If The Universe Is Expanding, What's It Expanding Into?

http://io9.com/5526583/if-the-universe-is-expanding-whats-it-expanding-into

by instantramen • 16 years ago

45 55 16 years ago

So the galaxies are sitting still, the space inside it is expanding ... at faster than the speed of light.

The way my puny layman mind can figure out ... if 2 objects move away from one another at slightly more than half the speed of light, then one would see the other as apparently moving away at greater than the speed of light.

Also, iirc, the expanding balloon metaphor was more to give an idea how all points are moving away from one another, and each sees itself as the center. It was not meant to talk of dimensions, or moving into ... (btw, thanks a lot for the link. great read)

tydok2• 16 years ago

> 2 objects move away from one another at slightly more than half the speed of light, ...

IIRC, mass grows towards infinity as it approaches the speed of light. So that's not possible if the objects have mass.

ErrantX4• 16 years ago

Which bit are you questioning - making it to half the speed of light (I believe that is entirely possible) or the relative speed being more than the speed of light?

If it is the latter; it's perfectly possible. Ultimately it is just mathematics - as pointed out on Wikipedia apparent "FTL" travel is allowed by most models.

http://en.wikipedia.org/wiki/Faster-than-light

JoeAltmaier2• 16 years ago

Unless you change the "size" of the space between them - then they are getting further apart without moving, which is what is happening.

crazydiamond3• 16 years ago

So are you saying there is a maximum speed which is less than half the speed of light, for objects that have mass. What is that max speed ??

From what i know, a great deal of energy is required to accelerate it, as the speed increases, but its still a finite amount.

lmkg2• 16 years ago

Warning: special relativity hurts your brain.

First, there's no such thing as objective space. There is no objective Frame of Reference. You can pick a frame of reference, and it should probably be an inertial (non-accelerating) frame of reference, but it's just as valid as any other frame of reference. Inertial frames of references move with respect to each other at constant, less-than-C velocity.

Ok, so you picked a frame of reference, and in that frame of reference you have two objects A and B moving away from each other, each with a velocity of 0.5c, for a total relative velocity of c. What does this look like to either of the objects? Well, to answer that question, we have to jump into their frames of reference, which are moving at 0.5c with respect to ours.

In this new frame of reference, one of the objects A is stationary. Object C that was stationary in the previous frame of reference is now moving at 0.5c. And the other object B, that in the previous frame of reference was moving away at c? In this frame of reference, it is moving away at less than c. Why? A combination of time dilation and space compression, which you probably know happens in special relativity.

In the original frame of reference, object A was moving at 0.5c because it took 1 year to travel from one point to another than are half a light year apart. In the new frame of reference, the object A is standing still and the two points are moving, and they're more than half a light-year apart, and it takes less than 1 year for the new one to get here. Similarly, time & distance separation between points that the other object B is passing have altered in the new frame of reference. There's some math which isn't that complicated but is a bit onerous, but the punchline is that it appears to be traveling less than the speed of light in the new frame of reference.

Why does space and time contort like this? So that a light particle traveling will appear to be moving at the speed of light in any inertial reference frame, regardless of the relative velocities of those reference frames.

crazydiamond3• 16 years ago

> The expanding universe is due to gravity, which means that in regions of high density, the dominant gravitational effects are entirely local.

Is this the conventional view. I always thought the universe was expanding as a result of the big bang momentum. This expansion would keep going unless there was a critical mass to pull it back. (that was the way things were when i was a kid). Or am i misreading.

Devilboy• 16 years ago

The expansion is accelerating though

a-priori• 16 years ago

From http://en.wikipedia.org/wiki/Dark_energy: "dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe"

retube6• 16 years ago

not sure that quote is correct. Gravity is an attractive force, it should slow expansion. However recent observations indicate the expansion is accelerating. This has (I beleive - been out of modern physics for a little while) lead to the positing of "dark flow", missing or dark energy.

AngryParsley2• 16 years ago

While the IO9 post is better than most pop-physics articles, I feel a need to repeat myself...

Your question contains invalid assumptions. There is no analogy that gives both a satisfying and accurate answer. You want to know about physics? Understand the math. The closest you can get without differential equations is Feynman's QED: The Strange Theory of Light and Matter

gaius• 16 years ago

Did you see Agora yet? In it the mathematician Hypatia in 4th century Egypt (played by Rachel Weisz) makes the radical announcement that circles are not really shapes in their own right, they're just a type of ellipse, and everyone wigs out - everything they know about astronomy and physics will have to be rethought. Anyway, awesome film.

AngryParsley2• 16 years ago

I watched Agora tonight. It was good. I didn't know a movie could cause me to dislike religion even more than I already did.

ErrantX4• 16 years ago

Don't believe the metaphors. In every science special worth its salt, you'll see some image of a balloon being inflated.

I've always considered this metaphor really good; it's just it is badly explained. The go to guy for decent pop physics explanations is, in my mind, Dr Brian Cox [1] (he does some really excellent TV shows here in the UK). His explanation with the balloon notes that while the surface area increases (expansion of the universe) the volume of the balloon rubber does not. You have to ignore the air inside and outside of the balloon as having any importance.

Then it should become clear how the universe can "expand" without actually expanding into anything.

1. http://en.wikipedia.org/wiki/Brian_Cox_(physicist)

michael_dorfman3• 16 years ago

You have to ignore the air inside and outside of the balloon as having any importance.

And that's the bit of mental acrobatics that makes the metaphor break down. Which is why the advice above ("Don't believe the metaphors") is relevant.

ErrantX4• 16 years ago

I don't know - as long as you say, during the explanation, that the air inside/outside is irrelevant then it's a good demonstration of expansion!

stcredzero9• 16 years ago

If any given model of the universe involves n dimensions, why can't we simply have it expand into the n+1 dimension? When you inflate a balloon, it's 2d area is increasing by the balloon expanding into the 3rd dimension.

In any model of the universe as a brane of n dimension, its expansion can be modeled as expansion in the n+1 dimension. Perhaps "universe" is a paradoxical word that cannot have a meaning? It's hard to imagine a world of infinite dimension.

retube6• 16 years ago

The question doesn't really mean anything. Space (and time) and attributes/characteristics/features of our universe. So there is no "outside" (multiverse models aside). Likewise you cannot ask what was "before" the big bang, as time only came in to existence with the big bang.

euroclydon2• 16 years ago

Likewise you cannot ask what was "before" the big bang, as time only came in to existence with the big bang.

If there were ever a reason not to like cosmology, it's statements like that.

retube6• 16 years ago

Definitely have to disagree. I think this sort of understanding is what makes physics and cosmology awesome. It's hard to imagine as we experience 3 spatial dimensions and 1 of time (and can only experience that time dimension in one direction). There could be universes with no time, or a dozen space dimensions, or 2 time dimensions. How would you define "before" for a universe with multiple time dimensions?

Edit: the human perception of time is an interesting subject. Physics (GR) holds that there is a 4D space-time. What we experience as separate constructs is really just too sides of the same coin. Although thermodynamics seems to define a "direction" to time, there's no real reason why you couldn't move in both time directions. For example, the production of an anti-particle in a particle accelerator (e.g a positron) is really just a particle (electron) travelling back in time to the production point. You can get around cause and effect issues if you assume the universe is in fact a 4D static environment, where time travel (forward or backward) has been predetermined by the static layout created during the big bang. This does of course have implications for free will... However, the multi-verse interpretation of QM gets you out of this.

tfh• 16 years ago

> time only came in to existence with the big bang

My brains segfaults imagining that there was a time where time didn't exist...

stcredzero9• 16 years ago

That's like trying to imagine what being dead "is like" for materialist atheists. If you don't believe in an afterlife, then your consciousness simply ceases. There is no experience of being dead.

bhousel• 16 years ago

Think of it this way: We don't have any way of measuring time that doesn't also involve space. Whether it's pendulums, or atoms vibrating, or photons bouncing around, any way we measure time is linked to (and relative to) whatever space is around it. If space is being deformed by a large mass being there, time will be affected (slowed down). Some theories of time say that what we think of as 'time' is really just a byproduct of space.

So if space didn't exist (in any meaningful way that we would understand today) before the big bang, time couldn't exist either.

praptak• 16 years ago

The question contains a hidden assumption: "Everything that expands, must expand into something else." This assumption seems obvious because everything that we can directly observe has this property. But this does not make it automatically true for the universe.

michael_dorfman3• 16 years ago

If it is true for matter, and the universe is the sum total of all existent matter, why wouldn't it be true for the universe?

Put another way: "Expands into something else" is what "expands" means.

ThomPete• 16 years ago

Words don't dictate reality.

And words sure don't represent reality.

The expanding universe theory is an attempt to visualize what is going on, not to actually describe what is really going on.

Language is a simplification of reality. They create linear narratives (expanding from something smaller to something bigger)

But just as time probably isn't what we think it is, these narratives aren't anything but descriptors of experienced phenomena.

Just as when Dawkins talks about the selfish gene. He isn't saying that they are really selfish (they don't have agency) he say they behave as if they are selfish.

There is absolutely nothing that dictates that we need to be able to express in language what is going on, case in point quantum physics where we have non locality.

The expanding universe theory makes sense because the metaphors used to describe it take our perspective into consideration.

But in reality there isn't necessarily any expansion taking place. In physical reality yes, but that is not all of reality.

stcredzero9• 16 years ago

When the balloon expands, its two dimensional surface are increases because the balloon is expanding into the 3rd dimension. Why can't a universe of n dimensions simply expand into the n+1 dimension?

Or, put in a better way - the universe humans can perceive is expanding, without expanding into anything else we can perceive.

Herring• 16 years ago

>If it is true for matter, and the universe is the sum total of all existent matter, why wouldn't it be true for the universe?

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

http://www.logicalfallacies.info/relevance/composition/

ghjrtyh• 16 years ago

This guy is either crap at explaining, or hasn't got a clue what he's talking about.

baddox• 16 years ago

"We're not actually sure whether the universe is infinitely large, or merely very, very big, but in even supposing it is, ... "

Forget the philosophical/physics questions, what do those words even mean?

Shorel• 16 years ago

I think it is a valid question, not because it is valid in a strict physical sense, but because a lot of people can have the same doubts.

Carl Sagan would have provided an interesting anecdote or analogy to explain the answer instead of ridiculing whoever ask this.

Sadly, my writing skills are way lesser than those of Sagan.

If The Universe Is Expanding, What's It Expanding Into?

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