"Speed of Light"

[freyar]

Is there no frame of reference into which to put co-expanding particles? Say, of one or of the other particle?

I'm not saying that it isn't different, but how is that frame different than other frames of reference? Why aren't the relative particle velocities meaningful in that frame?

Umbran already covered this, but I'll add to it. You need to be careful about the difference between coordinates and frames of reference in general relativity. A coordinate system can stretch throughout the universe (subtleties do exist, of course), but a reference frame is something that is attached to a single observer and only exists locally. An observer can only observe things at the same location, essentially. So you can't say, "I see that galaxy 50 megaparsecs away moving so fast." All you can say is that "That galaxy 50 megaparsecs away has a redshift of X," meaning we see the light from the galaxy at a longer wavelength than what it was when it was emitted (which we can tell by measuring "signposts" in the light from atomic transitions). Because we're used to the Doppler effect here on earth, people sometimes translate that redshift into a velocity, but it's not the same thing.

Right right. But, what we think of as forces is a little trickier than that: Note that the force of gravity can be viewed as a fictitious force. From:

http://en.wikipedia.org/wiki/General_relativity
<snip>
I'm thinking we need to be a big more careful to define what we mean by velocity to understand recessional velocity.

What we usually think of as the "force of gravity" that, for example, makes the moon's path circle the earth, is not a force in the theory of general relativity. It is the path that the moon takes in the absence of forces but in the curved spacetime caused by the presence of the earth (and sun).

Velocities already don't add in a simple linear fashion -- v_ab + v_bc != v_ac (v_xy == velocity of y measured by x) -- because of special relativity. That there are other non-linear phenomena should not be too much of a surprise.

That was one the points of my previous post --- even if you make the mistake of thinking of redshift as a velocity, the conversion isn't a linear one in relativity.

Ok, to get more funky:

http://en.wikipedia.org/wiki/Comoving_distance

See the section "Uses of the proper distance":

All I can say to this interpretation of "expansion velocity" is ugh. This isn't a useful way of thinking about things in any practical sense. Here is one:

Imagine there is a grid extending through space. Galaxies are at fixed points on the grid and, to a good first approximation, don't move with respect to the grid. What happens is that the grid itself gets bigger. We get a redshift because the light waves also expand as the grid expands --- basically the peaks and troughs of the wave are at fixed grid points.

Back to velocities: the galaxies can of course move around on the grid even as the grid expands. This is called "proper motion" and is usually only noticeable for closer galaxies.

Last point: that arXiv paper is really not a good source, I think. It makes some ok points, but absolutely no one talks about recessional velocities like that. Basically, they have made up a new way to define "recessional velocity" just to try to make a mathematical point. I don't doubt their math, but they aren't making a lot of sense in terms of physics. The key point is that "the motion is not in any observer's inertial frame," which says that they're definition has no observational meaning.

 

[Scott DeWar]

*head spins - world tilts* Ok, looks like I kicked an ants nest of information, there.

 

[Scott DeWar]

Not sure what you mean here with regards to FTL?

someone mentioned what I was thinking - about the phrase " - - - - - -" aw crumbs, I can't find it now. Basically it had something to do with an unintentional misunderstanding on my part about the actual velocity exceeding "M" instead it appeared to exceed "M" as the gravitational waves caused matter to expand in x^-nth seconds (can't remember the actual time)

 

[Scott DeWar]

FTL motion is not possible in our best understanding of physics, and it would take a truly revolutionary and unexpected discovery to change that.

The phrase "a burst of inflation that was seemingly faster than the speed of light" is possibly one of the most misleading and damaging that people can use when talking about the expansion of the universe. If a scientist said it, they either didn't know the subject, were being incredibly lazy in talking to the press, or weren't thinking. And, in this case, the "FTL mistake" doesn't even require inflation. Here's my explanation:

No matter how fast the universe is expanding, you can always see "FTL motion" if you just look far enough away. That's a feature of the expansion of space itself. You don't need inflation.

BTW, there's another thread on this subject.

yup. That is the phrase that screwed me up. And I can see by your commentary that this phrase should never have been "Put to print"

I think I am getting the feeling tha fro a fixed point to any one direction expansion is moving at "M", but to two points in the universe that are moving from the starting point at the same velocity, they see the opposite point looking like the other is moving at "M + x" velocity or even (2M) velocity. Is that the general idea?

 

[freyar]

yup. That is the phrase that screwed me up. And I can see by your commentary that this phrase should never have been "Put to print"

I think I am getting the feeling tha fro a fixed point to any one direction expansion is moving at "M", but to two points in the universe that are moving from the starting point at the same velocity, they see the opposite point looking like the other is moving at "M + x" velocity or even (2M) velocity. Is that the general idea?

That's basically right. Suppose we have two galaxies that are relatively "nearby" us (but still millions of light-years away) that happen to be the same distance away but in opposite directions. We would translate the redshift of each galaxy into a speed v moving away from us (v stands for the size of velocity, ie speed). Either of those galaxies would see us moving at v away from them and the other galaxy moving at 2v away. (Remember, I don't really mean a velocity in the normal sense here.) If we look at a 3rd galaxy which is twice as far away at the first two, it would have an apparent speed of 2v away from us.

That's the behavior expected if the universe is expanding at a fixed rate, which roughly holds for the universe that's nearby us (on cosmological scales). However, if you look really far a way, you find deviations from that linear distance-speed relation (which is called the Hubble law). That deviation is due to the fact that the expansion of the universe is changing rates. For example, gravity from normal matter acts to slow down the expansion. We also discovered in 1998 that there is some other source of energy (often called "dark energy") that acts to increase the expansion rate of the universe, and that effect is winning. The expansion of the universe is actually getting faster.

 

[tomBitonti]

yup. That is the phrase that screwed me up. And I can see by your commentary that this phrase should never have been "Put to print"

I think I am getting the feeling tha fro a fixed point to any one direction expansion is moving at "M", but to two points in the universe that are moving from the starting point at the same velocity, they see the opposite point looking like the other is moving at "M + x" velocity or even (2M) velocity. Is that the general idea?

That phrase shows a misconception: Any expansion will lead to apparently faster-than-light motion. The current expansion does so with a horizon measured in billions of light years. The very fast expansion during inflation did so with a much much smaller horizon.

Hmm, this puts a figure of about 10^-35 cm for the particle horizon during inflation. (I'm not sure if I'm reading that correctly.)

http://adsabs.harvard.edu/full/1991ApJ...383...60H

Anyways, the amount of expansion (between two points) is proportional to the distance between those points. The rate of expansion is a proportional amount over time, not a fixed velocity.

Thx!

TomB

 

[Scott DeWar]

dang, wonky internet just ate my previous post.

The expansion of the universe is actually getting faster.

is it getting faster at a constant increase? has it been doing this since the beginning? what kind of implications does this have for the age of the universe?

That phrase shows a misconception: Any expansion will lead to apparently faster-than-light motion. The current expansion does so with a horizon measured in billions of light years. The very fast expansion during inflation did so with a much much smaller horizon.

Hmm, this puts a figure of about 10^-35 cm for the particle horizon during inflation. (I'm not sure if I'm reading that correctly.)

http://adsabs.harvard.edu/full/1991ApJ...383...60H

Anyways, the amount of expansion (between two points) is proportional to the distance between those points. The rate of expansion is a proportional amount over time, not a fixed velocity.

Thx!

TomB

you know, it kind of is scary to say, but I think it is starting to make sense to me.

 

[tomBitonti]

is it getting faster at a constant increase? has it been doing this since the beginning? what kind of implications does this have for the age of the universe

The rate of expansion (often, the Hubble Parameter) is only recently known to any decent accuracy. (To about 2% is what I'm seeing, say, around 71-73. Previously, values between 50-80 seemed usual.)

This seems to make the basic case for acceleration:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC314128/figure/fig6/

That seems to show acceleration starting some time in the past, and increasing since then.

Given the uncertainties in the measurements, telling the rate of acceleration seems unlikely.

The basic implication is that a computation of the age of the universe cannot rely on a constant Hubble parameter values.

One measure of the age of the universe is telling the amount the background radiation is red-shifted: That shift basically integrates the effect of expansion over time. Then, knowing the rate of expansion, and inverting the integration, the age can be obtained.

An increasing Hubble parameter will make for the same shift over a smaller amount of time. Not sure quantitatively how much that would be, but I'm guessing on the order of 10%, or a small multiple thereof, for the measured acceleration, and not of the order of 100% or more. But, I could be very off on that estimate, and the estimate is very probably thrown off anyways by the basic uncertainty of the current parameter value.

Just to say, I find just a bit disturbing the image of an increasingly sparse universe, with its parts flying apart close to the speed of light, as an end-state of the universe. Hopping 100 trillion years forward would leave you in a very empty universe.

---

So many interesting links found while looking up various questions in this space:

http://www.google.com/url?sa=t&rct=...03RWYtg0GHSq8Zw&bvm=bv.62922401,d.dmQ&cad=rja

This one is amusing:

http://www.answersingenesis.org/articles/tj/v9/n1/hubble

Thx!

TomB

 

[Umbran]

Hopping 100 trillion years forward would leave you in a very empty universe.

It would look empty even if it weren't expanding. In 100 trillion years, the entire Universe will have completely used up all its hydrogen and so on - all stars will have burned out, and their corpses cooled down to the ambient temperature of vacuum. The only light to be seen will be the occasional flash as a stellar corpse encounters a black hole or neutron star.

 

[tomBitonti]

It would look empty even if it weren't expanding. In 100 trillion years, the entire Universe will have completely used up all its hydrogen and so on - all stars will have burned out, and their corpses cooled down to the ambient temperature of vacuum. The only light to be seen will be the occasional flash as a stellar corpse encounters a black hole or neutron star.

Over so much time*, all the baryonic matter will decay to nothing.

There would just be the occasional flash of black holes evaporating.

Such a dark and gloomy place.

Thx!

*100 trillion years might not be long enough. At least for black hole evaporation, I think we are up in the 10^100 years range