How can I see early light from the big bang? Have I travelled faster than it?

[psjmartin]

This has always puzzled me. I keep hearing that with increasingly powerful telescopes we can see further into the past. I completely get the concept of light years and the fact that we are seeing objects as they were when light started its journey but to see light that started shortly after the big bang wouldn't I have had to travel faster than it to be able to observe it?

[imatfaal]

Your question makes a lot of sense - we can see light from about 300000 years after the big bang; we call this the Cosmic Microwave Background But surely what ever age it is, how is it just reaching us now? - ie how did the part of the universe that we are in seem to travel faster than the light?

 

This is explained by a phenomenon we call expansion. Whilst nothing can outpace light - what has actually happened, and is still happening, is that space itself expands . It's a difficult concept to understand or explain - every point in the universe gets a little bit further away from everything else, more space is created in the gaps between the clusters and superclusters. Because this is not objects travelling through space (which is limited to less than light speed) - it is space itself which is getting bigger and thus the distances between objects can increase faster than light can bridge the gap. In the very early universe this expansion was very rapid and the gaps between objects in the universe increased very quickly.

 

It is hard to believe but when we detect photons of the cosmic microwave background radiation we are observing photons that have been travelling for 13.4ish billion years - but they are now over 44 billion light years from the area of the universe where they were emitted. This is the reason they are cold microwave radiation - when they were emitted they were hard ultra-violet light from white hot plasma; but as they travelled in the expanding universe they themselves were stretched! This stretching increased their wavelength ie decreased frequency and they red-shifted from ultra-violet to microwave.

 

In short - light can still be catching up with us because the path it had to take was stretched by universal expansion

[liver]

think of the light not coming from a point but from a point on an already expanded "bubble" and not from "You" , but another, already distant neighbor. the light you see is not the light that "You" emitted, and it is taking the LONGEST way, a Great Circle around that expanding bubble universe to get to you. Stick around, there is more light on the way. from every way.

[Iggy]

I know I'm playing catch-up in this topic, but it's an interesting topic and a good question.

 

If you look at a spacetime diagram,

 

 

 

you see the past light cone (the red lines) moving away from us for the first 4 billion years, so I agree with the conclusion of the OP and Imatfaal. The most distant light that we see now was moving away from us, even though it was pointed at us and trying to make its way here, for the first 4 billion years. Space was expanding that fast.

 

It always amazes me to think that the CMB we see now started out only 42 million lightyears away from us (closer than the virgo cluster is now) and it took light more than 13 billion years to cross what was initially such a small distance. Fascinating.

[krash661]

look at that hubble line,

I always enjoy images from it.

[imatfaal]

I know I'm playing catch-up in this topic, but it's an interesting topic and a good question.

 

If you look at a spacetime diagram,

 

 

 

you see the past light cone (the red lines) moving away from us for the first 4 billion years, so I agree with the conclusion of the OP and Imatfaal. The most distant light that we see now was moving away from us, even though it was pointed at us and trying to make its way here, for the first 4 billion years. Space was expanding that fast.

 

It always amazes me to think that the CMB we see now started out only 42 million lightyears away from us (closer than the virgo cluster is now) and it took light more than 13 billion years to cross what was initially such a small distance. Fascinating.

 

Nice diagram - it repays close study. Agree is it fascinating and slightly disturbingly counter-intuitive; the portion of space that emitted the CMB we see now was 42Mlyr away when emitted, and is now 44Glyrs away - and that light has taken 13Gyrs to cross the distance between there and here.

[michel123456]

Great diagram.

 

Something interesting is also to show what ancient people (if they existed) would have observed from their point of view as in the following

 

 

[Iggy]

Nice diagram - it repays close study. Agree is it fascinating and slightly disturbingly counter-intuitive; the portion of space that emitted the CMB we see now was 42Mlyr away when emitted, and is now 44Glyrs away - and that light has taken 13Gyrs to cross the distance between there and here.

 

No doubt. I envy cosmologists for the scope and, you could almost say majesty, of their field of study.

 

Something interesting is also to show what ancient people (if they existed) would have observed from their point of view as in the following

 

Yep. Agreed.

 

If they were at t = 5 billion years then I was trying to figure out what the comoving distance to their CMB would be... the size of their visible universe back then, but I can't figure it.

 

I know their Hubble parameter would have been 139.55 km/s/Mpc (ours is 71). If a = 1 then their scale factor would be 1/2.264.

 

Let me make a diagram...

 

 

imagining that the dotted green line is the CMB (I know it is too high up, but otherwise the diagram would be absurdly big) the purple line is 46 billion lightyears, the green is 42 million, the light purple 20.3 billion, but...

 

I can't figure the dark blue line marked x... :-/

[michel123456]

Why would the CMB remain at the same place on the diagram for observer today and then?

I would expect to be an exact scaling of the original diagram, including the scaling of CMB.

Edited May 1, 2013 by michel123456

[Iggy]

Why would the CMB remain at the same place on the diagram for observer today and then?

I would expect to be an exact scaling of the original diagram, including the scaling of CMB.

 

Right, the CMB spreads out along the light blue lines. It turns into galaxies. The line that I marked CMB is the time that the CMB was emitted.

 

Actually, I marked t = 3.75 billion years just because it would have been impossible to put it down far enough to represent 370,000 years when the CMBR was actually emitted. But, yes, the idea is that the CMB spreads out along the light blue lines and turns into galaxies. The dark purple line, for example, is meant to represent 46 billion lightyears distance meaning that the CMB which was originally 42 million lightyears away (the green line) has receded from us and turned into galaxies that are currently 46 billion lightyears away.