# Where is the edge of the universe

Are you younger today than you were yesterday? Due to the finite speed of light, everything we observe is a consequence of events that occured in our past - i.e., when the universe was younger than it is at present.

Everything we observe is the same amount of time. The age of the atom at emission of a photon plus the duration of the photon always add up to the same present. Say I look out in the night sky with my hand out in front of me. The age of the atom within my hand plus the duration of the young photon I receive from it should be the same amount of time as the young atom in a galaxy far far away plus the duration of the old photon I receive right next to it. So even it we are the oldest part of matter we observe we are still seeing back the same amount of time from our view of our shared present. How else do you think of phased space always being relative.

Chronos
Gold Member
You are missing the point. The atoms in your hand are older [relative to the age of the universe] than the atoms that emitted photons you observe from the distant universe. All photons you observe from the distant universe were emitted when the universe was younger than it is now.

Exactly the photons are travelling from the past to the present or future, the present is younger than the past.

mfb
Mentor
I think you are using two different concepts of "younger". It is impossible to resolve this.

1) Less time has passed since the big bang (age of the universe at that point)
2) Less time has passed until now (similar to the age of humans: measured from "birth"=event to now)

The duration of a photon from emission til reception added to the age of the emitting atom at emission will always equal the age of the atom that is receiving the photon in the present. Could you please show me the error in my thinking.

phinds
Gold Member
2019 Award
The duration of a photon from emission til reception added to the age of the emitting atom at emission will always equal the age of the atom that is receiving the photon in the present. Could you please show me the error in my thinking.
Sure. A photon is emitted from a billion year old atom a billion light years from here and thus by your reckoning is 2 billion years old when it gets here. It hits a newly formed H2O atom that is 1 second old.

HallsofIvy
Homework Helper
The duration of a photon from emission til reception added to the age of the emitting atom at emission will always equal the age of the atom that is receiving the photon in the present. Could you please show me the error in my thinking.
"The time it takes a letter to go from me to my nephew, added to my age, at the time I mail the letter, will always equal the age of my nephew." What is the error in my thinking?

Age of an emitting atom at emission, added to any duration, will be the age of that atom at the end of the duration, just the time it takes a letter to go from me to my nephew add to my age when I send it will be my age when he recieves it.

In other words "The duration of a photon from emission til reception added to the age of the emitting atom at emission" will be the age of the emitting atom at reception.

In asserting that it is also the age of the receiving atom, you appear to be thinking that all atoms are the same age. And that is not true.

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mfb post 29
I think you may be right or perhaps, if this makes sense, time today is younger than yesterday, however I am older today than I was yesterday.

phinds
Gold Member
2019 Award
mfb post 29
I think you may be right or perhaps, if this makes sense, time today is younger than yesterday, however I am older today than I was yesterday.
How do you define time in a way that makes it younger today than yesterday? Are you running time backwards?

In asserting that it is also the age of the receiving atom, you appear to be thinking that all atoms are the same age. And that is not true.
I was under the impression that all matter, in the form of atoms, formed just after the decoupling of photons we see as the cmbr. I did not know new atoms were formed all the time I thought that new atoms were just a recombination of existing atoms into heaver ones not new ones. Thanks.

I didn't want to start yet another thread, so I'll post this here, it's quite closely related.

I've only recently started to comprehend that our universe could be flat and infinite, and has no boundary, this is really crazy. I still don't understand how the universe could have a finite age, a beginning in time and yet be infinite in spatial extent.

There are some truly brilliant minds here, I was hoping somebody might be good enough to explain exactly how this is mathematically possible.

I am not classed as brilliant but basically what you see in this thread is what you get regarding boundaries of the universe, got no idea about the maths though. As far as being edgeless goes imagine there is nothing outside the universe then you cannot have an edge as there is no outside you require an inside and outside to define the edge.
Thats space and you should be able to see the arguments caused when you include time.

mfb
Mentor
explain exactly how this is mathematically possible.
It would have been infinite in its spatial size all the time.

Tu
I didn't want to start yet another thread, so I'll post this here, it's quite closely related.

I've only recently started to comprehend that our universe could be flat and infinite, and has no boundary, this is really crazy. I still don't understand how the universe could have a finite age, a beginning in time and yet be infinite in spatial extent.

There are some truly brilliant minds here, I was hoping somebody might be good enough to explain exactly how this is mathematically possible.
Observable Universe is flat and has finite size... Google Universe and change results to images...

So, based on what I read in this thread it seems Universe is considered to be infinite while Observable Universe is not.

I too would appreciate to see proper explanations.

It makes me also think that then we don't really know what's beyond that which we can observe (if anything), so, why do we need dark matter and dark energy theory to explain where the missing mass is? It could well be beyond that which we can observe, no?

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phinds
Gold Member
2019 Award
Observable Universe is flat and has finite size... Google Universe and change results to images...

Based on what I read in this thread it seems Universe is considered to be infinite while Observable Universe is not.

I'd appreciate too to see proper explanations.
The universe is undefined in shape and size. Might be infinite, might be finite and unbounded. Believed to NOT be finite and bounded as that causes a myriad of problems in physics and seems unreasonable.

The observable universe is a well-defined sphere based on you and extending out for about 50billion light years in all directions.

Tu

It makes me also think that then we don't really know what's beyond that which we can observe (if anything), so, why do we need dark matter and dark energy theory to explain where the missing mass is? It could well be beyond that which we can observe, no?
Dark matter and dark energy is not an explanation in regards to shape or edge of the universe.

dark matter explains the missing mass that attributes to the rate of galaxy spins, by newtonian physics the outer edge of a galaxy would spin slower than the outer edge. This does not occur, it was found that having dark matter the spin works out correctly. this explanation does not cover all the evidence of dark matter which is numerous. However it covers what led scientists to infer its existance.

dark energy is an energy term used to explain the rate of expansion. It was thought at one time that the universe was static. However Einsteins GR predicted either a contracting or expanding universe. Hubble later showed that it was expanding. Dark energy is said to be the cause of that expansion. There is still debate on the cause of expansion mechanism. Could be false vaccum as stated by A. Guths false vacuum model. Could be a bounce as LQG states (hopefully I have that part right LQC is not my familiarity).

In an infinite Universe the only things we can confirm is the observable portions only, no matter how far back in time we look we will never see an edge or border to the universe not that one exists,. The flat geometry was once considered indicative of an open universe (infinite) however later research showed that other flat shapes such as a torus can be flat and finite.
other classical geometries of a closed universe are circle or sphere, and saddle both these geometries are finite geometries.

here is a straightforward link on Universe geometry

http://abyss.uoregon.edu/~js/cosmo/lectures/lec15.html

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mfb
Mentor
Dark energy is said to be the cause of that expansion.
Dark energy is the cause for the acceleration of the expansion. The universe could expand without dark energy as well.

Yeah I should have worded that better lol

The universe is undefined in shape and size.
That seems to me to be the most certain thing we can say on this topic.

All else are just various possible scenarios/theories with higher or lower degree of probability to describe truth of nature.

Though, I find all of them fascinating, because we infer so much from so little (all from light and particles hitting our planet). Amazing.

Another question.
Draw a simple time-line for the universe from BB to present day.
1 Where on that time-line has the universe expanded to.
2 Where on that line do we sit.

The answer to both appears to be the same place. This appears to make no sense at least as far as the observable universe goes. Any explanations. Am assuming time and space expand at the same rate.

Time does not have a spacial component. It neither expands or contracts. Rather its observed rate changes as described by GR and SR.

If your referring to the relation of light speed of travel and the observable universe.
Yes the universe is 13.78 billion years old.
The observable universe today has a radius from our locale of 47 billion light years.
As light travels towards us space is expanding.
Thats how we see further than the age of the universe.
See the FAQ sub forum for more detail.
During the first second of the universe. The universe exponentially expanded. Then expansion slowed down during the matter dominated era. This matter dominated era lasted till roughly 7.3 billion years after the BB. during the matter dominated era gravity was dominant and caused a contraction. However enough vacuum or dark energy was created referred to as the cosmological constant or Lambda $\Lambda$ in the lambdaCDM. The cosmological constant became dominant leading to a lambda dominant era where expansion is increasing.

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Am assuming time and space expand at the same rate.
Well, I guess current science says that space and time are not separate, but one 'thing' as spacetime.

Barbour has different opinion though, that time doesn't really exist, that time is merely our perception of motion of all things.

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$${\scriptsize \begin{array}{|r|r|r|r|r|r|r|} \hline S=z+1&a=1/S&T (Gy)&R (Gly)&D (Gly)&D_{then}(Gly)&D_{hor}(Gly)&D_{par}(Gly)&a'R_{0}\\ \hline 1090.000&0.000917&0.000378&0.000637&45.731&0.042&0.056&0.000&20.164\\ \hline 849.077&0.001178&0.000574&0.000953&45.543&0.054&0.072&0.001&17.299\\ \hline 661.405&0.001512&0.000866&0.001419&45.325&0.069&0.092&0.002&14.917\\ \hline 515.214&0.001941&0.001301&0.002103&45.073&0.087&0.118&0.003&12.919\\ \hline 401.336&0.002492&0.001943&0.003106&44.782&0.112&0.151&0.005&11.230\\ \hline 312.629&0.003199&0.002891&0.004574&44.448&0.142&0.193&0.007&9.791\\ \hline 243.528&0.004106&0.004284&0.006717&44.066&0.181&0.246&0.011&8.558\\ \hline 189.701&0.005271&0.006328&0.009846&43.629&0.230&0.313&0.016&7.496\\ \hline 147.771&0.006767&0.009322&0.014408&43.131&0.292&0.399&0.024&6.576\\ \hline 115.109&0.008687&0.013699&0.021057&42.563&0.370&0.507&0.036&5.776\\ \hline 89.667&0.011152&0.020093&0.030744&41.917&0.467&0.644&0.053&5.079\\ \hline 69.848&0.014317&0.029424&0.044849&41.182&0.590&0.816&0.079&4.469\\ \hline 54.409&0.018379&0.043031&0.065385&40.348&0.742&1.032&0.116&3.935\\ \hline 42.383&0.023594&0.062863&0.095274&39.400&0.930&1.303&0.172&3.467\\ \hline 33.015&0.030289&0.091754&0.138771&38.325&1.161&1.640&0.253&3.056\\ \hline 25.718&0.038884&0.133830&0.202060&37.105&1.443&2.057&0.372&2.694\\ \hline 20.033&0.049917&0.195082&0.294128&35.722&1.783&2.572&0.547&2.376\\ \hline 15.605&0.064080&0.284235&0.428027&34.154&2.189&3.202&0.802&2.096\\ \hline 12.156&0.082263&0.413945&0.622664&32.377&2.663&3.964&1.176&1.850\\ \hline 9.469&0.105605&0.602597&0.905329&30.363&3.207&4.876&1.722&1.633\\ \hline 7.376&0.135570&0.876751&1.315024&28.084&3.807&5.950&2.520&1.443\\ \hline 5.746&0.174038&1.274606&1.906448&25.507&4.439&7.190&3.683&1.278\\ \hline 4.476&0.223420&1.850354&2.753067&22.602&5.050&8.581&5.378&1.136\\ \hline 3.487&0.286815&2.678630&3.944165&19.346&5.549&10.082&7.837&1.018\\ \hline 2.716&0.368198&3.856519&5.562897&15.738&5.795&11.615&11.390&0.927\\ \hline 2.116&0.472674&5.494908&7.622994&11.826&5.590&13.062&16.471&0.868\\ \hline 1.648&0.606794&7.688893&9.965115&7.742&4.698&14.289&23.622&0.852\\ \hline 1.284&0.778970&10.465993&12.220152&3.709&2.889&15.203&33.467&0.892\\ \hline 1.000&1.000000&13.753303&13.999929&0.000&0.000&15.793&46.686&1.000\\ \hline 0.848&1.178769&16.127949&14.832270&-2.203&-2.596&16.037&57.611&1.113\\ \hline 0.720&1.389495&18.618735&15.419791&-4.150&-5.766&16.198&70.616&1.262\\ \hline 0.611&1.637894&21.189539&15.814752&-5.855&-9.590&16.300&86.034&1.450\\ \hline 0.518&1.930698&23.813226&16.071344&-7.332&-14.156&16.363&104.265&1.682\\ \hline 0.439&2.275846&26.470879&16.234284&-8.601&-19.575&16.400&125.793&1.963\\ \hline 0.373&2.682696&29.149943&16.336240&-9.687&-25.986&16.420&151.192&2.299\\ \hline 0.316&3.162278&31.842341&16.399447&-10.612&-33.558&16.429&181.147&2.700\\ \hline 0.268&3.727594&34.543144&16.438244&-11.399&-42.492&16.438&216.465&3.175\\ \hline 0.228&4.393971&37.248853&16.462174&-12.069&-53.030&16.462&258.104&3.737\\ \hline 0.193&5.179475&39.957670&16.476843&-12.637&-65.454&16.477&307.189&4.401\\ \hline 0.164&6.105402&42.668392&16.485825&-13.120&-80.101&16.486&365.051&5.185\\ \hline 0.139&7.196857&45.380278&16.491324&-13.529&-97.369&16.491&433.258&6.110\\ \hline 0.118&8.483429&48.092876&16.494692&-13.877&-117.723&16.495&513.659&7.200\\ \hline 0.100&10.000000&50.805908&16.496757&-14.172&-141.718&16.497&608.434&8.487\\ \hline 0.085&11.787686&53.519206&16.498025&-14.422&-170.001&16.498&720.152&10.003\\ \hline 0.072&13.894955&56.232831&16.498643&-14.634&-203.342&16.499&851.842&11.791\\ \hline 0.061&16.378937&58.946390&16.499128&-14.814&-242.642&16.499&1007.074&13.898\\ \hline 0.052&19.306977&61.660009&16.499432&-14.967&-288.969&16.499&1190.057&16.382\\ \hline 0.044&22.758459&64.373666&16.499625&-15.097&-343.577&16.500&1405.752&19.311\\ \hline 0.037&26.826958&67.087345&16.499751&-15.207&-407.947&16.500&1660.006&22.763\\ \hline 0.032&31.622777&69.801037&16.499835&-15.300&-483.825&16.500&1959.712&26.832\\ \hline 0.027&37.275937&72.514739&16.499894&-15.379&-573.267&16.500&2312.997&31.628\\ \hline 0.023&43.939706&75.228445&16.499938&-15.446&-678.699&16.500&2729.438&37.282\\ \hline 0.019&51.794747&77.942154&16.499972&-15.503&-802.978&16.500&3220.325&43.947\\ \hline 0.016&61.054023&80.656031&16.499836&-15.551&-949.475&16.500&3798.968&51.804\\ \hline 0.014&71.968567&83.369743&16.499861&-15.592&-1122.161&16.500&4481.054&61.065\\ \hline 0.012&84.834290&86.083456&16.499884&-15.627&-1325.718&16.500&5285.075&71.981\\ \hline 0.010&100.000000&88.797170&16.499905&-15.657&-1565.665&16.500&6232.831&84.849\\ \hline \end{array}}$$

Time now (at S=1) or present age in billion years: 13.753301 'T' in billion years (Gy) and 'D' in billion light years (Gly)

Look at the 7.6 billion year timeline you can see where the change over occcurs in the last columm.

this graph is for a theoretical galaxy at the edge of the observable universe today. or stretch 1090 which is the first row. the S row 1.000 is when the BB occured at 10-43 seconds.

the a'Ro column shows the distance increase to the hypothetical galaxy. you can see the change in contraction and expansion related to its recessive velocity in S=1.648 or first column

the calculator is developed by forum members and is in the sticky or pinned thread above "look 88 years into the .." thread above.

http://www.einsteins-theory-of-relativity-4engineers.com/TabCosmo9.html

here is a quote from wiki thats appropriate

The metric expansion of space is the increase of the distance between two distant parts of the universe with time. It is an intrinsic expansion whereby the scale of space itself is changed. That is, a metric expansion is defined by an increase in distance between parts of the universe even without those parts "moving" anywhere. This is not the same as any usual concept of motion, or any kind of expansion of objects "outward" into other "preexisting" space, or any kind of explosion of matter which is commonly experienced on earth.

the keyword is scale of space itself is expanding

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Try to imagine 3 orthogonal (mutually perpendicular) planes.

That’s pretty easy; the corner of any cube shows how 3 orthogonal planes meet. It defines the 3-dimensional (3-D) space we live in. Look at the corner of your room, 2 walls and the ceiling are 3 mutually perpendicular planes (in most normal rooms).

Only, we don’t live in a 3-D space. Time is a dimension. We appear to live in a 4-dimensional (4-D) space.

We can’t actually visualize 4 orthogonal planes. The 4 planes of ordinary space and time are mutually perpendicular, but we have 3-D eyes and can’t see or draw 4-D space. We can draw 2-D spaces and build 3-D spaces, but images beyond that are not possible (yet).

However, we can describe it mathematically. It’s not too difficult to do, just add a 4th dimension to the X, Y & Z Cartesian coordinates, call it T if you will.

Our 3-D (X,Y, Z) space is within this 4-D (X, Y, Z, T) space.

Wouldn’t our 3-D space be expanding within 4-D space? What are the boundaries of this 4-D space? What would constitute an edge in 4-D space?

What if the universe is actually 9, or 11 or 12 dimensional?

The expansion of 3-D (and 4-D etc.) space would be subsumed by these higher dimensions, wouldn’t it?

There's an edge to a sheet of paper, but I'm not so sure there's an edge to the Universe.

You might enjoy reading Flatland by Edwin Abbott

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