How certain are we that space itself is actually expanding?

Pretty certain buddy.

Two separate teams were racing to figure out how fast inflation was slowing down. Instead they both independently discovered the opposite was happening. I used to work with one of the supernovae guys on one of those teams. Cool dude, super chill, didn't let the fame get to his head.

The sad part is that we don't know WHY the universe's expansion is accelerating. Is it because of virtual particles, the inherent curvature of the universe, someone summoning dragon dildos? No one knows.

Until we can figure out the WHY, we really can't answer your second question.

Potential of the mind is bound by organic limitations.

But are we sure that it's space that's actually expanding rather than just objects getting further apart?

Do we assume it's space expansion because of the fact that some galaxies are moving away from us faster than the speed of light? (because we have to reconcile our observations with relativity and the only way tot explain it with relativity is that they aren't moving through space faster than c and space is just expanding)?
Is that the only reason or are there others?

No it's more like the fact that the expansion is isotropic (looks the same in all directions). This could mean we're at the exact center of the universe, but that would presuppose we're super-duper special (scientists don't like to do that). The other possibility is if all of space is expanding. Then no matter where we are in the universe, it would look the same in all directions. Namely, from any given point it would like everything is rushing away from you, getting faster the further away you went.

If I had an overhead and some slides, I could show you how that works. Damn.

Space isn't real. Nothing's real. You're not real.

Fuck off Kant, we all stopped believing your shit when we realized it was pointless.

I get what you're saying, but I'm not sure why we wouldn't still get isotropic expansion for objects just moving away from each other (no space expansion) even if we weren't at the center of the universe.
Objects further away from the center are moving faster, objects closer in than we are are moving slower (so to us it still appears as if they are moving away). So everything would appear to recede from us on all sides

Cogito ergo sum.

Space is a hypothetical concept to enable us to come up with equations. Just like how you draw an X, Y, Z axis to explain the position of stuff within in. The same way, when we see objects are moving away from each other, we use the concept of expanding space so that we can scale our graph to avoid dealing with numbers so large that don't fit in a computer's memory.

It really doesn't matter how you visualize it as long as your equations work in the real world.

Einstein told us that nothing (with mass) can travel faster or at the speed of light so when we see that objects are moving away from each other faster than speed of light, there's an obvious contradiction to this statement and something needs to be fixed. We do that by concluding that space is expanding.

whats on the outside of space though

anti-space

chloes virginity

If the brain can do it, a machine can be built to do it.

prove it

No. Without the model of an expanding universe the redshifts observed would be treated relativistically, in which case they are not faster than the speed of light.

What's north of the north pole?

Why do we never hear about this possibility?
Wouldn't this have huge implications on the size and maybe even age of the universe?

Time doesn't exist like that, its just a measure of change in matter and energy you can't measure time alone.

>How certain are we that space itself is actually expanding?

Everything is moving away from us so either space is expanding or we smell really really bad.

>How certain are we
What do you mean by "we", Peasant?

He said harvest energy, not food:

1. Tie a rope to a planet (must be extremely far away, like 4,435 Mpcs).
2. Coil up the other end of the rope on the axle of a generator.
3. As the planet moves away (at the speed of light or faster), the rope will spin the generator.
4. Since the rope is spinning the generator at a speed faster than the speed of light, the generator will keep spinning forever, since all the laws of physics will have broken down and friction will no longer be a problem.
4 (again). Say it with me: Infinite. Free. Energies.

Here's something I could never understand about this argument: if space has always been expanding, and our galaxy/local group is the only superstructure in our region of the universe that will remain in our region of space due to gravity holding the structure together,

isn't it possible that eventually the supermassive black hole in the centre of the universe/various galaxies will acquire enough mass to reverse the expansion of space?

I mean, it would be a stupidly short-lived universe if gravity accelerated too quickly and crushed everything together, and it would be a ludicrously expansive universe if no physical factor was able to prevent the radical expansion of space. This seems a compromise to extend the timeline of the universe enough to allow the chemical factors of various celestial bodies to undergo their own little lifespans.

>isn't it possible that eventually the supermassive black hole in the centre of the universe
>the centre of the universe
>the centre
Universe does not have a center, because it's isotropic and (probably) infinite.
Also to answer your question. No, dark energy, as far as our current understanding goes, is going to prevail and will only further accelerate the expansion of space. The amount of baryonic matter will stay the same (conservation of energy, you can't create mass from nothing).

Why would a blackhole reverse expansion?

As far as I know there's no dividing line between regular blackhole and super massive blackhole that says they have different laws of physics.
The super massive one would still just contain all the same mass that's in our local region right now, so from the outside the gravitational field wouldn't really change at all from how it is now to how it would be all compressed inside a black hole.

How do local galaxies retain their structure, then? It is gravity holding these celestial bodies together, right? Don't blackholes grow and increase in mass based on the matter sucked into them? Essentially what I'm wondering is whether black holes operate as anchor points based on their gravity.

I'm entry-level to all of this shit, so feel free to speak down to me like a Plebeian.

Blackholes don't "suck" in anything. Their gravitational fields are just like any other object in space. Things can orbit them, blackholes can orbit each other etc..

There are 2 important results about the force coming from spherically symmetric distributions.

1) From the outside the field appears the same as if all the mass were concentrated at a single point at the center
2) From the _inside_ of a thin spherical shell all the fields cancel each other out.

Now, considering that the strength of the gravitational force is inversely proportional to the distance as you get closer and closer to a point charge the strength gets larger and larger right? As you approach r=0 the strength goes to infinity.

That's a black hole isn't it? Yet we use those equations all the time to model the gravitational field of objects.
It doesn't matter as long as you don't cross a certain radius (which every spherically symmetric object has, no matter how small and it differs from object to object) beyond which the strength is so large that not even light can escape. That's called the Schwarzschild radius.

As long as you stay outside that radius it doesn't matter whether it's a blackhole or a baseball. The importance of the second property I mentioned at the top is that even if you burrowed inside the Earth to enter its Schwarzschild radius there's no black hole because you are no longer outside all of the mass. You're inside a bunch of thin spherical shells now which all cancel each other out so the remaining mass that doesn't cancel is caused by a smaller spherical region inside the Earth with less mass and therefore a smaller Schwarzschild radius.

That's why blackholes depend on density, not mass. Which is why you may have heard of things like "micro blackholes".

literally nothing

>How do local galaxies retain their structure, then?
Gravity holds them together.
>Don't blackholes grow and increase in mass based on the matter sucked into them?
They do grow, yes, but the mass of a cluster remains the same, the only thing that changes is the density, the same amount of mass is concentrated in the smaller volume of spacetime. But the total amount of energy remains (actually it even gets smaller, due to the fact that some of the energy is released in the form of gravitational waves when two black holes merge).

Also google Schwarzschild radius for better understanding on how black holes work.

>maybe it's not expanding but rather is an orbit of sorts

l4
In dark mass there's a set of energies smaller in mass than light, they are relatively very small to humans and embraces the whole universe.

Unless you study cosmology you aren't going to be taught about various non-viable cosmologies, there are hundreds. Yes it would have a big impact, it would be a totally different cosmological model, one with many problems. For a start it would not pass the surface angular scale tests like baryon acoustic oscillations and the surface brightness test. Secondly it's very ugly, you've made a static cosmology, you have to explain what supports it under gravity and how a Hubble law came to be in the first place. It doesn't explain the presence of the cosmic microwave background and it's also incompatable with measurements of the CMB temperature though cosmic time.

It just doesn't work. GR makes it very difficult for a universe to exist which is not collapsing or expanding.