universe was the size of our solar system after 1 sec

kjamha

I've heard many times that the universe was the size of our solar system after 1 sec. But how is this possible if nothing can travel faster than the speed of light? Are there any theories on this?

Chronos

There is a widespread misperception the universe was this or that size shortly after the big bang. That is true only for the observable universe. The expansion of space is exempt from the light speed limit. Nothing is actually moving 'through' space faster than light.

cepheid

We have a FAQ thread for this:

http://www.physicsforums.com/showthread.php?t=508610

kjamha

Thank you - I found the article very interesting. The last paragraph says there are galaxies receding from us faster than c. Is that true? If it is, how could we ever detect the EM waves?

marty1

The universe wasn't complicated back then by all the machinery of atoms and molecules that take up a lot of empty space now.

cepheid

They are presently receding away from us faster than c. However, that wasn't always the case. They were receding more slowly in the past. Not only that, but keep in mind that they were much closer to us when the light from them was emitted than they are now. It's tricky thinking about these things in an expansing universe.

What are you talking about?

Drakkith

For some galaxies, the rate of expansion is too great for the light to ever catch up to us. (I say "catch up to us" because, from the emitting galaxies point of view, we are receding from it, not the other way around) For closer ones we are still close enough for light to eventually reach us. Even though a galaxy is receding faster than c, the light itself is moving against the expansion. Initially the light would recede from us, but as it moves the rate of recession gradually decreases until finally it reverses and the photon starts to actually move toward us, eventually making it's way here. I'm not sure on the details of why this happens this way. Something to do with light always traveling at c for all observers I think.

bahamagreen

There is something peculiar about the idea that the light is "moving against" the expansion... and will subsequently reverse its direction.
I guess it's a little better than saying the light is moving through a "head wind"...

- because the expansion is relative to non-expansion similar to comparing "moving" and "rest" position in space?

- because it seems to give space an "aetheral" aspect? Could a modern day M&M experiment use light to detect local motion thought the expansion?

RUTA

Special relativity (SR) holds locally in general relativity (GR), much like we can treat the surface of Earth as flat over small distances when we do projectile motion problems. According to SR, light always travels at speed c and so it does locally in GR. However, these locally flat spatial frames are carried apart by the expansion of the universe so the speed at which the light approaches us is given by c - Hd, i.e., the local frame is being dragged away from us at speed v = Hd (Hubble's law) while light is moving towards us through that frame at c, just as in simple Newtonian physics. In fact, for large red shifts, Hd for the emitting galaxy was greater than c at time of emission so the light was actually being dragged away from us. The emitting galaxy has sinced slowed, but may still be moving away faster than c. There is a nice article explaining this and other misconceptions about the big bang in Scientific American http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf. Here is a paper I wrote years earlier explaining it to physics undergrads: “Can Galaxies Exist within Our Particle Horizon with Hubble Recessional Velocities Greater Than c?” W.M. Stuckey, American Journal of Physics 60, No. 2, 142 – 146 (1992). In my paper there is a graph showing the recession velocity of the photon from emission to reception for redshift 4.73 and you can see that it starts with a positive recession velocity and ends with a recession velocity of -c in accord with SR.

kjamha

Thank you everyone! You have given me A LOT to think about. I did not know about the realm of expansion (and I've found it is a vast topic) - It is going to take me a while to digest just the simple basics.

cepheid, when I read your post, it really made sense, but after thinking about it, I wonder if contradicts a universe that is expanding at an increasing rate, but Ruta's article was really good and answered many of my new questions.

Based on everything I've learned, I do have another question - If space and time are interwoven, I'm wondering if time is affected in these areas of expansion. It seems it would have to play a part in analyzing red shifts. No?

RUTA

You are correct. See the explanation given in Gravitation by Misner, Thorne and Wheeler, pp 775-778 (also Exercise 29.2), Freeman, 1973.

marcus

RUTA, this is a nice explanation and I would like to see your plot of the photon speed of approach that starts out negative (dragged away) and ends up arriving at us at the ordinary speed of light c. Or the negative plot thereof, whichever.
I will look for a link to an online version of the pedagogical article you wrote for AmJPhys. If it is not at arxiv.org perhaps you coud supply a link?
No luck. Only version I could find was behind the pay wall ($30 to read). Some folks (e.g. Lineweaver) put a few of their pedagogical articles online at their faculty websites so that they are readily accessible. I'm not sure what the etiquette is about this, does an author have to ask the Journal's permission to post his own article for students at his institution or other educational purposes?

RUTA

Hi, marcus. I didn't know anything about arXiv in 1991 when I submitted this paper and I don't have any Word or WP file after 20 years so I copied it to my online repository http://users.etown.edu/s/stuckeym/AJP1992a.pdf. Fig 1 shows how far away the photon is as a function of time and Fig 2 shows its recession velocity as a function of time.

marcus

Thanks, figures 1 and 2 are good ways to get the idea across. the photon starts out towards us when U is 1 billion years old and initially gets swept back.
But finally reaches us today with the U being 13 billion years old.
I like figure 1, it tells a simple story that is good for the beginner to have clearly in mind.

Diagrams and simple stories certainly help make equations concrete. He starts out 4 billion ly from us and actually gets dragged back till almost 6 billion ly away before turning it around and beginning to make progress. A moral lesson for us all, the little photon that could .

RUTA

Hahaha (I wanted to post a rolling laughing face, but I can't figure the code for that).

marcus

it is : r o f l : without the spaces