# I Big Bang timing: according to what frame of reference?

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1. Apr 15, 2017

### Sophrosyne

My apologies if there are some related discussions on this topic in another thread here, but I could not find one specifically addressing this question. Big bang theory, as it currently stands, talks of some extraordinarily precise time measurements; you see numbers like that 10^-37 sec after the Big Bang, cosmic inflation happened.

But the question is, this is according to what frame of reference? Time has meaning only when there is propagation of electromagnetic waves. It really has no meaning in a black hole, or, I presume, at the time of the big bang, and for a good long time after that, until light could actually propagate.

It seems to me that these time measurements are being talked about from a theoretical frame of reference of an observer standing outside of the universe watching it undergo The process. Can such a frame of reference exist? But it seems Einstein would object to such a frame of reference. If such a frame of reference cannot exist, then where are these timing numbers coming from?

2. Apr 15, 2017

### phinds

All times of the sort you speak of are based on calculating backwards from now from the time of a co-moving observer. "Outside the universe" is not a meaningful concept. "T=0" is the place where the math breaks down and we don't know what was going on but we use that time as a baseline. Time is believed to have flowed at one second per second since about one Planck Time after T=0.

3. Apr 15, 2017

### Bandersnatch

The reference frame used is that of an observer who sees the cosmic microwave background radiation as isotropic (meaning: the same in every direction). I.e. a 'comoving observer' that phinds mentioned.

4. Apr 15, 2017

### Sophrosyne

I see. So the timing, though, is measured from OUR frame of reference observing the microwave background radiation. As an example, The first 10,000 years after the Big Bang is considered the radiation era: where the energy was dominated mostly by photons of different wavelengths. But that 10,000 years is according to our frame of reference. To a hypothetical observer going through that time period, where the mass density and gravity were much higher, the time period have been measured as much longer. Correct?

5. Apr 15, 2017

### PeroK

You could try this:

https://van.physics.illinois.edu/qa/listing.php?id=19268&t=measuring-time-after-big-bang

On a related point, consider the following scenario. A spaceship passes our solar system at high speed. They record an orbit of the Earth round the Sun. According to their ship's clock, it takes 3 months (about 9 million seconds), say, because of time dilation.

But, they wouldn't then return to their planet and say that our Earth has a 3-month year. They could and would calculate how long an orbit took in the rest frame of the solar system. And return with the correct figure for an Earth-year.

So, although measurements depend on relative speed, you can still have a natural frame in which to study a certain phenomenon - usually the rest frame of the thing you are observing. In cosmology, the comoving frame is the most appropriate frame to study the evolution of the universe.

6. Apr 15, 2017

### phinds

No. It is, as I said, from a co-moving frame. We are not co-moving.