0
$\begingroup$

I was reviewing various questions regarding the big bang, including:

The answer to the question here Did the Big Bang happen at a point? by @JohnRennie and the community that describes how the Big Bang is not a central point, but the beginning of the expanse of the universe, which occurs everywhere at the same time;

The answer @RonMaimon provided to this question Does (it make sense to say that ) the universe has a center? that describes a homogeneous Newtonian big-bang where every known location within the universe is the center of the universe;

And the answer here https://astronomy.stackexchange.com/questions/669/what-is-in-the-center-of-the-universe by @astromax and @JimHaddocc who both have a similar answers as Ron.

All of these answers center around a single concept: that the universe appears to be expanding away from every known point, and that all points can be conceived as the center of the universe. I've seen many references to a loaf of raisin bread, where no matter which raisin you are on the inside of the bread, all other raisins are expanding away from you.

However, those all rely on the idea that because the universe appears to be expanding equally away from all known points we are assuming that our tiny view of the universe is indicative of the whole.

I posed this idea on each of those other answers, but I figure it might be better as a question:

How can we be certain that there is not an initial point of origin, and that all galaxies are not moving away from one another for the same reason that pellets from a shotgun all move away from each other after leaving the barrel?

Could not we attribute this equal expansion the result of a blast that provided momentum in one direction, with all known galaxies spreading out as they move laterally? That would provide the expansion as we currently witness it, while still allowing for an origin.

Improve this question
$\endgroup$
1
  • $\begingroup$ An interesting article on the expanding universe came out today here: bigthink[dot]com/starts-with-a-bang/universe-speed-of-light/ $\endgroup$
    – mkinson
    Commented Nov 23, 2021 at 12:52

3 Answers 3

Reset to default
4
$\begingroup$

How can we be certain that there is not an initial point of origin, and that all galaxies are not moving away from one another for the same reason that pellets from a shotgun all move away from each other after leaving the barrel?

We cannot be certain, but the fact that the galaxies are all moving away from each other is not the key observation in this context. The key observation is that the universe is isotropic on large scales. Since we observe large scale isotropy and homogeneity the simplest assumption (one that is based on evidence) is that the universe really is isotropic and homogenous.

Any other model will require an unjustified assumption of some conspiratorial effect that acts to hide the true asymmetry. That conspiratorial assumption (one that is not based on evidence) will add complexity to the model but will not increase predictive power. So by Occham’s razor it will be discarded.

If there were an actual center then galaxies away from the center would not see an isotropic universe. Instead they would see fewer galaxies in one direction than in the other. Given the trillions of galaxies we see, it would be highly unlikely that ours just happened to be the one in the center.

Of course, there are other possible explanations for the observed isotropy. One is that we are in the center, that is unlikely but does not require any additional explanation. Another is that we are not in the center but we can see only a small portion of the universe that is isotropic. That requires an explanation about why we can only see a small portion.

All in all, the homogenous and isotropic assumption is the simplest one that explains our homogenous and isotropic observations. So until we have some observations that contradict it, we will use it. There would literally be no reason to use another until such evidence is discovered.

Improve this answer
$\endgroup$
4
  • $\begingroup$ Shouldn't the fact that we cannot see a limit to our known universe be suggestive by itself that we may not be in the middle, but that your second reason (seeing only a small portion of the universe) be assumed? $\endgroup$
    – mkinson
    Commented Nov 9, 2021 at 22:34
  • 1
    $\begingroup$ Sure, but again it is a more complicated assumption, so if there is no evidence to support it over the simpler assumption then we stick with the simpler one. $\endgroup$
    – Dale
    Commented Nov 9, 2021 at 23:04
  • $\begingroup$ In the expanding balloon example, all points on the surface are getting away from each other equally. But there is still a center, inside the balloon. And all points on the surface are equally far from it. Isn't this an option too? That there is a center of the universe and that all galaxies are on a surface around it? $\endgroup$
    – Juan Perez
    Commented Nov 9, 2021 at 23:35
  • 2
    $\begingroup$ @JuanPerez In general relativity the balloon analogy only goes so far. In GR we do not have a fifth dimension to locate the center in. In the balloon example, the center is in 3D space, but the surface of the balloon ("the universe") is only 2D. GR does not work that way - there's no extra spatial dimension to locate the center in. What the universe expands into is itself (which is hard to grasp). So no center. $\endgroup$
    – StephenG - Help Ukraine
    Commented Nov 9, 2021 at 23:48
2
$\begingroup$

We know that the expansion of the Universe is accelerating and we can measure this. This acceleration already rules out the incorrect shotgun model, wherein the expansion is carried out by inertia/momentum. The true process of accelerating expansion is best modeled by the Friedmann equations, a solution of General relativity. The unique success of these models in describing the observed evolution of the Universe is why we explain the Universe using these models.

Observing the Universe in all directions, we know the Universe is approximately homogeneous and isotropic. We know this from the distribution of galaxies and from the tiny (1 part in $10^{4-5}$) anisotropies in the Cosmic Microwave Background.

These answers rely on evidence which leaves only 1 model standing.

Improve this answer
$\endgroup$
6
  • $\begingroup$ Inertia slows due to forces pulling on it, but if the Big Bang occurred in a setting where the "space" in front of the blast were a true vacuum then there would be no friction to slow the spread out, and indeed the idea of entropy and diffusion would encourage the matter that exists to continually spread further apart. The acceleration affect may be like that of air being sucked into the vacuum of a broken light bulb. $\endgroup$
    – mkinson
    Commented Nov 9, 2021 at 22:30
  • 2
    $\begingroup$ Air is a collisional gas where pressure exists because air particles collide. The Universe is nothing like that, and trying to apply your intuition about pressure and vacuum is incorrect. Moreover, such an idea does not match the observed rates of expansion over time. $\endgroup$
    – Alwin
    Commented Nov 9, 2021 at 22:41
  • $\begingroup$ you are correct, there is a huge difference between the pressure of air being forced into a vacuum by their collective mass, and the effect of accelerating due to being "sucked" into a vacuum. However, is it possible that there is some form of internal pressure with the relative density of the universe that contains galaxies that it could be acting in the same manner? e.g. Could something like dark matter be attempting to spread out, pushing galaxies away from one another? Also.. how do I send something to chat? $\endgroup$
    – mkinson
    Commented Nov 10, 2021 at 11:35
  • 2
    $\begingroup$ In short, no. That is not how dark matter behaves and that is not how matter in the Universe behaves. Galaxies are relative overdensities pulled together by gravity, and do not exert an outward pressure. An overdensity only becomes more dense, and an underdensity becomes less dense: this forms filaments and voids that we see. This is the opposite behavior as any kind of pressure you might be supposing. $\endgroup$
    – Alwin
    Commented Nov 10, 2021 at 12:02
  • 2
    $\begingroup$ Dark energy providing pressure is according to general relativity and the Friedmann equations as I noted in my answer. Unfortunately, everyday intuition about 'pressure' is not very useful when thinking about dark energy pressure. The dark energy + general relativity model describes a Universe expanding everywhere at once with no center. Definitely not "shotgun blast" $\endgroup$
    – Alwin
    Commented Nov 10, 2021 at 13:17
0
$\begingroup$

Yes, but this explanation implies that we were at the center of that explosion. Only there you observe the situation as we observe it in the universe. I am aware of two main arguments against this explanation: One, the history of Copernican revolution tells you that you are betting on the wrong horse. Two, this is in violation of the homogeneity and isotropy of space, which in turn are the very foundations of our extremely successful Standard Model of Cosmology.

Improve this answer
$\endgroup$
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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