Question about Expanding Universe

[Chanakya]

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/forums/showpost.php?p=13113305&postcount=14
http://www.internationalskeptics.com/forums/showpost.php?p=13113318&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]

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]

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/forums/showpost.php?p=13113305&postcount=14
http://www.internationalskeptics.com/forums/showpost.php?p=13113318&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]

("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]

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.