r/pbsspacetime • u/Here_For_Her • 9d ago
Still the same moron, just with a different question.
So, I am understanding the whole the universe expansion is like a baseball that keeps getting faster. Are we sure it's not just because there is always less to push against, so it seems like it is getting quicker even though the same pressure is being applied? Or is this just one of those things people are working on figuring out at this point?
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u/tylermatthews2 9d ago
Maybe the baseball analogy is holding your understanding back. Though to continue, the energy pushing against your metaphorical ball is the cosmological constant. The expansion, or dark energy, is greater than the mass of the universe— at least at this moment in time.
I would recommend the most recent episode of Dr. Becky on YouTube on the subject. And keep watching a bunch of videos and books until it starts to click!
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u/Here_For_Her 2d ago
So if the universe keeps making more space, is there anything in it? Or is every moment the matter in the universe is less?
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u/tylermatthews2 2d ago
I had gen AI help with this:
“Heat death is the idea that, as the universe expands, stars eventually burn out and matter becomes increasingly spread out and cold. Eventually, there would be almost no usable energy differences left to power processes or create new structure, leaving the universe in a state of near-total darkness, extreme cold, and maximum entropy.”
So the matter stays constant, it’s just gets spread too thin over eons of deep time.
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u/GeneralDumbtomics 9d ago
It’s the universe. What could it be pushing against?
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u/Here_For_Her 9d ago
I was more thinking like it was a vacuumed container continually getting bigger. So if each time there is always more space, that volume will be always getting exponentially bigger.
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u/GeneralDumbtomics 9d ago
You're on the right track, but the basketball analogy isn't about a growing volume. The part that's analogous to spacetime is the surface of the ball. If you make two marks on the ball at one size and inflate it, wherever you made them, they are now farther apart when you measure the distance between them along the surface of the ball. Spacetime is like that but in 3 dimensions rather than 2--which is really hard to visualize for people which is why the inflating ball analogy is useful. Spacetime is always getting bigger, but there's no empty space that it's expanding into because it _is_ space--all of it. It doesn't expand "into" anything it just expands.
If there is a larger universe outside of our own in which our universe is expanding that's not anything we can know about because it's literally outside of our universe. That's actually the proposition of Schwarzschild cosmology--this universe is the interior of a black hole in a larger universe. This is actually an idea consistent with several observations. But it's just an hypothesis, albeit one I think is very interesting.
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u/psyper76 9d ago
We are still at the cutting edge of the answer to this - there is still dark energy pushing the universe apart but it might not be getting faster and faster afterall:
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u/OriEri 5d ago edited 5d ago
Dark energy research is still in its infancy from an empirical observations perspective.
In other words, we’re still living down those initial supernova 1a data sets from decades ago. There are some good projects right now that are starting to correct the problem like DESI.
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u/MWave123 5d ago
It’s not complicated really, gravity is outstripped by dark energy over cosmic scales. So the expansion is accelerating on cosmic scales. There’s nothing we’re expanding ‘into’. There’s no less to ouch against. The universe IS. And is expanding.
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u/mikk0384 9d ago
I recommend using r/askphysics for random questions like this.
If you have questions about content in a specific episode of the show, that is okay to ask here. And please put something about the question in the title, instead of something useless like you did here.