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DocJanItor t1_jam805g wrote

Reply to comment by nivlark in Why do cosmologists say that gravity should "slow down" the expansion of the universe? by crazunggoy47

That answer seems kinda counterintuititive to me considering that the universe is larger than the equivalent size it should be for its age and the speed of light. Thus, the expansion of the universe has obviously gone faster than light and has broken a major physical limitation of the universe itself. Why would we expect that expansion follow other in universe laws?

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nivlark t1_jamd3iu wrote

Sufficiently distant objects have apparent recession velocities greater than the speed of light, but this doesn't break any physical laws. Special relativity says that velocity measured in an inertial frame will never exceed the speed of light, but cosmologically distant galaxies are not inertial from our perspective.

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ableman t1_jammpqd wrote

> Special relativity says that velocity measured in an inertial frame will never exceed the speed of light, but cosmologically distant galaxies are not inertial from our perspective.

We are in an inertial reference frame, so we are measuring from an inertial reference frame, it shouldn't matter what frame they're in, SR works just fine on accelerating objects, as long as the observer is inertial. If you meant to say we're not in an inertial reference frame, it's a very poor explanation, because that's just a No True Scotsman fallacy, but also just seems wrong? The whole point of inertial reference frames is that you can tell whether you're on one or not with a local experiment.

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nivlark t1_jaogkfl wrote

Our reference frame is only locally inertial, where "local" means "close enough that the global geometry of spacetime can be ignored". In special relativity spacetime is flat everywhere and there is no such distinction, but in GR the same is not generally true.

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zutnoq t1_janifi1 wrote

Indeed, the inertial-ness of a reference frame is only a local property (local here meaning infinitesimal displacements).

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Aseyhe t1_jamocwu wrote

Repeating a response I made to a similar question elsewhere in the thread:

Relative velocities of distant objects aren't well defined in curved spacetimes. It's often said that distant objects are receding faster than light, and there are standard ways of writing down their distance such that the distance grows faster than the speed of light. However, there is no relativistically meaningful sense in which these objects are moving faster than light in relation to us. Also, the distance isn't uniquely defined either.

In intuitive terms, the relative velocity is the angle between two vectors in spacetime. Imagine drawing two arrows on a sheet. If those arrows are in the same place, you can measure the angle between them. If they are in different places, but the sheet is flat, you can also define the angle between them uniquely. However, if they are in different places and the sheet is not flat, the angle between the arrows is not uniquely defined.

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DocJanItor t1_jan4djg wrote

Doesn't matter about their current velocities. The universe is 14B years old. It's bigger than 28B light years wide. Therefore the universe had to expand faster than the speed of light.

Further, light goes through the universe. The universe expands outside of itself into who knows what. We have no idea if the speed of light applies outside of the universe. Same for thermodynamics.

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Ape_Togetha_Strong t1_jan9j7f wrote

It sounds like you're imagining "expansion" to be some outward expansion from a point in space, rather than spacetime expanding everywhere. There's no reason to tie the age of the universe to its size (other than just how its size changes proportionally, relative to itself with time). It could have been any size prior to inflation. The fact that the observable universe is larger in lightyears than the age of the universe in years is not surprising or particularly meaningful.

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Possible-Quail-7376 t1_jb1bhnu wrote

allright but the universe as we know it might be one string, layer of whatever. looking at the big picture, anything goes really. assuming the universe is same shape as the rest of stuff (toroidal) doing same stuff as atoms, quantum particles and stuff like that on what knows dimensions..

well, i guess we live in positive side of universe since stuff seems to expand. One string in a field. perhaps that is the part what scientists would call the universe part.

to be continued...

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