B How dense was the first starfield we might have seen?

[Grinkle]

I recall at some time in my past hearing an argument that if the universe is of infinite extent then the night sky should be completely filled with starlight and have no areas where there is no visible light. Of course this argument does not account for the possibility of light emitting objects that exist but are outside of our light cone.

But I am left wondering if a night view in the earliest possible universe in which we might have existed would have looked much denser than our contemporary star field. Was there ever a time when such a view could have existed for us?

 

[kimbyd]

I recall at some time in my past hearing an argument that if the universe is of infinite extent then the night sky should be completely filled with starlight and have no areas where there is no visible light. Of course this argument does not account for the possibility of light emitting objects that exist but are outside of our light cone.

But I am left wondering if a night view in the earliest possible universe in which we might have existed would have looked much denser than our contemporary star field. Was there ever a time when such a view could have existed for us?

Certainly no time when the Earth (or any life) existed would the sky have been bright in this manner.

In the early universe, there was a glow which filled the entire universe. We see this today as the Cosmic Microwave Background. When it was first emitted (when the early universe condensed from a plasma to a gas), the temperature was around 3000K. This would have appeared to be a reddish light which was pretty much the same in every direction (there would have been small differences in temperature of about 0.03K or so from place to place on the sky).

But this light never came from stars. By the time stars started to appear, the temperature of the CMB had cooled to around 30K, which isn't visible to humans. The light from stars never created the effect described here, because the expansion of the universe prevents it.

 

[Grinkle]

Thanks! To you for this answer and to all the folks who take the time in PF to answer questions like this - much appreciated.

 

[jbriggs444]

I recall at some time in my past hearing an argument that if the universe is of infinite extent then the night sky should be completely filled with starlight and have no areas where there is no visible light.

That is Olber's Paradox. It applies to a universe which is infinite and uniform in time and space

 

[kimbyd]

That is Olber's Paradox. It applies to a universe which is infinite and uniform in time and space

And also neither expanding nor contracting (i.e., no cosmological redshift).

 

[JMz]

But I am left wondering if a night view in the earliest possible universe in which we might have existed would have looked much denser than our contemporary star field. Was there ever a time when such a view could have existed for us?

To extend @kimbyd's answer: The MW apparently formed from mergers of smaller (proto-)galaxies, say, 10^10 years ago. At that time, there wasn't a lot of material ("dust") available to make rocky planets. But if you managed to find some, and if life got going quickly at that time, and if the life evolved eyes to see stars with (something that took ~3-4 GY on Earth), then those eyes would -- from typical places in the MW -- likely have seen lots more stars than the few thousand we see (a) now and (b) from Earth.

That's for two big reasons: (1) There was a great deal more star formation going on throughout the galaxy/-ies back then (more than 10x as much as now, as I recall), and (2) there was very little dust to absorb the star light. (If it weren't for dust, we'd be able to see the Galactic Center, and [my recollection] it would be about as bright as the full moon -- visible even by day.) Of course, "very little dust" means "very little chance of forming such a planet". ;-)

 

[Chronos]

The 'seeing' from Earth's surface during the Archean period was probably very poor. As noted here; https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1664689/; "A consequence of ‘significant’ amounts of CH4 photolysis in the early Earth atmosphere is the possible formation of an organic haze layer like that on Titan, as first explored in detail by Sagan and Chyba ...", suggesting it was filled with a prebiotic haze consisting largely of nitrogen and sulfur rich compounds" - IOW smog. The view from space, however, may have been spectacular - assuming the MW was still in a starburst phase and our sun formed as member of a star cluster that was not yet entirely dispersed. However, our galaxy was already several billion years old old by then, so already middle aged, and probably looked similar to its present appearance. Most of the other galaxies visible then would probably also have been well established and not grossly dissimilar from their present day appearance. Even the CMB was remarkably similar at Z~0.5 [5 billion years ago] as it is now [re:https://arxiv.org/pdf/1502.06707.pdf]

 

[JMz]

Quite right, which is why I took the question to refer to even earlier times in the MW's life. (Also, the same haze might have been present on most early candidate planets. But if eyes developed at all, I imagined that they didn't have to contend with haze.) All-in-all, probably an especially rare scenario. However, probably a great view to experience virtually!

 

[kimbyd]

To extend @kimbyd's answer: The MW apparently formed from mergers of smaller (proto-)galaxies, say, 10^10 years ago. At that time, there wasn't a lot of material ("dust") available to make rocky planets. But if you managed to find some, and if life got going quickly at that time, and if the life evolved eyes to see stars with (something that took ~3-4 GY on Earth), then those eyes would -- from typical places in the MW -- likely have seen lots more stars than the few thousand we see (a) now and (b) from Earth.

That's for two big reasons: (1) There was a great deal more star formation going on throughout the galaxy/-ies back then (more than 10x as much as now, as I recall), and (2) there was very little dust to absorb the star light. (If it weren't for dust, we'd be able to see the Galactic Center, and [my recollection] it would be about as bright as the full moon -- visible even by day.) Of course, "very little dust" means "very little chance of forming such a planet". ;-)

It's actually hard to draw a conclusion like that. If our planet was in a dense, star-forming region, then it would be bombarded by high-energy radiation from nearby supernovae periodically, and that radiation would sterilize the planet.

For you to get complex life, the planet has to live in a pretty sedate environment, which may make it difficult for life to arise in a location with lots of bright stars nearby (since bright stars tend to be massive stars which have a tendency to blow up).

Thus I suspect that there's a selection effect which would prevent civilizations from seeing skies that are too filled with bright stars.

 

[JMz]

It's actually hard to draw a conclusion like that. If our planet was in a dense, star-forming region, then it would be bombarded by high-energy radiation from nearby supernovae periodically, and that radiation would sterilize the planet.

For you to get complex life, the planet has to live in a pretty sedate environment, which may make it difficult for life to arise in a location with lots of bright stars nearby (since bright stars tend to be massive stars which have a tendency to blow up).

Thus I suspect that there's a selection effect which would prevent civilizations from seeing skies that are too filled with bright stars.

Good points. I tried to say what might be possible IF all the problems were absent on some very rare planet -- but as you point out, there are so many problems that the population of such planets might be empty.

 

[JMz]

If our planet was in a dense, star-forming region, then it would be bombarded by high-energy radiation from nearby supernovae periodically, and that radiation would sterilize the planet.

BTW, you actually understate the problem somewhat: In such a region, we should expect O & B stars that could sterilize the planet all by themselves, with no need to wait for them to go supernova.

I guess VR may be the only way to expect any species to see what that environment would look like.