I have been watching the Universe series on Netflix. It is a great show I love it. I have always been fascinated with gravity and how it works and all that… Anyway here is my question:
I understand that space is expanding and everything is moving away from eachother which explains how the observable universe is bigger in light years across then its age which is 13.7 billion years old. So far so good I just think of it as expanding from the inside out. I can totally wrap my mind around that.
The question I have is what would happen if matter was in the spot in space where the expansion is happening? Say I was there would I expand with the space and be ripped up or would objects in space just move away from me no matter where I was in the universe?
Does that question make sense? Here is an analogy if you can’t understand what I am asking. Sometimes after earthquakes the ground splits leaving a gap so that section is “longer” then it was before. If I was at that gap that split open I would fall into it and probably die.
Would the same thing happened if I was in the spot where the universe is expanding from or is it the same as dividing by 0
The universe isn’t expanding from a single point, it’s expanding from every point. No matter where you are in the universe, the space between you and everything else is expanding. There is no “spot where the universe is expanding from.”
I’m going on TV-documentary-learned knowledge, too, so someone else can correct me if I’m off, but as I understand it, the expansion is a property of space, and is also known as “dark energy”.
Instead of an earthquake’s fault line causing an increased seperation, imagine that the Earth itself is increasing in radius. Now you live on the surface; your world is confined to the crust of the Earth. Imagine what effects you would notice as the Earth kept expanding; everything on the surface (all the cities, mountain ranges, people) would kept moving further and further away from each other as new “space” was created. And if the earth was expanding fast enough, you’d never be able to get from, say, New York to Los Angeles at 60 miles per hour, if the expansion speed was too great.
And that would mean at one point in the past, the Earth had such a small radius that everything was effectively crunched together at a single point.
Not a perfect or even correct analogy, but helps a little.
You have it basically correct, Dahlnor. Chris, what you’re missing is that a better analogy to earth-torn-asunder is a divergent boundary between two tectonic plates where lava flows up from underneath and forms new crust. Only, the “lava” is flowing up at every point and there is no “underneath” (as far as we know). In short: as the space expands, new spacetime is created, and indeed it is this creation that is the mechanism of expansion.
So, let’s say you have two particles hanging onto each other by electromagnetic attraction. New space is created in between them, but it happens slowly enough that the particles can pull closer together, pull themselves back to their previous distance, rather than drift apart from each other. There is no hole to fall into, just a slight increase in distance, which all the ordinary particles compensate for with the regular forces of gravity and electromagnetism.
Matter is in the spot where expansion is happening. It’s happening everywhere. But the forces that hold matter together are stronger than the expansion of space, so matter stays together.
At the scale of the space between galaxies, the force holding galaxies together at those distances isn’t stronger than the expansion, which is what allows us to detect the expansion at all.
People seem to love to explain the expansion as the result of new space being created, but this gives a false view. For example, it suggests that our solar system should also be expanding, albeit at a slow rate. It suggests that two nearly massless particles that are at rest with respect to each other will separate as the universe expands. Neither of these things happen in the absence of a cosmological constant. In fact, the expansion occurs because the galaxies are already moving apart from each other at a rate that depends on how far away they are from each other. Of course, you can view this in different ways depending on what choice of coordinates you use to describe the universe, but the important thing to remember is that, locally, everything behaves as expected from the normal laws of physics. Nothing weird is happening. Things are a bit more complicated when you include the cosmological constant (closely related to “dark energy”).
I’m not so sure that these are incorrect. Obviously the Solar System is gravitationally bound, but that doesn’t mean there isn’t some effect of the universe’ expansion.
According to Wikipedia’s Metric expansion of space page, currently only about half the expansion is due to inertia, with the other half due to “a repulsive force of unknown nature, which may be a cosmological constant.” It’s this second half that makes me believe that two massless particles at rest will separate as the universe expands.
You need to include something, though, if you’re talking about our universe.
Toolie’s analogy is how I’ve always thought of it myself. If Earth was slowly increasing in size, it wouldn’t tear your body apart. Distant objects and lands would seem to be slowly drifting away, but no objects would be torn apart. Even if it was doubling in size every few seconds, your feet may slowly drift apart while you stand, but you could easily pull them back together.
Likewise with the expansion of space, it puts a very slight expansion force on the molecules in your body, but the electromagnetic force easily overcomes it and holds it together. Gravity also overcomes it in the solar system, galaxies, and even galaxy clusters. However, at greater distances, the expansion has greater effect and gravity can’t overcome it. However, if the Big Rip Theory is correct, the expansion will keep accelerating until galactic clusters fly apart, then the stars in galaxies, and eventually even the molecules in objects will be torn apart in the last moments of the universe. Not a very fun way to go out…
ZenBeam - Thanks for the link to the Wikipedia page on the Metric Expansion of Space.
That article explains much better than I did the point I was trying to make, namely that it is incorrect to attribute the expanding universe to new space being created, thus pushing the galaxies apart. Let me summarize.
It is easier to understand the situation in the absence of the acceleration of expansion caused by a cosmological constant or dark energy, so let’s talk about that first. In this case, the observed expansion of the universe is entirely due to the inertia of the matter we started with. It is false to think that space is expanding and pushing the galaxies further apart. It is the opposite, the galaxies are coasting apart, thus the extent of the universe is increasing. This expansion gradually slows down, due to the mutual gravitational attraction.
This is the model that we believed prior to the relatively recent discovery that the expansion is accelerating rather than decelerating. The origin of the repulsive force that is driving the acceleration is not understood. We can simply posit a “cosmological constant” in Einstein’s equations, or blame it on “dark energy” which we don’t understand. Either way, the effect was negligible in the early universe and is now roughly equal in size to the expansion caused by the original inertia of the expanding matter. Still, it is incorrect to think that somehow space is expanding and pushing the galaxies apart. On the local scale, the normal rules of physics still apply, no new space is appearing like lava from sea floor spreading. The solar system or hyrdrogen atom won’t get any bigger. Yes, you need to account for the extra force that is driving the acceleration, but nothing weird happens locally.
I find cosmology fascinating. The acceleration of expansion is a mystery which has given rise to the dark matter and dark energy (separate concepts) theories. However one explanation is that our local time is slowing down. If true, then far objects will appear to move faster.
Mmmm…so what’s pushing and pulling the galaxies about? And where did all the anti-matter go??
Just joking - I realise dark matter isn’t a major factor.
Never mind - the LHC and the Higgs Boson will finally resolve the sea of quantum foam and we’ll have a GUT.
You don’t have this quite right. It is, in fact, correct to think the space is expanding – that it exactly what the math describes. Wikipedia uses the word “repulsive force”, but the article should be more clear that the expansion of space results in an effective repulsive force.
It is true that on the local scale, the “normal rules of physics” still apply, but only because the rate of expansion of space is small. It only adds up on a very large scale. It is, in fact, very much like “new space is appearing like lava from sea floor spreading”, except that it is happening more slowly and more uniformly. But as you say, the solar system or hydrogen atom don’t get “bigger”; this is because 1) locally the expansion of space is insignificant, and 2) the forces between objects remain the same, so even though a little bit of space gets added between them, they continue to attract each other just as before, and so get “pulled” back into their previous positions.
To give the notion a little more heft at how slow this is on a local scale a megaparsec = 19,151,160,600,000,000,000 miles (19 quintillion).
You can see if you figure out the expansion locally, even just down to planet earth or even solar system “small” the expansion is negligible and not noticeable.
The universe is really big though so when the effect compounds itself at ultra long distances the speed of expansion is very substantial…faster than light even at (to us) the “edges” (air quotes because I am using that word very loosely here).
According to my trusty calculator, 80 km/s/Mpc is about 3.879 x 10^-7 m/s/au or 12.24 m/yr/au, where au means astronomical units (the distance between Earth and the sun). So clearly at such distances, the expansion is miniscule and easily overcome by the elemental forces.
Ok I get it now particals attract eachother stronger then the expansion is happening. Ok fair enough, do we have instruments powerful enough to see this happening? Do the particals show any movement at all before they pull back together or can we only detect it on the big scale
