How are we able to observe the early universe?

Submitted by four-lima-golf t3_111fyu5 in askscience

I understand the basic concept. It takes a long time for light to traverse great distances, which means the furthest objects in the observable universe are also the oldest.

The question is more about how this light is only now reaching us. If the universe expanded slower than the speed of light EM from this time would have passed us already. If it expanded faster, the light would never catch up. I doesn't seem like we should be able to see anything at all.

A search suggests this is possible because of a phenomenon called hyperinflation. The early universe was hot and dense, expanding faster than the speed of light. However, explanations of hyperinflation seem to indicate that it only lasted a few seconds immediately following the big bang, so this would have nothing to do with light from early stars.

Is heat and density the key here? I know it takes about 100,000 years for light created in the core of the sun to reach the surface. Are we talking about light that was trapped and released as the universe expanded and cooled?

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Aseyhe t1_j8h9e1k wrote

I think the point you are missing is that the universe is (statistically) the same everywhere. This means that there will always be light reaching you from some distance -- and hence some time -- and the objects that you see at that distance/time have similar statistics to what happened in our own past.

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_mizzar t1_j8r9mfl wrote

The above comment is the best answer because it focuses on your primary misunderstanding which is that the past we are seeing into is not the past of “our part” of the universe.

The universe is likely infinite. The observable universe is a sphere with us in the middle. The edge of the sphere is where we see the oldest parts of the universe because the light from these distant places is just now reaching us, showing us what things looked like back then.

This sphere is getting bigger for an obvious reason, more and more light from distant places is reaching us. However, the sphere is also getting bigger because the entire universe (not just the observable universe sphere) is expanding.

Careful here not to imagine the entire universe’s expansion as a sphere, but rather every galaxy that isn’t locally bound to another galaxy by gravity is moving away from one another.

An oversimplified way to imagine this is to visualize an infinite 3D space with tennis balls each 10 meters from one another in every direction. Move forward through time and as the universe expands they are now 20 meters away from one another. Move back in time and they are 5 meters away from one another and so on.

The interesting thing is that, though the speed of light is constant, this expansion of the entire universe seems to happen faster with the more space that there is between things, as if the space itself was causing the expansion (we call this mysterious force Dark Energy).

What this means is that eventually the expansion of the entire universe will outpace the speed of light, making galaxies we can currently see in the observable universe fade out of sight as they slip out of our observable universe. Eventually, only our own galaxy (at this point merged with Andromeda) will visible to us, everything else too far away and the universe expanding too fast for new light to reach us.

If humans still exist in this time, they would have no knowledge of other galaxies and the universe unless we managed to pass down the data from our time.

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Pharisaeus t1_j8hq86v wrote

> If it expanded faster, the light would never catch up. I doesn't seem like we should be able to see anything at all.

The mistake here is that you dismiss the fact that speed of the expansion is related to distance. What happens is: "space expands". Imagine that 1m of space at some point becomes 2m. This also means that 100m become 200m in the same timespan. Notice that this means that object which was 1m away is now 2m away (so moved away by 1m) but object which was 100m away is now 200m away (so moved away by 100m).

So while expansion makes everything further away from everything else, the distance change is greater the further the object is. So objects which are closer are getting away slower, and objects which are further are getting away faster (and even faster than the speed of light!).

It's true that light from things very far away won't ever reach us, because space expands faster than the light can travel, but there are lots of objects closer than that, and light from those objects will eventually reach us.

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[deleted] t1_j8gtc77 wrote

[removed]

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ncc81701 t1_j8gxtcw wrote

To put that into context, something that was 1mm apart is now 10.57 million light years apart after a 10^26 x expansion.

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Skarr87 t1_j8iuyal wrote

The inflation of the early universe is what put the material that will eventually become the stars far apart from each other. Imagine if you have an empty balloon and you put little dots on it close together. Then you inflate it, those dots will now be much further apart. Now anything that happens to those dots has to travel the distance in between.

I think the problem you may have is incomplete understanding of the time scale. Right after the Big Bang up to about 10^-36 seconds all the fundamental forces were one, after that they began to separate from each other. We think this is what caused cosmic inflation. From that time to about 10^-32 seconds cosmic inflation occurred. Nuclei would have began forming a bit later around 10^-6 seconds to 1 second after the Big Bang. By then everything was already spread out. Starts won’t form for somewhere between 100k to 100 million years later.

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fuerdiesache t1_j92psso wrote

for your 2nd para, how do we know all this is correct? isnt cmb the only information we have from early universe and it comes after 100k years of big bang? (if so, all this 10^-36, 10^-6 secs after the big bang sounds like a bunch of hokum, when there is zero evidence to verify it)

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Skarr87 t1_j93wpsi wrote

Correct we don’t have direct evidence from that early right after the Big Bang but we can use mathematical models we’ve developed from centuries from scientific experimentation. We then work these models back to conditions that we would expect that early in the universe. These models then make specific predictions that we can perform other experiments to see how accurate our models are. For example the standard model predicted that in conditions with very high temperatures the fundamental forces become one and at slightly lower temperatures something called quark gluon plasma would form where protons and neutrons themselves break down into almost a soup. When we were able to engineer particle accelerators to have enough energy we were able to actually produce quark gluon plasma and partially prove that at least the weak force and electromagnetic force combine into the electroweak force.

It’s all kind of like a big jigsaw puzzle. Every time we make a new discovery it’s like we add another piece telling us more and more about the nature of the whole picture. If you have 60% of the puzzle finished and you see a corn field it’s reasonable to assume the rest isn’t going to be something like an underwater scene.

We don’t know everything about what happened immediately after the Big Bang, but assuming the laws of physics weren’t simply different then it’s not going to be drastically different than what we think barring some new paradigm shifting discovery.

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