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How did scientists come to the conclusion that it is space that is expanding and not galaxies moving away from each other as in a giant explosion?

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  • $\begingroup$ What is (or do you think is) the difference between these two alternatives? $\endgroup$ – Walter Jun 20 '17 at 10:53
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    $\begingroup$ What's with the -1? $\endgroup$ – pela Jun 20 '17 at 11:32
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The simplest explanation relies on the statistical unlikelihood of us being at a special place and time in the cosmos. Let's say, for a moment, that we see galaxies moving away from us because of some cosmic explosion that set them in motion. In that case, we would expect to see the galaxies in one direction moving faster than the galaxies in another. Looking towards the center of the explosion, we would see galaxies on the other side moving much faster than if we looked in the other direction.

This, however, is not what we see. What we observe is all galaxies moving away from us, with a speed proportional to their distance from us. That means that the further away they are, the faster they are moving. It doesn't matter what direction we look in; a galaxy 500 Mpc away in one direction will have a recessional velocity approximately the same as one 500 Mpc away in the other direction.

This forces us into one of two conclusions: either we are at the exact center of the universe, which is statistically impossible, or space itself is expanding, so everything is moving away from everything else. The animation below shows how this would work. From any given point in space, it would look like everything is getting further away from you. An animation of the expansion of space.

Explaining the recession of galaxies due to an explosion also raises many questions: What caused the explosion? Why would it start at one particular point? What were things like before the explosion? All of these are questions which do not have accepted answers. Instead, the expansion of space ties directly into the idea of the Big Bang. The expansion of space itself is what drove the Big Bang. It is not that the universe is expanding into something. Instead, space itself is expanding. However, the topic of the Big Bang is a different subject, so you can look up information on that yourself.

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  • $\begingroup$ If galaxies moved away from a point at all possible velocities, any observer (who is not in one of the fastest galaxies) would actually see the same: faster galaxies would be seen as moving away from the point of explosion, and slower galaxies would be seen to be moving toward that point, i.e. also away from the observer. Similarly, galaxies with the same speed, but moving in slightly different directions, would be seen to move away from the observer. $\endgroup$ – pela Jun 20 '17 at 9:53
  • $\begingroup$ @pela Yes, but if you weren't at the center, you'd be able to see that. Only at the center would all galaxies have only radial motion. Observations anywhere else would be noticeably different. I'd have to think about exactly how, but I'm pretty sure you could work out your distance from the center of the explosion in such a universe by observing the radial velocities of the galaxies around you. $\endgroup$ – zephyr Jun 20 '17 at 14:34
  • $\begingroup$ @zephyr: No, as I attempt to explain in my answer, the velocity field would look isotropic from any galaxy (except if you're at the "edge" of the cloud of matter), for much the same reason as the velocity field in GR looks the same from the, say, the Milky Way and the Somrero Galaxy. You could in principle use my drawing to explain the velocity field in GR if you replace the red star with the MW and the small green circle with the Somrero Galaxy. $\endgroup$ – pela Jun 20 '17 at 18:07
  • $\begingroup$ In other words, this answer is not correct. In particular, this sentence is wrong: "Let's say, for a moment, that we see galaxies moving away from us because of some cosmic explosion that set them in motion. In that case, we would expect to see the galaxies in one direction moving faster than the galaxies in another. Looking towards the center of the explosion, we would see galaxies on the other side moving much faster than if we looked in the other direction". You will see if you do the transformation. $\endgroup$ – pela Jun 20 '17 at 18:09
  • $\begingroup$ Thought experiment: Suppose that the universe was created by an explosion like a giant supernova (or say that by some unknown science a super/ super/ super massive black hole explodes at a critical mass and sends matter spewing in all directions). We would be in the "shell" created by that explosion. What would you see looking into space from Earth in all directions? Not a shell? Are you saying that the explosion has lasted for 13.7 billion years (and is still exploding)? What would you see if that was the case? $\endgroup$ – Jack R. Woods Jun 28 '17 at 4:36
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This excellent question continues to confuse laymen as well as professional astronomers. In the standard, general relativistic (GR), cosmological model, the Universe is described as expanding, with galaxies lying approximately still in space and being "dragged along". Thus, every observer sees the other galaxies receding. The Universe started out in an $\sim$infinitely dense state — either finite or infinite in size — and has been expanding ever since, not into something, but "by itself".

Special relativistic model

A more intuitive, "explosion-like", description of the recession of galaxies would in principle be possible: In this model, Big Bang happened at a specific point in an otherwise empty space. Particles traveled outwards from this point at all possible velocities less than $c$ (left panel in the figure below). The slow particles are still close to this special point; the faster ones are farther away, and the fastest ones — traveling at almost $v=c$ — comprise a "surface" of the cloud of debris. In this model, the Universe is not isotropic. But to any observer who is not at the edge of this cloud of debris, it would look isotropic, at least for a sufficiently small region of the cloud, and the observer would still see galaxies recede at velocities proportional to their distance from him/her. All it requires is that the observer's observable Universe is sufficiently small.

Thus, the answer provided by Phiteros is unfortunately wrong, and is not a proof that the SR model is wrong.

expansion

Time dilation

The time dilation of various "standard clock" processes in distant galaxies (e.g. the time it takes for a supernova to fade away) has been proposed as a way to distinguish the GR model from the SR model. However, it turns out that the time dilation factor in the same in the two models, namely $1+z$, where $z$ is the redshift of the galaxy. So this doesn't work either. It does, however, rule out models that don't predict any time dilation at all, such as the "tired light" hypothesis.

Supernova magnitudes

The best "proof" (in quotes because nothing is ever really proven in physics, only verified or falsified) of GR is given in Davis & Lineweaver (2004), I think. Because of the mechanism that makes supernovae type Ia explode, these objects are thought to reach a known brightness at their peak luminosity, and can thus be used as "standard candles". Plotting their magnitudes (in the $B$ band, taken from Perlmutter et al. 1999) $m_B$ vs. their redshift, the result can be compared to the predictions of various cosmological models. In the standard GR model, the result depends on the history of the expansion, which in turn depends on the densities of the various constituents of the Universe (baryons, dark matter, dark energy, radiation). This is the reason that SN1a can be used to constrain the densities of the components. In the special relativistic (SR) explosion model, however, the result only depends on the velocity of the emitter at the time of emission, and the velocity of the observer at the time of observation. Whatever happened in between doesn't matter (in this models nothing special happened in between$^\dagger$).

The result is shown in the figure below for various GR models (thin black lines) and for the SR model (thick black line). The SR model is ruled out at $23\sigma$.

Mz relations

Figure 5. from Davis & Lineweaver (2004).


$^\dagger$As the authors note, "1) SR could be manipulated to give an evolving Hubble’s constant, and 2) SR could be manipulated to give a non-trivial relationship between luminosity distance, $d_L$, and proper distance, $d$. However, it is not clear how one would justify these ad hoc corrections".

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  • $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – zephyr Jun 20 '17 at 20:40
  • $\begingroup$ That's an interesting plot you've got showing the isotropic field in the different frames. Was there a source for this or did you produce it yourself? $\endgroup$ – zephyr Jun 21 '17 at 17:26
  • $\begingroup$ I produced it myself. The left one shows a velocity field where the velocity is proportional to the distance from the center. In the right one I subtracted the velocity of the black dot from all velocities. $\endgroup$ – pela Jun 22 '17 at 7:44
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Phiteros' answer is spot on. We believe it is space that is expanding since the alternative has a near zero chance of being true. But I wanted to provide another, related piece of the puzzle $-$ The Cosmological Principle. This is notion that

the spatial distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale.

What this means is that no matter where you are in the Universe, be it on Earth or in a galaxy far far away, the universe will appear to look the same to you in every direction you look. If this principle is true, it implies something very important about our universe: There is no "special" location in the Universe which distinguishes it from the rest of the Universe.

If the universe was expanding from a single point due to an explosion, that point of expansion would be the center of the universe and thus hold a special significance. If you were at that point, you'd know it. Any observations at that point would clearly look different than if you were anywhere else in the Universe. If the Cosmological Principle is true, that point can't exist.

If someone really spends the time to think about it, this isn't a great reason as to why we know space itself is expanding (Phiteros' answer is the great reason). What I'm basically saying is, I'm making up a principle that says there can't be a center of the universe and if that principle is true, the center of the universe can't exist. It's a little tautological, but there is good evidence that the Cosmological Principle is true. We've performed large scale observations of the distribution of galaxies and matter throughout our universe and the evidence shows that on large scales, the universe does indeed appear to be homogeneous and isotropic.

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  • $\begingroup$ The cosmological principle is rather like some guy who lives in a forest claiming every body else does. IMHO you shouldn't use a "principle" as some kind of substitute for hard scientific evidence. For all we know some guy on some planet 50 billion light years away is looking up wondering why half the night sky is black. $\endgroup$ – John Duffield Jun 20 '17 at 16:49
  • $\begingroup$ @JohnDuffield I tried to make your point clear in my question by saying it's somewhat tautological. I was mostly implying that this is another vantage point that gives credence to the expansion interpretation while pointing out that the other answer is real evidence. FYI, there is not some guy on some planet 50 billion light years away wondering why half the night sky is black because we'd be able to see that from our vantage point. From our observations, the visible universe is homogeneous and isotropic everywhere. $\endgroup$ – zephyr Jun 20 '17 at 17:50
  • $\begingroup$ The diameter of the observable universe is thought to be 93 billion light years, so the radius is circa 46 billion light years! :) $\endgroup$ – John Duffield Jun 24 '17 at 16:14

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