Why We Have Night: Exploring the Science Behind it

[discord73]

I think you're talking about Olber's paradox. Expansion plays some role, but the main thing to keep in mind is that the universe is thought to have only a finite age, so light from stars too far away just won't have had time to reach us.

If the universe is 14.5 billion years old and If some stars are so far away they haven't had time to reach us yet wouldn't that mean that the universe was at least 14.5 billion ly wide when it was formed. and if the universe was created with a big bang it would have had to expand awfully fast to get to 14.5 b ly wide before any stars formed to start emitting light. also if we look far enough away shouldn't we stop seeing galaxies and start seeing just stars if the universe was created by a big bang and stars formed first and then the stars formed into galaxies?

 

[neopolitan]

If the universe is 14.5 billion years old and If some stars are so far away they haven't had time to reach us yet wouldn't that mean that the universe was at least 14.5 billion ly wide when it was formed. and if the universe was created with a big bang it would have had to expand awfully fast to get to 14.5 b ly wide before any stars formed to start emitting light. also if we look far enough away shouldn't we stop seeing galaxies and start seeing just stars if the universe was created by a big bang and stars formed first and then the stars formed into galaxies?

Ok, I wrote out some stuff on this then accidentally navigated away and lost it all. So you are going to have to seek out the references yourself, because frankly I can't be bothered doing it again.

The observable universe is 46.5 billion ly in radius. The age of the universe is 13.73 billion years (give or take a bit). The earliest radiation we can see is CMB, which is about 3.35 billion years old. The actual size of the universe is thought to be at least 78 billion ly in radius. So your figures are out.

You have misunderstood how cosmological expansion works (not to mention star and galaxy formation).

Here are some figures which may help. The rate at which two points in space expand due to cosmological expansion is proportional to their separation (multiplied by the Hubble constant).

Alpha Centauri is 4.37 ly away, so we are separating from AC at 1mm/s. When light reaches us from AC, we have separated by about 30 km.

Omega Farus (I made it up) was 3.5 billion light years away when a photon was emitted. Once that photon has traveled 3.5 billion light years, assuming an invariant Hubble constant at its current value, then the separation (between the source and us) will have increased by about 1.5 billion light years. Note, this is hugely simplified - for example, 3.5 billion years ago the Hubble constant would have been about a third again more than what it is now, which means that we and Omega Farus were separating more quickly when the photon started out but on the other hand, the rate of separation between where the photon was and where we were would decrease as the photon approached us. The point is that a photon from a star that was 3.5 billion ly away at emission would take (very very roughly, assuming a Hubble constant of 70.8 throughout and doing some very rough element analysis) about 5 billion years to get to us.

I'm sure there is someone else out there who has done the more complicated maths :)

cheers,
neopolitan

 

[DaveC426913]

Is thought by whom? A lot of the research going on in modern cosmology extends the models back before the big bang.

While that may be true, I do believe that, by definition, our universe began with the BB. Whatever was before the BB is not part of this universe.

 

[austin123]

it seems that there are grounds for saying that the "edge of the universe" is expanding away from us at the speed of light, if we define the edge of the universe to be "that bounding everything that can or ever could have an impact on us" - it's not necessarily a real thing. Since the edge of the universe is 14 billion light years and the universe is suspiciously close to 14 billion years old.http://www.o2sensorsdirect.com"