Napier indicated that we were talking about astronomy and not astrophysics. I was replying that I thought Astrophysics was an appropriate term for this discussion as it’s not just about astronomical observation.
AllFree
Thank you, that is exactly the kind of thing I wanted to learn from this thread. I had no idea about the ‘inflationary period’.
AllFree
Then Napier was wrong. Sorry.
Do you understand the distinction between dark matter and dark energy, though?
I hate it when people say this, because it’s not only not right, it’s not even wrong. The rate of expansion of the Universe has units of frequency, not speed, so it’s not meaningful to compare it to the speed of light.
Yeah, but. Every single for-the-layperson article on the subject uses this phraseology. It makes much more sense for everyone but the technical expert. It’s like complaining that parameter is used incorrectly in everyday English.
Saying that space in the early universe expanded faster than the speed of light gives an explanation for what is otherwise senseless. I don’t know of any other formulation that works. Since everybody uses this one, I have to assume that nobody else has either.
If you have, I’d love to hear it because I would eagerly start using it.
Yes as said. In fact, there appears to be a large mass beyond the universe we can see affecting matter in the observable universe.
Yes I do, and I should have been explicitly mentioning it, however in my mind this discussion was somehow about the dispersal of mass in the universe, so I kept thinking in terms of matter, not energy.
AllFree
If you have an explanation for a Intelligent laymen (perhaps I’m flattering myself but I like to think I’m intelligent) of “The rate of expansion of the Universe has units of frequency, not speed” I’d love to hear it. I am a laymen, but my secret shame is that Astrophysicist was one of the jobs I regarded the highest in my personal system of value, but I was certain I was too dense to ever achieve competence at it and never even tried. I am very curious about cosmology and the universe in general though, and want to learn more before I shuffle off this mortal coil.
AllFree
No, it makes it even more senseless. If you want a simple statement for the layman, just say that the very early Universe expanded at a ludicrously great rate. If you want to be more precise than just “ludicrous”, then say that the Universe doubled in size every 10[sup]-36[/sup] seconds.
I think I can handle that. How fast something is moving away from us depends on how far away it is. So the expansion rate of the Universe has to be expressed in units of speed per distance (in practice, typically kilometers per second per megaparsec). But speed is distance per time, so the expansion of the Universe is a distance per time per distance, which cancels out to just a “per time”. And “per time” is a frequency.
The rate of expansion of the universe is the Hubble constant which has been measured to be roughly 75 kilometers per second per megaparsec. (A megaparsec is about 3.2 million lightyears.) What this means is that if you look at an object one megaparsec from earth, it will appear to be receding at about 75 kilometers per second because of cosmic expansion. The farther away an object is, the faster it is receding. So an object that’s 100 megaparsecs away will be receding at 7500 km/sec.
The speed of light is about 300,000 km/sec. So if an object is 4,000 megaparsecs away it will be receding at the speed of light and we’ll never be able to observe it.
So it’s not really correct to say that during the inflationary period the universe was expanding faster than the speed of light and now it isn’t. Very distant parts of the universe are moving away from us at faster than the speed of light right now. It’s more correct to say that during the inflationary period the Hubble constant was much, much higher, so locations that were much, much closer to each other receded at greater than the speed of light.
I think it depends by what you think “rate of expansion of the Universe” means. If you’re talking about the constant that defines relative rates of expansion between any two objects, then you’re absolutely right - we have to define the objects we’re talking about before we can talk about speed. That would be the most precise use of the term by a physicist.
However, I’ve always assumed that the expression is not talking about the constant. Rather, it’s talking about the speed of the edges of the universe relative to the center of the universe (or relative to the opposite edge). Now we’ve implied the distance and the units are those for speed. The actual speed of the expansion may be changing every second, but if we’re simply saying “the edge of the universe is moving away from the center faster than the speed of light” we’ve got a lot of leeway for that to be true.
Chronos, meet NASA.
Per second is a speed.
The galaxies were formed from the super hot mass left over after the big bang at the time that mass was the whole universe. When galaxies formed they were fairly evenly distributed in the space that was so now seeing the farthest galaxy IS in essence seeing the edge of the universe.
There have to be better pictures of it than this but it’s early an I haven’t had my coffee.
Um … “per second” is another way of saying “hertz” which is a unit of frequency.
“Meters per second” would be a speed.
Galaxies have never been fairly evenly distributed in space. Early inhomogeneities in the distribution of matter led to a runaway gravitational process where most of the matter in the universe collapsed into thin sheets and filaments with huge voids in between. (You can think of the structure as similar to a sink full of bubbles.) Galaxy formation occurred only in the matter-dense regions on the walls of the voids.
And it’s not true that farthest visible galaxy is the “edge of the universe”. There are almost certainly many, many more galaxies (perhaps an infinite number) beyond the range of what we can see.
WTF?
Per second, per se, says nothing. Hertz commonly denotes the number of sinusoidal cycles per second although it could be confusingly to measure speed as in the frequency mileposts are encountered per unit time.
adhay and Chronos, thanks for the great links. They help keep things in perspective.
After a week, wouldn’t we be millions of miles away from our original position, or are the distances that large that it doesn’t really matter?
eta - Sorry, I was thinking the Hubble telescope was stuck within our orbit. Is it?
From here, the NASA main Hubble page.
OK, now we’re into territory that’s meaningful, but even if you’re defining your terms that way, it’s still wrong. The only meaningful definition for “the edge of the Universe” is precisely that distance at which the recession speed is the speed of light. So by that standard, the Universe is expanding at exactly the speed of light, and always has and always will, including during inflation and right now.
The nearest star to the Sun is still far enough away that the amount of its shift in apparent position due to our orbit is less than a single arc-second, and other galaxies are incomprehensibly further than the nearest star. For very nearby stars, it is (just barely) possible to measure the change in apparent position, and this is how we measure distances to them (this is called the method of parallax). In fact, this defines one of the units commonly used for astronomical distances, the parsec: A parsec is the distance at which an object would have a parallax of one arcsecond (so alpha Centauri, with a parallax of a little less than an arcsecond, is a little more than a parsec away).