Current Events > How do we know the universe is expanding from a single point?

(+) Yeah! (+)

Three universe is obviously too big to observe. Maybe all the movement we've picked up is just some small cluster of movement in our observable view.

Or, instead of emanating or from a single point, all that matter could have just sort of moved past that point and kept going in the other direction.

Nah mean?
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Cool

It's possible.

But all we have to go on is shit moving away from each other. So them's the brakes.
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Dragonblade01 posted...

It's possible.

But all we have to go on is shit moving away from each other. So them's the brakes.

cool

I mean, we know basically the following: for the most part within our portion of the universe we can see (the observable universe),

- Everything is moving away from us to some degree
- Everything is moving away from everything else
- The further away something is from a reference point the faster it moves

This all supports the idea of a uniformly expanding universe (I say uniform​because we've even measured it, the Hubble constant H0, which IIRC is 70 km/s per mega-parsec from a reference point).

That doesn't mean it's certain: it could be possible we're just a blip in an otherwise nonexpanding universe for all we know. We just go with the expansion theory because we find evidence supporting it and testing observations or derivatives of theories supports it. We can say we're (probably) expanding from some point from our observable universe: we cannot say the whole universe is, though.

Hopefully that explanation was kinda clear?
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Red shift, CBM, and the fact that you can actually look back about 13 billion years and see how the Universe was in the beginning.
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you can actually look back about 13 billion years and see how the Universe was in the beginning.

Er, IIRC photons weren't emitted until like 300,000 years after the Big Bang.

I think it had something to do with the temperature cooling enough to separate electromagnetism from the other fundamental forces.
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KeyBlade999 posted...

you can actually look back about 13 billion years and see how the Universe was in the beginning.

Er, IIRC photons weren't emitted until like 300,000 years after the Big Bang.

I think it had something to do with the temperature cooling enough to separate electromagnetism from the other fundamental forces.

Huh? I meant looking back into the early Universe, around a few tens of hundreds of million years after the Big Bang and seeing the building blocks of the Universe at their simplest form.
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KeyBlade999 posted...

you can actually look back about 13 billion years and see how the Universe was in the beginning.

Er, IIRC photons weren't emitted until like 300,000 years after the Big Bang.

I think it had something to do with the temperature cooling enough to separate electromagnetism from the other fundamental forces.

To be perfectly fair, that basically is the beginning of the universe.
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Fair enough, I assumed you meant literal beginning. Sorry.
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KeyBlade999 posted...

I mean, we know basically the following: for the most part within our portion of the universe we can see (the observable universe),

- Everything is moving away from us to some degree
- Everything is moving away from everything else
- The further away something is from a reference point the faster it moves

This all supports the idea of a uniformly expanding universe (I say uniform​because we've even measured it, the Hubble constant H0, which IIRC is 70 km/s per mega-parsec from a reference point).

That doesn't mean it's certain: it could be possible we're just a blip in an otherwise nonexpanding universe for all we know. We just go with the expansion theory because we find evidence supporting it and testing observations or derivatives of theories supports it. We can say we're (probably) expanding from some point from our observable universe: we cannot say the whole universe is, though.

Hopefully that explanation was kinda clear?

so the only reason we go with the big bang theory is because it makes sense based on the small bit of the universe we can observe? That makes me feel way less sure about things :/
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I mean, we don't really know for certain how big the universe is. We know the observable universe is some 49 billion light-years in radius but that's really about it.

The actual universe could be 50 Gly or 1,000, we don't know.

Our observations are only necessarily accurate for just that: what we can observe. While logic could suggest that the universe beyond the particle horizon is ultimately no different, we'd be crossing a line in a sense because we cannot confirm such observations.
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(Actually, an addendum: IIRC measures of our spatial curvature suggest near flatness in line with an infinite universe, though again that's based on observations within our bubble. Just thought it relevant because I brought up size.)
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big bang is bollocks then tbh.
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Not necessarily - it's well evidenced, just not proven. A lot of predictions from the Big Bang theory are valid and supported after all: the fact that there's probably universe beyond our current observable range doesn't change that.

There could've after all been a really hot, small, dense point in the grander universe which began expanding, causing the Big Bang. There's no reason to say that the Big Bang gives birth to a universe.

Not trying to say you're right or wrong (the willingness to question the current status quo is always good). Just felt like you're quick to get on the "well it's not completely proven for the universe at whole so it's probably false" train.
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It just seems like it's a heck of a stretch to say that everything originated from one singular point when that singular point may be (and in all probability is) well outside our observable range. So we have evidence that stuff is moving away from something, but that relatively small chunk of the universe that we can see moving is not reflective of the rest of the universe, right?

or do we somehow have evidence that stuff outside of what we can observe is also moving away from the same point?

I get that the big bang isn't inconsistent with what we know, but I guess I don't see how that's is the leading theory right now when it's premised on the notion that everything is expanding, yet we don't know everything is expanding, or have any evidence for such a claim.

now I'm picturing a "little bang" where everything in our observable universe is maybe emanating from a single point, but the rest of the universe existed well before that.

Idk man, I was a poli sci major in college yo. I don't know about space and shit, but this just seems wack now that I take 10 seconds to think about it.
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So we have evidence that stuff is moving away from something, but that relatively small chunk of the universe that we can see moving is not reflective of the rest of the universe, right?

It may or may not be. We cannot observe that which is unobservable (which is limited to the aptly named observable universe). Theories suggest that the universe is homogeneous on very large scales and these have held true for our observable universe: whether they're true for the universe beyond is anyone's guess, but logically it'd make sense if it was.

Think of it like two separate rooms in your house. You can observe the temperature, humidity, etc., of the room you're in. You can logically assume these factors are mostly true for the other room, even though quantum mechanically there's a chance that all the particles in that room's air have bunched up into a really small space. Possible but logic dictates it unlikely. Similarly while the universe beyond our observable universe could be really anything, logically the homogeneity we observe holds true for the rest of the universe.

Though ultimately:

or do we somehow have evidence that stuff outside of what we can observe is also moving away from the same point?

We really can't say for sure beyond logical deduction (i.e. not necessarily with evidence).

Though in the end science is a lot of "best guess," in a sense. Take gravity for example. First it was "everything goes down." Then Newton formalized it. Then it eventually got found inconsistent but became the basis for Einsteinian relativity. Science forms hypotheses and theories from observed data, and eventually theories become widely accepted; once found inconsistent a new theory, either modified or altogether new, is made. Science accepts what we see until we find something that invalidates it: it all builds on itself. Right now, a Big Bang theory is the predominant theory; one day, it might not be. Really just depends on what theory is the most evidenced, you know?

I get that the big bang isn't inconsistent with what we know, but I guess I don't see how that's is the leading theory right now when it's premised on the notion that everything is expanding, yet we don't know everything is expanding, or have any evidence for such a claim.

While I get your point, that kind of fallacy tends to invalidate all science. Going on the idea of an infinite universe, there's no way we can observe it all. How do we know that gravity simply doesn't stop working/works as we know it at extremely large distances, as an example? (This is a proposed explanation for dark matter by the way.)

Well, we don't.

But it's like "what we have does work." We have evidence and tested theories for all the stuff that gets widely accepted in science. And sure, it could all be wrong. We could live in a steady-state universe made by God for all we know. Perhaps the Big Bang never did happen and all the proof for it can be explained away by another theory or set thereof. And if so, science will change in line with that. Science is about developing and modifying theories: what we accept as likely today might be invalidated tomorrow. But we stick with what we know because it does work until then.

I'm not sure if I'm making sense at this point. I know what I'm trying to explain but I'm having a hard time doing so, I think, so I'll stop before I make things any more confusing.
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KeyBlade999 posted...

It may or may not be. We cannot observe that which is unobservable (which is limited to the aptly named observable universe). Theories suggest that the universe is homogeneous on very large scales and these have held true for our observable universe: whether they're true for the universe beyond is anyone's guess, but logically it'd make sense if it was.

Think of it like two separate rooms in your house. You can observe the temperature, humidity, etc., of the room you're in. You can logically assume these factors are mostly true for the other room, even though quantum mechanically there's a chance that all the particles in that room's air have bunched up into a really small space. Possible but logic dictates it unlikely. Similarly while the universe beyond our observable universe could be really anything, logically the homogeneity we observe holds true for the rest of the universe.

I get that in theory, but the big bang doesn't seem logically as intuitive the same way as the temperature in another room does. I'm thinking more like tossing a pebble in the ocean, and all the microorganisms being like "holy smokes, there was a big bang and now all the water is emanating from that one point!" but these little guys can only see the part of the ocean that is affected by the pebble, and don't understand that the rest of the ocean don't give a damn about the pebble and is doing its own thing.

We can't talk about logic when there's some extraordinary causal event breaking the chain of logical inference. Deducing the temperature of a room is one thing because we can compare it to other stuff that we know, like the temperature of the room we are currently in. When the theory in question is supposed to explain the literal beginning of the universe, you can't really do that because we don't have anything to compare it to at that scale. What if the "big" bang was just a relatively small bang in a much larger universe? We're using what we know about our observable universe and applying it to the universe at large, despite the fact that we're the outlier. That's not logical at all, at least in my mind. Again, maybe there's some badass sciencey equation that makes sense of all this that I've never seen
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Science can only make observations and extrapolate from those observations. We cannot include unknown "potential" information in a model.
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I don't think you are grasping how cosmic expansion works. It's not centered like a explosion. You can't travel to a part of the universe and go "here is where the singularity was". That point WAS the universe.

Imagine taking a marker and making dots on balloon. If you inflate that balloon the dots each move away from each other, not from a single point of reference.
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Because physics.
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bump for more science peeps
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KeyBlade999 posted...

I mean, we know basically the following: for the most part within our portion of the universe we can see (the observable universe),

- Everything is moving away from us to some degree
- Everything is moving away from everything else
- The further away something is from a reference point the faster it moves

This all supports the idea of a uniformly expanding universe (I say uniform​because we've even measured it, the Hubble constant H0, which IIRC is 70 km/s per mega-parsec from a reference point).

That doesn't mean it's certain: it could be possible we're just a blip in an otherwise nonexpanding universe for all we know. We just go with the expansion theory because we find evidence supporting it and testing observations or derivatives of theories supports it. We can say we're (probably) expanding from some point from our observable universe: we cannot say the whole universe is, though.

Hopefully that explanation was kinda clear?

Very. One of the best explanations I've come across, actually.
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lilORANG posted...

I get that in theory, but the big bang doesn't seem logically as intuitive the same way as the temperature in another room does.

That tends to happen when you're dealing with the physics of the very small and the high energy.
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KeyBlade999 posted...

So we have evidence that stuff is moving away from something, but that relatively small chunk of the universe that we can see moving is not reflective of the rest of the universe, right?

It may or may not be. We cannot observe that which is unobservable (which is limited to the aptly named observable universe). Theories suggest that the universe is homogeneous on very large scales and these have held true for our observable universe: whether they're true for the universe beyond is anyone's guess, but logically it'd make sense if it was.

Think of it like two separate rooms in your house. You can observe the temperature, humidity, etc., of the room you're in. You can logically assume these factors are mostly true for the other room, even though quantum mechanically there's a chance that all the particles in that room's air have bunched up into a really small space. Possible but logic dictates it unlikely. Similarly while the universe beyond our observable universe could be really anything, logically the homogeneity we observe holds true for the rest of the universe.

Though ultimately:

or do we somehow have evidence that stuff outside of what we can observe is also moving away from the same point?

We really can't say for sure beyond logical deduction (i.e. not necessarily with evidence).

Though in the end science is a lot of "best guess," in a sense. Take gravity for example. First it was "everything goes down." Then Newton formalized it. Then it eventually got found inconsistent but became the basis for Einsteinian relativity. Science forms hypotheses and theories from observed data, and eventually theories become widely accepted; once found inconsistent a new theory, either modified or altogether new, is made. Science accepts what we see until we find something that invalidates it: it all builds on itself. Right now, a Big Bang theory is the predominant theory; one day, it might not be. Really just depends on what theory is the most evidenced, you know?

I get that the big bang isn't inconsistent with what we know, but I guess I don't see how that's is the leading theory right now when it's premised on the notion that everything is expanding, yet we don't know everything is expanding, or have any evidence for such a claim.

While I get your point, that kind of fallacy tends to invalidate all science. Going on the idea of an infinite universe, there's no way we can observe it all. How do we know that gravity simply doesn't stop working/works as we know it at extremely large distances, as an example? (This is a proposed explanation for dark matter by the way.)

Well, we don't.

But it's like "what we have does work." We have evidence and tested theories for all the stuff that gets widely accepted in science. And sure, it could all be wrong. We could live in a steady-state universe made by God for all we know. Perhaps the Big Bang never did happen and all the proof for it can be explained away by another theory or set thereof. And if so, science will change in line with that. Science is about developing and modifying theories: what we accept as likely today might be invalidated tomorrow. But we stick with what we know because it does work until then.

I'm not sure if I'm making sense at this point. I know what I'm trying to explain but I'm having a hard time doing so, I think, so I'll stop before I make things any more confusing.

@KeyBlade999 May I ask what you do? You did a phenomenal job breaking down the concepts into relatable concepts. Tbh I'm curious if you work in Acadmia because you would probably do quite well giving lectures.

Either way I thoroughly enjoyed your read an wanted to thank you. You cleared up a lot of confusion I had about certain aspects.
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lol

It's funny because I'm just a college student (junior this year). I was originally majoring in astrophysics and mathematics but I switched to just the latter this year. I originally wanted to become an astrophysicist but due to a number of things I decided that wasn't a good path for me: plus I love mathematics more.

I still have a lot of interest in the field though, don't get me wrong, but I just don't have the skills and motivation right now to hold my own in physics. (Though despite screwing up in general physics I did plenty well in my astrophysics class this semester. So it's weird but I guess part's just because my lecturer was amazing and because of a lot of prior exposure to material? Who knows to be honest. XD)

Still, that made me smile. Thanks. ^_^;
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KeyBlade999 posted...

lol

It's funny because I'm just a college student (junior this year). I was originally majoring in astrophysics and mathematics but I switched to just the latter this year. I originally wanted to become an astrophysicist but due to a number of things I decided that wasn't a good path for me: plus I love mathematics more.

I still have a lot of interest in the field though, don't get me wrong, but I just don't have the skills and motivation right now to hold my own in physics. (Though despite screwing up in general physics I did plenty well in my astrophysics class this semester. So it's weird but I guess part's just because my lecturer was amazing and because of a lot of prior exposure to material? Who knows to be honest. XD)

Still, that made me smile. Thanks. ^_^;

It was well deserved! Best of luck with all your future endeavors mate.
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