As God-Emperor, I declared that a Dyson sphere be constructed around Sol to power a really big UV laser that sends a 1-second-duration pulse every 10 seconds at, oh, 200 nm wavelength, aimed into deep space.
20 billion years from now, an astronomer way, way, way, way off in space sees some pulses on its radio telescope readout.
Does it see 1-second pulses every 10 seconds, of wavelength, oh, 0.5 meters, due to cosmological expansion? Or are the pulses themselves longer in duration and separated by longer periods as well?
They aren’t in the UV range by the time they get to the receiver due to redshift.
Redshift isn’t quite enough to get to radio, though. Max known redshift is z=~1000, which stretches out that 200 nm into 200 um. That’s about 1.5 THz, but radio tops out at ~300 GHz. Starting in the infrared would just make it, though.
Dammit! I was afraid that there would be a reason my attempt at snarky humor would prove to be stupid. Better to remain silent and be thought an idiot…
But 20 billion years from now the maximum possible z will be ~46% higher than it is today at t=13.7 billion years, right? But I do see that I put too much redshift in the original hypothetical.
So let’s assume instead that the future astronomer observes our (originally 200 nm UV) pulses with their orbiting infrared telescope instead, at 2200 nm, giving a z of 10 ((2200nm-200nm)/200nm).
Does that mean that they would also observe the pulses to be eleven seconds long, separated by a hundred and ten seconds, or am I applying the principle incorrectly?
The latter. The biggest observed redshift is light from the beginning of the universe, the Cosmological Microwave Background (CMB), which was emitted when the universe was only 380,000 years old. It has z = 1023 or thereabouts.
In theory, something could have a higher redshift, but it’d have to have an intrinsic (that is, non-cosmological) velocity away from us at speeds approaching the speed of light. It’s unlikely we’d ever observe something like that. It’s not that things don’t go that fast, but they’re all subatomic particles, not macroscopic things like stars that emit their own light.
I’ve read that some galaxies in our expanding universe are moving at almost the speed of light. If true, would the signal ever reach there if, in effect, the galaxy is “running away” from the signal?
Many galaxies are receding at near the speed of light, and even more (possibly infinitely more) are receding at greater than the speed of light.
There is a horizon beyond which light from a star will never reach us, but anything on this side of that horizon will eventually make it to us. The light that is reaching us right now from the furthest galaxies was actually emitted when that galaxy was much closer to us(like a few tens of millions of lightyears), and those galaxies are now much further away(around 45 billion light years).
Approaching, or would a significant fraction be enough? The light reflecting off of the push plate of a craft we accelerate to a fraction of lightspeed by blasting with a giant laser could do, perhaps?
It would have to be a very large fraction of light speed. In fact, it would have to be going so fast that from its perspective, the CMB is at a higher temperature than it actually was at the time of first scattering. So, probably in the 99.9…% range.
OTOH, if we are looking at an object that is in a galaxy that has a z>1000, then it could have a star traveling in it at a smaller fraction of light speed that would appear to us to have a redshift greater than the CMB.
Yes, but we don’t see any galaxies with a redshift anything like that high. They keep finding new redshift records for galaxies and quasars, so I’m not sure what the record currently is, but it’s around 11 or 12, I think. And those formed within a few hundred million years of the Big Bang.
Googling, the current record is 16.7, formed about 250 million years after the BB.
Agreed, I was just trying to come up with conditions that would satisfy @Babale’s question. The fact that those conditions are so extreme is the important part of the answer.
But I do see that I put too much redshift in the original hypothetical.