For some parts relative to us, only a billion years would have passed, for others maybe 20?
In: Physics
We’re simply judging by earth years not relativistic years. Sure technically, certain parts have advanced further in “time” due to the effect of gravity. But according to us here on earth, looking out. There has been approximately 13.8 billion years since the big bang. (A year being the time it takes for the earth to complete 1 full cycle around the sun)
For 99.9% of the conversation, the only perspective that everyone understands is that of humans living on earth. So it makes sense to use that perspective as a measurement point. It would not make a whole lot of sense to say it is xx “years” old from the perspective of someone living on another planet since pretty much no one understands anything from that perspective.
We can say that a “year” on Jupiter is about 12 of earth years. Would it make much sense to say that someone is 2 Jupiter years old? It is perfectly definable and measurable, but such a measure is pretty meaningless to everyone. Science is about discovery and the communication of discovery. So, where possible, it is logical to choose a way of communication that is relatable to most people.
I’m no expert but as I understand it everything comes down to frame of reference.
We are trying to measure the age of our part of the universe. The assumption is that physics works the same in every part of the universe so if we could instantly teleport to the farthest place we can see and take a measurement there we would get the same answer.
Of course time dilation, univers expansion and the like produced some interesting effects like Methuselah star:
And we’re not even so sure about the 13.8 billion years old part:
No no, he’s got a point, it’s not even a “we are humans so we describe it in a way humans understand”
I get the whole “from our point of view, the universe is x age” but what do we gain from saying? Like, if we find a rock, and we know this rock has existed since the start of the universe, we cant say that its 13.8 billion years old because we dont know the speed at which its travelled for the last 13.8 billion of our years, it could be older if it travelled slower than us, or younger if it travelled faster, and thats ignoring the effect of gravity
The oldest anything in the universe could be is 13.8 billion years old. This would be a hypothetical object that came into existence at the big bang and has been stationary (called comoving) relative to the cosmic microwave background for its entire existence. You are correct that there is no universal time for the whole universe, and any reference frame is valid, but using the CMB makes the most sense since it’s the leftover radiation from the big bang.
It’s also important to note that most parts of the universe are pretty close to being comoving with the CMB, so most of the universe is pretty close to this age. The only places where you’d expect a large difference in the measure of elapsed time would be close to massive objects like black holes and things that have been moving at relativistic speeds for most of the existence of the universe.
I’m not smart enough to offer an answer, but this video on the [Twin Paradox](https://youtu.be/UInlBJ4UnoQ) might offer some additional insights.
There are several good answers here, but I think it’s worth mentioning how rare relativistic things are. Most things in space tend to be moving at about 0.1% the speed of light. Now that seems fast, but it turns out the relativistic effects that make clocks move differently are very small until you hit about 90% the speed of light. You can also change the clock’s speed with heavy gravity, but again you need to be near a black hole for that to matter.
Overall, you get that for pretty much all the clocks out there, the age of the universe is going to be the same, give or take a few thousand years.
Last thing worth noting is the cosmic microwave background. Basically there was a time the universe was full of gas that was so hot we can still see the glow from it today. You can tell from this glow whether you are moving with respect to that gas, so you can use it as a reference point for a standard speed, and so a standard clock, for the universe. As I describe above, it doesn’t make much difference to account for this, but it’s pretty cool.
Don’t think of time as something measurable in and of itself. What we experience as time is the effects of entropy. Entropy is how things change (super simplified version) and time is how much things change.
In places where time doesn’t move as quickly, entropy sort of slows down. For example, if you have a nail that is rusting, it will rust more slowly in places with higher gravity or speed than others.
If you have two rusting nails and one of them is near you and the other is 1000 lightyears away, it makes no sense to say “The entire universe is x amount of rust on my nail.” and expect all of the nails in the universe have rusted the same amount. All of the nails have been rusting at different rates.
Instead, you would have to say, “My nail rusts at this rate, my nail has rusted this much, therefore the local ‘time’ is x.” You would then say that the universe is x years old compared to your nail.
For the same reason ELI5 exists… Going into a complex explanation every time discussing the subject will confuse most people. If someone asks “how old is the universe?” the answer “13.8 billion years” is sufficient for the huge majority of conversations. From Earth’s perspective, that’s how old the universe appears (or at least that’s our best prediction to date).
Physicist here. Here’s a copy/paste from [my answer to an old /r/askscience thread on the topic that included lots of good discussion.](https://www.reddit.com/r/askscience/comments/1m3yql/since_time_is_relative_how_do_we_define_it_when/cc5kh6z)
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It depends on how we measure it, but all reasonable reference frames give about the same value.
The most precise measurements are based on the [Cosmic Microwave Background (CMB)](https://en.wikipedia.org/wiki/Cosmic_microwave_background). There is a convenient reference frame called the [comoving frame](https://en.wikipedia.org/wiki/Comoving_coordinates#Comoving_coordinates), in which the CMB light coming from all directions is equally redshifted. This is also the reference frame in which the universe is the oldest, and is the reference frame we use when doing most cosmology.
Our solar system is moving at about 371 km/s relative to the comoving frame, which gives a time dilation factor of only 1.0000008, which is why it doesn’t matter much what (reasonable) reference frame we pick. In this frame the universe is only about 10,000 years younger, out of 13.8 billion years.
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