The "observable" universe

[campp]

@levdrakon

One simple way to look at is to pretend that the speed of light is 100mph. You’re in a car traveling 50 mph in one direction, and your friend is in a car traveling 50mph in the opposite direction. Neither of you is violating anything, but the space between you is getting greater at 100mph. Still no violation. Now add a mysterious energy to the expansion of space that makes the expansion speed up. Call it Dark Energy. It’s making space expand at 10mph. No violation, but now the space between you and your friend is getting bigger at 120mph? Turn your telescopes toward your friend and you’ll find he is outside your observable universe, because the light coming from him can only go 100mph. It will never get to you. He’s gone from you forever.

Excellent explanation, thanks. This is the kind of stuff my friends and I talk about sitting aound a campfire late at night…

[Chronos]

As far as I know, we haven’t detected gravity waves at all, except maybe indirectly. But even if we could, we still couldn’t “see” them further away than the Big Bang.

That’s correct on both counts (though we really are getting pretty close to direct detection-- Perhaps 2-5 years away). But light is much more limited than “can’t see past the Big Bang”, due to the opacity of the early Universe. With gravitational waves (in the right wavelength band and with enough sensitivity, which will probably take 30 or 40 years), we should be able to see clear back to the end of Inflation.

[Fiendish_Astronaut]

@Whack-a-Mole

In addition to the above realize that no particular part of space (or anything) is moving faster than light speed.

The effect is cumulative.

Every little part is expanding but not very fast. Imagine adding 1 inch per year to a yardstick. Pretty slow. However, if you have enough yardsticks laid end-to-end and each one adds one inch per year eventually someone at the far end of that line of yardsticks would seem to be receding from you at faster than light speed (and from their perspective you would seem to be receding at FTL speeds). That said neither of you are actually moving FTL so there is no violation.

ETA: Check out this page for a nifty animated picture of how expanding space affects the observable edge of the universe and a simple explanation how different distance calculations get different answers.

So does this mean that if two space craft each raced away from each other in opposite directions at 0.5c the distance between them would be growing at a tiny tiny tiny bit faster than the speed of light?

[The_Hamster_King]

@Fiendish_Astronaut

So does this mean that if two space craft each raced away from each other in opposite directions at 0.5c the distance between them would be growing at a tiny tiny tiny bit faster than the speed of light?

It depends on the reference frame. If I’m standing on a planet watching two spacecraft heading off in opposite direction at 0.6c, then from my perspective they’re moving apart at greater than the speed of light. However, from the point of view of an observer on either spaceship, the gap between them is growing at less than the speed of light.

[SurveyorAlpha]

@The_Hamster_King

It depends on the reference frame. If I’m standing on a planet watching two spacecraft heading off in opposite direction at 0.6c, then from my perspective they’re moving apart at greater than the speed of light. However, from the point of view of an observer on either spaceship, the gap between them is growing at less than the speed of light.

It seems to me that from the point of view of an observer on a planet between them, they would each be travelling away from the planet at 0.6c, and this distance between them would be increasing at 1.2c (though neither ship is moving faster than c). But from the point of view of an observer on either of the spaceships, the other ship is now moving away at 1.2c.

[The_Hamster_King]

@SurveyorAlpha

But from the point of view of an observer on either of the spaceships, the other ship is now moving away at 1.2c.

Not possible. No matter which reference frame you observe from, nothing can ever travel faster than the speed of light relative to you.

Due to relativistic effects, the spaceships would see each other receding at about 0.88c, not 1.2c.

[Stranger_On_A_Train]

@SurveyorAlpha

It seems to me that from the point of view of an observer on a planet between them, they would each be travelling away from the planet at 0.6c, and this distance between them would be increasing at 1.2c (though neither ship is moving faster than c). But from the point of view of an observer on either of the spaceships, the other ship is now moving away at 1.2c.

Nope. At relativistic speeds (those at a significant fraction of c) you have to apply Lorentz corrections to obtain the apparent relative velocity from one observer to another. In the case of two observers in inertial reference frames moving on colinear paths at 0.6c away in opposite directions from a fixed point of reference, the apparent velocity of either craft from the other is w~0.882c.

While I say “apparent” (as in how it would appear to the fixed observer in the middle if he just adds the vector) those magnitudes are very, very real to the observers in the moving crafts themselves. In order to make that work, they are also experiencing time slower (by a factor of 0.8) than the guy in the middle. Strange, but true, or perhaps just truly strange. But that is the way the world works when you’re living in the fast lane.

Stranger

[The_Hamster_King]

@Stranger_On_A_Train

In order to make that work, they are also experiencing time slower (by a factor of 0.8) than the guy in the middle.

Only from the perspective of the guy in on the planet. From the perspective of the observers on the spaceships, the clocks back on the planet are ones that are running slow. However, from the perspective of the spaceships the entire universe is also squished in their direction of travel, so it all works out … .

[dtilque]

@The_Hamster_King

I believe the current thinking is that the Local Group of galaxies is bound together tightly enough by gravity to not be pulled apart by the expansion. However, eventually everything else will redshift off into invisibility.

Recently, I read or heard that someone had found evidence of cosmic expansion in some of the outer galaxies of the Local Group. Can’t remember where I read or heard that, though.

Note that long before cosmic expansion threatens to isolate the Local Group, the Milky Way and Andromeda will have collided and merged. I suspect that M33 will also be sucked into the resulting mess, as, no doubt, will many of the smaller irregular galaxies of the Local Group. In the absence of the Big Rip (which I understand will probably not happen), gravity should have little problem keeping one elliptical galaxy together.

[Chronos]

In the absence of the Big Rip (which I understand will probably not happen)

Hard to put a probability on it: The data slightly favor the Big Rip possibility. The reason it’s not taken very seriously is mostly because of Occam’s Razor, since the data are also consistent with simpler models.

[MonkeyMensch]

@dtilque

… In the absence of the Big Rip (which I understand will probably not happen), gravity should have little problem keeping one elliptical galaxy together.

But I don’t wanna live in an elliptical galaxy, I want my Milky Way!

[levdrakon]

@Chronos

Hard to put a probability on it: The data slightly favor the Big Rip possibility. The reason it’s not taken very seriously is mostly because of Occam’s Razor, since the data are also consistent with simpler models.

Speaking of the Big Rip, apparently stars and planets won’t be ripped apart until the last few minutes. I assume about that same time, it’s going to be Big Rip vs. super massive black holes, like the ones supposed to live in the centers of large galaxies.

So, what happens?

[Chronos]

Short answer, it gets messy. The long answer probably wouldn’t fit in one post, and I’m not certain I’m qualified to answer it anyway. Just keep in mind that the Rip is a kind of singularity, and singularities always make things go screwey.