Why is there a normal galaxy sitting at the edge of the Universe? • Hacker News Board

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#121

Post by ddevault » Wed, Sep 09, 2020, 7:32 PM UTC

iwonderwhy wrote: post #113

ddevault wrote: post #70

Earlier quoted context omitted.

In case anyone is confused, I prefer to explain the expansion of space without metaphors. Space itself is getting larger, everywhere, all at once. The scale factor of the universe is getting larger. New space is constantly being "made". Every second, every kilometer in the universe grows by 2.2×10^-18 centimeters. If no other forces were in play, it'd carry everything along with it and pull everything a bit further a…

Very interesting. So the I wonder if everything expands along with the universe, does it mean that from outside, things moving at the speed of light in our universe, are potentially moving at speeds greater than the speed of light within our universe? If i was watching from outside, a rectangle moving at the speed of light in our universe, would one of its edges be moving away from me at the speed of light + the spee…

That question is too abstract for me to reason about. A "rectangle" couldn't move at the speed of light; light is point-like (oh dear, saying that could get me in trouble). Anything with mass cannot move at the speed of light, and anything without mass must move at the speed of light, and all known objects without mass are point-like. And then there's the question of observation in the first place - observation itself is governed by fundamental particles, and observing a fundamental particle at its scale is theoretically impossible.

I know you want me to cheat and set aside all of these complications and just imagine a rectangle moving at the speed of light, but such an answer would be nonsensical.

But the answer to the more specific question, "does the expansion of space make something move faster than the speed of light?", is "no", but it does cause the distance between objects to increase faster than the speed of light. To an external observer (which is inherently difficult to reason about, for the record), these objects would appear to "move" away from one another faster than lightspeed. However, the very idea of "movement" is rooted in the context of spacetime, and is bounded by the speed of light, so it's a confusing/nonsensical question.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#122

Post by iwonderwhy » Wed, Sep 09, 2020, 7:33 PM UTC

ddevault wrote: post #102

iwonderwhy wrote: post #55

All right hackernews. Genuine question, not sure where else to ask. If galaxies and stars at the other “end” formed out of matter resulting from the big bang, just as the matter we are made of, how did we get here at the same time that that matter got there? Is it because speeds faster than light are possible, and around the big bang all matter was spread near instantly at that speed? I don't understand how matter of…

I don't fully understand your question to give a direct answer, but I think that's mostly due to a number of misconceptions you hold about the universe. So I'll just try to clear those up instead. First of all, the big bang didn't happen somewhere , it happened everywhere . There's no "center" of the universe that everything is moving away from. The universe did not start from a point, it started from a small scale f…

Ok, thanks for clarifying - indeed i seem to have had quite a few misconceptions. But from outside the universe, are things inside of our universe moving faster than the speed of light due to our universe expanding, and as such, things traveling at the speed of light when seen from the outside are traveling at the speed of light + speed of expansion?

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#123

Post by ddevault » Wed, Sep 09, 2020, 7:34 PM UTC

thaumasiotes wrote: post #117

ddevault wrote: post #115

Earlier quoted context omitted.

>And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule? The dominant factor on this scale is the strong nuclear force, which governs interactions at the scale of quarks - much smaller than molecules.

But the scale of quarks increases just as much as every other scale, doesn't it? Why did the iron rod shrink while space grew? Why are the iron atoms closer together now than they were before? Why are the protons in each iron atom closer together now than they were before?

The iron rod didn't shrink, it's more like it just held together. If I have a rubber sheet with a ball on it, and I pull at the ends of the rubber sheet, it's not going to pull the ball apart. The binding force of the ball's constituent atoms is sufficient to overcome the pulling force. And in the case of dark energy, the pulling force is absoultely miniscule at those scales.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#124

Post by ddevault » Wed, Sep 09, 2020, 7:35 PM UTC

iwonderwhy wrote: post #122

ddevault wrote: post #102

Earlier quoted context omitted.

I don't fully understand your question to give a direct answer, but I think that's mostly due to a number of misconceptions you hold about the universe. So I'll just try to clear those up instead. First of all, the big bang didn't happen somewhere , it happened everywhere . There's no "center" of the universe that everything is moving away from. The universe did not start from a point, it started from a small scale f…

Ok, thanks for clarifying - indeed i seem to have had quite a few misconceptions. But from outside the universe, are things inside of our universe moving faster than the speed of light due to our universe expanding, and as such, things traveling at the speed of light when seen from the outside are traveling at the speed of light + speed of expansion?

The answer is similar to the one I gave for your other question:

https://news.ycombinator.com/item?id=24424912

But I'll give you another analogy anyway. Say you put two rocks into a mass of water in an environment without gravity (e.g. the ISS), then freeze the water, causing it to expand. The expanding ice will cause the rocks to become more distant from each other, but they're not "moving" in local terms - the manifold in which they exist is itself expanding. That said, the frozen water and the rocks both exist in space, and the expanding ice moved the rocks through space. But, in this metaphor, there is no second, external "space" - the water itself represents space.

And so to for our universe. Space itself isn't a part some other, second "space" that it's expanding into (at least not so far as we're aware). The existence of somewhere for space to expand "into" is not necessary for the expansion to take place. Remember, it's the scale factor which is changing, not a discrete object (i.e. the universe) physically expanding.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#125

Post by saagarjha » Wed, Sep 09, 2020, 7:36 PM UTC

onlyrealcuzzo wrote: post #95

z92 wrote: post #78

Earlier quoted context omitted.

> 547 AU For comparison Voyager-1 is now at a distance of 40 AU only.

Is Voyager-1 still accelerating?

Nope, both are slowly decelerating as they leave the solar system: https://upload.wikimedia.org/wikipedia/commons/0/0c/Voyager_...

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#126

Post by thaumasiotes » Wed, Sep 09, 2020, 7:38 PM UTC

ddevault wrote: post #123

thaumasiotes wrote: post #117

Earlier quoted context omitted.

But the scale of quarks increases just as much as every other scale, doesn't it? Why did the iron rod shrink while space grew? Why are the iron atoms closer together now than they were before? Why are the protons in each iron atom closer together now than they were before?

The iron rod didn't shrink, it's more like it just held together. If I have a rubber sheet with a ball on it, and I pull at the ends of the rubber sheet, it's not going to pull the ball apart. The binding force of the ball's constituent atoms is sufficient to overcome the pulling force. And in the case of dark energy, the pulling force is absoultely miniscule at those scales.

The iron rod started out being 1 meter long, and now it's 0.952 meters long. In what sense did it not shrink?

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#127

Post by saagarjha » Wed, Sep 09, 2020, 7:39 PM UTC

thaumasiotes wrote: post #101

ben_w wrote: post #93

Earlier quoted context omitted.

Just to add to your question: The usual response of “it only affects space between galaxies not inside galaxies” is deeply unsatisfying, and feels like it must be an oversimplification. What I think is meant, but I absolutely want someone wiser to confirm or refute , is that the rate of expansion is proportional to distance, and therefore so small inside a single galaxy it can be ignored. What I don’t know is: can an…

> What I think is meant, but I absolutely want someone wiser to confirm or refute , is that the rate of expansion is proportional to distance, and therefore so small inside a single galaxy it can be ignored. I am not that person. But I feel like this can't work -- if all space is expanding equally, in proportion to its own extent, then we would perceive no expansion at all. If two points start out 1 light year apart,…

You'd notice a slower speed of light, wouldn't you?

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#128

Post by dumpsterdiver » Wed, Sep 09, 2020, 7:40 PM UTC

outworlder wrote: post #71

dumpsterdiver wrote: post #62

Earlier quoted context omitted.

I have a question: Just over 20 years ago when the accepted view was that the expansion of the universe was slowing down, would you have backed that position with as much certainty as you are backing this one? That the universe is a mere 13.7 billion years old? We cannot prove the absence of light in a place that is already so far away that it's light will never reach us. Do you agree? We can prove what we can observ…

> Just over 20 years ago when the accepted view was that the expansion of the universe was slowing down Was it ever a mainstream view? I thought it was just a conjecture, leading to the 'big crush', and was disproven long ago, way more than 2 decades ago. > The entire premise of dating our universe is based on its observed rate of expansion Also there's the cosmic microwave background radiation. We have mapped it, an…

> Unobservable, therefore unfalsifiable, therefore unscientific. Philosophy again.

I completely agree. Apologies if my comment above rubbed anyone the wrong way. The point you just made was my overall point. Our concept of time is confined to our current existence / cycle - that's what I meant by the sun revolving around the Earth comment, but others were correct when they responded that as a current metric that's not really useful.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#129

Post by ddevault » Wed, Sep 09, 2020, 7:43 PM UTC

thaumasiotes wrote: post #126

ddevault wrote: post #123

Earlier quoted context omitted.

The iron rod didn't shrink, it's more like it just held together. If I have a rubber sheet with a ball on it, and I pull at the ends of the rubber sheet, it's not going to pull the ball apart. The binding force of the ball's constituent atoms is sufficient to overcome the pulling force. And in the case of dark energy, the pulling force is absoultely miniscule at those scales.

The iron rod started out being 1 meter long, and now it's 0.952 meters long. In what sense did it not shrink?

>The iron rod started out being 1 meter long, and now it's 0.952 meters long. In what sense did it not shrink?

In the sense that it did not become 0.952 meters long. It didn't "become" anything - it did not change at all. The universe grew around it, or perhaps "underneath" it. More space appeared for it to exist in. It's like putting an iron rod in a 10 meter square house, then moving the walls out another meter.

Edit: let me phrase this another way. Let's imagine that you and I were pulling on the bar at either end. We could pull it forever and ever and it would never come apart (unless it rusted and became brittle or something, but let's imagine it doesn't). That's because we aren't putting in enough force to overcome the forces which are holding it together. Well, the expansion of the universe is putting like 10^-20 times less force into it than our arms would be.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#130

Post by johncolanduoni » Wed, Sep 09, 2020, 7:43 PM UTC

thaumasiotes wrote: post #101

ben_w wrote: post #93

Earlier quoted context omitted.

Just to add to your question: The usual response of “it only affects space between galaxies not inside galaxies” is deeply unsatisfying, and feels like it must be an oversimplification. What I think is meant, but I absolutely want someone wiser to confirm or refute , is that the rate of expansion is proportional to distance, and therefore so small inside a single galaxy it can be ignored. What I don’t know is: can an…

> What I think is meant, but I absolutely want someone wiser to confirm or refute , is that the rate of expansion is proportional to distance, and therefore so small inside a single galaxy it can be ignored. I am not that person. But I feel like this can't work -- if all space is expanding equally, in proportion to its own extent, then we would perceive no expansion at all. If two points start out 1 light year apart,…

The problem with that model is that as was previously said, the electromagnetic, nuclear, and gravitational forces easily countermand the expansion on non-inter-galactic scales, to the point of not being measurable on the scale of a meter. Ultimately the molecular structure of your meter-stick is determined by various structure constants that aren’t changing, so the expansion can’t affect it unless it starts ripping bonds apart.