Cosmology

Sir Cumference

Feb 22, 2016 23:22

Does anyone know the answer to this?

0

Q: Will gravitational waves too far away ever reach us?

Gravity is the curvature of spacetime, and its effects travel at lightspeed. However, space is expanding; eventually, light from distant galaxies will become more and more redshifted, and we will no longer be able to see them (source). As such, there is a limit to how far we could ever possibly ...

ACuriousMind

I tend towards saying no, stuff beyond the observable universe will not have gravitational influence on us. But stuff beyond the observable universe would not have gravitational influence on us simply because it is so ridiculously far away to begin with, so the question's moot.

user54412

@user36790 As a test, can you see this or this answer?

ACuriousMind

Note, though, that gravity is different from gravitational waves. Just because gravitational waves can or cannot reach something, that doesn't mean gravity can't reach it.

Sir Cumference

I was thinking yes, but for a different reason. Even if it's the slightest gravitational effect, a lot of distant objects still affect us

DanielSank

@Danu I've been reminded recently that philosophical questions are an absolutely essential part of science.

From that perspective, I do think there is a measurement "problem"... but only sort of.

Sir Cumference

Feb 22, 2016 23:26

My question is whether these objects will eventually have no effect on us

DanielSank

From a very strict scientific method point of view, there isn't really much of one to speak of.

Danu

@DanielSank Sure, but I think the "but wtf Y U NO deterministic" is just something that belongs in the past, so it doesn't properly qualify

DanielSank

@Danu Agreed.

Danu

I'm sensible to philosophy of science generally though, yeah

DanielSank

@ChrisWhite put it incredibly well a while ago. He said something like "Folks need to just come to accept that probability distributions do not always come from a lack of information".

Sir Cumference

Feb 22, 2016 23:27

@ACuriousMind Aren't gravitational waves just from the propagation of gravity?

user54412

@SirCumference One has to be very careful about what even "object" means in this case.

Sir Cumference

@ChrisWhite Anything with gravity

I'm clearly focusing on very massive objects

ACuriousMind

@SirCumference No, just as light is not just from the propagation of electromagnetic fields

Danu

Anything that contributes to the energy-momentum tensor?

Sir Cumference

Yep

Light, even

user54412

Feb 22, 2016 23:27

@SirCumference Too many people in cosmology questions like this implicitly have a notion of "present time" that gets wrapped up in their definition of an object.

DanielSank

Now @Danu, I do think it's important to try to understand what the hell happens if you put an "observer" in a closed box. When the density matrix diagonalizes from that observer's perspective, what happens from my perspective as the "outside" observer?

Sir Cumference

@ChrisWhite Er...I'm sorry?

DanielSank

If you try to concoct an experiment on this subject, you can't really find anything interesting though... you always just find that quantum mechanics gives you the right answer if you keep track of all of the degrees of freedom.

user54412

user54412

^ @Sir this diagram is your best friend

Sir Cumference

Feb 22, 2016 23:30

@ChrisWhite Er...what is this?

user54412

6

Q: Is the cosmic horizon related to the Big Bang event?

The Universe expands according to the Hubble's law: velocity is proportional to distance. There must be some distance, therefore, at which the velocity reaches the speed of light. This defines the horizon. The Doppler shift is so huge that the length of waves becomes infinitely long: We can not s...

cosmology universe big-bang

user54412

One of many, many questions that can be answered with that diagram

Danu

Lololol

Going way over da heads

user54412

@Danu there's no other way

user54412

In any event, focus on the third panel. The vertical lines are the worldlines of Hubble-flowing objects like galaxies. We're at the center where all lines meet.

user54412

Feb 22, 2016 23:33

Note our past light cone. The larger triangle labeled event horizon is what our light cone will approach as $t \to \infty$.

ACuriousMind

"We're at the center where all lines meet" Typical anthropocentrism!

user54412

Here's an important feature: for any galaxy whose worldline intersects our current past light cone (i.e. any galaxy we can see some version of), some part of that galaxy's worldline will always intersect our past light cone for all times in the future (i.e. we will always see some version of it).

Danu

@ACuriousMind "cosmologists"

user54412

Even if a galaxy's worldline is partially outside our event horizon, any part of it inside our current past light cone is inside our domain of dependence, and always will be, so in some sense you can't escape its influence any more than you can escape your past.

Sir Cumference

OH, it all makes sense!

user54412

Feb 22, 2016 23:39

Oh good. I'm kind of surprised actually. This stuff shouldn't make sense ;)

ACuriousMind

@ChrisWhite lol, while I agree with the comment that plain o is wrong, it at the same time uses "Göthe", which is also just wrong because that name is always written with oe.

Sir Cumference

Damn good answer

But I'm still a bit confused. Why does our light cone go beyond the hubble sphere?

Danu

@ChrisWhite This all is just saying that we're not traveling faster than light, right.

user54412

@Danu yes

user54412

@SirCumference let me think about a good way to answer that (or someone else can say something)

ACuriousMind

Feb 22, 2016 23:47

Easiest way is to turn the question around ;) @SirCumference, why do you think it shouldn't?

Danu

I recall that Mukhanov's book (chapter 3) has a very good discussion of this stuff.

Sir Cumference

Well, the hubble sphere is the point where astronomical objects recede faster than light

Danu

It's all just a big tumble of definitions, but if you take an hour to sit down and write stuff out everything is simple.

Sir Cumference

Yet the boundry of light cone is the farthest we can see today

Wait, crap

Now I'm confusing myself

Wait, our light cone will grow larger until it approaches the event horizon, right?

ACuriousMind

@SirCumference Now. Since the Hubble parameter is not constant, it shifts over time. Only if the Hubble parameter were constant would Hubble sphere and light cone coincide

Sir Cumference

Feb 22, 2016 23:54

Er...I'm guessing the Hubble parameter isn't the same as Hubble's constant?

Danu

It is

The thing is not constant in time, but constant throughout space

Sir Cumference

All right, but I still don't see why the Hubble sphere and light cone would only coincide if it were really constant

Sorry for my ignorance

Danu

Perhaps you should try to check out Mukhanov's book on cosmology

chapter 3

Sir Cumference

Er, any quick explanation?

Danu

Don't remember :)

"quick explanation" + physics = bad times

ACuriousMind

Feb 23, 2016 00:04

@SirCumference Because the light cone doesn't care how fast things are moving now

Sir Cumference

I know

ACuriousMind

The Hubble sphere looks at where things are now. The light cone looks at whether or not the spacetime distance between us and that event is timelike or not.

Sir Cumference

So we can see the objects beyond the Hubble sphere based on what they looked like when they were within the Hubble sphere?

ACuriousMind

Only if the Hubble parameter is constant you can infer from the distance now that they must always have been receding faster than light. But it isn't constant, so there is a priori no relation between the Hubble sphere and the light cone

@SirCumference Yes.

Sir Cumference

THERE WE GO

Dang, it should've been so simple

Can't believe I didn't realize it

Eventually, won't we be unable to see those objects?

ACuriousMind

Feb 23, 2016 00:09

Eventually, yes. Why is there a "but" there, does that contradict anything?

Sir Cumference

What are you talking about?

ACuriousMind

Being in this chat has made me realize why many chats do not allow editing :P

Sir Cumference

Anyways, back to my question. When objects go beyond the point when we won't be able to see them, will their gravity still affect us?

ACuriousMind

@SirCumference No, because they will be outside our light cone.

0celo7

@user36790 Ok, finished AC Rogue. One of the shortest ACs!

Sir Cumference

Feb 23, 2016 00:13

Dang, all this makes sense now!

Thanks a lot

The h Bar

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