Where did the early photons go?

[DaveC426913]

I interpret this as analogous to 'zooming' a view in an image. The coordinates are not changing but user's viewport is.

Except that it is not. Because there are things that do that change. For example: the length of time it takes light to travel from one coordinate to another.

If cosmological expansion were akin to zooming a viewport, then light would take the same length of the time to travel from one galaxy to another, no matter the 'zoom setting'.

Let's say the Virgo cluster is 65M light years way.
Let's say, in a billion years, it will be double that distance: 130M light years away.'

In your "viewport model", instead of the distance doubling, we are simply taking 1 billion years to zoom in by a factor of 2.

But if that were so, then it shoudln't change the lrngth of tiem of light propagtion. Light should still take 65M years to reach us from the Virgo cluster.

But that is not what our observations tell us. As things move apart the propagation of light is invariant. i.e. After a billion years, light really does take 130M years to reach us, and therefore it really is 130M light years away. That tells us the expanasion is real.

And a whole lot of other observations fall, if you assume the "viewport model", such as Doppler shifting and the Observable Universe boundary (there woudn't be a boundary in your viewport model).

 

[PeterDonis]

given the fact that the early universe was opaque and not open to visual investigation, how do we know anything about the universe's density during the first 380,000 years of its existence?

Because we have other ways of inferring what the properties of the universe were before it became transparent to radiation. For example, we measure the relative abundances of light elements in our present universe, and we apply our knowledge of nuclear reactions to infer what the density and temperature of the universe must have been to make those light elements. We know the density and temperature when the universe became transparent to radiation were way, way short of what's required for those nuclear reactions, so we infer that there must have been an earlier time when the density and temperature were much higher.

Of course that's just one line of reasoning. Overall, the answer is that we apply our knowledge of the laws of physics to back-calculate what must have happened before the universe became transparent to radiation. Those laws of physics include GR, meaning curved spacetime, and we have built a model of the universe's evolution in time, going back to the end of inflation, that makes use of those laws of physics and matches all the data we have. And that model tells us what the density and temperature were as a function of time.

We neither 'know' nor 'observe' what the early universe was actually doing. Instead we extrapolate backwards in time using physical principles we know well.

Not just "physical principles"--physical laws. As above, we have built a model using those laws. It's not just a vague extrapolation. It's much more than that.

Theoretical models of the early universe make certain predictions and some of these have been confirmed through observation.

Yes. I gave an example above (abundances of the light elements).

when scientists discuss the density of the early universe and its decrease through expansion, are they doing so solely on the basis of this kind of extrapolation?

They are doing it based on the model they have built using the laws of physics and the data we have. As above, it's not just a vague "extrapolation".

given the lack of a boundary to yield a frame of reference for density

You don't need a boundary to have a meaningful density. Density is a local concept, not a global one. You can measure the density of air around you without having to know the total volume of the Earth's atmosphere, or whether it has a boundary.

how do we know that the density at one time was different from that of an earlier time?

Because of the model we've built using the laws of physics and the data we have.

In this thread the use of a coordinate system has been discussed as a way of measuring expansion.

No, as a way of describing expansion. And it's not necessary, just convenient. There are invariants, independent of any coordinate system, that describe the expansion--I mentioned the expansion scalar in an earlier post.

what about density, which changes as a function of the expansion of the universe? Is that how this works? That if the universe is deemed to be expanding, then density must also be deemed to be falling?

When you construct a model of an expanding universe using the laws of physics, yes, this is what you find.

 

[Cerenkov]

Because we have other ways of inferring what the properties of the universe were before it became transparent to radiation. For example, we measure the relative abundances of light elements in our present universe, and we apply our knowledge of nuclear reactions to infer what the density and temperature of the universe must have been to make those light elements. We know the density and temperature when the universe became transparent to radiation were way, way short of what's required for those nuclear reactions, so we infer that there must have been an earlier time when the density and temperature were much higher.

Of course that's just one line of reasoning. Overall, the answer is that we apply our knowledge of the laws of physics to back-calculate what must have happened before the universe became transparent to radiation. Those laws of physics include GR, meaning curved spacetime, and we have built a model of the universe's evolution in time, going back to the end of inflation, that makes use of those laws of physics and matches all the data we have. And that model tells us what the density and temperature were as a function of time.

Thank you. I also take your point about BB nucleosynthesis. For that to happen the temperatures and densities must have been very high indeed. I begin to see the strength of the reasoning being done.

Not just "physical principles"--physical laws. As above, we have built a model using those laws. It's not just a vague extrapolation. It's much more than that.

Thank you for correcting my tentative language. I was trying to be as careful as possible when writing and not over-commit myself on matters that are far beyond me.

Yes. I gave an example above (abundances of the light elements).

Agreed.

They are doing it based on the model they have built using the laws of physics and the data we have. As above, it's not just a vague "extrapolation".

As mentioned above Peter, any implication of vagueness comes from my cautious wording.

You don't need a boundary to have a meaningful density. Density is a local concept, not a global one. You can measure the density of air around you without having to know the total volume of the Earth's atmosphere, or whether it has a boundary.

Then the early universe's density is inferred by applying physical laws and running the scenario backwards in time in strict accordance with those laws. Thank you.

Because of the model we've built using the laws of physics and the data we have.

And this model makes certain predictions about what we should observe now if the inferred conditions existed then. Predictions which have been well confirmed and which now constitute different, but agreeing, lines of evidence.

No, as a way of describing expansion. And it's not necessary, just convenient. There are invariants, independent of any coordinate system, that describe the expansion--I mentioned the expansion scalar in an earlier post.

I will have to go back and study that specific post. Above you use two terms that need a bit more explanation for the layman, invariant and scalar. After reading your post I'll see if I can discover more about them by myself and then try to put things together.

This may well lead to more questions. Seeing as they will pertain to the early universe, they should (I hope) be considered to be on-topic.

When you construct a model of an expanding universe using the laws of physics, yes, this is what you find.

Thank you. I think I see and understand more now.

I appreciate you taking the time to answer my questions, Peter.




Cerenkov.

 

[PeterDonis]

Above you use two terms that need a bit more explanation for the layman, invariant and scalar.

Briefly:

An invariant is a quantity that is independent of any choice of coordinates.

A scalar is an invariant that is simply a number--or rather, a number at each point of spacetime, i.e., a scalar function on spacetime.

 

[PeterDonis]

I appreciate you taking the time to answer my questions, Peter.

You're welcome!

 

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

A subthread on how the "speed of expansion" of a material object is defined has been moved to a separate thread in the relativity forum:

https://www.physicsforums.com/threads/how-is-speed-of-expansion-of-an-object-defined.1082773/

This thread is reopened, but please keep discussion here on the original thread topic.