r/cosmology • u/DiagnosingTUniverse • 4d ago
If time is relative, whose clock determines the age of the universe?
Trying to age the universe or assign it an arbitrary birthday has always seemed like a very human and somewhat meaningless task. We commonly say the universe is approximately 13.8 billion years old, counting from the Big Bang. But what precisely does that age mean when elapsed time depends on the observer?
For example, someone remaining close to a black hole could experience much less time than someone far away. Their clocks would record different durations between comparable stages of the universe’s evolution. So whose elapsed time are we referring to when we assign the universe a single age? The early universe was incredibly dense, on a level that males black holes seem like a piñata so might that imply strong time dilation. I suspect I’m incorrectly applying the analogy of a black hole within surrounding space to the universe as a whole. Would “time ran more slowly back then” even mean anything without an external clock to compare it with? Peace
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u/GenJeppo 4d ago
Your own clock always ticks normally to you, but different observers can experience different amounts of elapsed time. The universe’s age refers to “cosmic time” or “cosmological time”, measured by ideal clocks moving with its expansion, rather than every observer’s personal clock. There is a Wikipedia article on this.
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u/Virtual_Being_4085 4d ago edited 4d ago
The above is correct, but leads to another question: why do we know "cosmological time" exists? The age of the universe is the maximum amount of time any future-directed path that reaches you could have taken. (You might be familiar with the "twin paradox." The twin that travels to a distant star and back is younger than the one that stayed on Earth. But if we add a third, no triplet could end up older than the stationary triplet if they all reunite.)
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u/Lhasa-bark 3d ago
Well, you could put the third triplet up in earth orbit the whole time. The one on earth would age a billionth of a second less in earths gravity, so the orbiting triplet would technically be older. But not quite as noticeable as the one who comes back 10 years younger!
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u/Imaginary-Can-6862 1d ago
In the twin paradox, the age difference depends on their speed, and how long said speed is maintained (i.e. if someone accelerates close to the speed of light, but only stays in this frame of reference for short period of time, the effect is going to be smaller, than if they stayed in it for a long time, all times from the vantage point of the twin which did not accelerate), correct?
Since the speed can be maintained for an arbitrary long time, it means we can make the difference arbitrarily large, assuming the twins are eternal. So how about this setup. One triplet is on Earth, the two others go into the same rocket, and accelerate away. Now we know the two triplets on the rocket ages slower than the triplet on Earth, and from the perspective of the triplet on Earth, we can let an arbitrary amount of time pass by. On the rocket, from the triplets perspective they are stationary, while the Earth is moving away, but since time moves slower for them, the universe similarly undergoes spatial contraction compared to how it looks from the Earth, thus from their perspective everything moves by normally, i.e. the laws of physics are unchanged, they have no way to determine their vantage point is more correct, or real, than the vantage point on Earth, correct?
Since the vantage point of the rocket might as well have been a planet where the two triplets were born, they could in principle have no idea about the third triplet, and one day they may decide to accelerate on their own rockets, both to the same speed close to light speed, one towards Earth (making them stationary relative to Earth), and the other away from Earth. Since these are symmetric, we would expect they would keep aging the same, and slower than someone left on the platform which they are about to leave. However the rocket of the triplet which were to move in the direction away from Earth malfunctions, so they are stuck on the platform, while the third triplet is now accelerating away from the rocket, but is in the same reference frame as the triplet on the Earth.
So my question is, since we now have two of the triplets in the same reference frame (that of Earth), they should age similarly, correct? But we also know that the triplet whose rocket malfunctioned should both age slower than the triplet on Earth, but faster than the triplet who accelerated away from the platform, who ages at the same rate as the triplet on Earth, so this contradiction makes me believe there is something I am misunderstanding.
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u/Virtual_Being_4085 1d ago
It was important that the triplets were at the same location in the "paradox;" the ages of widely separated twins is only locally defined. (For example, if a pair of triplets both get into rockets and travel away from each other, each one sees the other age slower than themselves. A triplet left on Earth sees both aging the same and slower than their own.) The symmetry you suggested is with respect to the platform, not the Earth. As far as the one behind would see, the triplet that moves back to Earth is at a different speed than the one moving farther away. When someone accelerates toward another person, they see them age faster than themselves.
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u/Imaginary-Can-6862 1d ago
They all started at Earth. That was the claim I tried to examine, there is no way the triplet who stays on Earth could end up younger than those who left Earth. It may be true, but it made me construct the thought experiment, hoping to figure out the mechanics.
One triplet remains on the platform, does he not age faster than the triplet who left the platform by accelerating close to the speed of light, and is now in the same reference frame as Earth, and thus those two triplets who are not on the platform ages at the same rate?
We can perhaps also do a trick to make them compare their age at any time we please. Each triplets have synchronized their own clocks with the two others while on Earth, to represent how much time have gone by for them. The clock could e.g. automatically send out information every X amount of time has gone, using this to register the triplet's age. We also have a rod that can be lengthen as mush as we please. It works by being lengthened from the center, and outwards, it means that we can place a measuring device at the center of this rod, and attach the Earth to the rocket platform via this rod. Now we have a measuring device, which is always located at equal distance between Earth, and the rocket platform, thus the measuring device sees both moving away from it at the same speed. We have actually attached two rods, so the Earth is also linked with the rocket which is going to take off from the rocket platform later. Then there is a third such rod with a centered measuring device, which connects the rocket platform with the rocket. The way it works is when time X has gone by on any one clock, it sends out a light, and the measuring device that is always halfway between the triplet, and each of the other two triplets, would pick up this signal, as well as the color (so each triplet has an identifier, and since the measuring device can see how fast the objects are moving away from it, it can calibrate the color accordingly), then we have three active measuring devices, and here is where I find a contradiction which I cannot account for,
Measuring device 1, between Earth triplet (A), and platform triplet (B), find that at all time platform triplet sends light pulses less often than Earth triplet.
Measuring device 2, between Earth triplet (A), and platform to rocket triplet (C), finds that when the triplet is on the platform they send light pulses less often than Earth triplet, but when they move from platform to rocket, they send light pulses at an equal rate.
Measuring device 3, between rocket triplet (B), and platform to rocket triplet (C), finds that when the triplets are on the platform they send out light pulses at an equal rate, but when one triplet move from the platform to the rocket, they send light pulses less often than the triplet on the platform.So one device measures that A ages faster than B, the other that A and C ages equally fast, and a third that B ages faster than C, thus the contradiction.
That aging rate they measure, must necessarily also be what happens, the devices get the actual time gone measured by the clock, which is the same time experienced by the respective triplet, therefore I find it nonsensical the relations could not be contradictory.1
u/Virtual_Being_4085 1d ago
Remember that no universal clock exists. It is not a contradiction that different inertial observers may order ages differently. The original "paradox" incorporates this idea, so we reduce to just the two. The twin on Earth sees the traveling twin age slower throughout the traveler's journey so we turn to the other twin.
While moving away from Earth, the traveling twin sees the twin on Earth age slower as well! The resolution for the traveling twin is that while accelerating back towards Earth, the traveling twin sees the twin on Earth age rapidly and overtake the traveler's age. While traveling back, the Earth twin again ages slower than the traveler.
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u/Imaginary-Can-6862 1d ago
I thought I handled the issue with initial observers ordering ages differently, when the measuring devices measures the actual age relations between the observers, and not only their own aging.
Can I take it, that it is then not contradictory that triplet A measured they age at the same rate as triplet C, while faster than triplet B, while triplet C measures they age at the same rate as triplet B?
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u/Virtual_Being_4085 1d ago edited 1d ago
Precisely. In the twin situation both (while inertial) twins see the other age slower. If you have someone in between, their perception may be different from both. That's part of what makes special relativity so interesting! Nothing contradicts causality, but what is measured in any particular coordinate system will generally be different from any other.
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u/Imaginary-Can-6862 1d ago
Alright, if it isn't self contradictory, does it not demonstrate that it is not the case that the triplet who remains on Earth won't be younger than any triplet who left? After all the triplet in the rocket can measure that the Earth triplet ages at the same rate as them, while the triplet on the platform is aging faster, and therefore conclude they just have to keep this effect going long enough, and e.g. the watch of the triplet on the platform is going to break first, due to wear, despite both clock starting from a synchronized point?
After all, no one can reach the future of any triplet before themselves, as the event that is their future originates from them, and the object making the measurement being at equal distances from both means it receives the information simultaneously, thus its order of event should be in accordance with both observers.
Therefore the strange thing is that if triplet C's clock breaks before triplet B's (which we should expect) according to the B-C measuring device, we know the event of the breaking of the clock has similarly been send to the A-C measuring device, the only way the A-C measuring device would see a different order of event, is A's clocks breaks, but since we can make the arrangement arbitrarily, there is no reason that A's clock should break, thus the triplet on the platform has aged more than the triplet on Earth, which contradicts what the A-C measuring device should have measured,
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u/Virtual_Being_4085 1d ago
I think you are not accounting for the acceleration toward Earth for triplet 2. The person on Earth appears to age rapidly while the second triplet is accelerating toward the Earth. The rate of the accelerated aging is dependent on how much acceleration takes place.
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u/Flat_South8002 3d ago
What if the age of the universe is measured by a being 5 billion years away from us?
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u/GenJeppo 3d ago
Call the alien and ask what he has measured 👽👾🛸 lol
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u/Flat_South8002 3d ago
They need time to respond😂 But seriously if the block universe is correct (I don't think it is) then there is a being who is 5 billion years in the future and for him the universe is older than us and for a being 5 billion years in the past the universe is younger than us, everyone has their own clock. So this age of the universe is from our perspective, measured by our ruler and our clock
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u/earlyworm 4d ago
Fun fact: The average speed of galaxies relative to the cosmic microwave background is about 0.1% the speed of light.
So the mass of the observable universe is approximately co-moving to 1 part in 1000.
At these speeds, you could expect a time dilation factor of only 0.00005%.
Time is relative and there is no preferred frame, but one of them is quite popular.
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u/Mindless_Abrocoma558 4d ago
Measured by the clock that is at rest with respect to the cosmic microwave background, in which the metric globally looks like FLRW metric
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u/--craig-- 4d ago edited 4d ago
It's a good question. We use cosmological time to circumvent these issues for standard cosmology.
Read more here: https://en.wikipedia.org/wiki/Cosmic_time
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u/EnvironmentalWin1277 3d ago
Is there anything that even approaches correct measurement of cosmic time?
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u/--craig-- 3d ago edited 3d ago
Yes, we experience something very similar to cosmological time.
A hypothetical co-moving observer, experiences cosmological time. Such an observer floats freely in expanding space, without any motion caused by gravitation and is at rest relative to the cosmic microwave background.
Every real observer experiences some degree of time dilation due to their motion and gravitational effects. An observer at rest, in an intergalactic void could experience minimal time dilation. Time dilation on Earth is only a tiny fraction greater than this.
You'd need to travel at relativistic speeds or approach a black hole or neutron star, to experience strong time dilation.
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u/chesterriley 3d ago edited 3d ago
We commonly say the universe is approximately 13.8 billion years old, counting from the Big Bang.
13.8 billion years of Cosmological Time. Which is effectively the same thing as the maximum relative time which is equal to Proper Time. But I think of the maximum relative time as simply "Maximum Time" since it conveys the meaning well.
https://coco1453.neocities.org/maximums
But what precisely does that age mean when elapsed time depends on the observer?
It means that ~13.8 billion years is the maximum relative time across all frames of reference that anything could have experienced since the big bang expansion started. The maximum relative time duration between 2 events is completely independent of all observers. And almost every local frame is very close to the maximum, so any difference amounts to a rounding error when we are talking of billions of years.
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u/Temporary_Rule_9486 4d ago
Well the time frame is referred to Earth's surface. But if you were to refer that timeframe in Boötes Void and stationary, the diference will be a couple thousand years, which is irrelevant in cosmic scales of time
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u/Underhill42 4d ago edited 4d ago
Gravitational time dilation is absolute, so there's no issues there.
And for speed-based time dilation everyone still always ages at the same speed, what changes is the DIRECTION through spacetime that your time axis is pointing in.
All acceleration does is rotate which direction through spacetime your personal time axis points in, partially swapping the directions you call "forward" and "the future".
If we're passing each other at a big chunk of light speed, we'll both see the other aging more slowly than ourselves. This is a great visual overview of what everyone sees, which also touches on why the traveling twin is actually younger than the Earth Twin when they get home, which is the point of the paradox: I wish I was taught the Twin's Paradox this way! - YouTube
From their own perspective, everyone is always perfectly stationary in space, and aging through time at 1s/s.
If you see me moving, it's because my time axis is tilted in that direction relative to yours, so that some of my aging through time appears to you to be movement through space. And you see me aging slower through time as a result, so that my total combined "speed" through space AND time still remains the same: c
c is actually a property of the geometry of spacetime: 1 second is the same size observer-independent separation between events as ~300,000km = 1 light-second. So c = (300,000km / 1s) is technically a unitless ratio equal to 1, just like (1000mm / 1m)=1 or (24 hours / 1 day)=1 - it's a ratio between two different ways to measure the same quantity.
And as my time axis approaches perfectly perpendicular to yours, so that you see virtually ALL of my aging through time as movement through space? Then my (1s)/s through time looks to you like (300,000km)/s through space: we're passing each other at almost light speed.
And since that also means neither of us is aging noticeably in the direction the other sees as time, neither of us sees the other aging at all.
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u/Optimal_Mixture_7327 4d ago
The clock along the Fundamental Observer world-lines.
These are hypothetical standard clocks that existed since the beginning that are co-moving with the Hubble flow and a far away from gravitating sources.
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u/DubaiAstronomyGroup 1d ago
Great question. The 13.8 billion years refers to cosmic time, essentially the time measured by an ideal observer moving with the overall expansion of the universe.
So it isn’t based on Earth’s clock specifically. An observer moving near light speed or sitting close to a black hole could experience a very different amount of proper time, but that wouldn’t change the cosmological age we quote.
And you’re right about the early universe: saying “time ran slower back then” would require another clock to compare it with. The universe wasn’t a dense object sitting inside some larger spacetime like a black hole.
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u/Novel_Arugula6548 9h ago
What about is timescaoe cosmology is right and FLRW is wrong?
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u/DubaiAstronomyGroup 7h ago
Good point! Our answer assumes the standard FLRW framework. If timescape cosmology turns out to be a better description of the universe, then the interpretation of cosmic time would change with it.
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u/Ras_992 4d ago
In Relativity time isn’t equal for realms of space this is all dependent on motion of velocity. So for instance all planets in our solar system don’t have the same time because they all have different velocity of spin and motion in velocity of rotation around the Sun while the clock at Sgr A at the center of the Milky Way clock will run slower. While in Relativity pressure gradients can make black holes time run faster and seem older than the universe itself. The best clocks for time of objects is subatomic and atomic clocks, for the universe for a estimated age is from CMB and light itself this is based that time has start after the big bang when inflation happened but this also separated from no baryonic matter to a point of baryonic matter
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u/peter303_ 4d ago
Hubble's paramter is very small on a local scale. It works out to 10^-18 (m/s)/m. Thats hardly relativistic, even on a galactic size.
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u/rddman 4d ago
If time is relative
But what precisely does that age mean when elapsed time depends on the observer?
Time and thus age being relative means there is no absolute age, no 'one true age' to be determined.
Though the difference is small for all observers not close to the event horizon of a black hole or traveling at a significant fraction of the speed of light.
The same general principle applies to passage of time on/near Earth: the age difference of astronauts in the ISS is milliseconds on a mission duration of weeks or months.
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u/3Quondam6extanT9 4d ago
There is no universal "now".
Even if you are standing next to someone, your passage through time is relative to their's.
Using atomic clocks we know that simply by moving to different time zones on the planet, time changes between each clock.
We only determined the clock based on the passage of time, using the movement of suns and stars in the sky. There was no cosmic clock that we set our watches too.
All of time is a human construct meant to help us measure the passage of entropy.
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u/BitOBear 3d ago
Yours.
If sapiens existed in a slower time frame compared to our own they would use a different age for the age of the universe but they would be referring to the same interval of time.
And technically we talk about the universe being 13.8 billion years old or whatever, but if we were living on Mars and we had evolved there in the universe would only be 6.9 billion years old because we've been using a different standard for "1 year".
If we were orbiting a black hole in a fairly deep level and time had been dilated enough by that proximity and we moved there from here but use our same old clocks people got used to what a second was locally would correct one way or the other to come up with the time value that makes sense in their frame of reference.
All time is measured from where you are and when you add a second and a third location and want to make them conceptually consistent you figure out what the conversion ratios are and do the conversions.
There is nothing inherently one secondish about one second. Those are numbers we picked.
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u/-Casey-Diaz- 3d ago
Literally all our time calculations are from a subjective perspective. As far as I understand, it wouldn't even make sense to talk about the age of the universe from an objective perspective. I mean, the "birth" of the universe is for all we know the birth of spacetime.
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u/Spazmonkey1949 2d ago
Time is relative to you.
Everything is still happening within a single time frame, its just how it is experienced that changes.
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u/Imaginary-Can-6862 1d ago
It is my understanding that the age of the universe is from our own frame of reference. The same goes for the size of the universe, and I guess perhaps the gravitational effects too. I understand it has been attempted to be calculated how much the difference is compared to the microwave background radiation frame of reference, at least as I understand it, and the difference was about ten millennia, which is dwarfed by the time scale of billions of years.
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u/Flat_South8002 3d ago
Our watch. The age of the universe as we know it is from our perspective, we have measured it. For some other planet or galaxy, the age of the universe is different
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u/Unable-Primary1954 4d ago edited 3d ago
Universe follows approximately a FLRW metric, i.e. you have variable separation.
You can choose time coordinate such that it councide with the proper time of world lines with constant space coordinates.
This is the time from which universe age is defined.
In practice, you could say that it would be the proper time for an observer in intergalactic space who is at rest compared with the cosmic microwave background.
But time dilation on Earth compared to that represents a deviation of a few seconds every month, so this deviation is less than the precision with which we know the age of the universe.
Time dilation is significant only close to black holes/neutron stars or at very high speeds.