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Aside from unanswerable questions (has the universe started to fill it's container? Is a simulation property nearing "1"?), does this make long distance space

Aside from unanswerable questions (has the universe started to fill it's container? Is a simulation property nearing "1"?), does this make long distance space travel feasible again? I thought there was something around the universe is expanding too fast to visit places like Alpha Centuri (and preventing visitors to us).

indoordin0saur 11mo ago
The universe was always only expanding between galaxies, not within them.
- Razengan 11mo ago
  So wait, individual stars aren't getting further apart? Galaxies aren't getting "bigger"/more diffuse?
  - kmeisthax 11mo ago
    Galaxies have enough gravity to counteract the expansion of the universe.
    - Razengan 11mo ago
      So do we see the expansion cancelled out by the gravity, or do we only see the gravity? I mean, is it change = gravity or change = expansion - gravity Because this just made me wonder.. is "dark energy" simply the absence of gravity? i.e. just in regions where there is next to no matter/activity?
      - JumpCrisscross 11mo ago
        > do we see the expansion cancelled out by the gravity, or do we only see the gravity? We see gravity overpowering expansion. Same way you can’t launch yourself into orbit by throwing lots of pennies at one a second.
  - recursivecaveat 11mo ago
    Imagine the universe as a giant balloon. Inside are little miniature balloon stars floating around, tied with string into balloon galaxies. If we heat the air: the big balloon expands, the clusters of mini-balloons spread out from the other clusters, but the clusters don't get any more diffuse. The string is way way too strong to be overpowered by the separating force from the expansion of the gas over short distances.
    - Razengan 11mo ago
      I mean, this is tricky to even ask: is there still expansion INSIDE galaxies, BUT it's countered by gravity? Or is there no expansion within galaxies at all? i.e. is dark energy or whatever that causes expansion only present in the absence of matter, or is it present everywhere regardless of matter, but because matter also has its own gravity the expansion is not visible/relevant?
  - indoordin0saur 11mo ago
    No, stars stay just as close to each other. Also even the nearest galaxies (Andromeda) are either getting closer or staying at roughly the same distance.
oofbey 11mo ago
That limitation only counts for visiting other galaxies. Travel within the galaxy is always possible, regardless of the universe’s expansion. And Alpha Centauri is super close, even within our galaxy.
- dtech 11mo ago
  Specifically the local group, so Milky way + Andromeda and some dwarf galaxies
  - Sharlin 11mo ago
    Dozens of dwarf galaxies, even! Also, Triangulum is sort of borderline at around 70% of the Milky Way's diameter, although admittedly only 10% of its mass. But Mars is also around 10% of Earth's mass, for a comparison.
Sharlin 11mo ago
Edit: A big brain fart, ignore the retracted part below. Colonizing the universe is of course impossible in 100My, barring FTL. What the paper I referred to [1] says is that colonizing the Milky Way may take less than that, and if you can do that, spreading to the rest of the observable universe is fairly easy, very relatively speaking. <retracted> According to some calculations, it should in principle be possible to colonize the entire observable universe in less than a hundred million years. It's much too fast for the expansion to affect except marginally.</retracted> The relative jump in difficulty from interstellar to intergalactic is much smaller than from interplanetary to interstellar. Anyway, as others said, mere intragalactic (and intra-Local Group) travel is not affected by expansion in any way whatsoever. [1] https://www.sciencedirect.com/science/article/abs/pii/S0094576513001148 https://www.sciencedirect.com/science/article/abs/pii/S00945..., PDF at https://www.aleph.se/papers/Spamming%20the%20universe.pdf https://www.aleph.se/papers/Spamming%20the%20universe.pdf
- hn_acc1 11mo ago
  I found someone saying colonize the Milky Way Galaxy in ~90m years? Is that what you meant? The observable universe is ~93B LY - unless you're assuming FTL (and MUCH faster than light), I don't see how that's possible?
  - Sharlin 11mo ago
    Yes, my brain totally froze. Added a correction.
  - omnicognate 11mo ago
    Time dilation means that you can get anywhere while experiencing an arbitrarily small amount of time. You can cross the galaxy in a second as far as special relativity is concerned. (With the expenditure of insanely vast amounts of energy, ofc.) To an observer back home you'd look like you're travelling at merely extremely close to the speed of light, but to you the journey would take a second.
    - mr_toad 11mo ago
jfengel 11mo ago
The limit to space travel is the Rocket Equation, which says that you require exponential fuel to reach higher speeds. Alpha Centauri isn't going anywhere, but it will take millennia of travel even with wildly optimistic assumptions. Also note that there isn't any "container" to fill up. It could well be infinite. It's just that we will be forever limited to a finite subset, even in theory.
- floxy 11mo ago
  Roundtrip Interstellar Travel Using Laser-Pushed Lightsails https://ia800108.us.archive.org/view_archive.php?archive=/24/items/wikipedia-scholarly-sources-corpus/10.2514.zip&file=10.2514%252F3.8632.pdf https://ia800108.us.archive.org/view_archive.php?archive=/24... "The third mission uses a three-stage sail for a roundtrip manned exploration of Eridani at 10.8 light years distance."
- mr_toad 11mo ago
  There are theoretical designs using antimatter (pion rockets, or beam core engines) that could reach 0.4c - 0.7c, which puts Alpha Centauri at decades away, not millennia. https://arxiv.org/pdf/1205.2281 https://arxiv.org/pdf/1205.2281 https://ntrs.nasa.gov/api/citations/20200001904/downloads/20200001904.pdf https://ntrs.nasa.gov/api/citations/20200001904/downloads/20...

indoordin0saur 11mo ago

The universe was always only expanding between galaxies, not within them.

Razengan 11mo ago
So wait, individual stars aren't getting further apart? Galaxies aren't getting "bigger"/more diffuse?
- kmeisthax 11mo ago
  Galaxies have enough gravity to counteract the expansion of the universe.
  - Razengan 11mo ago
    So do we see the expansion cancelled out by the gravity, or do we only see the gravity? I mean, is it change = gravity or change = expansion - gravity Because this just made me wonder.. is "dark energy" simply the absence of gravity? i.e. just in regions where there is next to no matter/activity?
    - JumpCrisscross 11mo ago
      > do we see the expansion cancelled out by the gravity, or do we only see the gravity? We see gravity overpowering expansion. Same way you can’t launch yourself into orbit by throwing lots of pennies at one a second.
- recursivecaveat 11mo ago
  Imagine the universe as a giant balloon. Inside are little miniature balloon stars floating around, tied with string into balloon galaxies. If we heat the air: the big balloon expands, the clusters of mini-balloons spread out from the other clusters, but the clusters don't get any more diffuse. The string is way way too strong to be overpowered by the separating force from the expansion of the gas over short distances.
  - Razengan 11mo ago
    I mean, this is tricky to even ask: is there still expansion INSIDE galaxies, BUT it's countered by gravity? Or is there no expansion within galaxies at all? i.e. is dark energy or whatever that causes expansion only present in the absence of matter, or is it present everywhere regardless of matter, but because matter also has its own gravity the expansion is not visible/relevant?
- indoordin0saur 11mo ago
  No, stars stay just as close to each other. Also even the nearest galaxies (Andromeda) are either getting closer or staying at roughly the same distance.

oofbey 11mo ago

That limitation only counts for visiting other galaxies. Travel within the galaxy is always possible, regardless of the universe’s expansion. And Alpha Centauri is super close, even within our galaxy.

dtech 11mo ago
Specifically the local group, so Milky way + Andromeda and some dwarf galaxies
- Sharlin 11mo ago
  Dozens of dwarf galaxies, even! Also, Triangulum is sort of borderline at around 70% of the Milky Way's diameter, although admittedly only 10% of its mass. But Mars is also around 10% of Earth's mass, for a comparison.

Sharlin 11mo ago

Edit: A big brain fart, ignore the retracted part below. Colonizing the universe is of course impossible in 100My, barring FTL. What the paper I referred to [1] says is that colonizing the Milky Way may take less than that, and if you can do that, spreading to the rest of the observable universe is fairly easy, very relatively speaking. <retracted> According to some calculations, it should in principle be possible to colonize the entire observable universe in less than a hundred million years. It's much too fast for the expansion to affect except marginally.</retracted> The relative jump in difficulty from interstellar to intergalactic is much smaller than from interplanetary to interstellar. Anyway, as others said, mere intragalactic (and intra-Local Group) travel is not affected by expansion in any way whatsoever. [1] https://www.sciencedirect.com/science/article/abs/pii/S0094576513001148 https://www.sciencedirect.com/science/article/abs/pii/S00945..., PDF at https://www.aleph.se/papers/Spamming%20the%20universe.pdf https://www.aleph.se/papers/Spamming%20the%20universe.pdf

hn_acc1 11mo ago
I found someone saying colonize the Milky Way Galaxy in ~90m years? Is that what you meant? The observable universe is ~93B LY - unless you're assuming FTL (and MUCH faster than light), I don't see how that's possible?
- Sharlin 11mo ago
  Yes, my brain totally froze. Added a correction.
- omnicognate 11mo ago
  Time dilation means that you can get anywhere while experiencing an arbitrarily small amount of time. You can cross the galaxy in a second as far as special relativity is concerned. (With the expenditure of insanely vast amounts of energy, ofc.) To an observer back home you'd look like you're travelling at merely extremely close to the speed of light, but to you the journey would take a second.
  - mr_toad 11mo ago

jfengel 11mo ago

The limit to space travel is the Rocket Equation, which says that you require exponential fuel to reach higher speeds. Alpha Centauri isn't going anywhere, but it will take millennia of travel even with wildly optimistic assumptions. Also note that there isn't any "container" to fill up. It could well be infinite. It's just that we will be forever limited to a finite subset, even in theory.

floxy 11mo ago
Roundtrip Interstellar Travel Using Laser-Pushed Lightsails https://ia800108.us.archive.org/view_archive.php?archive=/24/items/wikipedia-scholarly-sources-corpus/10.2514.zip&file=10.2514%252F3.8632.pdf https://ia800108.us.archive.org/view_archive.php?archive=/24... "The third mission uses a three-stage sail for a roundtrip manned exploration of Eridani at 10.8 light years distance."
mr_toad 11mo ago
There are theoretical designs using antimatter (pion rockets, or beam core engines) that could reach 0.4c - 0.7c, which puts Alpha Centauri at decades away, not millennia. https://arxiv.org/pdf/1205.2281 https://arxiv.org/pdf/1205.2281 https://ntrs.nasa.gov/api/citations/20200001904/downloads/20200001904.pdf https://ntrs.nasa.gov/api/citations/20200001904/downloads/20...