What if we cut out the Big Bang and inflation?

[Cody Livengood]

TL;DR Summary

Are they needed to account for the large-scale structure of the universe, nucleosynthesis, baryon acoustic oscillations, and the anisotropies of the CMB? Can the universe not begin in an already expanded state and still have all those things happen just the same, especially considering how isotropic and homogeneous the universe was? If the universe began uniform and is not expanding, would the Big Bang and inflation still be needed? For this question, assume the universe isn't expanding.

Are the Big Bang and inflation really needed to account for the large-scale structure of the universe, nucleosynthesis, baryon acoustic oscillations, and the anisotropies of the CMB? Can the universe not begin in an already expanded state and still have all those things happen just the same, especially considering how isotropic, homogeneous, and ratiation-dominated the universe was? If the universe began uniform and is not expanding, would the Big Bang and inflation still be needed? For this question, assume the universe isn't expanding.

 

[Vanadium 50]

Could things be different? Sure. But without another model that explains at least as much as the present model, you're left with no more than that: "maybe things are different."

Not really a road to understanding.

 

[Cody Livengood]

Could thgings be different? Sure. But without another model that explains at least as much as the present model, you're left with no more than that: "maybe things are different."

Not really a road to understanding.

I'm not sure that really answers my question though. What additional function do they play after they have both taken place? Why are they needed?

 

[phinds]

@Cody Livengood I think probably you have not really understood the Horizon Problem, just to give one example.

Can the universe not begin in an already expanded state and still have all those things happen

simply does not make sense without an EXPLANATION for how that which has been experimentally verified could have occured. Handwaving and saying "it just is" doesn't cut the mustard.

 

[Cody Livengood]

@Cody Livengood I think probably you have not really understood the Horizon Problem, just to give one example.
simply does not make sense without an EXPLANATION for how that which has been experimentally verified could have occured. Handwaving and saying "it just is" doesn't cut the mustard.

I'm asking what additional function the Big Bang and inflation provided after taking place. Finding a replacement is a wholly separate task. I just want to know how or why they would be needed if the universe were already expanded.

 

[Drakkith]

Are the Big Bang and inflation really needed to account for the large-scale structure of the universe, nucleosynthesis, baryon acoustic oscillations, and the anisotropies of the CMB?

Yes. There are no other reasonable explanations that match our observations. While inflation is still fairly theoretical, the BBT, as a whole, is incredibly successful in making accurate predictions across a wide range of scales, environments, and time. To get rid of the BBT would be to get rid of a single, cohesive theory in favor of a plethora of disjointed, unrelated theories that aren't nearly as accurate and often don't make any sense in light of the huge amount of data we have gathered.

The BBT is so successful that it doesn't really even have any contender theories. The alternatives are that the universe is static or is contracting, both of which make no sense in light of redshift data and would require that virtually every other theory related to astronomy be abandoned or significantly altered to take into account the changed history of the universe. As an example, how can our theories on galactic formation be accurate if they depend on a denser universe and the specific abundance of matter as given by the BBT? How was star formation possible in the past few billion years if the universe was much less dense in the past? If the universe is static, how static is it? Where did all the matter and radiation in the universe come from? Why hasn't everything collapsed inward on itself under gravity by now? Where did the CMB come from if there wasn't a dense, hot 'fog' of matter to emit it like the BBT says?

The questions just go on and on.

 

[PeroK]

I'm not sure that really answers my question though. What additional function do they play after they have both taken place? Why are they needed?

The Big Bang Theory is the cornerstone of modern cosmology. Without it, you have nothing. It's not a question of what the BBT adds to cosmology; the BBT Iis modern cosmology. It's a bit like asking why we need Newton's second law and what does that add to classical mechanics.

Moreover, the BBT is predicted by General Relativity. A static universe is not a solution to the Einstein Field Equations; the universe must be expanding or contracting to be compatible with GR.

 

[Cody Livengood]

The Big Bang Theory is the cornerstone of modern cosmology. Without it, you have nothing. It's not a question of what the BBT adds to cosmology; the BBT Iis modern cosmology. It's a bit like asking why we need Newton's second law and what does that add to classical mechanics.

Moreover, the BBT is predicted by General Relativity. A static universe is not a solution to the Einstein Field Equations; the universe must be expanding or contracting to be compatible with GR.

I hope you realize that your answer doesn't actually answer my question. Pretend we never knew about the Big Bang and inflation and believed the universe began in an expanded state. What difference does that make? What problems would that lead to?

 

[PeroK]

Pretend we never knew about the Big Bang and inflation and believed the universe began in an expanded state.

We can't. It doesn't fit the data nor the established theory of gravitation (GR).

What difference does that make? What problems would that lead to?

You have no working theory of cosmology.

Science does actually all fit together. The BBT is linked to particle physics is linked to GR etc. You can't just pretend it's not so.

 

[Cody Livengood]

We can't. It doesn't fit the data nor the established theory of gravitation (GR).

You have no working theory of cosmology.

I think you understand that my question is both serious and hypothetical. Just assume (for the sake of this question) that the universe is not expanding. How would we know that something was wrong? What part of cosmology would fail to work without their existence?

 

[PeroK]

Just assume (for the sake of this question) that the universe is not expanding. How would we know that something was wrong?

We can measure the expansion! Or, at least, the pattern of redshift against distance over time.

 

[berkeman]

Just assume (for the sake of this question) that the universe is not expanding. How would we know that something was wrong?

Do you mean something like Hoyle's Steady State model? Can you say what observational evidence led to it being discredited?

https://en.wikipedia.org/wiki/Steady-state_model

While the steady-state model enjoyed some minority[citation needed] support in the scientific mainstream until the mid-20th century, it is now rejected by the vast majority of cosmologists, astrophysicists and astronomers, as the observational evidence points to a hot Big Bang cosmology with a finite age of the universe, which the steady-state model does not predict.[1][2]

 

[Cody Livengood]

We can measure the expansion! Or, at least, the pattern of redshift against distance.

I think you know what "hypothetical" means. I don't care if the universe is or isn't actually expanding or if we can measure it. I'm asking you to just forget about expansion for the sake of the question. You mentioned redshift. That's good. Are there any other reasons or effects that don't occur after the Big Bang and inflation took place?

 

[PeroK]

I think you know what "hypothetical" means. I don't care if the universe is or isn't actually expanding or if we can measure it. I'm asking you to just forget about expansion for the sake of the question. You mentioned redshift. That's good. Are there any other reasons or effects that don't occur after the Big Bang and inflation took place?

There are three main experimental supports for the BBT:

1) Redshift

2) CMBR (see posts by @kimbyd on here).

3) The percentage of light elements (H, He, Li).

There may be others, but those are three big ones that would take a lot of explaining otherwise.

 

[Cody Livengood]

Do you mean something like Hoyle's Steady State model? Can you say what observational evidence led to it being discredited?

https://en.wikipedia.org/wiki/Steady-state_model

I assume the observational evidence would be cosmological redshift. But this question is hypothetical in the sense that I'm asking you to disregard expansion for the sake of the question whether or not it's true or whether or not we can measure it. Because if we're pretending it doesn't exist for the sake of the question, then it's irrelevant.

 

[Cody Livengood]

There are three main experimental supports for the BBT:

1) Redshift

2) CMBR (see posts by @kimbyd on here).

3) The percentage of light elements (H, He, Li).

There may be others, but those are three big ones that would take a lot of explaining otherwise.

You would still have the CMBR and the same ratio of light elements if the universe were already spread out just the same as it was. You could cut out the Big Bang and inflation and just cut to the time after they took place and not change those things. Right? Do they have some other direct effect on the universe?

 

[PeroK]

You would still have the CMBR

The CMBR is very specific and needs an explanation. You can't just say it's there and needs no explanation.

Imagine you asked what if we didn't have gravity and I said we need to explain the Moon and you said well, the Moon could still orbit the Earth without gravity. That's not an answer or a satisfactory explanation.

and the same ratio of light elements if the universe were already spread out just the same as it was.

Yes, but if the BBT predicts the exact proportion and if your theory just says that's the way it is, then that's not a satisfactory answer either.

No one is going to give up the BBT that explains most things for something that doesn't explain any of those things!

You may have worked for seven years on something, but thousands of cosmologists have spent decades piecing together a comprehensive BBT. And they are neither fools nor hidebound adherents to tradition.

 

[pbuk]

Exactly the same problem here as in this closed thread: https://www.physicsforums.com/threads/how-do-we-know-gluons-exist.1044741/

There is no point in discussing any detail of science with someone who refuses to agree with what science is.

 

[Vanadium 50]

You would still have the CMBR and the same ratio of light elements if the universe were already spread out just the same as it was.

Sure, The entire universe could gave been created yesterday, including all our memories. You didn't actually have lunch yesterday - you just remember that you did.

No way to prove otherwise.

 

[Cody Livengood]

The CMBR is very specific and needs an explanation. You can't just say it's there and needs no explanation.

Imagine you asked what if we didn't have gravity and I said we need to explain the Moon and you said well, the Moon could still orbit the Earth without gravity. That's not an answer or a satisfactory explanation.

Yes, but if the BBT predicts the exact proportion and if your theory just says that's the way it is, then that's not a satisfactory answer either.

No one is going to give up the BBT that explains most things for something that doesn't explain any of those things!

You may have worked for seven years on something, but thousands of cosmologists have spent decades piecing together a comprehensive BBT. And they are neither fools nor hidebound adherents to tradition.

Talking about alternative explanations or why things in the scientific community are so hard to change is off-topic. We're just talking about the standard model here. What does the standard model say would be different about the universe if it is not expanding and began in an expanded state? I don't intend to reply comments not addressing this exact question.

 

[berkeman]

I don't intend to reply comments not addressing this exact question.

No need to. This thread is done.