# How are younger objects outside our observable universe when the older CMB is an observer's temporal "edge"?

We know the observable universe is limited to objects whose light has had time to reach us and that an observer very far away will see a slightly different (assuming cosmological principle) observable universe with a slightly different (yet similar) CMB. We know that the CMB (cosmic microwave background or surface of last scattering) was released everywhere in the universe at once and is considered one of the earliest images of the universe. If objects (galaxies, stars et.) created after the CMB are outside our observable universe, why is the CMB our temporal edge? In other words, how are younger objects outside the older CMB? I understand that the universe is expanding, but wouldn't the CMB get redshifted then too?

edit:grammar

• The CMB is the most red shifted em radiation you can think of. What is really meant by your Q "if objects.... why is CMB our temporal edge? “ I do not get the thinking behind. Sep 8 '20 at 5:33

The part of the universe that we can see is a cone in spacetime (our past light cone) and there are young and old parts of the universe both inside and outside the cone.

Here's a picture from Ned Wright's cosmology tutorial that shows what's going on.

We're at the top center. The red lines are (a slice through) our past light cone. Ignore the wavy $$\phi(x)$$ line and imagine a straight horizontal line there instead. Below that line the universe was opaque, so our observable universe effectively ends at that line (just like our view of the sun ends at its surface). There are young and old galaxies, stars, etc. both inside and outside the cone.

Your question isn’t well stated and I think it shows an unclear understanding about cosmic horizons, the consequences of the speed of light and in general and the notion that the universe has no center. I could suggest a few videos. “If objects created after the CMB are outside our observable universe, why is the CMB our temporal edge” makes no sense. First of all, it’s not IF objects created after the CMB are outside our observable universe. Objects ARE created after the CMB within our observable universe and outside our observable universe - no “If”. The CMB is usually used as a temporal line in physics but for your question you are really asking it as a distance line - “a horizon line” but you need to understand that and not say “temporal edge”.

You can’t compare a distance thing (objects created outside of something) to a temporal time thing -which seems to me to be how your major confusion is arising.

Fundamentally stuff we can’t see is because it’s in parts of the universe that was moving much to fast to send light that will ever reach us. Its not about time as much as physics doesn’t let matter/energy travel faster than the speed of light but it allows space to expand faster than light. You probably know that but keep it in mind.

To get to nuts and bolts then, the distance limit of the observable universe is 46 bly (billion light-years) away. “Today” there are galaxies, at that distance, that are 13.8 billion years old, just like here. Where light generated during the Last Scattering Event, for Earth observers, is today is somewhat closer still probably 30 bly away or so (I'm not going to do the math) and...again there are galaxies there that are, unsurprisingly, 13.8 billion years old!...just like here and this is because “the universe has no center”. And maybe it would also be useful for you to realize, observers there see us as the source of THEIR CMB! Think about your question a little more now and it should resolve.