# How do we know the current state of the universe by observing it from Earth?

Since light travels at the constant speed and takes more than a billion years to travel from the distant parts of the Universe, how do we know the state of the distant regions? Do those still exist or not? Is our picture of the Universe outdated?

Since light travels at a constant and finite speed, looking into the Universe means looking back in time.

No matter in which direction we look, on average we see the same: Locally, we see evolved galaxies, far away we see young galaxies, and even farther we see the so-called cosmic microwave background, which is light from so far back in time that galaxies hadn't yet formed.

That is, the Universe seems to be isotropic. Unless we occupy a special place in the Universe, this implies that the Universe is also homogeneous, i.e. it would look more or less the same if you were located in another part of it.

This is turn means that the current state of the Universe can be inferred by observering the local region. The most distant observed galaxies can then be concluded be in the same state (statistically speaking; of course they don't look exactly the same, but any given property, e.g. the distribution of their sizes, will be the same as in our neighborhood).

• Thanks for answering! I am new and my votes don't show up yet. You also answered a question I had about the CMB. BTW when you mentioned isotropic, is it a 360 degree view? Do we actually see the CMB kind of like the horizon whichever way we look? If yes, what would someone in the visual location of the CMB view? Sorry if I am not allowed to ask questions recursively. I can repost as another question(s) if those are the rules. Feb 6, 2016 at 1:06
• @signsgeek: Yes, the horizon on Earth is a good analogy of the CMB "horizon". But whereas the 2D surface of Earth makes the horizon a circle with you in the center, the 3D space of the Universe makes the CMB come from a sphere with you in the center. And where another person on Earth sees another — but similar — horizon, a person in a distant galaxy sees another — but similar — CMB surface. If that galaxy happens to be located at the CMB horizon as seen from us, we will be at his CMB horizon, just like for the sailor at the ship in your horizon, you'll be in his horizon.
– pela
Feb 6, 2016 at 1:26
• But yes, clarifications are fine in comments, but if they're too far from the original question, in general they should probably be posted as new. Wrt. the CMB, you may be interested in this answer, and possibly this.
– pela
Feb 6, 2016 at 1:28