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And now that they’re further apart, they separate even faster the next second.
That’s a common misconception! Barring effects of matter and dark energy, the two points do NOT separate faster as they get farther apart, the speed stays the same! The Hubble constant H0 is defined for the present. If you are talking about one second in the future, you have to use the Hubble parameter H, which is the Hubble constant scaled with time. So instead of 70 km/s/Mpc, in your one-second-in-the-future example the Hubble parameter will be 70* age of the universe / (age of the universe +1second) =69.999...9 and your two test particles will still be moving apart at 70000km/s exactly.
The inclusion of dark energy does mean that the Hubble “constant” itself is increasing with time, so the recession velocity of distant galaxies does increase with time, but that’s not what you meant. Moreover, the Hubble constant hasn’t always been increasing! It has actually been decreasing for most of the age of the universe! The trend only reversed 5 billion years ago when the effects of matter became less dominant than effects of dark energy. This is why cosmologists were worried about the idea of a Big Crunch for a while - if there had been a bit more matter, the expansion could have slowed down to zero and reversed entirely!
Oh wow thanks. You learn something new every day! I’m definitely an “armchair physicist”, and still find it hard to think about things in a nonstacically geometric way.
Sounds like the Hubble Constant ain’t so constant :)
That’s a common misconception! Barring effects of matter and dark energy, the two points do NOT separate faster as they get farther apart, the speed stays the same! The Hubble constant H0 is defined for the present. If you are talking about one second in the future, you have to use the Hubble parameter H, which is the Hubble constant scaled with time. So instead of 70 km/s/Mpc, in your one-second-in-the-future example the Hubble parameter will be
70 * age of the universe / (age of the universe + 1 second) = 69.999...9and your two test particles will still be moving apart at 70000km/s exactly.The inclusion of dark energy does mean that the Hubble “constant” itself is increasing with time, so the recession velocity of distant galaxies does increase with time, but that’s not what you meant. Moreover, the Hubble constant hasn’t always been increasing! It has actually been decreasing for most of the age of the universe! The trend only reversed 5 billion years ago when the effects of matter became less dominant than effects of dark energy. This is why cosmologists were worried about the idea of a Big Crunch for a while - if there had been a bit more matter, the expansion could have slowed down to zero and reversed entirely!
Oh wow thanks. You learn something new every day! I’m definitely an “armchair physicist”, and still find it hard to think about things in a nonstacically geometric way.
Sounds like the Hubble Constant ain’t so constant :)