Michael Strauss in his recent book "Welcome to the Universe" with authors Tyson & Gott describes that space is NOT expanding within galaxies but rather between galaxies. So then the expansion is local not universal?

Does the presence of matter impede the expansion of space?

  • 1
    $\begingroup$ Hi doc, is this related: physics.stackexchange.com/q/2110 $\endgroup$ – user146020 Mar 8 '17 at 18:57
  • 2
    $\begingroup$ @Countto10 Hi Count. I'll have to take time to read through the answers tonight, but maybe. Looks like Soulman never assigned the 'correct' answer. A quick scan shows the answers are addressing the question in terms of balanced forces. But space I believe doesn't exert force or 'drag' matter. Expanding space competing with electromagnetic or gravitational forces? How do you quantify that? Unless I'm big time missing what it means for space to expand. $\endgroup$ – docscience Mar 8 '17 at 21:33
  • $\begingroup$ I was going to ask this, but there's a (not very well received) dupe/answer: physics.stackexchange.com/questions/172953/… At some stage , if its big enough there might be a balance between the matter in the galaxy and the cosmic expansion, but I don't know the biggest size a galaxy can get too, best of luck with it anyway. $\endgroup$ – user146020 Mar 8 '17 at 21:57
  • 1
    $\begingroup$ @Bob Bee The OP question is precisely whether there is a space expansion fighting against gravity expansion inside galaxies or not. The reported theory does tell that space is not expanding inside galaxies, which I always found weird too. Moreover, the (first) question concerns the expansion of the space itself, not the objects therein. $\endgroup$ – user130529 Mar 9 '17 at 7:51
  • 1
    $\begingroup$ @claude chuber. It is the spacetime geometry (in this case of the universe) caused by the initial conditions (Big Bang), the matter- energy of the universe, and the equations of General Relativity which are Einstein's Field Equations (which are equation for gravitation) that one gets that result. A 'consequence of gravitation' is a short cut form of saying this more complicated version.... But yes it is basically the start(Big Bang) and the evolution due to the gravitation of the matter-energy of the universe. Thanks for your kind inducement to answer. $\endgroup$ – Bob Bee Mar 12 '17 at 2:59

This answer is spurred a little by @claude chuber's comment. I appreciate his clarifications of my comments, and his questioning of them as well.

As he states: the OPs statement from a book, 'space is not expanding within galaxies but rather within galaxies' "is wrong"

First, a description and explanation of what happens to the cosmological (what the OP calls space) expansion. The simplest description is that yes, there is cosmological expansion everywhere, inside galaxies, in between them in a cluster of galaxies, inside planets and the solar system, inside atoms and molecules, etc. But the expansion is a gravitational effect, and it can be counteracted by other forces - such as the gravitation due to the other bodies near them, electrical forces, nuclear forces, etc. And it turns out that for all those cases mentioned above (solar system, galaxies, clusters, atoms, planets, etc), except the forces between galaxy clusters, those other forces are stronger than the gravitational effect that causes the cosmological expansion. So, the cosmological expansion in those cases is so small in comparison as to be negligible, or if not negligible, makes a very small contributions. For expansion between galaxy clusters and any larger sizes the other forces have little effect, and the expansion proceeds and is measurable.

The reason why the expansion is more noticeable and effective for longer distances is that the expansion rate (i.e., velocity of recession between two objects at these large distances) increases with distance. That is what Hubble discovered, recession velocity is linearly proportional to distance. Remember that in the cosmological expansion everything is going away from everything else (imagine any two dots painted on the surface of an inflating balloon, as the ballon gets inflated they separate more and more), and the further away they are the faster they separate even more.

But two atoms, or two planets in the solar system, or two stars in a galaxy are not far enough from each other, and the expansion would be small. But it is even worse: their electrical and magnetic interactions (for atoms) and gravitational attractions (for the solar system, planets and galaxies) with each other are strong enough that the weak inflation effect is counteracted. They are called bound systems, and inflation does not affect them. So, for instance, for our galaxy, the Milky Way, and the Andromeda Galaxy, are about 1 Mpsec (about 3 light years)(number is approximate, could be off some) apart. Based on Hubble's constant of 67-70 Kms/sec/Mpsec they should receding from each other at about 67-70 Kms/sec. But what is measured is that they are going towards each other at a speed of about 119 Kms/sec. Their massive gravitational attraction is overwhelming the cosmological expansion at those distances. We will collide with the Andromeda Galaxy in about 3 billion years.

The gravitational effect at any small region of spacetime is the combination of many effects (for many cases one can treat one as a perturbation on the main effect, but if they are all about the same strength one has to treat them all together and it is more than we can do now. Now we treat them as perturbations on each other). The nearby objects have a large effect relative to each other, but on a large scale, call it cosmological scales, that whole group of galaxies that form our local cluster, are receding away from everything else due to the expansion. It is on scales of maybe 10 Mpsec or so, and certainly at the 30-50 Mpsec range, where the inflation takes over and is measurable. The linear relation of speed of recession to distance is good up to distances of a few billion light years, then some nonlinearities enter in.

So, yes, there is cosmological expansion at all levels, but it is very small and overwhelmed by local forces at scales less than about 10-30 Mpsec.

It is also worthwhile noting that the cosmological expansion is true, exactly as estimated and modeled, ON THE AVERAGE. There are irregularities at smaller scales, where the matter and energy density is higher, and other areas where it is less. At the 100 Mpsec scale the universe looks homogeneous and isotropic. But those over densities and under densities, make local groups of stars and galaxies have some peculiar velocities with respect to the overall expansion. From the earth, when we subtract our overall peculiar velocity in our measurements, we see, for instance, the cosmic background background (CMB), as homogeneous and isotropic (except for the remnants of the Planckian quantum super-microscopic perturbations right after the Big Bang that led later, as the universe evolved, to the galaxies and stars) - we've measured that on the CMB and it is very consistent with the predictions of the cosmological model.

THe cosmological model acceptEd today is the Lambda CDM model, with parameters best taken form the latest Planck satellite/collaboration data release.

See for references

Universe chronology https://en.m.wikipedia.org/wiki/Chronology_of_the_universe

Universe expansion, and small scale effects https://en.m.wikipedia.org/wiki/Metric_expansion_of_space

The Hubble law and constant. See the numbers from Wikipedia, at https://en.m.wikipedia.org/wiki/Hubble's_law, mostly 67 to 72.

See the numbers from the Planck data release, it shows less variation when averaged with various measurements. See it at https://en.m.wikipedia.org/wiki/Planck_(spacecraft)#2015_data_release

  • $\begingroup$ " those other forces are stronger than the gravitational effect that causes the cosmological expansion" wait, gravitational effect causes cosmological expansion? $\endgroup$ – Mockingbird Mar 11 '17 at 6:50
  • 2
    $\begingroup$ Gravitation and the matter energy in the universe define the spacetime geometry of the universe. That geometry is from the solution to the Einstein Field Equations (EFE). The solution for the universe is that it is expanding, with a Big Bang starting it all. Those are EFE ofor gravitation, also called General Relativity. It's a way of saying all of this when one says expansion is due to General Relativity, or gravitation, or the Big Bang and the matter energy content of the universe. That is how gravitation and matter energy is responsible for the expansion and its parameters. $\endgroup$ – Bob Bee Mar 12 '17 at 2:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.