The universe was smaller and more dense in the past. Why we can see the oldest galaxies in the world when we look at the outskirts then?

36 views
0

I just can’t wrap my mind around this. Can we see them anywhere? Why, if the universe was smaller? Or is there like one place in space where the aftermath big bang happened (I know there was no space at the time and big bang kinda went everywhere ofc) and we are pointing our telescopes at it?

Using human logic we should see the youngest galaxies (as their images in the past) far away and just won’t be able to spot the elders.

In: 25

>Using human logic we should see the youngest galaxies (as their images in the past) far away and just won’t be able to spot the elders.

This is correct, there’s a bit of a semantics issue here. When we talk about seeing the oldest galaxies, we mean the galaxies from the earliest times in the universe. This means that ‘now’ they will be incredibly old, but the image we are seeing of them is from when they were very young.

Two reasons, the farther something is away the longer it’s taken the light to reach us, so when we see something that’s millions of light years away we’re actually seeing what happened millions of years ago. Because the things we see normally are so close we don’t realize that light is not instantaneous, so a way to visualize it in your mind is to think about when lightning hits, there’s a delay with the sound. The time that it takes the sound wave to reach her ears is analogous to the time it takes light from an object millions of miles away to reach your eyes. Also the universe is expanding so everything is getting farther apart, so while certain things that would have been seeable eventually we will not actually be able to see because they are receding, other things we are able to see a little bit more in the past so to speak because that light has been delayed getting to us because that object is receding from us.

Unfortunately, this one just is unintuitive, there’s no way around that. I’m personally not aware if there is a universal, unambiguous answer to “why would something be almost 14b ly away if the universe was tiny”, that’s a question to an actual astrophysicist.

One possible answer I do know is that the universe could actually be infinitely big, so scaling infinity down by any number is still infinity.

If it’s not infinitely big, then I’m not sure what the correct answer is, although my moderetaly-educated guess is that our perceived 3D space behaves like the surface of a sphere, so it doesn’t really have an “edge” and light can just kind of… Go in circles until it interacts with something. But again, that’s my guess, not a professional answer. If an astrophysicist states otherwise, their answer is the valid one.

Okay here’s a mind bender for you — the Big Bang did not happen at a point in space, it happened to space itself. Which is very confusing but let’s look at it another way:

*Every* line of sight looking away from the earth is looking back in time toward the Big Bang, no matter which direction you’re looking. It’s weird but it *has* to be true, because every photon also emerged from the Big Bang — so if we can see it, it came from the same place. If you traced any two rays of light that reach our eyes (or telescopes) backwards along their timeline, they will get closer together than they are now, because the universe was smaller. If the Big Bang itself actually happened at a singular point (this is a huge “if” by the way), then those two rays would *have* to converge at that point at the moment of the Big Bang, because there is nowhere else for them to *be.* But we could (theoretically) see that single point no matter which direction we looked, as long as we looked far enough back — it would be stretched out across the entire night sky, like the inside of a sphere surrounding our observable universe.

So from our perspective, the past smaller, denser universe is not in a specific region of space that we have to look toward. That earlier state of the universe *surrounds us* like a shell. No matter where we look, if we look far enough away, we see younger stars and galaxies that are clustered together more closely than the older stars and galaxies closer to us. The furthest away we can see, the first light emitted by the universe, is the Cosmic Microwave Background radiation, which shows a picture of a universe so dense that it basically looks like the inside of a star. The fluctuations in temperature are about one part in a thousand. At the time the CMB was emitted, the observable universe was only about 100 million light years across, but from our perspective the CMB appears to create a sphere around us that is ~80 *billion* light years across, due to the expansion of the universe since then.

They are moving away from us faster than the light travelling towards us so we will never see them.

The rate of expansion of the universe over very large scales is faster than the speed of light, so once you get far enough away the light cannot travel fast enough to reach us and there is no way we could ever see those very far away, very young galaxies.