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This is what I don't get: "... if the universe expands today, this means if we look back in time the matter must have been squeezed together, so the density wa

This is what I don't get: "... if the universe expands today, this means if we look back in time the matter must have been squeezed together, so the density was higher. If the universe expands today, how does it follow that it was also expanding yesterday, or a million years ago? Maybe universe expands, then contracts again cyclically?

vkou 4y ago
> Maybe universe expands, then contracts again cyclically? It's possible, but you're going to have to come up with a theory for why whatever 'force' is driving the expansion goes from negative, to positive, and back.
- WXLCKNO 4y ago
  We live inside an engine piston?
  - vkou 4y ago
    And what wheel drives the inner workings of that piston?
    - eis 4y ago
      Maybe a better analogy would be a spring that goes from contraction to expansion and back again? A lot of things behave like waves so could it be some space field swinging in a wave-like pattern? Is there some evidence for the expansion to have also happened say 10B years ago in the CMBR?
- galaxyLogic 4y ago
  Is there a good theory as to why Universe is expanding and acceleration its expansion? Dark Matter? Dark Energy? Cosmological Constant? Those are more like "We can not really explain it so we assume there is something like "Dark Energy" which causes it all.
SiempreViernes 4y ago
The reason expansion yesterday is plausible if we see expansion is simply that's it's far more plausible nothing so special happened yesterday as to change the motion of the entire universe. The universe is not a little trickle of water down the wall in the bathroom you can easily redirect with your little finger, the universe it really big, so you need something equally big to make it change behaviour.
- Kerrick 4y ago
  The oceans used to seem really big to us too, but each rising tide wasn’t destined to continue rising—the tides turned out to be cyclical, even though we couldn’t imagine anything but gods or magic to be powerful enough to cause the change. Perhaps we’re in a similar trough of understanding here?
  - SiempreViernes 4y ago
    I'm not sure I get your point, are you saying the moon and the sun, the things we now know cause the tides, are small things?
    - Kerrick 4y ago
      My point is that you could replace a couple words in your comment to get the following, and it would've been conceivable for a person to say a thousand years ago on their first day at the ocean. And yet it wouldn't have been accurate. > The reason tides always rising is plausible if we see tides rising now is simply that's it's far more plausible nothing so special ever happened as to change the motion of the entire ocean. > The ocean is not a little trickle of water down the wall in the bathroom you can easily redirect with your little finger, the ocean is really big, so you need something equally big to make it change behaviour.
      - SiempreViernes 4y ago
        Are you aware that the tides rise and recede twice a day? Nobody has ever tough a tide would go on for ever, simply by the definition of what a tide is. But even beside that, the reasons why the ocean has tides are very big indeed. You could try making the argument with a Tsunami, but those also require huge motions of the Earth to happen at any impressive scale. Finally, the ocean remain big even now! Honestly I don't get what you were going for, it like an argument version of when sideshow bob is trapped in a field of rakes, every step brings pain.
aaaddaaaaa1112 4y ago
brabel 4y ago
> If the universe expands today, how does it follow that it was also expanding yesterday The universe is not only expanding, it's expanding faster and faster... it's very hard to see how anything could explain the rate of expansion could have started off negative, then instead of just collapsing into itself, it reverted somehow and then started accelerating. The simple explanation is that it has always expanded, though at a slower rate, just like in the future it seems it will continue to expand at a faster rate.
- galaxyLogic 4y ago
  I agree it is a simple plausible explanation and Occam's Razor argues that simple explanations are the most likely. So yes maybe it is the most likely explanation. But that is not the same as saying we know it is the explanation. Big Bang itself is a counter-intuitive explanation: Something came out of nothing.
  - brabel 4y ago
    > But that is not the same as saying we know it is the explanation. No scientist says that. If evidence ever comes up against any scientific theory (and that's why we call it a theory) it's readily replaced with the most seemingly correct theory. > Big Bang itself is a counter-intuitive explanation: Something came out of nothing. Can a theory of creation exist that does not involve something coming out of nothing?
    - galaxyLogic 4y ago
      Perhaps not. I'm just pointing out we should not dismiss theories simply because they are counter-intuitive. There is another theory, which says universe expands then contracts, then expands and so on and it has always, always been like that. I understand many scientists today don't vote for that theory but I think it is less counter-intuitive than something coming out of nothing.
      - eis 4y ago
        IMHO it is equally hard to wrap our head around the concept of inifity (has always been there) versus there being a start or end to time and space. It's a bit weird because if we try to think about the end of space our brain immediately asks "well what is beyond the end?" and so implicitly expects there to be more and more - ad infinitum. But on the other hand if we think about something being infinite then our brain immediately tells us "hold on! that seems fishy, I've never experienced something infinite, it has to start somehow" and so implicitly expects there to be a limit to everything.

vkou 4y ago

> Maybe universe expands, then contracts again cyclically? It's possible, but you're going to have to come up with a theory for why whatever 'force' is driving the expansion goes from negative, to positive, and back.

WXLCKNO 4y ago
We live inside an engine piston?
- vkou 4y ago
  And what wheel drives the inner workings of that piston?
  - eis 4y ago
    Maybe a better analogy would be a spring that goes from contraction to expansion and back again? A lot of things behave like waves so could it be some space field swinging in a wave-like pattern? Is there some evidence for the expansion to have also happened say 10B years ago in the CMBR?
galaxyLogic 4y ago
Is there a good theory as to why Universe is expanding and acceleration its expansion? Dark Matter? Dark Energy? Cosmological Constant? Those are more like "We can not really explain it so we assume there is something like "Dark Energy" which causes it all.

SiempreViernes 4y ago

The reason expansion yesterday is plausible if we see expansion is simply that's it's far more plausible nothing so special happened yesterday as to change the motion of the entire universe. The universe is not a little trickle of water down the wall in the bathroom you can easily redirect with your little finger, the universe it really big, so you need something equally big to make it change behaviour.

Kerrick 4y ago
The oceans used to seem really big to us too, but each rising tide wasn’t destined to continue rising—the tides turned out to be cyclical, even though we couldn’t imagine anything but gods or magic to be powerful enough to cause the change. Perhaps we’re in a similar trough of understanding here?
- SiempreViernes 4y ago
  I'm not sure I get your point, are you saying the moon and the sun, the things we now know cause the tides, are small things?
  - Kerrick 4y ago
    My point is that you could replace a couple words in your comment to get the following, and it would've been conceivable for a person to say a thousand years ago on their first day at the ocean. And yet it wouldn't have been accurate. > The reason tides always rising is plausible if we see tides rising now is simply that's it's far more plausible nothing so special ever happened as to change the motion of the entire ocean. > The ocean is not a little trickle of water down the wall in the bathroom you can easily redirect with your little finger, the ocean is really big, so you need something equally big to make it change behaviour.
    - SiempreViernes 4y ago
      Are you aware that the tides rise and recede twice a day? Nobody has ever tough a tide would go on for ever, simply by the definition of what a tide is. But even beside that, the reasons why the ocean has tides are very big indeed. You could try making the argument with a Tsunami, but those also require huge motions of the Earth to happen at any impressive scale. Finally, the ocean remain big even now! Honestly I don't get what you were going for, it like an argument version of when sideshow bob is trapped in a field of rakes, every step brings pain.

aaaddaaaaa1112 4y ago

brabel 4y ago

> If the universe expands today, how does it follow that it was also expanding yesterday The universe is not only expanding, it's expanding faster and faster... it's very hard to see how anything could explain the rate of expansion could have started off negative, then instead of just collapsing into itself, it reverted somehow and then started accelerating. The simple explanation is that it has always expanded, though at a slower rate, just like in the future it seems it will continue to expand at a faster rate.

galaxyLogic 4y ago
I agree it is a simple plausible explanation and Occam's Razor argues that simple explanations are the most likely. So yes maybe it is the most likely explanation. But that is not the same as saying we know it is the explanation. Big Bang itself is a counter-intuitive explanation: Something came out of nothing.
- brabel 4y ago
  > But that is not the same as saying we know it is the explanation. No scientist says that. If evidence ever comes up against any scientific theory (and that's why we call it a theory) it's readily replaced with the most seemingly correct theory. > Big Bang itself is a counter-intuitive explanation: Something came out of nothing. Can a theory of creation exist that does not involve something coming out of nothing?
  - galaxyLogic 4y ago
    Perhaps not. I'm just pointing out we should not dismiss theories simply because they are counter-intuitive. There is another theory, which says universe expands then contracts, then expands and so on and it has always, always been like that. I understand many scientists today don't vote for that theory but I think it is less counter-intuitive than something coming out of nothing.
    - eis 4y ago
      IMHO it is equally hard to wrap our head around the concept of inifity (has always been there) versus there being a start or end to time and space. It's a bit weird because if we try to think about the end of space our brain immediately asks "well what is beyond the end?" and so implicitly expects there to be more and more - ad infinitum. But on the other hand if we think about something being infinite then our brain immediately tells us "hold on! that seems fishy, I've never experienced something infinite, it has to start somehow" and so implicitly expects there to be a limit to everything.