3 ms·
Well spotted yes, by now that object will be about 45 bn ly away. The light reaching us from it has only traversed 13.5 bn ly though.
by simonh 5y ago
Well spotted yes, by now that object will be about 45 bn ly away. The light reaching us from it has only traversed 13.5 bn ly though.
- sdeframond 5y agoDefine "now" https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity
- simonh 5y agoWhile that’s a problem in theory, to moderate accuracy it’s not a practical issue. Most objects in the universe are roughly at rest relative to the cosmic background radiation (in other words they don’t see it as Doppler shifted in any particular direction), so we can use that as a common frame of reference. When we’re talking about ballpark integer billions of years, or even to a few decimal places, that’s easily good enough.
- uoaei 5y agoAccuracy? "Now" is an inaccurate (nonsensical, actually) concept except in extremely local conditions.
- simonh 5y agoIt's perfectly sensible given a common frame of reference, which we have thanks to the CMB. Frames of reference aren't local. We have clocks and can measure time on them just fine, and 'now' is just a time on a clock.
- deleted 5y ago[deleted]
- cygx 5y agoWhat are you basing the 45 bn ly on? A redshift of z~13 should correspond to a comoving distance of about 33 bn ly, whereas 45 bn ly would correspond to z~600.
- simonh 5y agoYou may be right, I've just seen ~45 bn ly given as the radius of the observable universe accounting for expansion. The discrepancy could be due to changes in the rate of expansion over time, as it hasn't been constant.
- cygx 5y ago45.5 bn ly is the approximate comoving distance of the surface of last scattering (the source of the microwave background and the point where the universe becomes opaque). This is the boundary of the universe observable via electromagnetic radiation.