I How Does Space Expansion Affect the Velocity of Galaxies?

[phinds]

What I meant is that if you're looking that far away with a telescope then galaxies had not yet formed.

You really need to think this through. Having been formed and being visible are TOTALLY different things. Think about what a comoving observer at that location would see vs what we see of that location.

 

[Bandersnatch]

You really need to think this through.

There's nothing wrong with his thinking that I can see.

 

[PeterDonis]

What I meant is that if you're looking that far away with a telescope then galaxies had not yet formed.

Had not yet formed at the time the light we're seeing now was emitted, yes. But in the rest of your post you're not talking about an observer at that location at the time when that light was emitted. You're talking about some observer there "now", seeing light emitted from whatever existed at our galaxy's location then. And according to our best current model, what that observer would see "now" in light emitted then from our location would be similar to what we see now in light emitted then from their location.

 

[phinds]

There's nothing wrong with his thinking that I can see.

Well, the phrasing is ambiguous. He got what WE see right but a comoving observer would not see what he seems to think would be seen.

 

[Dan White]

Well, the phrasing is ambiguous. He got what WE see right but a comoving observer would not see what he seems to think would be seen.

Sorry for confusing everybody. I'm new here and only have a superficial knowledge of this stuff. I signed up here to learn more. I can see that one needs to be precise when commenting here. My original post was like a mixed metaphor. Let me restate so I know I understand correctly:

We can view light from the earliest time of the universe because of the distance light has to travel to get to us. There were no galaxies that far back in time so we won't see any, or maybe just proto galaxies. If we now consider present day, that same region is now populated with galaxies, although we won't see them because it takes so long for the light to get here. Also, due to expansion, whatever was in that region billions of years ago is now much farther away. In my original comment I was merely considering two observers, each in their own galaxy, at the edge of each other's observable universe.

 

[PeroK]

Sorry for confusing everybody. I'm new here and only have a superficial knowledge of this stuff. I signed up here to learn more. I can see that one needs to be precise when commenting here. My original post was like a mixed metaphor. Let me restate so I know I understand correctly:

We can view light from the earliest time of the universe because of the distance light has to travel to get to us. There were no galaxies that far back in time so we won't see any, or maybe just proto galaxies. If we now consider present day, that same region is now populated with galaxies, although we won't see them because it takes so long for the light to get here. Also, due to expansion, whatever was in that region billions of years ago is now much farther away. In my original comment I was merely considering two observers, each in their own galaxy, at the edge of each other's observable universe.

It's worth mentioning that the farthest back we can see using light is the CMBR (Cosmic Microwave Background Radiation). This is from the earliest time that the conditions in the universe allowed light to propagate effectively indefinitely. Prior to that the universe was so dense that light tended to get absorbed by collisions with elementary particles.

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

That said, reasearchers are now trying to detect gravitational waves from even earlier times, as an alternative means of studying the very early universe. Here's something on this from MIT.

https://news.mit.edu/2020/universe-first-gravitational-waves-1209

 

[Dan White]

It's worth mentioning that the farthest back we can see using light is the CMBR (Cosmic Microwave Background Radiation). This is from the earliest time that the conditions in the universe allowed light to propagate effectively indefinitely. Prior to that the universe was so dense that light tended to get absorbed by collisions with elementary particles.

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

That said, reasearchers are now trying to detect gravitational waves from even earlier times, as an alternative means of studying the very early universe. Here's something on this from MIT.

https://news.mit.edu/2020/universe-first-gravitational-waves-1209

Is there something specific about the CMBR that makes it stand out from other radiation that did not originate from the Big Bang? They say something like 1% of the static on your TV comes from the CMBR. How do we know?

 

[Ibix]

Is there something specific about the CMBR that makes it stand out from other radiation that did not originate from the Big Bang?

Yes and no. It's just microwave radiation and totally undistinguished in some senses. However, it's black body radiation at a very nearly constant temperature coming from all parts of the sky. So there's nothing special about the radiation, but the source and the spectrum are fairly distinctive.

They say something like 1% of the static on your TV comes from the CMBR. How do we know?

I'd take anything "they" say with a pinch of salt, unless it's backed up by some maths. A quick Google of the frequency of TV frequencies suggests that they are considerably below the peak frequency of the CMB (a few hundred MHz versus 160GHz), but there's some CMB power in the TV bands. I don't know about 1% though - I'd tend to think local interference sources vary wildly in strength so I'd be very suspicious of a blanket figure like that.

 

[Bandersnatch]

Isn't the old TV static mostly thermal emissions from the atmosphere? You'd then get that 1% from relative temperatures of the sources.

 

[Vanadium 50]

What is this "static" of which you speak?

 

[Vanadium 50]

so I'd be very suspicious of a blanket figure like that.

We can at least see if it's in the ballpark. The sun's disk is 7 x 10^-5 steradians. Or 10^-5 of the sky. It's 2000x hotter than the CMBR so it puts out 2000x as much energy per unit frequency per unit area, or in total 2%.

You definitely can see the sun in static, by looking at day vs. night. I don't think it's a factor of 2 - maybe 1.5, but I'm just spitballing. I am going to call it a factor of 2. And that takes our 2% down to 1%.

So it's not a crazy number. My estimate/guess is that it's a little high but not crazy high.

 

[Vanadium 50]

Boy did I goof. The CMBR actually carries more energy than the sun: 50x as much, not 1/50 as much.

Therefore the day-night effect on static is not due directly to the sun. It's got to be the ionosphere, either directly, or indirectly (e.g. RF signals from far away lightning or other TV stations/multipath).

I now wonder if the 1% is an underestimate. At 1% that means there is a source 5000x brighter than the sun in RF that isn't the CMBR. And it has to be very, very cold (about 1K).

 

[Hornbein]

Ehm guys I don't have the background to fully grasp your replies but are you saying that GR even explains dark energy?

GR is a set of constraints. There are many states of the Universe that are consistent with these constraints. If your theory doesn't meet these constraints then doubt is darkly cast upon it. If your thesis does meet these constraints that's nice, but doesn't give any evidence that your idea is correct.

It appears to me they are saying that dark energy is consistent with GR.

 

[Bandersnatch]

What is this "static" of which you speak?

That I know not. But I heard tell of the mythical 'teevos' roaming the land in the days long past, when phones were rotary and news were made of paper. It was said that a teevo, if left to its own devices, would speak in 'static' to the befuddlement of onlookers.

 

[pinball1970]

Something is confusing me

Bob is on Earth and Alice is one of those galaxies at the edge of the observable universe which is C or greater than C (post 13)

What does that do to their respective clocks?

You plug velocities in the time dilation equation at or above C and things get strange.
I am getting something wrong here- not sure what it it

 

[Orodruin]

You plug velocities in the time dilation equation at or above C and things get strange.
I am getting something wrong here- not sure what it it

You cannot use the time dilation formula from special relativity. The description of the expanding universe requires general relativity.