13 Billion Year Old Planet of the Milky Way

[Chris Peterson]

Calling the Big Bang the "center" of the space time universe opens up a whole
can of semantic worms (especially as regards the inflationary time period).

Perhaps. But I don't see that it opens any functional cans of worms. Indeed, the situation as I described it is just a restatement of well accepted cosmological theory into largely unmathematical terms.

In any event, you should know better than to talk about: "x=0, y=0, and z=0."

Why? (Actually, you've left off t=0, which can't be ignored in considering the location of the Big Bang.)

[neufer]

In any event, you should know better than to talk about: "x=0, y=0, and z=0."

Why? (Actually, you've left off t=0, which can't be ignored in considering the location of the Big Bang.)

"x=0, y=0, z=0, and t=0." makes no more sense than:

"x=0, y=0, z=0, and t=now"

which was desired answer to the original question.

[pleas]

This is good stuff. If you set t at t=13.7B years, what is the basis for saying the universe is spherical? Is that an assumption? If you set x, y, z and t all at 13.7B, does that define the surface of a sphere?

Actually I think the original question is what are the x, y and z of Earth given that t= now.

[Chris Peterson]

"x=0, y=0, z=0, and t=0." makes no more sense than:

"x=0, y=0, z=0, and t=now"

which was desired answer to the original question.

I don't see how either of those don't make sense. The origin of the Universe is very properly regarded as x=0, y=0, z=0, t=0, and x=0, y=0, z=0, t=now corresponds to some physical point in the Universe.

[Chris Peterson]

This is good stuff. If you set t at t=13.7B years, what is the basis for saying the universe is spherical? Is that an assumption? If you set x, y, z and t all at 13.7B, does that define the surface of a sphere?

It doesn't. A sphere is a 3D structure. The analogous structure in four dimensions is called a hypersphere, and may or may not represent the shape of the Universe (there are other 4D manifolds, as well). If we assume the Universe is a hypersphere, than the radius is time (which is the axis of expansion), and the surface of the hypersphere is the three spatial dimensions we observe. Only t can be set to 13.7 by; the others are spatial units, like meters.

Actually I think the original question is what are the x, y and z of Earth given that t= now.

Unknown, and presumably unknowable.

[rstevenson]

And to confuse the matter further...

Time Was Never the 4th Dimension

Rob

[Chris Peterson]

Actually I think the original question is what are the x, y and z of Earth given that t= now.

Actually, thinking about this some more, the only physical requirement is that the four axes of spacetime are orthogonal to each other. The choice of direction is arbitrary- in the same way we define Greenwich as 0° of longitude. If we discover that there is some sort of intrinsic property of the Universe analogous to spin, that might define some sort of physical axes... but until then, we can consider any point in the Universe to be x-0, y=0, z=0, t=now, with something like the center of the Earth or the center of the Sun being logical choices (amongst others).

[Beyond]

And to confuse the matter further...

Time Was Never the 4th Dimension

Rob

For what it's worth, if anything, i've always viewed time as a measurement of rate of decay. But it's good for keeping track of other things also, like boiling eggs for instance. Then i guess you could call it rate of decook.

[pleas]

If the Earth is at the center of the universe or is one of many choices for the unknowable center of the universe, would the rest of the universe be accelerating away from us in all directions at the same rate? Does the evidence show that to be the case?

[Chris Peterson]

If the Earth is at the center of the universe or is one of many choices for the unknowable center of the universe, would the rest of the universe be accelerating away from us in all directions at the same rate? Does the evidence show that to be the case?

That is what theory predicts, and is exactly what the observational evidence shows. (Not sure if by "the same rate" you mean exactly that; the speed at which we see things moving away from us increases with distance, becoming c at our local event horizon, the edge of the observable universe.)

[pleas]

Are you saying that everything more than 13.7B light years away from us is speeding away faster than the speed of light, so we cannot see it? If so, we have no idea how much of the universe is beyond that horizon, which makes me wonder how we can peg the age of the universe by that horizon.

[Chris Peterson]

Are you saying that everything more than 13.7B light years away from us is speeding away faster than the speed of light, so we cannot see it? If so, we have no idea how much of the universe is beyond that horizon, which makes me wonder how we can peg the age of the universe by that horizon.

Everything more than ~46 billion light years away from us is moving faster than c. It's greater than 13.7 billion light years because the Universe has been expanding during that time.

The age of the Universe is determined by looking at the change in velocity with distance, and extrapolating backwards. It isn't necessary to see the entire Universe to do this (or even to the edge of the observable universe). You only have to do a slope calculation, and calculate an intercept.

[pleas]

Chris, thanks so much for all your explanations. I do not have sufficient mathematical prowess to follow some of it, but I get the gist. I appreciated hearing from Neufer too, but your explanations show patience and a total command of the subject. Over and out for me.

[The Code]

Everything more than ~46 billion light years away from us is moving faster than c. It's greater than 13.7 billion light years because the Universe has been expanding during that time.

Nothing can go faster than light, As stated in the above context. It is the empty space between the two distant objects that it growing larger, faster than c .

Which brings me to ask a question... If ( Z=Speeding photon, Y=Black Hole, D= Time Discrepancy) Could the two ends of the Space/Time Discrepancy "Rope" Explain Space/time Expansion ? Z <------ T -------> Y

tc

[Chris Peterson]

Nothing can go faster than light, As stated in the above context. It is the empty space between the two distant objects that it growing larger, faster than c.

Not so. There is no constraint on things moving apart at faster than c. What you can't have is information transfer at greater than c, which we don't have in the case of parts of the Universe outside our observable universe. And you can't physically accelerate one massive body away from another and end up with a relative velocity greater than c, because that would require infinite energy. But the expansion of space can carry them apart faster than c, and those physical bodies really do have a relative velocity greater than c.

Which brings me to ask a question... If ( Z=Speeding photon, Y=Black Hole, D= Time Discrepancy) Could the two ends of the Space/Time Discrepancy "Rope" Explain Space/time Expansion ? Z <------ T -------> Y

I don't understand the question. I don't think it is framed in a way that can be answered.

[pleas]

So if we were at the center of the universe and if the maximum velocity of an object from its point of origin or from any point through which it passes is c, then nothing would be escaping from us faster than c. Therefore, because things are escaping from us faster than c, we cannot be at the center of the universe. In other words an object moving away from a point it passed through could be moving faster than c/2 and an object going in the opposite direction could be moving faster than c/2, so that the two objects would be moving away from each other faster than c, right?

As you can see, I am still fixated on the center of the universe; but for me the above is a satisfactory proof that we are not at the center of the universe, leaving the still frustrating question of how to determine where we are relative to the center of the universe. Back to unknowable answer.

[rstevenson]

I don't think anyone said we are at the center of the universe, but we are at the center of the observable universe - that portion of the universe within which nothing is moving away from us faster than the speed of light. And anyway, you shouldn't be too concerned about things moving in the usual sense. What is happening is simply that the universe itself is expanding, taking everything within it along for the ride. So there may well be galaxies within the universe whose relative speed is greater than the speed of light, but they can't see each other so no causality rules are being broken. They are each outside of the other's observable universe.

Rob

[pleas]

Should we be seeing things entering the observable universe from the unobservable universe? Is there an assumption that none of the unobservable universe is expanding toward the observable universe?

[Ann]

Should we be seeing things entering the observable universe from the unobservable universe? Is there an assumption that none of the unobservable universe is expanding toward the observable universe?

The image shows a part of the Hubble Deep Field. All the objects we can see are galaxies. They are very distant, and they are all moving away from us at very high speeds.

No distant very objects (much farther away than, say, the Virgo Cluster) that are moving toward us have ever been identified.

Ann

[Chris Peterson]

So if we were at the center of the universe and if the maximum velocity of an object from its point of origin or from any point through which it passes is c, then nothing would be escaping from us faster than c. Therefore, because things are escaping from us faster than c, we cannot be at the center of the universe. In other words an object moving away from a point it passed through could be moving faster than c/2 and an object going in the opposite direction could be moving faster than c/2, so that the two objects would be moving away from each other faster than c, right?

As you can see, I am still fixated on the center of the universe; but for me the above is a satisfactory proof that we are not at the center of the universe, leaving the still frustrating question of how to determine where we are relative to the center of the universe. Back to unknowable answer.

So I'll use the balloon analogy again, because it's so good. Instead of a 4D universe, suppose we were in a 3D universe. Spacetime would now consist of two spatial dimensions and the time dimension. This universe can be modeled as a balloon that is expanding. The center of the universe is the center of the balloon- that's where the Big Bang happened, at t=0. As the universe expands, its surface moves away from that center. Nothing on the surface (which is "now") can see the center, because it's in the past, not in a spatial direction that any 2D beings on the surface can look towards (neither can they look "outwards", which is the future). From their perspective, in any direction they can see, things are moving away from them, and the further away, the faster things are moving. At some distance, things are moving faster than c, and can no longer be observed. Depending on how you want to define "center", there is either no center at all on the surface, or any point can be considered the center. This has nothing to do with the little circles defining observable universes around each point. For those, it is clear that there is a physical center: the observer.

In our 4D universe, the actual center is at the Big Bang, in the past. That's a real point, with real coordinates, but it can't be seen; we are limited to looking around the three spatial dimensions (just like the balloon people are limited to two). And in our spatial universe, we can either say that "center" has no physical meaning, or we can say every point is the center. Either can work, depending again on exactly how we want to define "center". And as in the 3D universe of the balloon, this center has nothing to do with the center of an observable universe.

The edge of the observable Universe is defined by the point where space (and the material within it) is moving away from us at greater than c. Every point in the Universe (not just our observable bit) has its own sphere of observability because of the speed of light and expansion of space.

[Chris Peterson]

Should we be seeing things entering the observable universe from the unobservable universe? Is there an assumption that none of the unobservable universe is expanding toward the observable universe?

The Universe is expanding. No part of it is moving towards any other part. Locally, material may be moving towards other material, either by chance or by the attraction of gravity. But these velocities are small compared to the rate of universal expansion at cosmic distances.

[ErnieM]

Chris wrote:

This universe can be modeled as a balloon that is expanding. The center of the universe is the center of the balloon- that's where the Big Bang happened, at t=0. As the universe expands, its surface moves away from that center. Nothing on the surface (which is "now") can see the center, because it's in the past, not in a spatial direction that any 2D beings on the surface can look towards (neither can they look "outwards", which is the future). From their perspective, in any direction they can see, things are moving away from them, and the further away, the faster things are moving. At some distance, things are moving faster than c, and can no longer be observed. Depending on how you want to define "center", there is either no center at all on the surface, or any point can be considered the center. This has nothing to do with the little circles defining observable universes around each point. For those, it is clear that there is a physical center: the observer.

In our 4D universe, the actual center is at the Big Bang, in the past. That's a real point, with real coordinates, but it can't be seen; we are limited to looking around the three spatial dimensions (just like the balloon people are limited to two). And in our spatial universe, we can either say that "center" has no physical meaning, or we can say every point is the center. Either can work, depending again on exactly how we want to define "center". And as in the 3D universe of the balloon, this center has nothing to do with the center of an observable universe.

The edge of the observable Universe is defined by the point where space (and the material within it) is moving away from us at greater than c. Every point in the Universe (not just our observable bit) has its own sphere of observability because of the speed of light and expansion of space.

Another way is to imagine a "bubble or cavity" universe and the Big Bang is center or nucleus and all the energies and matter (dark and baryonic, visible and invisible) are "floating in space" within and not on the surface of the expanding bubble. Time is not a straight line but the period it took galaxies to develop and travel away from the center of the expanding bubble. The "film or shell that envelopes" this bubble universe is unknown, not necessarily the dark matter but just as invisible to existing sensors in addition to the old stand by - gravity. The size of this bubble universe is at least double what the microwave background radiation shows as existing sensors can not see beyond the "curtain" of the big bang radiation. In turn this bubble universe maybe also be in motion in another dimension of space and such motion or motions contributes to the "expansion" in addition to the bubble expansion due to still unknown and undetected force(increasing dark energy?) within the bubble.

There may also be other "bubble" universes far beyond what existing sensors can detect.

[rstevenson]

[speaking of the balloon analogy...] Depending on how you want to define "center", there is either no center at all on the surface, or any point can be considered the center. This has nothing to do with the little circles defining observable universes around each point.
[... and then ...]
And in our spatial universe, we can either say that "center" has no physical meaning, or we can say every point is the center. Either can work, depending again on exactly how we want to define "center". And as in the 3D universe of the balloon, this center has nothing to do with the center of an observable universe.

Hi Chris. I don't quite get this. I think I understand the first statement in each part quoted above, that any point can be defined as the center of the universe, but I don't understand when you say this point "has nothing to do with the center of an observable universe for each observer."

Rob

[Chris Peterson]

Another way is to imagine a "bubble or cavity" universe and the Big Bang is center or nucleus and all the energies and matter (dark and baryonic, visible and invisible) are "floating in space" within and not on the surface of the expanding bubble. Time is not a straight line but the period it took galaxies to develop and travel away from the center of the expanding bubble. The "film or shell that envelopes" this bubble universe is unknown, not necessarily the dark matter but just as invisible to existing sensors in addition to the old stand by - gravity. The size of this bubble universe is at least double what the microwave background radiation shows as existing sensors can not see beyond the "curtain" of the big bang radiation. In turn this bubble universe maybe also be in motion in another dimension of space and such motion or motions contributes to the "expansion" in addition to the bubble expansion due to still unknown and undetected force(increasing dark energy?) within the bubble.

You may imagine it this way if you like, but be aware that it is... imaginary. The universe you are talking about doesn't match either our theory or observation of the actual universe we live in.

[Chris Peterson]

Hi Chris. I don't quite get this. I think I understand the first statement in each part quoted above, that any point can be defined as the center of the universe, but I don't understand when you say this point "has nothing to do with the center of an observable universe for each observer."

Consider the balloon universe, since most of us can deal with three dimensions better than four.

In the balloon universe, "now" is the surface of the balloon. That's the only place you find matter or energy. Because the radius of the balloon (time) is increasing, so is its area (space). That's just the ordinary expansion of space that we observe through redshift, and which the little flatlanders living on the balloon could observe the same way. Looking at the surface of the balloon, is there any center? Not really... geometrically, any point on the surface is the same as any other. There's no geometrical property unique to any location. But in some sense, any point could be considered a center, since it would be equidistant from its antipoint on the other side of the balloon, or even from itself if you went all the way around. But on the balloon surface, there's no center of expansion; any point you pick will see the same thing: the farther away you get, the faster things are moving away, making it appear to be at the center of the spatial universe.

These sort of "centers" are completely different from the centers of observable universes. On the balloon surface, any point is bounded by a small circle (small in area compared with the area of the entire balloon surface). The radius of this circle is defined by the distance you need to get from the point in order for a distant point to be moving faster than c. That's all an observable universe is. It has no real physical meaning in terms of the structure of the entire universe, it just defines a region of causality for any given point. But any point really is at the center of its observable universe- either a circle in balloonland, or a sphere in our universe.