First, I don’t see what the local curvature of space has to do with measuring your velocity relative to the frame in which CMB radiation has zero dipole moment. What difference does it make whether your local space is flat or curved?
Second, I don’t see where changes in the laws of physics enter into the question. Absolute frames of reference do not require the laws of physics to be unique in them, and different elsewhere. They only require certain observations to be unique in them, so that all observers, whatever their own reference frame, can agree on whether a certain observer is or is not in the absolute frame. The frame in which the CMB has zero dipole moment certainly qualifies: all observers will agree on whether or not a particular observer is in that frame – it is therefore unique, and you can define “absolute” velocity as velocity measured in this frame, if you like.
I didn’t say it was necessarily useful to do so. I just said it can be done, and that therefore there is a way to define absolute velocity if you want to.
Within that section is a pisser of a multi-image graphic/photo reconstructions, with ever larger scales of the placement of the solar system relative to six ever-larger stellar groupings, ending with the “observable Universe.” (Note that the image can be enlarged with a click.)
Side question: What is the difference between the “observable” universe and the one witout the adjective? Is it playing hide-and-seek somewhere?
I think it has to do with the amount of light getting to us as well as whether or not the light has had enough time to get to us.
The first is easy enough to understand: The fact that something is bright doesn’t mean it’s bright enough for us to see it. Far enough away, things might be too faint for us to see.
The second one is that if the universe is about 15 billion years old, we can only see 15 billion light years away and no more because the light that would allow us to see 16 billion light years away hasn’t gotten here yet.
Theoretically, you can use any reference frame you want. In practice, it’s often convenient to pick some large object, and say that you’re using a reference frame where that large object is at rest. In everyday life, the largest object we interact with directly is the Earth, so we often treat the Earth as being at rest, and measure velocities relative to the Earth. If you’re studying the Solar System as a whole, then this becomes inconvenient, and so we instead treat the largest object in the system, the Sun, as being at rest, and measure velocities of other things relative to it. Likewise, if you’re working on yet larger scales, you might treat the Galaxy as at rest, or a cluster of galaxies, or whatever. Well, the material emitting the cosmic microwave background radiation is the biggest object we’ll ever be able to see, so on extremely large scales, it’s most convenient to consider velocities relative to the CMB frame. But it’s still just another big object, qualitatively no different than the Earth, Sun, or Galaxy.
Well, it probably rotated for a bit before it initially formed, but why does that seem odd? It’s freaking huge. To paraphrase from HHG, the galaxy…it’s big. Really, REALLY big. You just wouldn’t BELIEVE how vastly, hugely, mindblowingly vast and fully of hugery it is! You might think it’s a long way to get to Poughkeepsie, but that’s nuffin compared to the size of the galaxy, let me tell ya!
I suppose it underlines the youth of the galaxy. That such structures only exist because of relatively rapid rotation that balances the force of gravity, could have been created so recently as to only rotate 56 times since creation.
Well, keep in mind that if you were closer to the core things would be substantially different and you would have made many more rotations on the merry go round. We’re pretty far out on the edge here…a backwater more than halfway out of our spiral arm.
Also, and I’m not sure of this point, but I think our galaxy is actually the concatenation (if that’s the right word) of several proto-galaxies, which probably had a few more spins in them prior to crashing into ours early on. IIRC, that’s our eventual fate in the future as well…another huge galaxy (Andromeda?) is currently en route to smacking us and forming an even bigger galaxy a few billion years in the future. Which might suck for us, depending on how it plays out, and whether there are any of ‘us’ (meaning any life from Earth) at that time, which seems doubtful.
Our Galaxy and the Andromeda galaxy are currently getting closer together, but that’s about all we can say. Yeah, maybe they’ll collide, but it’s far more likely that they’ll just swing past each other and continue to orbit. That depends on the other components of the motion, which are much more difficult to measure. And even if they do collide, it probably won’t have much effect, if any, on anything on our scale. Space is mostly empty, and colliding galaxies are mostly just a matter of stars passing through the vast gaps between other stars.
Well, maybe I’m misremembering then…I thought it was a pretty sure thing. As to space being vast, that’s true enough, but there will be some serious gravitational effects with suns being flung in all sorts of crazy ways…not to mention that there is a huge supermassive blackhole presumably at the center of Andromeda as well, which would be slamming through our galaxy, even as ours slams through theirs. Could be…interesting. Though it’s only likely to be academic, assuming it happens, since it’s billions of years from my IIRC.
My understanding is that a collision with Andromeda is pretty sure. If they don’t collide directly, then they’ll make a short loop and collide soon after. Of course “soon” here is relative, say half a billion years later. A direct collision will be in a bit under 3 billion years.
In order for them not to collide at all, they would have to be gravitationally unbound, so they’d go sailing past each other at quite a distance. If they get even sort of close, they’ll raise tides in each other which will reduce their orbital energy so they collide eventually.
As for what happens when they collide, several things. One is that they could cause long streams of gas, dust and stars to extend from each other. See the Antenna Galaxy for an example of this (you’ll have to find your own link – I’m too tired tonight to provide one). Also, while the stars won’t collide, the gas and dust clouds will. Which means they’ll collapse into stars. See star-burst galaxies for examples. Since all the gas and dust gets used up in this, the result after everything settles down is an elliptical galaxy. The black holes will eventually merge with each other.
FWIW, according to Wikipedia the size of the observable universe is what can be observed in principle because its’ light has had time to reach us, regardless of whether there are technological limitations on how well we can see it.
I just learned from that article that there is a distinction between the visible universe (dating from a time when the universe was no longer filled with a plasma opaque to photons, approximately 300,000 years after the Big Bang) and the observable universe (dating from the end of the inflationary epoch, approximately 10[sup]-33[/sup] seconds after the Big Bang). Also, due to the expansion of space, we can see things that are now further away than the age of the universe.
They won’t? :dubious:
Although I am in no way qualified to dispute your statement, (my) simple logic says that they would.
I mean, in the event of a galactic collision it stands to reason that there are going to be at least a few stars that will meet head on, if for no other reason than their immense gravitational pull.
Or am I missing something?
And on a different note… Whose insurance is going to have to pay for damages?
