The discussion revolves around the concept of the age of the universe and whether it differs for observers in expanding space, particularly focusing on the implications of relative motion and the nature of comoving observers. The scope includes theoretical considerations of cosmology and the effects of expansion on time perception.
Participants express multiple competing views regarding the implications of relative motion and the nature of time for observers in expanding space. The discussion remains unresolved, with differing interpretations of how separation speed and comoving status affect time perception.
Limitations include the dependence on definitions of comoving observers and the implications of spatial expansion on measurements. The discussion also highlights the complexity of time dilation as it relates to different coordinate systems.
More precisely, the age of the universe according to comoving observers, i.e., observers who always see the universe as homogeneous and isotropic, is about 13.8 billion years.mister i said:The age of the universe is said to be about 13.8 billion years.
If both galaxies are comoving (which galaxies are on average), yes. The "separation speed" you refer to has nothing to do with the "rate of time flow"; it is a coordinate speed and has no direct physical meaning.mister i said:would it be the same for a possible inhabitant of a planet in a galaxy about 10,000 million light years away that is separating from us at 60% of the speed of light?
And what happens regarding the light (which is indeed moving), from our point of observation it travels in this expanding space at 300,000 km/s or do we have to add the part of the spatial expansion?Ibix said:"neither galaxy is moving, but the space between them is expanding" is the simplest description.
One can think, by analogy, of an ant crawling down the length of an expanding rubber band.mister i said:And what happens regarding the light (which is indeed moving), from our point of observation it travels in this expanding space at 300,000 km/s or do we have to add the part of the spatial expansion?
Locally, you will always measure it to be doing ##c##. In the "expanding space" interpretation, the distance light still has to go to reach us is decreasing at less than ##c## because the space between us and it is expanding. Sufficiently distant light will never reach us because the total remaining distance is increasing faster than even light can decrease the distance.mister i said:And what happens regarding the light (which is indeed moving), from our point of observation it travels in this expanding space at 300,000 km/s or do we have to add the part of the spatial expansion?
Sorry, I was reading this thread. With an hypothetical comoving observer co-located with Earth do you mean a comoving observer that at a given point/event along its geodesic worldline is spatially co-located with the Earth ?PeterDonis said:What can affect the "rate of time flow", although in practice the effect is small, is the fact that particular observers might not be comoving. For example, we here on the Earth are not comoving; we observe a dipole anisotropy in the CMBR that tells us that, relative to a hypothetical comoving observer co-located with Earth, we are moving at about 600 km/s.
No. A comoving observer, as you would know if you just looked it up, is:cianfa72 said:do you mean a comoving observer ...
Yes.cianfa72 said:With an hypothetical comoving observer co-located with Earth do you mean a comoving observer that at a given point/event along its geodesic worldline is spatially co-located with the Earth ?
Yes.cianfa72 said:such a comoving observer will be spatially co-located with the Earth just at that "event/point" along its worldline.
Read more carefully, and see my response in post #9 just now.phinds said:No.
Ok, but in FRWL spacetime in standard coordinates assuming on average galaxies as comoving "observers" then their spatial coordinates are fixed. Therefore such "separation speed" is nothing but the derivative w.r.t. the cosmological time of the proper distance between galaxies evaluated on spacelike hypersurfaces of constant cosmological time.PeterDonis said:The "separation speed" you refer to has nothing to do with the "rate of time flow"; it is a coordinate speed and has no direct physical meaning.
In the idealized model, yes. But in actual fact we know that most galaxies are not exactly comoving. That includes our own galaxy. The closest things we have in actual fact to something physical that is comoving to the best of our knowledge are the centers of mass of galaxy clusters.cianfa72 said:in FRWL spacetime in standard coordinates assuming on average galaxies as comoving "observers" then their spatial coordinates are fixed.
Yes, that is a valid interpretation. (Note, though, that this "proper distance", and the "separation speed" that goes along with it, is not something we actually measure. We have to calculate it, and the calculation is model dependent.)cianfa72 said:Therefore such "separation speed" is noting but the derivative w.r.t. the cosmological time of the proper distance between galaxies evaluated on spacelike hypersurfaces of constant cosmological time.
Yes, the proper time ##\tau## of comoving observers is the FRWL cosmological time ##t## so there is not any "time dilation" between them.PeterDonis said:However, it is still the case that this "speed" does not imply any "time dilation" of comoving observers relative to each other.
More precisely, this is the case if we choose the simultaneity convention of FRW coordinates. "Time dilation" always depends on a choice of coordinates.cianfa72 said:the proper time ##\tau## of comoving observers is the FRWL cosmological time ##t## so there is not any "time dilation" between them.
No, they can't, because the proper distance between them is increasing. Exchanging radar signals to check time dilation only works between observers if the proper distance between them is constant.cianfa72 said:I believe such comoving observers can check there is not any time dilation relative to each other by exchanging radar signals.
You mean that picking as simultaneity convention that implied from FRW coordinates, then the proper time of comoving observers "evaluated" at the same coordinate time ##t## is the same for both comoving observers.PeterDonis said:More precisely, this is the case if we choose the simultaneity convention of FRW coordinates. "Time dilation" always depends on a choice of coordinates.
The proper time since the Big Bang, yes.cianfa72 said:You mean that picking as simultaneity convention that implied from FRW coordinates, then the proper time of comoving observers "evaluated" at the same coordinate time ##t## is the same for both comoving observers.
No it isn't. The age of the universe is the time measured on clocks that see the CMB as isotropic.feynman101 said:The age of the universe is calculated by assuming that we are at the centre of the universe and everything is moving away.
There is no such place.feynman101 said:Therefore we are at the centre of the universe.
Do you have a reference (a proper journal, not a pop sci article) for that?feynman101 said:The current figure that you cited is now out of date due to observations from the James Webb telescope.
This sounds like a misunderstanding of standard cosmological models. The correct statement is that the CMB was emitted around 14bn years ago when the universe cooled enough to become transparent. We still see this simply because it happened everywhere so light from distant parts of this event has been travelling for 14bn years.feynman101 said:The perimeter of the universe that we observe as the cosmic background radiation is when the big bang coalesced into matter. Due to time dilation the edge of the universe is very young because it has been expanding at the speed of c.
And travelling since Big Bang, now its spectrum is at microwaves (CMB) and it is homogeneous and isotropic for comoving observers.Ibix said:We still see this simply because it happened everywhere so light from distant parts of this event has been travelling for 14bn years.
Observations by the JWST absolutely did no such thing. In fact, they do the opposite. They give even more evidence (and more accurate evidence) in support of the current cosmological model as a whole. That is, the big bang theory, specifically the LCDM model, with an expanding universe that is approximately 13.7 billion years old without any known 'center'. One of the things that the JWST observations HAVE shown us that wasn't predicted/known was to show that galaxy formation happened earlier than expected, which isn't exactly surprising since we didn't know very much about early galaxy formation to begin with. Its observations have also helped up better understand the expansion rate of the universe, atmosphere's of nearby exoplanets, planetary formation in protoplanetary disks, and much more.feynman101 said:Yup, it depends on your inertial frame of reference. The age of the universe is calculated by assuming that we are at the centre of the universe and everything is moving away. Therefore we are at the centre of the universe. The current figure that you cited is now out of date due to observations from the James Webb telescope.
Not true. Roughly the same amount of time has passed for the edges of our observable universe as has passed here on Earth.feynman101 said:Due to time dilation the edge of the universe is very young because it has been expanding at the speed of c.