If every part of the universe has aged differently owing to time running differently for each part, why do we say the universe is 13.8 billion years old?

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For some parts relative to us, only a billion years would have passed, for others maybe 20?

In: Physics
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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:

https://youtu.be/l_WOxn2Oct8

And we’re not even so sure about the 13.8 billion years old part:

https://youtu.be/RdJx34szLjs

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.

First of all, ask yourself the question: why is the universe so dark? If the universe is infinite and contains an infinite number of stars, shouldn’t it be brighter? A lot of smart people asked this question too and it’s know as [Olber’s paradox](https://en.wikipedia.org/wiki/Olbers%27_paradox).
Next, convince yourself that the further you look in the universe, the further you look in time. If you don’t understand this, let me try to explain it with a simple analog.

Suppose you are giving a small party. One of your friends lives 2km away and the another one 15km. they both travel by bike, because they care about the environment, with a constant velocity of 20km/h and arrive at the same time at your place. which one of them has left his house the earliest?
The same reasoning can be done for a [photon](https://en.wikipedia.org/wiki/Photon) (fancy name for light particles, “why is light a particle?”, that’s a whole other story). A photon that comes from a distant star will have traveled a lot longer than a photon from the sun and thus has to leave the star at an earlier time than the photon from the sun.

Now back to Olber’s paradox. The paradox can be solved by stating that the universe has a finite age (13.8 billion year). This means that we can only see photons that have traveled less than 13.8 billion years. Or in other words, and this is the clue, we can only see photons from stars that are less than 13.8 billion light years away (a light year is the distance a photon can travel in one year). Remember: the further you look in the universe the further you look in time. We don’t see every star in the universe and thus the universe is a lot darker (less stars ==> less light). Saying that the universe is 13.8 billion years, is thus a way the explain why the universe is so dark.

I now, it’s a lot to read, but I need to explain just one more thing before I can answer your question, so bear with me 😉

The universe is not completely dark. If we look at a dark spot on the hemisphere, we can still detect some radiation. This is known as the [cosmic microwave background](https://en.wikipedia.org/wiki/Cosmic_microwave_background). Smart people have stated that this is the light from the [Big Bang](https://en.wikipedia.org/wiki/Big_Bang) (the beginning of the universe) that has “dimmed” over time. This background light is almost the same at every place in the universe.

Aha! so now we can understand why we say the universe is 13.8 billion years old. The finite age of the universe depends on how dark the universe is and because the universe is equally dark at every point, we can find that the universe is 13.8 billion years old no matter where we look.

I know that this isn’t a direct answer to your question, but in order to give you that, I think that we need to talk about special relativity, redshift, [Hubble’s law](https://en.wikipedia.org/wiki/Hubble%27s_law), … Stuff that makes your head spin, or at least mine, and I don’t think I can do that in a reddit comment.
Just keep in mind that a lot of smart people are looking at stars and that they figured out how to account for a lot of stuff that can change a picture of the night sky, like relative motion, and after all those calculations, they find the same cosmic microwave background.

Quick note: I put some links to wikipedia in this explanation to be complete, but be aware that it can melt your brain.

Sorry for the long post, hopefully someone can explain it in less words 😉

That’s not exactly how relativity works. It’s about differences in time flow. Like your perception of the time flow will not differentiate, but to your frame of reference other people’s would be aging much faster if you were moving at the speed of light. But that is a very hypothetical scenario. It does not mean that there necessarily are different times that exist at the same time, but simply that it’s possible to experience time at different rates.

The theory of general relativity does not really settle what now is, or if that question even makes sense.

It just explains how time flow changes based on mass, and speed.

However, what you are asking about in your question is essentially a time block (now’s) multiverse, and not only would that would mean that the universe is deterministic. Meaning there is no free will, and every action ‘will’ happen no matter what – because rules on a planck scale.

But also that what we perceive as “now” can be different for you, and your mother. Like theoretically she could be in her now (time block) as a 10 year old, where as your now (time block) is reading this message!

In terms of the universe a “now” is a really really complicated issue. We all have our own perceivable/observable universes (although on a universal scale earth is about the same) and now (time block) theoretically could be different for everyone in the universe, but for anything to interact in any way, the now must be synchronized – otherwise the universe is pre-determined.

Your true question is, what is “now”. And is it the same?

[I recommend you watch this](https://youtu.be/EagNUvNfsUI)

But this video is not an ELI5, and I’m definitely not clever enough to ELI5 this issue.

Cosmologist here.

You are right! Every point in the universe has aged differently. I’ll give two levels of explanation:

Easy: 13.8 by is actually the average over all the different points

Hard: Have you ever heard of the expansion of the universe? Everything is moving away from everything else due to the expansion of space itself. This is called the Hubble Flow. If you don’t move but just go with the flow , ie only see the distances of galaxies change because space is expanding and not because you are moving in it, you are said to be a comoving observer.
So 13.8 by is the time as measured from someone who has been moving with the hubble flow since the big bang.

We measure the age of the universe by looking at the Oldest Light we can perceive: The Cosmic Microwave Background.

Light moves at a constant speed, and the expansion of the universe is also constant. The wavelength of Light traveling over such distance “expands” at a steady rate as a side effect of spatial expansion, producing a “red shift.” By measuring the amount of red shift, we know exactly how long the light has been traveling.

We have measured the age of the universe by measuring the amount of red shift in the oldest light we can see. There’s a possibility that there’s older light beyond that boundary, but it hasn’t reached us yet… and that means that we can say that the universe is *at least* as old as the light from the CMB.

A guy by the name of hubble figured out that some of the smudges in the nights sky were actually other galaxies and not just nebulea.

In figuring this out he also discovered that the ones which were further away from us were also moving away from us the fastest. Through plotting these galaxies on a graph of distance and speed he determined the rate of expansion called hubble’s constant.

Through a little mathematics (1/hubbles constant) we can determine how long the universe has been expanding for, approximately 14.4 billion years.

Simple, because time is actually just a construct. It isn’t a physical thing, merely a label attached to relativistic perception.

So while the interaction between molecules is effected by their relativistic speed and thus “age” differently, the pure time of existence of the components is the same.

If a 20 year old human departs earth on a round trip spacecraft and returns 300 years later with the body of a 21 year old, it doesn’t matter that the human only perceived 1 year, his body still exists for those 300 years and so it is 320 years old from the earth standard.

DISCLAIMER: I am not a physicist,,, at all.

But iirc they say this because of the “cosmological constant” which is something in math that describes the rate to which the universe expands (because it is always expanding).

And by assuming everything started as 1 point in space (directly prior to the big bang) we can use that constant to determine when most of what we observed would be around the same point.

Again, not a physicist, so I apologize if that’s completely wrong.

Man, i just happen to be almost done reading Einsteins booklet on relativity. Think of it this way… if we view the spacetime continuum, or just, the universe, as a long sheet of rubber, like bubble gum flattened and stretched out as a big sheet, then we can also imagine that gravitational fields that exist on that sheet can cause some parts of it to ‘bubble up’ a bit and stretch, others to contract. If you were to stand on one area of the sheet of bubble gum from the estimated start of the universe, your own gravitational field would allow you to perceive the length of “time” that has passed for you until now. Remember however, that this big sheet of bubblegum is connected everywhere, and those ‘distortions’ are all connected to in the curves of spacetime. But here’s the big problem with our conception of this. You immediately want to think about how being in a different part of the universe would affect you and how you would age vs your friends on earth, etc… this makes things hard to conceptualize. If we don’t call time ‘time’ anymore, and instead just perceive it as another dimension, just another numerical value, you can then start to understand… Let’s now think of the universe as a 2D finite plane. With the third dimension as time, we can start to piece things together. If we look at the plane just from directly above, it appears perfectly flat; however, looking at this from the edge reveals something odd. The plane has bumps and differences in height in different places. We can now see ‘valleys and peaks and mountains’ of varying heights. Let’s consider this depth, the third dimension in a seemingly 2D universe ‘time2D’. For the beings that live on that universe, ‘time2D’ affects their perceptions of life and rates of change from different perspectives. We however only see it as different heights on what looks like the aforementioned stretched bubblegum sheet. We know, that regardless of how those beings have perceived ‘time2D’ in different locations, the seemingly 2D universe has existed for a certain period of our human earth time which we know is true and measurable. To them, it is only measurable by relative time from each individual perspective because of the bumps and stretches that define ‘time2D’ as their third dimension. Just apply that to our perceptions of the 3 dimensional world. We can only measure the universe’s age from this location in the universe, oherwise we might perceive a different measure.

I assume that scientists mean that every single particle of the worlds existence, has been created 13.8 billion years ago in a single event, thus the age of the cosmos is practically all the same for the different objects it consists from.

Yes very true. We just say that because on average, it is that old but truly, different sections would be different years old. Section 7A56IP- in the universe would be way younger than our section.