# Is there preferred reference frame for motion in the universe.

1. Aug 25, 2010

### shomas

For a long time, I have wondered if space has a preferred reference frame.
As one looks at the cosmic microwave background radiation, ask your self what would it look like if I accelerated myself in one direction. You would expect a bluer shift in one direction and redder shift in the other. Because of the difference in temperature of the cosmic background radiation, there would arise a difference in the radiation pressure, a sort of brake on movements with respect to the preferred reference frame.

If a braking effect exists, how large would the effect be? further more if it did exist, it should be quite small, but the cumulative effect over billions of years may make it pronounced, and so, should be factored into cosmic theories. On the scale of a galaxy or clusters of galaxies over billions of years, does it have the potential to do away with the need for dark matter theories? Also, when the universe was fairly young and much hotter (after photon decoupling), could it have been stronger and helped contribute to the early formation of galaxies?

2. Aug 25, 2010

### Dmitry67

Yes, there is a frame which is in rest to CMB
But it not universal, as different places won't agree on it.

3. Aug 25, 2010

### shomas

I meant a preferred reference frame with respect to motion but not location.

My bigger question is will it lead to radiation pressure that would put the brakes on motion with respect to this preferred reference frame.

4. Aug 25, 2010

### Ich

Yes, with the http://en.wikipedia.org/wiki/Greisen%E2%80%93Zatsepin%E2%80%93Kuzmin_limit" [Broken]as an extreme example.

Galaxy clusters are essentially comoving, i.e. at rest wrt the CMB. That's what Dmitry67 meant when he said that every local comoving frame is in motion wrt such frames at different positions.

Last edited by a moderator: May 4, 2017
5. Aug 25, 2010

### shomas

I Imagen a dust cloud circulating around a galaxy in motion with respect to the CMB radiation. The CMB radiation should exert a pressure difference on this cloud slowing its rotation around the galaxy.

I don't have the math to prove that, or by how much, but my gut tells me that there is a pressure and that it is not constant, but would have been greater in the past, and could have been vital in the early formations of galaxies when the pressure from hotter CMB radiation would have been higher .

My next big question is could it do away with the need for dark matter theories.

Last edited: Aug 25, 2010
6. Aug 25, 2010

### Ich

That's negligible.
No. Even if the effect were large enough, it would not mimick DM signatures like too large velocities for the observed mass distribution.

7. Aug 25, 2010

### marcus

You are correct about CMB providing a universal criterion for being at rest. At rest with respect to CMB. Some facts about the universe are most convenient to state from the standpoint of observers who are assumed to be at CMB rest. There is an approximate concept of "universe time" that goes along with this.

But this does not have the potential to do away with the need for dark matter. And anyway the concentrations of dark matter can be seen and mapped by their weak lensing effect. The various shaped dark matter clouds distort the background galaxies that are seen through them.

Talking about "doing away with the need for dark matter" has gradually become a maverick activity. Five years ago it was more in the mainstream but it has drifted out to the fringe.
I am not trying to tell you what is true to Nature, only how things have gone in the community of working cosmologists.

8. Aug 26, 2010

### shomas

I made a mistake and should have seen dark matter signature loosely stated as accelerating matter with out regular matter to account for it, while CMB radiation pressure differentials would put the breaks on motion with respect to the CMB and may end up requiring even more Dark matter to explain the differences between observation and theory.

The sun's diameter is about 1,392,000 kilometers. Our galaxy is moving with respect to the CMB in the direction of constellation Hydra at 550 km/s, Due to the Milky Way's rotation the sun's resultant velocity with respect to the CMB is about 370 km/s in the direction of Leo. The Galaxy's rotational velocity averages near 230km/s

Some questions:
All velocities are with respect the CMB
CMBRPD = Cosmic Microwave Background Radiation Pressure Differential

1. As the CMB changes temperature over time, does the same velocity generate different CMBRPD?
2. With the sun's current velocity, how many newtons would the CMBRPD exert?
3. If the sun were at a different location in the galaxy, what would be its maximum velocity with respect to CMB and corresponding CMBRPD?
4. Would CMBRPD effect solar winds and thereby indirectly effect the sun through the sun's magnetic field? (y/n)

9. Aug 26, 2010

### Chalnoth

For non-relativistic matter, the CMB pressure differential can be safely ignored. The larger effect is the damping of motion from expansion.

To see how this works, consider a one-dimensional scenario. The velocity to distance relationship is:

v = Hd.

H here is the same no matter where you go in space. So a galaxy a distance d away will, on average, be receding at a velocity Hd. A galaxy at a distance 2d will be, on average, receding at twice that velocity.

But what happens if we imagine a galaxy that, just due to chance, is only a distance d away, but is receding at a velocity 2Hd?

After some time, that galaxy will have moved with respect to the other nearby galaxies. In particular, it will have moved further away from us than the other local galaxies will have. So as the universe expands, this galaxy that is moving faster (from our perspective) catches up to matter that is further away. But that matter that is further away is also moving faster, so this unusually fast receding galaxy is no longer moving as fast with respect to its own nearby galaxies. It hasn't slowed down any, it's just caught up with the expansion.

This very rapidly causes matter to slow down with respect to the expansion, until the matter falls into some gravitational potential well or other.

As Ich notes, however, the interaction with the CMB itself does become a significant effect for extremely fast-moving matter. But this isn't noticeable for things like galaxies and whatnot, because the CMB has cooled dramatically since the early universe, and these objects haven't gotten a chance to accelerate appreciably since then.

10. Aug 26, 2010

### shomas

I accept that the effect is very small for non relativistic matter, even more so as the CMB has cooled down to what it is today, but I have not seen it accounted for in any model. I could have over looked some ones work. Every one has an idea for a model.

I read on Wikipedia, the earth and sun with non relativistic velocities looses 200 joules per second through gravitational radiation. Yes rather small when compared to the total angular momentum between the sun and earth, apparently leading to a decay in the orbit by about 10^−15 meters per day or roughly the diameter of a proton.

How many Joules would the sun loose from CMB pressure differences today? Or how many joules would it have lost 4.5 billion years ago when it was first formed and the CMBR was hotter?

How much of an effect would CMBRPD have on neutral hydrogen from the moment of photon decoupling to the forming of the earliest galaxy at 750 million years after the big bang, from there to the more abundant number of galaxies only 200 million years after that. Could CMBRPD have slowed the earliest galaxy formation? It is my understanding that neutral hydrogen is not as transparent as re-ionized hydrogen after the fist star creations. The first suns re-ionizing hydrogen may have helped further reduce the already decreasing CMBRPD effect, allowing galaxies to grow faster. Understanding how much of an effect CMBRPD had may help us better understand galaxy formation during that period.

11. Aug 26, 2010

### Chalnoth

Well, the pressure is 1/3 the energy density. The energy density of a thermal photon gas is:

$$\rho =\frac{k^4}{\hbar^3 c^3}\frac{\pi^2}{15}T^4$$

So we can approximate the pressure on an object by taking the difference in the pressure in one direction versus another. With our own motion with respect to the CMB, for instance, the difference in temperature in opposite directions is 6.74mK. This would correspond to a difference in pressure of, approximately, $2 \times 10^{-12}$ pascals.

For the Earth, this would be a force of about 500N, which translates to an acceleration of $8 \times 10^{-23} m/s^2$.

This isn't exact, mind you. I didn't take into account a number of factors, but I think it's correct within an order of magnitude. In any case, this acceleration is way, way too small. After a billion years, it would account for a change in the velocity of the Earth of around $10^{-6} m/s$.

I'm being a bit sloppy here, but I think this shows rather accurately that it's completely negligible: the acceleration is around ten orders of magnitude too small to make any noticeable difference.

12. Aug 26, 2010

### shomas

At todays temperature the effect is truly very small indeed. What would the effect have been with the same velocity just after photon decoupling? Wikipedia says the universe's temperature was about 454,000K then

13. Aug 26, 2010

### Ich

You're right, that's what they write. I changed it to 3000 K.
As the effect is propotional to T^4, Chalnoth's numbers suggest some 500 bn N, and an acceleration an order of magnitude smaller than the Pioneer anomaly. Not our main concern in such a situation, I presume.
BTW, IIRC neutral Hydrogen is quite invisible for microwaves (except the 21 cm line).

14. Aug 26, 2010

### Chalnoth

I think you're off by a few orders of magnitude. Should be a bit more than $10^{12}$ larger at recombination (since the effect scales as $(1+z)^4$, and $z_{rec} = 1089$).

However, at that point, the calculation is pretty ridiculous, because there were no planets at all. There was basically nothing but a nearly-uniform gas of mostly hydrogen, some helium, and trace elements of everything else. As you mention, this gas would have been very transparent to the photons traveling around at the time, so it wouldn't be until much later, when stars started to form, that these sorts of considerations become remotely reasonable. And that started to happen probably no earlier than $z=20$ or so, at which point the effect would have been exceedingly negligible.

The intergalactic medium didn't reionize until around $z=11$, so until then this gas wouldn't really have felt the CMB much at all. But by that time, it was only around 30K, and thus nearly irrelevant.

15. Aug 26, 2010

### shomas

Chainoth and Ich suggest that the neutral hydrogen was transparent to the back ground radiation. Is the light the article was referring different then the back ground radiation, like say light from hydrogen fusion.

wikipedia also says decoupling took place over roughly 115k years, and was complete, when the universe was roughly 487k years old. I guess this means at first neutral hydrogen would from and then ionize again after a collision or two. After 115k years very few collisions would ionize a neutral hydrogen atom.

Last edited by a moderator: Apr 25, 2017
16. Aug 27, 2010

### Chalnoth

At the time, the light from the CMB would have only been about 30K, as I mention above, while the light from stars would frequently be far above 3000K in temperature, in other words above the temperature at which hydrogen becomes a plasma.

Right, so the ionization events get slower and slower until there is no more ionized gas.

Last edited by a moderator: Apr 25, 2017
17. Aug 27, 2010

### Ich

Sorry, long scale. billion->trillion. At least the Pioneer comparison should be correct.