# The speed of our sun

1. Feb 4, 2008

### John Richard

Hi,

Is it posible for us to determine the velocity of our sun?

The Hubble constant gives a referent for the exspansion of the universe but do we have any way of knowing if the point at the centre of the big bang was, (or is), stationary?

Do we know in which direction the centre of our universe was, (or is)?

I understand that we go through a very complex series of motions here on Earth, orbits round our sun, sun orbits galaxy, galaxy orbits other galaxies and on. So I appreciate that putting this in anything other than purely circumstantial language might be a bridge too far. but what is know of this?

In an expanding universe, I can understand the doppler shift of light in one direction due to the overlay of motion causing an increase in frequency, (please correct me if I am wrong), but a reduction in frequency would suggest motion opposing expansion.

Is this considdered to be a result of relative motion due to celestial dynamics?

Any help is greatly appreciated.

John

2. Feb 4, 2008

### chroot

Staff Emeritus
The universe has no center. The big bang happened "everywhere at once," and not at all like the explosion of a bomb in space already-extant space. A two-dimensional analogy is the expansion of the surface of a balloon while it is being inflated. The surface of a balloon also does not have a center.

The universe has no concept of absolute motion, so it's meaningless to ask what the absolute motion of the Sun is. If you provide some other reference point, a reasonable estimate of velocity with respect to it can be worked out, but it will not be of much use to you, or anyone else.

The fact that (essentially) every galaxy in the universe appears to be receding away from our own is an indication that the universe is expanding. Any observer in any other galaxy would observe the same thing: all other galaxies moving away.

- Warren

3. Feb 4, 2008

### John Richard

Thank you for the reply.

I find it staggering that such missconceptions are allowed to propagate. Every single public reference to the big bang makes it seem that it started as a tiny micro-dot in one place. That it was all compressed together into one single point "in" time and space and then boom.

What it seems like you are trying to suggest is that there was no extention at all, prior to the big bang! That there was no precursor of anything at all!

No dimensions, even if void of any content, (I don't care about time). Is that what you are saying?

4. Feb 4, 2008

### chroot

Staff Emeritus
The popular press is responsible for literally hundreds of staggering misconceptions in science. I'm not going to write a list, but several dozen pop into my head after only a moment of thought.

That's correct. We currently have no idea what existed before the big bang, or what exists outside the universe. According to current understanding, these aren't even reasonable questions to ask. The answers may be physically unknowable, and may not be of any consequence for our own universe anyway.

Space itself (at least, the space that comprises our universe) began with the big bang. There was no space before the big bang, nor did the big bang expand into some existing, enclosing space. At least, that's the current model.

- Warren

5. Feb 4, 2008

### John Richard

Thanks Warren

Gob smacked

John

6. Feb 4, 2008

### Staff: Mentor

So much of that is easy to misconstrue. Ie, if "it [the universe] was all compressed together into one single point" is generally correct. It's just that that point wasn't a point at the center of some void.

7. Feb 4, 2008

### EL

Well, at least if we're talking about the observable universe.

8. Feb 4, 2008

### tony873004

I believe that the cosmic microwave background radiation is redshifted in one direction and blueshifted in another by an amount that reveals that we are moving about 600 km/s relative to something that would observe no red and blue shifts.

9. Feb 4, 2008

### chroot

Staff Emeritus
You are correct, Tony, but there's no reason to believe that reference frame, the comoving reference frame, is any more important than any other.

- Warren

10. Feb 4, 2008

### marcus

Yes, our sun has an absolute velocity with respect to what is called the "Hubble flow" and this has been determined fairly accurately by the COBE microwave background mapping satellite in the mid 1990s.

This is not what you were asking about, I suspect.

You were asking about expansionary motion. However there is no centerpoint of expansion.
We have no absolute recession velocity as part of universal expansion. Distant objects appear to recede from us as if we were the center. And from the standpoint of a distant object we would be receding. Everybody sees himself as the center, and there is no center.

this expansion is often called the "Hubble flow" and it gives a criterion for when something is STATIONARY with respect to it. If you are stationary then the recession of distant objects is symmetric, the same in all directions.

If you are moving with respect to the universal expansion, then objects ahead of you will not be receding so fast----they will seem to have a few km/second deducted from the speed they should have according to Hubble law. And objects astern of you will have a few km/second added to the recession speed that the law says they should have.

Hubble was aware of this in the 1930s, but it is a very slight thing to measure. Actually our sun is moving at about 370 km/second with respect to the universal expansion, in the direction marked in the sky by the constellation Leo.

Redshifted galaxies in that direction do not look quite as redshifted as they should, given their distance. Galaxies behind us (opposite Leo position in the sky) are slightly more redshifted. In other directions there is little or no effect.

Our sun has several velocities depending on what you take as standing still. It is circling the center of the Milkyway galaxy at a couple of hundred km/second. The galaxy is moving, our local group of galaxies is moving. But what I have told you is, in some sense, more absolute because it is 370 km/second with respect to the universe as a whole.

This speed was measured in the 1990s by the COBE team, when they found that the microwave background sky was just slightly warmer in the Leo direction (and colder in the reverse direction). they were able to pinpoint it, and to say rather precisely the solar system would have to be moving in order to cause that slight milliKelvin temperture difference.

If you want a link to the paper that they published about this in the mid 1990s let me know. I or someone will get it. If I remember the lead author was Charles Bennett.

the expansion has no center. It is a largescale average increase distances between stationary objects---objects which are essentially not moving with respect to the Hubble flow or equivalently the CMB. Ignoring small random motions, basically stationary objects. The current rate of expansion of distance is about one percent every 140 million years.

11. Feb 4, 2008

### marcus

that is right, but it is not 600 km/second

If you are talking about the sun's motion relative to CMB, the speed is about 370 km/second.

If you are talking about the Milkyway galaxy, it's speed is higher. I recall something like 500-600 km/second.

the two vectors are in different directions because the sun also has orbital motion around the center of the galaxy. there is some cancelation too.

=======================

I see Chroot has already taken care of almost all the questions. I had to do something else halfway thru and didn't get my reply posted. So now I just basically agree with Warren, except possibly on some minor details (maybe he doesnt like the 370 figure or something.)

12. Feb 4, 2008

### John Richard

Thank you Marcus.

I will search out Charles Bennett on the web but if you do come across the link that would be great!

Am I right then, in saying that the expansion is isotropic.

Can I consider the expansion as a scalar deflection of distance. (Expansion in proportion to the distances between the celestial bodies)?

John

13. Feb 4, 2008

### chroot

Staff Emeritus
Space expands everywhere at the same rate, as far as we know. The further two objects are from each other, the faster they will measure their speed of mutual recession. This is Hubble's law:

$v = H_0 d$

http://en.wikipedia.org/wiki/Hubble's_law

- Warren

14. Feb 4, 2008

### John Richard

Thank you Warren

Marcus, I have found the following pdf:

First Year Wilkinson Microwave Anisotropy Probe (WMAP1) Observations:
Preliminary Maps and Basic Results

Big file with sky maps the lot, thank you

John

15. Feb 4, 2008

### John Richard

I see from the WMAP data that CBM are isotropic and I can see how that points to a voidless origin. If the big bang had been in the manner of an explosion in space then CBM would point to that explosion.

What I am struggling to understand is why CBM don't interact with themselves.

Do we know anything of the evolution of CBM. Are there any conjectures or theories that look at changes in CBM over the life of the universe.

I think what I'm asking is, as the universe expands, is the wavelength of the CBM increasing?

I appreciate that this might be a little ambitious since I am guessing we have only really been studying it for the past 8 years. (It may also be implicit in the WMAP data in some way that I am over looking)

Any thoughts

Thank you.

16. Feb 4, 2008

### DaveC426913

I think the popular press is only partly to blame. The public take some responsibility for only caring to know as much as fits into a 20 second news clip. Things like the Big Bang and SR and GR simply can not be described in 20 seonds using every day concepts - especially without any foundation of knowledge to build on.

Too many people are too comfortable thinking about little electron moons orbiting proton planets...

17. Feb 4, 2008

### Staff: Mentor

Most of the rest didn't make much sense to me, but yes!, the CMB's wavelength increases as the universe expands. The wavelength of the CMB was predicted by a team of scientists while another team was discovering it.

18. Feb 5, 2008

### John Richard

Thanks Russ,

The problem I'm having is with understanding how an isotropic wave system manages not to interfere with itself. I think it is a question about wave dynamics that I perhaps should ask over on the physics section.

Sorry, it was a bit wishy washy.

John

19. Feb 5, 2008

### John Richard

I was just writing out a question about wave dynamics over on the general physics section when something occured to me.

Is the entire universe beating as one, so to speak. Its pulse being the frequency of the CBM's? Is the entire background of the universe in harmony so that when the upper crest of a wave is detected in orbit around Earth, we can assume the upper crest will also be detectable over at Alpha Centauri?

As well as being isotropic, do the CBM's also posses spatial symmetry?

Thanks for your help

John

20. Feb 5, 2008

### Staff: Mentor

The CMB isn't a single uniform, phase-synchronized wave, it is random noise centered around a particular frequency. It works pretty much like the light we see from the sun (the universe was at one time an opaque clump of hot matter). It's a hot object with a bell-curve shaped spectrum of light, centered around a particular frequency.