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

20 Answers

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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 😉