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Comment by __blockcipher__ | Hacker News Reader

This might be obvious to most, but at first it wasn't clear to me why in 150 billion years the vast majority of the known universe will be unreachable. The answer is in the acceleration of expansion (which the blogpost mentioned but I didn't connect the dots on at first); eventually the rate of expansion ("velocity") will greater than the speed of light.

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Parsing this, I’d like to match it up with the Hubble horizon, at least for your stated condition that the velocity of expansion is greater than the speed of light: That already holds for us relative to points across the Hubble horizon. Their “past-emitted” light is still reaching us, meanwhile their “presently emitted” light never will. I need to check the proper definitions but that’s the gist. Kinda erie!

> eventually the rate of expansion ("velocity") will greater than the speed of light.

What will that do to the speed of light as measured on Earth?

And will this affect, say, the correct operation of computers?

The speed of light ought to remain constant (for inertial observers). So nothing would be different locally as far as I'm aware.

The speed of light (in vacuum) is constant for accelerated observers too. :-)

The dipole anisotropy of the Cosmic Microwave Background will continue to be a good tool to see the constancy of the speed of light for all observers into the practically infinite future, assuming the infinite future looks a lot like de Sitter space (which is essentially what the standard cosmology predicts).

http://www.astro.ucla.edu/~wright/CMB-dipole-history.html

The far future of the universe will still have generally covariant physics, as does its far past, sharpening up your second sentence.

At that point in time (assuming time has any meaning in that reference frame,) everything will be moving away from everything else faster than any force, even information, could possibly travel between them. There won't be computers. There won't even be atoms to make matter to make computers with.

I'm still wondering if the change would be instantaneous or gradual. Will the speed of light be constant and then suddenly impossible to measure?

I'm not precisely sure what you're referring to as "the change" here. The rate of expansion is increasing continuously though, if that's what you're asking.

As far as the measurability of the speed of light, I don't think anything changes in principle. It should remain measurable and constant. However, it gets a lot more complicated in practice, since it's pretty unclear who or what would actually be doing the measuring and how they might go about it.

No. The expansion of the universe does not mean that atoms themselves will fly apart

Actually it might. If expansion accelerates indefinitely, which is one of the theories, then eventually expansion will be stronger than the binding force of atoms and then sub-atomic particles. This is called the "Big Rip".

https://en.wikipedia.org/wiki/Big_Rip

We have lots of observations of spiral galaxies at ever-increasing redshifts, and they don't look very different at different times -- closer galaxies are not collapsing more slowly than the distant ones.

We also have good views of the Lyman-alpha forest through galaxy clusters at different redshifts, and the distribution of the spectral lines are another line of evidence that members of galaxy clusters closer to us have smaller radial components in their peculiar velocities than those at higher redshift/smaller angle/lower brightness.

To get to a big rip you have to impose a further time derivative and figure out how to keep it small now and make it big later. AFAIK nobody who has tried this has avoided significant complexity to match the observational data we have today, except by pushing the Big Rip so enormously far into the future that we don't have enough data to constrain that future. The wikipedia article refers to WMAP9, which ended 2013 (~ 9 years after WMAP turned on in 2003). There is more recent data from Planck and other observatories, and they make it harder and harder to justify a Big Rip for the reasons half-admitted in the "Observed universe" section of that wikipedia article.

Moreover, MESSENGER and other spacecraft sent to parts of our solar system have given us a lot of data about the metric of our solar system, and it is non-expanding; orbiting clocks support this more locally. Likewise, there are plenty of sensitive terrestrial experiments that highly constrain any expansion term in plausible Standard Model + gravity Lagrangians.

It can't be ruled out but it has been effectively pushed back so far that it's the least of our worries about "what happens hundreds of trillions of years in the future" : if one accepts there there is a Big Rip field, there's no compelling reason why it should have significant effects on matter in the local group that soon, and several arguments about why it shouldn't.

You'll find a bit more substance (and higher difficulty) at https://en.wikipedia.org/wiki/Quintessence_(physics)#Tracker...

Thanks for this

Oh! Well then. Thanks for pushing back. I learned something new today.

essentially it's similar to a black hole.. nothingness

The opposite, actually.

i saw a kind of circularity between so expanding it becomes nothing and so attractive it appears as nothing

Distant regions of the universe already are receding from us faster than light. Due to the accelerating rate of expansion, that horizon beyond which this is true is shrinking inwards towards us, so that in 150 bn years even galaxies outside our local group will be beyond that horizon.

Hurry up Elon, we're running out of time!

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