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[I've read, among others, other questions on this, don't believe this is a duplicate]

I (think I) understand the evidence that distant galaxies are moving away from us faster than closer galaxies. What I don't understand is why this even calls for an explanation..

Take a naive picture, where the big bang was an explosion of matter within space. Matter began spreading with some distribution of velocities, in all directions. Now we observe the universe billions of years later - it seems only natural that the galaxies who traveled the furthest during this time are exactly those with largest initial velocities.

Isn't the same true if we consider the big bang to be a dilation of space itself? The parts of space that 'are dilating' the most (not only them, but the entire portion of space between them and us), must be those that accumulated the most distance from us in the time since the bang.

This naive interpretation doesn't call for 'acceleration' of any kind. What am I missing?

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  • $\begingroup$ In uniform (i.e., unaccelerated) expansion, the expansion speed of 2 points is proportional to the distance between them. Eg, if 2 galaxies 1 Gpc (1 gigaparsec) apart recede from one another with a speed of v then 2 galaxies 10 Gpc apart recede at 10v. See en.wikipedia.org/wiki/Hubble%27s_law#Idealized_Hubble's_law $\endgroup$
    – PM 2Ring
    Commented Feb 16, 2020 at 12:42
  • $\begingroup$ @PM2Ring The idealized Hubble law starts from a premise of 'Any two points which are moving away from the origin, each along straight lines and with speed proportional to distance from the origin', and states that there's nothing special about the origin. What I'm asking is - isn't this seemingly weird premise, actually very natural. $\endgroup$
    – Ofek Shilon
    Commented Feb 16, 2020 at 14:02
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    $\begingroup$ Yes, idealized Hubble expansion is natural. But we don't observe that. Instead, it appears (from analysis of type 1a supernova "standard candles") that the expansion is accelerating. That was quite unexpected: the previous assumption was that the expansion should be slowing down (to some extent) due to the influence of gravity. $\endgroup$
    – PM 2Ring
    Commented Feb 16, 2020 at 14:09
  • $\begingroup$ @PM2Ring analysis of supernovae standard-candles shows that the farther a galaxy is, the faster it moves away. Seems to me this alone does not imply acceleration - these can be exactly the galaxies with initial velocity in the top of the distribution. I.e., instead of saying "the more distant a galaxy is, the faster it travels away", think the opposite: "the galaxies that happened to be traveling faster are exactly those that are now more distant". $\endgroup$
    – Ofek Shilon
    Commented Feb 16, 2020 at 14:28

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Your description would fit with relative velocities that are constant with time, or decreasing with time, but the evidence suggests they are increasing with time.

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  • $\begingroup$ Isn't the evidence for this acceleration exactly that farther galaxies travel away with larger speeds? Is there independent evidence? $\endgroup$
    – Ofek Shilon
    Commented Feb 16, 2020 at 13:59
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    $\begingroup$ @OfekShilon There are two issues: expansion and possible acceleration of the expansion. The evidence you mention is mention for the expansion itself. It might be that you used the word "acceleration" the wrong way; it means change of velocity with time not with distance. The evidence for change of velocity with distance is partly red shift, and partly a complete cosmological model with precise equations and further evidence from microwave background and the abundances of different nuclei. $\endgroup$
    – Andrew Steane
    Commented Feb 16, 2020 at 14:51
  • $\begingroup$ @safesphere I too have reservations about the state of the evidence for cosmic acceleration, but I felt it was not the first thing to say here. As I understand it the evidence is suggestive but not conclusive. $\endgroup$
    – Andrew Steane
    Commented Feb 16, 2020 at 20:39
  • $\begingroup$ Cosmic acceleration was discovered in the 1998, but Hubble expansion has been the accepted paradigm since the 40's or so. $\endgroup$
    – Eric David Kramer
    Commented Feb 17, 2020 at 12:40
  • $\begingroup$ @EricDavidKramer The phrase "cosmic acceleration was discovered in 1998" is not quite right; what was discovered was important and interesting signatures of high-red shift supernovae which, together with other evidence, makes a good case for cosmic acceleration. However, the case is not conclusive. See for example doi.org/10.1051/0004-6361/201936373 which says that after local flow is accounted for the data is consistent with no acceleration at 1.4 sigma. Acceleration is suggested but not sufficiently conclusively for us to say "discovered", I think. I would be happy to be corrected. $\endgroup$
    – Andrew Steane
    Commented Feb 17, 2020 at 13:01
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Good question!

I think perhaps the situation you are describing would imply that we are in the center of the universe, i.e. that all of matter is expanding out from some small center, with some distribution in velocities.

This would be inconsistent with the dominant viewpoint in science, the Copernican principle, that says that our vantage point is not special. Indeed, of the the main assumptions in cosmology is that our universe is homogeneous and isotropic on large scales. I agree that this is not an argument. It is in fact circular reasoning. But it reflects the common view of scientists today.

(From a physics point of view, you could argue that gravity would cause the matter to slow down as it spreads out, but I'm fairly certain that this would reduce to the ordinary Friedman equations, so I don't think that would lead to any observable difference.)

Edit: All this is considering only the evidence from redshifts. However, there is other evidence for the standard cosmology. These are nucleosynthesis (BBN) and the microwave background (CMB). I think that BBN would be unchanged, for the same reason as above, that the expanding matter just follows the same Friedman equations. On the other hand, the microwave background is radiation reaching us from some "last scattering surface" about 14 billion light years away (according to standard cosmology) in all directions. If we were all the matter in the universe, we wouldn't be seeing anything coming from there. There is also evidence that most of the energy density comes from a cosmological constant, which cannot be described by matter, only vacuum energy that really does cause expansion.

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    $\begingroup$ "the situation you are describing would imply that we are in the center of the universe" - I can't see how. Similar distribution of velocities would be seen by our reference frame even if it isn't the 'center'. Moreover, there is no center to the bang, all points in space experienced it identically. $\endgroup$
    – Ofek Shilon
    Commented Feb 16, 2020 at 11:46
  • $\begingroup$ The matter would scatter and not expand into vacuum. Also, matter from neighboring patches would reach us with equally high velocities as we see far away. $\endgroup$
    – Eric David Kramer
    Commented Feb 16, 2020 at 11:51
  • $\begingroup$ What evidence is there that vacuum energy really causes expansion ? $\endgroup$
    – cumfy
    Commented Feb 16, 2020 at 13:49
  • $\begingroup$ @cumfy The hypothesis is that dark energy accelerates the expansion. $\endgroup$
    – PM 2Ring
    Commented Feb 16, 2020 at 14:02
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    $\begingroup$ @Ofek if the initial distribution of velocities was the same everywhere, we should observe both slow and fast galaxies near us, moving in all directions. But that's not what we observe. $\endgroup$
    – Javier
    Commented Feb 16, 2020 at 14:12

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