How Fast Is the Milky Way Moving Relative to the Cosmic Background Radiation?

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    Milky way Speed
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Discussion Overview

The discussion centers on the speed of the Milky Way galaxy relative to the cosmic microwave background radiation (CMBR), specifically focusing on its motion through the universe rather than its rotational speed. Participants explore various measurements and sources related to this topic.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about the speed of the Milky Way relative to the CMBR, emphasizing that they are not interested in its rotation speed.
  • Another participant cites a measurement indicating that the Solar System travels at approximately 390 km/sec relative to the CMB, and suggests that the Milky Way's speed could be around 603 km/sec relative to the CMB, although they express disagreement with the idea that the CMBR defines a preferred frame of reference.
  • A different participant mentions a 2002 source that reports the speed of the Local Group as approximately 627 km/sec towards the constellation Crater, suggesting that the Milky Way's speed could be similar.
  • This participant also notes that part of the observed motion could be influenced by temperature fluctuations in the CMB, but asserts that this does not significantly alter the rough estimate of about 600 km/s.
  • They further clarify that the Milky Way's motion relative to the center of mass of the Local Group is small, implying that equating the Milky Way's speed with that of the Local Group is reasonable.
  • Additional references to other threads and sources are provided, including a link to an Astronomy Picture of the Day that discusses the Local Group's speed, although it does not specify the Milky Way's speed directly.

Areas of Agreement / Disagreement

Participants express varying views on the exact speed of the Milky Way relative to the CMBR, with some suggesting values around 600 km/s while others cite different measurements. There is no consensus on a definitive speed or the implications of the CMBR as a reference frame.

Contextual Notes

There are limitations in the discussion regarding the accuracy of the measurements cited, the dependence on definitions of reference frames, and the potential influence of CMB temperature fluctuations on observed velocities. These factors remain unresolved.

kingofjazz
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how quickly is the milky way relative to the cosmic backround radiation on its way through the universe ?

I mean not the rotation speed


thank you
 
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We had a very simlair question, so I'll quote Garth (even though I strongly disagree with his conclusion the CMBR could provide a frame that is 'prefered' by the laws of physics, as there is no reason to think this is so) as I know that the 390 km/sec for the solar system is approx. correct:

When the Earth's velocity around the Sun has been taken into account the Solar System is traveling at 390 +- 60 km/sec relative to the surface of last emission of the CMB. However when the Sun's motion around the Galaxy is also taken into account this translates into the fact that the Galaxy is traveling relative to the surface of last emission of the CMB, which probably defines the C.M. reference frame of the universe, at 603 km/sec or about 0.2%c! (Nature, Vol 270, 3 Nov 1977, pg 9)
 
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kingofjazz said:
how quickly is the milky way relative to the cosmic backround radiation on its way through the universe ?

I mean not the rotation speed


thank you
As you are new to PF, you may not be aware that the administrators here discourage multiple posting of the same post.

I think jcsd has given a good response, but I would ask the Mentors to move his response to the other thread (or move mine to this), and delete the redundant koj post.
 
kingofjazz said:
how quickly is the milky way relative to the cosmic backround radiation on its way through the universe ?

I mean not the rotation speed


thank you

hello king, you should have asked the direction as well :smile:
I have a fairly recent (2002) online source that may help.
http://arxiv.org/astro-ph/0210165

This source is a journal article (Phys. Rev. Series D, 2003) reporting an earlier measurment (Smoot et al) of the speed and direction of the Local Group (as usual wrt CMB)
the result IIRC is that motion is in direction of the constellation Crater
(a small faint constell in southern half of sky) or if you want a brighter marker try Corvus. And it gives the speed as 627+/-22 km/sec.
Part of this could be due, instead, to temperature fluctuation in the CMB itself but not enough to affect the rough idea of about 600 km/s.

the motion of Milky Way wrt center of mass of Local Group is small. It would not be too bad an approximation just to say equate Milky's speed wrt CMB and that of the Local Group. So one can say Milky is going about 600 km/s and in the direction of constellation Crater.

(the constellation stars just mark the direction in space, the other stars in the galaxy are not getting closer to them because they are moving along with the galaxy as well)

I am posting from memory, so will have to check the online source I gave to make sure.

--------------
I see in another thread Nereid gave a link to an Astronomy Picture of the Day which gives a speed for Local Group. Nereid's APOD does not give speed of Milky, but it says speed of Local Group is about 600 km/s. Again you can assume Milky speed is about the same.
https://www.physicsforums.com/showthread.php?p=303392#post303392

Also here jcsd quotes a Garth post which cites a Nature article roughly to the same effect----about 600 km/s. However there is no link. Maybe the Nature article is not online.

what would be nice is to have an online source giving speed and direction specifically for Milky. We don't have this yet. Here is a stopgap tho:
https://www.physicsforums.com/showthread.php?p=137931&posted=1#post137931

the celestial coords given for the Local Group velocity vector are
276 degrees, -33 degrees
so it is in the south hemisphere but not as far south as people usually associate with hydra and centaurus. the "Great Attractor" is in hydra/centaurus. therefore Local Group is not plunging directly at the Great Attractor. Virgo cluster might be, I don't know for sure, but Local deviates a bit from that.
 
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