How far.

[Bird11dog]

Can we see stars in all directions at 13 billion light years?

[Strange]

Yes.

[Externet]

Except in the directions blocked by celestial bodies; like behind the moon, sun, behind earth itself...

[Bird11dog]

Thirteen billion years ago the Universe was about 1/13 of it's size today. If we look at a galaxy 13 billion light years away that means we are looking at light coming out of a gravity well created by the entire mass of the Universe. When we make measurements on that light do we take into account the redshift caused by that gravity well?

[swansont]

Thirteen billion years ago the Universe was about 1/13 of it's size today.

 

 

It is?

[Bird11dog]

Notice the word about? That means, I don't know, I'm guessing! I suppose you can't answer my main question?

[imatfaal]

Notice the word about? That means, I don't know, I'm guessing! I suppose you can't answer my main question?

 

your main question was answered. yes there are stars in every direction. in one direction there is the bulk of the galactic disc (the milky way band that you can see on a clear night). for the rest of the sky there are minimal variations from galaxy to super-clusters in the order of 200 megaparsecs across - there are also voids; but from earth, everything looks pretty homogeneous till you look very carefully.

 

with respect "about" means it is an estimate, has errors, might not be exact etc. the rate of expansion from the first seconds of the universe till now means that you cannot make straight line predictions about the size versus the age of the universe. 13.8 billion years ago the observable universe was small (note the observable bit - the entire universe could well have been infinite) - now it is is about 46Glyrs radius. Hopefully someone will correct me if I am wrong but I think the observable universe was about one eighth of its present size 13 billion years ago - which shows how much expansion took place in those first 800Myrs

[Strange]

Notice the word about? That means, I don't know, I'm guessing! I suppose you can't answer my main question?

 

 

If you look at the most distant objects observed, they have red-shifts that indicate the universe was about 1/8th to 1/12th the size it is now (the scale factor).

https://en.wikipedia.org/wiki/List_of_the_most_distant_astronomical_objects

[Airbrush]

Can we see stars in all directions at 13 billion light years?

 

No we can not see stars in all directions. We can only see stars in our own galaxy with the naked eye. Can we see individual stars in nearby galaxies using powerful telescopes? If so, how far away does a galaxy need to be so that it is impossible to see its' individual stars? I cannot belief we have the ability to see individual stars at a distance of 13 billion light years, right?

Edited February 15, 2017 by Airbrush

[Strange]

 

No we can not see stars in all directions. We can only see stars in our own galaxy with the naked eye. Can we see individual stars in nearby galaxies using powerful telescopes? If so, how far away does a galaxy need to be so that it is impossible to see its' individual stars? I cannot belief we have the ability to see individual stars at a distance of 13 billion light years, right?

 

 

Fair point. We can see individual supernova in distant galaxies because they emit so much light. The most distant observed was at z=3.8993 (something like 12 billion light years, I think).

https://en.wikipedia.org/wiki/SN_1000%2B0216

 

But when I said we can see stars in all directions, I wasn't thinking of individual stars. There are trees as far as I can see when I look out of my window. But I can only see a few individual trees. The rest just make up various woods and copses. I still think I am seeing trees when I see a large green mass.

 

Similarly, I still think we are seeing stars when all we see is a small blob which is a galaxy.

[Bender]

We cannot see individual stars at that distance because the photons are spread out to much.

 

In my rough estimate, a one square meter space telescope would receive only 10^-27 W of radiation from a star like the sun at that distance, but a photon in the visible spectrum has an energy of 10^-19 J, so the telescope would only be able to catch one photon every couple of years.

(calculation of fraction of solar luminosity of 3.8e26 W based on Taylor expansion of cos: 1/(2*13e9*300e6*3600*24*365)^2 in [math]\Omega = 2\pi\left(1 - \cos\theta\right)\,\mathrm{sr}[/math])

Perhaps with a gravitational lens?

 

A supernova has a luminosity billions of times larger, so several photons each second.

Edited February 15, 2017 by Bender

[Janus]

Thirteen billion years ago the Universe was about 1/13 of it's size today. If we look at a galaxy 13 billion light years away that means we are looking at light coming out of a gravity well created by the entire mass of the Universe. When we make measurements on that light do we take into account the redshift caused by that gravity well?

When we are looking at a galaxy which appears to be 13 billion light years distant, that is the light travel time distance. So while the Universe was smaller when that light left, this means that "now" that galaxy is much further away. So for example an object at a light travel time distance of 13 billion light years with a red shift of 8.2 is actually, at this moment, 30 billion light years away.

[Bird11dog]

The question in my second post was unanswered. Sense the gravity of a star or galaxy red shifts the light they emit would not the light being emitted by the early Universe be red shifted much more by the mass of the entire Universe pulling on it thus changing the way we calculate distance using red shift?

Edited February 27, 2017 by Bird11dog

[Strange]

The question in my second post was unanswered. Sense the gravity of a star or galaxy red shifts the light they emit would not the light being emitted by the early Universe be red shifted much more by the mass of the entire Universe pulling on it thus changing the way we calculate distance using red shift?

 

 

It is not gravity that causes red-shift but the change in gravitational potential. As the (average) density of the early universe was the same everywhere and is still the same everywhere, I don't think that there is any change in gravitational potential.

 

However, the theory that calculates gravitational red-shift (GR) is also the theory that calculates cosmological red-shift. So if there were any gravitational effect, it would be included in the model automatically.

[Bird11dog]

I made an error in my second post, red shift should have been blue shift. If I have three stars orbiting each other won't the light being emitted by each be slightly blue shifted to a far away observer? If the people doing the calculations are unaware of the blue shift then it would not be automatically included.

[Bender]

They would shift between red shifted and blue shifted. Or between more red shifted and less red shifted or between more blue shifted and less blue shifted, if their overall speed away from or towards us is larger than their circumferential speed.

[Strange]

I made an error in my second post, red shift should have been blue shift. If I have three stars orbiting each other won't the light being emitted by each be slightly blue shifted to a far away observer? If the people doing the calculations are unaware of the blue shift then it would not be automatically included.

 

I can't see this effect being significant. It would also be about the same for all stars and groups of stars. So it wouldn't have any effect on the measurements related to Hubble's Law.

[Bird11dog]

1. Suppose we had a triple planet system that orbit each other and of quite different masses. We could find an average clock rate for the system by checking how fast a cock runs at the surface of each. We could do the same thing for our galaxy or a volume of space with a radius of a hundred million light years. If we look at the clock rate for the Universe thirteen billion years ago when it's volume was much smaller it's clock rate would be slower than the clock rate for today's Universe. This brings up an interesting question. We all know that velocity is determined by dividing the distance traveled by time. If this is true then the speed of light in the early Universe would be greater than it is today. If we were to go back to just after the BB, say 10^-42 seconds then the speed of light could have been millions if not trillions of times faster than today a neat explanation for inflation.

Edited March 2, 2017 by Bird11dog

[Strange]

If we look at the clock rate for the Universe thirteen billion years ago when it's volume was much smaller it's clock rate would be slower than the clock rate for today's Universe.

 

 

I don't think that is true.

 

And, how could you compare clocks then and now? The only way of comparing clock rates is to communicate between them, or bring them together. Neither of this is an option for a clock 13 billion years ago.

 

 

 

If this is true then the speed of light in the early Universe would be greater than it is today.

 

I have been told that you can choose a coordinate system where there is no expansion of space, but a shrinking of rulers and a changing speed of light. This is generally not used because it is more complex and less intuitive.

 

It would also be exactly equivalent to the current model (as it is just a coordinate transform). So if inflation is required (and it isn't clear that it is required) then it would still be required using a different coordinate system.

[Bird11dog]

I don't think that is true.

 

Does gravity slow a clock down? Yes

 

And, how could you compare clocks then and now? The only way of comparing clock rates is to communicate between them, or bring them together. Neither of this is an option for a clock 13 billion years ago.

 

My reply to your first answers your second.

 

I have been told that you can choose a coordinate system where there is no expansion of space, but a shrinking of rulers and a changing speed of light. This is generally not used because it is more complex and less intuitive.

 

It would also be exactly equivalent to the current model (as it is just a coordinate transform). So if inflation is required (and it isn't clear that it is required) then it would still be required using a different coordinate system.

 

If you use logic as a basis for thinking I don't see how you can give that response. It s logically intuitive that what i have said is true.

Edited March 3, 2017 by Bird11dog

[Strange]

If you use logic as a basis for thinking I don't see how you can give that response. It s logically intuitive that what i have said is true.

 

 

Things that are logically intuitive are very often wrong.

[Bird11dog]

Yes but very rarely.

[Strange]

Yes but very rarely.

 

 

Citation needed.

[Bird11dog]

But you don't need a citation?

[Delta1212]

Yes but very rarely.

 

They are very rarely very often wrong?