Microwave Background Radiation

[igosaur]

The MBR, I'm reliably informed, is light emanating from when the Universe was just 380,000 years old.

 

We are now just picking up this light in the form of microwave radiation.

 

However, surely when the light first started its journey the Universe was much, much, much smaller than it is today and of course, the matter that would one day make up our solar system and the detectors that see this radiation were part of it.

 

How is it then that over 13 billion years later we can 'see' this light? Surely by this time the light from this distant era of the Universe would have long since past the position in the Universe that we now occupy.

 

I know that I am probably missing a fundamental point about the shape of the universe and inflation etc. but if someone could help me expand my consciousness on this point I'd be really grateful!!

[swansont]

The radiation was everywhere, was going in all directions, and still is, i.e. it's isotropic (or very nearly so). The radiation we see now started off somewhere else. It's not like a train that left the station, going in one direction.

[Jacques]

What was the diameter of the universe when it was 380,000 years old ?

How long does it take for light to go throught that distance ?

Me too I am missing something....

[igosaur]

The problem probably arises because in this scenario I am imagining the Universe as a huge explosion at a point in an infinite void of space.

 

380,000 years after this explosion the first light shone. Now, imagine that somehow, I'm there in a spaceship and I'm surrounded by particles that will some day make up the Sun, Earth etc.

 

I decide to have a race with the furthest photon of light in this small, brand new Universe to see who can get to the ultimate location that the Sun now occupies.

 

My spaceship can only drift with the particles that surround it but the light of course, travels at the speed of, well, light.

 

After about 13 billion years I find myself far, far away from my starting point, orbiting the Earth. How is it then that I can observe the light I decided to have a race with?

 

Surely the furthest away it could possibly have been is 380,000 light years, so, after 13 billion years how is it that I have arrived here before the light?

[Cap'n Refsmmat]

It's not a "point in an infinite void of space." It's a point. That's all there is. The entire universe expanded, with all the light in it - not just the cloud of atoms, the entire universe. Space itself expanded.

 

An atom could be in that small cloud of atoms in the beginning and never move - yet end up thousands of light-years away from the rest.

[igosaur]

So basically it's like a closed system where no matter how fast or long light travels it can never escape.

 

It just goes round and around getting more and more stretched as the Universe expands further.

[swansont]

After about 13 billion years I find myself far, far away from my starting point, orbiting the Earth. How is it then that I can observe the light I decided to have a race with?

 

Ypu aren't seeing the light you were racing with. You're seeing light that was elesewhere in the universe.

[losfomot]

Surely the furthest away it could possibly have been is 380,000 light years, so, after 13 billion years how is it that I have arrived here before the light?

 

First off, I don't think this is true, the distances could have been further apart than that, because the expansion can result in two points moving apart faster than the speed of light. So 380,000 years after the big bang, the universe could have had a diameter larger than 380,000 light years, but anyway...

 

Pick a photon at that time in the universe and the relative spot in spacetime that the Earth will eventually occupy (we'll call this spot E). Let's say the two are 100,000 light years apart. The photon is racing toward E at the speed of light, but the universe is expanding, so E is moving away from the photon at a considerable speed. 100,000 years later, and the photon is still 98,000 light years away from E. 98,000 years after that, and the photon is still 96,000 light years away from E... still moving toward E at the speed of light, but having a hard time catching up to E because of expansion... 13 billion years or so later, the photon has finally caught up to E.

 

So basically it's like a closed system where no matter how fast or long light travels it can never escape.

 

escape what? the universe?

 

It just goes round and around getting more and more stretched as the Universe expands further.

 

Not round and around, it goes in as straight a line as spacetime allows it to go in, it just has a hard time catching up to other parts of the universe because of expansion... there is a part of the universe (that the photon is moving toward) that is moving away (because of expansion) faster than light... so the photon will never catch up to that point in the universe (unless the dynamics of the universe change in the future, and the expansion slows down)

[swansont]

there is a part of the universe (that the photon is moving toward) that is moving away (because of expansion) faster than light

 

Was that always the case, though? I think not. (Hadn't thought about this before)

 

v = H₀d

 

Even though the Hubble parameter is not a constant, under the (simple and incorrect) assumption of linear expansion you have some d representing the edge of the expansion for which expansion was sublight. Even with inflation this should probably hold.

[igosaur]

First off, I don't think this is true, the distances could have been further apart than that, because the expansion can result in two points moving apart faster than the speed of light. So 380,000 years after the big bang, the universe could have had a diameter larger than 380,000 light years, but anyway...

 

You're quite right, using this example it should be no bigger than 760,000 light years across.

 

Pick a photon at that time in the universe and the relative spot in spacetime that the Earth will eventually occupy (we'll call this spot E). Let's say the two are 100,000 light years apart. The photon is racing toward E at the speed of light, but the universe is expanding, so E is moving away from the photon at a considerable speed. 100,000 years later, and the photon is still 98,000 light years away from E. 98,000 years after that, and the photon is still 96,000 light years away from E... still moving toward E at the speed of light, but having a hard time catching up to E because of expansion... 13 billion years or so later, the photon has finally caught up to E.

 

This would then mean that objects are moving apart from each other at a speed that is at least aproaching the speed of light.

 

escape what? the universe?

 

Yes, or at least all the matter that was created.

 

Not round and around, it goes in as straight a line as spacetime allows it to go in, it just has a hard time catching up to other parts of the universe because of expansion...

 

When I said round and around I meant that if a photon, or anything else for that matter, were to travel in a straight line for long enough it would eventually arrive back at its starting point.

 

there is a part of the universe (that the photon is moving toward) that is moving away (because of expansion) faster than light... so the photon will never catch up to that point in the universe (unless the dynamics of the universe change in the future, and the expansion slows down)

 

 

Surely the Universe is expanding at the same rate throughout, so, an observer on the Earth would see the same rate as an observer 10 billion light years away. And anyway, isn't the speed of light the end of the line?

[losfomot]

You're quite right, using this example it should be no bigger than 760,000 light years across.

 

No, you are just taking the distance light can travel in that time, using it as a radius, and doubling it for a maximum diameter. That is not what I meant.

 

This would then mean that objects are moving apart from each other at a speed that is at least aproaching the speed of light.

 

Yes, the farther apart they are spacially, the faster they are moving away from each other. If they are far enough apart, they will be moving away at speeds approaching light speed. If they are even farther apart, they will be moving away at speeds faster than light.

 

When I said round and around I meant that if a photon, or anything else for that matter, were to travel in a straight line for long enough it would eventually arrive back at its starting point.

 

I understood what you meant.

 

Surely the Universe is expanding at the same rate throughout, so, an observer on the Earth would see the same rate as an observer 10 billion light years away.

 

Yes. (although that rate changes over time (for everyone))

 

And anyway, isn't the speed of light the end of the line?

 

Not regarding co-moving objects (objects moving away from each other due to the expansion of space) It is one of the few times objects can go faster than light.

 

Was that always the case, though? I think not. (Hadn't thought about this before)

 

v = H₀d

 

Even though the Hubble parameter is not a constant, under the (simple and incorrect) assumption of linear expansion you have some d representing the edge of the expansion for which expansion was sublight. Even with inflation this should probably hold.

 

I thought that there has always been super-luminal expansion. I am not sure what the Hubble parameter was when the universe was 380,000 years old, but I think it was pretty high.

 

I calcu-guess an answer of 2,300,000 km/sec/Mpc

 

Which should mean (by v = H₀d) that anything more than 425,478 LY away would have been moving away faster than light.

 

Of course, I am probably way out in left field with these calcu-guesses.

[Martin]

Swansont has be giving you the straight dope about the CMBR, I will just add a note to what's been said.

 

A good handle to get on the CMB is its REDSHIFT z = 1100

 

this is the factor by which the universe has expanded (more exactly distances have expanded) since the CMB was released

 

it is also the factor by which the TEMPERATURE has decreased

since it is now 2.76 kelvin you can multiply by 1100 and find out the temp of the hot glowing gas that made the original light (when expansion was some 380,000 years old). The gas was about 3000 kelvin.

 

the redshift z = 1100 can also allow you to compute the distance with any one of several cosmology calculators available on the web

 

it should tell you how far away the patch of glowing atoms was THEN when the light departed on its way to us, and it should also tell you how far that same material is away from us NOW

 

one way to get a redshift cosmology calculator is to google "ned wright" and his website will give you a choice of several

you just type in 1100 and press some button like "flat" for the flat universe case, and it will tell you distances

 

also it may tell you what the Hubble parameter was THEN which is kind of interesting because the H used to be a lot bigger than it is today, but that is a side issue

 

have fun

[losfomot]

there is a part of the universe (that the photon is moving toward) that is moving away (because of expansion) faster than light...

 

Was that always the case, though? I think not. (Hadn't thought about this before)

 

v = H₀d

 

Even though the Hubble parameter is not a constant, under the (simple and incorrect) assumption of linear expansion you have some d representing the edge of the expansion for which expansion was sublight. Even with inflation this should probably hold.

 

Swansont has be giving you the straight dope about the CMBR, I will just add a note to what's been said.

 

one way to get a redshift cosmology calculator is to google "ned wright" and his website will give you a choice of several

you just type in 1100 and press some button like "flat" for the flat universe case, and it will tell you distances

 

I like to use this one.

 

When I enter z=1100 this calculator most definitely tells me that objects were moving at super-luminal speeds at that time in the universe.

[Martin]

I like to use this one.

 

When I enter z=1100 this calculator most definitely tells me that objects were moving at super-luminal speeds at that time in the universe.

 

when you use that one (Morgan's)

remember to put in 0.27 for matter

and 0.73 for cosmo constant

and 71 for Hubble parameter

 

(I expect you do losfomot but in case anyone else wants to try it.

 

then if you put in z= 1100 you get that the recession speed THEN was 57 c

 

and the recession speed NOW is 3.3 c.

 

interestingly it says that the distance THEN to the gas that sent us the CMB light was only 40 million lightyears. Small world back then, relatively close by today standards

[swansont]

I like to use this one.

 

When I enter z=1100 this calculator most definitely tells me that objects were moving at super-luminal speeds at that time in the universe.

 

 

when you use that one (Morgan's)

remember to put in 0.27 for matter

and 0.73 for cosmo constant

and 71 for Hubble parameter

 

(I expect you do losfomot but in case anyone else wants to try it.

 

then if you put in z= 1100 you get that the recession speed THEN was 57 c

 

and the recession speed NOW is 3.3 c.

 

interestingly it says that the distance THEN to the gas that sent us the CMB light was only 40 million lightyears. Small world back then, relatively close by today standards

 

Ah, of course, the expansion speed is slowing down (duh). One thing that gives me pause about this calculator though, is that the age of the universe is given as 0 for any z greater than ~ 100. Is that just a highly nonlinear function and a lack of precision of the display?

[Martin]

...age of the universe is given as 0 for any z greater than ~ 100. Is that just a highly nonlinear function and a lack of precision of the display?

 

lack of precision in display, mostly, I should think. the display is probably in billions of years

 

ned wright's has better precision in the display, and we could compare them

 

he is more well known and has 3 calculators at his site so I'd be more apt to trust ned wright numbers if there is a difference

 

let's see what both do with z = 100

 

(be sure with Morgan that you put in .27, .73, and 71)

 

Morgan says age THEN was 0.01 billion years

 

on the other hand Wright

http://www.astro.ucla.edu/~wright/CosmoCalc.html

says

 

"# The age at redshift z was 16.771 Myr.

# The light travel time was 13.649 Gyr. "

 

Clearly Wright shows more precision and I would be inclined to trust it. Morgan probably has the algorithm implemented much more crudely.

If Morgan were more precise she would be saying something more like 0.01677 billion years, instead of 0.01 billion years.

 

My attitude is use whatever calculator you like but realize that

1. we dont know the parameters like 0.27, 0.73, 71 perfectly so anything you calculate is approximate anyway

2. a numerical algorithm has limits and if you push those limits then not to use for more than a qualitative impression===rough idea.

3. this is all within the standard mainstream model LambdaCDM which at least for the time being is widely accepted (but could always be challenged in future)

[Severian]

I think the problem most people are having with this stems from the idea of the big bang happening at a point. It didn't - at least not in the sense that people think of a point.

 

The big bang happened everywhere at once. We only say 'a point' because in the limit of [math]t \to 0[/math] all distance scales shrink to zero and there is no distance between anything in the galaxy. So you think of everything being in the same place.

 

But this is just a limit and probably isn't very physical. At [math]t=\epsilon[/math] where [math]\epsilon[/math] is vanishingly small, there were already finite distance scales and the universe had infinite extent.

[YT2095]

so assuming the Matter (mass) of universe is not expanding at light speed, then Eventually there will come a time when the Background Big-Bang radiation and radio noise will have left us?