My latest discussion with ChatGPT
Ron: How can we comprehend that the universe is expanding, without having reference to a ‘something’ that it is expanding into?
forbes.com
ChatGPT said:
1. The Universe is All of Space Itself
The universe doesn’t expand into anything because the universe is not an object within space; it is space itself. When we say the universe is expanding, we mean that the fabric of space between galaxies is stretching, not that galaxies are moving outward into some pre-existing empty space. In other words, space itself is growing, and the distances between unbound objects (like galaxies) are increasing as a result.
2. No External “Edge” or “Boundary”
The expansion is not like a balloon inflating into a larger room or into some pre-existing void. The universe doesn’t have an “outside” or “edge” in the conventional sense because space is self-contained. As space expands, more of it is created between objects, but this doesn’t imply that space is pushing into anything external.
One helpful analogy is the surface of a balloon. Imagine the universe as the two-dimensional surface of a balloon. As the balloon inflates, the surface area increases, and points on the surface move farther apart. However, from the perspective of beings living on the balloon’s surface (restricted to 2D), there’s no “outside” or “edge” into which the surface expands. Similarly, in our 3D universe, space itself expands, but there is no need for it to expand into something external.
3. Understanding Through General Relativity
Einstein’s general theory of relativity describes how space and time are interconnected and how the geometry of space can evolve over time. In this framework, the “expansion” of the universe refers to the change in the geometry of space itself. The mathematical description doesn’t require an external space for our universe to expand into—just a change in the distances between points within the universe.
4. The Observable Universe
Our comprehension of the universe is limited to the “observable universe,” which is the region we can see because light from distant objects has had time to reach us. Beyond this, there could be more universe that we cannot observe yet, but the idea of the universe expanding is about the increasing separation of galaxies in this observable region. The important point here is that as space expands, the observable universe gets larger over time, but we’re not looking at it as expanding into something else.
5. The Infinite or Finite Universe
Whether the universe is infinite or finite is still an open question. If the universe is infinite, it has always been infinite, even during the Big Bang, and it simply keeps expanding. If it’s finite, the universe might still be boundless, like the surface of a sphere (but in higher dimensions), and there’s no external space beyond its boundary. In either case, there’s no need for something outside the universe.
6. Expansion of Distances, Not Movement
When we say galaxies are moving apart, it’s not the galaxies traveling through space like cars on a road. Instead, the space between galaxies is stretching. This is fundamentally different from motion through space; rather, the fabric of space is expanding, causing galaxies to recede from each other without needing a background space to move through.
Conclusion: No “Outside” is Needed
The universe’s expansion is a growth of space itself, and it doesn’t require an external reference point or some “outside” into which it is expanding. Our difficulty in imagining this comes from our everyday intuition, which is based on objects moving in space, but the expansion of the universe is about the geometry of space evolving over time. There is no “container” for the universe to expand into—space itself is expanding.
Second Question:
If space is expanding everywhere, it must be expanding locally too; that is, within our solar system. Is it possible to measure the effect of this expansion locally?
Here’s why the expansion of the universe isn’t noticeable locally and can’t be measured within our solar system:
- Gravitational Bound Systems Resist Expansion: The expansion of space becomes noticeable only on large, intergalactic scales. On smaller scales—such as within galaxies, solar systems, or even galaxy clusters—gravity dominates. The gravitational attraction between objects (like planets orbiting the Sun, or stars orbiting the center of the galaxy) is much stronger than the effect of cosmic expansion, effectively preventing space from stretching between them.For instance, the gravitational pull between the Earth and the Sun is vastly stronger than the minuscule effect of the universe’s expansion at this scale.
- Cosmic Expansion’s Rate: The rate of expansion is very small at human scales. The current expansion rate, given by the Hubble constant, is about 70 kilometers per second per megaparsec (1 megaparsec = about 3.26 million light-years). This means that objects a million light-years apart move away from each other at around 70 km/s. However, objects within our solar system or galaxy are much, much closer, meaning the expansion rate between them is effectively zero.
- No Observable Local Expansion: On local scales, such as within our solar system, objects are gravitationally bound, so we don’t observe any measurable expansion. Even on the scale of our galaxy (the Milky Way), the gravitational forces holding it together far outweigh the very tiny effects of cosmic expansion.
- Cosmic Expansion vs. Other Forces: In addition to gravity, other forces such as the electromagnetic force (which holds atoms and molecules together) and nuclear forces (which bind atomic nuclei) are also vastly stronger than the cosmic expansion. This is why objects like atoms, planets, and stars don’t “expand” as the universe expands.
Why Local Expansion Can’t Be Measured:
Even though space is expanding on the largest scales, the effect of this expansion in smaller, gravitationally bound systems (like the solar system) is so tiny that it is unmeasurable with our current technology. The gravitational binding between planets and the Sun, for example, completely overrides any effect from the expansion of space on that scale.
In fact, only when you look at distances between galaxies that are not gravitationally bound to each other do you start to see the effect of cosmic expansion. This is why the universe’s expansion is mainly observed through the redshift of distant galaxies, rather than any changes in local distances.
In short, cosmic expansion doesn’t affect local systems like the solar system or even our galaxy in any measurable way, and this effect is far too small to be measured at these scales. The observable effects of expansion only become relevant at intergalactic distances.
4o
The Pavellas Perspective 
Remarkable what we have learnt. Even more remarkable what remains to be learnt.
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