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A popular misconception in the layman public is that the Big Bang was some sort "explosion" at a single point of space, where originally all matter was concentrated and then it "exploded" outwards. This is of course different from the modern general-relativity understanding of reality, which is that it is space itself which expands - not the content of the space moving, and the Big Bang did not start at a single point, but everywhere.

My question is - what experimental evidence do we have that can convince people that the explosion model can't be right.

Note that I'm not asking why GR guarantees that the space-expanding is the correct model, not the "explosion". I know that. I also know that GR has a lot of experimental evidence for its correctness at least in smaller scales. Rather I'm asking which evidence we have from astronomy, CMB measurements, or whatever, showing directly that the "explosion" model simply cannot be a valid explanation of the universe's history.

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    $\begingroup$ Note to potential answerers: inferred faster-than-light recession rates are not evidence against an "explosion from a point" model. Neither is the fact that the observable universe is larger than the age of the universe times the speed of light. Both of these happen in a very simple "explosion from a point" model. $\endgroup$
    – Sten
    Jun 13, 2023 at 9:11
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    $\begingroup$ As a more general point, I don't think it's possible to say the Universe did not originate from something like a point, because there are indeed viable models where it did (namely in the context of eternal inflation, where the Universe is taken to be our own non-inflating bubble). And I should note also that this is not in conflict with the GR description. "Expanding space" shouldn't be taken too literally; the difference between that and "everything is flying apart" is just a coordinate transformation. $\endgroup$
    – Sten
    Jun 13, 2023 at 9:16
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    $\begingroup$ Why the insistence on experimental evidence? (I'm smelling religious resistance, in which case no amount of evidence will suffice: if you want to believe, you'll just keep moving the goalposts on the required amount of evidence.) $\endgroup$
    – RonJohn
    Jun 14, 2023 at 5:38
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    $\begingroup$ No religious issues at all :-) I'm asking about experimental (or observational) evidence because I know how to explain space expansion using GR (the Friedmann equations, and so on), the GR model works well - but I started thinking how we know the "explosion" model doesn't work well also. For example Susskind's "the theoretical minimum" video lectures on cosmology start by looking at expansion of the universe as traditional movement in space, without GR, and he gets pretty far with this analogy. $\endgroup$ Jun 14, 2023 at 6:46
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    $\begingroup$ A rubber sheet is a 2D surface embedded in flat 3D space. However, the theory of GR is a theory of intrinsic curvature: our 4D spacetime isn't some "sheet" inside a higher dimensional space. Instead, we define curvature by how geodesics converge/diverge (tidal forces and sums of triangle angles). There is no "outside" space, so it loses meaning to say the big bang exploded "from" anywhere. $\endgroup$ Jun 14, 2023 at 7:34

5 Answers 5

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I am not going to add to the conventional list of differences between standard cosmology and the explosion picture, but I will say a few things about the general problem with these kinds of discussions in the public.

The first fundamental problem with the question is that it contains at least two fallacies. One is the false dichotomy fallacy that there are only two models and that therefor one of them has to be right and the other one has to be wrong. That is completely false. We simply do not know what the universe really looks like outside of the observable universe, hence any statement about its shape is local and therefor incomplete. If we take this limit to our knowledge seriously, then it should be obvious that we have to allow for an infinite variety of cosmological models that may or may not turn out to fall into some broad equivalence classes upon further analysis.

I don't know of anybody who has done a logically self-consistent analysis of the global situation so far (and I doubt we have nearly enough knowledge about the necessary theory, yet, to even perform such an analysis), hence I can't tell you what the result of such a broader approach would be. We can be assured, though, that it contains far more than just these two trivial cases.

In all likelihood we can assume that even the local expansion model is merely an approximation, so even if you manage to convince the layman that "expansion it is", did you really educate them about proper scientific thought or did you merely enforce an orthodoxy that stems from a rather limited view of the problem that was predominantly popular in the 1960s and maybe 1970s? I would suggest it's the latter. Personally I do not believe that this is a service either to the public or to science PR. It would be far better to educate them that cosmology is a discipline that keeps an open mind and that is aware of the approximative nature of statements about the cosmos. If we do that, then it becomes much easier to motivate active areas of research like inflation and more daring models like cyclical conformal cosmology etc..

The second major fallacy is the implicit assumption that both you and the layman understand the dynamics of explosions and that only the layman is struggling with the phenomenology of expansions. I would instead claim that neither of you know enough about the dynamics of explosions and that this lack of knowledge also applies to the majority of cosmologists. Those who do know more are most likely the ones who are also actively working on supernova physics and they happen to know that the phenomenology of real explosion is neither homogeneous nor isotropic. It is a turbulent and messy process that leaves a complicated aftermath. In other words: the cosmic explosion imagery is mostly based on a cartoon version of an explosion rather than on physically observed processes that show instabilities and asymmetries. In a serious scientific context explosion cosmology would therefor have to explain the detailed mechanism by which this special explosion would have yielded a CMB with a homogeneity of one part in 100,000 when no other explosion ever observed is even remotely that "smooth".

Incidentally this problem also exists for expansion and it is, to the best of my knowledge, still not solved satisfactorily. Inflation is one attempt to solve it, but you can find serious arguments in the literature (at least serious in my eyes as a cosmology layman) that indicate that inflation might actually be the wrong approach entirely. To me this is yet another reason to keep an open mind and to explain the problems in cosmology ("What are the observations that we have to explain?") rather than to force a toy model on the public that may or may not explain these observations well enough.

You are, by the way, not the only one who struggles with physics communication. Take Steven Weinberg, who is probably one of the smartest physicists ever. He wrote a beautiful book called "The First Three Minutes" that, to a large extent, gets most of this right... but it ultimately turned out to be wrong about the big picture completely. Weinberg preferred, if I remember correctly, a cyclical big bang model in that book. That model has been completely ruled out by now.

So what are you supposed to do? I would say, try to keep an open mind and motivate to your audience why and how you are doing that. That, if anything, is the best approach to science IMHO. We are trying to get answers rather than trying to give them.

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    $\begingroup$ A fundamental problem with trying to analyze things like the Big Bang is that physical laws describe an idealized version of the Universe which is generally close enough to reality to make reasonably accurate predictions in most typical situations, but sometimes fail to predict how things will behave in atypical situations. Given that the Big Bang would be about the most atypical situation imaginable, it would hardly be surprising if estimates of how things would behave are so far off as to make any estimates about initial conditions meaningless. $\endgroup$
    – supercat
    Jun 13, 2023 at 22:30
  • $\begingroup$ @supercat Even the assumption that the big bang has special initial conditions is already problematic. Imagine that we are out at sea in a storm. Does the storm end at the horizon? No. But is there blue sky somewhere else? Obviously. We just can't see the remainder of the ocean. That is what the universe is: a mostly invisible ocean. To make special assumptions about the "beginning" of "our" slice of the visible universe is a very 20th century way of thinking about it, already. It's stuck between horizons and false choices. We need to cultivate longer vision, even in lay people. $\endgroup$ Jun 13, 2023 at 23:01
  • $\begingroup$ It didn't read to me like they were saying the Big Bang necessarily had special initial conditions -- more that our current understanding of physical laws is woefully incomplete, and thus we can't accurately model such a massive and massively complex series of interactions. I think that meshes quite well with your (excellent) answer. $\endgroup$ Jun 14, 2023 at 15:55
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    $\begingroup$ I guess to use an analogy, trying to use the existing laws of physics to analyze the big bang may be like trying to use Boyle's gas laws to predict how gases will behave at extremely higher pressures or lower temperatures than have been observed. Trying to predict how CO2 would behave at 2,000psi when all experiments involving the substance have involved pressures below 200psi. $\endgroup$
    – supercat
    Jun 14, 2023 at 16:26
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    $\begingroup$ @MatthewRead There are people who like to call our universe "fine tuned", which is what I was referring to. The cheap solution to that is the anthropic principle, which I dislike a lot because it has no explanatory power. I agree with you. We are still very far away from understanding what is really going on. I mentioned Weinberg because I grew up with a different cosmology than the one we have today. I have witnessed the difference that additional data can make to our understanding of the universe. I expect at least one more fundamental shift in the field before we "get it right". $\endgroup$ Jun 14, 2023 at 17:22
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The evidence is in the distribution of velocity and the distribution of matter as a function of position, evolving over time.

The observed distribution (at the large scale) is like a swelling loaf of bread or a solid undergoing thermal expansion: each part moves away from its neighbouring parts with the same relative speed, for a given separation at any given time, no matter which region you examine. Also the matter is distributed uniformly over space at any given time.

For an explosion into space from a point, the region outside the expanding ball of debris is empty, and the velocity distribution within the ball of debris is typically not like the one outlined in the previous paragraph. So this differs from what is observed in both these respects.

The empirical evidence has not so far ruled out that the universe may have a small positive average curvature and a finite volume. It might also have a finite volume even if the average curvature is zero or negative. These aspects are not known. But we do know that the mathematics of General Relativity allow these possibilities. I mention it because if the total volume of space is finite but changing with time, then clearly the space itself is changing, not just the stuff in it.

We cannot directly observe the farthest regions so in the end we don't know for sure that the matter content does not fall away to zero outside some region, but that seems less plausible and less simple than the standard scenario where space is filled up roughly uniformly.

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    $\begingroup$ The velocity distribution is a tricky point. For example, if we simplify the "explosion from a point" by neglecting self-gravity, a particle is displaced today by an amount proportional to its velocity. That is, we get automatically the kind of expansion that we observe (namely Hubble's law). $\endgroup$
    – Sten
    Jun 13, 2023 at 10:57
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If the Universe originated from a single event in spacetime, then that event lies in our causal past, and the worldline of every particle in the Universe crosses that event. Therefore, every particle's worldline would enter our causal past. We would in principle be able to observe every particle in the Universe.

Does every particle lie in our observable universe? Well, naturally we cannot make a definitive statement about whether there are particles that we can't observe. Also, there is a practical limit to how far we can see, because the early universe was opaque.

Still, we have established observationally that the Universe was dominated by radiation (relativistic particles) from a time of about 1 second up to a time of around 52000 years. We do not know what happened before 1 second, but the simplest assumption is that the Universe was also radiation dominated before then. If radiation domination indeed extends indefinitely into the past, then the "true" observable universe is only about 1 percent larger in radius than the portion of the universe that we can practically observe (which terminates at the surface of last scattering of the cosmic microwave background).

Therefore, if the Universe originated from a point, and if nothing else very exotic happened before a time of about 1 second, then we can currently observe about 99% of the distance to the edge of the Universe's mass distribution. That is not really plausible; given that we see no large-scale inhomogeneity, it would imply an incredibly specific distribution of initial momenta (to produce a mass distribution that is uniform right up to the edge) and that we reside within 1% of the true center of the Universe.

Of course, maybe something else exotic did happen (e.g. inflation) that would lead to our "practically observable" universe being much smaller than our "observable in principle" universe (defined by the particle horizon).

So to conclude, we cannot say for certain that the Big Bang was not an explosion in space. But we can be pretty sure that it was not just an explosion in space.

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    $\begingroup$ "We have established ... up to a time of around 52000 years"-- of course, the relevant evidence (primordial abundances, CMB anisotropies, and lss) is compelling, but it's also indirect. My understanding is that the inference relies on the dynamics of GR's perturbative FLRW cosmology. Meanwhile, the "exploding universe" seems to be more of a vague idea than a specific and complete cosmological model. Am I mistaken here, and do you know of any references discussing such inferences in that context? Are the dynamics of the radiation (even its density scaling) actually constrained in this picture? $\endgroup$
    – jawheele
    Jun 14, 2023 at 15:58
  • $\begingroup$ @jawheele I'll just clarify that I'm not viewing an explosion as an alternative to the GR description, because I think that's a mistaken premise (as noted in the comments on the question). I accept perturbative FLRW as applicable because (1) we find GR to be accurate and (2) we observe homogeneity. Not to dismiss your concern -- it's certainly fair to ask how theory-laden an inference is. $\endgroup$
    – Sten
    Jun 14, 2023 at 16:13
  • $\begingroup$ Why would the radiation era extend indefinitely into the past? What does "past" and the kind of world line causality that applies to a transparent universe with strong energy density gradients (clocks) even mean in a system that does not have such local clocks and in which "communication" is blocked by the background? Strictly speaking none of the ideas of relativity apply in such a scenario. We are simply using familiar language to describe a causally unfamiliar situation. The conceptual problems here run very, very deep. $\endgroup$ Jun 14, 2023 at 18:13
  • $\begingroup$ @FlatterMann No reason other than that it's the simplest assumption. The point is that if you suggest that a localized explosion created the hot plasma of the early universe, you might run into issues. But through more creative scenarios, there's not necessarily any issue with a localized origin. $\endgroup$
    – Sten
    Jun 14, 2023 at 18:19
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    $\begingroup$ (And relativity of course still applies in situations where light can't stream freely. Why wouldn't it? Do you argue that relativity does not apply within opaque everyday objects? Regardless, neutrinos and gravitational waves can still stream freely through the plasma and can carry causal information.) $\endgroup$
    – Sten
    Jun 14, 2023 at 18:27
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There can be several approaches to this. But my point would be to compare Hubble law of receding galaxies ("debris of explosion") and true explosion in a spherical charge, like grenade covered by Gurney equations.

Hubble law states, that galaxy receding speed is :

$$ v = H_0~D \tag 1,$$

while grenade shell receding initial velocity is (assuming charge mass is a lot greater than shell mass) :

$$ v_0 \approx 1.3 ~\mathcal G \tag 2,$$

Where $\mathcal G$ is the Gurney constant for a given explosive. It depends on how effective explosive is and is related to a potential energy per mass unit of explosive which gets converted into shell kinetic energy.

Main differences between those two explosions is that grenade shell after acquiring initial maximum speed $v_0$,- starts to loose kinetic energy with the distance covered in the environment due to air drag force and similar reasons. Galaxy, on the contrary as (1) shows, with the distance covered $D$ just increases it's speed. Hence, universe is in ongoing and accelerated "explosion", for which we don't even have an analogy in real-life explosions, because every of it ends fast enough and with decreasing shell speed over time.

Other also important difference is that grenade spatially has center of detonation, because shell particles which are closer to the center of explosion - will have greater initial receding speeds and particles which are further away form it,- will have lower escape speeds. Hence, if you would detect grenade debris speeds with slow-motion camera or other instrument - you could find out debris velocities gradient and following that,- find out coordinates $x,y,z$ where explosion in space has started. In contrary, universe has no such property,- there is no "special" direction in outer space where galaxies would not commit to Hubble law (except some disturbances from local galaxy group gravitation, which may "override" global law). Or if you like - whatever galaxy habitant will deduce the same Hubble law and some may try to falsely make a conclusion that everything is receding from them. Hence, every point in the universe is "center of BigBang" and has participated in the early universe all historical events.

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  • $\begingroup$ When I mentioned "explosion" I didn't mean literally an explosion of an explosive, and obviously there wouldn't be any air drag of any of the "exploding" matter. I had in mind more a point with a huge amount of matter with random velocity distribution (due to quantum fluctuations), leading some matter to fly out faster than other matter, or something like that. $\endgroup$ Jun 13, 2023 at 13:43
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    $\begingroup$ IMHO, matter explosion detonated from a single point in space, does not differ much from a spherical shell explosion in explosives, hence my analysis should be valid as well for denying such analogy as you wanted. If there's no drag or any resistance,- matter should fly apart with constant speed,- not dependent on distance covered. This is different from Hubble law, which states galaxies speed gradient. So analysis still valid. $\endgroup$ Jun 13, 2023 at 13:57
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    $\begingroup$ If gravity is neglected, Hubble's law necessarily holds when the matter originates from a point, no matter what the velocity distribution is. This is because particles' present-day positions are set by their velocities. What would happen instead, if you had a highly concentrated distribution of initial speeds, is that the matter today would be spatially concentrated in a ring around the initial point, as opposed to the homogeneous distribution that we observe. (Observers in the ring would see an inhomogeneous distribution that nevertheless obeys Hubble's law.) $\endgroup$
    – Sten
    Jun 13, 2023 at 14:02
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    $\begingroup$ Sorry, hollow shell, not ring. $\endgroup$
    – Sten
    Jun 13, 2023 at 14:08
  • $\begingroup$ Agree about hollow shell. Similar debris inhomogeneous distribution should be found on the ground after grenade or any spherical charge explosion. Hence, it also could be used for disproving shell model of universe "explosion". $\endgroup$ Jun 13, 2023 at 14:40
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The CMB is the best experimental evidence we have that there was not an explosion, but rather an expansion of spacetime. It is mostly isotropic: it comes from all directions at the same time.

Not in your question, but it also helps to imagine what such an "explosion" would look like: a scattering of mass from a central point. This does not make a lot of sense: what was there in that point before? What remnants would remain of that point? Why would there be a preferred location? The absence of any clustering of galaxies around a central point (as far as we can see) is also experimental evidence that it was space itself that was expanding, not just the mass contained.

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