freyar
Extradimensional Explorer
Wowsers, this thread has turned into something else.
Ok, full disclosure. I'm a theoretical physicist (newly minted professor this year), and my research primarily deals with cosmology, so I work on or around these topics most days.
That said, Umbran's and Morrus's answers have mostly been spot-on. So it's been really nice reading through those.
There are a couple of points that might be good to clear up, since it seems there are still some questions or confusions for people. So here's my shot at them.
First, if we see light that has been traveling to us for, say, 13 billion years, the object that emitted it was not 13 billion lightyears away at the time it emitted the light. If we could somehow take an omniscient point of view, that object (galaxy, whatever) would have been a shorter distance away. But while the light traveled to us, the space in between got bigger, meaning the light itself traveled 13 billion lightyears. Similarly, if we could take that omniscient point of view now, the emitting galaxy would be further away than 13 billion lightyears.
Ok, next, I saw a comment that the cosmological constant is why the universe is expanding. Actually, the cosmological constant (or something similar, the measurements don't have it quite precisely nailed down yet) is what makes the expansion of the universe accelerate. The universe has always been expanding; even without the cosmological constant it would still expand, but the expansion would slow down. Eventually, the expansion could become a contraction, but only in some cases; that's a matter of initial conditions essentially. Incidentally, the discovery that the expansion actually increases won the Nobel prize this week.
On measuring distances: as noted, our solar system, our galaxy, etc, are held together by local gravity, and the expansion of the universe doesn't really affect that. Similarly, a meter stick is held together primarily by electromagnetic forces and would not expand with the expansion of the universe. At the same time, if you could somehow omnisciently lay down a bunch of meter sticks between two galaxies, you'd find that some time later you'd have to add more meter sticks. Space expanded. Of course, that's not how we really measure distances. We measure the apparent brightness of objects and infer the distance based on how bright those objects would be close up. Or we measure the apparent size on the sky of objects when we know how big they are. So if we can compare how fast those objects are expanding away from us to their distance (measured in one of these ways), we can plot out the expansion history of the universe.
Oh, and related to that, the "space" part of the universe is actually flat within our ability to measure (again, based on those distance measurements). It's spacetime together that's curved.
And, just for jonesy above: I don't know about firing a laser at Pluto. But I can tell you about the moon. There are experiments that bounce lasers off mirrors that the Apollo astronauts left on the moon. Using a pretty strong laser (there might be stronger ones, I'm not sure of the specs off hand), you get just a couple of photons back from the moon per pulse. It's a pretty high loss rate. Another way to put it: the laser light is like a usual laser dot when it leaves the earth. It's several meters in diameter by the time it reaches the moon.
Ok, full disclosure. I'm a theoretical physicist (newly minted professor this year), and my research primarily deals with cosmology, so I work on or around these topics most days.
That said, Umbran's and Morrus's answers have mostly been spot-on. So it's been really nice reading through those.
There are a couple of points that might be good to clear up, since it seems there are still some questions or confusions for people. So here's my shot at them.
First, if we see light that has been traveling to us for, say, 13 billion years, the object that emitted it was not 13 billion lightyears away at the time it emitted the light. If we could somehow take an omniscient point of view, that object (galaxy, whatever) would have been a shorter distance away. But while the light traveled to us, the space in between got bigger, meaning the light itself traveled 13 billion lightyears. Similarly, if we could take that omniscient point of view now, the emitting galaxy would be further away than 13 billion lightyears.
Ok, next, I saw a comment that the cosmological constant is why the universe is expanding. Actually, the cosmological constant (or something similar, the measurements don't have it quite precisely nailed down yet) is what makes the expansion of the universe accelerate. The universe has always been expanding; even without the cosmological constant it would still expand, but the expansion would slow down. Eventually, the expansion could become a contraction, but only in some cases; that's a matter of initial conditions essentially. Incidentally, the discovery that the expansion actually increases won the Nobel prize this week.
On measuring distances: as noted, our solar system, our galaxy, etc, are held together by local gravity, and the expansion of the universe doesn't really affect that. Similarly, a meter stick is held together primarily by electromagnetic forces and would not expand with the expansion of the universe. At the same time, if you could somehow omnisciently lay down a bunch of meter sticks between two galaxies, you'd find that some time later you'd have to add more meter sticks. Space expanded. Of course, that's not how we really measure distances. We measure the apparent brightness of objects and infer the distance based on how bright those objects would be close up. Or we measure the apparent size on the sky of objects when we know how big they are. So if we can compare how fast those objects are expanding away from us to their distance (measured in one of these ways), we can plot out the expansion history of the universe.
Oh, and related to that, the "space" part of the universe is actually flat within our ability to measure (again, based on those distance measurements). It's spacetime together that's curved.
And, just for jonesy above: I don't know about firing a laser at Pluto. But I can tell you about the moon. There are experiments that bounce lasers off mirrors that the Apollo astronauts left on the moon. Using a pretty strong laser (there might be stronger ones, I'm not sure of the specs off hand), you get just a couple of photons back from the moon per pulse. It's a pretty high loss rate. Another way to put it: the laser light is like a usual laser dot when it leaves the earth. It's several meters in diameter by the time it reaches the moon.
