# Gravitational reference point for every object in the universe

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Hey folks,

well. The title speaks kind of for itself: We all know that speed is relative to any reference point we measure it. But it there a reference point for every object in the universe? The universe expands. Every object orbits anything else. But every object also moves away from one point. I'm aware of the little point in which the big bang happened. But it this maybe the reference point I'm asking for? And where the hell is that thing? There has to be 1 single object or place everything has a relative speed to. Which is it and where?

Greetings

Ariggeldiggel

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Posted (edited)
4 minutes ago, Ariggeldiggel said:

But every object also moves away from one point.

No.  Very roughly speaking all objects are moving away from each other. (This is a ridiculous oversimplification . . . ). This is very much not  the same thing as "every object moves away from one point". The point you are looking for does not exist.

Edited by Kerwood Floyd
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Posted (edited)

If you imagine the expansion of the universe to be coming from a particular point, that point isn't inside the universe.

Edited by cubinator
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Posted (edited)

My understanding of this, influenced by a book by Katie Mack that I happen to be reading this week:

This goes to the heart of the "cosmological principle". The idea is that there is no special point in the universe. There is not even a "center" of the universe that everything is expanding away from. At any location in the universe, the entire universe will be seen to be expanding away from you. Furthermore, while there is a "center of the observable universe" (us), that's only a function of how observation works. Any other place in the universe would also be "the center of the observable universe" to someone who was there to observe from that location.

Despite the intuitive idea that the big bang happened at some location, some point, and then everything expanded away from it, that is not the current theory. The big bang had no location and did not take place at any moment in time, because space and time as we know them in our universe only started to exist after the big bang. How can we say that there has been a finite amount of time since the big bang, but that the big bang did not happen at any point in time? That's kind of like how the function 1/x can be defined for any positive x as close or far from 0 as you like, but does not actually exist at x=0.

Another way to say it is that since the entire universe was all together at the big bang, then the big bang happened at every location in the universe.

Edited by mikegarrison
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Quote

Gravitational reference point for every object in the universe

The observer him/herself.

Wherever he/she is.

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37 minutes ago, mikegarrison said:

My understanding of this, influenced by a book by Katie Mack that I happen to be reading this week:

This goes to the heart of the "cosmological principle". The idea is that there is no special point in the universe. There is not even a "center" of the universe that everything is expanding away from. At any location in the universe, the entire universe will be seen to be expanding away from you. Furthermore, while there is a "center of the observable universe" (us), that's only a function of how observation works. Any other place in the universe would also be "the center of the observable universe" to someone who was there to observe from that location.

Well... So far I imagined our universe like following: As the center of the observable universe we see that everything expands away from us. Okay. But what if some objects are just expanding faster from "point zero" and other objects slower. And these objects that expand slower seem to expand away from us - But the don't do that at all. They just expand with a lower speed than we do from point zero. So they're moving away from us not because they're technically speeding away, but we do.

Is that understandable in any shape or form what I'm writing down here? Sorry if the explanation sucks, I'm no native english speaker. If anything is a confusing please tell me so I maybe can make it clearer...

Greetings and thanks for the thought experiment so far.

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Posted (edited)

@Ariggeldiggel I'm no expert on this, but I do have an analogy that may help:

Imagine space as a three-dimensional grid, with galaxies sitting in it. This grid is constantly expanding in every direction, but the galaxies aren't physically moving away from each other - they're simply sitting in space and being carried along for the ride as every point on the grid becomes further and further from every other point.

Now, this doesn't prevent the galaxies from moving about in space normally, as we do see some galaxies such as Andromeda moving towards us due to gravity. However, it does provide an explanation for why everything in the Universe seems to be running away from us no matter where we look - objects aren't necessarily physically moving, they're just becoming further apart as space itself becomes physically bigger.

The interesting upshot of this is that the fabric of space doesn't need to obey the 'cosmic speed limit' aka the speed of light, because it's not made from matter or energy. This is why the observable universe appears to be about 93 billion light years across, despite being only 13 billion years old.

Hope this helped!

Edited by RealKerbal3x
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1 hour ago, Ariggeldiggel said:

Well... So far I imagined our universe like following: As the center of the observable universe we see that everything expands away from us. Okay. But what if some objects are just expanding faster from "point zero" and other objects slower. And these objects that expand slower seem to expand away from us - But the don't do that at all. They just expand with a lower speed than we do from point zero. So they're moving away from us not because they're technically speeding away, but we do.

Is that understandable in any shape or form what I'm writing down here? Sorry if the explanation sucks, I'm no native english speaker. If anything is a confusing please tell me so I maybe can make it clearer...

Greetings and thanks for the thought experiment so far.

There is no point zero. The big bang happened at every point in the universe. At the same time.

Trippy, right?

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3 hours ago, Ariggeldiggel said:

I'm aware of the little point in which the big bang happened

The Big Bang did not happen from a "little point", it happened everywhere. 2D beings on the surface of that balloon cannot determine a point on that balloon that everything is expanding from because that point does not exist in their two dimensional world - as far as they are concerned, and as far as their two dimensional physics is concerned, the balloon's big bang happened everywhere.

3 hours ago, Ariggeldiggel said:

1 single object or place everything has a relative speed to

The fabric of spacetime itself can be considered an absolute reference frame - a photon barely on the event horizon of a black hole barely has any velocity if its position is calculated using a distant static point as a reference frame, but it does move at the speed of light through spacetime. Gravity can be measured in the single digit Gs on some very large black holes because while the gravity isn't strong at the event horizon, spacetime itself is moving inwards near the speed of light because it has had so much time to accelerate.

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Posted (edited)

When the Big Bang happened, you were there. I was there. We were all there, and you were crammed tightly into every part of every galaxy that ever was and ever will be! You existed in every part of the universe at once for just a moment until space got large enough for things to settle down and stay mostly in their own place. Or, at least, the energy that eventually turned into matter and became you was there.

Edited by cubinator
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1 minute ago, cubinator said:

You existed in every part of the universe at once

How so? We don't know what happened in the first few instants of the Universe.

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As I understand it, in an expanding universe the universe will always appear to be expanding in all directions away from you personally, and this effect will appear the same no matter where in the universe you as the  the observer happen to be. So you cannot ask where the 'real' centre of the universe is because the universe is always centred on you. Try this for a bit of fun.  Close one eye. The eye that remains open is the centre of the universe.

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9 minutes ago, Bej Kerman said:

How so? We don't know what happened in the first few instants of the Universe.

We know that everything was very close together. In the earliest moments, they were so close together that particles could jump from one place in the universe to another in a quantum probabilistic manner. I believe the amount of time during which this was possible is one Planck time. It was very short, but as a result there are pieces of you that could have just as easily been transported to the other side of the universe.

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Just now, cubinator said:
18 minutes ago, Bej Kerman said:

How so? We don't know what happened in the first few instants of the Universe.

We know that everything was very close together. In the earliest moments, they were so close together that particles could jump from one place in the universe to another in a quantum probabilistic manner. I believe the amount of time during which this was possible is one Planck time. It was very short, but as a result there are pieces of you that could have just as easily been transported to the other side of the universe.

We do not yet have a theory adequate for explaining what happened that close to the Big Bang.

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This reminds me of a different thought I had; If speed is relative, why can't things go faster than light? If you and another object are moving at lightspeed relative to the earth in the same direction, you and that other object are stationary relative to each other. So, why can't you accelerate to lightspeed relative to that object?

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12 minutes ago, LHACK4142 said:

This reminds me of a different thought I had; If speed is relative, why can't things go faster than light? If you and another object are moving at lightspeed relative to the earth in the same direction, you and that other object are stationary relative to each other. So, why can't you accelerate to lightspeed relative to that object?

This is very literally a famous  thought experiment that Einstein considered. And his answer was that time itself starts to change and run at different rates.

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5 hours ago, cubinator said:

you were there. I was there. We were all there

Spoiler

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people like to think of the universe as a plane. but its really kind of a big ball of wibbly wobbly spacey timey stuff.

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Posted (edited)
9 hours ago, LHACK4142 said:

This reminds me of a different thought I had; If speed is relative, why can't things go faster than light? If you and another object are moving at lightspeed relative to the earth in the same direction, you and that other object are stationary relative to each other. So, why can't you accelerate to lightspeed relative to that object?

I'm not sure who said speed is relative, because speed isn't relative; space and time are relative. Speed is relative to the fabric of spacetime - you can't move through spacetime faster than light regardless of anything else that happens to be laying around in spacetime. That's just my understanding, though.

Edited by Bej Kerman
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56 minutes ago, Bej Kerman said:

I'm not sure who said speed is relative, because speed isn't relative; space and time are relative. Speed is relative to the fabric of spacetime - you can't move through spacetime faster than light regardless of anything else that happens to be laying around in spacetime. That's just my understanding, though.

I might have the misunderstanding here, but by the way I read your interpretation, it completely eliminates the existence of something like Cherenkov radiation, which we know to be a real thing.

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This is not my analogy but I quite like it.

Imagine a balloon. Paint a bunch of dots on it to represent stars (or galaxies - whatever). Now inflate the balloon.

The radius of the balloon represents the time axis. The surface of the balloon represents 3D space. As we move forward in time, all the stars move away from each other but they’re not moving away from any single point in space although they are moving away from a single point in time.

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If the balloon bursted, now you also have seen the Big Rip.

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39 minutes ago, kerbiloid said:

If the balloon bursted, now you also have seen the Big Rip.

Probably best not to imagine what happens if someone lets the balloon go and all the air shoots out of one end.

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2 minutes ago, KSK said:

Probably best not to imagine what happens if someone lets the balloon go and all the air shoots out of one end.

There is a scientific theory and on this case.

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Posted (edited)

Expansion is an interesting topic - and subject of fierce debate by cosmologists: https://www.science.org/content/article/recharged-debate-over-speed-expansion-universe-could-lead-new-physics

The old belief / assumption was that the expansion had to be uniform - but we keep finding 'structures' (massive, gravitationally bound clusters and filaments of galaxies that force us to change those perceptions:

If you really want to dig in, you will find that while the Cold Dark Matter theory has been successful in describing large scale structures - the model is not without its problems:

Edited by JoeSchmuckatelli

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