# Is the age of the universe different for a returning space traveller?

When an astronaut returns to the planet he came from, he would then count the exact same number of galaxies as an observer on that planet, but they could have had experienced wildly different amounts of elapsed time. Possibly billions of years if he travelled very near a black hole.

If it is true that we can imply the age of the universe by counting how many galaxies we can see, then would that mean that galaxies would have been disappearing very quickly from view for the astronaut?

• The galaxies merge; this leads to a decrease in the number of galaxies over time. – Yashas Feb 28 '17 at 13:49
• I'm talking about when galaxies disappear from view because they have moved beyond the observable universe. – Paul Feb 28 '17 at 15:07
• The observable universe is, basically, everything whose light has had time to reach us. So it grows larger as time passes, by one light year per year. A galaxy would have to travel faster than light to move beyond the observable universe. – Seeds Feb 28 '17 at 16:32
• @Seeds Lots of galaxies travel faster than light, also ones that we have no problem seeing. – Thriveth Mar 1 '17 at 3:07
• @Seeds Depends on how you define "travel". It is definitely possible for regions of an expanding Space to recede from each other faster than light. In fact, if said Space is infinite and homogeneously expanding, it is inevitable that all regions of Space will recede from a given point faster than light, except from a finite bubble around the point. All galaxies of redshift $z \gtrsim 1.5$ (if I recall correctly), have always beenr eceding faster than light. – Thriveth Mar 2 '17 at 19:35

The Universe's age does not depend on the elapsed time experienced by a moving astronaut. The astronaut will have aged less, but to this person, the surrounding Universe will just seem to have aged more quickly. So to the original question, the answer suggested by the OP is partly correct: To the astronaut, the aging of the Universe will seem to have been much quicker.

However, the idea of counting galaxies rests on some shaky assumptions. You cannot simply tell the age of the Universe by "counting galaxies". Galaxies do not "disappear" over the event horizon, they get increasingly redshifted towards infinity in an infinite amount of time, as seen from our point of view. That's for galaxies "leaving" our zone of visibility. At the same time, there are more and more galaxies for which we can see the earliest times in their history, as more and more light reach us from the earliest parts of the Universe. But these galaxies will also be increasingly redshifted as time passes by and the Universe's expansion accelerates. So if anything, we should measure the Universe's age by mapping the redshifts of the galaxies we observe. That is also easier said than done, but that's a story for another day.

• Thanks. I guess your statement that "The astronaut will have aged less, but to this person, the surrounding Universe will just seem to have aged more quickly" doesn't violate the theory that the laws of the universe are the same for all observers in uniform motion - because the astronaut was not in uniform motion with the planet. So I guess it's ok for them to have experienced two different ageing rates of the universe? I think that would answer my question. – Paul Mar 1 '17 at 23:00
• @P.Doe It's the good old Twin Paradox over again. In terms of relativity, a round trip to and from the Earth would require acceleration, which certainly is not inertial motion. Therefore the astronaut's perceived time is not equivalent to that of the Earth. – Thriveth Mar 17 '17 at 11:45

I'll try to answer this with limited physics and General Relativity vocabulary, hoping that my underlying understanding and interpretation of it is not severely distorted....

Time on earth would have elapsed much faster compared to the clock that the astronaut carried to his destination closer to a black hole. Meaning, the astronaut would have aged lesser when he returns. However, the space itself is consistently expanding. This would imply that both — the observer on earth and the astronaut — would see the galaxies receding away. The difference though would be in the "rate" at which they'd find them drifting away.

Both will have the same basis, before the astronaut's journey starts, to believe that the universe is 13.8 billion years old. Now, from the point that the astronaut's journey begins, they'd start measuring everything related to time, length, distance and speed differently — including their own and universe's age — in their own reference frames. Assuming that he returns back to the earth's reference frame after a few billion earth years (and assuming that the earth is still habitable), his calculations would show that the universe hasn't aged as much as it has for the then residents of earth. Even though both would agree that the universe has since expanded, they would disagree on the rate at which they "saw" it expanding.

There is one very serious catch though... The distant galaxies are moving away faster than the speed of light and at some point in the future there will be nothing else to see in our "increasingly expanding" sphere of the observable universe, aside from our own galaxy. So, if the astronaut, in his venture, goes far beyond into the space — into some other galaxy — and stays there in the vicinity of a massive stellar body for a "while", the expanding spacetime could even ensure that his observable universe would nowhere have Milky Way / Solar system / earth in his sphere of influence and sight, putting us and him out of each other's reach, forever; even if he had the means to travel with near-light-speed necessary to commence the journey in the first place.

Hopefully this describes the situation somewhat realistically. I also hope that if I've been incorrect, then someone would point out, comment on and refine the misjudgements, as appropriate.

• I don't think this is correct: "Assuming that he returns back to the earth's reference frame after a few billion earth years... his calculations would show that the universe hasn't aged as much as it has for the then residents of earth.". If you can calculate the age of the universe by counting the number of observable galaxies, then they both must agree on the age of the universe at the start and end of his journey. They therefore must agree on how much the universe has aged. – Paul Mar 1 '17 at 0:09
• While they'd both agree on the "state" that the universe would be in, the disagreement will be on the measurements done in their distinct reference frames. The reasons being: the velocity with which the travel had to be undertaken to get that far and the effects of gravity, owing to the proximity to a black hole. So, even though the astronaut's clock would start ticking like an earth based clock post his return, it just wouldn't have registered as many ticks when traveling at high speeds or being positioned close to a black hole. – Dhruv Saxena Mar 1 '17 at 0:36
• But that would mean the astronaut had aged less, not the surrounding Universe, which has not moved along with the astronaut. – Thriveth Mar 1 '17 at 3:18