For years I’ve tried to get my head around the idea of spacetime expanding. If space itself is expanding, and everything is rushing apart, can we detect this expansion on relatively close scales, such as Earth to Alpha Centauri? Is the entire galaxy expanding? Or is it only detectable at scales of millions of light years? And if space itself is expanding, why doesn’t the space within all matter expand, making everything bigger, not just making macroscopic objects farther apart?
Or is it the case that more space is being created, rather than just stretched out?
The other mind boggler is that time is part of all this and didn’t exist “prior to” the big bang (in fact “prior to the big bang” is undefined). Because space and time are inextricably linked, is time expanding too? Are there any telltale signs in time that are analogous to the red shift that tells us that everything is rushing apart?
I took classic mechanics and quantum mechanics in college (30 years ago), and have read a lot of popular treatments of the subject, but haven’t the faintest clue about the mathematics involved in cosmology and theoretical physics, and I suspect a certain amount of that is necessary to really get a foothold.
A lot of ?s…
From what I understand of it, & I’m still reading my Einstein & Hawking, on a relative average, everything (on a cosmological scale) is expanding. The farther away things are, the faster they tend to be moving away from everything else, all else being equal. It is my understanding that it’s the space between bodies that’s expanding, not solids or liquids (of which we are mostly composed, for instance). & even if that were so, if the ratio of expansion were proportional to the dimensions involved, I doubt that we’d be able to detect it, considering. It is also my understanding that the only primary fashion that we’ve been able to determine the distances involved is by calculating the shift in light frequency compared against the C constant. Created vs. stretched, is there any real difference? Don’t know about time… but from what I understand, since C is constant, there must have been an original Center & if that is so, we should be able to deduce just where the Center Of Everything is; we should also be able to find the Edge Of Everything, & if we were able to reach that Edge, we’d be able to see Creation, considering that reality cannot travel any faster than causality (i.e., we’d be unable to be someplace before reality had already been there, being unable to travel faster than C).
But matter is mostly space…it wouldn’t seem that space would discriminate as to where it expands…
The constant c is, I think, beside the point. The trick is that everything appears to be moving away from us, but we pretty much know we are not the center of the universe, so therefore everything must be moving away from everything else.
The short answer is that electromagnetic and other forces are much more powerful than the expansion of space on small scales. That’s why we don’t see matter itself expand. The physicists will be along with far more details. Time does not expand, or need to, by current theories.
However, everything The Central Scrutinizer says is wrong. In particular, there is no center and no edge to the universe. You can find lots of explanations for this in this recent thread.
You may also want to check out Before the Big Bang: The Prehistory of Our Universe, by Brian Clegg. Although the title is marketing hype - he doesn’t spend much time on what came before the big bang - he goes over all the current theories for universe creation and whether a big bang is necessary.
In a simplified model without dark energy, the expansion rate is linear with distance:
d = Hv
where d is the distance between two objects in space, v is the velocity they will appear to be moving away from each other, and H is the Hubble constant.
In common units, the Hubble constant is 70 kilometers per second per megaparsec. So two objects 1 megaparsec apart will appear to be expanding away from each other at 70 kilometers per second.
This tells us a few things. First, 1 megaparsec is a really long distance, about 3 million light years. That’s about the distance from here to the Andromeda galaxy. So this would tell us that Andromeda is being pushed away from us at 70 km/s. But while on a human scale 70 km/s is very fast, on the scale of galaxies (that are trillions of kilometers across), 70 km/s is barely noticeable. In fact, Andromeda is actually moving toward us, as the gravity between our two galaxies is causing it to move much faster than 70 km/s. So one has to go to distances much larger than 1 megaparsec in order for the expansion rate to be faster than anything caused by gravity.
And of course, the same works in reverse. Going down to scales below 1 megaparsec, the expansion speed becomes so small that it’s not even measurable. If I remember correctly (the calculation is fairly simple), the expansion rate between two objects 1 meter apart is something like 10^-22 m/s. At that speed it would take about a year to traverse one hydrogen atom. So, pretty slow.
Marketing hype notwithstanding, does it make any sense at all to talk about “before” the big bang if time and space are integrated in our universe? “Before” the universe there was no time. I have not heard of a theory of some sort of meta-time that transcends the linear time we know about.
All the latest theories that Clegg talks about posit some sort of eternal meta-universe. Our universe is one of many that appear because of various events. Space-time is a single whole that is created for each new universe, so time cannot be tracked before its appearance. Whether time as we know it even exists at all in these other universes is a subject he doesn’t cover.
It doesn’t really matter if these theories are true, though. If our universe was a singular event created from a bubble in vacuum energy, then there was a “before” even if time as we know did not exist.
The problem is that humans have a very limited notion of what time must be because time is part of our universe. That’s really no different from not being to understand how a photon can be both a wave and a particle and neither at the same. Humans’ everyday experiences break down on cosmic and micro scales. The multiverses don’t care what your conception of time is.
It should be noticed that one possible scenario for the end of the Universe is something called a “big rip”. Currently, there’s this phenomenon which we call “dark energy” (about which we understand almost nothing), that’s causing the expansion of space to accelerate. Our current best guess, based on our observations, is that this dark energy has a more-or-less constant strength as time passes, in which case the cosmological expansion will never be significant for anything on the scale of a single galaxy. But it’s also possible, based on our observations, that the dark energy might be getting stronger with time (why? I dunno). If that’s the case, then it could eventually get strong enough to rip everything apart, even the subatomic particles that make up atoms. Likely, probably not, but it can’t be ruled out.