Fair enough. I’ll watch my terminology.
Sorry, but I think you’re wrong. The units of frequency are [1/s], which is exactly 1 Hertz. By definition.
Yes, but aren’t we also on the end of an “arm” of a galaxy, which I presume is revolving and is not parked up in space somewhere, but is actually moving as fast as all the other galaxies are moving?
So, in a week, we have moved as far as a galaxy can travel in that time, yes? So why is it all these stars remain in the same position, unless they are also moving along relative to us, which sounds weird, to say the least?
missed the edit window
Does anyone ever get the feeling that we are giving these astronomers too much credit?
The def of 1 Hertz is 1 cycle/sec.
Ivan, not sure if I understand your points, but the stars within a galaxy are practically moving with each other, held together by orbiting the same effective mass in the center of the galaxy, and usually also orbiting other closer objects. For example, most stars are part of binary or more complex star systems. Also, some stars are in clusters that swarm around their common local center of mass. That great orbiting cloud, as a galaxy, is floating in space, and galaxies themselves are typically floating in some gravitationally bound way with other local galaxies. On a scale large compared to the typical distance between neighboring galaxies, they are all mutually fleeing one another due to the expansion of space.
The arms of the galaxy aren’t made of stuff, exactly, they are regions in which star formation is happening rapidly and there are many new massive stars that shine brightly. It’s like that thing where all the people in a stadium jump up and sit back down in a loosely organized way that forms a wave traveling around the stadium. The wave is made of people, but it travels much faster than the people in it do. At least, this is as I remember it from quite a few years ago…
The stars in our galaxy ARE moving along relative to us. It’s like riding a carousel with your friends. You’re all going round and round but your positions relative to each other stay the same.
Objects outside the carousel WILL appear to shift position. But objects that are very far away won’t shift very much. If I’m riding a carousel and look off toward a distant mountain range, it won’t appear to change very much at all even though I’m going round and round.
Looking at distant galaxies from inside our rotating galaxy is like looking at a distant mountain range from a carousel. The change in our angle of view from the rotation is so tiny compared to the distance of the thing we’re looking at, it appears completely stationary.
A couple of interesting points:
The size of the universe: There are two pieces of data which suggest the universe is MUCH larger than our observable ‘light bubble’. The first is that the measured curvature of space is very flat. There are two ways that can happen - one is that the energy density of the universe is exactly enough to keep space flat through sheer chance, which seems unlikely. The other is that the universe is so huge that measurements of curvature in our ‘light sphere’ approximate flatness - just as measuring a distance of a few feet on earth will make you think the Earth is flat even though it’s spherical. Some theories suggest that the expansionary phase after the big bang lasted until the universe grew so large that it became essentially flat - it was driven into ‘flatness’. That would mean it is much, much bigger than what we can see. Maybe even infinite.
The fate of our ‘light bubble’: Over time, as the unvierse expands more and more objects that we can see today will wind up outside our ‘light bubble’. Eventually, all we’ll be able to see from earth is our local group of galaxies, which are gravitationally bound together. Everything else will be outside our light bubble, and astronomers in the very distant future will have no physical evidence of the big bang at all. They’ll just see a collection of galaxies floating through what appears to be an empty void. Of course, the sun will be long gone by then, so maybe this isn’t a problem for us. But other civilizations that may come along hundreds of billions of years from now will have no way of seeing the big bang.
Given the evidence we have today, the best guess is that the universe is either infinite or incredibly large - a googol of our light bubbles in size, maybe. That leads to all sorts of interesting philosophical questions.
adhay, you and hobscrk777 are both right. The two definitions are interchangeable as far as SI units are concerned. Look no further than the fact that “cycle” is not an officially defined unit in the SI or any other system. A cycle is whatever we define it to be relevant to the system in question.
This has been your friendly neighborhood physics grad student, at your service.
“SI”?
The International System of units, known as SI from the French nomenclature. It’s what scientists use 90% of the time. More info here: International System of Units - Wikipedia
Système International, which is more often referred to as the metric system in the US. There are technically some differences between SI and metric.
All the representational models of the Big Bang I’ve seen, seem to show the universe to be expanding like the ripples on a pond surface… I take it this is just a simplification and that the universe didn’t expand along a flat plane?
Cycle could of course be anything but Hertz is defined (WIKI again)
No. It expanded in all three dimensions.
Again, it says it right there: “Hertz are inverse, s[sup]-1[/sup].” If you want to attach a specific cycle to that, you can, but you don’t need to. You can use Hertz with or without referring to an established oscillatory cycle.
At the risk of getting into a semantics argument…
And the other 10 percent of the time they just make shit up 
I am not sure exactly where the OP is going, but let me say this.
I get the impression the OP thinks that we may be able measure the position and velocity of most/many/all galaxies, then use this to run time forward or backwards and learn sometime specific.
Chaos theory comes in play here.
We know the movements of the planets WAY WAY WAY WAY more accurately than we do for even the nearest galaxy, much less the ones that arent close. Even then, astronomers have no idea what our solar system will look like billions of years from now. Predictions range from planetary collisions to planets being ejected to nothing at all happening.
Its way worse for knowing how all these galaxies are going to interact in the future.
If that point is irrelevant to the OP’s interest…carry on.
Billfish’s interest in considering what the OP’s interest was seems a breath of fresh air.
Is the point of the OP to get around the problem that we see distant galaxies as, and where, they were billions of years ago? That is, does all this discussion really facilitate getting at a current map, as opposed to one that is current at its center and very obsolete at its periphery?
If so, the more interesting tidbit might be that there are actually several different versions of what “current” should mean. That is, it seems easy and obvious to think about “right now”, but the concept of “now” breaks down for locations that are not “here”. Things being simultaneous really only has physical meaning if they are at the same location, surprising though this may be. In human experience, the distinction is trivial, but in cosmology it is very big.
A distant galaxy is in several different locations currently, depending on how we define “currently”.
And, it’s not just the issue of how long it took the light to reach us. Of course we presume the light travel time issue will be managed. The problem I am discussing is beyond the light travel time issue.