r/cosmology • u/AutoModerator • 19d ago
Basic cosmology questions weekly thread
Ask your cosmology related questions in this thread.
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u/MedullaOblongata_dj 15d ago
I have a naive question.
Imagine you're in a super powerful rocket and out of the milky way, with a super powerful camera. From out there, is it possible to observe the solar system moving around the milky way ?
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u/--craig-- 15d ago edited 15d ago
Yes but you'd need to have incredible durability and patience. It takes about 250 million years for our solar system to orbit the Milky Way
With existing technology you'd only be able to see the Sun. Provided that you weren't too far from the Milky Way, you might be able to detect the Sun dimming very slightly whenever one of our system's planets passes between it and your rocket.
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u/Kind_Of_A_Dick 18d ago
If this is not the right place for this question, hopefully I can be directed to a better place.
The question is whether the universe is “flattening out”. I’ve heard that matter and energy are deformations of space, however it seems that eventually the heat death of the universe means that these dense pockets of deformations will just flatten out, in a sense. Barring interference from some as-of-yet-unknown outside, eventually…. I really don’t know, I don’t know the math or concepts for it yet.
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u/--craig-- 18d ago edited 18d ago
The universe is becoming less dense as it expands but less uniform due to gravitational attraction, so at the moment, space is becoming less flat on galactic scales. On the largest of scales, space is already very flat, perhaps perfectly so.
In the long term, we project heat death as the most likely scenario, after all the black holes have evaporated. In this circumstance, the universe would become very flat on all but the most microscopic of scales, where quantum fluctuations dominate.
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u/Kind_Of_A_Dick 17d ago
Thank you and I have another question but I don't know where it actually fits. The question is whether or not the universe itself can be seen, conceptually because I don't think a limited element of a system can fully define the whole of the system itself(just using words that make sense to me, I can explain more if need be), as a chemical equilibrium equation. I don't think I see it as a "normal" function where there's a definite end, and the only other kind of equation I can think of is an equilibrium equation.
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u/--craig-- 17d ago edited 17d ago
A chemical equilibrium equation denotes reversibility. Our best physical models, General Relativity and Quantum Field Theory, obey the Second Law of Thermodynamics and as such, are irreversible. The heat death of the universe is the extreme future of irreversible physical processes.
We could argue that the heat death of the universe would put it in an equilibrium state and everything which happens from that state is reversible. We might even question what the arrow time actually is at this stage.
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u/Kind_Of_A_Dick 17d ago
I may be wrong, but I thought the direction of time’s arrow was arbitrary. I need to do a bit of digging but I’m going to leave this comment here so I can remember later what I was thinking.
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u/--craig-- 17d ago edited 17d ago
The equations which we use, work both forwards and backwards but the arrow of time is from a hot, dense, ordered state, towards a cold, sparse and disordered state.
In thermodynamics, it's only how well ordered the system is which determines the direction of time. In cosmology, we also need to include how space behaves over time.
Why the universe was in a hot, dense, well ordered state at the Big Bang, and why the thermodynamic arrow of time aligns with the cosmological arrow of time, remain somewhat mysterious.
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u/Kind_Of_A_Dick 16d ago
Thank you. Here's one last question before I move on, and this one is due to a lack of understanding of the higher level concepts. I've heard of brane theory and the idea the universe is the result of some collision. Does that mean matter itself, energy or whatever, could be an intrusion of other "stuff" to the universe? I'm not sure if I'm explaining what I'm thinking well enough though, especially because I doubt there's any way I can think of to check.
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u/--craig-- 16d ago edited 16d ago
Brane cosmology is a non-standard cosmology based upon superstring theory which posits that the space and time which we are familiar with, were created by an interaction between higher dimensional objects. In this type of cosmology, the initial energy at the big bang was introduced to our spacetime by this interaction, however matter formed from that energy in the same way as standard cosmology.
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u/Kind_Of_A_Dick 16d ago
Much appreciated. I’m coming to a conclusion, or the start of a conclusion, that while definitions are important they are just snapshots or data points because everyone seems to be using the same words to mean slightly different things due to our own perspectives. It’s like how Chris Knight tells Mitch in Real Genius about Lazlo and answers. All science, no philosophy; and I’m starting to feel that learning other disciplines and avenues of thought/science helps me understand how everything else works. I’m just talking for the sake of talking now so I’ll stop. Thanks again for your responses.
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u/Ras_992 16d ago
The arrow of time comes from Penrose’s Past hypothesis where T=0 when the big bang happened. The other hypothesis he came up with was the Conformal Circle Cosmology which would state that universe doesn’t die of heat death. It uses bounce theory to state as one universe is at its end where time arrow is zero will once again spread its information back out again creating another universe. Just remember as long as fits the 2nd law of thermodynamics there is many different theories and hypothesis on this. The biggest problem is really the gaps of information when talking the expansion, dark matter and dark energy these are big unknowns within the arrow of time.
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u/hypotaxis_ 17d ago
What are the origins of structures like The Great Attractor or The Sloan Great Wall and what is their significance? I'm curious how structures like these are significant at all, seeing how homogenous the universe is. Like, why would we notice them at all?
I was once reading (maybe Penrose) and encountered this idea that an infinitely large universe with nothing but photons inside is identical to an infinitely small universe full of energy. I'm misrepresenting the problem statement here, but hope someone familiar with the concept might be able to elucidate.
And, just tossing this out, what are the chances that the supermassive black holes in the center of galaxies are primordial, or does that question not make sense because Sagittarius A* exists right now and PBH describes black holes in the early universe? How the hell would be prove their existence experimentally? Is JWST capable of resolving gravitational lenses in the early universe.
Cosmology is wild, yo. What would be some good texts someone not trained in physics but computationally adept would benefit from reading if they're interested in the subject and willing to dig in the weeds a bit?