Since the big bang, the universe is expanding, it doesn’t mean new space is being inhabited that was previously empty, its the existing space spreading out. How measurable is this on the human scale (meaning human dimensions and human timeline)?
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Space itself it’s expanding. Empty space is growing. The more space there is between two objects, the more that space expands, the faster objects grow apart.
Since there is relatively no space within us or in the planet (yes, atoms have a lot of empty space, but not enough to matter in this context) we aren’t expanding. Only the practically empty space expands.
Gravity counteracts the expansion, so there’s nothing observable on a human scale. Or even really the Solar System or galaxy scale. All of that’s being held together against the expansion; the space is technically still expanding, it’s just not carrying any of the matter along with it.
Hall space is getting bigger and that doesn’t include the space that we occupy.
However, the bonding forces between our atoms is what ultimately determines how big we are, because it determines the distance between all our molecules.
So we are staying the same size, and basically we are occupying and ever increasingly small percentage of the universe.
No, we aren’t. Neither is our galaxy.
Mathematical models that cosmologists use to predict universal expansion make two **huge** assumptions: That the universe is *homogeneous* (“the same everywhere”) and *isotropic* (“the same in all directions”). At first glance, these assumptions seem strange. From where I’m sitting, the universe is clearly *not* the same in all directions. Galaxies are clearly distinct from intergalactic space.
The key is that if you zoom out far enough, this assumption holds *really well*. If you zoom out far enough, the universe looks like a uniformly dense gas. The thing is, you have to zoom out *really* far, on the scale of billions of galaxies. Models that discuss universal expansion are not valid on smaller scales than that.
The biggest misconception about universal expansion appears to be the idea that there is some fabric-like backdrop to spacetime that “expands” . Balloon analogies don’t help: they give the impression that there is some mystical “fabric of spacetime” that is “stretching” and dragging matter with it.
This is not the case. The universe (ignoring dark energy) is expanding because matter has momentum leftover from the big bang, that carries it away from other matter. Galaxies and galactic clusters are parts of the universe where gravity has won out over that momentum and managed to draw everything back together.
I mean, I’m expanding a little, but that has more to do with my workout habits and diet.
When you hear about the universe expanding, generally we’re talking about galaxies being carried away from one another. Within a galaxy, internal gravitational forces are more than counterbalancing the universe’s overall expansion. The universe’s expansion is something that happens at the very largest scales; if we could only see our own solar system, or even just our own galaxy, we wouldn’t have evidence to suggest the overall universe was expanding.
>How measurable is this on the human scale (meaning human dimensions and human timeline)?
Expansion does not occur near or within human-like structures at all.
We know space is expanding because light from distant objects is red-shifted at a very predictable rate proportional to the distance of the object. We do not, however, observe this same red shift in bodies within our galaxy or in our local group. (To be clear: you can find plenty of red-shifted objects near us, but they won’t follow this same red-shift/distance relationship that we see with distant galaxies.)
Digging into the math, what appears to be happening is that the baseline metric of spacetime is gradually changing in such a way that space expands. In relativity, an object called a “metric tensor” is used to describe the structure of a given point of spacetime. You can think of this as an array of values that describe the structure of spacetime (from which we can define things like distance, time, etc.). The “baseline” metric is the metric describing (practically) empty, gravitationally flat space, and that’s what appears to be expanding, in both our models and our observations.
And indeed we would not expect an expansion of the baseline metric to have any significance on the structure of something like a human or a galaxy. Spacetime in these structures is decidedly *not* empty or gravitationally flat, so we can’t use the baseline metric to describe it. The local metric is going to be completely dominated by local gravity (even in conditions that we humans would consider gravitationally weak). So whether your system is a baseball or a galaxy cluster, it’s not going to experience any expansion because it is gravitationally bound.
The standard ELI5 for expanding space is to draw a diagram on the outer surface of a balloon. Then add more air to the balloon.
when you do this demonstration, you see two things.
1. The images on the surface of the balloon grow as “space expands”. Lines grow thicker (and less dark because they thin out).
2. The distance between distant points on the balloon also grows as “space expands”
There are many reasons why we don’t notice the expansion on a human or even small scale level.
* All our reference points are also growing.
* If the growth is a small percentage, it is not measurable on a small scale.
* On a large scale (say 100,000 light years) even a 0.000000000% change is easily seen and measured (as speed or red shift of things moving away from us in all directions.)
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