect example -- will just pass right through the table without even noticing it's there. This also means it won't accrete in the same way as normal matter, and will just go around in a blob.

"Dark energy", as was said, is just whatever pushes the universe apart. We don't have the foggiest idea what that is.

It's also WEIRD, whatever it is. Normal interactions are generally subject to the inverse square law. E.g., gravity or electric fields. If you double the distance between two things, the interaction drops to a quarter. Well, some also have a hard distance limit (e.g., the weak force needs the neutrino to be practically on top of a nucleus to be able to happen, because the particle carrying the interaction only lives an extremely short time) but even those are still subject to the inverse square law over that distance.

Dark energy is the opposite. The farther away two things are, the harder it pushes them apart. If you double the distance, you double how hard they're pushed apart.

This kinda rules out any particle or mechanism we know of. Or really, that we can even imagine at the moment.

ok, so then dark matter is kind of a thing. While dark energy, like Pixel42'd said earlier on, and as you expand on it now, is just a name, apparently, for everything we don't know about how everything's expanding!

Farther things being pushed apart further, that's more than merely things on the outer periphery expanding more, is it? More than merely the angular-geomtric thing?

[Chanakya]

Originally Posted by theprestige

("Dark matter" is the name for a gravitational effect we have observed, that is not accompanied by a visible concentration of mass that would cause such a gravitational effect. There are a number of such observations, One is the lensing of light with no visible intervening object to explain it. Another is the orbital speed of stars around a galactic center, where the speed does not correspond to the estimated mass of the visible matter in the galaxy. A third is the behavior of stars in colliding galaxies, where their trajectories are influenced by gravitational effects without a corresponding visible mass to exert that gravitational effect. Hence, "matter" because it has mass and exerts a gravitational effect, and "dark" because it doesn't absorb or radiate light.)

Thanks, that clearly and concisely explains the 'what' I was wondering about, as far as dark matter, the specific observations it relates to.

Clearly dark matter and dark energy are two whole different categories of unknowns! The former apparently is fairly evidenced, then, even if we don't know the details of it; while the latter we seem to know nothing at all about.

[HansMustermann]

Originally Posted by Chanakya

ok, so then dark matter is kind of a thing. While dark energy, like Pixel42'd said earlier on, and as you expand on it now, is just a name, apparently, for everything we don't know about how everything's expanding!

Farther things being pushed apart further, that's more than merely things on the outer periphery expanding more, is it? More than merely the angular-geomtric thing?

Well, no, "dark energy" is literally what we call (the explanation for) the geometry expanding, or rather the fact that the expansion is actually accelerating. The farther something is from us, the faster it accelerates away from us. It's as if some force pushed us harder apart, the farther away we are from each other. That's really the behaviour I was describing.

But, as I was saying, we don't know of any interaction that actually acts like that, nor does it really fit any sane field behaviour we can currently imagine. I mean, if some particles were involved -- as they are for any other interaction we know of -- the number that hit the other thing should decline with the square of the distance. And that's not even getting into the fact that any wave should get redshifted on top of that.

So, yeah, as was said before, we have a name for that explanation, but we don't have the foggiest idea what that explanation might actually be.

Personally I'm still rooting for the black hole universe explanation, because then it's nothing mysterious and unusual, but rather just plain ol' gravity after a coordinate transform. But as I was saying (I think in the other thread) it has its own problems, and the biggest one is that according to the data we have at the moment about the density of the universe, the maths doesn't seem to check out. Just barely, mind you, but still, it puts the kibosh on that idea at the moment.

[Gord_in_Toronto]

Is it possible that the Universe is not expanding but that we are shrinking?

[Delvo]

Geometrically, there's no real distinction between those two things.

But one of the two descriptions not only yields easier, more straightforward calculations of that geometry, but also only requires one input assumption: expanding space.

The alternative requires a set of new input assumptions for a variety of physical constants, from particle radii (and possibly masses) to the parameters of the forces between them to the speed of light, to all be shifting, and not just shifting but shifting in a very particularly coordinated way which just happens to yield results identical to expanding space without other side effects.