You can also fire a bullet into the sky so fast that it will never fall back down.
And given the observations we’ve made so far, it looks as if the expansion of our universe has enough energy that it will never fall back down into a big crunch.
That is not true. Think carefully about it.
Rockets, of course, can change trajectory after initial launch, which is how they can end up in a stable orbit. Maybe that is what you were thinking of.
Please tell me then where does all this gravitational potential energy reside?
If we know energy cannot be created out of nothing and that matter and energy are equivalent, then all this energy has to exist in the form of mass somewhere right?
Do you have any idea how much gravitational potential energy a single tiny piece of matter has? There are all sorts of gigantic Black Holes in the universe it could fall into.
You might try to argue, “Well, the energy must exist in these Black Holes then”, but Black Holes are just a collection of ordinary matter just like our tiny piece of matter in question.
So where is all this mass hiding??
All I am proposing is that this mass is hiding in plain sight: the rest mass of the particle itself.
Do you have another explanation?
I’m afraid you’ll have to explain yourself, because it certainly is true. A bullet fired at greater than escape velocity won’t come back down to earth.
It is true that you can’t fire a bullet into the sky and without any course correction have the bullet orbit the Earth. Either it will fall back to Earth or it will leave Earth. It will never orbit Earth.
But it can clearly leave Earth forever.
I will point out that you are making the assumption that the Cosmic expansion actually reached escape velocity during the Big Bang. If the energy of expansion was just under the escape velocity, the origin of the universe could still have been able to explode.
btw, it doesn’t matter if some of the matter exceeded escape velocity, it is about the average energy of all the matter, because the gravitational force will still be able to eventually pull back all the other matter if the average is less than the full escape velocity value.
During the period of inflation - which is separate from the Big Bang, although often conflated in popular speech - space itself expanded at many times the speed of light. The expanding space pulled matter with it. There was no explosion, therefore. That’s a bad term left over from earlier popular attempts to explain the origin.
Escape velocity is also a meaningless term in this case. As you were told in another thread, escape velocity is the speed needed for an object to leave a gravity well and not have it fall back. There was no gravity well involved during inflation. The entire universe inflated simultaneously and everywhere. That’s why there is no center to the universe today.
It would be possible for the combined mass in the universe to overcome expansion and retract, what is called the Big Crunch. Big Crunch theories are out of favor these days with the discovery that the rate of expansion is accelerating.
It’s not an assumption. It’s an observation. Not one made by me, and it may be that we are misinterpreting what we observe.
It could be that we live in a universe that will experience a big crunch. Or it could be that we live in a universe that will continue to expand indefinitely. It turns out that the evidence currently shows that it is very likely that we live in the second kind of universe.
That isn’t an assumption. It’s just that we now think that the expansion of the universe is actually increasing. If there were going to be a big crunch we should see expansion slowing–just like when you throw a ball in the air it starts out fast and gets slower and slower until it reverses direction and starts to fall back to earth.
The cause of the measured increase in the rate of expansion is given the name “dark energy”, which just means “we don’t know what it is, but there’s a Nobel Prize for the team that figures it out”.
Logical fallacy, the math does not match your claim. If a projectile is approaching the escape velocity, it can still go into space, it just will eventually fall back down at some point. Mathematically, it’s kind of meaningless to say that an object is going at exactly the escape velocity; then it would be theoretically indeterminate whether that object would ever fall back down.
That may just be because space is expanding along with the rest of the matter in the universe.
Maybe it has not slowed yet. Also keep in mind that when we look at redshift of distant points in the universe we are effectively looking backwards in time billions of years. The time scale required to observe any change in expansion within our more local (spatially and temporally) area of the universe would be too long.
Most likely vacuum energy, probably exists as extremely ultra-long wavelength electromagnetic energy pervading space. When the wavelength is so much longer than the scale of the measurement in space and time, it is hard to observe this energy in the conventional sense. Of course this energy exists in equilibrium with other forms of matter/energy. Then there are coherence effects that prevent any of the quanta from individually dumping out their energy to local matter.
Escape speed may or may not be a good analogy for a universe without dark energy. But we live in a Universe with dark energy, and it’s certainly not a good analogy there.
If you throw a ball up from the Earth at less than escape speed, it’ll start off fast, slow down, and eventually stop and come back down. If you throw a ball up at exactly escape speed, it’ll start off fast, slow down, and eventually approach arbitrarily close to zero speed, but never quite reach it. If you throw a ball up at greater than escape speed, it’ll start off fast, slow down, and eventually approach but not quite reach some nonzero asymptotic speed. But in all three cases, it will slow down.
The Universe is not slowing down. It’s as if you threw a ball upwards, and as it climbed, it kept on going faster and faster. Clearly, if the ball is going to do that, it’s never going to come back down, no matter what speed you initially threw it at.
Now, the wildcard in this is that we don’t really know much at all about dark energy. For all we know, it might some day turn off, or even reverse, and if that happens, we certainly could get a Big Crunch. But by far the simplest assumption, in our ignorance, is that it’s just going to keep going forever like it is now.
I’ve always wondered what we can’t observe anymore because of expansion and other changes in the Universe. Like for example what created Dark Matter. For all we know Dark Matter could be the degenerate matter remnants of a decayed form of matter we can no longer observe.
- Which could make the Universe much older…
Yes, because the farther we try to look in the universe, the further back in time we are looking at (because light takes so long to reach us), and at some point looking back in time we no longer see anything. Or rather it is obscured by the cosmic microwave background radiation that resulted from the first moments of the Big Bang. All we can see is an ever expanding edge of all this radiation, which marks the end of the observable universe.