where are we in the 'universe'?

[bwanaaa]

From my reading, it seems we are at the periphery somewhere; certainly not near the center. Now when we look out into space there are some observations a little puzzling to me with regards to this position we are in.

The microwave echo of the big bang seems distributed pretty evenly everywhere. Smoot et. al. have have talked about dipoles, quadrupoles and higher order wrinkles (Actually I dont really know what these 'pole' moments represent) but wouldn't you expect this map to be distorted by the fact that we are not in the middle of the universe? Shouldn't the microwaves from the universe's center be redshifted? After all, there is more gravity there so EM radiation should redshifted. I make this statement based on the observation that gravity bends light. If gravity can bend light then it should also be able to slow light down as the light leaves a gravity well to a region of lesser gravity. This is analogous to 'gravitational lensing'- just as light travels slower in a lens, it would travel slower in a region of higher gravity. This should be evident as a redshift. The same should be true of stars in the center of the universe. They should be relatively more redshifted than stars that are just as far away from us but not in the universe center.

Also, what is the distribution of redshifting of stars compared to the microwave echo of the big bang?

Is there any data to support or refute these hypotheses?

 

[Biftheunderstudy]

First point, we are not located near the periphery. From Big Bang Cosmology and indeed observations there is no "center". So asking where we are relative to it is meaningless, it is the same as asking where the center is on the surface of a baloon. This is because if the universe started at a single point and space expanded from that then a better question is when is the center, to which the answer is about 13.7B years ago. Along this vein of logic, when we look out in any direction, the universe looks the same. The deeper we look, the older things appear because of light travel time. Looking deeper like this is the same as looking back in time to when the universe was smaller and younger.

A fundamental observation is Hubble's law, this states that the further you look into space the faster the object is moving. In almost all cases the object is moving away from us which implies that at one point everything was in the same place. We can extrapolate when this occurred and put a rough estimate on the age of the universe. There's a catch to this though, it turns out that the objects aren't actually moving but rather the intervening space is getting bigger and they are simply attached to that space and are along for the ride. This has the interesting effect of producing what is known as the cosmological redshift caused by the wavelength of light stretching from the expansion of space as it propagates.

Now we can talk about he CMB or Cosmic Microwave Background. In the early universe, photons a.k.a light was coupled to matter. That is to say, light could not travel very far before hitting something and thus the universe was opaque. As the universe expanded, light could travel further and further before hitting something until it reached a point when the universe became transparent. This is called a few different names but lets call it "the surface of last scattering". Now all of these high energy photons were free to travel the entire universe without hitting anything.
So now lets go back to Hubble's law and the cosmological redshift. If there was a time when these photons just became decoupled and before this they were completely isotropic and if looking deep into the universe is the same as looking back in time then we should be bathed in a completely isotropic sea of photons from the surface of last scattering. Now taking into account the cosmological redshift, these photons should be redshifted way into the microwave spectrum and indeed this is the CMB.

Now the dipole aspect. The CMB, in theory should be the same anywhere we look since the photons were isotropic. However there is a catch, we aren't just stuck to the expanding space. Our galaxy is part of a group of galaxies which are gravitationally interacting, thus we have motion relative the expanding space called peculiar motion. (Our current heading is towards the andromeda galaxy, on a collision course in fact.) This motion causes a redshift/blue shift in the ,regular motional sense, of the CMB photons since we are moving relative to them. Thus, in the direction we are moving the photons appear to be blue shifted and away from us they are redshifted. From this dipole term we can actually tell which direction we are moving and how fast.

 

[silverpig]

Originally posted by: Biftheunderstudy
First point, we are not located near the periphery. From Big Bang Cosmology and indeed observations there is no "center". So asking where we are relative to it is meaningless, it is the same as asking where the center is on the surface of a baloon. This is because if the universe started at a single point and space expanded from that then a better question is when is the center, to which the answer is about 13.7B years ago. Along this vein of logic, when we look out in any direction, the universe looks the same. The deeper we look, the older things appear because of light travel time. Looking deeper like this is the same as looking back in time to when the universe was smaller and younger.

A fundamental observation is Hubble's law, this states that the further you look into space the faster the object is moving. In almost all cases the object is moving away from us which implies that at one point everything was in the same place. We can extrapolate when this occurred and put a rough estimate on the age of the universe. There's a catch to this though, it turns out that the objects aren't actually moving but rather the intervening space is getting bigger and they are simply attached to that space and are along for the ride. This has the interesting effect of producing what is known as the cosmological redshift caused by the wavelength of light stretching from the expansion of space as it propagates.

Now we can talk about he CMB or Cosmic Microwave Background. In the early universe, photons a.k.a light was coupled to matter. That is to say, light could not travel very far before hitting something and thus the universe was opaque. As the universe expanded, light could travel further and further before hitting something until it reached a point when the universe became transparent. This is called a few different names but lets call it "the surface of last scattering". Now all of these high energy photons were free to travel the entire universe without hitting anything.
So now lets go back to Hubble's law and the cosmological redshift. If there was a time when these photons just became decoupled and before this they were completely isotropic and if looking deep into the universe is the same as looking back in time then we should be bathed in a completely isotropic sea of photons from the surface of last scattering. Now taking into account the cosmological redshift, these photons should be redshifted way into the microwave spectrum and indeed this is the CMB.

Now the dipole aspect. The CMB, in theory should be the same anywhere we look since the photons were isotropic. However there is a catch, we aren't just stuck to the expanding space. Our galaxy is part of a group of galaxies which are gravitationally interacting, thus we have motion relative the expanding space called peculiar motion. (Our current heading is towards the andromeda galaxy, on a collision course in fact.) This motion causes a redshift/blue shift in the ,regular motional sense, of the CMB photons since we are moving relative to them. Thus, in the direction we are moving the photons appear to be blue shifted and away from us they are redshifted. From this dipole term we can actually tell which direction we are moving and how fast.

Yup. I'll just add a few things.

1. One could say we are at the center of the observable universe as we can look in any direction and see the surface of last scattering.

2. The multipole moments the OP was asking about do certainly have meaning. The dipole is as you explained it. The purpose of the WMAP mission was to map out the anisotropy in the CMB and plot its power spectrum in terms of multipole moments. The scientists basically recorded the CMB, did a fourier transform of the data to get a power spectrum in terms of spherical harmonics, and then looked at what this told them. The power spectrum tells you what multipole moments are present in what powers to produce the CMB. These moments then translate into an angular size on the sky, which then translates into some physical size when you hit the rewind button on the universe. This physical size represents some scale of physics which was important way back when, and this can tell you the relative amounts of matter, radiation, dark matter, and dark energy in the universe.

 

[mindless1]

Universe is an arbitrary human definition. Until we find the edge of anything more tangible we have no point of reference. It would be like an ant asking where are we in Mr Smith's yard when there is no map of where Mr Smith's yard ends and no way to get there and back to tell anyone what we found. Ignorant bliss, until we try to define our artificial constructs instead of accepting they are just a concept since we don't actually define the principles upon which space and time exist.

I suppose my point might be, what does our dimensional reference to a big bang tell us about where we are? It would be like having the tire on your car blow out and that event telling you where you are.

 

[bwanaaa]

ok, i see i am using the wrong words. the center of the universe at this moment in time does not exist- the balloon analogy is clear.
(a long and entertaining 'twitterstream' about this is here:
http://blogs.discovermagazine....enter-of-the-universe/
)

like asking where is the center of the earth-that information is relatively useless (no pun intended) we map our position on the earth's surface irrespective of the center.

i should ask:

is the cmb radiation redshifted the same amount in ALL directions? as i understand the data, redshift is a function of distance between 2 points only(scalar quantity). is there a an angular component as well (vector quantity?)

does the fact that there is a 'dipole' mean that the universe is expanding more along a particular axis? in other words, the universe is shaped like a weiner (would Einstein laugh?) The ends of the weiner are further apart so light coming from the ends would be redshifted more than light from any other direction.

 

[silverpig]

Originally posted by: bwanaaa
ok, i see i am using the wrong words. the center of the universe at this moment in time does not exist- the balloon analogy is clear.
(a long and entertaining 'twitterstream' about this is here:
http://blogs.discovermagazine....enter-of-the-universe/
)

like asking where is the center of the earth-that information is relatively useless (no pun intended) we map our position on the earth's surface irrespective of the center.

i should ask:

is the cmb radiation redshifted the same amount in ALL directions? as i understand the data, redshift is a function of distance between 2 points only(scalar quantity). is there a an angular component as well (vector quantity?)

does the fact that there is a 'dipole' mean that the universe is expanding more along a particular axis? in other words, the universe is shaped like a weiner (would Einstein laugh?) The ends of the weiner are further apart so light coming from the ends would be redshifted more than light from any other direction.

The answer about the redshift is essentially yes. It is the same in all directions.

Technically it isn't. There are very very minute anisotropies due to inflated quantum fluctuations.

Back when the universe was extremely tiny and quantum mechanics was important at the size of the universe, it wasn't perfectly isotropic - there were minute quantum fluctuations. The universe then decayed a scalar potential and inflated extremely quickly. Parts of the universe expanded away from each other at much faster than the speed of light (nothing MOVED that fast, just the space expanded so quickly it seemed like it was that fast). Because these little quantum fluctuations didn't have enough time to talk to each other and re-equilibrate during the inflation these fluctuations were frozen out in the plasma phase of the universe. Slightly denser areas became galaxies and clusters. Slightly less dense areas became voids.

Aside from other minute nit-picky details, yes, the expansion is isotropic to an extremely high precision.

The dipole term is due to our proper motion through space. If you are in a large field with speakers surrounding the field that are very far away from you, and the speakers all start playing a note of "C", you'll hear "C" from all directions. If you start moving quickly in one direction and use a directional microphone you'll record a "C" to the sides of you, but the pitch in front of you will be shifted to a "C#" and the pitch behind you will be shifted to a "B".

 

[bwanaaa]

so how do we know that the dipole is due to our motion through space and not the anisotropic expansion of space? the only way to know this would be to make measurements from two places that are far apart (one measurement taken within our galaxy and another from outside).

 

[silverpig]

Originally posted by: bwanaaa
so how do we know that the dipole is due to our motion through space and not the anisotropic expansion of space? the only way to know this would be to make measurements from two places that are far apart (one measurement taken within our galaxy and another from outside).

It's opposite in sign in June and December (when the Earth is moving in an opposite direction through space).

 

[bwanaaa]

so am i to understand that the dipole is due to our revolution around the sun affecting the net perceived velocity of us in the universe? We are in a solar system and galaxy cruising through the universe each with their own motions. I assume that galactic motion is the fastest so the dipole is principally due to the vector sum of our solar orbital velocity and that of the milky way blasting through the universe.? Shouldn't there be a 'blueshift' and a 'redshift'. Is that what you mean by the 'opposite sign'?

 

[silverpig]

Originally posted by: bwanaaa
so am i to understand that the dipole is due to our revolution around the sun affecting the net perceived velocity of us in the universe? We are in a solar system and galaxy cruising through the universe each with their own motions. I assume that galactic motion is the fastest so the dipole is principally due to the vector sum of our solar orbital velocity and that of the milky way blasting through the universe.? Shouldn't there be a 'blueshift' and a 'redshift'. Is that what you mean by the 'opposite sign'?

Well, what is meant by dipole is one half of the sky is blueshifted, the other half is redshifted.

And yes there is some permanent dipole associated with our galactic motion, and another one associated with our orbit around the galactic core, and another one associated with our movement around the sun etc...

You can just subtract all those out though.

The dipole is the second pair of images.

 

[bwanaaa]

tnx for that "AHA" moment.

 

[silverpig]

Originally posted by: bwanaaa
tnx for that "AHA" moment.

np

<--- astrophysics degree

I took a cosmology course taught by a guy who wrote the article in the particle data handbook about the CMB, and one of the scientists for WMAP taught me some physics as well.

 

[sao123]

im still a little fuzzy on the baloon thing..

If space (ignoring times briefly) is 3 dimentional, has finite length, finite width, and finite height... therefore it must have volume.

we may not be able to observe it, or know its dimentions
however, provided that you are not on the surface of the edge of the universe, you should be able to travel in a straight line and if you go far enough, you will come to a rigid boundary...

correct?

 

[silverpig]

Originally posted by: sao123
im still a little fuzzy on the baloon thing..

If space (ignoring times briefly) is 3 dimentional, has finite length, finite width, and finite height... therefore it must have volume.

we may not be able to observe it, or know its dimentions
however, provided that you are not on the surface of the edge of the universe, you should be able to travel in a straight line and if you go far enough, you will come to a rigid boundary...

correct?

No one says it is finite.

And you may never come to a boundary. If you start in Seattle and head east, you'll eventually end up in Seattle again.

 

[sao123]

Originally posted by: silverpig

Originally posted by: sao123
im still a little fuzzy on the baloon thing..

If space (ignoring times briefly) is 3 dimentional, has finite length, finite width, and finite height... therefore it must have volume.

we may not be able to observe it, or know its dimentions
however, provided that you are not on the surface of the edge of the universe, you should be able to travel in a straight line and if you go far enough, you will come to a rigid boundary...

correct?

No one says it is finite.

And you may never come to a boundary. If you start in Seattle and head east, you'll eventually end up in Seattle again.

the earth is not a hollow volumous sphere, nor is the universe the outer surface of a sphere like the earth.

If I start at seattle and dig down far enough, eventually I will come out through the other side. (ignoring the crushing/melting physics in the core)... since the earth is not sitting on the surface of the universe if I travel in a striaght line from it, I will have to come to a boundary.

 

[f95toli]

Originally posted by: sao123
the earth is not a hollow volumous sphere, nor is the universe the outer surface of a sphere like the earth.

It very well might have the shape of a sphere, but a HYPERsphere; i.e a sphere in four dimensions.
Remember that a hypersphere is to a sphere what an sphere is to a circle; i.e. the same basic "shape" but in +1 dimension.
It is this extra dimension that might mean that one could end up a the starting point if traveling far enough in one direction.

Also, remember that we KNOW that space isn't "flat" (meaing "flat" in 3D) ; there are plenty of nice demonstrations of the curvature of space-time; the most obvious example being gravitational lensing. Hence, the curvature isn't just some far-fetched esoteric idea; it is an observable fact.

 

[silverpig]

Originally posted by: sao123

Originally posted by: silverpig

Originally posted by: sao123
im still a little fuzzy on the baloon thing..

If space (ignoring times briefly) is 3 dimentional, has finite length, finite width, and finite height... therefore it must have volume.

we may not be able to observe it, or know its dimentions
however, provided that you are not on the surface of the edge of the universe, you should be able to travel in a straight line and if you go far enough, you will come to a rigid boundary...

correct?

No one says it is finite.

And you may never come to a boundary. If you start in Seattle and head east, you'll eventually end up in Seattle again.

the earth is not a hollow volumous sphere, nor is the universe the outer surface of a sphere like the earth.

If I start at seattle and dig down far enough, eventually I will come out through the other side. (ignoring the crushing/melting physics in the core)... since the earth is not sitting on the surface of the universe if I travel in a striaght line from it, I will have to come to a boundary.

f95toli pretty much answered this but yes, our 3d space can be considered a "surface" in a higher dimensional space. We are stuck to this surface and can travel in a straight line in our world, but travel a curved path around a higher dimensional object, ending back up where we start.

We are stuck to this 3d surface much like a person is stuck to the 2d surface of the earth. Your suggestion of tunneling through the earth is akin to telling someone to use some kind of wormhole to travel from here to the other side of the universe.

 

[Atheus]

Stick a pin in your desk. Write YOU ARE HERE beneath it. These are words of the Buddha.

 

[silverpig]

Originally posted by: Atheus
Stick a pin in your desk. Write YOU ARE HERE beneath it. These are words of the Buddha.

What if the words said: "YOU ARE NOT HERE"?

 

[WildHorse]

Originally posted by: silverpig

Originally posted by: Atheus
Stick a pin in your desk. Write YOU ARE HERE beneath it. These are words of the Buddha.

What if the words said: "YOU ARE NOT HERE"?

wherever you go

there you are


A little something to calm you

 

[Eureka]

Its like those video games, like asteroid. You travel to one end of the screen and come out the other.

 

[sao123]

Originally posted by: silverpig

Originally posted by: sao123

Originally posted by: silverpig

Originally posted by: sao123
im still a little fuzzy on the baloon thing..

If space (ignoring times briefly) is 3 dimentional, has finite length, finite width, and finite height... therefore it must have volume.

we may not be able to observe it, or know its dimentions
however, provided that you are not on the surface of the edge of the universe, you should be able to travel in a straight line and if you go far enough, you will come to a rigid boundary...

correct?

No one says it is finite.

And you may never come to a boundary. If you start in Seattle and head east, you'll eventually end up in Seattle again.

the earth is not a hollow volumous sphere, nor is the universe the outer surface of a sphere like the earth.

If I start at seattle and dig down far enough, eventually I will come out through the other side. (ignoring the crushing/melting physics in the core)... since the earth is not sitting on the surface of the universe if I travel in a striaght line from it, I will have to come to a boundary.

f95toli pretty much answered this but yes, our 3d space can be considered a "surface" in a higher dimensional space. We are stuck to this surface and can travel in a straight line in our world, but travel a curved path around a higher dimensional object, ending back up where we start.

We are stuck to this 3d surface much like a person is stuck to the 2d surface of the earth. Your suggestion of tunneling through the earth is akin to telling someone to use some kind of wormhole to travel from here to the other side of the universe.

and if I move in a straight line in the 4th dimention (time) - IE by traveling at normal speeds, then what?

Im not even sure I understand/agree with seeing back in time with a telescope such as hubble.
i do recognize that it takes time for light to travel... but

Light is always traveling. Some light moves toward us, some away. if you consider the *location* of the singularity event, with respect to our position now, at some point, that light will have past us... and we can no longer see it. So, if we are trying to look to the edge of the universe to see the beginning, that light will be traveling away from us, and we will never see it. so we may be able to see some events in the past, but there has to be a theoretical limit as to what we will ever be able to see, which seems to be less and less as time progresses becasue light which passes us is *lost*.

 

[silverpig]

Originally posted by: sao123

Originally posted by: silverpig

Originally posted by: sao123

Originally posted by: silverpig

Originally posted by: sao123
im still a little fuzzy on the baloon thing..

If space (ignoring times briefly) is 3 dimentional, has finite length, finite width, and finite height... therefore it must have volume.

we may not be able to observe it, or know its dimentions
however, provided that you are not on the surface of the edge of the universe, you should be able to travel in a straight line and if you go far enough, you will come to a rigid boundary...

correct?

No one says it is finite.

And you may never come to a boundary. If you start in Seattle and head east, you'll eventually end up in Seattle again.

the earth is not a hollow volumous sphere, nor is the universe the outer surface of a sphere like the earth.

If I start at seattle and dig down far enough, eventually I will come out through the other side. (ignoring the crushing/melting physics in the core)... since the earth is not sitting on the surface of the universe if I travel in a striaght line from it, I will have to come to a boundary.

f95toli pretty much answered this but yes, our 3d space can be considered a "surface" in a higher dimensional space. We are stuck to this surface and can travel in a straight line in our world, but travel a curved path around a higher dimensional object, ending back up where we start.

We are stuck to this 3d surface much like a person is stuck to the 2d surface of the earth. Your suggestion of tunneling through the earth is akin to telling someone to use some kind of wormhole to travel from here to the other side of the universe.

and if I move in a straight line in the 4th dimention (time) - IE by traveling at normal speeds, then what?

Im not even sure I understand/agree with seeing back in time with a telescope such as hubble.
i do recognize that it takes time for light to travel... but

Light is always traveling. Some light moves toward us, some away. if you consider the *location* of the singularity event, with respect to our position now, at some point, that light will have past us... and we can no longer see it. So, if we are trying to look to the edge of the universe to see the beginning, that light will be traveling away from us, and we will never see it. so we may be able to see some events in the past, but there has to be a theoretical limit as to what we will ever be able to see, which seems to be less and less as time progresses becasue light which passes us is *lost*.

Okay a few things:

First we can describe a 4th dimension of space which is not time. A lot of people say "the 4th dimension is time" and they're kind of right, but it's a matter of semantics as to whether you call the 4th dimension time or not. Most physicists I know say we live in 3+1 dimensional spacetime; 3 of space, 1 of time. When you talk about a 4th spatial dimension you're talking about some direction orthogonal to x,y,z in space and not time.

As to seeing back in time, well it definitely is true. When you look at the wall in your room you see the wall as it was 1/100000000 of a second ago as that is how long the light took to reach you. You see the sun as it was 8 minutes ago, not as it is. You really are looking back in time. If you look far enough away, you're seeing stars as they were hundreds of thousands of years ago - galaxies are millions of years older than they appear. Looking back to the cosmic microwave background, that light has taken 13 billion years or so to get to us.

Sure, light passes us, but because the universe is seemingly infinite there is always more light passing us currently from some farther place away.

 

[bsobel]

Im not even sure I understand/agree with seeing back in time with a telescope such as hubble. i do recognize that it takes time for light to travel... but

Disturbing.

Light is always traveling. Some light moves toward us, some away. if you consider the *location* of the singularity event, with respect to our position now, at some point, that light will have past us... and we can no longer see it. So, if we are trying to look to the edge of the universe to see the beginning, that light will be traveling away from us, and we will never see it. so we may be able to see some events in the past, but there has to be a theoretical limit as to what we will ever be able to see, which seems to be less and less as time progresses becasue light which passes us is *lost*.

Your still stuck on the idea that their was a 'location' of the big bang in respect to our current position. You can't think that way, as that implies that space existed prior to the big bang.