The speed of light, etc

[idi banashapan]

Imagine the universe like a giant, stretchy balloon with tiny dots drawn on it. These dots represent galaxies.


1. The Universe Expands Like a Balloon

At the very beginning, the universe was tiny, hot, and dense. Then, it started expanding, like blowing up a balloon. The galaxies (dots) weren’t moving through space; instead, space itself was stretching between them.


2. The Light We See Started Its Journey Billions of Years Ago

When we see light from a galaxy that’s 13 billion years old, it means that light left that galaxy 13 billion years ago. But back then, that galaxy was already far away from where we are now, the universe had expanded a lot even by that time.


3. Space Has Stretched the Light on Its Way to Us

Light always travels at the same speed (about 300,000 km per second), but because space itself is expanding, the journey the light had to take got stretched over time. This also makes the light appear redder (this is called redshift). Light moving away appears bluer. This is the light version of a doppler in audio (where an police car siren appears to change frequency and pace as the car approaches and then goes past).


4. The Galaxy Is Much Further Away Now

Even though the light has taken 13 billion years to reach us, the galaxy it came from isn't 13 billion light-years away today. It's much further, maybe 30+ billion light-years away! That’s because space has been expanding the whole time.


So, Why Can We See It?

Even though space was smaller back then, there was already a path for the light to travel. That galaxy wasn’t in the exact spot where we are now, but its light had enough time to reach us as space stretched.


Think of it like this:

If a friend sends you a balloon with a drawing on it, but the balloon inflates while the drawing is traveling to you, the picture still arrives, it just looks stretched out.

That’s what happens to light in our expanding universe!

[mrmistoffelees]

The other way to think of the universe expanding is the raisin bread baking analogy
It’s not the raisins moving but the dough between them expanding

[Pierre]

By definition we are at the centre of the observable universe.

[Sephiroth]

Originally Posted by idi banashapan

Imagine the universe like a giant, stretchy balloon with tiny dots drawn on it. These dots represent galaxies.


1. The Universe Expands Like a Balloon

At the very beginning, the universe was tiny, hot, and dense. Then, it started expanding, like blowing up a balloon. The galaxies (dots) weren’t moving through space; instead, space itself was stretching between them.


2. The Light We See Started Its Journey Billions of Years Ago

When we see light from a galaxy that’s 13 billion years old, it means that light left that galaxy 13 billion years ago. But back then, that galaxy was already far away from where we are now, the universe had expanded a lot even by that time.


3. Space Has Stretched the Light on Its Way to Us

Light always travels at the same speed (about 300,000 km per second), but because space itself is expanding, the journey the light had to take got stretched over time. This also makes the light appear redder (this is called redshift). Light moving away appears bluer. This is the light version of a doppler in audio (where an police car siren appears to change frequency and pace as the car approaches and then goes past).


4. The Galaxy Is Much Further Away Now

Even though the light has taken 13 billion years to reach us, the galaxy it came from isn't 13 billion light-years away today. It's much further, maybe 30+ billion light-years away! That’s because space has been expanding the whole time.


So, Why Can We See It?

Even though space was smaller back then, there was already a path for the light to travel. That galaxy wasn’t in the exact spot where we are now, but its light had enough time to reach us as space stretched.


Think of it like this:

If a friend sends you a balloon with a drawing on it, but the balloon inflates while the drawing is traveling to you, the picture still arrives, it just looks stretched out.

That’s what happens to light in our expanding universe!

Thanks for the logical approach - which is how I like it to be.

First, though - a correction: Blue shift occurs when an object is approaching an observer, not receding. Local galaxies might be approaching but expansion of the universe outside any locality is generally accepted due to the red shift observations.

I've got a problem with the balloon analogy where it is stated that the objects on the edge aren't moving - just the space between them. That's not logical to my mind. As the balloon expands, if that's what happening, the objects are moving further from the notional centre. Indeed, the red shift itself indicates relative distance and thus relative motion of the objects being observed.

SO that leaves still with the conundrum that light leaving somewhere close to the centre of the universe that was released closer to the time when the so-called Big Bang occurred and our region of space had not yet been expanded into cannot be seen here! That would knock the Big Bang theory on the head unless it occurred at least 2x the 13.5 billion years that the light we're seeing was emitted.

Plus, relatively, a galaxy on the "other side" of the "balloon" would be even further away and we have no tools with we we could see the so far red-shifted EM waves. It's even been postulated on this thread that such galaxies are receding from us at a speed faster than light itself (hence Einstein's relativity theory).

Logic versus the balloon?

[GrimUpNorth]

If the universe is expanding, and everything is moving away from everything else, how can we be on a collision course wth Andromeda?

I've watched every episode of How The Universe Works on Discovery and still don't understand it.

[Sephiroth]

Originally Posted by GrimUpNorth

If the universe is expanding, and everything is moving away from everything else, how can we be on a collision course wth Andromeda?

I've watched every episode of How The Universe Works on Discovery and still don't understand it.

I've got that one! In our "local" region, the force of gravity plays its part. That'll be going on everywhere; clusters of stuff interacting and expanding away from other clusters of stuff.

I watch "How the Universe Works" too and hold it in disrespect somewhat (not that what I think matters). It's full of "Space-Time" shit which explains nothing.

[joglynne]

If the universe can be likened to an expanding balloon, with everything travelling away and apart, is there still something at the centre and what is surrounding those galaxies that have travelled the furthest, is there no limit to the 'space' we are all expanding into. Hope it's not finite eventually causing the balloon to implode
Blooming heck, another headache and am due to at hospital soon.

Off to have a lie down in a dark room.

[Sephiroth]

The expanding balloon analogy is totally flawed, imo. One has to consider what is causing the universe to expand. It could be one of three choices:

1. A big bang, so big and sufficiently "recent" for expansion to still be occurring.

2. An unknown process where matter is being spontaneously created and filling in the universe. Why the latter (so-called Dark Matter) should cause expansion when gravity might suggest otherwise is beyond me; and I haven't found that question addressed by the scientists.

3. We haven't go a bloody clue what's really going on. Hence my reversion to logic.

[Jaymoss]

Originally Posted by Sephiroth

The expanding balloon analogy is totally flawed, imo. One has to consider what is causing the universe to expand. It could be one of three choices:

1. A big bang, so big and sufficiently "recent" for expansion to still be occurring.

2. An unknown process where matter is being spontaneously created and filling in the universe. Why the latter (so-called Dark Matter) should cause expansion when gravity might suggest otherwise is beyond me; and I haven't found that question addressed by the scientists.

3. We haven't go a bloody clue what's really going on. Hence my reversion to logic.

You have missed 1 unless you throw creation in as an unknown process

[Sephiroth]

Originally Posted by Jaymoss

You have missed 1 unless you throw creation in as an unknown process

Yeah - but who/what is the creator? If there was one, I moot that the creator is now dead. If there wasn’t one, the universe has always been there. If there was one, who created the creator? Etc.

I would like to think that a good dose of logic goes a long way! But, thank you.

Or…

As put forward by Lee Smolin in his book "The Life of the Cosmos".

- Every time that a black hole is created, a new Universe is formed springing from the BH singularity
- Therefore, the Big Bang of our universe is interpretated as one of such black hole creations
- All the material that falls into the Black hole, ends up in the Baby Universe

From - https://www.physicsforums.com/thread...plained.58067/

[tweetiepooh]

Originally Posted by daveeb

Don't worry about it. We live in a universe inside a gigantic black hole which is expanding in an external universe which is also expanding. I had this confirmed by Keith at my local the other night. Keiths also captain of the darts team so has many strings to his bow.

Archery darts is real but not that common, even then a bow only has one string.

[downquark1]

Originally Posted by GrimUpNorth

If the universe is expanding, and everything is moving away from everything else, how can we be on a collision course wth Andromeda?

I've watched every episode of How The Universe Works on Discovery and still don't understand it.

Because Andromeda can be moving faster than the expansion. Imagine you are running down a corridor that is getting longer, as long as you can run faster than the expansion you will get to the end.

The noggin scratcher is if the edge of the universe is expanding faster than light then you can't even SEE what is beyond, let alone get to it.

I can answer some questions but I'm not going to be defending the truth of the big bang theory - if you don't believe fine I don't care. All cosmology is nuts -- scientific or otherwise, but some of it has empirical backing.

[Sephiroth]

Originally Posted by downquark1

Because Andromeda can be moving faster than the expansion. Imagine you are running down a corridor that is getting longer, as long as you can run faster than the expansion you will get to the end.

The noggin scratcher is if the edge of the universe is expanding faster than light then you can't even SEE what is beyond, let alone get to it.

I can answer some questions but I'm not going to be defending the truth of the big bang theory - if you don't believe fine I don't care. All cosmology is nuts -- scientific or otherwise, but some of it has empirical backing.

But not the Big Bang.

[downquark1]

Originally Posted by Sephiroth

But not the Big Bang.

The universe is seen to be expanding, if you run it in reverse it looks like it came from a point. That is empirical backing.

There are other problems with the model but that bit is uncontroversial.