crazunggoy47
crazunggoy47 OP t1_jajwm2d wrote
Reply to comment by Any-Broccoli-3911 in Why do cosmologists say that gravity should "slow down" the expansion of the universe? by crazunggoy47
>The expansion of the universe is kinetic energy. [...] Also, the observed speed is always less than the speed of light, so everything has a finite energy.
Is that true? I thought that for distant galaxies, the recessional speed was often greater than c, since c is only a local speed limit, and does not apply to space time inflation.
Consider the rapid inflation of the universe, which went from electron-sized to golf-ball sized in 10–35 seconds; applying a naïve speed calculation would yield speed = distance / time = 43 mm / 10^(–35) seconds >>>>>> 3 x 10^(–8) m/s.
This is all to say, I'm mostly questioning whether the perceived recessional velocity can really correspond to kinetic energy.
Submitted by crazunggoy47 t3_11fkfeq in askscience
crazunggoy47 t1_j5wv2lv wrote
crazunggoy47 t1_j5wu80j wrote
Reply to comment by Drzhivago138 in Why do sample return missions such as OSIRIS-REx use their own reentry vehicles instead of just going to the space station for pickup and return with ISS equipment? by PromptCritical725
In a pinch, they can also lithobrake!
crazunggoy47 OP t1_irrkvo7 wrote
Reply to comment by oodelay in How fast do bubbles rise in water? by crazunggoy47
It looks like it. See my comment to TexasPop. Air matches the density of water at a water depth of around 5.3 miles.
crazunggoy47 OP t1_irrkn04 wrote
Reply to comment by TexasPop in How fast do bubbles rise in water? by crazunggoy47
Wow, I plugged in a pressure of 16000 PSI into this calculator and you're right! It exceeds the density of water. That's weird.
crazunggoy47 OP t1_irpfue9 wrote
Reply to comment by mfairview in How fast do bubbles rise in water? by crazunggoy47
It seems like an evacuated container with high volume and low mass could feel greater buoyant force than air. Especially because an air bubble should reduce in volume due to the surrounding pressure, and therefore reduce its buoyancy (I think).
I’m still looking to better understand how ambient liquid pressure affects bubble velocity. It feels like on the one hand, higher pressure should impact greater force to the bubble. But on the other, higher pressure would contract the bubble and reduce its volume and buoyancy. Does that mean there is a particular optimal water depth that causes the greater bubble velocity?
crazunggoy47 OP t1_irpf37g wrote
Reply to comment by atomfullerene in How fast do bubbles rise in water? by crazunggoy47
Good point about the square-cube discrepancy here.
And yes, there surely are bubbles nearly everywhere there is liquid. Presumably on Europa (water), Enceladus (water), and Titan (methane), just to name a few.
Submitted by crazunggoy47 t3_y00ioa in askscience
crazunggoy47 OP t1_jajyba4 wrote
Reply to comment by Any-Broccoli-3911 in Why do cosmologists say that gravity should "slow down" the expansion of the universe? by crazunggoy47
Hmm. So if I understand you correctly, you're saying that an object that's moving away from us due to cosmic expansion has a finite kinetic energy (relative to us). So, from our perspective we should "expect" that kinetic energy to be falling, as our own gravity pulls them back in.
And then that galaxy, will also see the exact same thing. From its perspective every other galaxy is fleeing *it*. And if every galaxy sees this, and it just so happens that every trajectory has too little KE, then every galaxy would see the other galaxies crashing down on them.
Is that right?