APOD: CMB Dipole: Speeding Through the... (2014 Jun 15)

[APOD Robot]

CMB Dipole: Speeding Through the Universe

Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the cosmic microwave background radiation (CMBR). In the above all-sky map from the COBE satellite, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

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[Beyond]

Speeding through the universe. Watch out for the galactic cops and make sure you have your radiation belts fastened!!

[bystander]

Click to play embedded YouTube video.

[Beyond]

I never realized that the Moody Blues were so astronomical.

[Ann]

Quo vadis? Where are you going? Quo vadum(?)? Where are we going?

Well, look at the CDM dipole and you'll see!

Ann

[JohnD]

That diagram is not orientated along the direction of travel.
Is it with Galactic North up, and the plane of the Solar system horizontal?

If it were redrawn along the line of travel, I would expect an ellipse with one pole indigo, the other orange, and band of green across the middle.
So what are the blobs along the system plane?
They appear to show a component of the CMB that is slower along a line a a slight angle to the line of travel.

JOhn

[mario]

It is an accepted knowledge today, that nothing can be said to move, if not
with respect to a given frame of reference, and that there is no favoured
frame of reference in the Universe, provided it is inertial.
So in the Universe nothing really can be said to be at rest.
However, this APOD seems to tell us that there might be a frame where
there is no redshift in any direction of the sky. Anything at rest with
respect to this frame could be said to be at rest with the CMBR.
If this is so, could that be THE universal frame of reference? We could
finally say whether something moves or is at rest, absolutely
(i.e. not relatively).
It would be similar to the old idea of an ether filling the space, with
respect to which the Earth moves in its orbital and galactic motion.
Only, it is not the light speed that changes depending on the
direction, but the CMBR wavelength, but the point is: there exists
one absolute frame of reference in the Universe.
Is that correct?

[hoohaw]

That diagram is not orientated along the direction of travel.
Is it with Galactic North up, and the plane of the Solar system horizontal?

It is presented in Galactic coordinates, with the North Galactic Pole at the top. The plane of the solar system is not horizontal, it is at a large angle and is irrelevant.

If it were redrawn along the line of travel, I would expect an ellipse with one pole indigo, the other orange, and band of green across the middle.
So what are the blobs along the system plane?
They appear to show a component of the CMB that is slower along a line a a slight angle to the line of travel.

The blobs are emission from sources in our own Galaxy.

JOhn

[hoohaw]

If this is so, could that be THE universal frame of reference? We could
finally say whether something moves or is at rest, absolutely
(i.e. not relatively).
It would be similar to the old idea of an ether filling the space, with
respect to which the Earth moves in its orbital and galactic motion.
Only, it is not the light speed that changes depending on the
direction, but the CMBR wavelength, but the point is: there exists
one absolute frame of reference in the Universe.
Is that correct?

Yes and no. There is no absolute frame of reference, but the cosmic microwave background comes close!

[mario]

Yes and no. There is no absolute frame of reference, but the cosmic microwave background comes close!

Can you explain, please?

[hoohaw]

Yes and no. There is no absolute frame of reference, but the cosmic microwave background comes close!

Can you explain, please?

Special Relativity taught us that there is no such thing as absolute motion. But of course there are reference frames that are convenient to consider as "absolute" even though they are not. For example, the room in which I am sitting, I treat as an absolute frame of reference in daily life - things move relative to it, but it seems absolutely stationary. Of course it is not, it is moving, even rotating ...
In exactly the same sense the microwave background is VERY convenient as a frame of reference, not just for us, but for everyone in the Universe. Pretty darn good! But Special Relativity is still true, and there is no absolute frame. Is the microwave background moving? Rotating?

[Tszabeau]

Makes me wonder if motion causes the condensation of matter and gravity or vice versa?

[Cousin Ricky]

Quo vadum(?)? Where are we going?

My best guess is "vadimus," but Latin still throws me for a loop.

[Psnarf]

Was the 600km/s speed adjusted to account for space-time expansion? My guess is that since space-time expansion is constant in all directions, it would not affect the speed relative to the CMB.

quo vadis - where are you going (present indicative singular)
quo vadimus - where are we going (present indicative plural)
speculative - not authoritative, guess-work, sans latin dictionary

[Chris Peterson]

Was the 600km/s speed adjusted to account for space-time expansion? My guess is that since space-time expansion is constant in all directions, it would not affect the speed relative to the CMB.

We tend to get in trouble when we think of the Universe expanding at some speed. That's why it's best to stick with redshift. Speed seems more like a Doppler issue, and that's not what's going on.

It is not certain that expansion is uniform. Indeed, we know it is not. There is no expansion within our galaxy, but there is far outside it. Expansion varies with scale. We cannot see most of the Universe, only the tiny piece of it we call the observable universe, that bit which hasn't moved out of our zone of causality. The cosmological model that we currently believe describes things best tells us that at the scale of the observable universe, expansion should be uniform and we should not see this sort of structure. But it's likely that our model, while broadly correct, remains incomplete.

[Chris Peterson]

In exactly the same sense the microwave background is VERY convenient as a frame of reference, not just for us, but for everyone in the Universe.

Observers in other parts of the Universe see an entirely different microwave background at their local horizon. While our theory predicts a uniform background at the scale of the CMB, it would only require subtle modifications to that theory to change that. Certainly, the CMB is at best a convenient frame of reference for us. For the rest of the Universe, it probably is not.

[JohnD]

Frame of rference = non starter.
So what are the blobs?
(see my post above)
John

[Chris Peterson]

So what are the blobs?

Well, they lie on the galactic plane, so it's likely they are local artifacts. Curious that they seem to lie in the direction we are orbiting in the galaxy, and directly opposite that.

[Guest]

Observers in other parts of the Universe see an entirely different microwave background at their local horizon.

Why? And in what is it different?

[Chris Peterson]

Observers in other parts of the Universe see an entirely different microwave background at their local horizon.

Why? And in what is it different?

They are seeing photons produced in a different part of the Universe. In broad structure, they're probably seeing something similar. In detail, however, not at all.

[mario]

Observers in other parts of the Universe see an entirely different microwave background at their local horizon.

Why? And in what is it different?

Sorry, the post above (by "Guest") was mine.

[mario]

Observers in other parts of the Universe see an entirely different microwave background at their local horizon.

Why? And in what is it different?

They are seeing photons produced in a different part of the Universe. In broad structure, they're probably seeing something similar. In detail, however, not at all.

Ok, but the effect that we are seeing here is indeed a "broad structure", not, for example the non uniformities due to changes in density at the early universe, if that is what you mean. So the frame in which there is no average redshift in one direction and blueshift in the other should still be a good frame for both.
However, I guess that two observers very far apart could both see no redshift but still be moving away from each other because of the expansion, so maybe that is why there is no absolute frame. (and maybe that is what you meant.)

[Chris Peterson]

Ok, but the effect that we are seeing here is indeed a "broad structure", not, for example the non uniformities due to changes in density at the early universe, if that is what you mean.

How do we know it is broad structure, though? Certainly, it is large scale in terms of the CMB that we locally observe. But we have no idea (outside of where the LCDM model leads us... and as good as that model is, it's certainly not complete) what the broad structure of the entire Universe looks like. Statistically, anomalies like this may occasionally show up at this scale, and we just happen to be encountering a rare example in our locally visible CMB.

Since we are likely only seeing the tiniest fraction of the entire universe, it is perhaps a bit risky to make too broad of inferences from our limited observational capacity.

[mario]

we have no idea [...] what the broad structure of the entire Universe looks like.

Well, we may not be sure of course, but I thought that the belief today, as far as the CMBR is concerned, is that the entire Universe is just filled with almost uniform black body radiation and that there is no reason to believe that one part of the Universe is different from the other. Nobody can prove that, but this is the most logical and simple hypothesis, which is always the best choice.
So my point is: if it was true (and if I understand it cannot be ruled out more than it can be proved), then an absolute reference system could be at least "defined" in a consistent way, and this would be a great change in our point of view.

[Chris Peterson]

Well, we may not be sure of course, but I thought that the belief today, as far as the CMBR is concerned, is that the entire Universe is just filled with almost uniform black body radiation and that there is no reason to believe that one part of the Universe is different from the other.

However, it is precisely the sort of data presented in today's APOD that has led cosmologists to look at the very real possibility that we don't fully understand all size scales of structure in the Universe. It is very difficult to observationally assess scales on the order of size of the observable universe. But data like this can hint at what we can't see directly.

Nobody can prove that, but this is the most logical and simple hypothesis, which is always the best choice.

Limiting our ideas to our easiest observations is not necessarily logical, and does not necessarily lead to the simplest hypothesis.

So my point is: if it was true (and if I understand it cannot be ruled out more than it can be proved), then an absolute reference system could be at least "defined" in a consistent way, and this would be a great change in our point of view.

Perhaps there is an absolute reference system for the Universe. But that certainly is not the CMB, because that is a local phenomenon, dependent upon the location of the observer. It is quite certain that the CMB I observe is different from the one you observe (although by an extremely small amount). It is certain that the CMB as observed from billions of light years away looks entirely different from the one we observe here. Whether it looks qualitatively the same is an unanswered question.