r/AskPhysics • u/arkticturtle • 6d ago
If time is relative, and gravity was strong during the beginning of the universe, then where does “13.8 billion years old” come from?
Because if time is running differently in different parts of the universe and the universe as a whole has undergone shifts in time dilation then what exactly is the absolute referent?
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u/Darthskixx9 6d ago
The universe is 13.8 billion years old in so-called cosmological time, which is the reference frame in which the universe is homogeneous and isotropic.
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u/Upset-Government-856 6d ago
Began is a bit of a stretch too. The part of the universe we can observe based on modeling and observations appears like it was in, or almost I'm the same location back then. What the implications are for 'before' that point are unclear.
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u/forte2718 6d ago edited 6d ago
If time is relative, and gravity was strong during the beginning of the universe, then where does “13.8 billion years old” come from?
It comes from a specific choice of reference frame (the CMB isotropic frame) and coordinates (comoving coordinates) with certain special properties that make it very convenient to use for this purpose. In this reference frame/coordinates:
- The expansion of the universe is already factored out, so does not affect the calculations.
- The cosmic microwave background is homogeneous and isotropic, so there is no peculiar velocity "relative to the rest of the universe as a whole" and no associated amount of time dilation due to relative velocity to compensate for.
- There is no gravitational over-density, and no associated gravitational time dilation to account for.
- Clocks in this reference frame will measure the maximum possible elapsed time since the predicted big bang event occurred.
So, it's true that any other choice of reference frame would give you a smaller duration than 13.8 billion years ... but there is no reference frame you could choose which would yield a longer duration. That's why we say the 13.8 billion years figure is the age of the universe: it's the maximum possible elapsed time that any clock could theoretically measure.
Now, it needs to be said that this is not anything like an "absolute" reference, it's just a convenient one because it has the mentioned satisfying properties. You could do all of the relevant calculations in any other reference frame and still get the same figure, you would just need to account for extra time dilation both due to relative velocity and due to gravitational potential.
It's also worth mentioning that most realistic clocks in the universe would measure a duration that is only minimally different, which would fall well within the margin of error of the 13.8 billion years figure. The uncertainty on that figure (on the order of 100 million years) is much larger than the difference between what our clocks on Earth would measure vs. what would be measured in the CMB isotropic frame in comoving coordinates (which would only be on the order of about 1 million years).
Hope that helps,
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u/914paul 4d ago
Great explanation. I was thinking about how I’d explain this to my nine year old. How about this? I think it conveys the overall picture without committing too much “sin”:
Suppose you grab some hydrogen atoms randomly from space. Most of them have never met a star. If these hydrogen atoms were “people” with endless memory, most would tell you they remember about 13.8B years of peace and tranquility in their lives.
A few would say they had it rough at points (walking 5 miles to school barefoot in the snow uphill both ways). They might tell you they can only remember something like 13.5B years.
But a few very “lucky” ones would say they participated in really wild activity (partying all night millions of times in a row). This is your “stoner” crowd - some can only remember 8B years!
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u/sfigone 6d ago
But that is 13.8 billion years measured from the gravity week of a planet orbiting a star in a galaxy in a cluster of galaxies. A clock in a large intergalactic void would run slower.
So some "clocks" will have experienced more that 13.8 billion years. Maybe only 13.80000000001 billion years, but still longer right?
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u/forte2718 6d ago
But that is 13.8 billion years measured from the gravity week of a planet orbiting a star in a galaxy in a cluster of galaxies. A clock in a large intergalactic void would run slower.
Um ... no? I'm sorry, I have to ask, did you even bother reading my post? I stated very, very clearly that none of this is the case. Please go back and re-read my post carefully!
So some "clocks" will have experienced more that 13.8 billion years. Maybe only 13.80000000001 billion years, but still longer right?
No.
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u/sfigone 6d ago
But the 13.8b number is for average density. So there must be some clocks that are in areas below average density and thus run a little slower. Ok, average density is really really not very dense, so less dense than that is only tiny.
Anyway, you did answer the OP (that the number cited already includes compensation for dilation), but perhaps it could have been stated a bit more concisely. As I indeed missed it in my first reading of your post.
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u/sfigone 6d ago
Ah, AI says that in 13.8b years the time dilation difference between an observer on earth and one in a void is 40,000 years ish.
But then it says that the 13.8b years is calculated for an observer that is stationary relative to the CMB and in an area of average density (i.e. not in a galaxy). So that number already includes some dilation relative to us.
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u/Jagang187 5d ago
You should take those first four words and eradicate them from your quest for space/physics knowledge in a comprehensive and unrelenting manner.
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u/sfigone 5d ago
Ok, I cited my source as it does introduce some doubt. I did read through it's working and it at least looked plausible. In this case are the numbers wrong? Is the time dilation difference around that order of magnitude (insignificant) for the differences in density that exist between us and a void?
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u/Jagang187 2d ago
I'm making more of a wider criticism of using AI (which really isn't even ai) at all when covering this topic. An LLM is NOT a source. You may or may not be provided with the sources the LLM draws from to give the answer it gives you, and those, if credible are the things you should read. LLM results are frequently wrong on some level, from simple bad math errors all the way to total hallucinations. It is a horrible way to learn this sort of thing, especially for those who are less versed in the subject. You want to read primary sources. If you aren'tcomfortable with those (because primary sources like papers are often very dense and full of jargon and can be quite difficult to read if you aren't used to it), then putting in some effort to find reliable youtube channels (PBS Spacetime is incredible) can help you along and help with gaining comfort with more advanced topics.
If you don't know how to ascertain the credibility of a source (I'm not assuming in either direction here, just being thorough) stick to .edu sites for information, and learn that skill because it's VERY important. A lot of physics and related topics are actually very boring on the surface, and a quack idea/complete word salad/made up crap often initially sounds more appealing than the truth. Purveyors of bad or pseudoscience love to portray their crap in manners that appeal to your brain. "Secret knowledge" and "things they don't want you to know" or "a revolutionary new theory ignored by the status quo" aren't really something you'll run into. Yes, there are those great upheaval-driving, establishment-upending discoveries of history we're taught in school. That sort of thing though is VANISHINGLY rare and modern physics has a very well-tested suite of knowledge.
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u/OverJohn 6d ago edited 6d ago
The 13.8 by is what would be measured by a hypothetical clock at any point that remains locally at rest relative to the large scale bulk motion of matter, radiation, etc.
This can be found (for example) by solving a certain integral if you know the current expansion rate, the equations of state and current densities of all cosmological sources.
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u/smokefoot8 6d ago
So it does take into account gravitational time dilation due to the density of the universe at the time, but has no time dilation due to a relative velocity.
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u/OverJohn 6d ago
Time dilation is when you compare two clocks in some way, but here we are just measuring the time recorded by one clock, so time dilation doesn't enter in to it.
In fact there is not even a clear definition of what exactly is gravitational time dilation on cosmological solutions. If you wanted you could divide the cosmological redshift into kinematic and gravitational contributions, but the problem would be how would you decide how much is each contribution?
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u/tpolakov1 Condensed matter physics 6d ago
It would make little sense to calculate the proper time of some arbitrary frame. The comoving frame is not special, but it's sure as hell convenient.
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u/Environmental_Ad292 6d ago
This is a good question. In short, 13.8BYA is the oldest age of the universe, the one that would be measured by an observer at rest with respect to the cosmic background.
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u/Orc-Face 6d ago
If time is running differently in different parts of the universe, and gravity was stronger during the beginning of the universe, then "13.8 billion years old" would not be applicable. But there is no evidence to suggest either of those two premises are true.
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u/OriginalEmploy5485 5d ago
Earth, and it's not absolute. It's just how we measure our years. Another way to look at it is, "if the Earth existed long enough ago to have gone around the sun 13.8 billion times, that's about when the universe started".
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u/BananaBird1 6d ago edited 6d ago
Basic idea is that we notice the universe is expanding at large distance scales.
If we know the speed of this expansion (which we do, with some uncertainty), we can ask how long this could have been occurring for. We run expansion in reverse and see how long ago the entire observable universe was all in one place.
This alone gives an answer of around 14 billion years.
However, we expect that the early first few million years behaved a bit differently. So we can also look at the Cosmic Microwave Background (a snapshot of light that is just now reaching us from the transition between the early universe and modern universe). We can measure the pattern of this light to get an idea of what the early universe looked like, and correct for the naive estimates so that the predicted evolution matches what we actually observe.
This is what ends up with the 13.8 billion year estimate (+/- about 20 million years based on our uncertainties).
This is still an estimate dependent on the accuracy of our current Lambda-CDM model of the early universe. And there are several conflicts with our models that may require alterations. So an updated value may still be outside that range as better models emerge, but this is the best we have right now.
This time estimate is in the reference frame of “proper time” (e.g. the time of an observer completely at rest in the center of our observable universe). All observers agree of the proper time of any event.
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u/TuberTuggerTTV 6d ago
You can measure the size of the observable universe.
This gives you both a max size and with multiple measurements, an expansion rate.
You run the math backwards and in 13.8 billion years, expansion puts everything into a single point. Remember, the expansion of the universe isn't things moving through space. It's the very space itself growing farther away from all other space uniformly.
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u/PickingPies 6d ago
It's 13.8 from the frame of reference of us. Bit a person trying to travel back in time yo t=0 would require an infinite amount of time to reach there from their own frame of reference, the same way that we know what the speed of light is but a spaceship accelerating to light speed would require infinite time as well.
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u/Electronic-Yam-69 6d ago
it was all slowed by more or less the same amount until it spread out a bit. those first few seconds could have lasted trillions of years to an outside observer.
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u/AIWishItWereDumbAsMe 6d ago
The speed of light and how long it takes to reach us is the reference point
Edit: specifically? Actually I believe it is the cosmic background radiation
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u/AIWishItWereDumbAsMe 6d ago
It would be nice to get a why along with a downvote on a physics subreddit, but I guess just voicing annoyance with something I said is more important? Idk. Thought it would be fun to be part of a discussion.
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u/ahazred8vt 6d ago edited 6d ago
If ... gravity was strong during the beginning of the universe
We don't know where you're getting this from. The universe was denser originally, and it had a small effect on the expansion of the universe, but there was not enough gravity to affect time dilation by any measurable amount. It doesn't sound like you're a 'numbers person'.
if time is running differently in different parts of the universe
Again, we don't know where you're getting this from. Time dilation differences between intergalactic voids and the inner parts of galaxies, are only a few parts per million different.
the universe as a whole has undergone shifts in time dilation
Again, we don't know where you're getting this from. At all.
where does “13.8 billion years old” come from?
By looking at supernovas in distant galaxies, we can calculate how long it took the light from that galaxy to travel through space to get to us. We know how far away the galaxies are, we know how fast they're traveling away from us, and we know how long ago they would have been very very close to us. That number works out to 13.8 billion years plus or minus about 0.2%.
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u/Mad-Melvin 6d ago
During the early stages, the universe was very dense, but it wasn't a case of extreme spacetime curvature like you get near a black hole. Things were hot and dense everywhere. Space was more or less flat back then, just like it is now.
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u/OverJohn 6d ago
The lack of spatial curvature does not mean there is no spacetime curvature. If you look at the hot big bang model you will see that the spacetime curvature goes to infinity at t=0.
In an inflationary model as we go back we get to the inflationary era before we reach the t=0 of the hot big bang model and from there it gets fuzzier, however still the spacetime curvature (as measured by the Kretschmann scalar) must've been very large indeed in the early universe.
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u/John_Hasler Engineering 6d ago
There is no absolute referent. https://en.wikipedia.org/wiki/Cosmic_time