Relativistic Reference Frames and the Big Bang?

[jahzam]

Peeling this out into its own thread for clarity:
How is time dilation of extreme reference frames (photons, black holes, intergalactic space-time) taken into account in Big Bang cosmology? Since from the POV of a singularity or a photon, their clocks have effectively stopped and any lower inertial reference frames have already run their whole timelines (reached max entropy).

Doesn't relativity mean that from the POV of any photon or black hole, all slower or lower-g bodies instantly age near-infinitely (and thus photons & black holes perceive the Big Bang as already over)?

How can there be any *absolute age of a universe with physically infinite options of inertial reference frames an "object" can attain?

 

[PeterDonis]

How is time dilation of extreme reference frames (photons, black holes, intergalactic space-time) taken into account in Big Bang cosmology?

The "age" of the universe as that term is used by cosmologists is, by convention, the age according to "comoving" observers, i.e., observers who always see the universe as homogeneous and isotropic. The age according to observers who are in some other state of motion will be smaller.

Since from the POV of a singularity or a photon, their clocks have effectively stopped

No, that's not correct. There is no valid "POV of a singularity or a photon". (A photon at least has a well-defined worldline with zero arc length; the fact that the arc length is zero is often incorrectly described as "time being stopped" for a photon. A singularity doesn't even have a well-defined worldline, so I don't know what a "clock" for a singularity would be, even in a mistaken interpretation.)

any lower inertial reference frames have already run their whole timelines (reached max entropy).

I have no idea what this means.

Doesn't relativity mean that from the POV of any photon or black hole, all slower or lower-g bodies instantly age near-infinitely

No. The time since the Big Bang according to comoving observers does not depend on how much time has elapsed on anyone else's clock.

(and thus photons & black holes perceive the Big Bang as already over)?

I don't know what this means either.

How can there be any *absolute age of a universe with physically infinite options of inertial reference frames an "object" can attain?

There isn't, if by "absolute" you mean "age according to any clock, regardless of its state of motion". But that's not what cosmologists actually mean by "the age of the universe". See above.

 

[Ibix]

The reference frame of a photon or a singularity isn't a concept that makes sense. Time isn't defined in any meaningful sense for a photon, and a black hole singularity isn't meaningfully "at the same time as" anything else.

The 13.9bn year age usually quoted for the universe is that measured by a so-called co-moving observer, one who always sees the cosmic microwave background as isotropic. This is the maximum possible age. All other observers will measure less time since the Big Bang. Observers who move very close to the speed of light relative to a co-moving observer may measure an arbitrarily short age.

 

[phinds]

Doesn't relativity mean that from the POV of any photon or black hole, all slower or lower-g bodies instantly age near-infinitely (and thus photons & black holes perceive the Big Bang as already over)?

There is no such thing as a "POV of a photon" so the question is meaningless.

EDIT: I see I'm late to the party

 

[Ibix]

EDIT: I see I'm late to the party

Me too. Time dilation effects from the speed and information density of Peter's post, I think.

 

[phinds]

Me too. Time dilation effects from the speed and information density of Peter's post, I think.

 

[kimbyd]

Peeling this out into its own thread for clarity:
How is time dilation of extreme reference frames (photons, black holes, intergalactic space-time) taken into account in Big Bang cosmology? Since from the POV of a singularity or a photon, their clocks have effectively stopped and any lower inertial reference frames have already run their whole timelines (reached max entropy).

It has no effect, really.

The earliest that we can see is around 300,000 years or so after the "Big Bang". No photons from the really early times are visible (because the universe before roughly 300,000 years was opaque).

Extremely compact objects like black holes are so small that very few lines of sight ever intersect with them. Those that do have an effect usually do so because they're really bright (e.g. quasars). When attempting to measure the signal that was emitted at around 300,000 years, the Cosmic Microwave Background, these bright sources are cut out of the data and do not contribute to the result.

Time dilation around such extreme objects is more a question of astrophysics, where people might be interested in, for instance, the dynamics of accretion disks around black holes, or how black holes grow over time. It has no impact in cosmology beyond understanding those objects.