Observable universe question.

My mental model is that as we look at things further and further away we are seeing then as they were when the light left them. The further out we look in space the further back we are looking in time and newer better telescopes like JWST allow us see further back in time.

However

There is also a limit as to how much we can see as there are parts of the universe so far away from us that no light from them has reached us yet.

The conclusion is no matter how powerful the telescope there is only so far out (or back) we can see? How long after the Big bang is that threshold?

Is FTL travel possible in theory

I understand that there are numerous ways of arriving at the idea of multiple universe's and they have their appropriate definitions.

My question is related to Astronomy:

Given that black holes were hypothesized but not observed for such a long time, yet we still put coincidence of them existing, what level of confidence is considered to be appropriate when thinking about other universe's (with the same, or different laws of physics/initial conditions), or areas of the universe that are not directly observable?

Is this something one should have confidence in based on mathematical conclusions? (Ex: Max Tegmarks mathematical universe hypothesis: https://en.m.wikipedia.org/wiki/Mathematical_universe_hypothesis)

Or is the inability to have direct observations something that should hold us back?

As a physicist I have an idea of an answer but am really curious about what an astronomer thinks about it as I have almost no background in astronomy.

What does 99.9% of the universe is made up of plasma actually mean? Is there matter in the universe that can't be identified as elements from the periodic table and if so how much?

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After the Big Bang, the matter was so dense that any light that was emitted was instantly reabsorbed by the surrounding matter. So we cannot see light from that time. It's only when Universe had expanded enough that some photons were able to travel without being disturbed. These photons were emitted approximately 380'000 years after the Big Bang, and they constitute what is called the "cosmic microwave background". You cannot see further than that (with light).

But still, that's very far, and the first galaxies are no more than a few pixels in our better telescopes. So building better telescopes does help us finding further and further objects (the JWST for example, has already found a galaxy further than all the ones previously known, dating from around 300 million years after the Big Bang).

There is a difference between hypothesising something that could be observed, we just don’t have the means yet, and hypothesising something which is fundamentally unobservable.

An atom is composed of a nucleus, containing protons and neutrons, around which there are electrons. The number of protons determines what element this atom is (one proton, it's hydrogen, 6 protons it's carbon, 92 protons it's uranium...)

Plasma is a state of matter, like gas, liquid and solid. It is composed of elements. Plasma is matter that is so hot that the electrons are not bounded to atoms anymore and start moving around. But the atoms are still there, they've just become ions (atoms which have lost electrons). For example, the Sun is made up of a plasma of mainly hydrogen and helium, with some other trace elements.

The vast majority of observable matter in the universe is in the stars. The Sun makes up 99.85 % of the mass in our Solar System. So yes, the observable matter is nearly all in a plasma state, and is mainly composed of hydrogen and helium.

Now you may start to wonder why I talk about "observable matter". Well, because there may actually exist another type of matter, called dark matter, that has been hypothesized to explain why galaxies are heavier than they seem to be. This dark matter may be more than 5 times more abundant in the Universe than common matter, but we haven't observed it yet.

In the observable Universe, there are also things that are not elements.

First, elementary particles : electrons, protons or neutrons not in an atom, photons (particles of light) and other more exotic particles like muons or neutrinos. These are things smaller than atoms. Particles don't stick together to form bigger things like atoms do (except protons, neutrons and electrons that make up atoms), so you can't have something visible made of muons.

Second, matter that have been crushed so much that in the atoms, electrons and protons have fused together to make up neutrons. This requires an enormous gravity and is only happening in really massive stars when they die : they form what is called a neutron star.

And finally, black holes. Matter crushed so much by gravity than it's not even matter anymore. It's just a point with an enormous mass.

Plasma just means the substance is mostly charged particles rather than being mostly neutral. In practice, this means a lot of hydrogen and helium nuclei that have been separated from their electrons. We know where it is on the periodic table, it's just ionized.

This isn't too surprising. Most of the matter in the universe is in stars or dispersed in near vacuum, both of which are conducive to plasma formation.

Also, this means 99% of the visible ordinary matter in the universe.

What are your thoughts on "limits" of our universe? It seems there is always something bigger or always something smaller no matter what perspective the observation is being made from.

Indeed, although if there are unobservable conclusions from our best models of reality, how much confidence should be put into the conclusions that aren't directly observable?

In other words, should we be of the mindset that our current models of reality are incorrect and there are more appropriate ones which don't predict or result in unobservable conclusions, or are we on the right track and there are in fact things which we will never be able to directly observe

Ok so it's not like the Big Bang threw things apart so fast and so far that light hasn't got to us yet?

Probably dumb question: I always hear about the universe expanding. And as a proponent of the "big bounce" idea that the universe will expand and then contract in a cycle (as with the other cycles we see in nature) this is disappointing. My question is, being that we perceive time in a linear fashion, and being that space and time are the same thing (space-time) doesn't it simply naturally follow that the perception of the universe expanding is a condition of our experience of time?

Yes it is, too ! What you said in your message was perfectly true. The further we look, the further back we are looking in time, and we cannot see further and older than around 380'000 years after the Big Bang because the Universe was opaque before that. So an object that formed and emitted its first light some millions of years after the Big Bang, but whose light has to travel more than 13.8 billion light years (the Universe is 13.8 billion years old), is not visible by any telescope.

What would happen if the impact of chicxulub or another similar sized asteroid happened on a massive flat target circle made of pure diamond? Of course thickness matters, so how much would it take to not be obliterated? 100 feet? 1000 feet? How much damage would it do to a 1000 foot thick, 7 mile diameter (or just big enough for the whole asteroid to hit directly without much extra surface) disc? The disc would be sitting on top of the ground rather than buried in it, in case that makes a difference. This definitely is not a practical science question, just a sort of physics thought experiment I thought of but don't have the expertise to actual stove.

So i tried posting this last time but I guess I'll ask again here. How come there aren't any large bodied predators that use venom. I mean there's the komodo dragon but I'm talking about mega fauna large, is there an evolutionary reason for this? I'm asking because I'm genuinely curious if dinosaurs were venomous but most animal species I know that have venom are usually small.

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Why does something get stuck when you tighten it a lot Ex: chuck, screw nut, etc

I know this is a dumb question, but I’ve always wondered-

What’s stopping us from building a huge spaceship, loading it up with a bunch of nuclear waste/trash and shooting it into the Sun or off into space? Are we not capable of building a spaceship large enough, are we too afraid of what would happen if the launch failed, would the ship burn up leaving the Earth’s atmosphere and release its’ contents, etc.? Would something like this be a viable solution for disposing of our waste in the future?

As a non-expert here, launching nuclear waste is going to be a massive risk. Rates of rocket failure mean you're taking a gigantic risk of just blowing a bunch of waste up over the water, or even worse, the land.

It also take a lot more energy than you might think to get a lot of weight into orbit, and then you have to use a truckload more fuel to get it to the sun.

It is not, no. Nothing rules out wormhole travel connecting extremely vast distances though

Very simply you are driving something to a place where atoms are touching one another and normal forces are increasing: these normal forces result in very high friction