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Recently, I started to ponder again with the question of how do we know the age of the universe.

From my research, the answer is something like: "We measure the expansion rate directly with type ia supernovea, and extrapolate it from CMB data, and with that, we calculate when the universe would have size zero according to the expansion history".

But as far as I understand this explantion, we also need to know the current size of the universe independently of the current age of the universe (because we are trying to calculate its age, and if we need the age for the size, we have a circular dependency).

So my question is how do we know the size of the universe? Is it because we know the universe is infinite so we know it's size (although as far as I understand, this would lead to the question of how do we know the distance to the CMB, because we use that distance to prove that the universe is flat, and therfore infinite)? Is it somehow with the friedmann equations and additional parameters we measure with the CMB? Does it has something to do with the size of the visible universe (although as far as I understand, we know it thanks to the combanition of the age of the universe and its expansion rate)? Or do I misunderstand this explanation?

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You don't need to know the size of the universe to calculate its age.

The age of the universe or cosmic time is a function of the current density parameters and the Hubble constant,

$$t_{uni}(H_0, \Omega_{m,0},\Omega_{\Lambda,0},\Omega_{r,0})=\frac{1}{H_0}\int_{0}^{\infty}\frac{dz}{(1+z)\sqrt{\Omega_{r,0}(1+z)^4+\Omega_{m,0}(1+z)^3+\Omega_{\Lambda,0}+\Omega_{\kappa}(1+z)^2}}$$

We measure the expansion rate directly with type in supernovae, and extrapolate it from CMB data, and with that, we calculate when the universe would have size zero according to the expansion history.

It's partly true. From the CMB measurements, we can calculate many cosmological parameters. As you can see from the above equation the most important parameters are $H_0$ and the density parameters.

For instance, $H_0$ measured by using two methods, the early universe (by using CMB and BAO) and the late universe (by using type Ia Supernova, TRGB etc,). Measurements showed that there is approximately 6 sigma difference between these two methods, which is called the Hubble Tension

But as far as I understand this explanation, we also need to know the current size of the universe independently of the current age of the universe. So my question is how do we know the size of the universe?

We cannot know the size of the universe, however, we can calculate the size of the observable universe (see particle horizon)

$$\eta(H_0, \Omega_{m,0}, \Omega_{\Lambda,0}, \Omega_{r,0}) = \frac{1}{H_0}\int_{0}^{\infty}\frac{dz}{\sqrt{\Omega_{r,0}(1+z)^4 + \Omega_{m,0}(1+z)^3 + \Omega_{\Lambda,0} + \Omega_{\kappa}(1+z)^2}}$$

As you can see they have the same parameters as the cosmic time.

Is it somehow with the Friedmann equations and additional parameters we measure with the CMB?

Yes by obtaining the cosmological parameters from various measurements we can calculate the age of the universe or the particle horizon.

Can we obtain the cosmic time from the particle horizon?

Well, I am not sure but I do not think it's possible. Expansion of the universe is governed by the Friedmann Equations and Friedmann Equation depends on the density parameters. Without knowing how the universe evolves and by just looking at a distance we cannot calculate its age. Because we do not know "how fast it evolved"

For more information about the measurements I found this wiki page

If you want to know where these equations come from I made the derivation in another question

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  • $\begingroup$ Okay, thanks, I assume you get this equation from some derivation with the Friedman equations. But I still have a few questions: is the process I described in my question, a valid way to calculate the age of the universe? If it is does this calculation needs to know the current size of the universe? And how would such calculation would look like? If it isn't, then why does it seem to me, like every YouTube channel describe this way, as the way we measure the age of the universe. $\endgroup$ – Ofek Tevet May 5 at 21:01
  • $\begingroup$ You said "From my research, the answer is something like:" Could you share the link = $\endgroup$ – Layla May 5 at 22:31
  • $\begingroup$ I edited my answer $\endgroup$ – Layla May 5 at 23:04
  • $\begingroup$ It is mostly the videos of the YouTube channels of scisowspace and PBS space time. youtu.be/tCn96DbBnB4 scishow space. youtu.be/Y6Vhh70Lw9w PBS space time. $\endgroup$ – Ofek Tevet May 6 at 7:49
  • $\begingroup$ @OfekTevet I just looked briefly but as far as I can see they are only talking about the measurement of the $H_0$ which as I showed its a parameter that we put in to calculate the age of the universe. If you can give a timestamp maybe I can help you better. $\endgroup$ – Layla May 6 at 8:18

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