# How do we know for certain that space is expanding?

How do we know for certain that space is expanding?

Let's say that in the year 1950, we observe that galaxy 1 is 5 billion light years away from us and galaxy 2 is 10 billion light years away from us, putting both galaxies at a distance of 5 billion light years from each other. Then in 2013, we observe that they are now 7 billion light years away from each other so we conclude space is expanding.

We see that galaxies are move away from each other but how does that prove that space is expanding? Could this illusion of expanding space simply be due to something larger in mass pulling the farther galaxy 2 away from galaxy 1? This larger mass could be accelerating the velocity of galaxy 2 faster then it's accelerating the velocity of galaxy 1.

• On that last part, see Dark Flow and Great Attractor Commented Mar 29, 2013 at 17:58
• Re the second paragraph, there is no need to posit the existence of gravitational forces. Simple cosmological models are homogeneous and isotropic, so by symmetry any such force would have to vanish. Actually GR doesn't even describe gravity as a force. You can have tidal effects, but those vanish as well because of isotropy and homogeneity.
– user4552
Commented Mar 30, 2013 at 1:41

The expansion of the universe is not based on the observation of one or two galaxies moving away from us. Basically all stars and galaxies we have observed seem to follow the pattern known as Hubble's law, i.e. the further away they are the faster they are moving from us. Sure, for the observation of a single galaxy, there could be some unknown local effect, but it's not very likely that there would be such an explanation for every single galaxy. There are also additional proof of an expanding universe.

• Hubble observed that the speed of all objects is proportional to their distance. What does that mean? Commented Mar 29, 2013 at 13:54
• @SSpoke It means either everything is expanding uniformly as we believe, or else the Earth lies at the center of the universe, and that beyond the most distant galaxies there are invisible clumps of matter pulling everything away from us in a cosmic conspiracy to imitate the expansion of space as seen from our vantage point only.
– user10851
Commented Mar 29, 2013 at 15:14

How do we know for certain that space is expanding? The answer is that we don't. Expansion of space is just one possible verbal description of cosmological models. General relativity doesn't provide any way of unambiguously describing the velocity of object A relative to distant object B. It's purely a pedagogical issue whether to describe it one way or another. Some people, e.g., Francis et al., think it's better pedagogically to describe space as expanding, while others such as Bunn and Hogg feel the opposite way. All we can say as a matter of empirically testable fact is that the distance between A and B is increasing over time. The fact that Francis disagrees with Bunn doesn't mean that there's a scientific controversy, only a pedagogical one.

Francis et al., "Expanding Space: the Root of all Evil?," http://arxiv.org/abs/0707.0380v1

E.F. Bunn and D.W. Hogg, "The kinematic origin of the cosmological redshift," American Journal of Physics, Vol. 77, No. 8, pp. 694, August 2009, http://arxiv.org/abs/0808.1081v2

Actually, the proposal of this "expansion of universe" date back to almost a century when Friedmann tried to explain GR's prediction of an non-static universe. Then, Hubble's observation of distant stars proved it. Hubble's law gives the velocity of the expansion of universe. $v=Hd$ as told, where $H$ is his constant, which has been corrected from 72 to 69 km/s per Mpc by the Planck results. Though the value seems to be large in normal scales, it is very small. The universe, when taken as a whole (or atleast in scales of Mega parsecs) - this factor is negligible. As the distance $d$ increases, the velocity (with which it's moving away) increases. So, the farther apart the galaxy is from us, the faster it's moving. That's all...

Your assumption is very wrong. We don't say that two galaxies are moving away from each other, while we sit and observe. Instead, they're moving with respect to one-another. All celestial objects move away from you at similar velocities (based on how far or closer they are). There's no center for this expansion. The expansion is the same from wherever you look. Any location in the universe can be a center (which is difficult to assume). For instance, our Earth can be considered a center.