Orbits, Universe Expansion, Center ```Name: Marc Status: other Grade: other Location: CT Country: N/A Date: 10/16/2005 ``` Question: It has always been said that the Galaxies are moving away from us, and that space is "stretching". But is that really the case? From what I know, mass floating in space always tends to orbit something due to gravitational pull. Starting on a small scale, Moons and other Satellites orbit Planets. Planetary Systems orbit the sun. The Solar Systems that make up the Galaxies appear to move in a circular motion as if orbiting a central mass. Could it be that the Galaxies (including ours) are orbiting a central mass? Possibly the center of the universe? Is it possible that Galaxies are not moving away from us, but merely traveling along their orbits? Replies: On smaller local scales, various celestial bodies can/do orbit as you describe. However, on the inter-galactic scale this is not the case. No matter in what direction one observes galaxies, they are moving away from us. We know this because of the Doppler shift (also called the "red shift"). When one measures the wavelength of light (no matter what wavelength) the wavelengths are shifted to longer wavelengths compared to the wavelength one measures for the same emissions in a laboratory on Earth. If all galaxies were rotating about a single central point of reference one would expect to see some galaxies moving away and some galaxies moving toward us in a relative sense. So roughly speaking half the galaxies should be "red" shifted and half should be "blue" shifted. In addition, the "center of the Universe" model could not explain the (now well established) cosmic background radiation. This is not to say that the motions of galaxies is a "solved" problem. There are definitely unanswered questions. For example, if you calculate the orbit of stars in a galaxy, you would "expect" that stars further away from the center should be moving slower than those near the center, because the force of gravity is less (It falls of like 1/R^2). However, that is not the case. The distant stars move at the same speed as nearer stars, so that the galaxy is behaving like "a rigid body". But no one has been able to find the "stuff" that would make this motion follow from the laws of physics. Hence the introduction of "dark matter" and "dark energy". These topics are too long to take up in a format such as NEWTON BBS but there are any number of books at any level of sophistication you desire that discuss these unanswered unobserved quantities. Vince Calder Marc, First of all, we need to understand how we determine that galaxies are in motion relative to us. The main indicator -as I am sure you already know- is the Doppler Effect on light. Thus, objects that are moving away from us are red-shifted, and objects moving toward us are blue-shifted. Second, not all galaxies are red-shifted. Some members of the Milky Way's local group (the Magellanic Clouds, etc.) are blue-shifted because, as you noted, the local gravity is affecting them and causing them to move around each other. Third, while it is true that gravity is a very long-ranged force, it is also the weakest. If some earlier effect (such as the Big Bang) caused an expansion, then galaxies could be moving away from us at a decreasing rate. If there is some other force (it is being theorized that vacuum itself has an expansive nature) then that force could easily overpower gravity. Thus, only members of our local group are affected strongly by our local gravity. Finally, if all the galaxies are moving around a universal center, then around half the galaxies that we can see should be moving toward us while the other half should be moving away from us. The fact that practically all of the galaxies that we can see are red-shifted suggest that there is an expansion. Addendum: It is probably not a good idea to visualize the expansion of the universe as a 3D expansion into 3D space - but I will leave that for later. Greg (Roberto Gregorius) Click here to return to the Astronomy Archives

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