How can we see all the way back toward the singularity?

Question

According to Brian Greene, The Fabric of the COSMOS (his CDs), if the whole universe could be reduced to the size of earth, the part of it that would be available to us for observation would be the size of a grain of rice; that the light produced by distant planets and galaxies have not yet had time to reach us, and won't until well after the earth and our solar system has long gone out of existence.

If that is so, then how can astronomers say that they have looked as far back as the first star systems at the edge of the universe?

So far, I have seen some really good answers, but I just don't get it. At the singularity, just after the big-bang, inflationary theory says that "stuff" expanded out very, very rapidly filling what was then the flat universe. The universe cooled, matter formed, and the space fabric began to stretch; thus, spacetime. There is no center or point of origin to the universe. Then, if a galaxy formed, the tadpole galaxy, and it was so far away that its light hasn't had time to reach us. We would have no idea that the tadpole galaxy even exists. If we haven't seen the tadpole galaxy yet, what does looking back to 13.5 billion years into an area of space that we can see, really mean? Wouldn't that suggest that the tadpole galaxy is much further away and older than 13.5 billions years of light travel? Given "zero space," and the Calabi-yau manifold at the Planck length; and the singularity happened uniformly throughout the universe at once---what does it all mean! LOL

Answer 1

Because the light from the very first moments of space and time have already passed us. We can only see back to a certain point where galaxies where very young. About 15 billion light years to be more precise.

Answer 2

If you look 13 billion light years away, you don't see the universe as it is now, but as it was 13 billion years ago, since that is the time it took light to get to us. That is where we directly observe the first star systems.

We cannot see all the way to the Big Bang, because when the universe was younger than 100,000 years or so, it was opaque. We CAN see the Cosmic Microwave Background, which is the leftover radiation from this hot opaque era.

There is no edge of the universe. But there is a horizon to the observable universe. We can't see more than 13.7 billion light years away, because light from those regions has not had time to reach us. This horizon is filled with the CMB and it is opaque.

Answer 3

Astronomers can see star systems that are co-eval with the very early universe. Yes, they we can only see part of the universe as it was at a given point in time; we see things at other distances at other stages in the life of the universe. When astronomers say they are seeing back to "the first star systems" they mean they are seeing star systems that formed around the same time as the first star systems, but of course they cannot see ALL of the star systems that formed at the time.

Astronomers often make the assumption that if we sample a large enough volume of space at a given distance/point in time in the universes past, that that chunk of space pretty much looks like any other chunk of the universe at that same time period. Given that the microwave background points to a universe that appears pretty much the same in every direction, this is probably a reasonable assumption to make.

So, these early star systems are thought to be very much like the star systems that existed in our neighborhood of the universe 13 billion years ago(*), and even though we can't see the very ones that were 'here' then, we can still know what the local neighborhood was like then from the systems that we CAN see.

Answer 4

If the CMB is the remnant of the big bang and that opaque area was expanding, does it not make sense to assume that that radiation is now 13.5 billion light years from where it was and in fact it now makes up the edges of the universe and that is why it detected from all points in the sky? Further, if this is so it would make the universe 27 billion years in diameter.

Answer 5

I'm not sure you got his messae right. We see the microwave background which is the red shifted fireball from the Big Bang - the start of the universe. And as we look far out into the cosmos, we see early galaxies and formations. We can measure the age of the light reaching us by using "standard candles" - types of stars whos luminocities are well know. By looking for them in the very distance formations, we can estimate how far away, hence how old, they are.

Answer 6

No light or electro-magnetic radiation existed in the universe until the first stars lit up.
It was 100 million years before this happened so you would not be able to look back beyond this point.