That question has occupied my mind for a long while.
Finding a brief rendition is not easy.
However, Prof Jim Al-Khali who is often on TV wrote a book on Gravity.
It's a Ladybird book for adults (2019).
You'd expect it to be brief and clear.
On page 22, he says:
the distant galaxies are receding away from us equally in every direction. ...
Think of us as a point somewhere on the surface of a balloon.
As you blow the balloon up, ... all points around us move away equally. ...
The image is on page 23.
If you want more the going becomes tougher.
A
- Come on, admit it, you’ve had this question. “Since astronomers know that the Universe is expanding, what’s it expanding into?
- What’s outside of the Universe?” Ask any astronomer and you’ll get an unsatisfying answer. ...
- The definition of the Universe is that it contains everything. If something was outside the Universe, it would also be part of the Universe too. Outside of that? ... Also more Universe. It’s Universe all the way ....
- Either the Universe is infinite, going on forever, or its finite, with a limited volume. In either case, the Universe has no edge.
B
- When we imagine the Universe expanding after the Big Bang, we imagine an explosion, with a spray of matter coming from a single point.
- But this analogy isn’t accurate.
- A better analogy is the surface of an expanding balloon. Not the 3 dimensional balloon, just its 2 dimensional surface.
- If you were an ant crawling around the surface of a huge balloon, and the balloon was your whole universe, you would see the balloon as essentially flat under your feet.
- Imagine the balloon is inflating. In every direction you look, other ants are moving away from you.
- The further they are, the faster away they’re moving. Even though it feels like a flat surface, walk in any direction long enough and you’d return to your starting point. This is not explained.
- Your 2-dimensional ant brain can’t comprehend an expanding 3-dimensional object. There may be a center to the balloon, but there’s no center to the surface. Just a shape that extends in all directions and wraps in upon itself. This is not explained.
- And yet, your journey to make one lap around the balloon takes longer and longer as the balloon gets more inflated.
- A great analogy comes from .... Dr. Pamela Gay. Instead of an explosion, imagine the expanding Universe is like a loaf of raisin bread rising in the oven.
- From the perspective of any raisin, all the other raisins are moving away in all directions.
- But ... you could travel in any one direction within the bread and eventually return to your starting raisin. This is not explained. Also, it's 3D now.
- I hope this helps you understand why there’s no answer to “what is the Universe expanding into?” With no edge, it’s not expanding into anything, it’s just expanding.
Fraser Cain source
A 3
Ask any astronomer and you’ll get an unsatisfying answer. ... Such as a lot of this is!
B3 is contentious.
Find this text for an interesting cube image. here
Even if you could move faster than the speed of light, you’d just return to your starting position more quickly. Why would you? Light travels in straight lines until an object with high gravity bends it.
We see other galaxies moving away from us in all directions just as our ant would see other ants moving away on the surface of the balloon. Give me the bread!
Time to relax but the topic is not quite a raisin (but they are relatives) here.
There's a big time gap between the Big Bang and the formation of galaxies.
Find the image with a b c d e f g
under it here.
How do they know if a galaxy is moving away or towards us?
The Doppler effect is . . observed in the visible spectrum. When a source of visible light moves away from an observer, the incoming light tends to move towards the red region (higher wavelength region) of the spectrum. This is called redshift.
Similarly, if the source is moving towards the observer, the incoming light waves tend to shift nearer to the blue region (lower wavelength) of the visible spectrum. This is called blueshift.
The Dopler effect
Have you ever seen a police car speed by? As it nears you, the siren is heard louder (invoking fear in the hearts of criminals). As the car moves away, the intensity of the sound diminishes as well. This is known as the Doppler effect.
There is, however, no sound is to be heard in the vacuum of deep space except in a specific case.
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The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye.
The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation.
In physics, electromagnetic radiation ... refers to the waves ... radiating through space.
Now you explain it to me!