Submitted by _bidooflr_ t3_11isl13 in askscience

Just thought about it so my thoughts are a bit confused.

I know time depends on gravity force as time-space is a field. When you are next to a heavy body time is faster. When we calculated the age of the universe we used thermodynamic equations that ruled how it will expands and reversed them to find a single point, but that only applies to calculations and observations made on earth right? So is our universe 13.7 billion years old only for a constant earth gravity? Would it be anither result somewhere else in the universe? Could it be shorter as in the beginning of expansion everything was very dense and thus happened faster?



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Aseyhe t1_jb0odnt wrote

When we say that the universe is 13.7 billion years old, this is actually in the rest frame of the cosmic microwave background, not that of the Earth. However, the difference due to gravitational time dilation (mostly due to the galactic potential) and kinematic time dilation (since we're moving at ~370 km/s with respect to the cosmic microwave background) is of order one part in a million, so any ambiguity in the age of the universe due to time dilation is much smaller than the measurement uncertainty in the "13.7 billion years" value.

More generally, the question of whether the age of the universe depends on where you are depends entirely on what convention you adopt. There is no such thing as a universal "now". If you wanted, you could define that "now" means the elapsed time, in the cosmic microwave background frame, is 13.7 billion years. This convention is called "synchronous gauge" and is commonly used in cosmology calculations. Under this convention, the age of the universe does not depend on position.

For other conventions, like the "Newtonian gauge" that is also commonly used in calculations, the age of the universe does depend on position.


7eggert t1_jb0vkzk wrote

>There is such thing as a universal "now".

Is this missing a negation or did I miss something?


Aseyhe t1_jb0x01j wrote

yes, fixed, thanks!


pepe_silvia_12 t1_jb151yf wrote

I’m sorry could you explain that more please? There is no universal “now”?


Anonymous_Otters t1_jb1crzq wrote

The flow of time is relative, so time is flowing at different rates at different places depending on the curvature of the universe in respective locations and on relative velocity. Only when you get into the same gravity well in the same place can two objects be considered to be more or less at the same time. As soon as they leave the same gravity well or travel at different relative velocities, they are technically now in different times until they meet up again. That said, every single molecule, atom, subatomic particle is technically running at slightly different times even within the same object. Space and time are, under general relativity, considered inextricably linked, so can be thought of as the same thing. Changing space changes time, so being in even a slightly different spaces means you are in a slightly different timr. This is why the cosmic microwave background radiation is used as a universal resting position to make some sort of standard time.


Melkor15 t1_jb2286m wrote

This is truly amazing, thanks for your time explaining it.


JasonDJ t1_jb30ffx wrote

So, wait a tick…

If you’ve got two watches in perfect sync on earth, and one leaves at a low rate of speed (well below speed of light), where is “out” of our gravity well that they are significantly losing sync, and would that still hold true with atomic clocks?

Assuming we had some sort of hypothetical instantaneous radio communication, would the communications be distorted once one side were in a different gravity well?


Anonymous_Otters t1_jb313dn wrote

Atomic clocks are routinely used to measure the time difference between things on the surface of earth and things in orbit. The difference of the flow of time needs to be calibrated or else things like GPS wouldn't work right.

If communication were instant, how it would work would I guess depend on what you were using, since that isn't supposed to be possible.


Hapankaali t1_jb70cm0 wrote

Instantaneous communication leads to quite a few problems of the "grandfather paradox" type. For example, the relativity of simultaneity means that according to some observers, the response to a query will be given before the query itself.


Grimyak t1_jb26yje wrote

I think of the "now" as a kind of unbroken fabric of causality. Although I guess with the universe expansion and stuff there are places that will float past the horizon where anything connected to our specific causality cannot interact.

Anyways in my mind "now" is more tied to cause and effect than it is strictly a time based measurement. As in "now" is simply the period where the "cause" side of cause and effect can be manipulated.


Anonymous_Otters t1_jb27sbx wrote

Causality is the reason for there being different nows since if causality existed irrespective of time then things outside of now, from the perspective of an observer, could causes changes too far away to have actually been caused by the observer if, say, information could propagate faster than light. The reason light speed is what it is is because light speed in a vacuum is the speed of causality.

Your definition doesn't make sense as the period where the causes can be manipulated since, for example, my observation of the light from a distant galaxy is completely unaffected by anything happening in my now since the "now" of the galaxy I'm observing occured billions of years ago from my now. Now is entirely relative. The only way I can see the now of the galaxy I'm looking at would be to go there, and by then the now I want to be part of would have passed.


Grimyak t1_jb2evbg wrote

I apologize for my poor explanation. I understand that there are regions in our universe where causality cannot apply due to distance and time limitations.

My intention was not to suggest that there is a single shared "now" across the universe. Rather, I meant that the local "now" we experience is the time period where object interactions and state changes occur.

In hindsight, my use of the phrase "unbroken fabric of causality" may have been misleading. What I intended to convey is that within its sphere of influence, causality remains unbroken and could be considered to have one "now" that bends and conforms to the fabric of space/time in that region. However, beyond a certain distance, causality no longer applies, as even light emitted from our location will never reach those areas. In my mind that place would have its own separate discreet "now" to ours.


criminally_inane t1_jb2sb76 wrote

But then there is a place in between here and there that shares a "now" with both.


Waste_Bin t1_jb3i7wb wrote

I prefer to think of it as three distinct points with three different horizons dependant on relative position and acceleration.

The "now" in-between the two is an artifact.


Grimyak t1_jb30k5j wrote

From a technical standpoint, every location has its own distinct area of influence, even if they have points that overlap each other. However, this doesn't contradict my earlier point as far as I can tell.

To illustrate this concept, imagine three circles that overlap in a linear fashion. The outer two circles extend inward, but do not touch. While an outer circle can interact with the portion of the inner circle that it overlaps with, the inner circle cannot transmit this information beyond the point where it is overlapped by the outer circle. Essentially, although each circle has the potential to overlap with another's influence, it is still unable to communicate or affect regions outside of its own radius.


OberonsGhost t1_jb3i8rp wrote

Can't causes be interconnected instantaneously due to quantum entanglement?


SewFine69420 t1_jb3yev6 wrote

This time stuff has been gnawing on my brain for years, as a fiction writer. In my writing I have beings that were born in different parts of the galaxy, some born on things that weren’t even planets or near a star, so I have a hard time trying to put forth how old they are, except to say they are ancient or ageless or something vague like that. If I try to put a number of years on them I immediately think “okay well they aren’t from earth nor are they on earth, so what exactly is a year in this context”. I have yet to settle on a solution.


andreasbeer1981 t1_jb24q7l wrote

So if a tree falls and there is noone to hear, it doesn't make a sound.


Anonymous_Otters t1_jb274k3 wrote

So, what I described it the opposite of that assertion. No one has to be around to observe difference in time, it is inherent to the fabric of reality.

Also, if a tree falls in the woods and there is no one to hear it, it still makes a sound as sound is the propagation of a fluid compression wave, which occurs regardless of an observer. What it doesn't make is a noise, since noise is the conscious experience constructed by the brain using the stimulus of sound.


Aseyhe t1_jb17c9w wrote

Perhaps I should say there's no unique choice for a universal "now". "Now" would be a 3D surface in 4D spacetime that intersects the point "earth in the year 2023". But there are lots of possible choices you could make in defining this surface, and none of these choices are more correct than the others.


Teo_Filin t1_jb1dzzk wrote

What about simultaneity of entangled particles? Same "now"?


Aseyhe t1_jb1pypu wrote

Entanglement doesn't have any connection to time. It just means that the outcome of a measurement of one particle is correlated with the outcome of a measurement of the other. Those measurements could be performed at any times.


Teo_Filin t1_jb1quc6 wrote

And entangled particles are "connected" via their past moment of interaction?


[deleted] t1_jb1tzq5 wrote



Aseyhe t1_jb1vjul wrote

This is a good way to think of entanglement, as long as you also keep in mind that not every configuration of entangled particles corresponds to a set of local hidden variables in this way (those would be the spooky outcomes you're referring to). But most entanglement scenarios people think of are indeed equivalent to local hidden variables.


OldWolf2 t1_jb3un0o wrote

Your scenario is not entangled, it's just a lack of knowledge on the part of the other person .

Lack of knowledge is NOT the explanation of entangled particles.


PercussiveRussel t1_jb1kli4 wrote

I don't know a lot about cosmology, but I do know about quantum and relativity.

This is a very interesting question! The best way I 'mangle' entangled particles into relativity is to avoid it all together. This is based on the fact that, in order to know about the simultaneity of entangled particles, you'd need a classical channel to transfer information.

Take this experiment: you and I have an entangled particle pair (that's either in AA or BB) and we're lightyears apart. I measure my particle and measure it to be in state A. This means I know yours is in state A too. However, I don't know whether mine is in state A because you collapsed it to be in A, or whether I was the first and have therefore collapsed yours to A. There is no "now your particle is in A" because it could've been there for years (depending on how long ago we separated the particles). The only thing that my experiment proves is that now I know that your particle is in state A. In this way, it's safe to say there is no real simultaneity occurring. This is one of the more prevalent explanations at the moment and one that I personally believe is the most robust since it doesn't require us to completely rip up either relativity or quantum (which are both among the most verified theories we have)


In other words: to me relativity describes the speed of information. Entangled particles don't break this speed as no information is changing hands. This means there is no shared now between the particles.

(The math also shows this, the order of observations is symmetric (they commute), so they have no impact with regards to causality. No causality = no "now")

Edit: To clear something up, for those who don't live and breathe this stuff: entangled particled are two or more particles that share a common set of states. A "quantum object" (which is not a scientific term, but I'll use it anyway) can occupy a quantized (meaning discrete) set of states. When the object is said to be in a superposition, this object is in an undefined state, where it has an x% chance to be in state A, a y% chance to be in state B etc. When we measure the state of this object, it "chooses" one of these states at random and will be that state.

An entangled state means that two seperate objects share a combined superposition. This means that there are less possible states than just all possible permutations. Eg: two objects who can each be in state A or B and are not entangled if they're in the following (equally likely) superposition: AA, AB, BA and BB. Knowing that the first one is A leaves us with the following possible equally likely states: AA and AB. We know precisely nothing about the second object from knowing the first.

Now an entangled object removes some of these possible states. For example the particles are in a superposition of AA, AB or BB. Now knowing that the first one is B tells us that the second one has to be B too. Knowing the first one is A leaves us with no knowledge on the second one. (they're still entangled though, even if the AA and AB states are really useless)

This is "all" entanglement is. It's not something whereby moving the first from state A to B automatically moves the second one from A to B too.

Since you can't control the outcome of a random measurement, there is no way to encode any information in the entanglement itself that is transported on observation. It's pure probabilistics. I mean, you could make the states in a "superposition" of AA and BA so you know the second person always gets a state A after measuring, but you need to send them the particle which goes with locality, so that's just sending an email with extra steps.

The reason that so called "quantum communication" is so interesting though, is that no one can listen in on your conversation. Take the following scheme:

We agree on the following protocol: We have an entangled pair that's in a superposition of AA and BB. I measure mine, then send you a phone call with my measurement. Then you measure your particle (and therefore verify my phone call, beceause our measurements have to be the same).

If I lie, I mean to transmit a 0, if I speak the truth I mean to transmit a 1

No one knows what we are talking about, because they don't know what our entangled pairs are doing. For them, when I tell you "A", it's either a 0 or a 1, with 50% chance each. However notice I do need to say something to you, because I can't affect the outcome of the observation.

I think this explanation clears up a lot about what entanglement actually is. Some people imagine it's like two switches where if you flip one, you instantaneously flip the other one. It's not.


BedrockFarmer t1_jb22lez wrote

This made sense to me, a non mathematician and non physicist. So basically the entangled particles will behave identically when observed. So there is no information linking the particles across spacetime.

So like if I had two cans and two six sided die and I “entangled” the die and closed the lid. I could then send one can to the moon and keep the other and when opened, both die will show the same result because of entanglement when normally there would be a 1/6 probability of what is observed for a single die.


PercussiveRussel t1_jb25fro wrote

Bingo! This is effectively the same thing.

However, I have to be a bit pedantic here, in your example the dice might always have been "destined" to be the same, becauase a simple explanation could be that I glued the dice to the bottom of the can, both facing the number 3 up, and that you and a friend measure the same thing because the dice were always going to show 3. This is what we'd call a 'hidden variable theory' and is almost surely not how quantum probability works.

But yeah, entanglement simply means that knowing the outcome of 1 of the experiments gives you some sort of knowledge about the other experiment (like I said, this could be knowing the exact outcome of the other, or just give you better odds than pure luck for guessing the other experiment). The key concept is that you can't control the outcome of the experiments, you just improve your chances of guessing the other experiment correctly, which is exactly what happens in your dice example.


Teo_Filin t1_jb1o0zq wrote

Cool! So no way for instant info exchange? Such a pity.

Though I can't accept "information" as physical measure at macro-level (it's our description of some properties), just at quantum-level (numbers defining a particle).

I even doubt that time is a coordinate meaning some "future" and "past" do exist as places we can visit (present is real with some intensity of processes influenced by gravity and speed; future is our anticipation, past is our memory).

I'm not deep in advanced physics and higher mathematics, just evolving my perception and checking cosmology with common sense sometimes.


PercussiveRussel t1_jb240js wrote

>So no way for instant info exchange?

Reddit was being reddit, so while I could see your reply in my replies, I couldn't find it to reply to. Reply.

I've added a bit at the bottom of my post to explain why. This isn't just aimed at you, I don't know your understanding of quantum mechanics, but I notice that entanglement is a pretty misunderstood topic. And to be honest, actual quantum researchers talking calling their incredibly cutting edge cool research "quantum teleportation" isn't exactly helping this.

Not accepting or even understanding information at a macro level is perfectly understandable. I'm a physicist and I can't marry my knowledge of quantum with the macro world, I don't understand "information" as a quantum concept in macro scale either. (to be honest, I don't think I fully accept "energy" on the various scales)

I think the most simple way to accept "the speed of information" is to just imagine someone transmitting "Hello world", with laser beams through the vacuum of outer space. Since we know that the speed of light is finite, and furthermore light is the fastest thing in the world, no message can get to us faster than a message sent with light. There are lots of caveats to this of course (there always are), but this conceptually at least made sense to me when I first started to learn about this. Once you start to trust this concept, just flip it on its head: the speed of information is 3.00e8 m/s, so there is no way light is faster than information.

(this is a way to get to grips with the concept, this is in no way a proof nor meant as such)


IAmEnteepee t1_jb2ca9j wrote

In simpler terms, time on earth and on Mars will never be in sync. People traveling back and forth will experience time differently as well, relative to their counterparts on Earth or Mars.


mywan t1_jb2d1v2 wrote

One way to see this is to consider the clock paradox. Spaceships A and B are moving about 86% the speed of light relative o each other. Spaceship A look at B and says the clock at B has slowed down by half. Time slows down for moving objects. But B says no, they are not moving, A is moving. So B says the clock that the clock at A is the one that has slowed down by half.

So how do we test who is right? If A goes and parks next to B to prove they are right then B would appear to be right. But if B goes and parks next to A to prove they are right then A would appear to be right. “Now” is not any more universal than up and down.


sirgog t1_jb3yhjl wrote

> I’m sorry could you explain that more please? There is no universal “now”?

The technical term is "relativity of simultaneity"

Consider two events A and B that happen at "about" the same time, separated by a great distance. They are close enough to simultaneous that light from A cannot get to the location of B until after B has happened, and light from B arrives at A after A has happened.

Let's assume that A occurs at 1230 GMT on 11-Mar-2023, and is a political speech given worldwide coverage made in Scotland. And let's assume that B occurs at 1500 GMT on 11-Mar-2023 by Earth reckoning, and is a critical failure of the Voyager-1 probe which is currently 21 light hours away from Earth.

Observers of both events can calculate the time of each, and deduce which was 'first'.

However, these observers will not necessarily agree with each other.

An observer on Earth who notes both events will conclude, after light arrives 21 hours after Voyager's incident, that the speech was made 2½ hours before the probe failed.

But an alien probe moving through the solar system at 99.98% of lightspeed which saw both events might well conclude that the speech occurred 7 hours after the probe failed.

Both would be correct.

It's a completely counterintuitive mess, but the math checks out on it.


LazyLizzy t1_jb1m5vr wrote

But when will then be now?


TransparentMastering t1_jb0ya8q wrote

How is the rest frame of the CMBR determined if light travels at the same speed in all reference frames? Is it to do with the red and blue shift?


Aseyhe t1_jb0yzuq wrote

It's the center-of-momentum frame for all of the CMB photons (within some volume of space), not the frame of individual photons. It's also the frame in which the CMB temperature is the same in all directions. If you're moving with respect to that frame, you'll find that the CMB is hotter (blueshifted) in the direction of your motion and colder (redshifted) in the opposite direction. That's what we find, and the magnitude of this effect tells us that we are moving at 370 km/s with respect to the CMB.


Void_vix t1_jb2ssty wrote

So accelerating with respect to any other object causes the CMB to Doppler shift for the observer?

Wouldn’t that mean that a truly non inertial frame would have to be at the center of a mass?


azkedar_ t1_jb19cn0 wrote

Is the age of the universe in the CMB frame the maximum it can be in any reference frame? If not, in what sort of reference frame could it be older?


Aseyhe t1_jb1y8p1 wrote

I'll first note that when thinking about the possible trajectories that reach a given spacetime point (e.g. earth in 2023), there is a trajectory that maximizes the elapsed time since the beginning of the universe.

Does that trajectory correspond to the CMB frame? If the universe is homogeneous, then yes. In the presence of peculiar gravitational fields, like those of our galaxy, that doesn't generally remain true, although I think you could pick a "time threading" such that it does. That's another gauge freedom I didn't mention. Along with the freedom to pick the "different positions at the same time" surfaces in spacetime, there's also freedom to pick the "same position at different time" lines.


Ralph_Shepard t1_jb1xfya wrote

Wait, you can really use the cosmic microwave background as a sort of "absolute" reference frame? (Since it is certainly more universal than the Sun or even galactic core). Wow, that is interesting.


SharkFart86 t1_jb203b1 wrote

What you’re asking is kind of a weird question, you can use anything you want as your reference frame. The CMB is often used as the reference frame because it has some convenient qualities that make it more a “fair” reference in cosmology, but there isn’t anything inherent about it that makes it more absolute than any other frame of reference.

And keep in mind that the reference frame matters within the context of the subject. Using CMB as a reference in regards to the orbit of the planets in our solar system makes way less sense than using the sun as the frame of reference. The frame of reference should be whatever makes the model clearer to accurately understand. The CMB works great when looking at the movement or rotation of galaxies, because otherwise those values are very difficult to describe in a clear way.


TaiVat t1_jb2cwhd wrote

I think the question there in terms of "absolute" is that the CMB is "special" in the sense of being the most distant observable "object", and thus is kind of "absolute" in the sense that its the most encompassing of all possible frames. I.e. all of the rest of the observable universe is within it. Kinda like a skybox in a video game.


Ralph_Shepard t1_jb2epci wrote

That is why I used the quotation marks and "better than", but I can't think of any other better reference point


CarolBaskinDidntDoit t1_jb1985y wrote

In a similar (and possibly dumb) question: When we say that the expansion of the universe is accelerating, is it possible that the objects at the far edges of the universe are actually travelling at constant velocity from their reference, but as they get further away from the gravitational center of the universe, the time dilation reduces what a second is and therefore appears from our reference to be accelerating?

E.g. if a=dv/dt. If we measure the velocity of the distant object for an arbitrary amount of time - say 10,000 seconds. And let’s say the object is travelling at constant velocity, but as the object gets away from the gravitational center, a delta 10,000 seconds from the object’s reference would be shorter than a delta 10,000 seconds from our reference. So therefore we perceive this as a change in velocity?


bluesam3 t1_jb1txud wrote

The universe expanding isn't anything to do with objects having velocities: it's the space between them getting bigger.


Howrus t1_jb2mmeo wrote

> objects at the far edges of the universe are actually travelling at constant velocity from their reference

That's the neat trick - they are not traveling at all! Expansion of Universe is happening because space between everything is growing, not because something is flying away.

Closest analogue would be - draw two points on a balloon and start inflating it. Distance between this two points would start to increase without them moving by themselves.


ViralVortex t1_jb2gy10 wrote

You’re looking at now, sir. Everything that happens now is happening now.

What happened to then?!

We passed then.


Just now.


clocks212 t1_jb109x2 wrote

Does the cosmic background radiation have a different velocity relative to the earth in different directions, or as far as we can tell we can consider all of the CMB in every direction to be identically “stationary”.


Aseyhe t1_jb12c8v wrote

The CMB frame is different in different places. It's also the frame of a comoving observer -- that is, one who is moving only due to the expansion of the universe and does not have further ("peculiar") motion. So if we consider distant galaxies in some direction, they are receding in that direction, and so is their CMB frame.


Drops-of-Q t1_jb1gjma wrote

Can you also explain why the age of the universe is dependant on position with the Newtonian gauge?


Aseyhe t1_jb1tjn1 wrote

Gravitational time dilation, due to the -(1+2Φ)dt^2 term in the metric. (In synchronous gauge, that term is just -dt^(2)). The result is that the elapsed time is shortened by a factor of about 1-Φ, where Φ is the Newtonian gravitational potential, of order 10^-6 in our galaxy.


lGrayFoxl t1_jb1h2mn wrote

Is it possible to misinterpret a small detail that made us arrive at 13.7 billion years old that would make us be billion of years off or is it 13.7 give or take a few million for sure.


Aseyhe t1_jb1qtgd wrote

A billion years off is likely possible, given the Hubble tension. There's about a 7% difference between the present-day expansion rate favored by the CMB (~68 km/s/Mpc) and the present-day expansion rate favored by supernovae (~73 km/s/Mpc). 7% of 13.7 billion years is a billion years.


fizzbish t1_jb1rjan wrote

Thanks for this response. But I've always been confused about the CMB frame of reference. Isn't the CMB moving at the speed of light as microwave radiation? How is it the "rest frame" that is always used? Wouldn't its frame be the same as that of a random photon from a star?


bluesam3 t1_jb1v0ev wrote

It's something of a misnomer: it's not the rest frame of the radiation itself, but the rest frame in which the CMB appears the same in all directions: in most rest frames, you'll see it redshifted in one direction, and blueshifted in the other (this is what this looks like for us, for example: the overall hot/cold spots (NB: on this diagram, red is blue-shifted and blue is red-shifted, because humans like red to be hot, even though blue is hotter) are due to our velocity reshifting it, the funky lumps are local effects. If you adjust that to account for shifting the velocity of the observer, you can get it to the point at which that looks almost exactly flat (this famous image scales up the differences by orders of magnitude in comparison to the previous one - actual differences are on the order of one part in 100,000). The reference frame where that image is flattest (modulo a few adjustments for local effects) is the CMB reference frame.


fizzbish t1_jb63gbx wrote

Thanks this clears it up a whole lot for me!


Aseyhe t1_jb1txf9 wrote

An individual photon doesn't have a rest frame, but the CMB rest frame is the center-of-momentum frame of a collection of photons. Since the photons have all different momenta, their center of momentum doesn't move at the speed of light.


Atlein_069 t1_jb2pmzt wrote

How different are the two ages? Like is we consider position does it change significantly if we don’t use Newtonian gauge?


Aseyhe t1_jb2qrs7 wrote

Gravitational time dilation is at most of order one part in a million in most contexts. It's only significantly larger near black holes, neutron stars, and white dwarfs.


DrMaxwellEdison t1_jb34z1i wrote

A follow up question, if I may.

Is there some consensus on a more precise age for the universe than "13.7 billion"? Certainly it's not something that matters in the context of our own life spans, but at some point that 13.7 has to go up. Like the joke of a guard working in a museum who says a dinosaur is 65 million and 4 years old, because it was 65 million when they started working there 4 years ago.

So, is there some agreed-upon starting point we can count up from?


Aseyhe t1_jb47564 wrote

The Planck 2018 paper gives 13.787 ± 0.020 billion years, so that's an uncertainty of 20 million years. That's from the cosmic microwave background.

However, supernova-based measurements of cosmic expansion favor about a 7% higher expansion rate, which could imply the universe is younger by of order a billion years. This discrepancy is the "Hubble tension", a major current research topic.


PvtDeth t1_jb3eo5n wrote

How can you use the CMB as a reference frame if it is roughly uniform in every direction?


Aseyhe t1_jb46ate wrote

It's only isotropic (the same in every direction) if you're in the CMB rest frame. Otherwise, due to the Doppler effect, one direction will be blueshifted and the other redshifted.


CallMePyro t1_jb0ngvu wrote

Yes, however we’re also able to take the age of the universe with respect to the rest frame of the cosmic microwave background. This is the age commonly given when someone asks the age of the universe. From the perspective of Earth, the universe is about 250k years younger.


Something_Else_2112 t1_jb0w3tv wrote

The part that really gets me is that the oldest part of the universe that we can see, has changed and moved for over 13 billion years since it's original light left in our direction. Everywhere we look we are seeing ancient history with our telescopes. The actual distant universe is not visible as it exists now, even if it does exist. So many of those stars could have died by now, we only see their distant past.


[deleted] t1_jb0ynig wrote

Not only is that information old, but you will never be able to receive information from those places as they are today. They are moving from us faster than the speed of light due to the expansion of space, and will never again be connected to us (unless of course the expansion is only local in the larger universe and will later reverse, or some other unknowns). It's groovy to think about.


TheDoctorIsInane t1_jb1ut2z wrote

So where will that information end up? It'll never get to us even though it is heading in our direction. What is the right way to think about it?


ibringthehotpockets t1_jb21oiq wrote

That light is traveling towards us at the speed of light. Always. Eventually the distance between us will be expanding faster than the speed of light. That light will never reach us. You could say it’s in a limbo of some type.


[deleted] t1_jb2kmm7 wrote

It continues to travel through space towards us but the space expands to fast for it to "catch up". Unless something is tightly enough connected to counteract this, it will happen to everything separated by space. The current models even predict that in the far future, the only information available will be that from the local cluster of galaxies. The space between our cluster and the other ones will, although it is relatively small today compared to the visible universe, expand at a faster and faster rate. 1 unit of space becomes 2 units of space in some time, then 4 units of space, 8 etc...

We are actually fortunate that we may observe light all the way back to where it started! The "observable horizon" is still behind the beginning of time, so to speak.


Energylegs23 t1_jb2vji4 wrote

You know one of those hallways in a dream that stretches out so you never actually reach the end? Light is basically trapped in one of those where the "hallway" is the fabric of space-time


zman0313 t1_jb3ivm5 wrote

In this case, can we really consider them part of the “observable” universe?


EcchiOli t1_jb15di4 wrote

Others have a lot more competence to answer than me, but I'll mention the existence of an elegant hypothesis, , the eternal inflation.

A TL;DR would be the "what if" the cosmic inflation, after the big bang, wasn't properly identical in every direction and, well, while areas "cooled off" most others remained subjected to inflation for a while longer, etc, leading to a weird multiverse or perhaps oddly patchwork-based idea of a universe or multiverse. Our universe being one of the areas in which inflation ended up before the rest, even though, most likely, the largest part of the matter and energy from the big bang is still in its inflation phase; we'd be like shards off, with a headstart.

And in such a context, the answer to your question would be a weird "no".

It may be scientifically-based, but it's perhaps less provable than a science-fiction novel, so it's to be taken with a grain of salt, but, eh, it's still an original take on the answer to give you, hehe, I always found it fascinating that such notions will be possible to imagine. It doesn't need to be true to be awesome.


PeculiarAlize t1_jb3dlfz wrote

Yes because space is time and time is space, they are interconnected. Also at the same time no because boundary conditions are relative to a point of origin or reference. So more specifically which age of the universe you are in depends on the location you exist at however that age stays the same size regardless of where you are.


dedokta t1_jb3l6sr wrote

Here's an analogy that's going to fall down if you think about it too much, but it's still valid.

Consider the universe as the surface of a spherical balloon. Don't consider the void in the centre, just the surface. As the balloon expands all parts of the surface move away from you in all directions. The universe expands. If you were to measure the circumference of the balloon and you knew the starting size and the rate of expansion then you'd know the age.

You could do this from any point on the balloon.


randomcanyon t1_jb1yd6q wrote

No. The age of the universe is the same everywhere it is only how we can see it due to the speed of light and our position in the universe. If you were standing 10 billion light years from the Sun on a planet you would be seeing the space from before the Sun or Earth even existed.


DevinVee_ t1_jb3f4t5 wrote

The thing I've never understood is "we're looking at the first light after the big bang" this is what they say every time we look deeper into space. If this light is just now hitting us and it's 13 some odd billion years old. How the hell did we get here before the light after the big bang. So whatever light we're seeing is actually NOT the first light after the big bang. Not by a long shot or am I overthinking this?


Narwhal_Assassin t1_jb3mcuo wrote

Imagine taking a picture of yourself and mailing it to your grandma. That picture shows how you look on the day you take it. It then travels through the postal system for a couple days (or weeks, or months, depending on how bad the system is). In that time, you could change a lot: you could shave your eyebrows, or dye your hair, or get a tattoo, or go tanning. However, the photo still shows the you from a couple days ago, so when your grandma sees it she only knows what you looked like then. The further away she lives, the more stuff can change in the meantime.

In the same way, photons are like pictures of the stuff that emitted them. When the Big Bang happened, a bunch of photons got shot out in all different directions. At the same time, space itself expanded, and it expanded a lot. The distance those photons had to travel went from almost zero to millions of light years faster than they could traverse it. Imagine if the postman was walking the 20 feet to your grandmas mailbox, when suddenly it grew into a 30 mile hike. He’d take a lot longer to bring her the mail. Eventually though, he would make it, and grandma would finally see your lovely face. In the same way, these photons would eventually make it to us, and we could see the early universe: it just takes 13 billion years to cross that gap, since space just keeps expanding while the photons move.

So yes, those photons are from the early early universe, because space itself expanded and made them travel for longer to reach us.


DevinVee_ t1_jb4hg63 wrote

No I understand how the speed of light works. What I don't understand is how earth, sun, solar system, physical matter. Beat photons "from the beginning of the universe" so if it took this light 13 billion years to reach us , it must've taken far longer for us to get to this exact location we are at when we saw it. Meaning a far greater amount of light has already gotten to this point and past. Nothing travels faster than light. So how is it we as physical objects are seeing the fastest thing in the universe just now get here? It's like if grandma left at the same time as the mailman and somehow getting to her house first.

The only other explanation is that all the matter of the universe did not come from the big bang and "we" we're already here.

Please note I use we not as humans but as a place holder for our solar system or our location.


Narwhal_Assassin t1_jb58rm4 wrote

Ok I see what you mean now. Ordinarily, you would be exactly right: there’s no way we could’ve gotten that far away from the photons that they would take 13 billion years to reach us. However, early universe is anything but ordinary.

Going back to grandma and the mailman, imagine they’re both on the sidewalk, but grandma is on the side closer to the house. At the same time, they both start to move towards the house, but as grandma steps off the sidewalk, it suddenly quadruples in size. Grandma is fine since she already got off the sidewalk, but the mailman suddenly has to walk four times as far to catch up. Then, as he hits the halfway point, it quadruples again. And as he keeps going, it keeps getting bigger and bigger. Grandma already made it to the porch and is knitting a sweater, but the mailman hasn’t even gotten off this piece of sidewalk.

This is how the early universe looked. In the first instants after the Big Bang, some photons were going the same direction but from different places. Because space expanded so rapidly, the photons with a “head start” in their direction got a lot further ahead than the others, and this head start kept growing as space continued to expand. Except instead of doubling or quadrupling, it was expanding by millions and billions. Even though they started so close in the beginning, a gap of a nanometer could expand to a light year faster than the photons could cross it, so they got left in the dust.

Remember that all the stuff we see and interact with started off as photons too, just running a shorter race, so after they got here and started settling down into planets, the other photons were still trying to overcome the vast distance that space itself had made by expanding


DevinVee_ t1_jb5c59e wrote

but isn't "space expanding" just matter/objects spreading apart from each other? space isn't a physical thing it's the absence of physical things. So, space expanding in this spot faster than this spot just means these two objects are moving away from each other faster than these other two things are moving away from each other. This still doesn't solve the issue that if the Earth is x distance away from the hypothetical center of the universe (something afaik we haven't determined its location) where we are seeing this light come from, that light theoretically can't be from the big bang, other wise it would imply that the Earth was in this location before the big bang occurred, or that light would've passed through here already.


If you're saying space expands faster than light in certain spots in the universe fine that happens, but saying that means the distance between Earth and the hypothetical center of the universe has expanded faster than the speed of light meaning Earth has traveled faster than the speed of light. Which is impossible according to our understanding of physics.


So with that, I see it as a few possible explanations..


  1. The light we are seeing today from 13 billions years ago (i.e. 13 billion light years away) was, in fact, not the first lights after the big bang.

  2. The lights we are seeing are just "the earliest lights after the big bang" that we've seen and "The first lights after the big bang" is simply click-bait titles. This means that we will never see any light source from the initial big bang or shortly there after because that light has already passed us by. (hypothetically we could see the refraction from a large gravitational force bending said light back but for it to bend back in exactly the same way to reflect something understandable seems impossible.)

  3. The lights we are seeing are from the opposite side of the universe from a source that is expanding just as rapidly (but not at the speed of light) from us and we can actually see the center of the universe -- We know we've seen things that are moving away from us, but have we seen anything moving the exact opposite direction from us? Specifically multiple things?

  4. the big bang is a lie, everything is just floating around nothing is moving away from a specific point.


These are the things that confuse me every time some one mentions the "First lights of the universe" how can that possibly be? regardless of the expansion of space moving faster than the speed of light.


Narwhal_Assassin t1_jb66t7l wrote

Space is a real thing that can expand. If you’ve heard phrases like “the fabric of spacetime” or “the spacetime continuum”, these are actually real, not just some sci-fi mumbo jumbo. You can imagine a big rubber sheet, on which all the planets and stars and everything are sitting. If you label this sheet with a grid and stretch it out, you’ll see that stuff gets further apart, but it doesn’t change position on the grid. That’s how space expands: it doesn’t move things, it just makes the distance between them bigger. (Note: don’t take this analogy too far: unlike rubber, space can stretch infinitely, and it doesn’t “snap back” into place).

So space expanding makes distances bigger, but it doesn’t make objects move any faster. Nothing ever moves faster than light, even when space expands. It just travels a shorter distance, so it can get places earlier.

Also, there is no “center” of the universe. No matter where you are, whether on Earth or on Jupiter or floating somewhere in the middle of the Andromeda Galaxy, if you take the measurements and do the calculations, you’ll find that you are at the center. Every single point in the universe can be treated as the “center”, and every single one of those points would be perfectly accurate for any tests or measurements or calculations you could think of. So, either everything is the center, or nothing is, but there’s not one singular point we can look at and say “yeah that’s the literal exact center and nothing else is.”


DevinVee_ t1_jb6tvq0 wrote

But if the grids don't get bigger they are the same distance, always. Otherwise the two objects are, in fact, moving. If there is no center of the universe then where'd the big bang happen?

Btw I'm really not trying to sound like I'm arguing. I'm actually enjoying this conversation most people I talk to just go "oh, huh, yea that's did you want to order something?"


_mizzar t1_jb8si63 wrote

Your primary misunderstanding is that the past we are seeing into is not the past of “our part” of the universe.

The universe is likely infinite. The observable universe is a sphere with us in the middle. The edge of the sphere is where we see the oldest parts of the universe because the light from these distant places is just now reaching us, showing us what things looked like back then.

This sphere is getting bigger for an obvious reason, more and more light from distant places is reaching us. However, the sphere is also getting bigger because the entire universe (not just the observable universe sphere) is expanding.

Careful here not to imagine the entire universe’s expansion as a sphere, but rather every galaxy that isn’t locally bound to another galaxy by gravity is moving away from one another.

An oversimplified way to imagine this is to visualize an infinite 3D space with tennis balls each 10 meters from one another in every direction. Move forward through time and as the universe expands they are now 20 meters away from one another. Move back in time and they are 5 meters away from one another and so on.

The interesting thing is that, though the speed of light is constant, this expansion of the entire universe seems to happen faster with the more space that there is between things, as if the space itself was causing the expansion (we call this expansion Dark Energy).

What this means is that eventually the expansion of the entire universe will greatly outpace the speed of light, making galaxies we can currently see in the observable universe fade out of sight as they slip out of our observable universe. Eventually, only our own galaxy (at this point merged with Andromeda) and perhaps a few others in our local group will visible to us, everything else too far away and the universe expanding too fast for new light to reach us.

If humans still exist in this time, they would have no knowledge of other galaxies and the universe unless we managed to pass down the data from our time.


DevinVee_ t1_jb95v2p wrote

So then there's parts of the universe currently that we will never see ever unless wormholes etc.


Aseyhe t1_jb7e5vt wrote

> Space is a real thing that can expand. If you’ve heard phrases like “the fabric of spacetime” or “the spacetime continuum”, these are actually real, not just some sci-fi mumbo jumbo. You can imagine a big rubber sheet, on which all the planets and stars and everything are sitting. If you label this sheet with a grid and stretch it out, you’ll see that stuff gets further apart, but it doesn’t change position on the grid. That’s how space expands: it doesn’t move things, it just makes the distance between them bigger. (Note: don’t take this analogy too far: unlike rubber, space can stretch infinitely, and it doesn’t “snap back” into place).

This is kind of a problematic way of thinking, because there isn't any objective sense in which space or spacetime can move or stretch. Those kinds of effects only ever represent subjective choices, often made to simplify a mathematical problem. They are coordinate choices, specifically. The only objective property of a point in spacetime is its (tensor) curvature.

For example, the idea of space expanding is a coordinate choice. It's equally valid to just say that objects are moving apart.

(How, then, can things recede "faster than light"? Just as it's not possible to uniquely define the angle between arrows drawn at different places on a curved sheet, relative velocities of distant objects in curved spacetimes are not meaningful.)


Aseyhe t1_jb7kk6l wrote

The answer is time dilation, essentially. Even though our spacetime is globally curved, it's more intuitive to think about a flat spacetime, since that removes ambiguity about the definitions of distances and relative velocities. In this scenario, the most distant objects are receding at velocities arbitrarily close to the speed of light. That means that even though they might have traveled a great distance from their origin point, arbitrarily little time has passed for them, due to time dilation. So we can receive light from these objects that tells us the state of the universe at arbitrarily early times, even though they could be quite distant from the origin point for the universe.

(I'll also note that a common answer to this question is that the universe didn't begin at a point. While that would also resolve the problem, it's not something we can say for certain.)


[deleted] t1_jb260dh wrote



Monkfich t1_jb2idvd wrote

When people say “the edge of the universe” they actually mean “the edge of the visible universe”. Why? It’s a combination of the (constant) speed of light and the rate that objects (galaxies for this example) fly away from each other in the universe.

Say a galaxy beside us is moving away at 2 kilometres (we’ll use km, 1000m, instead of miles as miles doesn’t really fit in with science very well) per hour, then another galaxy further on is moving at 4 km per hour from our viewpoint in galaxy 1. Galaxy 2 sees both galaxy 1 and 3 moving away from it at 2 km per hour, and galaxy 3 has a similar view to galaxy 1, but for it it looks like we’re the ones travelling away from it at 4 km per hour.

And as these galaxies move away from each other, that 2 km/hr is no longer true, and speeds start to get faster and faster.

So objects in the universe are not just moving away from each other, they are also accelerating away from each other.

Now let’s take that logic and take us thousands of km/ph, then millions of km/ph, then billions of km/ph, and beyond…

Then we get to the speed of light. The speed of light per second is easy to find but gets even more massive when talking about light in km/hr.

1.08 x 10 to the power of 12 km/hr or 1,080,000,000,000 km/hr

That’s a trillion km/hr. And we’re now at the edge of the visible universe - 46.5 billion light years away.

When objects are flying away from us an inch / cm / etc faster than that, we will never see them again. And of course, that object is continually accelerating, so of course it is going faster than that already.

And why don’t we see them? If an object is moving away from us faster than the speed of light, then the light emitted by that object will continue to be emitted, but it will never ever reach us … it didn’t and it can’t go fast enough.

What we see as things get closer to the edge of the universe is that they get massively red shifted, then when they get to the edge, it’s a bit like a black hole - things will appear to get fainter, then as they approach and cross horizon, they will infinitely slow down and appear to stop, then fade away. Or I assume they can’t fade completely, so all objects that ever passed the horizon will be visible on it, just incredibly faint, and effectively gone.

That galaxy that has just sadly disappeared from our view is … ok (or at least it wasn’t affected by a visual effect only experienced by us). That galaxy will have it’s own visible universe, and has just seen our Milky Way slow down, stop, and get continually fainter. And similarly, every object in the very likely infinite universe (very likely, as we can’t observe it for ourselves, snd never will) will experience it’s own individually experienced visible edge of the universe.

Over time, we will see more and more galaxies travel over that horizon, and one day in the far flung future, we’ll not see any other galaxies or objects outside of the milky way (or Milky Way and a few other galaxies linked together by a gravitational force stronger than the expansion rate - these galaxies including the milky way are known as the Local Group). Beyond the stars of the Local Group, it will be dark, and it always will be.

Will that force pulling apart galaxies impact the galaxies themselves - will galaxies be destroyed as stars find that expansion force to be higher than the gravitational force acting on them?

The current theory here is that the expansion rate isn’t big enough for that to happen, but scientific debate changes - or is enhanced - in this field all the time. It’s really interesting stuff!


mishaxz t1_jb2irbt wrote

Is the edge really 90 billion light years away or just 45? For some reason I was thinking 90 sounds more like the width


bullevard t1_jb2wy86 wrote

You are right. 45 billion LY is the furthest distance we can see in any direction, meaning the diameter of the observable universe is 90 billion LY across.


TaiVat t1_jb2dwyh wrote

"They" cant tell us because nobody knows. Chances are, it'll always be impossible to know. It doesnt matter that much though, whichever answer it is, its gonna be too weird to wrap our heads around. That's already the case with a lot of physics we do somewhat understand.