What experimental evidence shows that the "explosion" model of the Big Bang with an explosion at a single point of space is wrong?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.